Systems and methods for monitoring and / or controlling short pulse laser welding

Abstract

The systems and methods can be used to monitor and / or control a short-pulse laser welding process involving the formation of a series of heat piles in the welded material. The systems and methods use an imaging system, such as an ICI system, that can take measurements of the inside of the steam channel during the formation of the heat piles to provide pile measurement data that is indicative of the characteristics of the piles. The pile measurement data can be used to monitor and / or control the short-pulse laser welding process.

Description

[Technical Field]

[0001] [CROSS-REFERENCE TO RELATED APPLICATIONS] This application claims the benefit of U.S. Provisional Patent Application No. 63 / 440,334, filed January 20, 2023, which is incorporated herein by reference in its entirety.

[0002] FIELD OF THE DISCLOSURE The present disclosure relates to monitoring and / or controlling laser welding processes, and more particularly to systems and methods for monitoring and / or controlling short pulse laser welding with heat stake formation. [Background technology]

[0003] Lasers are often used as an effective manufacturing tool for joining applications. Fiber lasers have been established as the tool of choice for high-power welding of thick materials as well as for microwelding applications of thin metals. Short-pulse infrared fiber lasers with pulse widths in the nanosecond range have demonstrated high capabilities in challenging laser welding applications such as microwelding of thin metals (less than 0.5 mm thick), welding of metals with high reflectivity at near-infrared wavelengths of 1 μm, and / or high-strength welding of dissimilar metals.

[0004] These challenging welding applications can be used in many different industries, including consumer electronics, e-mobility, medical devices, energy storage, renewable energy, and aerospace / defense. Laser welding processes used in these industries often involve high levels of process control to obtain reliable, high-quality products. Furthermore, some products manufactured using short-pulse infrared fiber lasers are mass-produced, so a high-performance in-situ monitoring system can be beneficial.

[0005] Welding processes for metals that are highly reflective in the near-infrared (e.g., copper, aluminum, brass, silver, gold) generally require higher laser intensities to allow for sufficient absorption power and meaningful bonding for effective processing. Introducing thin thicknesses and dissimilar metal combinations increases the overall welding challenges and problem complexity. As a result, the process window for thin dissimilar welding of reflective materials is often narrow. High-intensity beams produced from short-pulse lasers at high repetition rates (e.g., in the 20 kHz to 500 kHz range) can be detrimental if the process is not carefully optimized, resulting in defects such as incomplete or excessive penetration, the formation of intermetallic compounds that cause fracture, distortion, and ablation rather than an effective weld. Intermetallic compounds are alloys formed by mixing two or more metallic elements. The formation of an intermetallic alloy layer in a weld joint can result in a brittle, low-strength weld. The thicker the intermetallic layer, the less ductile the weld.

[0006] Short-pulse, high-brightness fiber lasers can overcome the reflectivity of shiny metals using high peak power while maintaining low overall heat input compared to continuous-wave (CW) lasers of comparable wavelengths. Furthermore, by using certain advantageous temporal pulse shapes, the high peak power of a substantially single-spatial-mode laser beam operating at a high pulse repetition rate creates a liquid-vapor channel known as a "keyhole" weld. The formation of the keyhole results in a series of spikes or piles in this welding style, and the overall depth of the piles is directly related to the total penetration.

[0007] Detailed characterization of the pile formation and understanding the mechanics of the keyhole formed by high-repetition short pulses will help achieve a successful welding process. Existing ex-situ destructive and non-destructive testing methods are time-consuming, not comprehensive, and cannot affordably support the level of quality assurance required in a high-volume production environment. Some conventional imaging process monitoring systems lack the ability to fully characterize and monitor such welds.

[0008] Therefore, there is a need for a sophisticated in-situ weld monitoring solution that can image, characterize, and process keyhole data obtained from short-pulse laser welding processes that addresses the welding challenges described above. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] U.S. Patent No. 1,142,6816 [Patent Document 2] U.S. Patent No. 1,132,7011 [Patent Document 3] U.S. Patent Application Publication No. 2020 / 0198050 [Patent Document 4] U.S. Patent Application Publication No. 2021 / 0323086 Summary of the Invention [Means for solving the problem]

[0010] In accordance with one aspect of the present disclosure, a system for performing, monitoring, and / or controlling a short-pulse laser welding process is provided. The system includes a pulsed laser source configured to generate a process beam for performing the short-pulse laser welding process at a weld site to join at least a first workpiece material and a second workpiece material. The pulsed laser source is configured to generate the process beam having a pulse width of less than 1000 ns and a repetition rate such that the process beam can penetrate at least the first workpiece material and generate a series of vapor channels extending into the second workpiece material. Each vapor channel in the series of vapor channels is subsequently refilled to form a series of pilings extending into the second workpiece, and a series of pulses of the process beam form each of the vapor channels and each of the pilings.

[0011] The system also includes an imaging system configured to generate an imaging beam that reflects at the weld site, generate an interference output from the reflection of the imaging beam, and detect the interference output to generate measurement data representative of a characteristic of the weld site. The measurement data includes at least pile measurement data representative of a characteristic of a pile in a series of piles extending into the second workpiece material. A beam delivery system is coupled to the pulsed laser source and the imaging system to direct the process beam and the imaging beam to the weld site on the first workpiece. An electronics system is programmed to receive the measurement data from the imaging system and to monitor and / or control the short-pulse laser welding process based on the measurement data.

[0012] In accordance with another aspect of the present disclosure, a method for performing and monitoring a short pulse laser welding process is provided, the method including: generating a process beam having a pulse width of less than 1000 ns; generating an imaging beam; directing the process beam and the imaging beam at a weld site on a workpiece including at least a first workpiece material and a second workpiece material such that the process beam generates a series of vapor channels extending into the second workpiece material, each vapor channel of the series of vapor channels subsequently refilling to form a peg within a series of pegs extending into the second workpiece material, the series of pulses of the process beam forming each of the vapor channels and each of the pegs; generating an interference output from reflection of the imaging beam from the weld site; and detecting the interference output to generate measurement data representative of a characteristic of the weld site, the measurement data including at least peg measurement data representative of a characteristic of the peg within the series of pegs extending into the second workpiece material.

[0013] These and other features and advantages will be better understood from the following detailed description taken in conjunction with the drawings. [Brief explanation of the drawings]

[0014] [Figure 1]FIG. 1 is a schematic block diagram of a system for monitoring and / or controlling a short pulse laser welding process with pile formation consistent with an embodiment of the present disclosure. [Figure 1A] FIG. 2 is a schematic block diagram of an inline coherent imaging (ICI) system that may be used as the imaging system in FIG. 1 to monitor and / or control a short-pulse laser welding process with pile formation consistent with an embodiment of the present disclosure. [Figure 2] 1 is a schematic cross-sectional view of an example of pile formation using a short pulse laser welding process consistent with embodiments of the present disclosure. [Figure 3A] 1 is an image showing keyhole data (depth vs. weld length) measured by an imaging system while performing a short pulse laser welding process on stainless steel using a method consistent with an example of the present disclosure. [Figure 3B] 3B is an image showing a subset of the keyhole data from FIG. 3A. [Figure 4A] 10 is an image showing keyhole data (depth vs. weld length) measured by an imaging system while performing a short pulse laser welding process on copper using a method consistent with another example of the present disclosure. [Figure 4B] 4B is an image showing a subset of the keyhole data from FIG. 4A. [Figure 5] 1 is a flowchart illustrating a method of performing and monitoring a short pulse laser welding process consistent with an embodiment of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0015] The present disclosure involves monitoring and controlling high-repetition-rate short-pulse laser welding using imaging systems, such as inline coherent imaging (ICI) systems. The mechanics of short-pulse laser welding are not always accurately described and are sometimes misunderstood. Furthermore, there has been uncertainty as to whether processes involving nanosecond-duration laser pulses can be effectively observed with imaging systems whose integration times are measured in microseconds. If the vapor channel collapses between each pulse, as previously described, it is impossible to measure its progression / penetration into the material using such imaging systems. However, the exemplary systems and methods described herein use imaging techniques that provide microsecond- and micrometer-level temporal and spatial resolution, respectively, to reveal the welding process window in which the vapor channel and melt pool are maintained for hundreds of pulses. Measuring these mechanics using the systems and methods described herein has broad potential for engineering, monitoring, and regulating such processes, including real-time applications.

[0016] As used herein, a "thermal pile" (also referred to as a "pile") refers to a weld fusion zone that extends from a first material into a second material below the first material, with a series of piles separated (at least at their deepest ends) by unmelted material. The piles may taper to a point at their deepest ends, although the piles may have other blunt shapes or configurations. While the piles extend into the second material, they do not necessarily extend through the second material.

[0017] As used herein, "short pulse" refers to a pulse width of a pulsed laser that is less than 1000 nanoseconds (ns), and "high repetition rate" refers to a repetition rate of a pulsed laser that is greater than 10 kHz. As used herein, in-line coherent imaging (ICI) refers to a process in which an imaging beam is directed along with, or "in-line" with, a process beam at a workpiece for the purpose of measuring process and / or workpiece characteristics. The term "in-line" does not require that the imaging beam and the process beam be coaxial. The imaging beam may be coaxial with the process beam, or it may be offset or angled relative to the process beam.

[0018] 1 , a system 100 for controlling and / or monitoring a short-pulse laser welding process on a workpiece 102 is shown and described in more detail. System 100 generally includes a pulsed laser source 110, an imaging system 120, a beam delivery system 130, and a control and / or monitoring electronic system 140. As described in more detail below, pulsed laser source 110 generates a process beam 111, imaging system 120 generates an imaging beam 121, and beam delivery system 130 directs process beam 111 and imaging beam 121 toward workpiece 102. Pulsed laser source 110 and imaging system 120 may be coupled to beam delivery system 130 using, for example, optical fibers 113, 123 and optical fiber collimators 115, 125. Other structures and configurations for coupling the imaging system and pulsed laser source to the beam delivery system are also contemplated. The workpiece 102 includes at least two workpiece materials, such as dissimilar metals, that are to be welded, and the upper workpiece material may be reflective and relatively thin.

[0019] The pulsed laser source 110 generates a process beam 111 for performing a short-pulse welding process at a weld site on the workpiece 102 to join at least a first workpiece material and a second workpiece material. The pulsed laser source 110 may include a short-pulse fiber laser configured to generate a process beam having a wavelength in the infrared (IR) range, a pulse width in the range of 1 ns to 1000 ns, and a high repetition rate in the range of 20 kHz to 1000 kHz. The pulsed laser source 110 may have a pulse width as short as 50 fs to 100 ps. As described in more detail below, the pulse width and repetition rate are set, for example, so that as the process beam travels across the first workpiece material, the process beam can penetrate at least the first workpiece material (e.g., a thin upper material) and generate a series of vapor channels (also known and referred to as keyholes) extending into the second workpiece material. Each vapor channel in the series of vapor channels is subsequently refilled to form a series of pegs extending into the second workpiece. Thus, a series of pulses of the process beam form each of the vapor channels and each of the piles as the process beam advances against the first workpiece material.

[0020] The imaging system 120 generates an imaging beam 121 that reflects off the weld site, generates an interference output from the reflection of the imaging beam, and detects the interference output to generate measurement data representative of characteristics of the weld site, such as the depth of penetration at each point along the length of the weld. As described in more detail below, the measurement data includes at least pile measurement data representative of characteristics of piles within a series of piles extending into the second workpiece material. The pile measurement data may include, but is not limited to, the number of piles formed, the frequency with which piles are formed, and a statistical distribution of the depth of the formed piles. Other measurement data may include, but is not limited to, surface signal data representative of reflections of the imaging beam from the surface of the workpiece. In the embodiment described herein, the imaging system 120 is an inline coherent imaging (ICI) system, although one skilled in the art will understand that an optical coherence tomography (OCT) system may also be used.

[0021] FIG. 1A illustrates one embodiment of an inline coherent imaging (ICI) system 120′ that can be used as imaging system 120 in FIG. 1 . In this embodiment, ICI system 120′ includes an imaging beam source 122, such as a semi-coherent light source, for generating an imaging beam 121. ICI system 120′ also includes an interferometer 124, such as a Michelson interferometer, for generating an interference output from a reflected imaging beam reflected from workpiece 102. The interference output may be based on at least one optical path length to the workpiece and at least one other optical path length (reference path). ICI system 120′ further includes a detector 126, such as a spectrometer and camera, for detecting the interference output and generating interferogram data, and an ICI image processor 128 for processing the interferogram data and generating measurement data. Examples of ICI systems and methods for monitoring keyhole welding processes are described in further detail in, for example, the aforementioned U.S. Patents 6,113,529; 6,114,529; 6,115,796; and 6,116,796, which are commonly owned and incorporated herein by reference in their entireties.

[0022] A beam delivery system 130 coupled to the pulsed laser source 110 and the imaging system 120 delivers the imaging beam 121 along with the process beam 111 to the weld site on the workpiece 102. The beam delivery system 130 can include scanning optics, such as deflecting elements (e.g., mirrors 132, 134), to move the process beam 111 and the imaging beam 121 along with the process beam 111 for substantially continuous advancement over the workpiece 102. Advancement of the process beam 111 relative to the workpiece 102 can be at a speed ranging from approximately 10 mm / s to 2000 mm / s. In other embodiments, the workpiece 102 can be moved (e.g., on a workpiece stage 150) to achieve advancement of the process beam 111 and the imaging beam 121 across the workpiece 102. In the embodiments described herein, the process beam 111 advances continuously across the workpiece 102; however, the systems and methods for monitoring short pulse laser welding according to the present disclosure may also be used to monitor and / or control welding processes in which the process beam 111 is moved discontinuously (e.g., stopped and fired).

[0023] In the illustrated embodiment, the beam delivery system 130 is a welding head (i.e., a dichroic beam combiner) that includes at least a first mirror 132 for deflecting the imaging beam 121 and at least a second mirror 134 (e.g., a dichroic mirror) for deflecting the process beam 111 and allowing the imaging beam 121 to pass along with the process beam 111. The deflecting elements (e.g., mirrors 132, 134) may be separately movable or adjustable (e.g., using motors to rotate mirrors 132, 134) to align the imaging beam 121 and process beam 111 directed at the workpiece 102. The beam delivery system 130 also includes an objective or focusing objective 136 that focuses the beam onto the workpiece 102. Other embodiments of the beam delivery system 130 may include deflecting elements (e.g., mirrors) for deflecting both the process beam and the imaging beam together.

[0024] A monitoring and / or control electronic system 140 may be connected to the pulsed laser source 110, the imaging system 120, the beam delivery system 130, and the workpiece stage 150. The electronic system 140 may be programmed to control laser parameters of the pulsed laser source 110, receive measurement data from the imaging system 120, and control the movement of the process beam 111 and / or the imaging beam 121 relative to the workpiece 102 (e.g., by controlling the movement of mirrors 132, 134 and / or the workpiece stage 150). The electronic system 140 may include known hardware (e.g., a general-purpose computer or microcontroller) and software for use with an imaging system, such as an ICI system, and for controlling lasers and scanners. The electronic system 140 may include a user interface (e.g., a keyboard and display or touchscreen) for providing user input to the electronic system 140 and output to a user.

[0025] In some embodiments, the computerized system 140 may include separate controllers for each of the subsystems (e.g., the pulsed laser source 110, the imaging system 120, the beam delivery system 130, and the workpiece stage 150) and a process master controller for controlling and coordinating all of the subsystems and associated controllers. In some cases, one of the subsystem controllers may act as the process master controller. The computerized system 140 may also include known data storage media for storing measurement data, such as pile measurement data, for use in monitoring and / or controlling the laser welding process.

[0026] In one embodiment, the pulsed laser source may include an ytterbium nanosecond pulsed fiber laser such as the IPG YLPN-1-30x240-300-R laser available from IPG Photonics, the beam delivery system 130 may include a scanner head such as the IPG Mid-Power Scanner available from IPG Photonics, and the imaging system 120 may include an in-line coherent imaging (ICI) system such as the IPG LDD-700 In-line Welding Process Monitor available from IPG Photonics. This system may be used in conjunction with various clamps / fixtures, air knives, shielding gases, and other accessories known to those skilled in the art of welding, particularly laser welding, to join one or more workpieces together by pulsed laser welding.

[0027] The formation of a pile in the materials being welded will be described in more detail with reference to Figure 2. As shown, the workpiece 102 includes at least a first workpiece material 204 being welded to a second workpiece material 206 below or below the first workpiece material 204. The process beam 111 is directed at the first workpiece material 204 and is continuously moved relative to the workpiece 102 in the direction of arrow 2 while being pulsed as described above. Parameters of the process beam 111, such as output power, repetition rate, pulse duration, and velocity, can be determined based on the first workpiece material 204 and second workpiece material 206 being welded.

[0028] In one embodiment, the workpiece material 204 is metal and arranged in an overlapping configuration, with the top sheet or workpiece material 204 being thin enough to allow a laser pulse to penetrate completely to deliver laser energy onto or into the bottom sheet or workpiece material 206. The laser and motion parameters are set so that the welding process creates a melt 208 (i.e., molten metal), such as a weld pool, and vapor channels 209, which serve to trap the laser pulses (increase energy coupling) and allow the laser energy to penetrate deeper into the material stack. As a scanner (or other device that causes relative motion between the laser aimpoint and the workpiece) continuously advances the process beam 111 across the workpiece, the vapor channels 209 may not be continuously supported by individual laser pulses, nor may they be one-to-one supported by individual laser pulses. Instead, the weld 210 (ie, molten zone) that is formed is characterized by a series of spikes or pegs 212 emanating from the steam channel 209, each of which is opened by multiple pulses and then collapses.

[0029] As the vapor channel 209 opens and material is delivered to the process beam 111, successive pulses are offset from the center of the vapor channel 209, but only by a small amount, so that most of their energy is still accumulated and trapped inside the vapor channel 209, causing it to deepen and expand. However, the rate at which the vapor channel 209 expands does not keep pace with the progress of the process beam 111, and eventually, the absorbed energy portion of each pulse begins to drop as more of it misses the vapor channel 209 entrance / exit. At this point, the vapor channel 209 begins to collapse, and the surrounding melt 208 begins to refill the vapor channel 209, forming a weld 210. Soon after, a new vapor channel begins to open, and the process repeats. Thus, the formation of a series of vapor channels 209 by the successive advancement of the process beam 211 relative to the workpiece 102 forms a series of heat piles, and each of the vapor channels 209 and heat piles 212 may be formed by multiple pulses of the process beam 111.

[0030] The first workpiece material 204 and the second workpiece material 206 may be dissimilar metals, and at least the first workpiece material 204 may be a metal that is reflective at infrared wavelengths. The first workpiece material 204 may also be relatively thin (e.g., less than 0.5 mm) so that the process beam 111 can penetrate through the first workpiece material 204. Although only two workpiece materials, the first workpiece material 204 and the second workpiece material 206, are shown, the systems and methods described herein may be used to weld more than two workpiece materials (e.g., multiple layers of material).

[0031] For example, during a short-pulse laser welding process involving pile formation, as shown in FIG. 2, an imaging system 120, such as the ICI system 120′ shown in FIG. 1A, can be used to measure the dynamics of the process by directing an imaging beam 121 into the vapor channel 209 along with the process beam 111. Although shown in FIG. 2, the imaging beam 121 need not be axially aligned with the process beam 111. The imaging beam 121 can be reflected from inside the vapor channel 209 back to the ICI system 120′, where an interferometer 124 generates an interference output from the reflected imaging beam based, for example, on the optical path length to the workpiece 102 and at least one other optical path length (e.g., a reference path).

[0032] The interference output from interferometer 124 may then be detected by detector 126 and processed by ICI image processor 128 to generate measurement data, which may include measurement signals representative of keyhole data measured during the welding process. The intensity or brightness of the measurement signals may be indicative of various features measured by the reflected imaging beam, such as the surface of the workpiece or the bottom of the keyhole. As described in more detail below, the measurement data generated from a short-pulse laser welding process involving pile formation also represents characteristics of the pile 212 formed by the process (i.e., pile measurement data).

[0033] Figures 3A and 3B show images of measurement signals representing keyhole data measured during welding of stainless steel workpiece material. In this example, the system included an IPG YPLN-1-30x240-300-R laser with an IPG mid-power scanner providing the beam for welding and an IPG LDD-700 in-line welding process monitor for acquiring the measurement data. In this example, the laser was used with an average power of 300 W, a repetition rate of 300 kHz, a pulse duration of 120 ns, and a speed of 100 mm / s to weld the stainless steel over a 30 mm length. Figure 3A shows the measured depth over the weld length, and Figure 3B shows a subset of the data within the dashed rectangle in Figure 3A. The strong downward slope in Figure 3B indicates the evolution of the pile.

[0034] Figures 4A and 4B show images of measurement signals representing keyhole data measured during welding of copper workpiece material. In this example, the system included an IPG YPLN-1-30x240-300-R laser with an IPG mid-power scanner to provide the beam for welding and an IPG LDD-700 in-line welding process monitor to acquire the measurement data. In this example, the laser was used to weld copper over a 20 mm length at an average power of 300 W, a repetition rate of 300 kHz, a pulse duration of 120 ns, and a speed of 100 mm / s. Figure 4A shows the depth measured over the weld length, and Figure 4B shows a subset of the data within the dashed rectangle in Figure 4A. The wide vertical region of the measurement signal in Figure 4B indicates that multiple internal reflections frequently occurred within the pile as it formed. As the wide vertical distribution of the measurement signal in Figure 4B shows, an ICI image processor can be used to detect the presence of multiple scattering events and determine the exact depth by finding the brightest peak in the distribution or by fitting / convolving a statistical distribution model to the data distribution.

[0035] The measurement data generated by imaging system 120 and ICI system 120′ (e.g., ICI image processor 128) may be further processed by computerization system 140 to monitor and / or control the short-pulse laser welding process and pile formation. Computerization system 140 may be used to monitor the process by outputting (e.g., displaying) the generated measurement data relative to the laser welding process data or by outputting monitoring information generated by further processing the measurement data. Computerization system 140 may be used to control the process by processing the measurement data and determining control signals to pulsed laser source 110 to control laser parameters, to beam delivery system 130 (e.g., motors that rotate mirrors 132, 134) to control beam delivery, and / or to workpiece stage 150 to control movement of workpiece 102.

[0036] In some embodiments, measurement data (i.e., surface measurement signals) representing the reflection of the imaging beam from the surface of the workpiece can be used to control and / or monitor the process. Figures 3A and 4A show the effects of uncorrected chromatic and / or other optical aberrations that gradually misalign the imaging beam from the center of the keyhole location during a short-pulse laser welding process involving pile formation. This effect is most evident in the surface measurement signal at a depth indicated by 0 μm and gradually becomes stronger as the process moves from the position indicated by approximately 8000 μm in Figure 3A and from the position indicated by approximately 3000 μm in Figure 4A. In some embodiments, these aberrations can be corrected by pre-calibration and subsequent adjustment of the imaging beam's deflection as the process beam moves through the scan field. Such deflection adjustments can be performed by the same deflection element (e.g., a mirror) that aligns the imaging beam. Examples of static and dynamic calibration of coherence imaging measurement systems are described in more detail in the aforementioned U.S. Patent Application Publication No. 2009 / 0129994, which is commonly owned and incorporated herein by reference.

[0037] According to one example of using metrology data to control a process, the alignment of the process beam with the imaging beam may be optimized by minimizing the intensity of a surface measurement signal representative of the reflection of the imaging beam from the surface of the workpiece. According to one example of using metrology data to monitor a process, the presence or intensity of the surface measurement signal may be used to monitor the health and / or focusing conditions of the process by comparing the intensity of the surface measurement signal to a reference level.

[0038] By way of further example, the spot size of the imaging beam may be selected to generate a surface measurement signal sufficient for use as a surface reference and / or to monitor process health / focus conditions as previously described. The spot size of the imaging beam may be expanded to simultaneously or dynamically cover both the top surface and the keyhole bottom, for example, as described in the aforementioned commonly owned U.S. Patent Application Publication No. 2007 / 0129994, which is incorporated herein by reference.

[0039] In other embodiments, measurement data indicative of pile formation within the workpiece (i.e., pile measurement data) may be used to control and / or monitor the process. The pile measurement data may include, for example, a statistical distribution of the number of piles formed, the frequency with which piles are formed, and / or the depth of the piles formed. The computerized system 140 may store the pile measurement data for one or more processes in a storage medium. The computerized system 140 may then compare the stored and aggregated pile measurement data with pile measurement data generated for subsequent processes using statistical comparison techniques known to those skilled in the art.

[0040] According to one example of using pile measurement data to monitor a process, the shape of a histogram plotting the number of piles versus depth from a baseline process may be compared to the target process using the histogram's width, centroid location, and / or area to determine whether the target process conforms to the baseline process. The computerized system 140 can perform this comparison and communicate the results of this comparison to other automated equipment, such as an alert to a user or a signal indicating a conforming or non-conforming weld has been detected. For example, in the electric mobility market, this knowledge may be used to require additional welding or rework on a workpiece to ensure it functions properly during service.

[0041] According to one example of using pile measurement data to control a process, the pile measurement data can be used to provide process depth stabilization. Obtaining pile measurement data can include directly measuring the frequency of pile formation. For example, from the data collected in FIGS. 3A and 4A, the pile formation frequency occurs at a rate different from the pulse repetition rate. In these examples, the observed pile formation frequency was measured to be two to three orders of magnitude longer than the pulse repetition rate produced by the short-pulse laser used in the process. In one embodiment, the electronic system 140 can use the measured pile formation frequency to define an external modulation signal for gating laser emission from the pulsed laser source 110, such as a short-pulse laser. In this embodiment, the gating frequency can be equal to or less than the pile formation frequency, and the formation of individual piles can be intentionally stopped to minimize pile depth variation across the weld. In another embodiment, the electronic system 140 can use the pile measurement data to optimize the laser gating duty cycle and frequency for the purpose of stabilizing the weld pile depth.

[0042] Referring to FIG. 5, an example of a method 500 for performing and monitoring a short-pulse laser welding process is shown and described. In the ICI system 120′ described above, a process beam having a pulse width of less than 1000 ns is generated (step 510) using a pulsed laser source 110, such as the short-pulse laser described above, and an imaging beam is generated (step 512) using, for example, an imaging beam source 122. The process beam may have a high repetition rate in the range of 20 kHz to 1000 kHz. The imaging beam may be coaxially aligned with the process beam or may be offset or angled relative to the process beam.

[0043] The process beam and the imaging beam are directed 514, for example, using the beam delivery system 130 described above, to a weld site on a workpiece including at least a first workpiece material and a second workpiece material, such that a series of pulses of the process beam form a series of pilings extending into the workpiece. In particular, the process beam generates a series of vapor channels extending into the second workpiece material, each vapor channel in the series of vapor channels is subsequently refilled to form a piling within the series of pilings extending into the second workpiece, and the series of pulses of the process beam form each of the vapor channels and each of the pilings. Directing the process beam and the imaging beam may include traveling the process beam and the imaging beam across the workpiece at a speed, for example, in a range of 10 mm / s to 2000 mm / s, such that the process beam forms each of the vapor channels and each of the pilings as the process beam travels.

[0044] Interference output is generated from reflections of the imaging beam from the weld site (step 516), for example, using the interferometer 124 of the ICI system 120' described above. The interferometric output is then detected (step 518), for example, using the detector 126 and ICI image processor 128 of the ICI system 120' described above, to generate measurement data representative of characteristics of the weld site. The measurement data includes at least pile measurement data representative of characteristics of piles within a series of piles extending into the second workpiece material. The pile measurement data may include, for example, a statistical distribution of the number of piles formed, the frequency with which the piles are formed, and the depth of the formed piles.

[0045] The pile measurement data can then be used to monitor and / or control the process (step 520). The process can be monitored using the pile measurement data, for example, by comparing it to pile measurement data previously stored during a reference process, as described above. The process can be controlled using the pile measurement data, for example, by controlling the duty cycle and frequency of the process beam in response to the frequency at which the piles are formed, as described above.

[0046] Thus, systems and methods consistent with embodiments of the present disclosure can be used to monitor and / or control a short-pulse laser welding process involving the formation of a series of heat piles in the welded material. The systems and methods use an imaging system, such as an ICI system, that can take measurements of the inside of the steam channel during the formation of the heat piles to provide pile measurement data that is indicative of the characteristics of the piles. The pile measurement data can be used to monitor and / or control the short-pulse laser welding process.

[0047] While the principles of the present invention have been described herein, it should be understood by those skilled in the art that this description is made by way of example only and is not intended to limit the scope of the invention. In addition to the exemplary embodiments shown and described herein, other embodiments are contemplated within the scope of the present invention. Modifications and substitutions made by those skilled in the art are considered to be within the scope of the present invention, and the present invention is not to be limited except as by the following claims.

Claims

1. 1. A system for performing, monitoring, and / or controlling a short pulse laser welding process, the system comprising: a pulsed laser source configured to generate a process beam for performing the short pulse laser welding process at a weld site to join at least a first workpiece material and a second workpiece material, the pulsed laser source configured to generate the process beam at a pulse width and repetition rate such that the process beam can penetrate at least the first workpiece material and can generate a series of vapor channels extending into the second workpiece material, each vapor channel in the series of vapor channels being subsequently refilled to form a series of pilings extending into the second workpiece material, a series of pulses of the process beam forming each of the vapor channels and each of the pilings, and the pulse width is less than 1000 ns; an imaging system configured to generate an imaging beam that reflects off the weld site, generate an interference output from the reflection of the imaging beam, and detect the interference output to generate measurement data representative of a characteristic of the weld site, the measurement data including at least pile measurement data representative of a characteristic of a pile in the series of piles extending into the second workpiece material; a beam delivery system coupled to the pulsed laser source and the imaging system for directing the process beam and the imaging beam to the weld site on the first workpiece material; an electronic system programmed to receive the measurement data from the imaging system and to monitor and / or control the short pulse laser welding process based on the measurement data; A system comprising:

2. The system of claim 1 , wherein the repetition rate is greater than 10 kHz.

3. The system of claim 1 , wherein the imaging system is an in-line coherent imaging (ICI) system configured to generate the imaging beam coincident with the process beam.

4. The system of claim 1 , wherein the pulsed laser source is a nanosecond infrared fiber laser.

5. 10. The system of claim 1, wherein the beam delivery system is configured to continuously advance the process beam across the first workpiece material at a speed in a range of 10 mm / s to 2000 mm / s.

6. The system of claim 1 , wherein the beam delivery system includes a scanner configured to deflect and scan the process beam and the imaging beam across the first workpiece material.

7. 10. The system of claim 1, wherein the beam delivery system includes a first mirror for deflecting the imaging beam and a second mirror for deflecting the process beam, the second mirror being a dichroic mirror.

8. 10. The system of claim 1, wherein the beam delivery system includes an adjustable deflection element configured to scan both the process beam and the imaging beam together and configured to scan the imaging beam relative to the process beam to adjust alignment of the imaging beam relative to the process beam.

9. The system of claim 1 , wherein the measurement data comprises surface measurement signal data representative of a reflection of the imaging beam from a surface of the first workpiece material.

10. The system of claim 9 , wherein the computerized system is configured to monitor the short pulse laser welding process using the surface measurement signal data.

11. 10. The system of claim 9, wherein the computerized system is configured to control the short-pulse laser welding process by adjusting an alignment of the imaging beam relative to the process beam in response to the surface measurement signal data.

12. 10. The system of claim 9, wherein the computerized system is configured to control the short pulse laser welding process by selecting a spot size of the process beam based on the surface measurement signal data.

13. 2. The system of claim 1, wherein the computerized system is configured to compare the pile measurement data with pre-stored pile measurement data representative of characteristics of piles formed during a reference process, and determine, based on the comparison, whether the short-pulse laser welding process complies with the reference process.

14. 14. The system of claim 13, wherein the pile measurement data represents at least one pile characteristic selected from the group consisting of a statistical distribution of the number of piles formed, the frequency with which piles are formed, and the depth of the piles formed.

15. 2. The system of claim 1, wherein the pile measurement data represents at least one pile characteristic selected from the group consisting of a statistical distribution of the number of piles formed, the frequency with which piles are formed, and the depth of the piles formed.

16. 2. The system of claim 1, wherein the pile measurement data represents at least a frequency at which piles are formed, and the computerized system is configured to control a duty cycle and a frequency of the process beam in response to the frequency at which the piles are formed.

17. 1. A method for performing and monitoring a short pulse laser welding process, the method comprising: generating a process beam having a pulse width of less than 1000 ns; generating an imaging beam; directing the process beam and the imaging beam at a weld site on a workpiece including at least a first workpiece material and a second workpiece material such that the process beam generates a series of vapor channels extending into the second workpiece material, each vapor channel in the series of vapor channels being subsequently refilled to form a series of pegs extending into the second workpiece material, and a series of pulses of the process beam forming each of the vapor channels and each of the pegs; generating an interference output from reflection of the imaging beam from the weld site; detecting the interference output to generate measurement data representative of a characteristic of the weld site, the measurement data including at least pile measurement data representative of a characteristic of the piles in the series of piles extending into the second workpiece material; A method comprising:

18. 20. The method of claim 17, wherein the process beam is generated at a repetition rate greater than 10 kHz.

19. 18. The method of claim 17, wherein the process beam is generated at a repetition rate in the range of 20 kHz to 1000 kHz.

20. 18. The method of claim 17, wherein the process beam is generated with a pulse width in the range of 1 ns to 1000 ns.

21. 18. The method of claim 17, wherein directing the process beam and the imaging beam comprises translating the process beam and the imaging beam across the first workpiece material such that the process beam forms each of the vapor channels and each of the pilings as the process beam travels.

22. 22. The method of claim 21, wherein the process beam travels across the first workpiece material at a speed in the range of 10 mm / s to 2000 mm / s.

23. 22. The method of claim 21 , wherein translating the process beam and the imaging beam across the first workpiece material comprises scanning the process beam and the imaging beam across the first workpiece material.

24. 22. The method of claim 21, wherein translating the process beam and the imaging beam across the first workpiece material comprises moving the workpiece relative to the process beam and the imaging beam.

25. The method of claim 17 , further comprising using the measurement data to monitor the short pulse laser welding process.

26. 26. The method of claim 25, wherein the measurement data includes surface measurement signal data, and the short pulse laser welding process is monitored using the surface measurement signal data.

27. The method of claim 17 further comprising controlling the short pulse laser welding process based on the measurement data.

28. 28. The method of claim 27, wherein the measurement data includes surface measurement signal data, and the short-pulse laser welding process is controlled by adjusting alignment of the imaging beam relative to the process beam using the surface measurement signal data.

29. 28. The method of claim 27, wherein the measurement data includes surface measurement signal data, and the short pulse laser welding process is controlled by selecting a spot size of the process beam based on the surface measurement signal data.

30. 20. The method of claim 17, further comprising: comparing the pile measurement data to pre-stored pile measurement data representative of characteristics of piles formed during a reference process; and determining whether the short pulse laser welding process complies with the reference process based on the comparison.

31. 31. The method of claim 30, wherein the pile measurement data represents at least one pile characteristic selected from the group consisting of a statistical distribution of the number of piles formed, the frequency with which piles are formed, and the depth of the piles formed.

32. 18. The method of claim 17, wherein the pile measurement data represents at least one pile characteristic selected from the group consisting of a statistical distribution of the number of piles formed, the frequency with which piles are formed, and the depth of the piles formed.

33. The method of claim 17 , wherein the pile measurement data represents at least a frequency at which piles are formed, and further comprising controlling a duty cycle and a frequency of the process beam in response to the frequency at which the piles are formed.

34. 20. The method of claim 17, wherein at least the first workpiece material is a reflective metal and has a thickness of less than 0.5 mm.

35. The method of claim 17 , wherein the first workpiece material and the second workpiece material are dissimilar metals.

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