METHOD FOR JOINING METAL WORKPIECES

Abstract

A method for joining metallic workpieces (12, 14, 150), the method comprising: a workpiece stack (10) is provided having three metal workpieces (12, 14, 150) overlapping to define a welding area (16), wherein the welding area (16) of the workpiece stack (10) has a top surface (20) and a bottom surface (22) and further establishes a butt interface (34) between each pair of adjacent metal workpieces (12, 14, 150) included in the workpiece stack (10), and wherein all of the metal workpieces in the workpiece stack (10) are magnesium workpieces; an optical scanning laser head (42) of a remote laser welding device (18) is operated to direct a first laser beam (24') onto the upper surface (20) of the workpiece stack (10) and additionally to move a beam spot (44) of the first laser beam (24') relative to the upper surface (20) of the workpiece stack (10) within the welding area (16) and along a linear beam path pattern (80) to form a keyhole (70) and to displace a surrounding molten metal weld pool (68) along a corresponding path within the workpiece stack (10), wherein a single translation of the keyhole (70) and the surrounding molten metal weld pool (68) creates a linear laser seam weld joint (66) extending into the workpiece stack (10) and intersecting each butt interface (34) formed between the top and bottom surfaces (22) of the workpiece stack (10) to weld the three metallic workpieces (12, 14, 150) together, the laser seam weld joint (66) having an initial top surface (76) adjacent the top surface (20) of the workpiece stack (10); the scanning optical laser head (42) of the remote laser welding device (18) is operated to impinge on the initial upper surface (76) of the laser seam weld joint (66) with a second laser beam (24") and additionally to move the second laser beam (24") sinusoidally along the initial upper surface (76) of the linear laser seam weld joint (66) to melt an upper portion of the laser seam weld joint (66) including the initial upper surface (76) of the laser seam weld joint (66), wherein the upper portion melted by the second laser beam (24") constitutes between 10 vol.% and 30 vol.% of the laser seam weld joint (66); and the second laser beam (24") is removed from the laser seam weld joint (66) to allow the upper portion of the laser seam weld joint (66) to resolidify and to provide the laser seam weld joint (66) with a modified upper surface (84) that is smoother than the initial upper surface (76) of the laser seam weld joint (66) wherein the first laser beam (24') is a solid-state laser beam, wherein the first laser beam (24') is moved relative to the upper surface (20) of the workpiece stack (10) along the beam travel pattern (80) at a travel speed ranging from 2 m / min to 50 m / min, while a power level of the first laser beam (24') ranges from 2 kW to 6 kW and a focus position of the first laser beam (24') ranges from +10 mm above the upper surface (20) of the workpiece stack (10) to -10 mm below the upper surface (20) of the workpiece stack (10); and wherein the second laser beam (24") is a solid-state laser beam, wherein the second laser beam (24") is moved along the initial upper surface (76) of the laser seam weld joint (66) at a travel speed ranging from 50 m / min to 130 m / min, while a power level of the second laser beam (24") ranges from 1 kW to 3 kW and a focus position of the second laser beam (24") ranges from 0 mm to -50 mm below the upper surface (20) of the workpiece stack (10).

Description

TECHNICAL FIELD

[0001] The present invention relates generally to a method for joining metallic workpieces and, more particularly, to a method for smoothing the top surface of a laser weld joint using a laser beam to melt an upper portion of the laser weld joint having the top surface.

[0002] DE 10 2009 057 997 A1, for example, describes a method for welding two metal components, in particular made of aluminum alloys, in which the metal components to be joined are melted in a welding area by means of a laser beam to form a weld seam, whereupon the surface of the weld seam is smoothed by a further laser beam to partially melt the weld seam in the area of ​​the surface. BACKGROUND

[0003] Laser 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 creating a weld joint between the individual metal workpieces. Generally, complementary flanges or other joining areas of two or more metallic workpieces are first aligned relative to each other, assembled, and stacked such that their butt surfaces overlap and oppose each other to form one or more butt interfaces. A laser beam is then directed at an accessible upper surface of the workpiece stack within a weld area spanned by the overlapping portion of the workpieces. The heat generated by the energy absorption from the laser beam initiates the melting of the metallic workpieces, forming a molten metal weld pool within the workpiece stack.The molten metal weld pool penetrates the stack and intersects at least one, and usually all, of the produced butt interfaces. If the power density of the laser beam is high enough, a keyhole is created beneath a laser beam spot within the molten metal weld pool. A keyhole is a column of vaporized metal, which may contain plasma originating from the metallic workpieces. The keyhole effectively absorbs the laser beam's energy, allowing deep and narrow penetration of molten workpiece metal into the stack.

[0004] The molten metal weld pool and, if present, the keyhole are created very quickly once the laser beam impacts the top surface of the workpiece stack. After the metallic workpieces are initially molten, the laser beam spot can be advanced relative to the top surface of the workpiece stack, which typically involves moving the laser beam along a beam path pattern with a relatively simple or complex geometric profile as projected onto the top surface of the stack. As the laser beam advances along the top surface of the stack, molten workpiece metal flows from the molten metal weld pool around and behind the advancing beam spot within the workpiece stack. This penetrating molten workpiece metal cools and rapidly solidifies into resolidified metallic workpiece material as the laser beam advances.The laser beam transmission to the upper surface of the workpiece stack is finally stopped once the laser beam has finished tracing the beam travel pattern. At this point, the keyhole, if present, collapses and the molten workpiece metal still in the stack solidifies. The joint, resolidified composite workpiece material obtained by the laser beam operation forms a laser weld joint that autogenously welds the overlapping metallic workpieces together.

[0005] Many industries use laser welding as part of their manufacturing practices, including the automotive, aerospace, marine, railroad, and construction industries. Laser welding is an attractive joining process 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 assembly, minimizing thermal distortion of the metallic workpieces. For example, in the automotive industry, during the manufacture of the body-in-white (BIW), laser welding can be used to join metallic workpieces as well as finished add-on parts that are mounted on the BIW prior to painting. Specific applications of laser welding include the design and attachment of load-bearing body structures within the BIW, such as rail assemblies, rocker arms, A-, B-, and C-pillars, and underbody cross members.Other specific cases where laser welding can also be used are non-structural fastenings within the BIW, such as the attachment of a roof to a side wall, and the connection of overlying flanges, as occurs in the construction of doors, hood and trunk.

[0006] The practice of laser welding can present challenges for certain types of metallic workpieces. For example, when the metal workpieces included in the workpiece stackup are steel, aluminum, or magnesium, the turbulence created in the molten metal weld pool during laser welding and the tendency for gases to become trapped in the weld pool, which leads to porosity defects as the molten workpiece material cools and solidifies, can result in a disturbed and roughened top surface in the final laser weld joint.A rough upper surface of the laser weld joint not only indicates a poor weld, even if the weld joint is structurally sound and has satisfactory mechanical properties, but can also create crack-prone residual stress concentration points and, in particular, stress corrosion cracking if the joint is subjected to tensile loading in a corrosive environment. A rough upper surface of the laser weld joint can also damage weatherstripping that may be applied over the joint if the joint is located on a vehicle door or along a door or window opening of the BIW. SUMMARY OF THE INVENTION

[0007] According to the invention, a method for joining metallic workpieces is presented, which is characterized by the features of claim 1.

[0008] Another method for joining metallic workpieces may comprise multiple steps. In a first step, a laser weld joint is formed in a workpiece stack, welding two or more overlapping metal workpieces together. The laser weld joint extends into the workpiece stack from a top surface of the stack to a bottom surface of the stack and intersects at least one butt interface established between the top and bottom surfaces of the workpiece stack. The initially formed laser weld joint has an initial top surface adjacent to the top surface of the workpiece stack. In a second step, a laser beam is applied to the laser weld joint, and the laser beam is moved along the initial top surface of the laser weld joint to melt an upper portion of the laser weld joint.The molten upper part of the laser weld joint exhibits the initial upper surface. In a third step, the laser beam is removed from the laser weld joint to allow the upper part of the joint to resolidify, giving the laser weld joint a modified upper surface. The modified upper surface of the laser weld joint is smoother than the original upper surface.

[0009] The workpiece stack of the method of this particular embodiment may include two or three overlapping metal workpieces. In one implementation of the method, each of the two or three overlapping metal workpieces is a steel workpiece. In another implementation of the method, each of the overlapping metal workpieces is an aluminum workpiece. And in yet another implementation of the method, each of the overlapping metal workpieces is a magnesium workpiece. Furthermore, the laser weld joint initially formed in the workpiece stack may be a laser spot weld joint or a laser seam weld joint, regardless of the number of overlapping metal workpieces included in the stack and the composition of those metal workpieces.The initial top surface of the laser weld joint, whether a spot weld joint, a seam weld joint, or another structure, may have a surface roughness (Ra) of 5 µm to 10 µm, and the subsequently derived modified top surface of the laser weld joint, which is lower than that of the initial top surface, may have a surface roughness (Ra) of 0.5 µm to 3 µm.

[0010] The laser beam that impacts the initial upper surface of the laser weld joint and is moved along it to ultimately melt the upper part of the weld joint can be a solid-state laser beam. Such a laser beam can be directed by a remote laser welding device onto the initial upper surface of the laser weld joint and moved along it.

[0011] For example, the laser beam can be moved along the initial upper surface of the laser weld joint at a travel speed of 50 m / min to 130 m / min. During the time the laser beam is moved along the initial upper surface of the laser weld joint, the power level of the laser beam can be between 1 kW and 3 kW, and the focus position of the laser beam can be between 0 mm and -50 mm. The molten upper part of the laser weld joint can, of course, be realized using other types of laser welding devices and different laser beam characteristics. In any case, the upper part of the laser weld joint that is melted by the laser beam can, in certain embodiments, account for between 10 vol% and 30 vol% of the laser weld joint.

[0012] The laser weld joint, which is initially formed within the workpiece stack and welds two or more metallic workpieces together, can be obtained by using a laser beam from the same or a different laser welding device. More specifically, the laser weld joint can be formed by first directing a laser beam onto the top surface of the workpiece stack, creating a molten metal weld pool that penetrates the workpiece stack and intersects the at least one butt interface between the top surface and the bottom surface of the stack. Once the molten metal weld pool is created, a beam spot of the laser beam is advanced relative to the top surface of the workpiece stack along a beam travel pattern to translate the molten metal weld pool along a corresponding distance within the workpiece stack to form the laser weld joint.In this way, the laser weld joint is formed or solidified from a composite workpiece material derived from each of the metal workpieces penetrated by the molten metal weld pool. The laser weld joint can also be formed in keyhole welding mode. A keyhole is created below the laser beam spot. The keyhole is surrounded by the molten metal weld pool. Thus, the keyhole is displaced within the workpiece stack along with the molten metal weld pool as the laser beam spot advances along the beam path pattern.

[0013] Another method for joining metallic workpieces may comprise multiple steps. In a first step, a workpiece stack is provided comprising two or more metallic workpieces that overlap to define a weld zone. The weld zone of the workpiece stack has a top surface and a bottom surface and further forms a butt interface between each pair of adjacent metallic workpieces included in the stack. Any two or more metallic workpieces in the workpiece stack are steel workpieces, aluminum workpieces, or magnesium workpieces. In a second step, a first laser beam is directed onto the top surface of the workpiece stack to create a molten metal weld pool that penetrates the workpiece stack and intersects at least one butt interface between the top surface and the bottom surface of the stack.In a third step, a beam spot of the first laser beam is advanced relative to the top surface of the workpiece stack along a beam travel pattern to displace the molten metal weld pool along a corresponding distance within the workpiece stack to form a laser weld joint consisting of composite material derived or reconsolidated from each of the metal workpieces penetrated by the molten metal weld pool. The initially formed laser weld joint has an initial top surface adjacent to the top surface of the workpiece stack. In a fourth step, the laser weld joint is impinged upon by a second laser beam, and the second laser beam is moved along the initial top surface of the laser weld joint to melt an upper portion of the weld joint including the initial top surface.In a fifth step, the second laser beam is removed from the laser weld joint to allow the upper part of the joint to solidify again, giving the laser weld joint a modified upper surface. The modified upper surface of the laser weld joint is smoother than the original upper surface.

[0014] The aforementioned embodiment of the method for joining metallic workpieces can be further defined. For example, the workpiece stack may comprise two or three overlapping metal workpieces, all of which are steel workpieces, aluminum workpieces, or magnesium workpieces. In a further implementation, the initial top surface of the laser weld joint may have a surface roughness (Ra) of 5 µm to 10 µm, and the subsequently derived modified top surface of the laser weld joint, which is lower than that of the initial top surface, may have a surface roughness (Ra) of 0.5 µm to 3 µm, although there may well be situations where the surface roughness measurements of the initial or modified top surface of the laser weld joint lie outside one or both of these ranges.Furthermore, the second laser beam used to melt the upper part of the laser weld joint may be a solid-state laser beam directed at and moved along the initial upper surface of the laser weld joint by a remote laser welding device. The second laser beam may be moved along the initial upper surface of the laser weld joint at a travel speed of 50 m / min to 130 m / min. During the time the second laser beam is moved along the initial upper surface of the laser weld joint, the power level of the second laser beam may be between 1 kW and 3 kW, and the focus position of the second laser beam may be between 0 mm and -50 mm. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a general illustration of a workpiece stackup comprising overlapping metal workpieces along with a remote laser welding apparatus capable of performing the disclosed method for joining the overlapping metal workpieces; Fig. 1A is an enlarged view of the Fig. 1, showing a focal point and a longitudinal axis of a general laser beam representative of both the first and second laser beams employed in one embodiment of the disclosed method; Fig. 2 is a cross-sectional view of a workpiece stack during the formation of a laser weld joint according to one aspect of the present invention, wherein a first laser beam is directed onto the workpiece stack and maneuvered relative to a top surface of the stack by a scanning optical laser head of a remote laser welding device, and wherein the workpiece stack assembly includes two overlapping metal workpieces; Fig. 3 is a cross-sectional view of a workpiece stack during the formation of a laser weld joint according to another aspect of the present invention, wherein a first laser beam is directed onto the workpiece stack and maneuvered with respect to a top surface of the stack by a scanning optical laser head of a remote laser welding device, and wherein the workpiece stack comprises three overlapping metal workpieces instead of two as in Fig. 2; Fig. 4 is an elevated perspective view of a workpiece stackup showing a laser spot weld joint welding the overlapped workpieces together with a beam path pattern that can be traced by a second laser beam to fuse an upper portion of the laser spot weld joint to an initial upper surface of the laser weld joint, according to one aspect of the present invention, and wherein the beam path pattern shown here consists of a single spiral weld path; Fig. 5 is an elevated perspective view of a workpiece stackup showing a laser seam weld joint welding the overlapped workpieces together with a beam path pattern that can be traced by a second laser beam to fuse an upper portion of the laser seam weld joint to an initial upper surface of the laser weld joint, according to one aspect of the present invention, and wherein the beam path pattern shown therein consists of a single continuous sinusoidal weld path; Fig. Figure 6 is a generalized cross-sectional view of the laser weld joint created by the first laser beam according to one aspect of the present invention, and which is representative of the Fig. 4-5, wherein the laser weld joint has an initial top surface; Fig. 7 is a generalized cross-sectional view of the laser weld joint previously created by the first laser beam according to one aspect of the present invention, along with a second laser beam impinging on the initial upper surface of the laser weld joint and being moved relative to the initial upper surface to melt an upper portion of the laser weld joint including the initial upper surface; and Fig. Figure 8 is a generalized cross-sectional view of the laser weld joint after the molten top portion of the weld joint was allowed to resolidify to give the laser weld joint a modified top surface that is smoother than the initial top surface of the originally formed laser weld joint. DETAILED DESCRIPTION

[0015] The disclosed method for joining two or more stacked metallic workpieces consists of melting an upper portion of an already formed laser weld joint with a laser beam. The molten upper portion has an initial laser weld joint upper surface that may be somewhat rough due to the dynamics created by the displacement of a molten metal weld pool through the workpiece stack and the solidification of the resulting molten workpiece material. Upon solidification, the molten upper portion of the laser weld joint settles, and the surface tension of the instantaneously present molten metal results in a modified laser weld joint upper surface that is smoother than the original upper surface.By providing the laser weld joint with a smoother, modified top surface, which is essentially the exposed surface of the laser weld joint adjacent to a top surface of the workpiece stack, residual stress concentration points that can be prone to crack initiation and propagation are removed, and the laser weld joint is less susceptible to damage from sealing strips that may be applied in close proximity to the joint. The smoother, modified top surface also gives the laser weld joint a more aesthetic appearance.

[0016] The formation of the laser weld joint and the subsequent melting of the upper part of the weld joint, including its upper surface, can be performed separately by any type of laser welding device, such as a remote laser welding device or a conventional laser welding device. The laser beam used to form the laser weld joint and the laser beam used to provide the joint with a smoother modified surface can each be a solid-state laser beam or a gas laser beam, depending on the properties of the metallic workpieces to be joined and the laser welding mode (conduction, keyhole, etc.) to be used.Some notable solid-state lasers that may be used are a fiber laser, a disk laser, a direct diode laser, and an Nd:YAG laser. One notable gas laser that may be used is a CO2 laser, although other types of lasers can certainly be used. In a preferred implementation of the disclosed method, described in more detail below, a remote laser welding device is employed to both form the laser weld joint and immediately thereafter melt the top of the laser weld joint. In this context, the terms "first laser beam" and "second laser beam" are used herein to identify (1) the laser beam that forms the original laser weld joint and (2) the laser beam that melts the top of the previously formed laser weld joint, although the same welding device emits each of these laser beams.

[0017] The disclosed method for joining two or more metallic workpieces can be performed on a variety of workpiece stack configurations. For example, the disclosed method can be used in conjunction with a "2T" workpiece stack ( Fig. 1 - 2) which has two overlapping metal workpieces, or it can be used in conjunction with a “3T” workpiece stack ( Fig. 3) having three overlapping metal workpieces. Furthermore, in some cases, the disclosed method may be used in conjunction with a "4T" workpiece stack (not shown) having four overlapping metal workpieces. The two or more metallic workpieces included in the workpiece stack may be any of steel, aluminum, or magnesium, and they do not necessarily have to have the same composition or thickness as the others in the stack. The disclosed method is performed in substantially the same manner to achieve the same results whether the workpiece stack has two overlapping metal workpieces or more than two overlapping metal workpieces. Differences in workpiece stack configurations can be readily accommodated by adjusting the characteristics of the laser beams employed.

[0018] With general reference now to Fig. 1, a workpiece stack 10 is shown, in which the stack 10 includes at least a first metal workpiece 12 and a second metal workpiece 14 that overlap to define a welding area 16. A remote laser welding device 18 capable of performing the disclosed workpiece joining method is also shown. Within the boundaries of the welding area 16, the first and second metal workpieces 12, 14 form an upper surface 20 and a lower surface 22 of the workpiece stack 10. The upper surface 20 of the workpiece stack 10 is made available to the remote laser welding device 18 and is accessible by a laser beam 24 emanating from the remote laser welding device 18. Furthermore, since only one-sided access is required for laser welding, the need to similarly expose the lower surface 22 of the workpiece stack 10 is eliminated.The terms “upper surface” and “lower surface” as used herein are relative terms that identify the surface of the stack 10 (upper surface) that is closer to and faces the remote laser welding device 18 and the surface of the stack 10 (lower surface) that faces in the opposite direction.

[0019] The workpiece stack 10 may comprise only the first and second metallic workpieces 12, 14, as shown in the Fig. 1-2. Under these circumstances and as described in Fig. 2, the first metal workpiece 12 has an outer outer surface 26 and a first abutment surface 28, and the second metal workpiece 14 has an outer outer surface 30 and a second abutment surface 32. The outer outer surface 26 of the first metal workpiece 12 forms the upper surface 20 of the workpiece stack 10, and the outer outer surface 30 of the second metal workpiece 14 forms the opposite lower surface 22 of the stack 10. And because the two metal workpieces 12, 14 are the only workpieces present in the workpiece stack 10, the first and second abutment surfaces 28, 32 of the first and second metal workpieces 12, 14 overlap within the weld area 16 and oppose each other to form a butt interface 34. In further embodiments, one of which is described below in connection with Fig. 3, the workpiece stack 10 may include an additional third metal workpiece disposed between the first and second metal workpieces 12, 14 to provide the stack 10 with three metal workpieces instead of two.

[0020] The term "butt interface" is used broadly in the present invention and is intended to encompass a wide range of overlapping relationships between the opposing first and second butt surfaces 28, 32 of the first and second metal workpieces 12, 14 that may accommodate laser welding practices. For example, the butt surfaces 28, 32 may establish the butt interface 34 by being in direct or indirect contact. The butt surfaces 28, 32 are in direct contact with each other upon physical contact and are not separated by a discrete intervening layer of material or gaps that are outside of normal assembly tolerance ranges.The abutting surfaces 28, 32 are in indirect contact when separated by a discrete intervening layer of material such as a sealant or adhesive—and thus are not subject to the type of interfacial engagement typical of direct contact—but are in sufficiently close proximity suitable for laser welding. As another example, the abutting surfaces 28, 32 may establish the abutting interface 34 by being separated by predetermined gaps. Such gaps may be imposed between the abutting surfaces 28, 32 by creating protruding features on one or both of the abutting surfaces 28, 32 by laser scribing, mechanical denting, or otherwise. The protruding features maintain intermittent contact points between the abutting surfaces 28, 32 that keep the surfaces 28, 32 apart by up to 1.0 mm outside and around the contact points.

[0021] With reference to Fig. 3, the first metallic workpiece 12 includes a first base metal substrate 36, and the second metallic workpiece 14 includes a second base metal substrate 38. The first and second base metal substrates 36, 38 may all be made of steel, aluminum, or magnesium, i.e., the first and second base metal substrates 36, 38 are both made of steel, both of aluminum, or both of magnesium. At least one of the first or second base metal substrates 36, 38 may include a face coating 40. The face coating(s) 40 may be used on one or both of the base metal substrates 36, 38 for various reasons, including corrosion protection, strength enhancement, and / or to improve processing, and the composition of the coating(s) 40 is largely based on the composition of the underlying base metal substrates 36, 38.Taking into account the thickness of the base metal substrates 36, 38 and their optional surface coatings 40, each of a thickness 121 of the first metal workpiece 12 and a thickness 141 of the second metal workpiece 14 is preferably between 0.4 mm and 6.0 mm at least within the welding area 16. The thicknesses 121, 141 of the first and second metal workpieces 12, 14 may be the same or different.

[0022] Each of the first and second base metal substrates 36, 38 may be coated with a surface coating 40 as shown here in Fig. 2. The face coatings 40, in turn, provide the metal workpieces 12, 14 with their respective outer surfaces 26, 30 and their respective abutment surfaces 28, 32. In another embodiment, only the first base metal substrate 36 has a face coating 40, while the second metal substrate 36 is uncoated or bare. Under these circumstances, the face coating 40 covering the first base metal substrate 36 provides the first metal workpiece 12 with its outer face and abutment surfaces 26, 28, while the second base metal substrate 38 provides the second metal workpiece 14 with its outer face and abutment surfaces 30, 32. In yet another embodiment, only the second base metal substrate 38 has the face coating 40, while the first base metal substrate 36 is uncoated or bare.Consequently, in this case, the first base metal substrate 36 provides the first metal workpiece 12 with its outer facing and abutting surfaces 26, 28, while the surface coating 40 covering the second base metal substrate 38 provides the second metal workpiece 14 with its outer facing and abutting surfaces 30, 32.

[0023] The base metal substrates 36, 38 may take a variety of metal forms and compositions belonging to the broadly defined base metal groups of steel, aluminum, and magnesium. For example, when made of steel, each of the base metal substrates 36, 38 (currently referred to as the first and second base metal substrates 36, 38) may separately be made of a variety of steels, including a low carbon (mild) steel, an interstitial free (IF) steel, a bake hardening steel, high strength low alloy (HSLA) steel, two phase (DP) steel, complex phase (CP) steel, martensitic (MART) steel, transformation induced plasticity (TRIP) steel, twinning induced plasticity (TWIP) steel, and boron steel, such as when the workpiece(s) 12, 14 comprise press hardened (PHS) steel.Furthermore, each of the first and second base steel substrates 36, 38 may have been treated to obtain a specific set of mechanical properties, including heat treatment processes such as annealing, quenching, and / or tempering. The first and second base steel substrates 36, 38 may be hot- or cold-rolled to their final thickness and may be prefabricated to obtain a specific profile suitable for assembly into the workpiece stack 10.

[0024] The surface coating 40 present on one or both of the base steel substrates 36, 38 is preferably composed of a zinc-based material or an aluminum-based material. Some examples of a zinc-based material are zinc or a zinc alloy, such as a zinc-nickel alloy or a zinc-iron alloy. A particularly preferred zinc-iron alloy that may be used has an average mass composition comprising 8 wt.% to 12 wt.% iron and 0.5 wt.% to 4 wt.% aluminum, with the remainder (in wt.%) being zinc. A coating of a zinc-based material may be applied by hot-dip galvanizing (hot-dip galvanized zinc layer), electrogalvanizing (electrodeposition of zinc-iron alloy), or electroplating (electrodeposition of zinc-iron alloy), typically in a thickness of 2 µm to 50 µm, although other methods and thicknesses of the resulting coating(s) may be employed.Some examples of suitable aluminum-based materials include aluminum, an aluminum-silicon alloy, an aluminum-zinc alloy, and an aluminum-magnesium alloy. A coating of an aluminum-based material may be applied by dip coating, typically in a thickness of 2 µm to 30 µm, although other coating methods and thicknesses of the resulting coating(s) may be used. Taking into account the thicknesses of the base steel substrates 36, 38 and their surface coating(s) 40, if any, the total thickness of each of the first and second steel workpieces 12, 14 is preferably in the range of 0.4 mm to 4.0 mm, or more narrowly from 0.5 mm to 2.0 mm at least through the weld area 16.

[0025] If the first and second base metal substrates 36, 38 are made of aluminum, each of the base metal substrates 36, 38 (currently referred to as the first and second base metal substrates 36, 38) may separately be made of unalloyed aluminum or an aluminum alloy comprising 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. Additionally, each of the base aluminum substrates 36, 38 may be provided separately in wrought or cast form. For example, each of the base aluminum substrates 36, 38 may be formed from 4xxx, 5xxx, 6xxx, or 7xxx series wrought aluminum alloy sheet, extruded, forged, or other machined article, or a 4xx.x, 5xx.x, or 7xx series aluminum alloy casting.x. Some more specific types of aluminum alloys that can be used as the first and / or second base aluminum 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-cast alloy. The first and / or second base aluminum substrates 36, 38 can be used in a variety of hardnesses, including annealed (O), work-hardened (H), and solution-treated (T).

[0026] The face coating 40 present on one or both of the aluminum base substrates 36, 38 may be a native refractory oxide coating composed of aluminum oxide compounds that forms passively when fresh aluminum from the aluminum base substrate 36, 38 is exposed to ambient air or another oxygen-containing medium. The face coating 40 may also be a metallic coating composed of zinc or tin, or it may be a metal oxide conversion coating composed of oxides of titanium, zirconium, chromium, or silicon, as disclosed in U.S. patent application Ser. No. 2014 / 0360986A1. A typical thickness of the face coating 40, if present, is between 1 nm and 10 µm, depending on the composition of the coating 40 and the manner in which the coating 40 is derived, although other thicknesses may be used.For example, passively formed refractory oxide coatings often have thicknesses in the range of 2 nm to 10 nm when the underlying aluminum material is an aluminum alloy. Considering the thicknesses of the aluminum base substrates 36, 38 and their surface coating(s) 40, if any, the total thickness of each of the first and second aluminum workpieces 12, 14 is preferably in the range of 0.4 mm to 6.0 mm, or more narrowly, 0.5 mm to 3.0 mm, at least through the weld area 16.

[0027] If the first and second base metal substrates 36, 38 are made of magnesium, each of the base metal substrates 36, 38 (currently referred to as the first and second base metal substrates 36, 38) may separately be 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 base magnesium substrates 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 substrates 36, 38 may be separately provided in wrought (sheet, extrusion, forging, or other machined article) or cast form.Some specific examples of magnesium alloys that can be used as the first and / or second base magnesium substrates 36, 38 include AZ91D die-cast or wrought magnesium alloy (extruded or clad), AZ31B die-cast or extruded (extruded or clad) magnesium alloy, and AM60B die-cast magnesium alloy. The first and / or second base magnesium substrates 36, 38 can be used in a variety of tempers, including annealed (O), work-hardened (H), and solution-treated (W).

[0028] The face coating 40 present on one or both of the base magnesium substrates 36, 38 may be a native refractory oxide coating composed of magnesium oxide compounds (and possibly magnesium hydroxide compounds) that forms passively when fresh magnesium from the base magnesium substrate 36, 38 is exposed to ambient air or another oxygen-containing medium. The face coating 40 may also be a metallic conversion coating composed of metal oxides, metal phosphates, or metal chromates. A typical thickness of the face coating 40, if present, is between 1 nm and 10 µm, depending on the composition of the coating 40 and the manner in which the coating 40 is derived, although other thicknesses may be used. For example, passively formed refractory oxide coatings often have thicknesses in the range of 2 nm to 10 nm when the underlying magnesium material is a magnesium alloy.Taking into account the thicknesses of the base magnesium substrates 36, 38 and their surface coating(s) 40, if any, the total thickness of each of the first and second magnesium workpieces 12, 14 is preferably in the range of 0.4 mm to 6.0 mm, or more narrowly 0.5 mm to 3.0 mm, at least through the weld area 16.

[0029] The Fig. 1 - 2 illustrate an embodiment of the workpiece stack 10 having two overlapping metal workpieces 12, 14 forming a single butt interface 34. Of course, as in Fig. 3, the workpiece stack 10 may include an additional third metal workpiece 150 having a thickness 151 located between the first and second metal workpieces 12, 14. The third metal workpiece 150, if present, includes a third base metal substrate 152, which may be bare or coated with a surface coating 40 (as shown). The third metal workpiece 150 is similar in many respects to the first and second metal workpieces 12, 14, and accordingly, the above description of the first and second metal workpieces 12, 14 (in particular, the composition of the base metal substrates, their possible surface coatings, and the workpiece thicknesses) applies entirely to the third metal workpiece 150. The welding area 16 in this embodiment of the workpiece stack 10 is now defined by the perimeter of the common overlap of all of the first, second, and third metal workpieces 12, 14, 150.

[0030] By overlapping stacking the first, second, and third metal workpieces 12, 14, 150 to provide the workpiece stack 10, the third metal workpiece 40 has two abutment surfaces: a third abutment surface 156 and a fourth abutment surface 158. The third abutment surface 156 overlaps and opposes the first abutment surface 28 of the first metal workpiece 12, and the fourth abutment surface 158 overlaps and opposes the second abutment surface 32 of the second metal workpiece 14. Within the welding area 16, the opposing first and third butt surfaces 28, 156 of the first and third metal workpieces 12, 150 form a first butt interface 160, and the opposing second and fourth butt surfaces 32, 158 of the second and third metal workpieces 14, 150 form a second butt interface 162. These butt interfaces 160, 162 are of the same type and include the same attributes as the butt interface 34 described above with reference to Figs. 1-2.Consequently, in this embodiment, the outer surfaces 26, 30 of the flanking first and second metal workpieces 12, 14 still face away from each other in opposite directions and form the upper and lower surfaces 20, 22 of the workpiece stack 10.

[0031] With reference to Fig. 1, the remote laser welding device 18 includes an optical scanning laser head 42. Generally, the scanning optical laser head 42 directs the transmission of the laser beam 24 toward the top surface 20 of the workpiece stack 10 (also the outer surface 26 of the first metallic workpiece 12). The directed laser beam 24 has a beam spot 44, which, as shown in Fig. 1A, the cross-sectional area of ​​the laser beam 24 is 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) that can quickly and precisely carry the laser head 42 to many different preselected locations within the weld area 16 in rapid, programmed sequence. The laser beam 24 used in conjunction with the optical scanning laser head 42 is preferably a solid-state laser beam operating at a wavelength in the near-infrared region (commonly referred to as 700 nm to 1400 nm) of the electromagnetic spectrum. Additionally, the laser beam 24 has a power level capability capable of achieving a power density sufficient to optionally create a keyhole within the workpiece stack 10 upon formation of the laser weld joint.The power density required to create a keyhole within the overlapping metal workpieces 12, 14 (and possibly 150) is typically in the range of 0.5 - 1.5 MW / cm. 2 .

[0032] Some examples of a suitable solid-state laser beam that can be used in conjunction with the remote laser welding device 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 in which the laser gain medium is a glass fiber doped with a rare earth element (e.g., erbium, ytterbium, neodymium, dysprosium, praseodymium, thulium, etc.). A preferred disk laser beam is a diode-pumped laser beam in which the gain medium is a thin laser crystal disk doped with a rare earth element (e.g., an 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.,A single-wavelength laser beam (combined) derived from multiple diodes in which the gain medium consists of multiple semiconductors, such as those based on aluminum gallium arsenide (AlGaAs) or indium gallium arsenide (InGaAs). Laser generators capable of producing each of these laser types and other variants are commercially available. Other solid-state laser beams not specifically mentioned here can, of course, be used.

[0033] The optical scanning laser head 42 has an array of mirrors 46 that maneuver the laser beam 24 and can thus transport the beam spot 44 along the upper surface 20 of the workpiece stack 10 within an operating boundary 48 that at least partially spans the welding area 16. Here, as in Fig. 1, the portion of the upper surface 20 spanned by the operating boundary 48 is referred to as the xy-plane, since 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 optical scanning laser head 42 also has a z-axis focus lens 50 which defines 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 perpendicular to the xy-plane in the Fig. 1. To prevent dirt and debris from compromising the optical system components and the integrity of the laser beam 24, a cover carriage 56 may be disposed beneath the scanning optical laser head 42. The cover carriage 56 protects the assembly of mirrors 46 and the z-axis focus lens 50 from the environment, but allows the laser beam 24 to exit the scanning optical laser head 42 without significant interference.

[0034] The arrangement of mirrors 46 and the z-axis focus lens 50 cooperate during operation of the remote laser welding device 18 to determine the desired movement of the laser beam 24 and its beam spot 44 within the operating boundary 48, as well as the position of the focus point 52 along the longitudinal axis 54 of the beam 24. In particular, the arrangement of mirrors 46 includes a pair of tiltable scanning mirrors 58. Each of the tiltable scanning mirrors 58 is mounted on a galvanometer 60. The two tiltable scanning mirrors 58 can move the position of the beam spot 44, thus changing the point at which the laser beam 24 strikes the top surface 20 of the workpiece stack 10, anywhere in the xy plane of the operating boundary 48 through precisely coordinated tilting movements of the galvanometers 60.Simultaneously, 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 with the correct power density and achieve the desired heat input both instantaneously and over time. All of these optical components 50, 58 can be indexed rapidly in a few milliseconds or less to advance the beam spot 44 of the laser beam 24 relative to the xy plane of the top surface 20 of the workpiece stack 10 along one or more beam path patterns with simple or complex geometry while controlling the position of the focal point 52.

[0035] One feature that distinguishes remote laser welding from other conventional forms of laser welding is the focal length of the laser beam 24. Here, the laser beam 24, as shown in Fig. 1, a focal length 62, which is measured as the distance between the focal point 52 and the last tiltable scanning mirror 58, which intercepts and reflects the laser beam 24 before the laser beam 24 leaves 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 that is typically between 100 µm and 700 µm. The focal length as well as a focal distance 64 are easily adjustable. The term "focal distance" as used herein refers to the distance between the focal point 52 of the laser beam 24 and the upper surface 20 of the workpiece stack 10 along the longitudinal axis 54 of the beam 24, as shown in Fig. 1A. The focal distance 64 of the laser beam 24 is thus zero when the focus point 52 is positioned on the upper surface 20 of the stack 10. Likewise, the focal distance is a positive distance value (+) when the focus point 52 is positioned above the upper surface 20 and a negative distance value (-) when it is positioned below the upper surface 20.

[0036] It should be noted that the Fig. 1 and the accompanying text describing the laser beam 24 are intended to be generally representative to facilitate the description of the remote laser welding apparatus 18. In the practice of the disclosed method for joining metallic workpieces, as described in more detail below, the laser beam 24 is sequentially applied in time as the first laser beam (reference numeral 24' in Fig. 2-3), which originally forms a laser welding joint, and then as a second laser beam (reference numeral 24" in Fig. 7) which melts an upper portion of the laser weld joint to impart a smoother modified upper surface to the joint. To this end, the designation of the laser beams as "first" and "second" below is not necessarily intended to indicate a difference in the type of laser beam—although such differences are not precluded in other alternative embodiments in which the first and second laser beams are transmitted from different devices—but rather to establish the order of transmission of the laser beams and to distinguish between their intended functions (i.e., to form the laser weld joint or to melt the upper portion of the previously formed joint). The laser beam identified by reference numeral 24 is thus an indicator of how each of the first and second laser beams is delivered and controlled by the remote laser welding device 18.

[0037] In the currently disclosed joining process, which now relates to the Fig. 2-3, a laser weld joint 66 is formed in the workpiece stack 10 by briefly melting portions of the metallic workpieces 12, 14 with a first laser beam 24'. To form the laser weld joint 66, the first laser beam 24' is directed from the scanning optical laser head 42 onto the upper surface 20 of the workpiece stack at a predetermined weld location within the weld area 16. The resulting impingement of the first laser beam 24' onto the upper surface 20 of the stack 10 creates a molten metal weld pool 68 within the stack 10 that penetrates into the stack 10 from the upper surface 20 to the lower surface 22 and intersects at least one butt interface. For example, in the embodiment shown in Fig. 2, the molten metal weld pool 68 cuts the butt interface 34 between the metal workpieces 12, 14 and can penetrate completely or partially into the workpiece stack 10. Likewise, the molten metal weld pool 68 cuts the butt interface 34 between the metal workpieces 12, 14 in the Fig. 3, the 3T stacking process will penetrate at least the first butt interface 160 and, in many cases, both butt interfaces 162 and may fully or partially penetrate the workpiece stack 10. A fully penetrating molten metal weld pool 68 will completely penetrate the workpiece stack 10 and breach the bottom surface 22, as shown, while a partially penetrating molten metal weld pool 68 will penetrate to some intermediate depth and thus not breach the bottom surface 22 of the workpiece stack 10.

[0038] The first laser beam 24' also preferably has a power density sufficient to vaporize the workpiece stack 10 directly below the beam spot 44. This vaporization action creates a keyhole 70, also in the Fig. 2-3, 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 penetrates the workpiece stackup 10 from the top surface 20 to the bottom surface 22 and intersects the butt interface 34 (or the first and / or second butt interfaces 160, 162) established between the first and second metal workpieces 12, 14 (or the first, second, and third metal workpieces 12, 150, 14).The keyhole 70 provides a conduit for the first laser beam 24' to deliver energy downward into the workpiece stack 10, thereby enabling a relatively deep and narrow penetration of the molten metal weld pool 68 into the workpiece stack 10 and a relatively small surrounding heat-affected zone. The keyhole 70 may penetrate completely (as shown) or partially into the workpiece stack 10 along with the molten metal weld pool 68.

[0039] When creating the molten metal weld pool 68 and preferably the keyhole 70, the beam spot 44 of the first laser beam 24' is advanced in the forward direction 72 relative to the upper surface 20 of the workpiece stack 10 in the xy plane of the operating boundary 48 along a beam path pattern. The beam path pattern may include one or more weld paths projected onto the upper surface 20, such as one of the patterns illustrated in US 10 953 497 B2, US 2019 / 224 781 A1, US 2020 / 047 285 A1, US 2018 / 304 405 A1, US 2018 / 214 983 A1, and WO 2017 / 035 728 A1, to name just a few possibilities. The advance of the beam spot 44 of the first laser beam 24' along the beam path pattern is controlled by precise control of the coordinated movements of the tiltable scanning mirrors 58 within the optical scanning laser head 42.As the beam spot 44 of the first laser beam 24' advances along the beam path pattern, the molten metal weld pool 68 (along with the keyhole 70, if present) is displaced along a corresponding route within the workpiece stack 10. As a result, the penetrating molten metal weld pool 68 flows around and behind the beam spot 44 within the workpiece stack 10, causing the molten metal weld pool 70 to elongate due to the progression of the first laser beam 24'.

[0040] After the beam spot 44 of the first laser beam 24' has finished tracing the beam path pattern 66, the transmission of the first laser beam 24' is stopped and the workpiece material created by the first laser beam 24' cools and solidifies into a resolidified composite workpiece material 74. The resolidified composite workpiece material 74 is derived from each of the metal workpieces 12, 14 (or 12, 150, 14) penetrated by the molten metal weld pool 68, and its composition (as well as the composition of the previously created molten metal weld pool 68) is determined by the composition of the penetrated metal workpieces.The collective resolidified composite workpiece material 74 obtained from the first laser beam 24' forms the laser weld joint 66, which may extend fully or partially into the workpiece stack 10, depending on whether the molten metal weld pool 68 has fully or partially penetrated the stack 10 and may be surrounded by a heat-affected zone (HAZ). The laser weld joint 66 thus extends into the workpiece stack 10 from the top surface 20 of the stack 10 to the bottom surface 22 while intersecting the butt interface 34 (or the first and / or second butt interfaces 162, 164) to autogenously weld the subject metal workpieces 12, 14 (or 12, 150, 14) together.

[0041] The laser weld joint 66 can take on a variety of shapes and structures depending on the geometry of the beam path pattern followed by the first laser beam 24' along the upper surface 20 of the workpiece stack 10. For example, as shown in Fig. 4, the laser weld joint 66 may be formed as a laser spot weld joint, which is a solidified weld nugget of resolidified composite workpiece material 74, which may be formed by maneuvering the first laser beam 24' along a beam path pattern consisting of a spiral weld path or a series of concentric circular or elliptical weld paths, such that the molten metal weld pool 68 substantially grows into a larger molten pool. In another example, as shown in Fig. 5, the laser weld joint 66 may be constructed as a laser seam weld joint, which is a track of reconsolidated composite workpiece material 74 formed by a single pass of the first laser beam 24' in either a linear weld path (as shown) or a non-linear weld path such as a C-shaped "clamp" path or a single circular or elliptical weld path with a diameter large enough to have a central, unwelded portion within the circular or elliptical path of reconsolidated composite workpiece material 74. Regardless of its shape and structure, the laser weld joint 66 has an initial top surface 76 adjacent the top surface 20 of the workpiece stack 10, as shown in the general and representative cross-sectional view of the laser weld joint 66 in Fig. 6 shown.

[0042] The initial top surface 76 of the laser weld joint 66 tends to be somewhat rough, due at least in part to the turbulence in the molten metal weld pool 68 during the laser welding process and the tendency for gases to be trapped in the weld pool 68. The initial top surface has a surface roughness, measured as the mean or arithmetic average roughness (Ra), which in many, but not necessarily all, cases is in the range of 5 µm to 10 µm. An overly roughened or disturbed initial top surface 76 can have several negative effects on the visual appearance and / or structural integrity of the laser weld joint 66. A rough initial top surface 76 of the laser weld joint 66 can give the impression of a poor joint, even though the weld joint 66 is actually structurally and functionally sound.Furthermore, a rough initial surface 76 can create crack-prone residual stress concentration points, and in particular, stress corrosion cracking, when the joint 66 is subjected to tensile loading in a corrosive environment. Furthermore, a rough initial upper surface 76 of the laser weld joint 66 can damage sealing tapes applied over the joint 66 in later manufacturing steps.

[0043] To remedy the potentially adverse effects of a rough initial upper surface 76 of the laser weld joint 66, the disclosed method provides that the initial upper surface 76 of the laser weld joint 66 is exposed to a second laser beam 24", which is also directed onto the stack 10 by the optical scanning laser head 42, and then the second laser beam 24", and in particular its beam spot 44, melts an upper portion 78 of the laser weld joint 66 along the initial upper surface 76. The melting of the upper part 78 of the laser weld joint 66 by the second laser beam 24" is in Fig. 7. The molten upper portion 78 of the laser weld joint 66 extends partially into the weld joint 66 and consumes the initial upper surface 76 of the weld joint 66, i.e., at least 85% of the initial upper surface 76 is melted, and preferably the entire initial upper surface 76 is melted, with acceptable unmelted portions of the initial upper surface 76, if any, typically being limited to peripheral edge regions along the interface between the weld joint 66 and the surrounding workpiece structure 10. The size and ratio of the molten upper portion 78 of the laser weld joint 66 to the remainder of the joint 66 can vary depending on the size, shape, and structure of the joint 66 as originally formed. In many cases, however, the upper portion 78 of the laser weld joint 66 that is melted by the second laser beam 24" represents between 10 vol.% and 30 vol.-% of the laser welding joint 66.

[0044] The second laser beam 24" may be moved along the initial upper surface 76 of the laser weld joint 66 in a beam path pattern 80 that covers at least the area of ​​the initial upper surface 76. For example, if the laser weld joint 66 is a laser spot weld joint, the beam spot 44 of the second laser beam 24" may be advanced along a beam travel pattern 80 projected onto the initial upper surface 76 consisting of a spiral weld path, as shown in Fig. 4, although other beam wander patterns, such as those comprising a series of concentric circular or elliptical weld paths, may also be used. As another example, if the laser weld joint 66 is a laser seam weld joint, the beam spot 44 of the second laser beam 24" may be advanced along a beam wander pattern 80 projected onto the initial upper surface 76 consisting of a single continuous sinusoidal weld path, as shown in Fig. 5, although other beam travel patterns, such as those having a zigzag weld path or a loop weld path, may also be used. The characteristics of the second laser beam 24" may differ from those of the first laser beam 24" because the second laser beam 24" is only intended to partially melt the laser weld joint 66. In many typical applications, the first laser beam 24" may have a power level of 2 kW to 6 kW and a focus position of +10 mm to -10 mm while being moved along its beam path at a travel speed of 2 m / min to 50 m / min, while the second laser beam 24" may have a power level of 1 kW to 3 kW and a focus position of 0 mm to -50 mm while being moved along its beam path pattern 80 at a travel speed of 50 m / min to 130 m / min.

[0045] After the upper portion 78 of the laser weld joint 66 has melted, the second laser beam 24" is removed from the laser weld joint 66 so that the melted upper portion 78 resolidifies into a shiny cap 82, as shown in Fig. 8. Removing the second laser beam 24" from the laser weld joint 66 may comprise stopping the transmission of the second laser beam 24 or simply moving the second laser beam 24" away from and away from the laser weld joint 66. When allowed to resolidify from a momentary molten state into the shiny cap 82, the upper portion 78 of the laser weld joint 66 settles, and the surface tension of the momentarily present molten metal results in a modified upper surface 84 of the laser weld joint 66 that is smoother than the initial upper surface 76, as generally shown in Fig.8. The modified upper surface 84 has a surface roughness, measured as before as the mean or arithmetic average roughness (Ra), which in many cases, but not necessarily all, ranges from 0.5 µm to 3 µm. The smoother modified upper surface 84 not only has a more aesthetic appearance than the initial upper surface 76 of the laser weld joint 66, but also removes residual stress concentration points that may be prone to crack initiation and propagation, making the joint 66 less susceptible to damage to the applied sealing strips.

Claims

[1] A method for joining metallic workpieces (12, 14, 150), the method comprising: a workpiece stack (10) is provided having three metal workpieces (12, 14, 150) overlapping to define a welding area (16), wherein the welding area (16) of the workpiece stack (10) has a top surface (20) and a bottom surface (22) and further establishes a butt interface (34) between each pair of adjacent metal workpieces (12, 14, 150) included in the workpiece stack (10), and wherein all of the metal workpieces in the workpiece stack (10) are magnesium workpieces; an optical scanning laser head (42) of a remote laser welding device (18) is operated to direct a first laser beam (24') onto the upper surface (20) of the workpiece stack (10) and additionally to move a beam spot (44) of the first laser beam (24') relative to the upper surface (20) of the workpiece stack (10) within the welding area (16) and along a linear beam path pattern (80) to form a keyhole (70) and to displace a surrounding molten metal weld pool (68) along a corresponding path within the workpiece stack (10), wherein a single translation of the keyhole (70) and the surrounding molten metal weld pool (68) creates a linear laser seam weld joint (66) extending into the workpiece stack (10) and intersecting each butt interface (34) formed between the top and bottom surfaces (22) of the workpiece stack (10) to weld the three metallic workpieces (12, 14, 150) together, the laser seam weld joint (66) having an initial top surface (76) adjacent the top surface (20) of the workpiece stack (10); the scanning optical laser head (42) of the remote laser welding device (18) is operated to impinge on the initial upper surface (76) of the laser seam weld joint (66) with a second laser beam (24") and additionally to move the second laser beam (24") sinusoidally along the initial upper surface (76) of the linear laser seam weld joint (66) to melt an upper portion of the laser seam weld joint (66) including the initial upper surface (76) of the laser seam weld joint (66), wherein the upper portion melted by the second laser beam (24") constitutes between 10 vol.% and 30 vol.% of the laser seam weld joint (66); and the second laser beam (24") is removed from the laser seam weld joint (66) to allow the upper portion of the laser seam weld joint (66) to resolidify and to provide the laser seam weld joint (66) with a modified upper surface (84) that is smoother than the initial upper surface (76) of the laser seam weld joint (66) wherein the first laser beam (24') is a solid-state laser beam, wherein the first laser beam (24') is moved relative to the upper surface (20) of the workpiece stack (10) along the beam travel pattern (80) at a travel speed ranging from 2 m / min to 50 m / min, while a power level of the first laser beam (24') ranges from 2 kW to 6 kW and a focus position of the first laser beam (24') ranges from +10 mm above the upper surface (20) of the workpiece stack (10) to -10 mm below the upper surface (20) of the workpiece stack (10); and wherein the second laser beam (24") is a solid-state laser beam, wherein the second laser beam (24") is moved along the initial upper surface (76) of the laser seam weld joint (66) at a travel speed ranging from 50 m / min to 130 m / min, while a power level of the second laser beam (24") ranges from 1 kW to 3 kW and a focus position of the second laser beam (24") ranges from 0 mm to -50 mm below the upper surface (20) of the workpiece stack (10).

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