Laser welding station and method for welding together elements of metal manufactured products

Abstract

Laser welding station comprising: a first laser (2) for emitting a main beam (B1) at a first wavelength (λ1) in the near infrared (NIR) spectrum; a second laser (3) for emitting an auxiliary beam (B2) at a second wavelength (λ2) in the visible spectrum; an optical head (4) configured to emit a laser beam (LB) combining the main (B1) and auxiliary (B2) beams to create a molten-metal pool to join together at least two elements (W1, W2) of a workpiece (W); and a monitoring device (5) configured to detect a reflected light emission (RL) from the workpiece and comprising a first detector (52), configured to detect the reflected light (RL) in the first wavelength (λ1) and a second detector (53), configured to detect the reflected light (RL) in the second wavelength (λ2) of the auxiliary beam (B2), thereby monitoring both the absorption of the main (B1) and auxiliary (B2) beams.

Description

[0001] 2

[0002] “LASER WELDING STATION AND METHOD FOR WELDING TOGETHER

[0003] ELEMENTS OF METAL MANUFACTURED PRODUCTS”

[0004] SCOPE OF APPLICATION

[0005] The present invention relates to a laser welding station for welding together at least two elements of a metal product and to a laser welding method. In particular, the welding station is configured to monitor and the laser welding comprises a step of monitoring the reflected light of the product due to the incidence of the laser beam responsible for the melting of the metal material, in manufacturing applications such as welding of electrochemical battery elements, welding of electrical terminals of a winding or the like.

[0006] STATE OF THE ART

[0007] In the field of welding products made of metallic materials, the use of laser sources configured to quickly transmit thermal energy to the elements to be welded to each other is known, thus obtaining a good compromise between product quality, plant productivity and welding process cost.

[0008] Due to mechanical design needs, in some cases, the weld to stably join two elements has to be highly resisting to a tearing force stress. Such mechanical property requirement is achievable thanks to the penetration capacity of the laser beam in the manufactured articles during the welding process. In fact, there is a specific laser welding method which is typically known as “keyhole welding”, wherein the high-power density of the laser source allows to generate a beam with a very small incidence area on the manufactured article to be worked, in contrast to the heat conduction welding method, which is characterized by presenting a lower power and energy density, resulting in a rather superficial and wider fusion compared to the keyhole welding method. The thermal welding method, on the other hand, is pursued in cases where a rather superficial welding is necessary, for example when two thin layers of material are overlapped with the first layer interposed between the laser source and the second layer to be welded reciprocally to the first.

[0009] In the keyhole welding mode, instead, the incident laser beam generates a narrow profile, a real excavation in the material due to the vaporization of metallic material, thus allowing the laser beam to penetrate deeply into the material to be welded. During the process, the excavation (keyhole) is surrounded by molten material generated in the meantime due to the heat transfer between the laser beam and the metallic material. This molten material tends to fill the excavated space as the laser beam is moved along a welding path in the workpiece to 3 be welded. In the keyhole welding mode, it is known to scan the laser beam for a plurality of repetitions in a predetermined welding path, in order to progressively generate the excavation and the molten material.

[0010] The keyhole welding method is now widely used in many industrial sectors. For example, in the automotive industry the use of infrared laser sources is known, in particular the near infrared or NIR type (Near InfraRed), typically with wavelengths between 700 and 1,400 nm, in particular between 900 nm and 1,080 nm.

[0011] The keyhole welding of metal products includes specific engine components, mechanical transmission components, fuel injectors, fuel filters, HVAC components such as air conditioning or structural parts of vehicle chassis and body.

[0012] Typically, the abovementioned applications are, at least in large part, made of ferrous alloy materials, such as steel and cast iron, wherein the absorptivity of infrared light generated by NIR sources by iron is relatively high (about 36.5% for the chemical element iron - Fe), making the use this technology particularly convenient, which is widely consolidated and has an excellent quality-price ratio and high productivity.

[0013] However, with regard to the ongoing technological evolution in the automotive sector, in particular the transition to electric mobility, the market is increasingly introducing a series of new components for electrified propulsion, which likewise require welding processes during their assembly phases. A significant portion of these new generation components are made of non-ferrous metal materials.

[0014] For example, reference is made to electromechanical motors for vehicle traction and electrochemical energy storage devices, such as lithium-ion batteries. In both cases, the need to transmit electrical energy with high efficiency requires the use of non-ferrous metallic materials with high electrical conductivity (low electrical resistivity) in order to reduce electrical transmission losses in use, essentially due to the Joule effect. Particular examples of these non-ferrous metallic materials are copper (Cu) and aluminum (Al) and respective alloys thereof, which present low electrical resistivity and are economically convenient compared to other materials.

[0015] However, such non-ferrous metallic materials, such as copper or aluminum, are characterized by presenting a low absorptivity to irradiation in the near infrared (NIR) spectrum. In particular, when copper is irradiated by a NIR laser source, the absorptivity is around 3-4%, in contrast to the above-mentioned ferrous materials, typically with an absorptivity of an order 4 of magnitude higher than copper or aluminum. Such property inherent to the aforementioned non-ferrous materials leads, consequently, to the need to oversize the single NIR laser source, so as to transmit enough power and energy density, in order to contrast the low absorptivity of the material to be welded.

[0016] For example, in the process of welding terminals of “hairpin” -type elements of the stator windings of an electric-vehicle (EV) motor, the use of a NIR laser source is known. These terminals are typically made of copper, therefore given the low absorptivity of copper in the infrared spectrum, the laser welding process is rather unstable, consequently, prone to the generation of “spatter” phenomena due to the implosion of molten material in contact with the cloud of vaporized metal during the solidification of the welding bubble.

[0017] Lately, it has been demonstrated the use of laser sources operating in wavelengths where the absorptivity by the aforementioned non-ferrous materials is greater. Referring to FIG. 3 of this document, it can be observed how the absorptivity of light in non-ferrous metallic materials is greater in the visible light spectrum (VIS). In other words, for wavelengths between 380 and 750 nm approximately, with particular reference to blue light, between 400 and 500 nm wavelength.

[0018] The great technical development of laser technology since the early 2000s has led to the technical maturity of Direct Diode Lasers (DDLs), which are applicable to mechanical machining processes and are applicable in different embodiments, such as using a blue laser diode source or the green laser diode source.

[0019] The abovementioned laser diode-based solutions can be applied on their own or in combination with the above-mentioned infrared (NIR) laser source to obtain a hybrid welding beam. The hybrid welding beam combines the advantages of both technologies. In such cases, two different sources are used, for example a first source operating in the near-infrared (NIR) spectrum and a second source operating in the visible spectrum, in particular with a blue laser diode light source.

[0020] On the one hand, the laser diode source operating in the blue light spectrum is mainly used to preheat the workpiece, stabilizing the welding process, thanks to the higher absorption rate, while the laser source operating in the near-infrared (NIR) provides a higher power, typically with a smaller incidence diameter than the blue source, which ensures a fast and precise melting of the metal to obtain an intrinsically more stable welding process. Furthermore, the presence of a blue laser beam once the metal has melted can contribute to a more controlled 5 cooling process, thus reducing the porosity of the welding joint and consequently increasing the mechanical resistance of such joint.

[0021] Anyway, regardless the chosen configuration, either with a single NIR source or with two sources operating at different wavelengths, the use of laser sources, with particular reference to the keyhole welding mode, the welding station requires a very fine setting of the welding process. Consequently, a parallel welding parameter control process is normally required, so as to be able to react to the various disturbance factors responsible for defects in the workpiece or malfunctions of the welding station.

[0022] In fact, hybrid welding systems, comprising two laser sources operating at different wavelengths, require a quality control process that ensures high repeatability and minimization of non-conformities and, at the same time, which reduces energy consumption due to high NIR laser powers and increases the process speed. In particular, such welding quality control process can be based on optical technologies that collect information from the welding process.

[0023] To this end, it is possible to monitor process signals in hybrid welding systems comprising two laser sources. For example, patent application US20230311243A1 (see FIG. 1 of this document) teaches the use of a process sensor in a welding station comprising two sources operating at different wavelengths, where each of these sources generates a respective laser beam. Each of the two laser beams is defined by different operating parameters, including the beam geometry, more specifically: the diameter incident on the workpiece or the focal distance, the pico power and the geometric distribution of the beam (e.g., Gaussian distribution). In US20230311243A1, the two laser beams are combined together in an optical head, typically concentrically superimposed between them. This optical head is configured to combine both beams and to generate a welding laser beam, incident on the particular workpiece on which the appropriate processes are to be performed.

[0024] Once the laser beam acts on the particular workpiece to be welded, that workpiece reflects a fraction of the incident light. In US20230311243A1, the reflected light is intercepted by an optical sensor, such as a camera or tomographic sensor, configured to detect the intensity of the optical signal in a specific spectrum of wavelengths. US20230311243A1 refers in particular to the use of a camera, in a particular infrared spectrum, to measure the temperature of the molten metal during the welding process.

[0025] Similarly, the Japanese patent application JP2004058141A teaches a hybrid welding station with a first fundamental wave YAG laser that emits laser light having a wavelength of 1,064 6 nm and a second harmonic YAG laser that emits laser light having a wavelength of 532 nm by doubling the frequency of an infrared Nd:YAG laser by means of a nonlinear material. The light reflected from the workpiece is monitored by a silicon-based photodiode (Si), which operates in the of 1064 nm wavelength spectrum. Furthermore, the welding station is provided with an additional photosensor made of InGaAs, which measures the thermal radiation light in the 1,300 nm wavelength range, thus looking to the temperature of the welding process.

[0026] With the configuration just described, a verification operation of the detected signal intensity will concern the whole of the reflected light or just one of the two laser sources involved, allowing only to identify anomalies detectable in a certain spectrum of the reflected light rather than simultaneously monitoring, individually, the correct performance of both laser sources generating the concentric welding beam.

[0027] There is therefore a need to improve a laser welding station that can overcome the drawbacks of the prior art. To do this, it is necessary to solve the technical problem of creating a laser welding station that is effective, fast and efficient in its operation, with particular reference to a monitoring device configured to detect the light emitted by the laser devices and the light reflected by the workpiece being processed.

[0028] The main task of the present invention is to solve the abovementioned problems by proposing a laser welding station and a related laser welding method that allows the detection of the components of the back-reflection signal in the wavelengths of the two laser sources incident on the workpiece to be welded.

[0029] Another aim of the present invention is to provide a laser welding station and a related laser welding method comprising two laser sources operating at different wavelengths which provide richer process information and are able to identify the causes of defects in a more direct way.

[0030] Another object of the present invention is to provide a laser welding station and a related laser welding method that provides process information relating to the different phases of the laser welding method.

[0031] Another aim of the present invention is to provide a laser welding station and a related laser welding method that allows monitoring the fluctuations of the components of the back- reflection signals in the wave frequencies of the main beam and auxiliary beam.

[0032] Another object of the present invention is to provide a laser welding station and a related multipurpose laser welding method, which allows to monitor defects and anomalies in welding 7 processes comprising two laser sources of different wavelengths for different welding modes and materials to be welded to each other.

[0033] A further aim of the present invention is to propose a laser welding station and a related laser welding method which guarantee high productivity thanks to the drastic reduction of waste or maintenance required due to unwanted effects such as the spatter phenomenon.

[0034] Another aim of this invention is to increase the speed of the welding process by using a second, appropriately monitored laser source.

[0035] This task and these objects are achieved by the laser welding station according to claim 1. This task and these objects are also achieved by means of the laser welding method according to claim 13.

[0036] To overcome the drawbacks of the known art and to obtain these and further purposes and advantages, the Applicant has studied, evaluated, and created the present invention.

[0037] SUMMARY OF THE INVENTION

[0038] The present invention is expressed and characterized in the independent claims. The dependent claims set forth other characteristics of the present invention or variants of the main solution idea.

[0039] In accordance with the above-mentioned purposes and to solve the above-mentioned technical problem in a new and original way, also obtaining significant advantages with respect to the prior art, according to the present invention a laser welding station is provided comprising a first laser device comprising a first oscillator configured to emit a main beam at a first wavelength; a second laser device, comprising a second oscillator configured to emit an auxiliary beam at a second wavelength different from the first wavelength; an optical head configured to emit a laser beam for generating a molten weld pool to join at least two elements of a workpiece, wherein the laser beam is formed by combination of said main beam and said auxiliary beam; first and second optical members configured to transmit the main and the auxiliary beams, respectively, to the optical head; and a monitoring device configured to detect reflected light emitted or reflected from the workpiece.

[0040] In accordance with an aspect of the present invention, the monitoring device comprises a first semiconductor sensor, configured to detect back-reflected light at a first wavelength spectrum substantially corresponding to the first wavelength, thereby monitoring absorption of the main beam by the workpiece, and a second semiconductor sensor, configured to detect back- 8 reflected light at a second wavelength spectrum substantially corresponding to the second wavelength, thereby monitoring absorption of the auxiliary beam by the workpiece.

[0041] Preferably, the optical head is further configured to at least partially transmit the reflected and emitted light from the workpiece to the monitoring device.

[0042] Preferably, the optical head comprises at least one optical lens configured to direct the laser beam onto the workpiece and to focus the reflected and emitted light from the workpiece towards the monitoring device: In particular, the at least one optical lens preferably comprises a plurality of lenses, in particular f-theta lenses.

[0043] Preferably, the optical head further comprises a scanning device, in particular of the galvo scanner type, positioned upstream of the at least one optical lens. The scanning device is configured to vary the orientation of the laser beam in two mutually perpendicular directions to scan the surface of the workpiece along a predetermined path.

[0044] Preferably, the first laser device is configured to emit the main beam in the near-infrared (NIR) spectrum. The near-infrared spectrum is defined as a wavelength range between 700 and 1,400 nm, preferably between 800 and 1,100 nm, even more preferably between 1,020 and 1,090 nm.

[0045] Preferably, the oscillator of the first laser device is of the solid-state type. In particular, the oscillator of the first laser device comprises at least one laser diode. Alternatively, the oscillator of the first laser device may comprise another type of laser element, such as an Nd:YAG type source, a fiber laser source, or a disk laser.

[0046] Preferably, the oscillator of the first laser device is configured to operate in single mode.

[0047] Preferably, the average power density of the main beam emitted by the first laser device is in the range between 15 and 80 MW / cm2, in particular between 20 and 60 MW / cm2and even more preferably between 25 and 50 MW / cm2. Average power density means the quotient between the average power of the beam and the area on which it is incident. To calculate the average power, the integral or sum of the distribution function based on the diameter is performed, i.e., the intensity profile, as shown in figures 8A, 8B.

[0048] Preferably, the second laser device is configured to emit the auxiliary beam in the visible light spectrum (VIS). In particular, the visible light spectrum is that of blue light. Blue light is understood to be a beam with wavelengths in the range between 400 and 500 nm, more preferably between 420 and 460 nm. 9

[0049] Preferably, the oscillator of the second laser device comprises at least one blue laser diode. Advantageously, the oscillator of the second laser device can comprise a plurality of blue laser diodes. In embodiments where the at least one blue laser diode is a plurality, the different emitted laser beams, with limited power, are combined into a single auxiliary beam thanks to dedicated optical elements, thus obtaining an auxiliary beam of sufficient power for the intended purposes and maintaining an acceptable beam quality.

[0050] Preferably, the average power density of the auxiliary beam emitted by the second laser device is in the range between 0.5 and 1.5 MW / cm2, even more preferably between 0.6 and 1 MW / cm2.

[0051] Preferably, the first laser device comprises, downstream of the respective oscillator, a first optical group. Preferably, the second laser device comprises, downstream of the respective oscillator, a second optical group. Preferably, the first and / or second optical group comprises a fiber optic cable. This fiber optic cable is configured to transmit the laser beam generated by the respective oscillator.

[0052] Preferably, the first optical group comprises a first collimating lens. Preferably, the second optical group includes a second collimating lens. The collimating lenses of the respective optical groups are configured to straighten the beams coming from the respective oscillators of the laser devices.

[0053] Advantageously, the first and / or second optical group may comprise a respective convex lens. Such convex lens is configured to converge the beam to a predetermined focal distance. Some embodiments provide for such convex lens to be mounted movably, so as to be able to vary the focal distance of the beam of the respective laser device.

[0054] Equally, the first and / or second optical group may comprise, respectively two mutually aligned fixed convex lenses at a fixed distance from each other, with a movable concave lens interposed between said fixed convex lenses. With such configuration, the focal distance may be varied of each laser beam may be varied.

[0055] Preferably, the first laser device is configured to emit a main laser beam having a substantially circular area. Preferably, the second laser device is configured to emit an auxiliary laser beam having a substantially circular area. Preferably, the diameter of the main beam emitted by the first laser device will have a smaller diameter than the diameter of the auxiliary beam emitted by the second laser device. 10

[0056] Preferably, the first laser device is configured to emit a Gaussian-type main beam. A Gaussian light beam is defined as a beam having an intensity profile perpendicular to the direction of beam propagation with a Gaussian distribution (Gaussian bell). Preferably, the second laser device is configured to emit an auxiliary beam of the Gaussian type.

[0057] Advantageously, the first and / or second optical group may comprise, downstream of the respective focal lens, a diffractive optical element (DOE). This diffractive optical element or DOE is configured to shape the profile of the main and / or auxiliary beam. The use of a diffractive optical element or DOE is particularly advantageous in ring-core configurations, in which two concentric beams of different diameters are superimposed. In such cases, it is possible to modify the propagation profile of the external beam in order to create an external ring, in which the distribution of the beam in the direction perpendicular to the propagation plane has a W shape, leaving the central part substantially empty. In such cases, the beam of smaller diameter can occupy the internal (empty) part of the ring and function as a core.

[0058] Preferably, the welding head comprises at least one welding lens configured to direct the laser beam towards the workpiece to be machined.

[0059] Preferably, the elements of the workpiece to be welded together will be positioned close together. According to preferred embodiments of the present invention, the elements of the workpiece to be welded to each other are both arranged facing the welding head, oriented so as to be directly hit by the laser beam. These embodiments allow the welding process to be carried out in a thermal welding mode or in a deep or keyhole welding mode.

[0060] Alternative embodiments of the present invention provide that the elements of the workpiece to be welded to each other are arranged facing the welding head and stacked on top of each other. In this way at least a first element will be interposed between the welding head and a second element, on which the first element will he. In such embodiments, the welding beam will hit the first element, the heat transfer by conduction between the first and the second element of the workpiece downstream of the incidence of the welding beam on the first element generating a superficial molten metal pool will determine the reciprocal welding between the two elements.

[0061] Preferably, the at least one welding lens of the welding head is constituted as a plurality of lenses. In particular, such plurality of lenses will be of the f-theta type. The f-theta lenses contain a plurality of lenses, typically three or four, pre-aligned with each other and housed in a robust case and are configured to focus the welding beam on a flat surface. This type of 11 configuration is particularly advantageous when the workpiece to be worked is arranged on a plane perpendicular to the welding beam at a predetermined distance.

[0062] Preferably, the welding head includes a scanning device upstream of the welding lens. This scanning device is configured to change the orientation of the welding beam, i.e., the main and auxiliary beams combined together in the welding head, so as to scan the welding beam along a path on the surface of the workpiece.

[0063] Preferably, the scanning device is a galvo scanner. Such a galvo scanner is configured to vary the orientation of the welding beam in two mutually perpendicular directions, so as to scan the welding beam in two mutually perpendicular directions of the surface of the workpiece to be machined.

[0064] In accordance with another aspect of the present invention, the welding station monitoring device further comprises a monitoring mirror configured to reflect the light of the main and auxiliary beams of the laser beam as the back-reflected light reflected from the workpiece, towards at least said first and second semiconductor sensors. In this manner, the back- reflection signals will be efficiently transmitted to the individual semiconductor sensors of the monitoring device.

[0065] In accordance with another aspect of the present invention, the first and / or second optical groups comprise respective first and second mirrors. The first and second mirrors of the respective first and second devices are configured to orient the respective main and auxiliary beams towards the optical head of the welding station. Preferably, such first and / or second optical group mirrors are configured to steer the main beam and the auxiliary beam coaxially.

[0066] In accordance with another aspect of the present invention, the monitoring mirror is aligned with the first and second mirrors of the respective first and second optical groups, so that the monitoring mirror transmits both the light of the main and auxiliary beams of the laser beam towards the optical head and the back-reflected and emitted light by the workpiece crossing coaxially the optical head to be analyzed by the monitoring device.

[0067] The monitoring mirror may be placed between the laser devices and the welding head. Alternatively, the monitoring mirror may be placed between the first and second mirrors of the respective first and second optical groups of the laser devices.

[0068] In accordance with another aspect of the present invention, the monitoring device further comprises a first optical filter interposed between the monitoring mirror and the first and second semiconductor sensors. Such first optical filter is configured to reflect at least a second 12 back-reflection signal towards the second semiconductor sensor and to transmit at least a first back-reflected signal to the first semiconductor sensor. By doing so, the first optical filter splits the light into two complementary spectra, facilitating separate detection of the signals.

[0069] Preferably, the first optical filter is a dichroic filter configured to operate as a low band pass filter below a certain threshold wavelength, selectively reflecting light with wavelengths shorter / longer than that threshold wavelength and transmitting light with wavelengths longer / longer than that threshold.

[0070] Alternative embodiments provide that such first optical filter comprises a beam splitter. Such beam splitter will be configured to divide the light incident on it into two or more beams, each beam having a certain spectrum of wavelengths. In embodiments in which beam splitters are used, it will also be necessary to use additional optical filters arranged downstream of the beam splitters. Alternatively, other implementations different from the use of a dichroic filter or a beam splitter just illustrated are not excluded.

[0071] In accordance with another aspect of the present invention, the monitoring device further comprises a third semiconductor sensor configured to detect emitted light from the welding process in a third, visible wavelength light spectrum, thereby monitoring the plasma of the molten weld pool generated by the laser beam. This third visible light spectrum is an indicator of the plasma generated by the incidence of the welding beam on the workpiece during melting. Typically, plasma is detectable in the visible light spectrum. Detection of light in the third visible light spectrum by the third semiconductor sensor will enable the welding process to be monitored with respect to the plasma of the molten metal pool generated by the incidence of the main and auxiliary beams, i.e., the combined welding beam.

[0072] In accordance with another aspect of the present invention, the monitoring device further comprises a second optical filter interposed between the first optical filter and the second semiconductor sensor. Such second optical filter is configured to reflect the second back- reflected signal of the light emission with a wavelength spectrum substantially corresponding to the second wavelength emitted by the second laser device. Such second optical filter is preferably a dichroic filter configured to operate as a low band pass filter.

[0073] In accordance with another aspect of the present invention, the second optical filter is further configured to transmit a third plasma signal of the emitted light in the third visible light spectrum. The third visible light spectrum has a wavelength range greater than those of the 13 second spectrum. Besides, the second optical filter is arranged to direct the third plasma signal towards the third semiconductor sensor.

[0074] In accordance with another aspect of the present invention, the monitoring device further comprises a fourth semiconductor sensor configured to detect emitted light from the welding process in a fourth, thermal infrared spectrum, thereby monitoring the temperature of the workpiece. Monitoring the temperature of the workpiece concerns the different phases of the laser welding process, from the heating of the workpiece by scanning it with the laser beam, measuring the temperature of the molten weld pool formed on the workpiece and monitoring the temperature of the welding joint during the subsequent cooling once the main and / or auxiliary beams have been turned off.

[0075] In accordance with another aspect of the present invention, the monitoring device further comprises a third optical filter interposed between the first optical filter and the first semiconductor sensor. Such third optical filter is configured to reflect the first back-reflection signal corresponding to the first wavelength. Such third optical filter is preferably a dichroic filter configured to operate as a low band pass filter.

[0076] In accordance with another aspect of the present invention, the third optical filter is further configured to transmit a fourth thermal infrared signal in the fourth thermal infrared spectrum which has a wavelength range greater than that of the first spectrum. Such third optical filter is arranged to direct the fourth thermal infrared signal towards the fourth semiconductor sensor.

[0077] Preferably, the first semiconductor sensor comprises a photodiode. Preferably, the second semiconductor sensor comprises a photodiode. Preferably, the photodiodes of the first and second semiconductor sensors have a first response band that is substantially equal to each other. Preferably, the third wire sensor is a photodiode. Preferably, the photodiode of the third semiconductor sensor has a response band substantially equal to the first response band. Preferably, the fourth wire sensor is a photodiode.

[0078] Preferably, the photodiode of the fourth semiconductor sensor has a second response band in a wavelength spectrum different from the first response band of the first and second semiconductor sensors.

[0079] Preferably, the welding station includes a controller. Preferably, the controller is operationally connected to the monitoring device. Preferably, the controller is operationally connected to the first and / or second laser device. Preferably, the controller is operationally connected to a 14 human-machine interface. Preferably, the controller includes a memory for storing data and programs and a processor for performing computational operations.

[0080] Preferably, the controller is configured to record predetermined welding programs. Such predetermined welding programs contain the operating parameters, timing, etc. of the first and second laser devices involved in the welding process as a function of the workpiece to be processed.

[0081] Preferably, the controller is configured to calculate a first tolerance band of the signal recorded by the first semiconductor sensor based on the operating parameters of the first laser device of the relevant welding program and the reflectance of the material on which the main beam in the first wavelength is incident. Preferably, the controller is configured to calculate a second tolerance band of the signal recorded by the first semiconductor sensor based on the operating parameters of the second laser device of the relevant welding program and the reflectance of the material on which the auxiliary beam in the second wavelength is incident. By doing so, it will be possible to check for anomalies, i.e. out-of-threshold values, within the welding process by analyzing the signals detected by the first and second semiconductor sensors, in the absence of a training phase or comparisons of real signals coming from the field with complex reference signals.

[0082] Preferably, the controller is configured to record real signals detected by the semiconductor sensors. Each such detected real signal is a time course consisting of one of a continuous time series and a plurality of consecutive discrete values of a discrete time series. Preferably, the controller is configured to record a first signal and a second real signal relating to a first wavelength and a second wavelength, substantially corresponding to the wavelengths of the main beam and auxiliary beam of the welding beam. Preferably, the controller is configured to further record a third real plasma signal in a third visible light spectrum. Preferably, the controller is configured to further record a fourth real thermal signal in a fourth thermal infrared light spectrum.

[0083] Preferably, the controller contains in memory reference signals of predetermined welding programs. Each of such reference signals is a time pattern consisting of one of a continuous time series and a plurality of consecutive discrete values of a discrete time series referred to a welding program. Preferably, the controller is configured to store a first reference signal and a second reference signal relating to a first light spectrum and a second light spectrum, substantially corresponding to the wavelengths of the main beam and auxiliary beam of the welding beam of a predetermined welding program. Preferably, the controller is configured to 15 further record a third plasma reference signal. Preferably, the controller is configured to further record a fourth thermal reference signal.

[0084] Preferably, the controller is configured to divide the real signals and the reference signals into a plurality of partial segments, depending on predefined instants according to the predetermined welding programs, suitable for identifying anomalies and defects in the time intervals in which they may occur.

[0085] Preferably, the controller is configured to extract features from the real signals and reference signals. Such features may refer to the entire time series, discrete or continuous, or to a predetermined partial segment.

[0086] Preferably, the controller is configured to compare characteristics of at least one real signal with respective characteristics of a reference signal. The result of this comparison will allow to verify whether the behavior of the detected signal is as expected or whether there is an anomaly in progress.

[0087] The present invention extends its scope of protection to a laser welding method for welding together at least two elements of a workpiece, comprising the steps of: positioning two elements of a workpiece in a welding zone and facing an optical head of a laser welding system; scanning at least one of the elements of the workpiece with a welding laser beam to generate a molten weld pool; turning off the laser beam to allow the molten weld pool to solidify and form a weld seam; and monitoring reflected light (RL) during scanning and cooling to assess weld quality.

[0088] The welding laser beam consists of at least one main beam, emitted by a first laser device at a first wavelength, and an auxiliary beam, emitted by a second laser device at a second wavelength. Each of the main and auxiliary beams will have a predetermined set of operating parameters, including average power density, beam distribution, focal length, etc.

[0089] Preferably, the first wavelength belongs to the near-infrared (NIR) light spectrum.

[0090] Preferably, the second wavelength belongs to the visible spectrum (VIS). According to an embodiment of particular interest, the second wavelength belongs to the light spectrum of blue light. The light spectrum of blue light is understood to mean light with a wavelength between 400 and 500 nm, more specifically between 420 and 460 nm.

[0091] In accordance with one aspect of the present finding, the monitoring phase is composed of the sub-phases of monitoring a first back-reflected signal in a first spectrum corresponding to the 16 first wavelength; and monitoring a second back-reflection signal in a second spectrum corresponding to the second wavelength.

[0092] In accordance with an aspect of the present invention, the laser beam, the first back-reflection signal and the second back-reflection signal are coaxial with respect to each other within the optical head, thereby the step of monitoring is carried out in real time with respect to the steps of scanning and turning off.

[0093] Preferably, also an emitted third plasma signal is coaxial with respect to the coaxial laser beam and the first and second back-reflection signals.

[0094] Even more preferably both an emitted third plasma signal and an emitted fourth thermal infrared signal are coaxial between each other and with respect to the coaxial laser beam and the first and second back-reflection signals.

[0095] In accordance with an aspect of the present invention, the sub-phase of monitoring the first back-reflection signal comprises calculating a tolerance band for the first signal based on operating parameters of the first laser device and the material reflectance at the first wavelength.

[0096] In accordance with an aspect of the present invention, the sub-phase of monitoring the second back-reflection signal comprises calculating a tolerance band for the second signal based on operating parameters of the second laser device and the material reflectance at the second wavelength.

[0097] By doing so, it is possible to detect the presence of an anomaly during the laser beam monitoring phase, if the first and / or second back-reflection signal goes outside the pre- established acceptability band. In this way, it will not be necessary to train a model based on a historical series of data or to use a reference signal to compare the back-reflection signals recorded by the semiconductor sensors.

[0098] Preferably, the sub-step of monitoring a first back-reflection signal and the sub-step of monitoring a second back-reflection signal are carried out contemporaneously.

[0099] In accordance with an aspect of the present invention, the monitoring step further comprises a sub-step of monitoring a third plasma signal emitted in a third, visible-light spectrum.

[0100] Preferably, the third plasma signal is an electromagnetic emission in a third spectrum of light with wavelengths shorter than the first wavelength of the first back-reflected signal and longer than the second wavelength of the second back-reflected signal. 17

[0101] In accordance with an aspect of the present invention, the monitoring step further comprises a sub-step of monitoring a fourth thermal infrared signal emitted in a fourth, thermal-infrared spectrum, distinct from the first wavelength.

[0102] Preferably, the fourth thermal infrared signal is an electromagnetic emission in a fourth spectrum of light with wavelengths longer than the first wavelength of the first back-reflected signal.

[0103] In accordance with an aspect of the present invention, each of the monitoring sub-phases comprises the steps of: detecting and acquiring signals originating from the workpiece during the scanning and cooling phases; extracting real signals corresponding to the respective spectral bands from the reflected and emitted signals, wherein each real signal is expressed as a temporal trend composed of either a continuous time series or a series of consecutive discrete values of a discrete time series; comparing features of at least one temporal trend of the real signals with corresponding features of at least one temporal trend of a respective reference signal, to identify defects in the welding process of the workpiece based on the relationship between said compared features; and classifying the real signals based on the comparison of their features with those of the respective reference signals, thereby identifying any defects and / or estimating mechanical properties of the weld in the workpiece, based on the unique relationship between the compared features.

[0104] In accordance with an aspect of the present invention, the laser welding method further comprises, before the scanning and cooling steps, the step of selecting a predefined welding program, wherein such welding program comprises operating instructions and parameters with which a controller executes the scanning and cooling steps, wherein such welding program further comprises at least one reference signal relating to the reflected and emitted light for the selected welding program, and at least one monitoring time interval ti - tf during such scanning and cooling steps, such time interval ti- tf corresponding to a specific stage of the welding process, defined by a start time and an end time.

[0105] In accordance with an aspect of the present invention, the monitoring step further comprises, before the comparing step, the steps of: extracting values corresponding to the specific stage from the real signals thereof; and calculating at least one physical indicator corresponding to each specific stage of the welding process, based on the values extracted from the at least one real and reference signals over the monitoring time interval. 18

[0106] In accordance with an aspect of the present invention step of comparing the at least one physical indicator derived from the respective real signals with the corresponding at least one indicator derived from the respective reference signal for the same welding stage.

[0107] According to embodiments of the present invention, the method includes a training phase in which real data of the welding process carried out with the first laser device and the second laser device will be used. In this training phase, the method includes a signal collection campaign, in particular the first back-reflection signal, the second back-reflection signal, the third plasma signal and the fourth thermal infrared signal.

[0108] This data collection campaign will be carried out for a predetermined welding program, i.e., the set of operating parameters for carrying out a weld of a particular workpiece. The data collection campaign will include both data relating to acceptable welding cycles, which will be labeled as such, and data relating to welding cycles with defects present or anomalies, which will also be labeled as such.

[0109] The combination of the data related to the first back-reflection signal, the second back- reflection signal, the third plasma signal and the fourth thermal infrared signal will allow to identify defects and / or anomalies and associate such defects and / or anomalies to their causes. From the analysis of historical data labeled as acceptable and anomalous / defective it will be possible to build a model to identify such anomalies / defects.

[0110] Preferably, a supervised machine learning approach, i.e., Machine Learning, will be followed. Such Machine Learning model will be compared with real signals coming from the welding process.

[0111] Preferably, the method involves using data from at least one of the signals (first, second, third and / or fourth) collected in the Machine Learning model.

[0112] In some embodiments, the signal (first, second, third and / or fourth) of the entire welding process cycle will be used.

[0113] In other alternative embodiments, partial data of the at least one signal (first, second, third and / or fourth) will be used, i.e., referring only to a predetermined temporary interval of the entire cycle. For example, a temporary interval means each of the welding stages, such as the pre-melting heating stage, the welding stage (molten metal), the joint cooling stage, the temporary interval of passage along the gap between two elements of the workpiece to be welded, etc. 19

[0114] Preferably, a supervised machine learning approach, i.e., Machine Learning, will be followed. Such a Machine Learning approach involves the use of at least one algorithm for detecting an anomaly and / or defect.

[0115] Some embodiments provide that the training phase includes the use of an unsupervised approach. This approach will be based on the data of the real signals, in particular the first back-reflection signal, the second back-reflection signal, the third plasma signal and the fourth thermal infrared signal. The data relating to the real signals will be used to define, for each type of anomaly / defect, the acceptability bands. Such acceptability bands will be based on the physical behavior of the real signals (physics-informed approach) for the predetermined welding program. In fact, these acceptability bands can be circumscribed within a specific time interval of a welding cycle, referring to one or more stages of the welding process (i.e., heating, melting, cooling). The deviation of the signal values (first, second, third and / or fourth) from the acceptability bands will indicate the presence of an anomaly / defect starting from the modeling of such anomaly / defect based on the acceptability bands.

[0116] DESCRIPTION OF THE DRAWINGS

[0117] Further features and advantages of the invention will become more evident in the light of the detailed description of a preferred but not exclusive embodiment of a laser welding station and method according to the invention, illustrated by way of a non-limiting example with the aid of the attached drawing tables in which:

[0118] □ FIG. 1 represents, in schematic view, a laser welding station present in the prior art;

[0119] □ FIG. 2 represents, in isometric view, a schematic representation of a laser welding process in keyhole mode;

[0120] □ FIG. 3 represents the absorptivity of incident light as a function of the wavelength of that light for a plurality of metallic materials in the visible (VIS) and infrared spectrum;

[0121] □ FIG. 4 schematically represents the laser welding station according to a preferred embodiment of the present invention;

[0122] □ FIG. 5 schematically represents the optics of a first laser source configured to emit a main laser beam.

[0123] □ FIG. 6A schematically represents the optics of a second laser source configured to emit an auxiliary laser beam according to a preferred embodiment of the present invention. 20

[0124] □ FIG. 6B schematically represents the optics of a second laser source configured to emit an auxiliary laser beam according to an alternative embodiment of the present invention.

[0125] □ FIG. 7 schematically represents the main beam and the auxiliary beam emitted by the welding station of the present invention according to a preferred embodiment.

[0126] □ FIG. 8A graphically represents the power density distribution of the main and auxiliary beams according to a preferred embodiment of the present invention.

[0127] □ FIG. 8B graphically represents the power density distribution of the main and auxiliary beams according to an alternative embodiment of the present invention.

[0128] □ FIG. 9 schematically represents the laser welding beam incident on the mutually adjacent elements of a product in keyhole welding mode.

[0129] □ FIG. 10 schematically represents the laser welding beam incident on a first element placed between the beam and a second element of the product to be welded in thermal welding mode.

[0130] □ FIG. 11 schematically represents a detail of the monitoring device of the laser welding station shown in FIG.12.

[0131] □ FIG. 12 schematically represents the laser welding station according to a preferred embodiment of the present invention;

[0132] □ FIG. 13 schematically represents the laser welding station according to an embodiment of the present invention;

[0133] □ FIG. 14 schematically represents the laser welding station according to a preferred embodiment of the present invention;

[0134] □ FIG. 15 schematically represents a detail of the monitoring device of the laser welding station shown in FIG.14.

[0135] □ FIG. 16 represents the components of the respective first back-reflection signal, second back-reflection signal, third plasma signal and fourth thermal infrared signal related to the operation of the first and second laser device.

[0136] □ FIG. 17 graphically represents the signals transmitted to the semiconductor sensors of the monitoring device. 21

[0137] □ FIGS. 18A, 18B, 18C and 18D graphically represent the different signals transmitted to the semiconductor sensors, the threshold wavelength of the respective optical fdter and the minimum and maximum signal responsiveness thresholds of the respective semiconductor sensors.

[0138] □ FIGS.19A, 19B and 19C graphically represent the components of the first back- reflected signal (in the first wavelength Xi) relating to the main beam emitted by the first laser device and to the auxiliary beam emitted by the second laser device, as well as the sum of both components.

[0139] □ FIGS. 20A, 20B and 20C graphically represent the components of the second back- reflected signal (in the second wavelength X2) relating to the main beam emitted by the first laser device and to the auxiliary beam emitted by the second laser device, as well as the sum of both components.

[0140] □ FIGS. 21A, 21B and 21C graphically represent the components of the third plasma signal (in a third visible light spectrum WB3) related to the main beam emitted by the first laser device and to the auxiliary beam emitted by the second laser device, as well as the sum of both components.

[0141] □ FIGS. 22A, 22B and 22C graphically represent the components of the fourth thermal infrared signal (in a fourth thermal infrared spectrum WB4) related to the main beam emitted by the first laser device and to the auxiliary beam emitted by the second laser device, as well as the sum of both components.

[0142] □ FIG. 23 represents the first real back-reflection signal detected by the first semiconductor sensor and the second real back-reflection signal detected by the second semiconductor sensor at different stages of the welding process.

[0143] □ FIG. 24 represents the first and second back-reflection signals, the mean value of the respective signals as well as the pre-established acceptability band for each of the two signals.

[0144] □ FIG. 25 represents the first and second back -reflection signals, and a series of physical indicators relating to specific stages of the welding process.

[0145] □ FIG. 26 represents the third plasma signal, in particular the real time series, the reference time series and the physical indicators relating to concrete stages of the welding process. 22

[0146] □ FIG. 27 represents the fourth thermal infrared signal, in particular the real time series, the reference time series and the physical indicators relating to concrete stages of the welding process.

[0147] □ FIGS. 28 and 29 represent the first and second back-reflection signals, their respective reference signals, and a pre-established acceptability band for each of the two reference signals.

[0148] □ FIG. 30 shows a tabulated list of the possible defects generated with a welding station according to the present invention and the respective signals for detecting such defects with a monitoring system according to the present invention.

[0149] DESCRIPTION OF AN EMBODIMENT OF THE PRESENT INVENTION

[0150] FIG. 1 shows a laser welding station known in the prior art. For simplicity, in the description of this station, symbols similar to those of the welding station according to the present invention will be used. The welding station 1 of the prior art comprises a first laser device 2, configured to emit a first laser beam Bl at a first wavelength X2, and a second laser device 3, configured to emit a second laser beam Bl at a second wavelength Xi. These first and second laser beams Bl, B2 are combined in a welding head configured to emit a welding laser beam towards a component to be machined. The welding station 1 further comprises a monitoring device 5 configured to detect the light reflected by this component in a predetermined wavelength spectrum. This configuration does not allow individual monitoring of the reflected light components relating to the first beam and the second beam.

[0151] With reference to FIG. 2, the representative elements of a laser welding operation in depth welding or keyhole mode are schematically represented, in which a laser beam LB penetrates deeply into a component to be machined, digging a narrow hole, known as keyhole, while at the same time, due to the interaction between the laser beam LB and a component W, a vaporization of the metal, an ionization of the surrounding environment in the form of plasma and a pool of molten metal are generated. The parts already subjected to the incidence of the laser beam LB on the component W cool quickly, originating a welding joint.

[0152] With reference to FIG. 3, the characteristic absorptivity of certain metallic materials to light emissions are graphically represented as a function of the wavelength of the incident light. This graph represents the values recorded in the visible and infrared spectrum. It should be noted that for non-ferrous metals and their alloys, the characteristic absorptivity to light in the near infrared (NIR) spectrum is particularly low. For example, in the case of copper (Cu) and 23 alloys thereof, the absorptivity in the spectrum between 900 and 1,000 nm is below 10%. Following the example of copper (Cu), for light in the visible spectrum (VIS) the absorptivity value is much higher, at rates above 60%. For this reason, the recent development of blue diode laser technology is gaining ground in combination with NIR laser sources for welding applications.

[0153] With reference to FIG. 4, a preferred embodiment of a welding station 10 according to the present invention is shown. This welding station 10 comprises:

[0154] - a first laser device 2, which comprises a respective oscillator 21 configured to emit a main beam Bl at a first wavelength Xi, wherein the first laser device 2 is preferably configured to emit such main beam B 1 in the near infrared (NIR) spectrum;

[0155] - a second laser device 3 comprising a respective oscillator 31 configured to emit an auxiliary beam B2 at a second wavelength X2different from the first wavelength Xi, wherein the second laser device 3 is preferably configured to emit such auxiliary beam B2 in the visible light spectrum (VIS), in particular in the blue light spectrum;

[0156] - an optical head 4 configured to emit a laser beam LB for generating a molten weld pool to join at least two elements Wl, W2 of a workpiece W, such laser beam LB being obtained by combining together the main beam Bl and the auxiliary beam B2;

[0157] - first 22 and second 32 optical members configured to transmit the respective first B 1 and second B2 beams to the optical head 4; and

[0158] - a monitoring device 5 configured to detect a reflected light emission RL from the workpiece W being processed; wherein such monitoring device 5 further comprises:

[0159] - A first semiconductor sensor 52, configured to detect reflected light RL in a first wavelength spectrum WB 1 substantially equal to the first wavelength Xi of the main beam B 1 , thereby monitoring the absorption of the main beam B 1 by the workpiece W; and

[0160] - A second semiconductor sensor 53, configured to detect reflected light RL in a second wavelength spectrum WB2 substantially equal to the second wavelength X2of the auxiliary beam B2, thereby monitoring the absorption of the auxiliary beam B2 by the workpiece W.

[0161] The first laser device 2 comprises, downstream of the respective oscillator 21, a first optical group 22. The first 22 optical group comprises a fiber optic cable 23. The second 3 laser device comprises, downstream of the respective oscillator 31, a second optical group 33. The second 32 optical group comprises a fiber optic cable 33. The fiber optic cables 23, 33 are configured 24 to transmit the laser beam Bl, B2 generated by the respective oscillator 21, 31 to elements arranged downstream of such fiber optic cables 23, 33.

[0162] The first 22 and / or second 32 optical groups comprise respective mirrors 24, 34. The mirrors 24, 34 of the respective first 22 and second 32 devices are configured to direct the respective main beam Bl and auxiliary beam B2 towards the optical head 4 of the welding station 10. Such mirrors 24, 34 of the first 22 and / or second 32 optical group are configured to direct the main beam Bl and the auxiliary beam B2 in a coaxial manner, as shown in FIG.17.

[0163] The welding head 4 comprises at least one welding lens 41 configured to direct the welding laser beam LB towards the workpiece W to be machined. The welding head 4 further comprises a scanning device 42 upstream of the welding lens 41. This scanning device 42 is configured to change the orientation of the welding beam, i.e., the main beams Bl and auxiliary beams B2 combined together in the welding head, so as to scan the welding beam LB along a path on the surface of the workpiece W to be machined.

[0164] With reference to FIGS. 5 and 6A, the first optical group 22 further comprises optics 25 of the first laser device. The optics 25 of the first laser device comprise first collimating lenses 26. The second optical group 32 further includes optics 35 of the second laser device. The optics 35 of the second laser device comprise second collimating lenses 36.

[0165] The first 26 and second 36 collimating lenses of the respective first 22 and second 32 optical groups are configured to straighten the respective main Bl and secondary B2 beam coming from the respective first 21 and second 31 oscillators of the respective laser devices 2, 3.

[0166] The first 22 and / or second 32 optical groups may also comprise a respective convex lens. Such convex lens is configured to converge the beam to a predetermined focal distance. Although not represented in the figures, some embodiments provide for such convex lens to be mounted movably, to be able to vary the focal distance of the beam of the respective laser device.

[0167] With reference to FIG. 7, the first laser device 2 is configured to emit a main laser beam B 1 having a substantially circular area. Furthermore, the second laser device 2 is configured to emit an auxiliary laser beam B2 having a substantially circular area. Preferably, the diameter D 1 of the main beam B 1 emitted by the first laser device 2 will have a smaller diameter than the diameter D2 of the auxiliary beam B2 emitted by the second laser device 3.

[0168] With reference to FIGS. 8A and 8B, the first laser device 2 is configured to emit a main beam B 1 of the Gaussian type . By Gaussian light beam is intended that the beam presents an intensity profile BS1 perpendicular to the direction of propagation of the beam Bl with a Gaussian 25 distribution (Gaussian bell). Similarly, the second laser device 3 is configured to emit an auxiliary beam B2 of the Gaussian type. In other words, the auxiliary beam B2 has an intensity profile BS2 perpendicular to the direction of propagation of the beam B2 with a Gaussian distribution (Gaussian bell).

[0169] With reference to FIGS. 6B and 8B, the first 22 and / or second 32 optical group may comprise, in an alternative embodiment, a diffractive optical element (DOE) arranged downstream of the respective lenses. This diffractive optical element or DOE is configured to shape the profile of the main and / or auxiliary beam. In particular, the second optical group 32 may comprise, downstream of the collimating lenses 36, a diffractive optical element 37 configured to modify the BS2 profile of the auxiliary beam B2. FIG. 8B shows this double-W profile, which acts as a ring for a core of the main beam Bl, with a Gaussian BS1 profile.

[0170] With reference to FIGS. 9 and 10, the elements Wl, W2 ofthe workpiece W to be reciprocally welded will be positioned close to each other. According to embodiments of the present invention, the elements Wl, W2 of the workpiece W to be welded to each other are both arranged facing the welding head 4, oriented so as to be directly hit by the laser beam LB (see FIG. 9). These embodiments allow the process to be carried out in a thermal welding mode, also known as keyhole welding mode.

[0171] Alternative embodiments of the present invention provide that the elements Wl, W2 of the workpiece W to be welded to each other are arranged facing the welding head and stacked on top of each other. In this way at least a first element W 1 will be interposed between the welding head 4 and a second element W2, on which the first element W 1 will lie. In such embodiments, the welding beam LB will hit the first element Wl, the heat transfer by conduction between the first W 1 and the second W2 element of the workpiece W downstream of the incidence of the welding beam LB on the first element generating a superficial molten-metal pool will determine the reciprocal welding between the two elements.

[0172] The monitoring device 5 of the welding station 1 further comprises a monitoring mirror 51 configured to reflect the light of the main beam B 1 and the auxiliary beam B2 and the reflected light RL from said component W towards the first 52 and second 53 semiconductor sensors of the monitoring device 5.

[0173] The monitoring mirror 51 is aligned with the mirrors 24, 34 of the respective first 22 and second 32 optical groups, so as to allow both the light of the main laser beams B 1 and auxiliary 26

[0174] B2, and the reflected light RL from the workpiece W and passing through the welding head 4, to be reflected.

[0175] The monitoring mirror 51 may be arranged between the two laser devices 2, 3 and the welding head 4. Alternatively, this monitoring mirror may be arranged between the two mirrors 24, 34 of the respective first 22 and second 32 optical groups of the laser devices 2, 3.

[0176] The welding station 1 further comprises a controller 6. This controller is operationally connected to the monitoring device 5. Furthermore, the controller 6 is operationally connected to the first 2 and / or second 3 laser device. The controller 6 may be operationally connected to a human-machine interface (HMI), not shown in the figures.

[0177] With reference to FIGS. 11 and 12, the monitoring device 5 further comprises a first optical filter 54 interposed between the monitoring mirror 51 and the first 52 and second semiconductor sensors 53. Such first optical filter 54 is configured to reflect at least a second back-reflection signal RL2 to the second semiconductor sensor 53 and transmit at least a first back-reflection signal RL1 to the first semiconductor sensor 52. By doing so, the first optical filter 54 splits the light into two complementary spectra, facilitating the separate detection of the RL1, RL2 signals.

[0178] With reference to FIG. 13, some embodiments of the invention provide that the monitoring device also comprises a third semiconductor sensor 55, configured to detect the emitted light EL3 in a third visible light spectrum WB3. Such emitted light EL3 in the third visible light spectrum WB3 it is an indicator of the plasma generated by the incidence of the welding beam LB on the product W to be worked.

[0179] Some embodiments of the present invention provide that the monitoring device also comprises a fourth semiconductor sensor 56 configured to detect the emitted light EL4 in a fourth spectrum WB4 of thermal infrared light so as to monitor the temperature of the workpiece W during the welding process, i.e. the temperature of the respective molten-metal pool formed on it and the temperature during the subsequent cooling once the main beams B 1 and auxiliary beams B2 of the welding station 1 have been turned off. The fourth infrared light spectrum WB4 comprises a range with wavelengths greater than the wavelengths of the first spectrum WB1.

[0180] With reference to FIGS. 14 and 15, the monitoring device 5 further comprises a second optical filter 57 interposed between the first optical filter 54 and the second semiconductor sensor 53. This second optical filter 54 is configured to reflect the second back-reflection signal RL2 of 27 the reflected light emission RL with a wavelength spectrum substantially coinciding with the second wavelength X2emitted by the second laser device 3.

[0181] The second optical filter 57 is advantageously configured to transmit a third plasma signal EL3 with a wavelength spectrum longer than that of the first wavelength Xi . Such third plasma signal EL3 is directed towards the third semiconductor sensor 55 to detect the third plasma signal EL3.

[0182] Similarly, the monitoring device 5 further comprises a third optical filter 58 interposed between the first optical filter 54 and the first semiconductor sensor. Such third optical filter 58 is configured to reflect the first back-reflection signal RL1 of the reflected light emission RL with a wavelength spectrum substantially coinciding with the first wavelength Xi emitted by the first laser device 2.

[0183] Furthermore, the third optical filter 58 is configured to transmit a fourth thermal infrared signal EL4 with a fourth thermal infrared spectrum WB4. Such fourth spectrum WB4 is referred to wavelengths longer than those of the first spectrum WB 1. The third optical filter 58 is oriented towards the fourth semiconductor sensor 56 to detect said fourth thermal infrared signal EL4.

[0184] With reference to FIG. 13, the reflected RL and emitted EL light relating to a laser process with two laser sources operating at different wavelengths Xi, X2and applied to a workpiece W is represented. By way of example and not limitation, an embodiment of particular interest is reported in which the main beam B 1 is generated by an oscillator 21 of the first laser device 2 in the infrared spectrum and in which the auxiliary beam B2 is generated by an oscillator 31 of the second laser device 3 in the blue light spectrum.

[0185] In particular, FIG. 13 reports the signal intensity of four channels RL1, RL2, EL3 and EL4 of the reflected and emitted light as a function of the wavelength incident on the respective semiconductor sensors 52, 53, 55, 56. The first channel RL1 represents the back-reflected signal of the light of the main beam B 1 (operating at a first wavelength Xi) incident on the first semiconductor sensor 52. The second channel RL2 represents the back-reflected signal of the light of the auxiliary beam (operating at a second wavelength X2) incident on the second semiconductor sensor 53. The third channel EL3 represents the process emission signal in the visible spectrum (VIS) incident on a third semiconductor sensor 55, where the relative peak of signal intensity lies in an intermediate wavelength between the peaks of intensity of the signals RL1, RL2 incident on the first 52 and second 53 semiconductor sensors. The fourth channel 28

[0186] EL4 represents the thermal emission signal in the infrared (IR) spectrum incident on a fourth semiconductor sensor 56.

[0187] Referring to FIG. 11, the first semiconductor sensor 52 comprises a photodiode. Similarly, the second semiconductor sensor 53 comprises a photodiode. Referring to FIG. 10, the third semiconductor sensor 55 comprises a photodiode. Similarly, the fourth semiconductor sensor 56 comprises a photodiode.

[0188] With reference to FIGS. 18A, 18B, 18C, 18D, the PDR1, PDR2 response curves are represented as a function of the wavelength of the respective first 52, second 53, third 55 and fourth 56 semiconductor sensors.

[0189] In particular, in FIG. 18A the first NIR back reflected RL1 signal is represented as a function of the wavelength and superimposed to the PDR1 response curve: on one side the intensity of the signal of the first reflected light channel RL1 (dimensionless, %) incident on the first semiconductor sensor 52 and on the other side the responsivity (electric current generated per incident power, i.e. AAV). This first semiconductor sensor 52 is of a first type and is able to detect signals in a predetermined band, presenting a respective lower band threshold LBT1 in a predetermined wavelength and a respective upper band threshold HBT1 in a predetermined wavelength.

[0190] Similarly, FIGS . 18B, 18C and 18D represent, respectively the second blue back reflected RL2 signal, the third plasma emitted signal EL3 and the fourth thermal emitted signal EL4 as a function of wavelength and superimposed: on one side the signal intensity of the respective second, third and fourth light channels RL2, EL3, EL4 (dimensionless, %) incident on the respective second sensor 52, third sensor 55 and fourth semiconductor sensor 56 and on the other side the respective responsivity curves PDR1, PDR2 (electric current generated per incident power, i.e. AAV).

[0191] With reference to FIG. 18B and according to an embodiment of particular interest, the second semiconductor sensor 53 has a first response curve PDR1 substantially equal to that of the first semiconductor sensor 52. Such first 52 and second 53 semiconductor sensors can be two photodiodes of the first type with similar technical specifications.

[0192] With reference to FIG. 18C and according to an embodiment of particular interest, the third semiconductor sensor 55 has a first response curve PDR1 substantially equal to that of the first 52 and second 53 semiconductor sensors. Such third semiconductor sensor 55 can be a 29 photodiode of the first type with similar technical specifications to those of the first 52 or second 53 semiconductor sensors.

[0193] With reference to FIG. 18D and according to an embodiment of particular interest, the fourth semiconductor sensor 56 has a second PDR2 response curve different from the first PDR1 response curve of the first 52, second 53 and third 55 semiconductor sensors. In such cases, the fourth semiconductor sensor 56 is a photodiode of a second type with technical specifications different from those of the first, second or third semiconductor sensors 52, 53, 55, which belong to the first type.

[0194] With regard to FIG.18A, a first threshold wavelength LPF 1 of the first optical filter (low-pass filter) 54 and a third threshold wavelength LPF3 of the third optical filter (low-pass filter) 58 are also represented. The concatenation of the first 54 and third optical filters 58 in combination with the intrinsic response curve PDR1 of the type 1 photodiode of the first sensor 52 will allow to detect the signal at a first wavelength band WB1 comprising the operating wavelength Xi of the oscillator 21 of the first laser device 2. This first band WB1 will be compressed between the first LPF1 and the third threshold wavelength LPF3.

[0195] Furthermore, FIG. 18B graphically reports a second threshold wavelength LPF2 of the second optical filter (low-pass filter) 57. The second optical filter 57 interposed between the second sensor 53 and the light source of the second channel RL2 will appropriately filter the light with a wavelength greater than the second threshold wavelength LPF2, detecting the signal at a second band WB2 of wavelengths between the lower band threshold LBT1 of the type 1 photodiode and the second threshold wavelength LPF1.

[0196] With reference to FIG. 18C, the above reasoning is reiterated, in which the first 54 and second 57 filters have threshold wavelengths LPF1 and LPF2, respectively. The concatenation of the first 54 and second 57 filters in combination with the intrinsic PDR1 response curve of the type 1 photodiode of the third sensor 55 will allow the signal to be detected in a third band WB3 of wavelengths in the visible spectrum.

[0197] With reference to FIG.18D, the third threshold wavelength LPF3 of the second optical filter (low-pass filter) 58 is also represented. The third optical filter 58 interposed between the fourth sensor 56 and the light source of the fourth channel EL4 will appropriately filter the light with a wavelength shorter than the third threshold wavelength LPF3, detecting the signal included in a fourth band WB4 of wavelengths compressed between the third threshold wavelength 30

[0198] LPF3 of the third optical filter 58 and the upper threshold band HBT2 of the photodiode of the second type.

[0199] With reference to FIGS. 16, 19A-19C, 20A-20C, 21A-21C, 22A-22C, the components of the light intensity signal relating to the incidence of the main beam B 1 of the first laser device 2, operating in the first wavelength Xi and of the auxiliary beam B2 of the second laser device 3 operating in the second wavelength Xi are represented as a function of time (time series). In particular, with respect to the signals recorded by the respective first semiconductor sensor 52, second semiconductor sensor 53, third semiconductor sensor 55 and fourth semiconductor sensor 56, in the respective first WB1, second WB2, third WB3 and fourth WB4 spectra of light.

[0200] In particular, FIG. 19A represents the component 61 of the time series of the first back- reflection signal RL1 due to the incidence of the main beam Bl on the workpiece W; while FIG. 19B represents the component 62 of the time series of the first back-reflection signal RL1 due to the incidence of the auxiliary beam B2 on the workpiece W, which is in this case negligible. With reference to FIG. 19C, the sum of the two components 61, 62 in the first wavelength Xi constitutes the time series 63 of the first back-reflection signal RL1 recorded by the first semiconductor sensor 52.

[0201] FIG. 20A represents the component 71 of the time series of the second back-reflection signal RL2 due to the incidence of the main beam Bl on the workpiece W; in this case negligible; while FIG. 20B represents the component 72 of the time series of the second back-reflection signal RL2 due to the incidence of the auxiliary beam B2 on the workpiece W. With reference to FIG. 20C, the sum of the two components 71, 72 in the second wavelength X2constitutes the time series 73 of the second back-reflection signal RL2 recorded by the second semiconductor sensor 53.

[0202] FIG. 21A represents the component 81 of the time series of the third EL3 plasma signal due to the incidence of the main beam Bl on the workpiece W, while FIG. 2 IB represents the component 82 of the time series of the third EL3 plasma signal due to the incidence of the auxiliary beam B2. With reference to FIG. 21C, the sum of the two components 81, 82 in the third visible WB3 light spectrum constitutes the time series 83 of the third plasma signal EL3 recorded by the third semiconductor sensor 55.

[0203] FIG. 22A represents the component 91 of the time series of the fourth thermal infrared signal EL4 due to the incidence of the main beam B 1 on the workpiece W, while FIG. 22B represents 31 the component 92 of the time series of the fourth thermal infrared signal EL4 due to the incidence of the auxiliary beam B2. With reference to FIG. 22C, the sum of the two components 91, 92 in the fourth thermal infrared spectrum WB4 constitutes the time series 93 of the fourth thermal infrared signal EL4 recorded by the fourth semiconductor sensor 56.

[0204] With reference to FIG. 23, the time series 63, 73 detected by the respective first semiconductor sensor 52 and by the respective second semiconductor sensor 53 are represented in a single graph, in which at least an initial time tstart is defined for a predetermined welding program coinciding with the start of the pre -melting stage Astart , a start time tmeitof the metal melting stage Amelt , a start time tsoiid of the cooling stage Asolid and a final time tend of the welding cycle. Each welding program will present predefined time values of the different welding stages, so that the time series 63, 73, 83, 93 of the signals RL1, RL2, EL3, EL4 detected by the respective semiconductor sensors 52, 53, 55, 56 can be cut into a plurality of segments to be analyzed separately.

[0205] With reference to FIG. 24, the mean values 66, 76 of the respective real time series 63, 73 of the respective back-reflection signals RL1, RL2 are also graphically represented. The calculation of the mean value 66, 76 is achieved by summing or integrating the values of the time series 63, 73 and dividing them by the time of the reference interval.

[0206] The calculation of the mean value 66, 76 is achieved by summing or integrating the values of the time series 63, 73 and dividing them by the time of the reference interval.

[0207] For example, if the entire duration of a welding cycle is taken as a reference interval, the average value will be calculated considering the initial time tstart, before the first and second laser devices 2, 3 start to emit their respective main beams B 1 and auxiliary beams B2 and tend, once the laser devices 2, 3 no longer emit their respective beams Bl, B2 at the end of the cooling stage Asolid of the welding joint. It is possible to define, for a predetermined welding program, a pre-established acceptability band with an upper threshold 64, 74 and a lower threshold 65, 75. The average values 66, 76 compressed within the acceptability bands defined by the thresholds 64-65, 74-75 will be considered as normal.

[0208] In the example illustrated in the upper part of FIG. 24, the mean value 66 of the time series 63 detected by the first semiconductor sensor 52 is included within the acceptability band defined by the upper threshold 64 and the lower threshold 65. In this sense, according to the criteria of the welding program, the first back-reflection signal RL1 (first wavelength Xi) does not present anomalous values. With regards to the example relating to the second back-reflection signal 32

[0209] RL2 (second wavelength X2), the mean value 76 of the time series 73 is located outside the acceptability band represented by the upper threshold 74 and the lower threshold 75, therefore, according to the criteria of the welding program, there would be an anomalous value to be evaluated.

[0210] Even if not shown in the figure, the same reasoning can be applied to the time series 83 and 93 of the respective third plasma signal EL3 and the fourth thermal infrared signal EL4. It is reiterated that, even if not shown in FIG.25, it will be possible to apply the reasoning just exposed to predetermined time intervals defined by the welding program. Each of these intervals will report a predetermined acceptability band and the calculated average value will represent a physical indicator to be further evaluated, in order to detect possible anomalies and / or defects related to the specific time interval (stage) of the welding process.

[0211] With reference to FIGS. 25-27, the welding method further provides for the calculation of welding quality indicators starting from the real signal 63, 73, 83, 93 detected by the respective semiconductor sensor 52, 53, 55, 56. The same reasoning applies to the reference signals 67, 77, 87, 97.

[0212] In general, a real signal 63, 73, 83, 93 or reference signal 67, 77, 87, 97 of the optical emission EL, RL from the workpiece W can be defined as a time-varying intensity I(t) at different instants of time t.

[0213] In general, a physical indicator can be defined as the energy of a respective real signal 63, 73, 83, 93 or reference signal 67, 77, 87, 97 within a respective time ti-tf interval, wherein each time ti-tf interval relates to a respective stage of the welding process, the duration of this stage varying according to the specific welding program (product to be manufactured). In other words, a physical indicator is defined as the energy E of a respective portion of the real signal 63, 73, 83, 93 or reference signal 67, 77, 87, 97, each signal portion being related to a respective stage of the welding process.

[0214] As mentioned, in a preferred embodiment, the controller 6 is configured to extract one or more physical indicators from the at least one real signal 63, 73, 83, 93 or reference signal 67, 77, 87, 97 of the optical emission RL, EL of the workpiece W.

[0215] Suitably, using a first mode of extracting the physical indicators of a signal, the controller 6 is configured to extract each of the physical indicators by calculating the energy E of a respective real signal 63, 73, 83, 93 or reference signal 67, 77, 87, 97 within a respective time period ti- 33 tf via the integral of the time-varying intensity I(t) from an initial time instant ti to a final time instant tf, according to the following mathematical formula: wherein:

[0216] - “E” is the total energy of the real signal 63, 73, 83, 93 or reference 67, 77, 87, 97 within a time period ti-tf;

[0217] - “Y” is a variable indicating the type of the observed real 63, 73, 83, 93 or reference 67, 77, 87, 97 signal, which can be “BRI” for the first back-reflection signal RL1, in particular the real 63 or reference 77 time series, “BR2” for the second back-reflection signal RL2, in particular the real 73 or reference 77 time series, “P” for the plasma emitted signal EL3, in particular the real 83 or reference 87 time series, and “T” for the thermal infrared emitted signal EL4, in particular the real 93 or reference 97 time series;

[0218] - “ positionX ” is a variable that qualitatively indicates the time interval ti-tf, i.e. a specific stage of the welding process, and therefore the time domain in which the integral is calculated, in particular positionX can be referred to an initial stage Astart prior to the melting of the metal, Amelt relating to the interval in which the metal melts while the laser sources are operating and Asolid the interval in which the welding joint on the workpiece W is cooled down;

[0219] - “I(t)” is the time-varying intensity of the real signal 63, 73, 83, 93 or reference signal 67, 77, 87, 97 of the optical emission EL, RL of the workpiece W;

[0220] - “ti” is the instant of time at which the integral begins, or rather at which the time domain in which the integral is calculated begins; and

[0221] - “tf ’ is the instant of time at which the integral ends, or rather at which the time domain in which the integral is calculated ends.

[0222] FIGS. 25-27 graphically show the extraction of the physical indicators BRlstart, BRlmeit, BRI solid, BR2start, BR2melt, BR2solid, Pstart, Pmelt, Psolid, THstart, THmelt, THsolid Of a real Signal 63, 73, 83, 93 and of a reference signal 87, 97 of the emission.

[0223] As reported above, in a preferred embodiment, the controller 6 is configured to extract one or more optical physical indicators, using such first extraction mode. 34

[0224] However, considering that the real signal 63, 73, 83, 93 or reference signal 67, 77, 87, 97 of the optical emission EL, RL coming from the workpiece W is preferably of the digital type, the integral of the time-varying intensity I(t) from an initial time instant ti to a final time instant tf, to calculate the energy E of a respective real signal 63, 73, 83, 93 or reference signal 67, 77, 87, 97 within a respective time period ti-tf, can be approximated.

[0225] Therefore, advantageously, using a second mode of extraction of the physical indicators of a signal, the controller 6 is configured to extract each of the physical indicators by calculating the energy E of a respective real signal 63, 73, 83, 93 or reference signal 67, 77, 87, 97 within a respective time period t, - tf by summing the instantaneous intensity li from an initial time instant 6 to a final time instant t multiplied by the sampling time At (i.e., related to the signal acquisition frequency), according to the following mathematical formula: wherein:

[0226] - “E” is the total energy of the real 60-72 or reference 50-58 signal within a tstart-tend time period;

[0227] - “Y” is a variable indicating the type of real signal 63, 73, 83, 93 or reference 67, 77, 87, 97 observed, and which can assume the value “BRI” for the first back-reflection signal RL1, in particular the real time series 63 or reference 77, “BR2” for the second back-reflection signal RL2, in particular the real time series 73 or reference 77, “P” for the plasma signal EL3, in particular the real time series 83 or reference 87, and “T” for the thermal infrared signal EL4, in particular the real time series 93 or reference 97;

[0228] - “positionX” is a variable that qualitatively indicates the time period tstart-tend, i.e., a specific stage Astart, Amelt, Asolid of the welding process, and therefore the time domain in which the summation is calculated;

[0229] With reference to FIGS. 26-29, together with the real time series 63, 73, 83, 93 of the respective back-reflection signals RL1, RL2, plasma EL3 and thermal EL4, the respective reference time series 67, 77, 87, 97 are represented. These time series 67, 77, 87, 97 correspond to the set of expected values for a predetermined welding program. The controller is further 35 configured to calculate, starting from such reference time series 67, 77, 87, 97, the corresponding acceptability bands. In particular, starting from the reference time series 67, 77, 87, 97 it will be possible to calculate, for each value of the reference time series 67, 77, 87, 97, a lower threshold value and an upper threshold value.

[0230] With particular reference to FIGS. 28 and 29, for the back-reflection signals RL1, RL2, in particular for the respective reference time series 67, 77, the respective upper threshold series 68, 78 and lower threshold series 69, 79 are advantageously calculated. Even if not specifically reported graphically in the figures, the same reasoning is valid for the reference time series of plasma 87 and thermal 97. The controller, in fact, is configured to detect point values or a series of anomalous values of the real time series 63, 73 that go outside the threshold series.

[0231] Referring to FIG. 28, the values of the series 63 relating to the first back-reflection signal RL1 are compressed within the respective upper threshold series 68 and lower threshold series 69. However, the values of the series 73 relating to the second back-reflection signal RL2 exceed the respective upper threshold series 78 and lower threshold series 79. In such cases, the controller is configured to signal an anomalous situation.

[0232] With particular reference to FIG. 29, the values of the series 73 relating to the second back- reflection signal RL2 are compressed within the respective upper threshold series 78 and lower threshold series 79 of the respective reference signal RL2rif. However, the values of the series 63 relating to the first back -reflection signal RL1 go outside the respective upper threshold series 78 and lower threshold series 79. In particular, a plurality of values of the real time series 63 go outside the upper threshold 68 in a particular time interval 631 during the metal melting stage. The controller 6 will be configured to identify the type of anomaly based on the threshold exceeded and the stage in which it occurs.

[0233] With reference to FIG. 30, a summary of the correlation between the different possible defects of a welding station 1 with two laser devices 2, 3 and the respective monitored RL1, RL2, EL3, EL4 signals is presented in a table. For example, monitoring the first RL1 and second RL2 back-reflection signals will allow to identify welding instability situations (spatter phenomenon or splashes due to implosion of the welding bubble) when the second RL2 signal reports higher values than the reference signal RL2rif. Furthermore, the welding method provides for a comparison between the recorded RL1, RL2 back-reflection signals and the expected values. The calculation of the expected values can be achieved thanks to the calculation of the respective threshold bands for the RL1, RL2 back -reflection signals. The threshold band relating to the first back -reflection signal RL1 will consider the operating 36 parameters of the first laser device 2, with which to calculate the emission of the main beam B 1 in the first wavelength Xi and the reflectance of the material on which the main beam B 1 is incident in such first wavelength Xi. The threshold band relating to the second back-reflection signal RL2 will consider the operating parameters of the second laser device 3, with which to calculate the emission of the auxiliary beam B2 in the second wavelength X2and the reflectance of the material on which the auxiliary beam B2 is incident in such second wavelength X2. For example, the fluctuation of the first RL1 and / or second RL2 back-reflection signals outside their respective threshold bands will indicate the presence of gaps.

[0234] It is clear that modifications and / or additions of parts or phases can be made to the laser welding station and method described herein, without departing from the scope of the present invention as defined in the claims.

[0235] In the claims that follow, the references in brackets are for ease of reading only and should not be considered as limiting factors with respect to the scope of protection underlying the specific claims.

Claims

1. 37CLAIMS1. A laser welding station comprising:- a first laser device (2) comprising a first oscillator (21) configured to emit a main beam (Bl) at a first wavelength (Xi), the first laser device (2) being preferably configured to emit said main beam (Bl) in the Near Infrared (NIR) spectrum;- a second laser device (3) comprising a second oscillator (31) configured to emit an auxiliary beam (B2) at a second wavelength (X2) different from the first wavelength (Xi), the second laser device (3) being preferably configured to emit said auxiliary beam (B2) in the visible (VIS) spectrum, in particular the blue light spectrum;- an optical head (4) configured to emit a laser beam (LB) for generating a molten weld pool to join at least two elements (Wl, W2) of a workpiece (W), said laser beam (LB) being formed by combination of said main beam (Bl) and said auxiliary beam (B2);- first (22) and second (32) optical members configured to transmit the main (Bl) and auxiliary (B2) beams, respectively, to said optical head (4); and- a monitoring device (5) configured to detect reflected light (RL) emitted or reflected from the workpiece (W); characterized in that said monitoring device (5) comprises:- a first semiconductor sensor (52), configured to detect back-reflected light (RL) in a first wavelength spectrum (WB1) substantially corresponding to the first wavelength (Xi), thereby monitoring absorption of the main beam (B 1) by the workpiece (W); and- a second semiconductor sensor (53), configured to detect back-reflected light (RL) in a second wavelength spectrum (WB2) substantially corresponding to the second wavelength (X2), thereby monitoring absorption of the auxiliary beam (B2) by the workpiece (W).

2. The laser welding station according to claim 1, characterized in that said monitoring device (5) further comprises a monitoring mirror (51) configured to reflect the main (Bl) and auxiliary (B2) beams of the laser beam (LB), as well as the back-reflected light (RL) reflected from said workpiece (W), towards at least said first (52) and second (53) semiconductor sensors,3. The laser welding station according to claim 2, characterized in that each of said first (23) and second (33) optical members further comprises a respective first (24) and second (34) mirror configured to orient the respective main (B 1) and auxiliary (B2) beam towards said optical head (4), and in that said monitoring mirror (51) said first mirror (24) and38 said second (34) mirror are aligned thereto, so that said monitoring mirror (51) transmits both the light of the main (Bl) and auxiliary (B2) beams of the laser beam (LB) towards the optical head (4) and the back-reflected (RL) and emitted (EL) light by the workpiece (W) crossing coaxially the optical head (4) to be analyzed by the monitoring device (5).

4. The laser welding station according to claim 2 or 3, characterized in that said monitoring device (5) further comprises a first optical filter (54) interposed between said monitoring mirror (51) and said first and second semiconductor sensors (52, 53), said first optical filter (54) being configured to reflect a second back-reflection signal (RL2) towards said second semiconductor sensor (53), and to transmit at least a first back reflection signal (RL1) towards said first semiconductor sensor (52).

5. The laser welding station according to any one of the preceding claims, characterized in that said monitoring device (5) further comprises a third semiconductor sensor (55) configured to detect emitted light (EL) from the welding process in a third, visible- wavelength light spectrum (WB3), thereby monitoring the plasma of the molten weld pool generated by the laser beam (LB).

6. The laser welding station according to claim 4 or 5, characterized in that said monitoring device (5) further comprises a second optical filter (57) interposed between said first optical filter (54) and said second semiconductor sensor (53), the second optical filter (57) being configured to reflect said second back-reflection signal (RL2) corresponding to the second wavelength (X2).

7. The laser welding station according to claim 6, when dependent on claim 5, characterized in that said second optical filter (57) is further configured to transmit a third plasma signal (EL3) of the emitted light (EL) in the third spectrum (WB3), which has a wavelength range greater that of the second spectrum (WB2), said second optical filter (57) being arranged to direct the third plasma signal (EL3) towards said third semiconductor sensor (55).

8. The laser welding station according to any one of the preceding claims, characterized in that said monitoring device (5) further comprises a fourth semiconductor sensor (56) configured to detect emitted light (EL) from the welding process in a fourth, thermal infrared spectrum (WB4), thereby monitoring the temperature of the workpiece (W).

9. The laser welding station according to claim 8, when dependent on claim 6 or 7, characterized in that said monitoring device (5) further comprises a third optical filter (58) interposed between said first optical filter (54) and said first semiconductor sensor (52), the third optical filter (58) being configured to reflect the first back-reflection signal (RL1) corresponding to the first wavelength (Xi).

10. The laser welding station according to claim 9, characterized in that said third optical filter (58) is further configured to transmit a fourth thermal infrared signal (EL4) in the fourth thermal-infrared spectrum (WB4), which has a wavelength range greater than that of the first spectrum (WB1), and is arranged to direct said fourth thermal infrared signal (EL4) towards said fourth semiconductor sensor (56).

11. A laser welding method for welding at least two elements of a workpiece using a laser welding station according to any one of claims 1-12, wherein said method comprises the steps of: a) positioning the elements (Wl, W2) of a workpiece (W) in a welding zone facing an optical head (4); b) scanning at least one of the elements (Wl, W2) with a laser beam (LB) to generate a molten weld pool, the laser beam (LB) being formed by a main beam (Bl) emitted at a first wavelength (Xi), and an auxiliary beam (B2) emitted at a second wavelength (X2), wherein the first wavelength is preferably in the NIR spectrum and the second in the visible (VIS), in particular the blue spectrum; c) turning off the laser beam (LB) to allow the molten weld pool to solidify and form a weld seam; d) monitoring reflected light (RL) during scanning and cooling to assess weld quality, characterized in that the monitoring step comprises: d.3) monitoring a first back-reflection signal (RL1) in a first spectrum (WB1) corresponding to the first wavelength (Xi); and d.4) monitoring a second back-reflection signal (RL2) in a second spectrum (WB2) corresponding to the second wavelength (X2).

12. The method according to claim 11, characterized in that the laser beam (LB), the first back-reflection signal (RL1) and the second back-reflection signal (RL2), preferably also an emitted third plasma signal (EL3), more preferably also both an emitted third plasma signal (EL3) and an emitted fourth thermal infrared signal (EL4), are coaxial with respect to each other within the optical head (4), thereby the step of monitoring is carried out in real time with respect to the steps of scanning and turning off.

13. The method according to claim 11 or 12, characterized in that sub-step (d.3) comprises calculating a tolerance band of the first signal (LR1) based on operating parameters of the first laser device (2) and the material reflectance at the first wavelength (XI), and in that sub-step (d.4) comprises calculating a tolerance band for the second signal (RL2) basedon operating parameters of the second laser device (3) and the material reflectance at the second wavelength (X2).

14. The method according to any one of claims 11-13, characterized by further comprising: sub-step d.5: monitoring a third plasma signal (EL3) emitted in a third, visible-light spectrum (WB3); and sub-step d.6: monitoring a fourth thermal infrared signal (EL4) emitted in a fourth, thermal-infrared spectrum (WB4), distinct from the first wavelength (Xi).

15. The method according to any one of claims 11-14, characterized in that each of said substeps d.3 to d.6 comprises:- detecting and acquiring said signals (RL1, RL2, EL3, EL4) originating from said workpiece (W) during the scanning (b) and cooling (c) phases;- extracting real signals (RL1, RL2, EL3, EL4) corresponding to the respective spectral bands (WB1, WB2, WB3, WB4), from said reflected and emitted signals (RL, EL), each real signal (RL1, RL2, EL3, EL4) being expressed as a temporal trend composed of either a continuous time series or a series of consecutive discrete values of a discrete time series;- comparing features of at least one temporal trend (63, 73, 83, 93) of the real signals (RL1, RL2, EL3, EL4) with corresponding features of at least one temporal trend (67, 77, 87, 97) of a respective reference signal, to identify defects in the welding process of the workpiece (W) based on the relationship between said compared features; and- classifying said real signals (RL1, RL2, EL3, EL4) based on the comparison of their features with those of said respective reference signals (RLlnf, RL2nf, EL3nf, EL4nf), thereby identifying any defects and / or estimating mechanical properties of the weld in said workpiece (W), based on the unique relationship between said compared features.

16. The method according to claim 15, characterized in that, prior to said scanning (b.) and cooling (c.) steps, said method comprises the step of:- selecting a predefined welding program, wherein said welding program comprises operating instructions and parameters with which a controller (6) executes said scanning and cooling steps, and further comprises: o at least one reference signal (RLlrif, RL2nf, EL3nf, EL4nf) relating to the reflected and emitted light (RL, EL) for the selected welding program, and o at least one monitoring time interval (t - tf) during said scanning and cooling steps, said time interval (ti - tf) corresponding to a specific stage (Astart,Amelt, Asolid) of said welding process, defined by a start timeand an end timein that said monitoring step further comprises, before said comparing step, the steps of:- extracting values corresponding to said specific stage (Astart, Amelt, Asolid) from said real signals (RL1, RL2, EL3, EL4); and- calculating at least one physical indicatorcorresponding to each specific stage (Astart, Amelt, Asolid) of said welding process, based on the values extracted from said at least one real and reference signals over the monitoring time interval (ti - tf); and in that said step of comparing features of the temporal trends further comprises the step of:- comparing the at least one physical indicatorderived from the respective real signal (RL1, RL2, EL3, EL4) with the corresponding at least one indicator derived from the respective reference signal (RLlnf, RL2nf, EL3nf, EL4nf) for the same welding stage (Astart, Amelt, Asolid).

Images