SPIRAL LASER WELDING METHODS FOR JOINING METAL.

Abstract

Laser welding methods include focusing laser radiation (120) onto a first sheet of metal (112) disposed on a metal part (114), optionally with one or more intermediate sheets of metal between them; the laser radiation (120) is directed to trace at least one spiral path for spot welding the metal parts (114); the laser radiation (120) includes a central beam (122C) and an annular beam (122A) to maintain a stable keyhole; one method is designed for welding aluminum parts, for example, with high gas content and / or different compositions, and the laser radiation (120) first traces an outward spiral path (810) and then an inward spiral path (830); the central beam (122C) is pulsed during a segment of the inward spiral path (830);Another method is designed for welding steel or copper parts that have a coating at an interface between them, and the laser radiation (120) traces an inward spiral path (830); the interface (414F) may be a zero-space interface, or there may be a non-zero space.

Description

Spiral laser welding methods for joining metal PRIORITY This application claims priority for United States application serial no. 17 / 338,109, filed on June 3, 2021, the descriptions of which are incorporated herein in full by reference. TECHNICAL FIELD OF THE INVENTION The present invention relates in general to the laser welding of metal sheets, in particular metal sheets with demanding material compositions. BACKGROUND OF THE INVENTION Laser welding uses a laser beam as a concentrated heat source to locally melt and join two parts, typically made of metal. The laser beam can be focused to a relatively small spot, resulting in high power density and a small heat-affected zone. Therefore, laser welding is an attractive technique when precision and a high degree of control are required. Furthermore, laser welding is well-suited to automation. In laser welding, the focused laser beam precisely locates each weld point or line, minimizing collateral heating. It is useful to distinguish between two main laser welding regimes. Conduction welding occurs with lower laser powers and power densities. The absorbed laser power heats the irradiated material, melting the material of each part to be joined, which then flows, mixes, and solidifies. Keyhole welding occurs with higher laser powers and power densities, sufficient to vaporize some of the irradiated material. The pressure of the vaporized material on the surrounding molten material creates a channel through the molten material.This channel, known technically as a keyhole, has a characteristic narrow and deep profile, allowing for deep penetration of the laser beam. Finished keyhole welds are generally narrower, deeper, and stronger than conduction welds. Laser welding has been successfully applied to a wide range of welding problems involving a variety of materials arranged in a variety of configurations. OCbt? I n / O7n7 / R / VIAI In some cases, laser welding replaces other welding techniques. In other cases, laser welding allows for the welding of structures that are unsuitable for welding using conventional, non-laser welding techniques. The automotive industry is one of several manufacturing industries increasingly adopting laser welding. In the automotive industry, laser welding is currently used to weld many different vehicle parts, such as chassis, body structures, doors, engine components, and batteries (for electric and hybrid vehicles). With the benefit of a small heat-affected zone and a generally well-controlled and finely tunable process, laser welding can be used to automatically and reliably weld thinner and smaller parts than with conventional welding techniques. In this way, laser welding has helped advance automotive manufacturing technology to meet the demand for lighter and more efficient vehicles.For example, laser welding facilitates the precise welding of lightweight body parts, as well as connections within and to electrochemical batteries (e.g., connections between thin metal sheet stacks and battery lugs, and connections between battery lugs and bus bars). For body parts, the materials welded are typically steel, aluminum, and / or aluminum alloys. For batteries, the materials welded typically include copper, but may also include aluminum or aluminum alloys. Spot welding is one of several welding methods that can be used to laser weld overlapping parts. When laser spot welding two overlapping parts, the laser beam strikes one of the parts and melts locally through it to the interface with the second part, and at least some distance into the second part. Laser spot welding can be applied to stacks of two, three, or more metal parts. Keyhole welding has proven to form strong spot welds in many applications. To create a larger weld spot than can be achieved with a stationary laser beam, the laser beam can be directed to trace a spiral pattern. BRIEF DESCRIPTION OF THE INVENTION This document describes spiral laser welding methods configured to spot weld a stack of metal parts by tracing spiral patterns with dual-beam laser radiation. The dual-beam laser radiation strikes the metal stack from one side and melts through the stack to the outermost metal part. The metal parts can be a stack of two or more metal sheets. Alternatively, the outermost metal part... OCbt? I n / C7n7 / R / VIAI distant from the stack, further away from the side of the stack that receives the laser radiation, may be a thicker metal structure, which is not sheet-shaped. The methods currently described use keyhole welding and are specifically designed to achieve a strong welded joint in certain particularly challenging scenarios. Although keyhole welding is effective for melting and mixing material, the quality of the resulting welded joint can be compromised by problems such as trapped gas, discrepancies in cooling rate, and material loss due to spatter. Spatter is an undesirable effect in keyhole welding, where the convection within the keyhole is violent enough to expel droplets of metal during the welding process. This expulsion of droplets reduces the volume of the weld nugget in an uncontrolled manner. Gas in the weld pool can potentially pose several problems. The presence of gas bubbles in the weld pool can cause spattering. When gas becomes trapped during the final cooling process, the residual stresses caused by the trapped gas can lead to cracking in the weld joint. If not released before or during cooling, the trapped gas forms substantial voids and / or smaller pores in the resulting weld nugget. Discrepancies in cooling rate are particularly likely to occur when dissimilar materials are welded together. When laser-welding parts of identical or similar materials, a true metallurgical bond can form between the parts at the weld joint, and the composition of the weld nugget material is relatively uniform. When laser-welding dissimilar materials, it may not be possible to form metallurgical bonds between the different materials. Instead, the weld may contain a mixture of the two materials. When the mixture is not uniform, any substantial discrepancy in cooling rate between the two materials can lead to stress-induced cracking after the weld pool has cooled. Cracking, voids, porosity, and material loss can compromise the strength of a welded joint. Furthermore, in scenarios where the welded structure is intended to carry electrical current, such as in battery applications, the conductivity of the welded joint can be negatively affected by these factors. In the present methods, laser welding is performed using dual-beam laser radiation consisting of two beams: a central beam and an annular beam surrounding the central beam. The respective powers of the central and annular beams are controlled independently to achieve the desired result. This dual-beam laser radiation can hold a keyhole. The OCbt? I n / C7n7 / R / VIAI method is more stable and well-controlled than a single laser beam. The greater stability of the present keyhole simultaneously (a) minimizes spatter, and (b) maximizes the keyhole opening duration, thus facilitating the release of trapped gas. The present methods trace the dual-beam laser radiation along spiral patterns to reliably create a brazed joint with minimal trapped gas and minimal (or no) cracking, even in scenarios involving materials that would otherwise be prone to gas trapping and cracking. Furthermore, to avoid or at least minimize cracking, the methods conclude with a controlled decrease in laser power as the laser progresses toward the center of the spiral pattern. Benefiting from the procedural characteristics mentioned above, these methods are capable of welding dissimilar metals, metals with trapped gas, and metals with coatings that evaporate during the welding process. One method is designed for welding aluminum parts with trapped gas and traces the same region with an outward spiral and an inward spiral, with pulses of the central beam along a portion of the inward spiral to properly release the trapped gas. The aluminum parts can have different compositions. Another method traces an inward spiral and is designed for welding coated metal parts, such as zinc-coated steel or nickel-coated copper. Conventional laser welding methods have difficulty reliably producing a good weld joint in the presence of such coatings, especially when there are no gaps between the parts.This method reliably achieves a high-quality welded joint even in gapless configurations. In fact, the weld quality of this method is essentially insensitive to gap size, ranging from zero gap to gaps of, for example, approximately 0.5 millimeters or possibly more, depending on the thickness of the parts. In one aspect, a laser welding method for joining aluminum includes steps of focusing laser radiation onto a first aluminum sheet positioned on an aluminum part, and controlling the focused laser radiation to trace a plurality of paths on the first aluminum sheet to weld the first aluminum sheet to the aluminum part. The laser radiation includes a central beam and an annular beam surrounding the central beam. The control step involves tracing an outward spiral path while maintaining the respective first powers of the central and annular beams. The outward spiral path begins at a central location and spirals around and away from the central location.The control step also includes, after tracing the outward spiral path, tracing an outer path while reducing the powers of the central and annular beams from their respective first powers to their respective second powers. The outer path is peripheral to the spiral path as viewed from the OCbt? I n / O7n7 / R / VIAI central location. In addition, the control step includes, after tracing the outer path, tracing an inward spiral path toward the central location, while first (a) increasing the powers of the central and ring beams from their respective second powers to their respective third powers, subsequently (b) maintaining the third power of the ring beam and repeatedly pulsing the central beam between its third power and a lower fourth power, and finally (c) turning off the central beam and reducing the power of the ring beam to zero. In another aspect, a laser welding method for joining a stack of metal parts having a coating at an interface includes focusing laser radiation onto the stack of metal parts and controlling the focused laser radiation to trace at least one path on a first metal sheet of the stack of metal parts to weld the stack of metal parts together, also at least partially evaporating the coating at the interface. The metal parts include (i) the first metal sheet, (ii) a more distant metal part, and (iii) zero, one, or several intermediate metal sheets between the first metal sheet and the metal part. At least one of the metal parts has a coating at an interface with a neighboring metal part. The interface is configured with direct contact between the two neighboring metal parts or with a gap between them.The laser radiation strikes the first metal sheet and includes a central beam and an annular beam surrounding the central beam. The control step involves tracing an inward spiral path and, while tracing the inward spiral path, first (a) maintaining the respective first powers of the central and annular beams, then (b) simultaneously reducing the power of the central beam from its first power to zero watts, and reducing the power of the annular beam from its first power to a non-zero second power, and finally (c) turning off the annular beam. BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings, which are incorporated in and form part of the specification, schematically illustrate the preferred embodiments of the present invention, and together with the general description provided above and the detailed description of the preferred embodiments provided below, serve to explain the principles of the present invention. Figure 1 illustrates a laser welding apparatus for welding metal parts with double-beam laser radiation, in accordance with one modality. Figure 2 shows the cross-sectional laser profile of the laser radiation generated by the apparatus in Figure 1 focused on a target, in accordance with a modality. OCbt? I n / C7n7 / R / VIAI Figure 3 is a flow diagram for a method for joining aluminum using spiral laser welding with the dual-beam laser radiation of Figure 2, in accordance with one modality. Figure 4 shows an example of a configuration of metal parts laser-welded using the method of Figure 3. In this configuration, a sheet of metal is arranged over a metal part with no other sheets of metal in between, and the metal melting caused by laser welding extends only partially into the metal part. Figure 5 shows another example of a configuration of metal parts laser-welded using the method in Figure 3. This configuration is similar to the configuration in Figure 4, except that the metal melting extends throughout the entire metal part. Figure 6 shows yet another example of a configuration of laser-welded metal parts using the method in Figure 3. This configuration is similar to the configuration in Figure 4, except that it includes an intermediate metal sheet. Figure 7 shows a schematic of the laser power used in the method of Figure 3, in accordance with one modality. Figures 8A, 8B, and 8C show three paths traced by the focused double-beam laser radiation in the method of Figure 3, in accordance with one modality. Figure 9 is a flowchart for a method for joining metal parts with a coating at an interface between them, according to a specific modality. This method uses spiral laser welding with the dual-beam laser radiation shown in Figure 2. Figure 10 illustrates an example of a two-layer stack of metal parts that can be welded together using the method in Figure 9. Figure 11 illustrates an example of a stack of metal parts, with more than two layers, that can be welded using the method in Figure 9. Figure 12 shows a schematic of the laser power used in the method of Figure 9, in accordance with one modality. Figure 13 shows the paths traced by the focused double-beam laser radiation in the method of Figure 9, in accordance with one modality. DETAILED DESCRIPTION OF THE INVENTION OCbt? I n / C7n7 / R / VIAI With regard to the drawings, in which similar components are designated by identical numbers, Figure 1 schematically illustrates a laser welding apparatus 100 for welding metal parts. The apparatus 100 is configured to focus the double-beam laser radiation. 120 about a target and trace one or more paths, such as a spiral path 130, about the target. In the scenario depicted in Figure 1, apparatus 100 welds together two overlapping metal parts: a sheet of metal 112 and a metal part 114. Herein, the term "sheet of metal" refers to a metal part with a thickness no greater than 10 millimeters, such that focused laser radiation with an average power in the kilowatt range can melt through its thickness. The term "sheet of metal," as used herein, includes thin sheets of metal with thicknesses less than 100 micrometers, as well as non-flat metal parts. The term "sheet of metal," as used herein, also refers to localized, sheet-like portions of metal parts having one or more portions with a thickness greater than 10 millimeters. Thus, the thickness 112T of the sheet of metal 112 is 10 millimeters or less.While joining metal parts 112 and 114 by welding requires that the laser radiation 120 melt through the thickness 112T of the metal sheet 112, it does not need to melt through the thickness 114T of metal part 114. Therefore, metal part 114 may or may not be a metal sheet, and its thickness 114T may or may not exceed 10 millimeters. Figure 2 shows the cross-section 200 of laser radiation 120 focused on a target, for example, focused on a metal sheet 112 as depicted in Figure 1. The laser radiation 120 includes a central beam 122C and an annular beam 122A surrounding the central beam 122C. At least most of the power of the annular beam 122A is outside the diameter of the central beam 122C. In the configuration depicted in Figure 2, the central beam 122C and the annular beam 122A are circular. The following discussion assumes circular beams but is easily extended to elliptical beams. The central beam 122C has a diameter 210C of 1 / e². The annular beam 122A has an outer diameter 212A of 1 / e² and an inner diameter 214A of 1 / e². The inner diameter 214A of the ring beam 122A exceeds the diameter 210C of the center beam 122C.The combined power of the central beam 122C and the ring beam 122A reaches a minimum along a circle 220 that is the outer diameter 210C of the central beam 122C and the inner diameter 214A of the ring beam 122A. In one example, the diameter 210C is in the range of 50 to 500 micrometers, and the outer diameter 212A is in the range of two to three times the diameter 210C. In another example, the diameter 210C is in the range of 15 to 50 micrometers, and the outer diameter 212A is in the range of four to ten times the diameter 210C. The laser radiation 120 can be near-infrared, for example, with a wavelength in the range of 900 to 1200 nanometers. Referring again to Figure 1, the apparatus 100 includes a laser source 170, a central power controller 172, an annular power controller 174, an optical fiber 178, and a beam delivery module 180. The laser source 170 generates laser radiation. The laser source 170 couples a portion of the generated laser radiation into a central optical fiber core 178. OCbt? I n / C7n7 / R / VIAI to form a central beam 122C, and another portion of the laser radiation generated in an annular core of optical fiber 178 to form an annular beam 122A. In order to couple the laser radiation from the laser source 170 to the optical fiber 178, the apparatus 100 may implement fiber coupling techniques similar to those discussed in United States Patent No. US 10,807,190 and the publication of United States Patent Application No. US 2019 / 0118299, which are incorporated herein in their entirety by reference. The central power controller 172 adjusts the power of the central beam 122C as required. The ring power controller 174 adjusts the power of the ring beam 122A as required. In one implementation, the laser source 170 includes at least one laser that is controlled by the central power controller 172 and dedicated to generating the central beam 122C, and at least one other laser that is controlled by the ring power controller 174 and dedicated to generating the ring beam 122A. The beam delivery module 180 receives laser radiation 120 from the optical fiber 178. The beam delivery module 180 focuses the laser radiation 120 onto the target and directs the laser radiation 120 as required, for example, to trace the spiral path 130. The beam delivery module 180 directs the central beam 122C and the ring beam 122A together as a whole and does not need to be able to spatially manipulate the central beam 122C and the ring beam 122A independently of each other. The beam delivery module 180 may include a focusing lens or objective and beam-directing optics, as is known in the art. The apparatus 100 may further include a master controller 190 that manages the operation of the central power controller 172, the ring power controller 174, and the beam supply module 180. The master controller is, for example, a computer containing machine-readable instructions that specify the operations to be performed by the central power controller 172, the ring power controller 174, and the beam supply module 180. In the scenario depicted in Figure 1, the beam supply module 180 focuses laser radiation 120 onto a surface 112S of the metal sheet 112. Surface 112S is on the opposite side of the metal sheet 112 from the metal part 114. The beam supply module 180 traces one or more paths, including the spiral path 130, with laser radiation 120. While the beam supply module 180 focuses and directs the central beam 122C and the ring beam 122A together, the central power controller 172 and the ring power controller 174 adjust the respective powers of the central beam 122C and the ring beam 122A independently of each other as necessary to weld together the metal sheet 112 and the metal part 114. For example, the power of the central beam 122C can be increased or decreased at a different rate than OCbt? I n / C7n7 / R / VIAI The power of the ring beam 122A, or the center beam 122C, can be pulsed or deactivated while the ring beam 122A is continuously activated. Figure 3 is a flowchart for Method 300 for joining aluminum using spiral laser welding with dual-beam laser radiation 120. Method 300 can be performed using Apparatus 100 and can be used to weld an aluminum sheet to one or more other aluminum sheets and / or another aluminum part. Each aluminum sheet / part is substantially made of aluminum or an aluminum alloy. Without departing from the scope of this document, the surfaces may exhibit some degree of oxidation and / or contamination prior to welding. Aluminum is relatively viscous when molten. Because aluminum typically contains some degree of trapped gas, and the high viscosity hinders the release of this gas, conventional laser welding of aluminum is particularly prone to producing weld nuggets with significant porosity and substantial gaps, and is also prone to spatter. Certain forms of aluminum, such as cast aluminum, tend to contain a relatively large amount of gas. Method 300 is designed to optimize the release of trapped gas. For this reason, at least, Method 300 is capable of forming high-quality, low-porosity welded joints between aluminum parts, even when one or more of the parts has a high gas content, such as when one or more of the parts is cast aluminum. For example, Method 300 can be used to weld one or more sheets of extruded aluminum to a cast aluminum part.In general, the 300 method facilitates the controlled release of trapped gas, and minimizes stress, thereby minimizing spatter and porosity, as well as the risk of cracking during cooling. Method 300 includes steps 310 and 320. Step 310 focuses laser radiation 120 onto a first aluminum sheet that is positioned on another aluminum part, optionally with one or more other intermediate aluminum sheets positioned between them. Step 320 controls the laser radiation 120, as focused, to trace a plurality of paths on the first aluminum sheet, thereby welding the first aluminum sheet to the aluminum part (and the intermediate aluminum sheets, if present). Before proceeding to discuss the details of steps 310 and 320, the present inventors address different configurations of aluminum parts that can be welded using method 300. Figures 4, 5, and 6 illustrate examples of metal part configurations to which method 300 can be applied, as well as examples of weld nuggets formed using method 300. Within the context of method 300, each of the metal parts shown in figures 4, 5, and 6 is an aluminum part. Figure 4 shows a configuration 400, in which the metal sheet 112 is arranged on the metal part 114 without any other metal sheets in between, and in the OCbt? I n / C7n7 / R / VIAI that the metal melting caused by the operation of method 300 extends only partially into the metal part 114. The metal sheet 112 may be in direct contact with the metal part 114 along an interface 414F between them. Without departing from the scope of this document, small gaps may exist in places along the interface 414F, for example, due to positioning tolerances or non-planar surfaces. The metal part 114 has a surface 414S on the side of the metal part 114 that is opposite the interface 414F. The metal sheet 112 and the metal part 114, arranged in configuration 400, have a combined thickness 410T between the surfaces 112S and 414S. In scenarios where the metal sheet 112 and / or the metal part 114 are not flat, the thickness 410T is a local thickness measured in the region where the laser radiation 120 impinges on the metal sheet 112 during welding. When method 300 is applied to configuration 400, method 300 directs laser radiation 120 onto the surface 112S of the metal sheet 112 to melt through the metal sheet 112, through the interface 414F, and into the metal part 114, but not completely through the metal part 114 to the surface 414S. In this way, method 300 forms a weld nugget 450 that starts at the surface 112S and ends within an inner portion of the metal part 114, such that the depth 450D of the weld nugget 450 is less than the thickness 410T. The weld nugget 450 has a width 450W that can exceed the depth 450D. For example, the width 450W can be in the range of one to five times the depth 450D.Since method 300, in configuration 400, does not seek to melt completely through the metal part 114 to the surface 414S, the thickness 414T of the metal part 114 can substantially exceed the thickness 112T of the metal sheet 112, provided that the corresponding heat sink provided by the metal part 114 does not prevent the laser radiation 120 from melting through the interface 414F. In an example of configuration 400, the thickness 112T is in the range of one to five millimeters, the thickness 114T is in the range of two to thirty millimeters, the width 450W is between three and fifteen millimeters, and the depth 450D extends into the metal part 114 by at least one millimeter. Alternatively, the depth 450D can extend into the metal part 114 by less than one millimeter, for example, when the metal part 114 is relatively thin, and it is preferred that the 414S surface not show any signs of the welding procedure.The 450 solder nugget will typically be wider at or near the 112S surface, so that a width of 450W is reached around there, and the width of the 450 solder nugget at the 414F interface is somewhat less. Figure 5 shows a 500 configuration that is similar to the 400 configuration, except that the metal melt and associated weld nugget 550, generated by the operation of method 300, extend along the metal part 114 to the surface 414S. In the 500 configuration, the metal part 114 can be a sheet of metal with a thickness 114T that is OCbt? I n / C7n7 / R / VIAI similar to thickness 112T. The width 550W of the weld nugget 550 can exceed the thickness 410T. However, since only the perimeter of the weld nugget 550 is supported by the solid portions of the sheet metal 112 and the metal part 114 during laser welding, it may be preferable to limit the width 550W to less than approximately three times the thickness 410T. If the width 550W is allowed to exceed this limit, the laser radiation 120 may eject substantial amounts of molten metal. Such ejection can compromise the size and strength of the 550 weld nugget and, in the worst case, even form an opening that extends through the 112 metal sheet and the 114 metal part. In an example of a 500 configuration, each of the 112T and 114T thicknesses is in the range of one to three millimeters, and the 550W width is less than three times the resulting value of the 410T thickness. Figure 6 shows a 600 configuration that is similar to the 400 configuration, except for the inclusion of an intermediate metal sheet 616 between the metal sheet 112 and the metal part 114. The metal sheet 616 has a thickness 616T, which is similar to the thickness 112T. The metal sheets 112 and 616 meet at an interface 616F, and the metal sheet 616 and the metal part 114 meet at an interface 614F. Each of the 616F and 614F interfaces has properties similar to the 414F interface. When method 300 is applied to configuration 600, laser radiation 120 melts through metal sheet 112, through interface 616F, through metal sheet 616, through interface 614F and into metal part 114, but not up to surface 414S.The resulting weld nugget 650 terminates within an inner portion of the metal part 114 and has a depth 650D that is less than the combined thickness 610T of the metal sheets 112 and 616 and the metal part 114 between surfaces 112S and 414S. In an example of configuration 600, each of the thicknesses 112T and 616T is in the range of one to three millimeters, the thickness 414T is in the range of two to thirty millimeters, and the width 650W is between three and fifteen millimeters. The extent of the depth 650D in the metal part 114 can be similar to the extent of the weld nugget 450 in the metal part 114 in configuration 400. Configuration 600 can be modified to melt metal, using method 300, which extends through metal part 114 to surface 414S, in a manner similar to modifying configuration 400 to arrive at configuration 500. In addition, configuration 600 can be extended to include more than one intermediate metal sheet 616 between metal sheet 112 and metal part 114. Referring again to Figure 3, in an example from step 310, the laser source 170 generates laser radiation 120, and the beam delivery module 180 focuses the laser radiation 120, including the center beam 122C and the ring beam 122A, onto the surface 112S of the metal sheet 112 arranged in any of the configurations discussed above with reference to Figures 4, 5, and 6. In a related example from step 320, the beam delivery module 180 directs OCbt? I n / C7n7 / R / VIAI laser radiation 120 to trace a plurality of paths on the surface 112S with the central beam 122C and the ring beam 122A. Step 320 includes steps 324, 326, and 328, performed in the order shown. Operation of step 320 involves manipulating the powers of the center beam 122C and the ring beam 122A while tracing the plurality of paths. Figure 7 shows the laser power scheme used in step 320, and Figures 8A, 8B, and 8C show three paths traced by the focused laser radiation 120 in steps 324, 326, and 328, respectively. The center beam 122C and the ring beam 122A can be continuous-wave beams. The powers of the center beam 122C and the ring beam 122A can be adjusted, as needed, by the center power controller 172 and the ring power controller 174, respectively. Step 324 traces an outward spiral path 810 shown in Figure 8A. The outward spiral path 810 starts at a central location Le at time t1, and spirals around and away from the central location Le to reach an outer location Lo at time t2. In the example depicted in Figure 8A, the spiral path 810 resembles an Archimedean spiral, such that successive revolutions around the central location Le are approximately equidistant and are characterized by a separation distance 812. The separation distance 812 can be determined by the diameter 212A of the ring bundle 122A and, for example, be at least as large as the diameter 212A, but no greater than twice the diameter 212A. Without departing from the scope of the present, the spiral path 810 may take a shape different from that of an Archimedean spiral. While tracing the outward spiral path 810, step 324 maintains a central beam power Pei 122C and an annular beam power Pai 122A (see Figure 7). Pai may be greater than Pei. These central beam powers 122C and annular beam powers 122A are configured to be sufficient to maintain a local melt pool with a keyhole extending from the surface of the aluminum sheet on which the laser radiation 120 is incident (e.g., the surface 112S of the metal sheet 112), through any intervening aluminum sheets, if present (e.g., the metal sheet 616), and into or through the farthest portion of the aluminum sheet on which the laser radiation 120 is incident (e.g., the metal portion 114). The keyhole and surrounding fusion bath move with the laser radiation 120 along the outward spiral path 810 during step 324.The keyhole is located towards the region where the 120 laser radiation impinges. The fusion bath will generally have a tail behind the 120 laser radiation. Conventionally, keyhole welding is performed by a single laser beam, typically with an approximately Gaussian or flat-surface transverse intensity distribution. The power density of this single laser beam is set OCbt? I n / C7n7 / R / VIAI high enough to form a keyhole. However, the keyhole convection mechanics are frequently so violent that significant spattering is inevitable, and the keyhole opens and closes unpredictably. Method 300, on the other hand, benefits from the presence of an annular beam to (a) control the temperature gradients imposed on the metal, and (b) reduce the power density requirements for the central beam. By tracing the outward spiral path 810 with laser radiation 120, a portion 824L of the annular beam 122A leads to the central beam 122C, and another portion 824T of the annular beam 122A follows the central beam 122C. The leading portion 824L preheats the material, so that the keyhole is established and maintained with relative ease.The heating provided by the 824T back portion serves to more gently lower the temperature of the material behind the keyhole, reducing temperature gradients and minimizing stress in the cooled material. In this way, Method 300 achieves a stable keyhole with very little or no spatter loss and minimal stress. The improved keyhole stability achieved by Method 300 helps release trapped gas from the aluminum, as the trapped gas can only escape through the keyhole when it is opened. The present inventors have found that step 324 alone is insufficient to achieve a satisfactory welded joint between aluminum parts when one or more of the parts contain substantial amounts of trapped gas. If step 324 is not accompanied by additional welding, the gas remains trapped in the molten material, typically resulting in a final weld nugget with substantial voids and stress-induced cracks. Similarly, they have found that step 324 alone is insufficient for welding aluminum parts of different compositions. In the present case, the single step along the outward spiral path 810 does not provide sufficient mixing, and therefore the weld nugget tends to crack upon cooling. Therefore, method 300 further includes step 328.Step 328 traces an inward spiral path 830, shown in Figure 8C, back toward the central location Le. However, since tracing the outward spiral path 810 in step 324 leaves the material at an elevated temperature, the material would overheat if step 328 were started immediately after step 324 and at the same power levels. Spattering would likely be unavoidable. To avoid such overheating, step 328 (a) starts at reduced power levels, and (b) is separated from step 324 by step 326, which irradiates an area peripheral to the spiral paths of steps 324 and 328. Step 326 traces an external path 820, shown in Figure 8B, between times tz and t3. The external path 820 begins at the external location Lo at time tz. The outer path 820 is peripheral to the outward spiral path 810, viewed radially from the central location Le (Figure 8B shows an example of a radial direction 890). While plotting the outer path 820, step 326 reduces the central beam powers 122C and the ring beam powers 122A from the powers Pei and Pai, respectively, to the powers Fish and Peace, respectively (see Figure 7). Fish and Peace are less than Pei. Fish and Peace can be zero. In one implementation, the outer path 820 includes a closed path, for example, a circle as shown in Figure 8B. In this implementation, step 326 can further ensure that the solder nugget formed by method 300 has a well-defined perimeter of a desired shape. Step 326 can trace this closed loop once, in which case the outer path 820 begins and ends at the outer location Lo. Alternatively, step 326 performs more than one circuit around the central location Le along the closed loop, in which case the outer path 820 terminates at the outer location Lo or another termination point Lt on the closed loop. When sufficient, a single circuit along the closed loop minimizes the total processing time in this implementation. In another implementation, the outer path 820 is an open path that terminates at the termination point Lt before completing a full circuit around the center location Le. In this implementation, the location of the termination point Lt can be defined by the time it takes to increase the power of the center beam 122C and the ring beam 122A to Fish and Peace, respectively. Terminating the outer path 820 before completing a full circuit results in a smaller solder nugget, which may be preferable in some scenarios. Without departing from the scope of the present, the outer path 820 may be a continuation of the outward spiral path 810, corresponding to step 326 which reduces the laser powers at the outer end of an extended version of the outward spiral path 810. Step 328 starts at time t3, and traces the inward spiral path 830 back to the central location Le, as shown in Figure 8C. The geometric properties of the inward spiral path 830 can be similar to those of the outward spiral path 810. The inward spiral path 830 begins where the outward path 820 ends. Thus, the inward spiral path 830 begins at the outward location Lo (as depicted in Figure 8C) or at the termination point Lt. The procedure for plotting the inward spiral path 830 takes place in three segments: (1) a first segment from the outward location Lo (or termination point Lt) at time t3 to a location Lp at time t4, (2) a second segment from location Lp at time t4 to a location Lr at time ts, and (3) a third segment from location Lr at time ts to the OCbt? I n / C7n7 / R / VIAI central location Le at time te. While plotting the first segment, step 328 increases the power of the Fish and Peace center beams 122C and 122A ring beams, respectively, to powers Pc3 and Pa3, respectively (see Figure 7). In the example depicted in Figure 7, Pc3 exceeds Pc2, and Pa3 exceeds both Pa2 and Pc3. However, other ratios may be advantageous in some scenarios. Then, while plotting the second segment, step 328 (a) maintains the power Pa3 of the 122A ring beam, and (b) repeatedly pulses the center beam 122C between Pc3 and a lower-power Pc4 (see Figure 7). Pc4 may be zero power, i.e., zero watts. The pulse rate of the center beam 122C may be in the range of 0.5 to 5 kilohertz. Finally, while tracing the third segment, step 328 reduces the powers of the center beam 122C and the ring beam 122A.In one form of this power reduction, step 328 deactivates the center beam 122C and reduces the ring beam 122A to zero watts (see Figure 7). When Pc4 is non-zero, step 328 can deactivate the center beam 122C by (a) changing its power to zero watts, for example, at the start of the third segment, or (b) reducing its power to zero watts during the tracing of the third segment. Without departing from the scope of the present, step 328 can conclude with a non-zero power in the ring beam 122A, for example, 20% or less of Pa3. The pulse of the central beam 122C during the second segment of the inward spiral path 830 has proven effective in releasing trapped gas from the molten material after traversing the outward spiral path 810 in step 324. The reduction of the annular beam 122A, as opposed to an abrupt shutdown, serves to slow the cooling of the material, thereby relieving stresses and preventing cracking of the weld nugget. The present inventors have found that reducing the power while the laser radiation 120 is stationary tends to produce a hole or indentation in the weld nugget. Therefore, step 328 performs the final reduction of the laser radiation 120 as it moves along the inward spiral path 830. Step 320 may further include a step 322 that precedes step 324. At time to, step 322 activates laser radiation 120 at the central location Le at the initial powers Peo and Pao of the central beam 122C and the ring beam 122A, respectively. Pao may exceed Peo. From time to to time ti, while maintaining laser radiation 120 directed at the central location Le, step 322 reduces the powers of the central beam 122C and the ring beam 122A to Pei and Pai, respectively. The energy deposited by laser radiation 120 in step 322 helps to form the fusion pool and establish the keyhole. Modalities that omit step 322 initially activate laser radiation 120 in step 324 with powers Pei and Pai of the central beam 122C and the ring beam 122A, respectively. In one mode, the duration of step 324 is between 150 and 300 milliseconds, the duration of step 326 is between 25 and 100 milliseconds, and the duration of step 328 is between 150 and OCbt? I n / O7n7 / R / VIAI 300 milliseconds, and the duration of step 322 (if included) is between 25 and 100 milliseconds. Method 300 can be completed in less than one second. In certain implementations, each of Peo, Pao, Pei, Pai, Pc3, and Pa3 exceeds one kilowatt of average power. For example, Peo, Pao, Pei, Pai, and Pa3 may be in the range of 2 to 4 kilowatts, Pc3 may be in the range of 0.5 to 2.5 kilowatts, and Pez, Pc4, and Paz may be in the range of zero to 0.2 kilowatts. The power levels can be adjusted according to the thicknesses of the aluminum parts involved and depending on whether the resulting weld nugget needs to penetrate the farthest part of the aluminum or terminate at an interior location. Method 300 may include providing a shielding gas to the weld zone to further help prevent porosity in the top layer of the weld nugget (e.g., the nearest surface 112S), to prevent plasma formation, and to minimize exposure to ambient oxygen. The shielding gas is, for example, argon or nitrogen. Paths 810, 820, and 830 connect to form a single continuous path. Pitch 320 can plot each of paths 810, 820, and 830 clockwise or counterclockwise. The direction need not be the same for each path. For example, the inward spiral path 830 can be identical to the outward spiral path 810, but plotted in reverse and inward instead of outward. The area plotted by the combination of paths 810, 820, and 830 can have an overall extent, for example, a diameter 870D as shown in Figure 8C, in the range of 3 to 15 millimeters. Figure 9 is a flowchart for a method 900 for joining metal parts that include a coating at an interface between them. Method 900 can be performed using apparatus 100. Method 900 can be used to weld zinc-coated steel or nickel-coated copper. The presence of a coating at the interface between the metal parts to be welded presents a challenge when the coating evaporates at temperatures lower than those required to melt the metal parts themselves. For example, the melting point of steel is typically around 1370 degrees Celsius, while the evaporation point of zinc is only 907 degrees Celsius. In the absence of an efficient gas escape path, the gas produced by the evaporating coating causes significant spatter during keyhole welding. In conventional keyhole laser welding of such coated metal parts, the metal parts are separated by a gap large enough to provide an alternative gas escape path. The 900 method does not require such a gap.In contrast, the 900 method is designed to allow the gas, produced by the evaporation of the lining, to escape efficiently through the keyhole. OCbt? I n / C7n7 / R / VIAI with minimal (or no) spatter. Therefore, Method 900 is able to minimize spatter and thus achieve a high-quality welded joint when the metal parts are in direct contact with each other. The present inventors have discovered that Method 900 also minimizes spatter and achieves a high-quality welded joint when the parts are separated by a certain amount of space. In the case of zinc-coated steel, they have found that the quality of the welded joint is insensitive to the existence of a gap, provided that the gap is relatively small. Even without modifying any parameters of the procedure according to the size of the gap, the same quality of welded joint is achieved with small gaps as with no gaps.Since the keyhole tends to force molten metal from the upper sheet (closer to the incoming laser radiation) into the lower sheet, the thickness of the upper sheet is the primary factor defining the maximum gap size to which the weld quality is unaffected. In certain scenarios, the weld quality is unaffected by the gap size, provided the gap is between zero (no gap) and approximately 60% of the upper sheet thickness. Method 900 includes steps 910 and 920. Step 910 focuses laser radiation 120 onto a stack of metal parts. The stack of metal parts consists of a first metal sheet positioned on a metal part, optionally with one or more intermediate metal sheets positioned between them. The metal part may or may not be a metal sheet. Step 920 controls the laser radiation 120, as focused, to trace at least one path on the first metal sheet to weld the first metal sheet to the metal part (and the intermediate metal sheet(s), if present). This welding causes the evaporation of coatings positioned in the path of the laser radiation 120, including coatings at the interfaces between the metal parts. Figures 10 and 11 illustrate examples of metal sheet / part configurations that can be welded using Method 900. Within the context of Method 900, each metal sheet / part is made of steel optionally coated with zinc or a zinc alloy, or each metal sheet / part is made of copper or a copper alloy, optionally coated with nickel or a nickel alloy. Without departing from the scope of this document, the surfaces may exhibit some degree of oxidation and / or contamination prior to welding. Figure 10 illustrates a 1000 configuration with a two-layer stack. The 1000 configuration is similar to the 400 and 500 configurations, except that (a) at least one of the metal sheet 112 and the metal part 114 has a coating on it at the 414F interface, and (b) a 1010G gap may exist at the 414F interface. The metal sheet 112 has a 1012C coating on the surface of the metal sheet 112 facing the 414F interface, and / or the metal part 114 has a 1014C coating on the surface of the metal part 114 facing OCbt? I n / C7n7 / R / VIAI towards interface 414F. Other surfaces of the metal sheet 112 and the metal part 114 may also be coated. In a typical scenario, all surfaces of at least one of the metal sheets 112 and the metal part 114 are coated. The gap 1010G may be in the range of zero (no gap) to one millimeter, or between zero and 60% of the thickness of the metal sheet 112. Weld nuggets formed by method 900 can have dimensions similar to those formed by method 300. The weld nuggets (not shown in figure 10) can penetrate the metal part 114 in a manner similar to weld nugget 550 in figure 5, or terminate in an inner portion of the metal part 114 in a manner similar to weld nugget 450 in figure 4. Figure 11 illustrates a configuration 1100 that includes an intermediate metal sheet 616 between the metal sheet 112 and the metal part 114. The configuration 1100 is similar to the configuration 600, except that (a) at least one of the metal sheet 112, the metal sheet(s) 616, and the metal part 114 has a coating at a corresponding interface, and (b) there may be a gap at one or more of the interfaces 616F and 614F. With respect to the coatings, the metal sheet 112 can have a 1012C coating at the 616F interface, the metal sheet 616 can have one or both of an 1116C(1) coating at the 616F interface and an 1116C(2) coating at the 614F interface, and the metal part 114 can have a 1014C coating at the 614F interface. Each of the 616F and 614F interfaces can be configured with a gap similar to the 1010G gap in Figure 10. The 1100 configuration is easily extended to include more than one intermediate metal sheet 616. Weld nuggets formed by method 900 in configuration 1100 (not shown in Figure 11) can penetrate the metal part 114 in a manner similar to weld nugget 550 in Figure 5, or terminate in an inner portion of the metal part 114 in a manner similar to weld nugget 650 depicted in Figure 6. Referring again to Figure 9, in an example of step 910, the laser source 170 generates laser radiation 120, and the beam delivery module 180 focuses the laser radiation 120, including the center beam 122C and the ring beam 122A, onto the surface 112S of the metal sheet 112 arranged in any of the configurations discussed above with reference to Figures 10 and 11. In an example of step 920, the beam delivery module 180 directs the laser radiation 120 to trace at least one path on the surface 112S with the center beam 122C and the ring beam 122A. The function of step 920 involves manipulating the powers of the center beam 122C and the ring beam 122A while tracing the path (at least one). The powers of the center beam 1220 and the ring beam 122A can be adjusted, as needed, by the center power controller 172 and the ring power controller 174, respectively.The central beam 122C and the annular beam 122A can be continuous wave beams. OCbt? I n / C7n7 / R / VIAI Step 920 includes step 924, which traces an inward spiral path. Step 920 may also include step 922, which precedes step 922 and traces a closed loop with laser radiation 120. The closed loop surrounds the inward spiral path and ends at the starting point of the inward spiral path. Thus, when step 920 includes step 922, the closed loop and the inward spiral path form a continuous path. Figure 12 shows the laser power scheme used in a mode of step 920 that includes step 922. Figure 13 shows the paths traced by the focused laser radiation 120 in steps 922 and 924. Step 924 plots an inward spiral path 1320. The inward spiral path 1320 is similar to the inward spiral path 830 in Figure 8C. The inward spiral path 1320 starts at the outer location Lo at time t1 and spirals around and toward the central location Le to reach the central location Le at time t3. The procedure for plotting the inward spiral path 1320 takes place in two segments: (1) a first segment from the outer location Lo at time t1 to a location Lr at time t2, and (2) a second segment from location Lr at time t2 to the central location Le at time t3. While plotting the first segment, step 924 holds the central beam powers 122C and the ring beam powers 122A at the powers Peo and Pao, respectively. Pao may be greater than Peo, as depicted in Figure 12.Then, while the second segment is being plotted, step 924 reduces the powers of the center beam 122C and the ring beam 122A to zero and a non-zero power Pai, respectively. Finally, upon reaching the center location Le at time t3, step 924 deactivates the ring beam 122A. The Peo and Pao powers of the central beam 122C and the ring beam 122A, respectively, are configured to maintain a local fusion bath with a keyhole extending from the surface of the first metal sheet (e.g., the 112S surface of metal sheet 112), through any intermediate metal sheet if present (e.g., metal sheet 616), and towards or through the most distant metal part (e.g., metal part 114). As discussed earlier in relation to step 324 of method 300, the keyhole and surrounding melt pool are moved with the laser radiation 120 along the inward spiral path 1320 during step 924. By virtue of including both the central beam 122C and the ring beam 122A, method 900 achieves a stable keyhole with very little or no material loss through spatter, as discussed earlier in relation to method 300.The improved keyhole stability achieved by Method 900 helps release trapped gas from the metal and provides an efficient escape route for gas generated by the evaporation of any coating at the interfaces in the metal stack. This applies when Method 900 is applied to copper. OCbt? I n / C7n7 / R / VIAI or copper alloys, the presence of the ring beam 122A can have an additional benefit, namely that the preheating provided by the leading portion 824L of the ring beam 122A induces a phase transition in the copper / copper alloy to a state characterized by a higher level of absorption of laser radiation 120. The ring beam 122A thereby further reduces the power requirements for the central beam 122C. The gradual decrease in laser power while tracing the second segment of the inward spiral path 1320 serves to slow the cooling of the material to relieve stress and prevent cracking of the weld nugget.This power reduction is performed while the laser radiation 120 is moving along the inward spiral path 1320, rather than being stationary, to prevent a hole or indentation from forming in the weld nugget as discussed above in relation to step 328 of method 300. When included, step 922 traces a closed loop 1310 between a time t₀ and a time t₀. The closed loop 1310 is peripheral to the inward spiral path 1320. The closed loop 1310 terminates at the outer location L₀ and may be a circle. Step 922 completes at least one full circuit of the closed loop 1310. The closed loop 1310 may spiral around the central location Lₑ in the same direction as the inward spiral path 1320 (as shown in Figure 13) or in the opposite direction. Step 922 serves primarily to ensure a well-defined perimeter of the solder nugget formed by method 900. When such a perimeter is not required, it may be advantageous to omit step 922, for example, to achieve a smaller solder nugget when subject to space constraints or to minimize the overall processing time. The Peo and Pao powers, applied using method 900, can range from 1.5 to 5 kilowatts, while Pai can range from 0.05 to 1.0 kilowatts. The area traced by the inward spiral path 1320 and the closed loop 1310 (if included) can have a general extent, for example, a diameter 1370D as shown in Figure 13, in the range of 3 to 15 millimeters. Method 900 can be completed in less than 500 milliseconds, with the power reduction portion of step 924 lasting from 30 to 100 milliseconds. Step 924, performed with constant laser powers (between times ti and tz), can occupy 60 to 100 percent of the processing time preceding the power reduction. Method 900 may include providing a shielding gas to the weld zone to further help prevent porosity in the top layer of the weld nugget. The shielding gas may be nitrogen. The present invention was previously described in terms of a preferred embodiment and other embodiments. However, the invention is not limited to the embodiments described and OCbt? I n / C7n7 / R / VIAI represented herein. Rather, the invention is limited only by the appended claims hereto.

Claims

1. A laser welding method for joining aluminum, characterized in that it comprises the steps of: focusing laser radiation onto a first aluminum sheet disposed on an aluminum part, the laser radiation including a central beam and an annular beam surrounding the central beam; and controlling the focused laser radiation to trace a plurality of paths on the first aluminum sheet to weld the first aluminum sheet to the aluminum part, the control step including: tracing an outward spiral path while maintaining the respective first powers of the central and annular beams, the outward spiral path starting at a central location and spiraling around and away from the central location, after tracing the outward spiral path, tracing an outward path while reducing the powers of the central and annular beams from the respective first powers to the respective second powers,the outer path being peripheral to the spiral path viewed from the central location, and after tracing the outer path, tracing an inward spiral path toward the central location, while first (a) increasing the powers of the central and annular beams from their respective second powers to their respective third powers, subsequently (b) maintaining the third power of the annular beam and repeatedly pulsing the central beam between its third power and a lower fourth power, and finally (c) reducing the power of the central and annular beams.

2. The method according to claim 1, further characterized in that the reduction step deactivates the central beam and reduces the power of the third power annular beam.

3. The method according to claim 2, further characterized in that the reduction step reduces the power of the annular beam to zero.

4. The method in accordance with any of the preceding claims, further characterized in that the aluminum part is a second aluminum sheet.

5. The method in accordance with any of the preceding claims, further characterized in that the outer path includes a closed loop that completes at least one full circuit around the central location.

6. The method in accordance with any of the preceding claims, further characterized in that the outer path is an open path that does not complete a full revolution around the central location. OCbt? I n / O7n7 / R / VIAI 7. The method in accordance with any of the preceding claims, further characterized in that the melting of the aluminum caused by the control step terminates at a depth that is within an inner portion of the aluminum part.

8. The method in accordance with any of the preceding claims, further characterized in that at least one of the first aluminum sheet and the aluminum part is cast aluminum or a cast aluminum alloy.

9. The method according to claim 8, further characterized in that the first aluminum sheet is extruded aluminum or an extruded aluminum alloy, and the aluminum part is cast aluminum or a cast aluminum alloy.

10. The method in accordance with any of the preceding claims, further characterized in that: one or more intermediate aluminum sheets are arranged between the first aluminum sheet and the aluminum part, and the control step welds together the first aluminum sheet, the intermediate aluminum sheets and the aluminum part.

11. The method in accordance with any of the preceding claims, further characterized in that the control step additionally comprises, at the central location and before tracing the spiral path outwards, reducing the powers of the central and annular beams from the respective initial powers to the respective first powers.

12. The method in accordance with any of the preceding claims, further characterized in that each of the first, second and third powers of the annular beam exceeds the corresponding first, second and third powers of the central beam.

13. A laser welding method for joining a stack of metal parts including a coating at an interface, characterized in that it comprises: focusing laser radiation onto the stack of metal parts, the metal parts including (i) a first sheet of metal, (ii) a more distant metal part, and (iii) zero, one or more intermediate metal sheets between the first sheet of metal and the metal part, at least one of the metal parts having a coating at an interface with a neighboring metal part, the interface being configured with direct contact between the two neighboring metal parts or with a gap between them, the laser radiation incident on the first sheet of metal and including a central beam and an annular beam around the central beam;and controlling the focused laser radiation to trace at least one path on the first sheet of metal to weld the stack of metal parts, thereby also at least partially evaporating the coating at the interface, the control step including: tracing an inward spiral path, and while tracing the inward spiral path, first (a) maintaining the respective first powers of the center and ring beams, then (b) simultaneously reducing the power of the center beam from its first power to zero watts and reducing the power of the ring beam from its first power to a non-zero second power, and finally (c) turning off the ring beam.

14. The method according to claim 13, further characterized in that the most distant metal part is a sheet of metal.

15. The method according to claim 13 or claim 14, further characterized in that each of the metal parts is made of steel, and the coating includes zinc.

16. The method according to claim 13 or claim 14, further characterized in that each of the metal parts is made of copper or a copper alloy, and the coating includes nickel.

17. The method in accordance with any of claims 13 to 16, further characterized in that the metal melting caused by the control step terminates at a depth in the pile that is within an inner portion of the most distant metal part.

18. The method in accordance with any of claims 13 to 17, further characterized in that the first power of the annular beam exceeds the first power of the central beam.

19. The method according to any of claims 13 to 18, further characterized in that the control step additionally comprises, before tracing the inward spiral path, tracing a closed loop with the central and annular bundles in the respective first powers, the closed loop being peripheral to the inward spiral path and ending at a starting point of the inward spiral path.

20. The method in accordance with any of claims 13 to 19, further characterized in that the space is no more than 60 percent of the thickness of one of the two neighboring metal parts closest to the side of the stack receiving the laser radiation.

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