METHOD FOR LASER WELDING TWO OR MORE LIGHT METAL WORKPIECES

Abstract

A method for laser welding two or more light metal workpieces together, the method comprising: Providing a workpiece stack (10) comprising two or more light metal workpieces (12, 14) which overlap to define a welding area (16), wherein the welding area (16) of the workpiece stack (10) has an upper surface (20) and a lower surface (22) and further establishing a butt interface (34) between each pair of adjacent light metal workpieces (12, 14) contained in the workpiece stack (10), and wherein all of the two or more light metal workpieces (12, 14) in the workpiece stack (10) are aluminum workpieces or magnesium workpieces; Directing a laser beam (24) onto the upper surface (20) of the workpiece stack (10) to create a keyhole (70) and a molten metal weld pool (68) surrounding the keyhole (70), each keyhole (70) and the surrounding molten metal weld pool (68) penetrating the workpiece stack (10) from the upper surface (20) of the stack (10) to the lower surface (22) of the stack (10); Advancing a beam spot (44) of the laser beam (24) relative to the upper surface (20) of the workpiece stack (10) such that the beam spot (44) is advanced a plurality of times along a closed curved weld path (72) at a beam path speed ranging from 8 m / min to 120 m / min to grow and develop a molten pool (76) extending inwardly and downwardly from the closed curved weld path (72) on the upper surface (20) of the workpiece stack (10), the molten pool (76) penetrating the workpiece stack (10) from the upper surface (20) of the workpiece stack (10) to the lower surface (22) and intersecting each butt interface (34) formed in the weld region (16) of the workpiece stack (10); Stopping the transmission of the laser beam (24) to allow the molten pool (76) to solidify into a laser weld joint (66), which is made of reconsolidated composite workpiece material (78), wherein the laser weld joint (66) welds the two or more overlapping light metal workpieces (12, 14) together within the weld area (16), and wherein the laser weld joint (66) further defines a central notch 84 extending from a top surface (20) of the laser weld joint (66) downward into the laser weld joint (66); and retransmitting the laser beam (24) and advancing the beam spot (44) of the laser beam (24) relative to the upper surface (20) of the laser weld joint (66) along a secondary beam path pattern (86), contained within the closed weld path (72) to melt a portion of the laser weld joint (66) and consume the central notch (84).

Description

Technical FieldThe present invention generally relates to a method of laser welding together two or more lightweight metal workpieces having a surface oxide coating, such as aluminum workpieces and magnesium workpieces.A method for laser welding two or more lightweight metal workpieces is known from U.S. Pat. No. 2015 / 0 104 244 A1, for example. Further prior art is evident from U.S. Pat. No. 2016 / 0 061 727 A1BACKGROUNDLaser welding is a metal joining process in which a laser beam is directed at an array of stacked metal workpieces to provide a concentrated heat source capable of producing a weld joint between the individual metal workpieces. Generally, complementary flanges or other joining regions of two or more metallic workpieces are first aligned, assembled, and stacked relative to each other such that their joint surfaces overlap and oppose each other to form one or more joint interfaces. A laser beam is then directed onto an accessible top surface of the workpiece stack-up within a welding area spanned by the overlapping portion of the workpieces. The heat generated from the laser beam by the energy input initiates melting of the metallic workpieces and forms a molten metal weld pool within the workpiece stack-up. The molten metal weld pool penetrates the stack and intersects at least one, and typically all, of the joint interfaces produced. And, when the power density of the laser beam is high enough, a keyhole is formed under a beam spot of the laser beam inside the molten metal weld pool. A keyhole is a vaporized metal column that may include plasma derived from the metallic workpieces. The keyhole is an effective absorber of the energy of the laser beam and thus allows a deep and narrow penetration of molten workpiece metal into the stack.The molten metal weld pool and, if present, the keyhole are created in a very short time as the laser beam impinges on the top surface of the workpiece stack-up. After the metallic workpieces are first melted, the beam spot of the laser beam may be advanced relative to the top surface of the workpiece stack-up, which is typically associated with moving the laser beam along a beam path pattern having a relatively simple or complex geometric profile as projected onto the top surface of the stack-up. As the laser beam is advanced along the top surface of the stack, molten workpiece metal from the weld pool flows around and past the advancing beam spot within the workpiece stack. This penetrating molten workpiece metal cools and rapidly solidifies as the laser beam advances to form a resolidified metallic workpiece material. The transmission of the laser beam at the top surface of the workpiece stack-up is ultimately stopped once the laser beam has completed tracking the beam migration pattern, at which time the keyhole, if present, collapses and the molten workpiece metal still in the stack-up solidifies. The joint resolidified composite workpiece material obtained by the operation of the laser beam forms a laser weld joint that autogenously welds the overlapping metallic workpieces together.Many industries use laser welding as part of their manufacturing practice, including the automotive, aeronautical, marine, railway and construction industries. Laser welding is an attractive joining method because it requires only one-sided access, can be practiced with reduced flange widths, and results in a relatively small heat affected zone within the stack-up assembly that minimizes the heat distortion of the metallic workpieces. For example, in the automotive industry, when producing the body-in-white (BIW) using laser welding, metal workpieces and finished add-on parts which are mounted on the BIW before painting can be joined to one another. Specific applications of laser welding include the construction and attachment of structural body structures within the BIW, such as rail structures, rockers, rocker arms, A, B, and C columns, and underbody cross-members. Other specific cases where laser welding may also be employed are non-load bearing attachments within the BIW, such as attachment of a roof to a side wall, and the connection of overlying flanges as occur in the construction of doors, hood and trunk.The practice of laser welding may present challenges for certain types of metallic workpieces. For example, when the light metal workpieces included in the workpiece stack-up are light metal workpieces having a surface oxide coating, as is typically the case with aluminum workpieces and magnesium workpieces, there is a possibility that welding performance may be impaired. The surface oxide coating on aluminum and magnesium workpieces is typically a native refractory oxide layer that is both thermally and electrically insulating and mechanically tough. Since the surface oxide coating is difficult to break and has a poor thermal conductor, it can suppress the heat transfer rate into the underlying aluminum or magnesium at least at the beginning of the laser welding process. In addition, the surface oxide coating and moisture from the immediate environment may be a source of hydrogen when the surface oxide coating is heated to elevated temperatures by the laser beam. Hydrogen has a relatively high solubility in both molten aluminum and molten magnesium. To this end, localized generation of hydrogen in close proximity to the molten workpiece material and the presence of oxide coating fragments even in the molten workpiece material can result in porosity within the final consolidated laser weld joint.SUMMARY OF THE INVENTIONAccording to the invention, a method for laser welding two or more lightweight metal workpieces to one another is presented, which method is distinguished by the features of claim 1.First, a workpiece stack-up is provided that includes two or more lightweight metal workpieces that overlap to define a welding area. The welding region of the workpiece stack-up has upper and lower surfaces and further forms an interface between each pair of adjacent lightweight metal workpieces contained in the workpiece stack-up. All two or more lightweight metal workpieces in the workpiece stack-up are either aluminum or magnesium workpieces. Second, a laser beam is directed onto the top surface of the workpiece stack-up to create a keyhole and a molten metal weld pool surrounding the keyhole. Each of the keyhole and the surrounding molten metal weld pool penetrates the workpiece stack from the top surface to the bottom surface of the stack. Third, a beam spot of the laser beam is advanced relative to the upper surface of the workpiece stack-up such that the beam spot is advanced a plurality of times along a closed curve welding path at a beam path speed of 8 m / min or more to grow and develop a molten pool extending inward and downward from the closed curve welding path. The molten pool penetrates the workpiece stack from the top surface to the bottom surface and intersects any joint interface defined within the welding area of the stack. Fourth, the transmission of the laser beam is stopped to cause the melt pool to solidify to a laser weld joint of resolidified composite workpiece material. The laser weld joint welds the two or more overlapping lightweight metal workpieces together within the weld area and further defines a central notch extending downwardly into the weld joint from an upper surface of the joint. Fifthly, the laser beam is retransmitted and its beam spot is shifted with respect to the top surface of the laser weld joint along a secondary beam travel pattern contained in the closed curve weld path. The advancement of the laser beam along the secondary beam path pattern melts a portion of the laser weld joint and consumes the central notch.The aforementioned embodiment of the method according to the invention for lasering together lightweight metal workpieces can be further defined. For example, the workpiece stack-up can have two or three overlapping lightweight metal workpieces. In addition, the closed curve welding path may be a circular welding path having a diameter of 4 mm to 12 mm. If so, the aforementioned embodiment of the method for lasering light metal workpieces together may use a second circular weld path as a pattern for the secondary beam path. The second circular weld path may have a diameter in the range of 0.5 mm to 6 mm and the laser beam may be advanced multiple times along the second circular path to melt a portion of the laser weld joint and consume the central notch.Further, another method for laser welding two or more light metal workpieces will be described. In this case, a laser beam is first directed onto an upper surface of a workpiece stack-up which comprises two or more overlapping lightweight metal workpieces. The workpiece stack-up has in particular at least one first lightweight metal workpiece and one second lightweight metal workpiece, which overlap within a welding region. The first lightweight metal workpiece provides the upper surface of the workpiece stack-up and the second lightweight metal workpiece provides a lower surface of the workpiece stack-up, and each pair of adjacent overlapping lightweight metal workpieces within the workpiece stack-up forms an impact interface therebetween. Second, a beam spot of the laser beam is advanced relative to the upper surface of the workpiece stack-up such that the beam spot is advanced a plurality of times along a closed curve welding path at a beam velocity of 8 m / min or more. Such advancement of the beam spot of the laser beam causes a molten pool to grow and develop which extends inwardly and downwardly from the closed curve weld path on the upper surface of the workpiece stack-up. The melt pool penetrates the workpiece stack-up from the top surface to the bottom surface and intersects any joint interface defined within the welding region of the workpiece stack-up. Third, the melt may solidify in a laser weld joint of resolidified composite workpiece material. The laser weld joint welds the two or more overlapping lightweight metal workpieces together within the welding region.In certain practices of the disclosed laser welding method, the workpiece stack-up may include two overlapping lightweight metal workpieces or three overlapping lightweight metal workpieces. For example, in a stack of two workpieces, the first lightweight metal workpiece has an outer surface and a first joint surface, and the second lightweight metal workpiece has an outer surface and a second joint surface. The outer surface of the first lightweight metal workpiece forms the upper surface of the workpiece stack-up and the outer surface of the second lightweight metal workpiece forms the lower surface of the workpiece stack-up. And thus, the first and second joint surfaces of the first and second lightweight metal workpieces overlap and oppose each other to provide a joint interface. A method substantially comparable in type is evident from the aforementioned U.S. Pat. No. 2015 / 0 104 244 A1.As another example, in a stack of three workpieces, the first lightweight metal workpiece has an outer exterior surface and a first joint surface, and the second lightweight metal workpiece has an outer exterior surface and a second joint surface. The outer surface of the first lightweight metal workpiece forms the upper surface of the workpiece stack-up and the outer surface of the second lightweight metal workpiece forms the lower surface of the workpiece stack-up. In addition, the workpiece stack-up has a third lightweight metal workpiece which is located between the first and second lightweight metal workpieces. The third lightweight metal workpiece has opposing third and fourth joint surfaces. To this end, the third joint surface overlaps and faces the first joint surface of the first lightweight metal workpiece to form a first joint interface, and the fourth joint surface overlaps and faces the second joint surface of the second lightweight metal workpiece to form a second joint interface.The aforementioned embodiment of the method for lasering together light metal workpieces can be further defined. It is certain that each of the two or more overlapping lightweight metal workpieces can be an aluminum workpiece or magnesium workpiece. Moreover, the closed curve welding path may be a circular welding path having a diameter of, for example, 4 mm to 12 mm. Still further, the beam spot of the laser beam can be displaced completely along the closed curve welding path - whether the closed curve welding path is a circular welding path, an elliptical welding path or another welding path - wherever it occurs four to eighty times. In this case, the laser beam can be moved along the closed curve welding path at a beam speed of 8 m / min to 120 m / min. The laser beam directed at the top surface of the workpiece stack-up and advanced along the closed curve weld path may be a solid state laser beam, the motion of which is controlled and performed by a remote laser welding apparatus.In some cases where the foregoing embodiment of the method for lasering light metal workpieces together is practiced, particularly where the closed curve weld path is of a certain size or greater, a central notch may be formed in the laser weld joint that extends downwardly from an upper surface of the joint. This can be effected by the stirring effect which is caused by the repeated advance of the beam point of the laser beam along the closed curve welding path and the resulting rapid solidification of the melt pool. To consume and eliminate such a central notch, the embodiment of the laser welding method may further and optionally require retransmission of the laser beam and translation of the beam spot of the laser beam with respect to the top surface of the laser weld joint along a secondary beam path pattern included in the closed loop weld path. Advancing the laser beam along the secondary beam path pattern causes a portion of the laser weld joint to be re-melted and thus the previously defined central notch to be filled and consumed. In a particular implementation, the secondary beam path pattern may be a second closed curve weld path, and the beam spot of the laser beam may be moved multiple times along the second closed curve weld path at a beam path velocity of 8 m / min or more. The second closed curve welding path can be, for example, a second circular welding path with a diameter in the range from 0.5 mm to 6.0 mm.Further, a further method for laser welding two or three lightweight metal workpieces to one another is described. Here too, a workpiece stack is first provided which has two or three lightweight metal workpieces which overlap in order to define a welding region. The welding region of the workpiece stack-up has upper and lower surfaces and further forms an interface between each pair of adjacent lightweight metal workpieces contained in the workpiece stack-up. All of the two or more lightweight metal workpieces in the workpiece stack-up are either aluminum or magnesium workpieces. Secondly, a laser weld joint is formed which welds the two or three overlapping lightweight metal workpieces together. Forming the laser weld joint includes operating a scanning optical laser head of a remote laser welding apparatus to direct a laser beam onto the upper surface of the workpiece stack-up and additionally advancing a beam spot of the laser beam relative to the upper surface of the workpiece stack-up such that the beam spot is advanced multiple times along a closed curve weld path at a beam path velocity of 8 m / min to 120 m / min. Such advancement of the beam spot of the laser beam causes growth and development of a molten pool extending inwardly and downwardly from the closed curve weld path on the upper surface of the workpiece stack-up.The aforementioned embodiment of the method for lasering together light metal workpieces can be further defined. In fact, the closed curve weld path may be a circular weld path having a diameter in the range of 4 mm to 12 mm, and the beam spot of the laser beam may be fully advanced along the circular weld path in the range of four times to eighty times. Moreover, in some implementations, the beam spot of the laser beam may also be advanced relative to the top surface of the laser weld joint along a secondary beam path pattern contained within the closed curve weld path to melt a portion of the laser weld joint and consume and eliminate a central notch defined within the weld joint. The secondary beam motion pattern may consist of one or more weld paths defining an area that is 50% or less than an area defined by the closed curve weld path on the top surface of the workpiece stack-up.BRIEF DESCRIPTION OF THE DRAWINGSFIG. 1 is a general illustration of a workpiece stack-up having two overlapping lightweight metal workpieces and a remote laser welding apparatus capable of performing the disclosed laser welding method; FIG. 1A is an enlarged view of the laser beam illustrated in FIG. 1, showing a focal point and a longitudinal axis of the laser beam; FIG. 2 is a plan view of an upper surface of the workpiece stack-up and a laser beam shown in FIG. 1, and a plurality of closed curve weld paths projected onto the upper surface of the workpiece stack-up according to an embodiment of the present invention, and wherein the laser beam is repeatedly advanced along at least the largest and outermost closed curve weld paths during the formation of a laser weld joint that welds the overlapping lightweight metal workpieces together within the workpiece stack-up; FIG. 3 is a cross-sectional view of the workpiece stack-up shown in FIG. 2 taken along section lines 3- 3 showing a molten metal weld pool and a keyhole generated by the laser beam, and wherein the molten metal weld pool and the keyhole enter the workpiece stack-up from the top surface to a bottom surface; FIG. 4 is a top view of the upper surface of the workpiece stack-up melt illustrating a larger melt pool formed inwardly and downwardly from the closed curve weld path as a result of the thermal conduction associated with the multiple advancement of the laser beam along the closed curve weld path; FIG. 5 is a cross-sectional view of the workpiece stack-up shown in FIG. 4 taken along lines 5- 5 showing the molten pool entering the workpiece stack-up from the top surface to the bottom surface; FIG. 6 is a cross-sectional view of the workpiece stack-up and a laser weld joint formed by repeatedly advancing the laser beam along the closed curve weld path substantially corresponding to the perimeter of the laser weld joint to be formed, as shown in FIGS. 2-5, and wherein the laser weld joint welds the two overlapping lightweight metal workpieces together; FIG. 7 is a cross-sectional view of the workpiece stack-up and a laser weld joint formed by repeatedly advancing the laser beam along the closed curve weld path substantially corresponding to the perimeter of the laser weld joint to be formed, as shown in FIGS. 2-5, and wherein the laser weld joint welds the two overlapping lightweight metal workpieces together and further includes a central notch extending downwardly from an upper surface of the laser weld joint; FIG. 8 is a cross-sectional view of the workpiece stack-up from the same perspective as FIG. 3, showing a molten metal weld pool and a keyhole generated by the laser beam, and wherein the molten metal weld pool and the keyhole enter the workpiece stack-up from the top surface toward a bottom surface, although here the workpiece stack-up includes three overlapping light metal workpieces instead of two as illustrated in FIG. 3 ; and FIG. 9 is a cross-sectional view of the workpiece stack-up and a laser weld joint formed by repeatedly advancing the laser beam along the closed curve weld path as shown in FIGS. 2 and 8, and wherein the laser weld joint welds the three overlapping lightweight metal workpieces together.DETAILED DESCRIPTIONThe disclosed method of laser welding two or more stacked lightweight metal workpieces is to displace a laser beam - and particularly the beam spot of the laser beam - multiple times along a closed curve welding path relative to a top surface of the workpiece stack-up until a melt pool of satisfactory penetration is developed that later solidifies to a laser weld joint. The closed curve weld path followed by the laser beam may be a circular weld path having a constant diameter over its circumference or an elliptical weld path having a large diameter extending between the two extreme points of its circumference and a small diameter extending between the two narrowest points of its circumference. The area defined by the closed curve weld path substantially corresponds to the area of the resulting laser weld joint. The laser beam can be advanced on the closed curved path multiple times at a relatively high travel speed of at least 8 m / min and in particular between 8 m / min and 120 m / min. By this laser welding method, a more efficient heat transfer rate between the laser beam and the workpiece stack-up can be realized, and the resulting laser weld joint rather has minimal, if any, porosity.The repeated scanning of the closed curve weld path as required to form the laser weld joint may be performed by a remote laser welding apparatus or a conventional laser welding apparatus, such as an apparatus in which a fixed laser head is supported by a high speed CNC machine. The laser beam used to form the laser weld joint may be a solid state laser beam or a gas laser beam, depending on the properties of the lightweight metal workpieces to be joined and the desired laser welding mode (conduit, keyhole, etc.). Some notable solid state lasers that can be used are a fiber laser, a disc laser, a direct diode laser, and an Nd" YAG laser, and one notable gas laser that can be used is a CO 2- laser, although other types of lasers can certainly be used as well. In a preferred implementation of the disclosed method, described in more detail below, a remote laser welder having a tiltable mirror scanning optical laser head and a z-axis focus lens is used to perform the disclosed laser welding method, although other types of laser welders having functionalities similar to a remote laser welder may be used.The disclosed method of laser welding two or more metallic workpieces together may be performed on a variety of workpiece stack-up configurations. For example, the disclosed method may be used in conjunction with a "2T" workpiece stack-up (FIGS. 1, 3, and 5-7) having two overlapping lightweight metal workpieces, or may be used in conjunction with a "3T" workpiece stack-up (FIGS. 8-9) having three overlapping lightweight metal workpieces. Moreover, in some cases, the disclosed method may be used in conjunction with a "4T" workpiece stack-up (not shown) having four overlapping lightweight metal workpieces. The two or more lightweight metal workpieces included in the workpiece stack-up may be all aluminum workpieces or magnesium workpieces, and need not necessarily have the same composition (within the same base metal class) or thickness as the others in the stack-up. The disclosed method is performed in substantially the same manner to achieve the same results, regardless of whether the workpiece stack-up includes two overlapping lightweight metal workpieces or more than two overlapping lightweight metal workpieces. Differences in the workpiece stack-up configurations can be easily compensated for by adjusting the properties of the laser beams used.Referring now generally to FIG. 1, a workpiece stack-up 10 is illustrated in which the stack-up 10 includes at least a first lightweight metal workpiece 12 and a second lightweight metal workpiece 14 that overlap to define a welding region 16. A remote laser welding apparatus 18 capable of performing the disclosed workpiece joining method is also shown. Within the boundaries of the welding region 16, the first and second lightweight metal workpieces 12, 14 form an upper surface 20 and a lower surface 22 of the workpiece stack-up 10. And since only one-sided access is required for laser welding, the need to make the bottom surface 22 of the workpiece stack-up 10 likewise accessible is eliminated. As used herein, the terms "top surface" and "bottom surface" are relative terms that identify the surface of the stack 10 (top surface) that is closer to and faces the remote laser welding device 18 and the surface of the stack 10 (bottom surface) that faces the opposite direction.The workpiece stack-up 10 can comprise only the first and second lightweight metal workpieces 12, 14, as illustrated in FIGS. 1, 3 and 5 to 7. In these circumstances, and as best shown in FIG. 3, the first lightweight metal workpiece 12 has an outer exterior surface 26 and a first impact surface 28, and the second lightweight metal workpiece 14 has an outer exterior surface 30 and a second impact surface 32. The outer surface 26 of the first lightweight metal workpiece 12 forms the upper surface 20 of the workpiece stack-up 10 and the outer surface 30 of the second lightweight metal workpiece 14 forms the opposing lower surface 22 of the stack-up 10. In further embodiments, one of which is described below in connection with FIGS. 8-9, the workpiece stack-up 10 may include an additional third lightweight metal workpiece disposed between the first and second lightweight metal workpieces 12, 14 to provide the stack-up 10 with three, rather than two lightweight metal workpieces.The term "joint interface" is used generally in the present invention and is intended to encompass a broad range of overlapping relationships between the opposing first and second joint surfaces 28, 32 of the first and second lightweight metal workpieces 12, 14 that can meet the practice of laser welding. For example, the impact surfaces 28, 32 may establish the impact interface 34 by directly or indirectly contacting them. The landing surfaces 28, 32 are in direct contact with each other upon physical impingement and are not separated by a discrete intervening layer of material or gaps that are outside of normal assembly tolerance ranges. The joint surfaces 28, 32 are in indirect contact when separated by a discrete intermediate layer of material such as a sealant or adhesive - and thus are not exposed to the type of interfacial engagement typical of direct contact - but are in sufficiently close proximity suitable for laser welding. As another example, the impact surfaces 28, 32 may establish the impact interface 34 by separating them by predetermined gaps. Such gaps may be imposed between the impact surfaces 28, 32 by creating protruding features on one or both of the impact surfaces 28, 32 by laser scribing, mechanical denting, or otherwise. The foregoing features maintain intermittent contact points between the abutment surfaces 28, 32 which maintain the surfaces 28, 32 outside and around the contact points apart by up to 1.0 mm.Referring to FIG. 3, the first lightweight metal workpiece 12 includes a first lightweight metal base layer 36 and the second lightweight metal workpiece 14 includes a second lightweight metal base layer 38. The first and second light metal base layers 36, 38 may all be made of aluminum or magnesium, i.e., the first and second light metal base layers 36, 38 are both made of aluminum or both of magnesium. At least one of the first or second light metal base layers 36, 38 and generally both of the base layers 36, 38 have a surface oxide coating 40. The surface oxide coating(s) 40 may be used on one or both of the light metal base layers 36, 38 for, among other things, various reasons including corrosion protection, strength enhancement, and / or to improve processing, and the composition of the surface oxide coating(s) 40 is largely based on the composition of the underlying light metal base layers 36, 38. Considering the thickness of the light metal base layers 36, 38 and their surface oxide coatings 40, each of a thickness 121 of the first light metal workpiece 12 and a thickness 141 of the second light metal workpiece 14 is preferably in the range of 0.4 mm to 6.0 mm at least within the welding region 16.The light metal base layers 36, 38 may take a variety of metal molds and compositions belonging to the broadly listed base metal groups of aluminum and magnesium. For example, if made of aluminum, each of the light metal base layers 36, 38 (currently referred to as the first and second aluminum base layers 36, 38) may be separately made of unalloyed aluminum or an aluminum alloy containing at least 85 wt % aluminum. Some notable aluminum alloys that may form the first and / or second base aluminum substrates 36, 38 are an aluminum-magnesium alloy, an aluminum-silicon alloy, an aluminum-magnesium-silicon alloy, or an aluminum-zinc alloy. In addition, each of the aluminum base substrates 36, 38 may be separately provided in forged or cast form. For example, each of the aluminum base substrates 36, 38 may be 4xxx, 5xxx, 6xxx, or 7xxx series aluminum wrought alloy sheet, extruded, forged, or other machined article, or a 4xxx, 5xxx, or 7xxx series aluminum alloy casting. Some more specific types of aluminum alloys that may be used as the first and / or second aluminum base substrates 36, 38 are AA5182 and AA5754 aluminum-magnesium alloy, AA6011 and AA6022 aluminum-magnesium-silicon alloy, AA7003 and AA7055 aluminum-zinc alloy, and Al-10Si-Mg aluminum die casting alloy. The first and / or second aluminum base substrates 36, 38 may be used in a variety of temper grades including annealed (O), work-hardened (H), and solution annealed (T).When the first and second light metal base layers 36, 38 are made of magnesium, each of the light metal base layers 36, 38 (currently referred to as the first and second magnesium base layers 36, 38) may be separately made of unalloyed magnesium or a magnesium alloy containing at least 85 wt % magnesium. Some notable magnesium alloys that may form the first and / or second magnesium base layers 36, 38 are a magnesium-zinc alloy, a magnesium-aluminum alloy, a magnesium-aluminum-zinc alloy, a magnesium-aluminum-silicon alloy, and a magnesium-rare earth alloy. Additionally, each of the magnesium base layers 36, 38 may be provided separately in forged (sheet, extruded, forged part, or other machined object) or cast form. Some specific examples of magnesium alloys that may be used as first and / or second magnesium base layers 36, 38 include AZ91D die casting or magnesium wrought alloy (extruded or clad), AZ31B die casting or extruded (extruded or sheet metal) magnesium alloy, and AM60B magnesium die casting alloy. The first and / or second magnesium base layers 36, 38 may be employed in a variety of tempers including annealed (O), work-hardened (H), and solution annealed (W).The surface oxide coating 40 present on one or both of the light metal base layers 36, 38, regardless of whether the light metal base layers 36, 38 are aluminum or magnesium, may be a native refractory oxide layer that passively forms when fresh metal of the base layer(s) 36, 38 is exposed to atmospheric air. This native refractory oxide layer may consist of aluminium oxide compounds or magnesium oxide compounds (and possibly magnesium hydroxide compounds), depending on whether the light metal base layers consist of aluminium or magnesium. A thickness of the surface oxide coating 40 is typically between 1 nm and 50 nm, although other thicknesses may be used, particularly when additional processing techniques are practiced that aim to grow the surface oxide coating 40, such as anodization. Passively formed refractory oxide coatings often have thicknesses in the range of 2 nm to 10 nm, for example, if the underlying light metal base layer consists of aluminum or magnesium. Such surface oxide coatings 40 are mechanically tough and electrically and thermally insulative.Referring to FIG. 1, the remote laser welding apparatus 18 includes a scanning optical laser head 42. In general, the optical scanning laser head 42 directs the transmission of the laser beam 24 to the upper surface 20 of the workpiece stack-up 10 (also the outer outer surface 26 of the first lightweight metal workpiece 12). The directed laser beam 24 has a beam spot 44 that, as shown in FIG. 1A, is the cross-sectional area of the laser beam 24 in a plane aligned along the top surface 20 of the stack 10. The optical scanning laser head 42 is preferably mounted on a robotic arm (not shown) which can rapidly and precisely carry the laser head 42 to many different preselected locations within the welding area 16 in a rapidly programmed sequence. The laser beam 24 used in conjunction with the scanning optical laser head 42 is preferably a solid state laser beam operating at a wavelength in the near infrared region (generally referred to as 700 nm to 1,400 nm) of the electromagnetic spectrum. Additionally, the laser beam 24 has a power capability that can achieve a power density sufficient to create a keyhole within the workpiece stack-up 10 when forming the laser weld joint, as desired. The power density required to create a keyhole within the overlapping lightweight metal workpieces 12, 14 is typically in the range of 0.5-1.5 MW / cm 2.Some examples of a suitable solid state laser beam that may be used in conjunction with remote laser welder 18 are a fiber laser beam, a disk laser beam, and a direct diode laser beam. A preferred fiber laser beam is a diode pumped laser beam, wherein the laser gain medium is a rare earth element doped glass fiber (e.g., erbium, ytterbium, neodymium, dysprosium, praseodymium, thulium, etc.). A preferred disc laser beam is a diode pumped laser beam in which the gain medium is a thin laser crystal disc doped with a rare earth element (e.g., a ytterbium-doped yttrium aluminum garnet (Yb: YAG) crystal coated with a reflective surface) and mounted on a heat sink. And a preferred direct diode laser beam is a combined laser beam (e.g., wavelength combined) derived from multiple diodes where the gain medium is multiple semiconductors such as those based on aluminum gallium arsenide (AlGaAS) or indium gallium arsenide (InGaAS). Laser generators capable of producing any of these types of lasers and other variations are commercially available. Other solid state laser beams not expressly mentioned herein may of course be used.The optical scanning laser head 42 includes an array of mirrors 46 that maneuver the laser beam 24 and thus can transport the beam spot 44 along the top surface 20 of the workpiece stack-up 10 within an operational boundary 48 that at least partially spans the weld area 16. Here, as shown in FIG. 1, the portion of the upper surface 20 spanned by the operational boundary 48 is referred to as the x-y plane because the position of the laser beam 24 within the plane is identified by the coordinates "x" and "y" of a three-dimensional coordinate system. In addition to the arrangement of mirrors 46, the scanning optical laser head 42 also includes a z-axis focus lens 50 that can move a focal point 52 (FIG. 1A ) of the laser beam 24 along a longitudinal axis 54 of the laser beam 24 so as to vary the position of the focal point 52 in a z-direction that is oriented perpendicular to the x-y plane in the three-dimensional coordinate system defined in FIG. 1. To prevent dirt and debris from affecting the optical system components and the integrity of the laser beam 24, a cover sled 56 may be disposed below the scanning optical laser head 42. The cover slide 56 protects the array of mirrors 46 and z-axis focus lens 50 from the environment, but leaves the laser beam 24 exit the scanning laser optical head 42 without substantial interference.The arrangement of mirrors 46 and z-axis focus lens 50 cooperate during operation of remote laser welder 18 to determine the desired movement of laser beam 24 and its beam spot 44 within operating boundary 48 as well as the position of focal spot 52 along longitudinal axis 54 of beam 24. In particular, the array of mirrors 46 includes a pair of tiltable scanning mirrors 58. Each of the tilting scanning mirrors 58 is mounted on a galvanometer 60. The two tiltable scanning mirrors 58 can move the position of the beam spot 44 and thus alter the point at which the laser beam 24 impinges on the top surface 20 of the workpiece stack-up 10 throughout the x-y plane of the operational boundary 48 by precisely tuned tilting movements of the galvanometers 60. At the same time, the z-axis focus lens 50 controls the position of the focal point 52 of the laser beam 24 to manage the laser beam 24 at the proper power density and achieve the desired heat input both instantaneously and over time. All of these optical components 50, 58 may be rapidly indexed in a few milliseconds or less to advance the beam spot 44 of the laser beam 24 with respect to the x-y plane of the top surface 20 of the workpiece stack-up 10 along one or more closed curve weld paths, as described in more detail below, while controlling the position of the focus spot 52.One property that distinguishes remote laser welding from other conventional forms of laser welding is the focal length of the laser beam 24. Here, as best shown in FIG. 1, the laser beam 24 has a focal length 62 that is measured as the distance between the focal point 52 and the last tiltable scanning mirror 58 that intercepts and reflects the laser beam 24 before the laser beam 24 exits the scanning optical laser head 42. The focal length 62 of the laser beam 24 is preferably in the range of 0.4 meters to 2.0 meters with a diameter of the focal point 52, which is typically between 100 μm and 700 μm. The focal length and a focal distance 64 can be easily adjusted. As used herein, the term "focal distance" refers to the distance between the focal point 52 of the laser beam 24 and the top surface 20 of the workpiece stack-up 10 along the longitudinal axis 54 of the beam 24, as best shown in FIG. 1A. The focal distance 64 of the laser beam 24 is thus zero when the focal point 52 is positioned on the top surface 20 of the stack 10. Similarly, the focal distance is a positive distance value (+) when the focal point 52 is positioned above the top surface 20 and a negative distance value (-) when positioned below the top surface 20.In the presently disclosed laser welding method, referring to FIGS. 1-7, a laser weld joint 66 is formed in the workpiece stack-up 10 by momentarily melting portions of the lightweight metal workpieces 12, 14 with the laser beam 24 in a particular manner. To form the laser weld joint 66, the laser beam 24 is directed by the scanning optical laser head 42 onto the upper surface 20 of the workpiece stack-up at a predetermined weld location within the weld area 16. The resulting impingement of the laser beam 24 on the top surface 20 of the stack 10 creates within the stack 10, as shown in FIGS. 2-3, a molten metal weld pool 68 that penetrates the stack 10 from the top surface 20 to the bottom surface 22 and may or may not initially intersect the joint interface 34 established between the first and second lightweight metal workpieces 12, 14. Indeed, in the 2T stack-up configuration shown in FIG. 3, the molten metal weld pool 68 may partially or completely penetrate the workpiece stack-up 10. A fully penetrating molten metal weld pool 68 fully penetrates the workpiece stack-up 10 and breaks through the bottom surface 22 of the stack-up 10, as shown, while a partially penetrating molten metal weld pool 68 penetrates to some intermediate depth between the top and bottom surfaces 20, 22 and therefore does not extend or break through to the bottom surface 22 of the stack-up 10.The laser beam 24 also preferably has a power density which is sufficient to evaporate the workpiece stack-up 10 directly below the beam point 44. This vaporizing action creates a keyhole 70, also shown in Figs. 2-5, which is a column of vaporized workpiece metal, often containing plasma. The keyhole 70 is formed within the molten metal weld pool 68 and exerts an outward vapor pressure sufficient to prevent the surrounding molten metal weld pool 68 from collapsing inward. And like the molten metal weld pool 68, the keyhole 70 also penetrates the workpiece stack-up 10 from the top surface 20 to the bottom surface 22 and first intersects or does not intersect the joint interface 34 formed between the first and second lightweight metal workpieces 12, 14. The keyhole 70 provides conduction for the first laser beam 24" to provide energy downwardly into the workpiece stack-up 10, thereby allowing relatively deep and narrow penetration of the melt pool 68 into the workpiece stack-up 10. The keyhole 70 may penetrate fully (as shown) or partially into the workpiece stack-up 10 along with the molten metal weld pool 68.In creating the molten metal weld pool 68, and preferably the keyhole 70, the laser beam 24, and more particularly its beam spot 44, is advanced along a closed curve weld path 72, as shown in FIG. 2, a plurality of times in a forward direction 74 relative to the top surface 20 of the workpiece stack-up 10 in the x-y plane of the operational boundary 48. The closed curve weld path 72 may be a circular weld path as shown in FIG. 2, in which case a diameter 721 of the weld path is constant over its circumference. However, in another embodiment, the closed loop weld path 72 may take a different geometric shape, such as an elliptical weld path having a large diameter extending between the two extreme points of its perimeter and a smaller diameter extending between the two narrowest points of its perimeter, in place of the circular weld path. However, regardless of its profile, the closed curve weld path 72 is sized to substantially conform to the desired perimeter of the laser weld joint 66 as viewed from the top surface 20 of the workpiece stack 10", i.e., a region defined by the closed curve weld path 72 substantially conforms to a region of the laser weld joint 66 that is ultimately formed. For this purpose, when the closed curve welding path 72 is circular, its diameter 721 is preferably in the range of 4 mm to 12 mm.The laser beam 24 may be advanced multiple times along the closed curve weld path 72, which, as previously indicated, substantially corresponds to the desired perimeter of the laser weld joint 66 to be formed. That is, the laser beam 24 is advanced more than once along the closed curve weld path 72 so that the laser beam 24 tracks the same weld path effectively over and over again a predetermined number of complete passes. The laser beam 24 may be moved along the closed curve weld path 72 at a beam velocity of at least 8 m / min (meters per minute), and preferably between 10 m / min and 50 m / min. The advancement of the laser beam 24 along the closed curve weld path 72 at such a travel speed is controlled by precisely controlling the coordinated movements of the tiltable scanning mirrors 58 within the scanning optical laser head 42 as described above. By repeatedly advancing the laser beam 24 along the closed curve weld path 72 at this relatively high speed, which is significantly faster than the beam path speeds usually realized during laser welding, i.e., 1 m / min to 5 m / min, the structural integrity of the laser weld joint 66 is considered positively affected, as will be explained in more detail below.The repeated advancement of the beam spot 44 of the laser beam 24 along the closed curve weld path 72 causes the molten metal weld pool 68 (along with the keyhole 70, if present) to be correspondingly displaced a similar distance within the workpiece stack-up 10, as shown in FIG. 4. Simultaneously, the energy of the laser beam 24 absorbed by the workpiece stack-up 10 generates heat which is in turn transmitted by conduction both radially inward from the closed curve weld path 72 and downward to the bottom surface 22 of the workpiece stack-up 10. As the laser beam 24 continues to traverse the closed curve weld path 72, this conductive heat transfer melts the portions of the first and second lightweight metal workpieces 12, 14 inward and downward of the closed curve weld path 72 to grow and develop a molten pool 76 that ultimately encompasses the entire area within the closed curve weld path 72, as shown in FIGS. 4-5. The number of passes of the laser beam 24 along the same closed curve weld path 72 to develop the larger melt pool 76 may vary depending on the composition of the lightweight metal workpieces 12, 14, the thicknesses 121, 141of the workpieces 12, 14, and the desired size of the laser weld joint 66. However, in many cases, the laser beam 24 may be propelled fully along the closed curve weld path 72 in the range of four to eighty times, or, more narrowly, eight to thirty times.The molten pool 76 has grown to intersect the joint interface 34 formed between the two lightweight metal workpieces 12, 14 while, as shown, fully penetrating through the workpiece stack-up 10 or only partially penetrating through the stack-up 10. The inward growth of the melt pool 76 and the stirring effect induced in the growing melt pool 76 by the repeated and relatively rapid advancement of the laser beam 24 along the closed curve weld path 72 not only results in effective and efficient heat transfer into the workpiece stack-up 10, but these actions also cooperate to guide or drive surface oxide coating fragments derived from the top surface 20, the joint interface 34, and possibly even the bottom surface 22, toward the center of the melt pool 76. And in addition to being directed or driven toward the center of the melt pool 76, the entrapped surface oxide coating fragments tend to rise to the upper portion of the melt pool 76, which is the exposed surface of the pool 76 that is closest to the upper surface 20 of the workpiece stack-up 10. After the beam spot 44 of the laser beam 24 has completed repeated tracking of the closed curve weld path 72 due to the satisfactory growth and penetration of the melt pool 76, the transmission of the laser beam 24 is stopped or the laser beam 24 is otherwise removed from the closed curve weld path 72. The consequent omission of the energy and heat transfer enables a rapid cooling and solidification of the melt bath 76 to form a resolidified composite workpiece material 78, as illustrated in FIG. 6.The collective resolidified composite workpiece material 78 obtained from the laser beam 24 forms the laser weld joint 66, which may extend fully or partially into the workpiece stack-up 10, depending on whether the preceding melt pool 76 has fully or partially penetrated into the stack-up 10 and may be surrounded by a heat-affected zone (HAZ). The laser weld joint 66 thus extends from the top surface 20 of the stack-up 10 to the bottom surface 22 into the workpiece stack-up 10 while intersecting the joint interface 34 to autogenously weld the lightweight metal workpieces 12, 14 together. The composition of the resolidified composite workpiece material material 78 comprising the laser weld joint 66 is determined by the compositions of the first and second lightweight metal workpieces 12, 14. Moreover, as representative and not necessarily drawn to scale in FIG. 6, the surface oxide coating fragments that have risen to the top of the melt pool 76 during the repeated advancement of the laser beam 24 along the closed curve weld path 72 may become resting as a film or other agglomerates 80 on a top surface 82 of the laser weld joint 66. Migration of these fragments of the surface oxide coating onto the top surface 82 of the laser weld joint 66, and thus from within the laser weld joint 66 where they may otherwise have remained, significantly reduces and may even eliminate porosity formation within the laser weld joint 66.In some embodiments of the disclosed laser welding method, particularly when the diameter 721 of the closed curve weld path 72= 6.5 mm or greater, a central notch 84 may materialize in the laser weld joint 66 extending downwardly from the top surface 82 of the joint 66, as shown in FIG. 7, as a result of the agitating action induced by repeated advancement of the laser beam 24 along the closed curve weld path 72 and rapid solidification of the melt pool 76. The presence of a central notch 84 generally does not adversely affect the mechanical properties (e.g., tensile strength, transverse tensile strength, etc.) of the laser weld joint 66. In cases where a central notch 84 remains in the laser weld joint 66, the laser beam 24 may be retransmitted and its beam spot 44 advanced along a secondary beam path pattern 86 that, as shown in FIG. 2, is projected onto the top surface 82 of the laser weld joint 66 after the laser beam 24 has completed its repeated advance along the closed loop weld path 72. The advancement of the laser beam 24 along the secondary beam travel time 86 melts a central portion of the laser weld joint 66, and thus consumes the central notch 84, thereby making the upper surface 82 of the joint 66 more visually pleasing.The secondary beam path pattern 86 is comprised of one or more weld paths 88 that span the central notch 84 and are located entirely within the closed curve weld path 72. The one or more welding paths 88 define an area that is preferably 50% or less than the area defined by the closed curve welding path 72 and may take a variety of geometric configurations. In a particular embodiment, for example, the one or more welding paths 88 of the secondary beam path pattern 86 may be a second closed curve welding path 90, such as the circular welding path shown in FIG. 2 or an elliptical welding path. As before, the circular weld path forming the secondary beam path pattern 86 has a diameter 901 which is constant about its circumference. And while the diameter 901 of the circular weld path of the secondary beam path pattern 86 may vary depending on the size of the laser weld joint 66, in many cases, the diameter 901 of the circular weld path preferably ranges from 0.5 mm to 6.0 mm. When a circular weld path is employed as the secondary weld pattern 86 as shown in FIG. 2, the laser beam 24 may be advanced multiple times along the weld path, such as between two and thirty times, at a beam path velocity that is preferably between 8 m / min and 120 m / min, or more narrowly, between 10 m / min and 60 m / min.The secondary beam path pattern 86 may take other arrangements of the one or more welding paths 88 besides the second closed curve welding path 90 (e.g., circular welding path or elliptical welding path) as shown in FIG. 2. Indeed, the secondary beam path pattern 86 may include a single spiral weld path, a series of concentric circular weld paths, a series of elliptical weld paths, a wavy weld path having a spiral, circular, or elliptical shape, or a star or cloverleaf weld path, just to name a few examples. Specific implementations of some of these types of alternative arrangements of the one or more welding paths 88 are illustrated and described in US 2019 / 224 781 A1, US 2020 / 047 285 A1, US 2018 / 304 405 A1, WO 2016 107 564 A1 and WO 2017 / 035 728 A1. When one of these alternative arrangements of the one or more weld paths 88 is used as the secondary beam path pattern 86, the one or more weld path(s) 88 may cover a similar large area on the top surface 82 of the laser weld joint 66 as the second closed loop weld path 90 described above as having a preferred diameter 901 of 0.5 mm to 6.0 mm. The laser beam 24 may also be advanced one or more times along any of the aforementioned alternative arrangements of one or more welding paths 88 at a beam travel speed that is preferably between 8 m / min and 120 m / min, or more narrowly between 10 m / min and 60 m / min.During the practice of the disclosed laser welding method, as described above, the laser beam 24 is advanced a plurality of times along the closed curve weld path 72 that creates the laser weld joint 66 with minimal, if any, porosity, and then optionally can be transferred to a secondary weld pattern 86 and advanced along a secondary weld pattern 86 contained within the previously traversed closed curve weld path 72 to eliminate the sometimes formed central notch 84. The characteristics of the operating laser beam 24 required to perform such a laser welding process, as well as the relatively fast beam path velocity that applies at least to the closed curve weld path 72, can be readily ascertained by those skilled in the art. The laser beam 24 may have a power level in the range of 1 kW to 50 kW and a focus position between -30 mm and +30 mm (relative to the top surface 20 of the workpiece stack-up 10) during repeated advancement along the closed-loop weld path 72, and further have a power level in the range of 0.5 kW to 20 kW and a focus position between -50 mm and +50 mm during advancement along the secondary beam path pattern 86 when the secondary beam path pattern 86 is part of the laser welding process.FIGS. 1, 3 and 5-7 illustrate an embodiment of the workpiece stack-up 10 that includes two overlapping lightweight metal workpieces 12, 14 that form a single joint interface 34. Of course, as shown in FIGS. 8-9, the disclosed laser welding method may also be applied to a workpiece stack-up 10 having an additional third lightweight metal workpiece 150 with a thickness 151 located between the first and second lightweight metal workpieces 12, 14. The third lightweight metal workpiece 150, if present, includes a third lightweight metal base layer 152 that may also be coated with a surface oxide coating 40 (as shown). The third lightweight metal workpiece 150 is similar in many respects to the first and second lightweight metal workpieces 12, 14 and, accordingly, the description of the first and second lightweight metal workpieces 12, 14 presented above (in particular the composition of the lightweight metal base layers, their possible surface oxide coatings and the workpiece thicknesses) applies entirely to the third lightweight metal workpiece 150. The welding region 16 in this embodiment of the workpiece stack-up 10 is now defined by the extent of the joint overlap of all the first, second and third lightweight metal workpieces 12, 14, 150.By stacking the first, second and third lightweight metal workpieces 12, 14, 150 in an overlapping manner to provide the workpiece stack-up 10 and as best shown in FIG. 8, the third lightweight metal workpiece 40 has two abutment surfaces", a third abutment surface 156 and a fourth abutment surface 158. The third impact surface 156 overlaps and faces the first impact surface 28 of the first lightweight metal workpiece 12, and the fourth impact surface 158 overlaps and faces the second impact surface 32 of the second lightweight metal workpiece 14. Within the welding region 16, the opposing first and third impact surfaces 28, 156 of the first and third lightweight metal workpieces 12, 150 form a first impact interface 160 and the opposing second and fourth impact surfaces 32, 158 of the second and third lightweight metal workpieces 14, 150 form a second impact interface 162. These shock interfaces 160, 162 are of the same type and include the same attributes as the shock interface 34 described above with respect to the 2T stack shown in Figures 1, 3 and 5-7. Thus, in this embodiment, the outer outer surfaces 26, 30 of the flanking first and second lightweight metal workpieces 12, 14 are further facing away from each other in opposite directions and form the upper and lower surfaces 20, 22 of the workpiece stack-up 10.The disclosed laser welding process is practiced in the same general manner as described above; i.e., the laser beam 24 is advanced multiple times along the closed curve weld path 72, preferably between four and eighty complete passes, at a beam velocity greater than 8 m / min or narrower, between 10 m / min and 50 m / min, thereby translating the molten metal weld pool 68 and keyhole 70 (if present) respectively within the stack 10, as shown in FIGS. 2 and 7. The inward and downward conductive heat transfer associated with such forward movement of the laser beam 24 along the closed curve weld path 72 causes the molten pool 76 to grow and develop, which here intersects each of the first and second impact interfaces 160, 162 formed between the lightweight metal workpieces 12, 14, 150 while passing entirely through the workpiece stack-up 10 or only partially through the stack-up 10, as shown. The eventual stopping of the transmission of the laser beam 24 causes the melt pool 76 to cool and transition to the resolidified composite workpiece material 78, which together forms the laser weld joint 66, as shown in FIG. 8. The laser beam 24 may then optionally be advanced along a secondary welding pattern 84 included in the previously tracked closed loop weld path 72 to eliminate the central notch 82, which may be formed depending on the size of the closed loop weld path 72.

Claims

A method of laser welding two or more lightweight metal workpieces together, the method comprising: providing a workpiece stack-up (10) having two or more lightweight metal workpieces (12, 14) that overlap to define a welding area (16), wherein the welding area (16) of the workpiece stack-up (10) has an upper surface (20) and a lower surface (22), and further establishes an interface (34) between each pair of adjacent lightweight metal workpieces (12, 14) contained in the workpiece stack-up (10), and wherein all of the two or more lightweight metal workpieces (12, 14) in the workpiece stack-up (10) are aluminum workpieces or magnesium workpieces; directing a laser beam (24) onto the top surface (20) of the workpiece stack-up (10) to create a keyhole (70) and a molten metal weld pool (68) surrounding the keyhole (70), each keyhole (70) and the surrounding molten metal weld pool (68) entering the workpiece stack-up (10) from the top surface (20) of the stack-up (10) to the bottom surface (22) of the stack-up (10); advancing a beam spot (44) of the laser beam (24) relative to the top surface (20) of the workpiece stack-up (10) such that the beam spot (44) is advanced a plurality of times along a closed curve weld path (72) at a beam path velocity ranging from 8 m / min to 120 m / min to grow and develop a molten pool (76) extending inwardly and downwardly from the closed curve weld path (72) on the top surface (20) of the workpiece stack-up (10), wherein the molten pool (76) penetrates the workpiece stack-up (10) from the top surface (20) of the workpiece stack-up (10) to the bottom surface (22) and intersects each joint interface (34) formed in the weld area (16) of the workpiece stack-up (10); Stopping transmission of the laser beam (24) to allow the molten pool (76) to solidify to a laser weld joint (66) comprised of resolidified composite workpiece material (78), wherein the laser weld joint (66) welds the two or more overlapping lightweight metal workpieces (12, 14) together within the weld area (16), and wherein the laser weld joint (66) further defines a central notch 84 extending downwardly from a top surface (20) of the laser weld joint (66) into the laser weld joint (66); and retransmitting the laser beam (24) and advancing the beam spot (44) of the laser beam (24) relative to the top surface (20) of the laser weld joint (66) along a secondary beam path pattern (86) contained within the closed weld path (72) to melt a portion of the laser weld joint (66) and consume the central notch (84).The method of claim 1, wherein the workpiece stack-up (10) comprises two or three overlapping lightweight metal workpieces (12, 14, 150).The method of claim 1, wherein the closed curve weld path (72) is a circle weld path (90) having a diameter (721) in the range of 4 mm to 12 mm.The method of claim 3, wherein the secondary beam path pattern comprises a second circular weld path (90) having a diameter (901) in the range of 0.5 mm to 6.0 mm, and wherein the beam spot (44) of the laser beam (24) is advanced multiple times along the second circular weld path (90).

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