METHOD AND DEVICE FOR PIERCING A WORKPIECE USING A LASER BEAM

Description

Technical area

[0001] The present disclosure relates to a method and apparatus for piercing a workpiece using a laser beam, and more particularly to a method and apparatus for controlling a piercing process in laser material processing. State of the art

[0002] In a device for laser material processing, e.g., in a laser processing head for laser cutting, the laser beam emitted from a laser light source or the end of a laser fiber is focused or bundled onto the workpiece to be processed using beam guidance and focusing optics. Typically, a laser processing head with collimation optics and focusing optics is used, with the laser light being supplied via an optical fiber, also known as the laser source.

[0003] In laser material processing, particularly in laser cutting, a piercing process can be carried out using the laser beam in the workpiece. The piercing process precedes the actual cutting process in laser cutting. During this process, an initial hole or puncture is created in the workpiece, which serves as the starting point for the cutting process. The piercing process or piercing into the workpiece can therefore also be referred to as a piercing process or piercing through the workpiece. Since, for example, a subsequent laser cutting process cannot begin without the puncture, a successful piercing through the workpiece plays an important role for laser cutting. The duration, quality, and stability of the piercing process depend on a variety of process parameters. The duration of the piercing process, i.e.The time span between the activation of the machining beam and the penetration of the material or workpiece (also called penetration time or penetration duration) is a critical factor for the efficiency of the machining process. The duration of the penetration process depends on the temporal rate of change of the penetration depth. The temporal rate of change of the penetration depth is referred to below as the penetration rate and also reflects the process efficiency.

[0004] The prior art describes various piercing processes in which the pulse frequency or the average laser power is increased within a piercing process until penetration in order to minimize the piercing duration or the piercing time for a given material thickness. US 5 434 383 A describes a piercing process with a shortened duration in which the pulse frequency and the relative pulse duty factor are increased step by step during piercing. Likewise, DE 11 2009 001 200 B4 describes a laser processing method for drilling and subsequent cutting, wherein drilling begins with a first frequency and drilling is continued with a second frequency that is higher than the first frequency. In US 6 693 256 B2, however, the maximum laser power is increased step by step during piercing.

[0005] Trends towards higher laser powers and greater sheet or workpiece thicknesses complicate the piercing process and lead not only to longer piercing times, but also more frequently to a piercing stop, where the piercing rate approaches zero and thus no piercing is possible.

[0006] WO 2016 / 143521 A1 describes a method for manufacturing a bonding structure, comprising using a laser beam source capable of repeatedly oscillating a laser pulse group at a constant period to form a plurality of perforations in a surface portion of a metal member constituting a bonding surface to be bonded to a resin member, and a bonding step of bonding the metal member and the resin member to each other by filling the plurality of perforations with the resin member. The laser pulse group is configured by combining a plurality of sub-pulse trains in the time axis direction, the peak power of the sub-pulses being different from each other.

[0007] US 2002 / 0185474 A1 describes a method for micromachining layered substrates, in particular for drilling holes in printed circuit boards, by means of a plurality of laser pulses.

[0008] US 8816246 B2 describes a method for forming holes in materials, in particular in printed circuit boards, by a series of laser pulse sequences. Summary of the invention

[0009] An object of the present invention is to provide a method and a device for piercing a workpiece by means of a laser beam (also called processing beam), by means of which a piercing process can be optimized, in particular for piercing thicker material thicknesses or thick workpieces, for example with a thickness greater than 10 mm or even greater than 20 mm.

[0010] A further object of the present invention is to provide a method and a device for piercing a workpiece by means of a laser beam, by means of which a piercing time can be minimized or a piercing rate can be maximized.

[0011] A further object of the present invention is to provide a method and a device for piercing a workpiece by means of a laser beam, by means of which a safe piercing can be ensured and a piercing stop can be prevented.

[0012] These objects are achieved by the features of the independent claims. Features of preferred embodiments are specified in the subclaims.

[0013] The present invention is based on the finding that a piercing process can be optimized by a targeted or controlled energy input of the laser beam into the processing zone, and in particular by the targeted or controlled reduction of an average pulse power of the laser beam radiated into the processing zone during the piercing process. The energy input can be controlled as a function of at least one of the following process parameters and boundary conditions: material thickness, material type, current piercing time, current piercing depth, process gas type, process gas pressure, focus position, imaging ratio of the optical system, nozzle type, nozzle diameter, intensity distribution of the laser beam, focus diameter and nozzle distance from the top of the workpiece. Based on the aforementioned parameters, the energy input in the piercing hole orin the processing zone can be adjusted by the pulse on time, pulse off time and / or the pulse peak power depending on the current piercing time and / or piercing depth.

[0014] According to one aspect, a method for piercing a workpiece by means of a laser beam (also called a processing beam) is specified. The method comprises: irradiating a pulsed laser beam onto a workpiece to form a piercing, wherein an irradiated average pulse power of the pulsed laser beam is reduced during the piercing or during a piercing time, e.g. at least once. The piercing time can here refer to a duration of the piercing or piercing process, ie a time span from switching on or first irradiation of the laser beam to piercing through the workpiece. In other words, a method for piercing a workpiece by means of a pulsed laser beam comprises: a first piercing step in which a pulsed laser beam with a first average pulse power is directed oris irradiated; and a second piercing step in which the pulsed laser beam is directed or irradiated onto the workpiece with a second average pulse power, the first average pulse power being greater than the second average pulse power. Preferably, the irradiated average pulse power is reduced at least once during piercing or during the piercing time. The irradiated average pulse power can be reduced discretely, i.e. stepwise, or continuously, or in any combination of discrete and continuous changes. The irradiated average pulse power can be reduced several times during piercing or during the piercing time. Thus, more than two piercing steps can be carried out, each with different average pulse powers, each of which is lower than an average pulse power in a previous piercing step.The irradiated average pulse power can be constant during a piercing step. The irradiated average pulse power of the pulsed laser beam can be reduced monotonically or strictly monotonically during the piercing. The irradiated average pulse power of the pulsed laser beam at the start of the piercing or the piercing time can be greater than the irradiated average pulse power during the piercing, i.e. at the end of the piercing or the piercing time. Preferably, a best-fit or regression line of the irradiated average pulse power during the piercing time, i.e. from the start of the piercing to the piercing, has a negative slope. The irradiated average pulse power of the pulsed laser beam can be reduced by at least one of the following settings or:Changes can be reduced: extending a pulse off-time, reducing a pulse peak power, shortening a pulse on-time, reducing a pulse frequency and reducing a relative pulse duty factor. In particular, the reduction of the irradiated average pulse power can be achieved by varying the pulse on-time and the pulse off-time: for example, the pulse off-time and the pulse on-time can be extended if the pulse on-time is extended less than the pulse off-time. In a further example, the pulse off-time and the pulse on-time can be shortened if the pulse on-time is shortened more than the pulse off-time. In a further example, the pulse off-time can remain constant if the pulse on-time is shortened. If the irradiated average pulse power is reduced by extending the pulse off-time, the pulse peak power and / or a pulse on-time and / or a pulse energy can be increased during the piercing or piercing process.kept constant during the piercing time. In one embodiment, the method can have at least two piercing steps, wherein the first average pulse power in the first piercing step is greater than or equal to an average limit pulse power and the second average pulse power in the second piercing step is less than the first average pulse power and / or the average limit pulse power. Optionally, the method can comprise a third piercing step in which a third average pulse power is less than the second average pulse power. The average limit pulse power can designate an average pulse power above which the piercing rate approaches zero or above which a piercing stop occurs or above which saturation occurs during piercing. The average limit pulse power can be predetermined depending on a material and / or a thickness of the workpiece.

[0015] According to a further aspect, a method for piercing a workpiece by means of a laser beam (also called a processing beam) is specified. The method comprises: irradiating a pulsed laser beam onto a workpiece to form a piercing, wherein a pulse-off time of the pulsed laser beam is extended during the piercing or during a piercing time, e.g. at least once. The piercing time can here refer to a duration of the piercing or piercing process, e.g. a time span from a first switching on of the laser beam until a piercing through the workpiece. In other words, a method for piercing a workpiece by means of a pulsed laser beam comprises: a first piercing step in which a pulsed laser beam is directed or irradiated with a first pulse-off time onto a workpiece.is irradiated; and a second piercing step in which the pulsed laser beam is directed or irradiated onto the workpiece with a second pulse off-time, the first pulse off-time being shorter than the second pulse off-time. Preferably, the pulse off-time is extended at least once during piercing or during the piercing time. The pulse off-time of the pulsed laser beam can be extended discretely, i.e. in steps, or continuously, or in any combination of discrete and continuous changes. The pulse off-time can be reduced several times during piercing or during the piercing time. Thus, more than two piercing steps can be carried out, each with different pulse off-times, each longer than a pulse off-time in a previous piercing step. The pulse off-time can be constant during a piercing step.The pulse off-time can be increased monotonically or strictly monotonically during the piercing. The pulse off-time of the pulsed laser beam at the start of the piercing or the piercing time can be shorter than the pulse off-time during the piercing, i.e. at the end of the piercing or the piercing time. Preferably, a best-fit or regression line of the pulse off-time during the piercing time, i.e. from the start of the piercing to the piercing, has a positive slope. The pulse off-time of the pulsed laser beam can be extended by reducing a pulse frequency and / or reducing a relative pulse duty factor. A pulse peak power and / or a pulse on-time and / or a pulse energy can be kept constant during the piercing or during the piercing time.In one embodiment, the method can have at least two piercing steps, wherein the first pulse off-time in the first piercing step is less than or equal to a limit pulse off-time and the second pulse off-time in the second piercing step is greater than the first pulse off-time and / or the limit pulse off-time. Optionally, the method can comprise a third piercing step in which a third pulse off-time is greater than the second pulse off-time. The limit pulse off-time can designate a pulse off-time below which the piercing rate approaches zero or a piercing stop occurs or saturation occurs during piercing. The limit pulse off-time can be predetermined depending on a material and / or a thickness of the workpiece.

[0016] According to another further aspect, a method for piercing a workpiece by means of a laser beam (also called a processing beam) is specified. The method comprises: irradiating a pulsed laser beam onto a workpiece to form a piercing, wherein a pulse frequency of the pulsed laser beam is reduced during the piercing or during a piercing time, e.g. at least once. The piercing time can here refer to a duration of the piercing or piercing process, e.g. a time period from a first switching on of the laser beam until a piercing through the workpiece. In other words, a method for piercing a workpiece by means of a pulsed laser beam comprises: a first piercing step in which a pulsed laser beam is directed or irradiated at a workpiece with a first pulse frequency.is irradiated; and a second piercing step in which the pulsed laser beam is directed or irradiated onto the workpiece at a second pulse frequency, the first pulse frequency being greater than the second pulse frequency. Preferably, the pulse frequency is reduced at least once during piercing or during the piercing time. The pulse frequency of the pulsed laser beam can be increased discretely, i.e. in steps, or continuously, or in any combination of discrete and continuous changes. The pulse frequency can be reduced several times during piercing or during the piercing time. Thus, more than two piercing steps can be carried out, each with different pulse frequencies, each of which is lower than a pulse frequency in a previous piercing step. The pulse frequency can be constant during a piercing step.The pulse frequency can be reduced monotonically or strictly monotonically during the piercing. The pulse frequency of the pulsed laser beam at the start of the piercing or the piercing time can be higher than the pulse frequency during the piercing, i.e. at the end of the piercing or the piercing time. Preferably, a best-fit or regression line of the pulse frequency during the piercing time, i.e. from the start of the piercing to the piercing, has a negative slope. The pulse frequency of the pulsed laser beam can be reduced by extending a pulse off-time. A pulse peak power and / or a pulse on-time and / or a pulse energy can be kept constant during the piercing or during the piercing time.In one embodiment, the method can have at least two piercing steps, wherein the first pulse frequency in the first piercing step is greater than or equal to a limit pulse frequency and the second pulse frequency in the second piercing step is less than the first pulse frequency and / or the limit pulse frequency. Optionally, the method can comprise a third piercing step in which a third pulse frequency is less than the second pulse frequency. The limit pulse frequency can designate a pulse frequency above which the piercing rate approaches zero or above which a piercing stop occurs or above which saturation occurs during piercing. The limit pulse frequency can be predetermined depending on a material and / or a thickness of the workpiece.

[0017] Each of these aspects can have one or more of the following features: Pulse parameters can include the irradiated average pulse power, the pulse off-time, the pulse on-time, the pulse frequency, the pulse period, the relative pulse duty factor, and / or the pulse peak power. At least one of the pulse parameters, selected from the group comprising the irradiated average pulse power, the pulse off-time, the pulse on-time, the pulse frequency, the relative pulse duty factor, and the pulse peak power, can be adjusted depending on a material and / or a thickness of the workpiece (also called material thickness) and / or a current piercing time and / or a current piercing depth. In particular, a first pulse frequency, a first average pulse power, and / or a first pulse off-time can be selected depending on the material and / or the thickness of the workpiece.A pulse frequency, an average pulse power, and / or a pulse off-time, particularly in a piercing step following the first piercing step, can be set based on a current piercing time and / or a current piercing depth. In particular, at least one of the pulse parameters selected from the group comprising the irradiated average pulse power, the pulse off-time, the pulse on-time, the pulse frequency, the relative pulse duty factor, and the pulse peak power can be changed based on at least one of the thickness of the workpiece, the material of the workpiece, a process gas type, a process gas pressure, a focus position, an imaging ratio of the optical system or the laser processing head, a focus diameter, and a nozzle distance from the top of the workpiece, and as a function of the current piercing time and / or piercing depth.

[0018] Preferably, the pulse duration is constant during the piercing or penetration time. The pulse duration can be adjusted depending on the material or thickness of the workpiece.

[0019] Preferably, the pulse peak power is constant during the piercing or piercing time. The pulse peak power can be adjusted depending on the material or thickness of the workpiece.

[0020] Preferably, the pulse energy is constant during the piercing or piercing time. In other words, the product of the pulse peak power and the pulse on time can be set to a constant value. The pulse energy can be set depending on the material or thickness of the workpiece. Optionally, a minimum value for the pulse energy can be set depending on the material or thickness of the workpiece.

[0021] Preferably, the duration of the individual piercing steps varies. For example, the duration of a first piercing step may be longer than that of a subsequent, e.g., second or third, piercing step. In the case of a discrete or stepwise change in the average pulse power, the pulse break time, and / or the pulse frequency, a first change, ie, a change from the first to the second piercing step, may be greater than a second change, ie, a change from the second piercing step to a third piercing step.

[0022] In one embodiment, a pulse frequency used at the beginning of the piercing process and / or in the first piercing step may be greater than or equal to a limit pulse frequency. The limit pulse frequency may be predetermined depending on the material and / or the thickness of the workpiece.

[0023] In one embodiment, a pulse-off time used at the beginning of the piercing process and / or in the first piercing step may be greater than or equal to a pulse-off limit time. The pulse-off limit time may be predetermined depending on the material and / or thickness of the workpiece.

[0024] Preferably, the pulse on time, e.g., during the puncture period, is in a range between 0.01 ms and 100 ms (0.01 ms ≤ t on ≤ 100 ms), in particular between 0.1 ms and 10 ms (0.1 ms ≤ t on ≤ 10 ms). Preferably, the pulse off time, e.g., during the puncture period, is in a range between 0.01 ms and 100 ms (0.01 ms ≤ t off ≤ 100 ms), in particular between 0.1 ms and 10 ms (0.1 ms ≤ t off ≤ 10 ms).

[0025] Preferably, a pulse on time or a pulse period of the pulsed laser beam, e.g. during the piercing period, is in a range of microseconds or milliseconds. In this case, piercing can primarily occur by melting the workpiece. A fiber, disk, or direct diode laser can be used. A wavelength of the pulsed laser beam is preferably in the range of 800 nm to 1300 nm. The pulse frequency is preferably set depending on the thickness of the workpiece. With a workpiece thickness of 10 mm to 15 mm, the pulse frequency, e.g. during the piercing period, can be in a range between 400 Hz and 3000 Hz (400 Hz ≤ f ≤ 3000 Hz), in particular between 600 Hz and 1500 Hz (600 Hz ≤ f ≤ 1500 Hz). Here, the pulse frequency can be changed at least once during piercing. If the thickness of the workpiece is greater than 15 mm to 20 mm, the pulse frequency can be increased, e.g.during the piercing period, in a range between 300 Hz and 2000 Hz (300 Hz ≤ f ≤ 2000 Hz), in particular between 400 Hz and 900 Hz (400 Hz ≤ f ≤ 900 Hz). In this case the pulse frequency can be changed at least once during piercing. With a workpiece thickness of greater than 20 mm to 25 mm the pulse frequency, e.g. during the piercing period, can be in a range between 300 Hz and 1500 Hz (300 Hz ≤ f ≤ 1500 Hz), in particular between 400 Hz and 800 Hz (400 Hz ≤ f ≤ 800 Hz). In this case the pulse frequency can be changed at least twice during piercing. For workpieces with a thickness greater than 25 mm, the pulse frequency, e.g., during the piercing process, can be in a range between 200 Hz and 1000 Hz (200 Hz < f ≤ 1000 Hz), particularly between 400 Hz and 700 Hz (400 Hz ≤ f ≤ 700 Hz). In this case, the pulse frequency can be changed at least twice during piercing.

[0026] The method will be used for piercing workpieces with a thickness of at least 10 mm, in particular with a thickness of at least 20 mm, for example with a thickness of 30 mm. The peak pulse power of the pulsed laser beam during piercing is preferably at least 4 kW, and can in particular be greater than or equal to 6 kW.

[0027] The material is a metallic workpiece. The workpiece may consist of metal or comprise metal. The workpiece may comprise or be a sheet metal. Furthermore, a material of the workpiece or sheet metal may comprise or be at least one of a structural steel alloy, a stainless steel alloy, an aluminum alloy, a copper alloy, a brass alloy, structural steel, stainless steel, aluminum, copper, and brass.

[0028] Preferably, an inert process gas, e.g., nitrogen, argon, or similar, is directed at the workpiece during piercing. The process gas can impinge on the workpiece coaxially with the laser beam.

[0029] The piercing method can serve to prepare a laser cutting method. In other words, the method can further comprise cutting the workpiece using the laser starting from the piercing. Accordingly, according to one embodiment of the present invention, a laser cutting method is also specified, comprising the piercing method according to one of the embodiments described in this disclosure; and cutting using the laser beam starting from the piercing. During the (entire) piercing method or during the (entire) piercing, the laser beam (also called the processing beam) can be pulsed. During laser cutting, the laser beam is preferably used continuously. Of course, the same laser beam can be used for laser cutting and piercing, possibly with different parameters.In other words, the laser beam for piercing and the laser beam for laser cutting can come from the same laser source.

[0030] According to a further aspect, a device for laser material processing of a workpiece is provided. The device comprises a laser source for generating a laser beam; a laser processing head for irradiating the laser beam onto a workpiece; and a control device configured to control the device to perform a piercing and laser cutting method comprising the following steps: irradiating the pulsed laser beam onto a metallic workpiece to form a piercing, wherein an irradiated average pulse power of the pulsed laser beam is reduced during piercing, and cutting using the laser beam starting from the formed piercing.

[0031] Within the scope of this disclosure, the pulse off-time can be defined as the time of a pulse period in which the irradiated power is below a predetermined threshold, for example, below 30%, 20%, or 10% of a maximum laser power or a pulse peak power. In other words, within the scope of this disclosure, the pulse on-time can be defined as the time of a pulse period in which the irradiated power is above a predetermined threshold, for example, above 30%, 20%, or 10% of a maximum laser power or a pulse peak power.

[0032] Furthermore, within the scope of this disclosure, a pulse parameter, for example the radiated average pulse power, a pulse peak power, a pulse frequency, a pulse period, a pulse off-time, a pulse on-time, a relative pulse duty factor and / or a pulse energy, may be in a range of ±0.2 or ±0.1 times the mean value of this pulse parameter, instead of being constant. Short description of the drawings

[0033] Embodiments of the disclosure are illustrated in the figures and are described in more detail below. They show: Figure 1 a schematic structure of a device for laser material processing according to embodiments of the present disclosure, Figure 2 a schematic representation of the pulse parameters, Figure 3A a graphical representation of measured values ​​of the penetration time as a function of the pulse frequency for different workpiece thicknesses, Figure 3BMicrographs of a piercing process in a 30 mm thick stainless steel sheet with different pulse frequencies, Figure 4A a schematic representation of a puncture process with a limit pulse frequency at a first time, Figure 4B a schematic representation of the puncture process of Fig. 4A at a second time after the onset of a puncture stop, Figure 5 a schematic representation of the pulse rate as a function of the puncture time during a puncture method with a stepwise reduction of the pulse rate according to embodiments of the present disclosure, Figure 6 a schematic representation of the irradiated laser power or the irradiated pulse sequence as a function of the puncture time during a method for puncturing with a stepwise reduction of the pulse frequency at a constant pulse on time according to embodiments of the present disclosure, Figures 7A to Dschematic representations of the relationship between pulse frequency and puncture depth during the procedure for piercing Fig. 6 , Figure 8 a schematic representation of the pulse rate as a function of the puncture time during a method for puncturing with stepwise lowering of pulse rate bands according to embodiments of the present disclosure, and Figure 9 a schematic representation of the pulse frequency as a function of the puncture time during a method for puncturing with lowering of a best fit line of the pulse frequency according to embodiments of the present disclosure. Detailed description of the invention

[0034] In the following, unless otherwise stated, the same reference symbols are used for identical and equivalent elements.

[0035] Figure 1shows a schematic representation of a device for laser material processing according to embodiments of the present disclosure. The device for laser material processing can comprise a laser processing head 100, in particular a laser cutting head, for irradiating a laser beam 10 onto a workpiece 1, a laser source 200 for generating the laser beam 10, and a control device 300. The control device 300 is designed or configured to control the device, in particular to control the device according to a method according to one of the embodiments described in this disclosure.

[0036] The laser source 200 emits a laser beam 10, also called a processing beam, which is guided and focused onto the workpiece 1 by processing optics. The processing optics and / or the laser source 200 are connected to the control device 300. In addition to the control function, the control device 300 can also have an evaluation and / or calculation function. The processing optics can have transmitting and / or reflecting optical elements for beam guidance and beam shaping. Furthermore, the device for laser material processing can have a gas supply for supplying a process gas into a processing zone on the workpiece 1.

[0037] During piercing, the laser beam 10 is directed in pulsed form onto the workpiece 1. A schematic overview of the pulse parameters is shown in Fig. 2 The pulse period or pulse period T results from the pulse on time t on and the pulse off time t off : T = t an + t aus .

[0038] The pulse on-time t on refers to the period of the laser pulse during which energy is radiated onto workpiece 1. Similarly, the pulse off-time t off refers to the period during which no or virtually no energy is radiated onto workpiece 1. The inverse of the pulse period T is referred to as the pulse frequency: f = 1 / T. Thus, the pulse frequency depends on both the pulse on-time and the pulse off-time. The relationship between the pulse on-time and the pulse period is referred to as the relative pulse duty cycle R: R = t on / T. The pulse peak power P peak can correspond to the maximum laser power provided by the laser source 200, hereinafter referred to as the maximum laser power, P max. The radiated energy during a pulse, i.e., the so-called pulse energy, is calculated from the product of the pulse peak power and the pulse on-time: E Puls = P peak * t an .

[0039] Accordingly, the radiated average pulse power is calculated from the product of the relative duty cycle and the pulse peak power or from the product of the pulse peak power and the pulse on time and pulse frequency: P mittel = P peak * R = P peak * t an * f = P peak * 1 − t aus / t an + t aus .

[0040] For the purposes of the present disclosure, not only the time of the pulse period during which no energy is radiated can be considered as a pulse off-time, but also a time of the pulse period during which the radiated energy or the radiated power remains below a threshold value. Fig. 2 Such a threshold value for the irradiated power is shown as P SW,off. This means that as soon as the current power irradiated onto the workpiece falls below this threshold value P SW,off, a pulse off time occurs. The threshold value can be, for example, 30% of the maximum laser power (P SW,off = 0.3 * P max ), 20% of the maximum laser power, or even 10% * of the maximum laser power.

[0041] According to the present disclosure, the control device 300 is configured to control an energy input into the processing zone or onto the workpiece 1 depending on one or more of the following process parameters and boundary conditions: material thickness or workpiece thickness, material of the workpiece, current piercing time, current piercing depth, process gas type, process gas pressure, focus position, imaging ratio of the optical system, focus diameter, and nozzle distance from the top of the workpiece. Based on at least one of the aforementioned parameters, the energy input can be controlled by the pulse on time (t on ), pulse off time (t off ), and / or the pulse peak power (P peak ) depending on the current piercing time or piercing depth. By knowing the causal relationships in the process zone, it is possible to specifically adapt the process parameters and thus increase process efficiency.

[0042] As a rule, the piercing rate (change in piercing depth per time) decreases with increasing piercing depth, and the piercing time, i.e. the time period from the first irradiation of the laser beam onto the workpiece until the piercing, increases with increasing piercing depth s. In Fig. 3AThe piercing time (in seconds) is shown as a function of a selected pulse frequency (in Hertz) for different material thicknesses or workpiece thicknesses (in millimeters). The pulse frequency remains constant throughout the entire piercing process. As the pulse frequency decreases, the piercing time increases disproportionately. This is due, among other things, to the increasing resolidification of the melt on the wall of the piercing hole. As a result, the piercing rate decreases and process efficiency falls. Furthermore, a saturation range, in which depth removal is stopped, is shown hatched, as is a so-called limiting pulse frequency as a function of the material thickness. The limiting pulse frequency describes a pulse frequency threshold above which reliable piercing is no longer possible under given process parameters and boundary conditions. The limiting pulse frequency shifts towards lower pulse frequencies as the material thickness increases.

[0043] In Fig. 3B micrographs of punctures in a 30 mm thick stainless steel workpiece with a pulse peak power of 6 kW are shown (see top curve in Fig. 3A ). At a pulse frequency of 350 Hz (constant), the penetration time is 3.6 s (first image from the left in Fig. 3B ), at a pulse frequency of 400 Hz (constant) the penetration time is 2.5 s (second image from the left in Fig. 3B ), and at a pulse frequency of 500 Hz (constant) the penetration time is 1.7 s (third image from the left in Fig. 3B ). At a pulse frequency of 588 Hz (constant), the piercing stops (no penetration, right in Fig. 3B ).

[0044] In Fig. 4A and 4BThe piercing hole 3 is shown schematically during piercing with a limit pulse frequency for two different points in time. From a certain piercing depth s, the time between the individual pulses (pulse off-time) is too short or the pulse frequency is too high to expel the molten material in sufficient quantity from the piercing hole 3, resulting in a piercing stop. A further cause for the piercing stop is the decreasing coupled energy of the laser beam 10 at the piercing base 5 with increasing piercing depth s and the increasingly greater distance that the molten material must travel until it exits the piercing hole 3 (cf. Fig. 4A ). As the penetration depth increases, the radiated energy also increasingly heats the sides of the piercing hole 3, which can lead to cave-like melting. For thick material thicknesses, e.g., greater than 20 mm, saturation of the piercing process can occur (see Fig. 4B), at which the piercing rate approaches 0 (piercing stop) if the pulse frequency is arbitrarily high or higher than a respective limit pulse frequency. The residual melt in the piercing hole 3 and the subsequent pulses lead to an ever greater heating of the processing zone from pulse to pulse. This creates a heat accumulation in the piercing hole (pulse-to-pulse mechanism), which leads to a certain material area being melted transversely to the piercing direction and repeatedly filling the piercing base 5 with material. The previously reached piercing depth s decreases and as a result a limit piercing depth s grenz is established (cf. Fig. 4B and Fig. 3B right). A safe penetration is not possible (piercing stop in the piercing direction).

[0045] To avoid a piercing stop and simultaneously shorten the piercing time, the invention reduces the average pulse power radiated onto the workpiece during piercing. This maximizes the piercing rate along the piercing depth while simultaneously ensuring a piercing. Several embodiments of the piercing method according to the present disclosure are described below.

[0046] According to embodiments, a method for piercing a workpiece is specified below, as well as a device for laser material processing with a control device configured to carry out this method. According to embodiments, the irradiated average pulse power is reduced by lowering the pulse frequency during piercing. However, the present invention is not limited thereto. Alternatively, the irradiated average pulse power can be reduced, for example, by extending the pulse off-time and / or shortening the pulse on-time and / or reducing a relative duty cycle.

[0047] In the Fig. 5In the embodiment shown, the pulse frequency is reduced in stages or in steps or discretely. For example, the pulse frequency is reduced at least once, preferably at least twice, during the piercing time. In this case, the piercing method can have at least a first piercing step (step 1) with a first pulse frequency f 1 and a second piercing step (step 2) with a second pulse frequency f 2, wherein the first pulse frequency f 1 is greater than the second pulse frequency f 2 . The first piercing step (step 1) therefore takes place at the beginning of the piercing and the second piercing step (step 2) takes place following the first piercing step (step 1). However, the method can also have any number of piercing steps, e.g. n, wherein the pulse frequency of each piercing step is lower than that of a preceding piercing step. The pulse frequency can be constant during a piercing step.Alternatively, as below, . Fig. 9 described, the pulse frequency during a piercing step is in a range of 80% and 120%, preferably 90% and 110%, of the average pulse frequency fn of this n-th piercing step (ie fn ± 0.2* fn or fn ± 0.1* fn ). The change in the pulse frequency between the piercing steps, i.e. Δf 1,2 , can be the same or different. Preferably, a first change, i.e. Δf 1,2 , is greater than a second change, i.e. Δf 2,3 . Likewise, a duration of the individual piercing steps can be the same or different. Preferably, a duration of the first piercing step is the longest.

[0048] As an alternative to a gradual or discrete reduction, the pulse rate can be reduced in any combination of discrete and continuous reductions during the puncture time (see also Fig. 8 ).

[0049] With regard to Fig. 5At least a change or reduction in the pulse frequency (f 2 < f 1 ) is described. The pulse frequency at the beginning of the piercing or at the beginning of the piercing time f 1 and / or the pulse frequencies f 2 to fn of the subsequent piercing steps can be adjusted depending on the thickness of the workpiece and / or the current piercing time and / or the current piercing depth.

[0050] In one embodiment, the piercing can be performed with a variable pulse frequency, with two pulse frequency changes. Preferably, the pulse frequency at the beginning of piercing and / or in the first piercing step is greater than or equal to the limit pulse frequency for this workpiece, which may depend on the workpiece thickness and the workpiece material.

[0051] Another embodiment is in Fig. 6 shown schematically. Just as in Fig. 5 The pulse frequency is reduced in n puncture steps until the puncture. In addition, the pulse rate remains Fig. 6 In the embodiment shown, the pulse on-time t on is constant or quasi-constant throughout the entire piercing process. In the latter case, the pulse on-time can deviate by ±20% or ±10% of the average pulse on-time during the piercing time. In other words, a quasi-constant pulse on-time can lie within a range from 0.8 * t on to 1.2 * t on or from 0.9 * t on to 1.1 * t on.

[0052] Preferably, the value for the pulse on-time t on is selected depending on the workpiece thickness. In this case, the average pulse power or the pulse frequency is adjusted by increasing the pulse off-time t off , for example, depending on the piercing time ( Fig. 6 ) or depending on the penetration depth ( Fig. 7 ). This ensures that the melt has sufficient time between pulses as the piercing depth increases, for example, to be expelled from the piercing hole by blown-in process gas.

[0053] At the in the in Fig. 6 In the embodiment shown, instead of a constant pulse on-time, the radiated pulse energy E Puls , ie the product of pulse peak power and pulse on-time, can be kept constant, the other pulse parameters can be the same as in the embodiment shown in Fig. 6 The embodiment shown can be selected. Optionally, the pulse energy can be set to a minimum value depending on the material and thickness.

[0054] In another in Fig. 7 In the illustrated embodiment, the workpiece thickness is at least 20 mm and the pulse peak power is at least 4 kW. The piercing method comprises at least three piercing steps with different pulse frequencies. The piercing process is started with a first pulse frequency greater than or equal to a limit pulse frequency dependent on the workpiece thickness ( Fig. 7A : f limit ≤ f 1 ). This can be step 1 of Fig. 5After a predetermined penetration time and / or penetration depth, the pulse frequency is reduced to a second pulse frequency f 2 , which is lower than the first pulse frequency or than the limit pulse frequency ( Fig. 7B : f 2 < f 1 or f 2 < f limit ≤ f 1 ). This can be step 2 of Fig. 5 After a further predetermined penetration time and / or penetration depth, the pulse frequency is reduced to a third pulse frequency f 3 , which is lower than the second pulse frequency ( Fig. 7C : f 3 < f 2 ) and held until penetration 9 (see 7D). In the example of piercing a 30 mm thick stainless steel workpiece at a pulse peak power of 6 kW from Fig. 3With this method, the piercing time could be reduced by more than 50%, namely from 1.7 s at a constant pulse frequency of 500 Hz to 0.8 s. The pulse frequency was changed twice during the piercing: from a first pulse frequency at the beginning of the piercing of f 1 = 588 Hz to a second pulse frequency of f 2 = 400 Hz and then to a third pulse frequency of f 3 = 350 Hz. The first pulse frequency f 1 of 588 Hz was greater than the limit pulse frequency (cf. Fig. 3A ). This resulted in an optimized piercing process in terms of short piercing time and safe piercing.

[0055] As mentioned above, the average pulse power or pulse frequency does not have to be reduced monotonically or strictly monotonically. Instead, the average pulse power or pulse frequency can be reduced stepwise or incrementally, whereby the average pulse power or pulse frequency can move within a power band or pulse frequency band in each step. This means that during each step, the average pulse power or pulse frequency lies within a band with a predetermined minimum value and a predetermined maximum value. The power band can be defined by a deviation of ±20%, in particular of ±10% of the average pulse power during the piercing step n. Analogously, the pulse frequency band can be defined by a deviation of ±20%, in particular of ±10% of the average pulse frequency fn during the piercing step n. Fig. 8Examples of pulse frequency bands are shown, in which the pulse frequency can move during the individual steps 1, ... n. The pulse frequency bands shift to smaller values ​​with each step. For example, the pulse frequency bands can decrease step by step. Fig. 8the pulse frequency bands are defined as follows: f 1,min = 0.8 * f 1 (or 0.9 * f 1 ); f 1,max = 1.2 * f 1 (or 1.1 * f 1 ); f 2,min = 0.8 * f 2 (or 0.9 * f 2 ); f 2,max = 1.2 * f 2 (or 1.1 * f 2 ); fn,min = 0.8 * fn (or 0.9 * fn ); and fn,max = 1.2 * fn (or 1.1 * fn ). In other words, slight increases in pulse rate can be neglected as long as the pulse rate is reduced on average over time until the puncture. If the average pulse power is to be reduced by changing another pulse parameter, for example by extending the pulse off-time, this change can also be made step by step, whereby this pulse parameter can be in each step within a band of ±20%, in particular of ±10% of the average value of this pulse parameter in this step.

[0056] Likewise, the average pulse power or pulse rate can be reduced continuously or quasi-continuously. For this purpose, it may be sufficient for the average pulse power or pulse rate to remain within a band that decreases on average over time (see Fig. 9 ). This means that the mean pulse power or pulse frequency decreases over time. In other words, the mean pulse power or pulse frequency can be considered as reduced if a corresponding regression line (dashed in Fig. 9) has a negative slope during the piercing time, i.e., from the start of piercing to the piercing. This is because a very brief increase in the average pulse power or pulse frequency can be negligible for the piercing method according to this disclosure. If the average pulse power is reduced by changing another pulse parameter, for example, by extending the pulse off-time, this change can also occur continuously or quasi-continuously, as long as a regression line for this pulse parameter has a negative slope during the piercing time.

[0057] According to the embodiments of this disclosure, a method and apparatus for piercing a workpiece using a laser beam are provided, whereby a piercing time can be minimized or a piercing rate can be maximized while ensuring a safe piercing and preventing a piercing stop.

Claims

1. A method for piercing a metallic workpiece with a thickness of at least 10 mm by means of a laser beam, comprising: irradiating a pulsed laser beam (10) onto a metallic workpiece (1) with a thickness of at least 10 mm to form a piercing breakthrough (9), characterized in that a irradiated mean pulse power (Pmittel) of the pulsed laser beam (10) is reduced during piercing.

2. The method according to claim 1, wherein the irradiated mean pulse power (Pmittel) is reduced discretely and / or in steps and / or continuously.

3. The method according to one of the preceding claims, wherein a line of best fit of the irradiated mean pulse power (Pmittel) has a negative slope during piercing.

4. The method according to one of the preceding claims, wherein the irradiated mean pulse power (Pmittel) and / or a pulse frequency (f) is reduced in steps and, during each step, lies within a band with a predetermined minimum value and a predetermined maximum value.

5. The method according to one of the preceding claims, wherein the irradiated mean pulse power (Pmittel) is reduced by at least one of the following changes in pulse parameters of the pulsed laser beam (10): lengthening a pulse off-time (taus), shortening a pulse on-time (tan), reducing a pulse frequency (f), and reducing a relative pulse duty cycle.

6. The method according to one of the preceding claims, wherein the irradiated mean pulse power (Pmittel) is reduced by lengthening the pulse off-time (taus) and keeping the pulse peak power (PPeak) and / or the pulse on-time (tan) and / or a pulse energy (EPuls) constant during piercing.

7. The method according to one of the preceding claims, wherein a first pulse frequency (f1) in a first piercing step at the start of piercing is greater than or equal to a predetermined limit pulse frequency.

8. The method according to one of the preceding claims, wherein the irradiated mean pulse power (Pmittel) is reduced in at least two steps, wherein a first pulse frequency (f1) in a first piercing step is greater than or equal to a predetermined limit pulse frequency and a second pulse frequency (f2) in a second piercing step is less than the first pulse frequency (f1) or than the limit pulse frequency.

9. The method according to claim 7 or 8, wherein the predetermined limit pulse frequency is based on a thickness and / or a material of the workpiece (1), and / or wherein the predetermined limit pulse frequency specifies a pulse frequency from which on a piercing stop occurs.

10. The method according to one of the preceding claims, wherein at least one of the pulse parameters of the pulsed laser beam (10), selected from the group comprising the mean pulse power irradiated, the pulse off-time, the pulse on-time, the pulse frequency, the relative pulse duty cycle and the pulse peak power, is adjusted based on a material and / or a thickness of the workpiece and / or on a current piercing time and / or on a current piercing depth.

11. The method according to one of the preceding claims, wherein a pulse off-time (taus) and / or a pulse on-time (tan) of the pulsed laser beam (10) is in a range of between 0.01 ms and 100 ms or between 0.1 ms and 10 ms; and / or wherein a pulse frequency of the pulsed laser beam (10) is in a range of between 200 Hz and 3000 Hz or between 400 Hz and 2000 Hz; and / or wherein, when piercing a workpiece (1) with a thickness of more than 20 mm, the irradiated mean pulse power (Pmittel) is changed in at least two steps.

12. The method according to one of the preceding claims, wherein the irradiated mean pulse power (Pmittel) is controlled as a function of at least one of the following parameters: thickness of the workpiece, material of the workpiece, current piercing time, current piercing depth, type of process gas, process gas pressure, imaging ratio of an optical system of a laser machining head (100), focal position, focal diameter, intensity distribution of the laser beam (10), nozzle type, nozzle diameter, and nozzle distance from the workpiece.

13. The method according to one of the preceding claims, wherein a metallic sheet is pierced.

14. A method of laser cutting, comprising: a method for piercing according to one of the preceding claims; and cutting by means of the laser beam (10) starting from the formed piercing breakthrough (9).

15. A device for laser material machining of a metallic workpiece (1) with a thickness of at least 10 mm, comprising: a laser source (200) for generating a laser beam (10); a laser machining head (100) for irradiating the laser beam (10) onto said workpiece; and characterized by a control device (300) configured to control said device to perform a method for piercing and for laser cutting with the following steps: irradiating the pulsed laser beam (10) onto a metallic workpiece (1) to form a piercing breakthrough (9), wherein an irradiated mean pulse power (Pmittel) of the pulsed laser beam (10) is reduced during piercing, and cutting by means of the laser beam (10) starting from the formed piercing breakthrough (9).