Method for resistance spot welding of a stack of workpieces comprising a steel workpiece and an aluminum workpiece

Abstract

A method for resistance spot welding of a stack of workpieces (10, 210, 310) comprising a steel workpiece (12, 212, 312) and an aluminum workpiece (14, 214, 314), wherein the method comprises: Attaching an aluminium filler piece (16, 216, 316) to a mating surface (32, 232, 332) of the steel workpiece (12, 212, 312); Positioning the aluminum workpiece (14, 214, 314) over the aluminum filler (16, 216, 316) and the steel workpiece (12, 212, 312) to assemble a workpiece stack (10, 210, 310), wherein the aluminum filler (16, 216, 316) is arranged between a mating surface (56, 256) of the aluminum workpiece (14, 214, 314) and the mating surface (32, 232, 332) of the steel workpiece (10, 210, 310), wherein the workpiece stack (10, 210, 310) has a first side (101) and an opposing second side (103), wherein the first side (101) is formed by an aluminum workpiece surface (101') and the second side (103) by a steel workpiece surface (103') is provided; Pressing a welding surface (64) of a first welding electrode (60) against the first side (101) of the workpiece stack (10, 210, 310); Pressing a welding surface (66) of a second welding electrode (62) against the second side (103) of the workpiece stack (10, 210, 310) in facial alignment with the welding surface (64) of the first welding electrode (60); Conducting an electric current between the welding surface (64) of the first welding electrode (60) and the welding surface (66) of the second welding electrode (62) to generate a molten aluminum weld pool (70) which penetrates through the aluminum filler (16) and into the aluminum workpiece (14, 214, 314), wherein the molten aluminum weld pool (70) consumes a section of the aluminum filler (16, 216, 316) to wet an exposed section (72, 272, 372) of the mating surface (32, 232, 332) of the steel workpiece (12, 212, 312); and The current flow is closed so that the molten aluminum weld pool (70) solidifies into a weld (74, 274, 374) which connects the aluminum workpiece (14, 214, 314) to the steel workpiece (12, 212, 312) through the aluminum filler piece (16, 216, 316), wherein the weld (74, 274, 374) forms a interface (76, 276, 376) with the exposed section (72, 272, 372) of the mating surface (32, 232, 332) of the steel workpiece (12, 212, 312), and wherein a notch root (82, 282, 382) is formed between the aluminum workpiece (14, 214, 314) and the aluminum filler piece (16, 216, 316). 216, 316) is generated and is displaced away from the mating surface (32, 232, 332) of the steel workpiece (12, 212, 312).

Description

introduction

[0001] A number of industries use resistance spot welding to join two or more metallic workpieces during the manufacture of a larger assembly. Resistance spot welding has long been used for fusion welding similarly composed metallic workpieces, such as stacking two or more steel workpieces or two or more aluminum workpieces. More recently, however, resistance spot welding processes have been developed that allow dissimilar metallic workpieces, such as a steel workpiece and an overlapping aluminum workpiece, to be joined.The ability to resistance spot weld stacks of dissimilar metal workpieces, as presented for welding, offers the automotive industry, for example, greater flexibility to use both ferrous and non-ferrous metals within the vehicle structure at specifically tailored locations without significantly increasing overall production complexity. Beyond the automotive industry, other sectors can also benefit from the ability to reliably resistance spot weld stacks of workpieces, including dissimilar metal components. These other sectors can include aerospace, marine, railway, construction, and industrial equipment, to name just a few.

[0002] Resistance spot welding is a metal joining process that relies on the instantaneous passage of an electric current through overlapping metallic workpieces to heat and join them at a weld point. To perform this welding process, two opposing welding electrodes are clamped at aligned points on opposite sides of a stack of workpieces, and an electric current is passed through the two or more overlapping metal workpieces between the opposing electrode welding surfaces. The resistance to the flow of this electric current generates heat within the metallic workpieces and at their interface. The resistively generated heat is produced rapidly and is sufficiently concentrated to melt one or more of the overlapping metal workpieces.If the workpiece stack includes a steel workpiece and an adjacent, overlapping aluminum workpiece, the heat generated at the interface between these two workpieces and within the electrically and thermally more resistant steel workpiece creates a molten aluminum weld pool within the aluminum workpiece. This molten aluminum weld pool does not consume the interface between the steel and aluminum workpieces, but rather expands and wets the adjacent mating surface of the steel workpiece. Finally, upon cooling, the molten aluminum weld pool solidifies into a weld seam that joins or brazes the steel and aluminum workpieces at their interfaces.

[0003] When adjacent steel and aluminum workpieces are joined by resistance spot welding, the elevated temperature reached in the steel workpiece and its exposure to the molten aluminum weld pool lead to the formation of Fe-Al intermetallic compounds. The solidified weld thus generally contains an intermetallic Fe-Al layer that is located along and adjacent to the mating surface of the steel workpiece. This layer is hard and brittle, especially compared to the softer and tougher aluminum nugget portion of the weld, which penetrates and often passes through the aluminum workpiece and constitutes the majority of the joint.If stress is transferred to the intermetallic Fe-Al layer when either the steel or aluminum workpiece, or both, are deformed, or when the weld is otherwise subjected to stress, the brittle intermetallic Fe-Al layer can crack and fracture, leading to interface failure of the joint and correspondingly low strength properties—especially in peel and transverse tensile tests—even though the joint may otherwise be structurally sound. Resistance spot welding processes, which can help isolate and protect the intermetallic Fe-Al layer while minimizing oxide layer defects, would therefore reliably contribute to producing welds with good mechanical properties.

[0004] DE 10 2012 020 223 A1 describes a method for joining at least two components made of different metallic materials, wherein the components are welded together using a welding aid. The method described therein comprises the following steps: providing a first component, a second component, and at least one welding aid; joining the welding aid to the first component by friction welding; and joining the second component to the first component by resistance welding, wherein the welding aid, which is firmly connected to the first component, serves as the joining partner for the second component, thus creating a strong connection between the components. Furthermore, a component composite produced by this method and a welding aid suitable for this purpose are described.

[0005] US Patent 2017 / 0297137A1 describes a method for joining an aluminum workpiece and an adjacent, overlapping steel workpiece by reaction metallurgical joining and the resulting metallurgical bond formed between the two workpieces. The method involves compressing a reaction material located between the aluminum and steel workpieces and briefly heating the reaction material to form a metallurgical bond that includes a bonding interface between the reaction material and the steel workpiece and a bonding interface between the reaction material and the aluminum workpiece. The reaction material is formulated to interact with both aluminum and steel to create the interfaces of the metallurgical bond.Furthermore, the reaction material can be applied to the steel workpiece using arc welding with an oscillating wire before the steel and aluminum workpieces are joined together to form a stack of workpieces. Description of the invention

[0006] The invention is defined by the claims.

[0007] A method for resistance spot welding a stack of workpieces, comprising a steel workpiece and an aluminum workpiece according to an embodiment of the present disclosure, includes several steps. In one step, an aluminum filler is bonded to a mating surface of a steel workpiece. In another step, an aluminum workpiece is positioned over the aluminum filler and the steel workpiece to form a stack of workpieces in which the aluminum filler is located between a mating surface of the aluminum workpiece and the mating surface of the steel workpiece. The stack of workpieces has a first side and an opposing second side. The first side is provided by an aluminum workpiece surface and the second side by a steel workpiece surface. In a further step, a welding surface of a first welding electrode is pressed against the first side of the stack of workpieces.In a further step, the welding surface of a second welding electrode is pressed against the second side of the workpiece stack in a facial orientation with the welding surface of the first welding electrode. In yet another step, an electric current is passed between the welding surface of the first welding electrode and the welding surface of the second welding electrode to create a molten aluminum weld pool that penetrates the aluminum filler and into the aluminum workpiece. The molten aluminum weld pool consumes a portion of the aluminum filler to wet an exposed portion of the mating surface of the steel workpiece. In a further step, the electric current is stopped, allowing the molten aluminum weld pool to solidify into a weld bead that joins the aluminum workpiece to the steel workpiece through the aluminum filler.The weld seam forms a connection interface with the exposed section of the mating surface of the steel workpiece, and additionally a notch root is created between the aluminium workpiece and the aluminium filler piece and shifted away from the mating surface of the steel workpiece.

[0008] The aforementioned method can include additional steps or be further defined. For example, the aluminum infill piece can be made of unalloyed aluminum, an aluminum-manganese alloy containing up to 1.8 wt.% manganese, or an aluminum-silicon alloy containing up to 15 wt.% silicon. Alternatively, the aluminum infill piece can be an aluminum disc with a first facial surface and an opposing second facial surface. Bonding the aluminum disc to the steel workpiece can involve pre-welding the aluminum disc to the steel workpiece to create a load-bearing weld that joins the aluminum disc to the steel workpiece. The aluminum disc can be made of unalloyed aluminum, an aluminum-manganese alloy containing up to 1.8 wt.% manganese, or an aluminum-silicon alloy containing up to 15 wt.% silicon.Furthermore, the molten aluminum pool generated by the electric current can penetrate the load-bearing weld, so that the resulting weld is surrounded by an unconsumed section of the load-bearing weld during hardening. The interface formed between the weld and the exposed portion of the mating surface of the steel workpiece is surrounded by a connection interface formed between the unconsumed portion of the load-bearing weld and the mating surface of the steel workpiece outside the weld. A notch root can be created between the aluminum disc and the steel workpiece outside and adjacent to the load-bearing weld.

[0009] In various implementations of the aforementioned process, an outer surface of the steel workpiece can form the steel workpiece surface on the second side of the workpiece stack, and an outer surface of the aluminum workpiece can form the aluminum workpiece surface on the first side of the workpiece stack. Furthermore, the aluminum filler can be an aluminum coating. Bonding the aluminum coating to the mating surface of the steel workpiece can involve applying the aluminum coating directly to the mating surface of the steel workpiece.In another implementation, the aluminum filler piece can be an aluminum disc having a first facial surface and an opposing second facial surface, and the adhesion of the aluminum disc to the mating surface of the steel workpiece comprises (1) the application of a curable adhesive layer between the second facial surface of the aluminum disc and the mating surface of the steel workpiece, and thereafter (2) the curing of the curable adhesive layer to bond the second facial surface of the aluminum disc to the mating surface of the steel workpiece after completion of the current conduction in order to solidify the molten aluminum weld pool into the weld seam joining the aluminum workpiece to the steel workpiece.

[0010] A method for resistance spot welding a stack of workpieces, comprising a steel workpiece and an aluminum workpiece according to a further embodiment of the present disclosure, includes several steps. In one step, an aluminum disc is pre-welded to a steel workpiece to form a load-bearing weld seam within the aluminum disc, which forms a connecting interface with a mating surface of the steel workpiece. In a further step, an aluminum workpiece is positioned over the aluminum disc and the steel workpiece to form a stack of workpieces in which the aluminum disc is arranged between a mating surface of the aluminum workpiece and the mating surface of the steel workpiece. The stack of workpieces has a first side and an opposing second side. The first side is provided by an aluminum workpiece surface and the second side by a steel workpiece surface.In a further step, the welding surface of a first welding electrode is pressed against the first side of the workpiece stack. In a further step, the welding surface of a second welding electrode is pressed against the second side of the workpiece stack in a facial orientation with the welding surface of the first welding electrode. In yet another step, an electric current is passed between the welding surface of the first welding electrode and the welding surface of the second welding electrode to create a molten aluminum weld pool that penetrates the aluminum disc within the load-bearing weld and into the aluminum workpiece. The molten aluminum weld pool consumes a portion of the load-bearing weld, including a portion of the interface of the load-bearing weld, to wet an exposed portion of the mating surface of the steel workpiece.In a further step, the flow of the electric current is stopped, causing the molten aluminum weld pool to solidify into a weld seam that joins the aluminum workpiece to the steel workpiece via the aluminum washer. The weld seam is surrounded by an unconsumed section of the load-bearing weld and forms a junction with the exposed portion of the mating surface of the steel workpiece. Additionally, a notch is created between the aluminum workpiece and the aluminum washer and pushed away from the mating surface of the steel workpiece.

[0011] The process of the aforementioned embodiment can include additional steps or be further defined. For example, the pre-welding of the aluminum disc to the steel workpiece can comprise several steps. In one step, the aluminum disc is placed against the mating surface of the steel workpiece. The aluminum disc has a first facial surface and a second facial surface, the second of which contacts the mating surface of the steel workpiece. In a further step, a welding surface of a first welding electrode is pressed against the first facial surface of the aluminum disc. In yet another step, a welding surface of a second welding electrode is pressed against an outer surface of the steel workpiece in a facial orientation with the welding surface of the first welding electrode.In yet another embodiment, an electric current is passed between the welding surface of the first welding electrode and the welding surface of the second welding electrode, and through the steel workpiece and the aluminum disc, to create a molten aluminum weld pool within the aluminum disc. This molten aluminum weld pool wets the mating surface of the steel workpiece. In a further step, the flow of the electric current is stopped, allowing the molten aluminum weld pool generated within the aluminum disc to solidify in the load-bearing weld seam.

[0012] The aluminum disc used in the aforementioned process can be made of unalloyed aluminum, an aluminum-manganese alloy containing up to 1.8 wt.% manganese, or an aluminum-silicon alloy containing up to 15 wt.% silicon. Furthermore, a notch root can be created between the aluminum disc and the steel workpiece outside and adjacent to the load-bearing weld. In some implementations, an outer surface of the steel workpiece can form the steel workpiece surface on the second side of the workpiece stack, and an outer surface of the aluminum workpiece can form the aluminum workpiece surface on the first side of the workpiece stack.

[0013] A stack of workpieces according to yet another embodiment of the present disclosure comprises several features. The stack of workpieces includes a steel workpiece with a mating surface, an aluminum workpiece that overlaps the steel workpiece and has a mating surface opposite the mating surface of the steel workpiece, an aluminum filler piece arranged between the steel workpiece and the aluminum workpiece, and a weld seam that joins the aluminum and steel workpieces together. The weld seam forms a connection interface with the mating surface of the steel workpiece and extends through the aluminum filler piece into the aluminum workpiece. In addition, the aluminum filler piece is bonded to the mating surface of the steel workpiece outside and around the weld seam.In one implementation, the aluminum infill piece is an aluminum disc welded to the steel workpiece by an unconsumed portion of a load-bearing weld that extends into the aluminum disc and surrounds the weld. The unconsumed portion of the load-bearing weld forms a junction with the mating surface of the steel workpiece, surrounding the interface established between the weld and an exposed portion of the mating surface of the steel workpiece. Brief description of the drawings Fig. Figure 1 is an increased cross-sectional view of a steel workpiece and an aluminium filler in the form of an aluminium disc during pre-welding of the disc to the steel workpiece by a pair of welding electrodes according to an embodiment of the present disclosure; Fig. 2 is an elevated cross-sectional view of the steel workpiece and the in Fig. 1. Aluminium disc shown, after they have been welded together, together with an aluminium workpiece positioned relative to the steel workpiece such that the aluminium disc is arranged between the aluminium workpiece and the steel workpiece to provide a stack of workpieces according to an embodiment of the present disclosure; Fig. 3 is an elevated cross-sectional view of the in Fig. 2 the workpiece stack shown together with a pair of welding electrodes which, during resistance spot welding of the stack, conduct an electric current through the workpiece stack in order to form a weld seam which joins the steel and aluminium workpieces together through the aluminium disk according to an embodiment of the present disclosure; Fig. 4 is an elevated cross-sectional view of the in Fig. 3 of the stack of workpieces shown, after the weld seam joining the steel and aluminium workpieces has been formed and the welding electrodes have been removed from the stack according to an embodiment of the present disclosure; Fig. 5 is a partially enlarged cross-sectional view of the in Fig. 4 of the workpiece stack shown, which shows a notch root formed around the weld seam between the aluminium disc and the aluminium workpiece according to an embodiment of the present disclosure; Fig. Figure 6 is an increased cross-sectional view of a steel workpiece and an aluminum filler in the form of an aluminum coating applied to a mating surface of the steel workpiece and metallurgically connected to it according to a further embodiment of the present disclosure; Fig. Figure 7 is a partially enlarged cross-sectional view of a workpiece stack, which includes the steel workpiece and the aluminum coating, as shown in Fig. Figure 6 shows another aluminium workpiece which is positioned relative to the steel workpiece such that the aluminium coating is arranged between the steel and aluminium workpieces, the stack of workpieces being shown here after spot welding and showing the notch base formed around the weld seam between the aluminium coating and the aluminium workpiece; Fig. Figure 8 is an elevated cross-sectional view of a stack of workpieces comprising a steel workpiece, an aluminum disc bonded to a mating surface of the steel workpiece by a cured adhesive layer, and an aluminum workpiece positioned relative to the steel workpiece such that the aluminum disc is positioned between the aluminum workpiece and the steel workpiece according to a further embodiment of the present disclosure; and Fig. Figure 9 is a partially enlarged cross-sectional view of the [structure]. Fig. Figure 8 shows the stack of workpieces after spot welding and shows the notch root created around the weld seam between the aluminum disc and the aluminum workpiece, as well as the notch root created around the weld seam between the aluminum disc and the steel workpiece. Detailed description

[0014] When resistance spot welding a stack of workpieces consisting of a steel workpiece and an opposing adjacent aluminum workpiece, a weld or braze joining these two adjacent workpieces can be made more uniform in its mechanical properties by using an interposed piece of aluminum to create a notch root between the aluminum piece and the aluminum workpiece above it. This notch root surrounds the weld and is pushed away from the mating surface of the steel workpiece.And, as explained in more detail below, precautions can be taken to divert stress to the notch root created between the aluminum filler and the aluminum workpiece when the weld is loaded. This, in turn, shields the intermetallic Fe-Al layer formed along a weld interface and the steel workpiece from some or all of the stress, thus preventing the formation of the intermetallic Fe-Al layer and premature failure of the joint. The aluminum filler also allows for the joining of a wider variety of aluminum workpieces to the steel workpiece than would otherwise be possible without it.

[0015] The aluminum infill piece is bonded to a mating surface of the steel workpiece, which is opposite the aluminum workpiece in the assembled and welded workpiece stack. The aluminum infill piece can, for example, be an aluminum disc connected to the mating surface of the steel workpiece by a load-bearing weld formed during pre-welding. In another example, the aluminum infill piece can be an aluminum disc bonded to the mating surface by a cured adhesive layer that cures after spot welding. Yet another example is the aluminum infill piece being an aluminum coating metallurgically bonded to the mating surface of the steel workpiece. Coating processes such as thermal spraying and cold spraying can be used to deposit the aluminum coating.By adhering the aluminum filler piece to the steel workpiece either before, during, or after spot welding the workpiece stack, the weld resulting from the spot welding process is surrounded by an unconsumed portion of the filler piece that adheres to the mating surface of the steel workpiece. This unconsumed portion of the aluminum filler piece insulates and protects the intermetallic Fe-Al layer of the weld and dissipates the stress exerted on the joint toward the notch root created between the aluminum filler piece and the overlying aluminum workpiece. Dissipating such stress at the notch root created between the aluminum filler piece and the aluminum workpiece is preferable because the notch root is generally displaced and does not cause cracking in the intermetallic Fe-Al layer of the weld.

[0016] A number of workpiece stacking configurations can be resistance spot welded using the aluminum filler piece according to the present disclosure. For example, the workpiece stacking method can involve only one steel workpiece and one adjacent, overlapping aluminum workpiece, with the aluminum piece being positioned between the opposing mating surfaces of the steel and aluminum workpieces. As another example, the workpiece stacking method can involve one steel workpiece and a plurality of aluminum workpieces, as long as the multiple aluminum workpieces are positioned side by side, or it can involve one aluminum workpiece and a plurality of steel workpieces, as long as the multiple steel workpieces are positioned side by side.In both scenarios, the workpiece stack can be a "3T" stack, containing one steel workpiece and two aluminum workpieces (steel-Al-Al) or one aluminum workpiece and two steel workpieces (Al-steel-steel), or the stack can be a "4T" stack, containing one steel workpiece and three aluminum workpieces (steel-Al-Al-Al), one aluminum workpiece and three steel workpieces (Al-steel-steel-steel), or two steel workpieces and two aluminum workpieces (steel-steel-Al-Al). If there is more than one aluminum workpiece and / or more than one steel workpiece in the stack, the aluminum filler piece is placed between the adjacent steel and aluminum workpieces, overlapping and facing each other.

[0017] With reference to the Fig. Sections 1-5 describe a first embodiment of a workpiece stack 10 according to the practices of the present disclosure. The workpiece stack 10, which is described in Fig. Figure 2, shown in the assembled state, comprises a steel workpiece 12, an aluminum workpiece 14 that overlaps and faces the steel workpiece 12, and an aluminum filler piece 16 that is positioned between the steel workpiece 12 and the aluminum workpiece 14. While the steel workpiece 12 and the aluminum workpiece 14 are shown here in a "2T" stack for demonstration purposes, it should be understood that one or more additional steel workpieces can be included in the stack 10 next to the steel workpiece 12 (away from the aluminum workpiece 14) and / or one or more aluminum workpieces can be included in the stack 10 next to the aluminum workpiece 14 (away from the steel workpiece 12).The additional steel and / or aluminum workpiece(s) do not necessarily affect the relationship between the opposing steel workpiece(s) and the aluminum workpieces 12, 14, which are joined by a weld extending from the steel workpiece 12 and into the aluminum workpiece 14 through the aluminum path 16, as explained in more detail below. Accordingly, the following description, as illustrated in the context of the "2T" workpiece stacking diagram, also applies to stacks containing additional steel and / or aluminum workpieces positioned outside the opposing steel and aluminum workpieces 12, 14, even if such additional workpieces are not shown.

[0018] The steel workpiece 12 comprises a steel substrate of various thicknesses and grades, which is either coated or uncoated. The steel substrate can be a hot-rolled or cold-rolled sheet and can consist of steel such as low-carbon (structural) steel, interstitial-free steel, flame-hardenable steel, high-strength low-alloy (HSLA) steel, dual-phase (DP) steel, complex-phase (CP) steel, martensitic (MART) steel, transformed plasticity steel (TRIP) steel, twin-induced plasticity steel (TWIP) steel, and boron steel, as is the case, for example, when the steel workpiece 12 comprises press-hardened steel (PHS). Preferred compositions of the steel substrate are structural steel, dual-phase steel, and boron steel, which are used in the production of pre-hardened steel. These three types of steel exhibit the highest tensile strengths, reaching from 150 MPa to 500 MPa, from 500 MPa to 1100 MPa and from 1200 MPa to 1800 MPa respectively.In its coated state, the steel substrate preferably comprises a surface layer of zinc (e.g., hot-dip galvanized), a zinc-iron alloy (e.g., electroplated or electro-deposited), a zinc-nickel alloy (e.g., electro-deposited), nickel, aluminum, an aluminum-magnesium alloy, an aluminum-zinc alloy, or an aluminum-silicon alloy, each of which may have a thickness of up to 50 µm on each side of the steel substrate. Taking into account the thickness of the steel substrate and any surface layer present, the steel workpiece 12 has a thickness 121 ranging from 0.3 mm to 6.0 mm or, more narrowly, from 0.6 mm to 2.5 mm. All additional steel workpieces included in the stack have the same general description.

[0019] The aluminum workpiece 14 includes an aluminum substrate that is either coated or uncoated. The aluminum substrate can consist of unalloyed aluminum or an aluminum alloy containing at least 85 wt.% aluminum. Some notable aluminum alloys that can form the coated or uncoated aluminum substrate are an aluminum-magnesium alloy, an aluminum-silicon alloy, an aluminum-magnesium-silicon alloy, and an aluminum-zinc alloy. If the aluminum substrate is coated, it may include a surface layer consisting of a refractory oxide material, such as the native oxide layer that forms naturally when the aluminum substrate is exposed to air, and / or an oxide layer that forms during exposure of the aluminum substrate to elevated temperatures during manufacturing, e.g., mill scale.The refractory oxide material typically consists of aluminum oxide compounds and possibly other oxide compounds, such as magnesium oxide compounds, for example, if the aluminum substrate is an aluminum-magnesium alloy. The aluminum substrate may also be coated with a layer of zinc, tin, or a metal oxide conversion coating, as described in US 2014 / 0360986A1. The surface layer may have a thickness ranging from 1 nm to 10 µm, depending on its composition, and may be present on either side of the aluminum substrate. Considering the thickness of the aluminum substrate and any surface layer present, the aluminum workpiece 14 has a thickness 141 ranging from 0.3 mm to approximately 6.0 mm, or more narrowly, from 0.5 mm to 3.0 mm.

[0020] The aluminum substrate of the aluminum workpiece 14 can be provided in forged or cast form. For example, the aluminum substrate can consist of wrought aluminum sheets, extrusions, forgings, or other machined articles from a 5xxx (“aluminum-magnesium alloy”), 6xxx (“aluminum-magnesium-silicon”), or 7xxx (“aluminum-zinc alloy”) series. Alternatively, the aluminum substrate can consist of an aluminum alloy casting from the 4xx.x, 5xx.x, or 7xx.x series. Some more specific types of aluminum alloys that can form the aluminum substrate include AA5754 and AA5182 aluminum-magnesium alloy, AA6111 and AA6022 aluminum-magnesium-silicon alloy, AA7003 and AA7055 aluminum-zinc alloy, and Al10SiMg aluminum die-casting alloy. The aluminium substrate can still be used in a variety of conditions, including annealed (O), hardened (H) and heat-treated (T), if desired.The term "aluminum workpiece" as used here encompasses unalloyed aluminum and a wide variety of aluminum alloys, whether coated or uncoated, in various spot-weldable forms, including forged sheet metal, extruded profiles, forgings, etc., as well as castings. All additional aluminum workpieces included in the stack have the same general description.

[0021] In this particular embodiment and with reference to Fig. In the aluminum infill piece 16, an aluminum disc 18 is pre-welded to the steel workpiece 12. The aluminum disc 18 has a first facial surface 20 and an opposing second facial surface 22. The first and second facial surfaces 20 and 22 define a thickness 181 of the disc 18, ranging from 0.1 mm to 2.0 mm or, narrower, from 0.2 mm to 1.0 mm. Additionally, the aluminum disc 18 has a width 183, perpendicular to its thickness 181 of at least 10 mm, or preferably at least 15 mm, extending in all directions through a vertical weld line 24 of the disc 18, which is parallel to the thickness 181 of the disc 18.Each of the first facial surfaces 20 and the second facial surface 22 of the aluminum disc 18 should extend beyond an imaginary circle having a radius of 5 mm or more, preferably 7.5 mm, and is oriented within a plane of the washer 18 perpendicular to the thickness 181 and centered on the vertical seam line 26 of the washer 18. The aluminum disc 18 is preferably rectangular, round, or egg-shaped and simultaneously meets the minimum width requirements mentioned above. Each of the first and second facial surfaces 20, 22 of the aluminum disc 18 is characterized by a refractory oxide layer 28 coating an underlying solid aluminum substrate 30.

[0022] The aluminum substrate 30 of the aluminum disk 18 preferably consists of a low-strength aluminum, i.e., an aluminum with a yield strength of less than 100 MPa or preferably less than 85 MPa in the annealed (O-temper) condition – such as an unalloyed aluminum with 99 wt.% or more aluminum, the remainder being impurities, or an aluminum-manganese alloy comprising aluminum as the main alloying element plus up to 1.8 wt.% manganese, or an aluminum-silicon alloy comprising aluminum as the main alloying element plus up to 15 wt.% silicon. The unalloyed aluminum can be an unalloyed aluminum of the 1xxx series. The aluminum-manganese alloy can be a sheet layer of a wrought aluminum alloy of the 3xxx series comprising aluminum and 0.3 wt.% to 1.8 wt.% manganese plus other optional alloying elements, including one or more of 0 wt.% to 0.3 wt.% copper, 0 wt.% to 0.7 wt.% silicon.-% iron or 0 wt.% to 0.6 wt.% silicon. The aluminum-manganese alloy may also include magnesium in an amount up to 0.5 wt.% or, more narrowly, up to 0.2 wt.%, although in preferred implementations magnesium is excluded from the alloy. The aluminum-silicon alloy may be a sheet layer of a 4xxx series wrought aluminum alloy comprising aluminum and 1 wt.% to 15 wt.% silicon plus other optional alloying elements, including one or more of 0 wt.% to 0.25 wt.% manganese or 0 wt.% to 0.25 wt.% magnesium.

[0023] Due to the low content or absence of readily oxidizable elements such as magnesium, the refractory oxide layers 28 derived from the aforementioned low-strength aluminum alloys are typically relatively thin, giving the aluminum disk 18 a relatively low contact resistance compared to other aluminum alloys such as the 5xxx and 6xxx series aluminum alloys. Specifically, each of the refractory oxide layers 28 consists mainly of aluminum oxide and has a thickness of 5 nm to 100 nm, and the aluminum disk 18 exhibits a contact resistance of less than 300 µΩ (microohms), or more precisely, less than 100 µΩ, measured between two sections of the aluminum disk material. The contact resistance is measured by placing two sections of the aluminum disk material in interfacial contact, typically at least 400 mm. 2Interfacial contact is established, and then the adjacent contact sections are clamped between a pair of welding electrodes with a high force of 1100 lb. Each of the welding electrodes incorporates a series of protruding ribs on its welding surface. Subsequently, an electric current is passed through the sections without initiating melting of the sections until the contact resistance has stabilized.

[0024] The aluminum disc 18 is pre-welded to the steel workpiece 12 by resistance spot welding. This pre-welding involves attaching the aluminum disc 18 against a mating surface 32 of the steel workpiece 12, such that, as applicable here, the second facial surface 22 of the aluminum disc 18 directly contacts the mating surface 32 of the steel workpiece 12 without the aid of an intervening adhesive layer. The steel workpiece 12 and the aluminum disc 18 are then clamped between a first welding electrode 34 and an opposing second welding electrode 36. In particular, a welding surface 38 of the first welding electrode 34 is pressed against the first facial surface 20 of the aluminum disc 18, and a welding surface 40 of the second welding electrode 36 is pressed against an outer surface 42 of the steel workpiece 12.And when they are pressed against their respective surfaces 20, 42 of the aluminum disc 18 and the steel workpiece 12, the welding surfaces 38, 40 of the first and second welding electrodes 34, 36 are effectively aligned with each other along the vertical weld line 24 of the aluminum disc 18 within acceptable manufacturing tolerances. Each of the welding surfaces 38, 40 of the first and second welding electrodes 34, 36 can have one of the most diverse weld geometries. Each weld surface 38, 40 can, for example, and as shown here, include a series of two to six ribs projecting outwards from a base surface of the weld surface 38, 40 as disclosed in one of US 2013 / 0 200 048 A1, US 2015 / 0 083 694 A1, US 2017 / 0 225 262 A1 and US 2018 / 0 234 302 A1, the entire contents of which are incorporated herein by reference.As a further example, each of the weld surfaces 38, 40 can be microstructured as disclosed in US 2017 / 0 304 928 A1, the entire contents of which are incorporated herein by reference.

[0025] An initial electric current is passed between the welding surfaces 38, 40 of the first and second welding electrodes 34, 36 and through the aluminum disc 18 and the steel workpiece 12. The initial electric current heats the thermally and electrically more resistant steel workpiece 12, causing the aluminum disc 18 to begin melting at the second facial surface 22 of the disc 18. As the passage of the initial electric current continues, a molten aluminum weld pool 44 is generated within the aluminum disc 18, as shown in Fig. Figure 1 illustrates this. The molten aluminum weld pool 44 grows laterally along the mating surface 32 of the steel workpiece 12 and penetrates the aluminum disc 18 to the first facial surface 20 of the disc 18 to a depth between 20% and 100% of the thickness 181 of the disc 18. In fact, the welding plan, which controls the passage of the first electric current, can be designed such that the molten aluminum weld pool 44 grows in such a way that it tapers inward within the aluminum disc 18 toward the first facial surface 20, away from the mating surface 32 of the steel workpiece 12, as generally indicated by dashed lines 46. The first electric current can be a continuous square wave, pulsed, or it can have another different waveform.An example of a suitable welding plan for the passage of electric current during the pre-welding of the aluminium disc 18 onto the steel workpiece 12 are those in US 10 058 949 B2 and US 2017 / 0 106 466 A1, the entire contents of which are incorporated herein by reference.

[0026] The molten aluminum weld pool 44 spreads along the mating surface 32 and wets the mating surface 32 of the steel workpiece 12. After the initial electric current has ceased flowing, the molten aluminum weld pool 44 solidifies into a load-bearing weld 48 that connects the aluminum disc 18 to the steel workpiece 12. In particular, the load-bearing weld 48 forms a connection interface 50 with the mating surface 32 of the steel workpiece 12 and includes an aluminum weld nugget 52 and an Fe-Al intermetallic layer 54, as shown in Fig. Figure 2 is best illustrated. The aluminum weld nugget 52 consists of reworked aluminum from the aluminum disk 18 and extends into the aluminum disk 18 to the first facial surface 20 of the disk 18 to a depth ranging from 20% to 100% of the thickness 181 of the disk 18. The intermetallic Fe-Al layer 54 is located between the aluminum weld nugget 52 and the mating surface 32 of the steel workpiece 12 and is adjacent to the interface 50 formed by the load-bearing weld 48. The intermetallic Fe-Al layer 54 may contain FeAl3 compounds, Fe2Al5 compounds, and possibly other intermetallic Fe-Al compounds and typically has a thickness of 0.5 µm to 10 µm. The load-bearing weld 48 has a diameter 481 which can range from 5 mm to 15 mm or narrower from 6 mm to 10 mm at the joint interface 50 and preferably tapers inwards, like the aluminium melt pool 44.

[0027] The load-bearing weld 48 is wide and, due to the composition and thickness 181 of the aluminum washer 18, exhibits good adhesion to the steel workpiece 12. The composition of the aluminum substrate 30 of the aluminum washer 18, for example, is responsible for both its low, high-temperature resistance and its clean and relatively thin refractory oxide layers 28. The oxidation resistance of the washer 18 is ensured by the low content of readily oxidizing elements, such as magnesium, in the aluminum substrate 30. By avoiding or at least limiting such readily oxidizing elements in the aluminum base 30, the formation of additional oxides (e.g., magnesium oxides), which tend to thicken the refractory oxide layers 28, is prevented.The combination of the low strength of the aluminum substrate 30—the low strength allows the washer 18 to deform under load—and the thinner refractory oxide layers 28 leads to a rapid degradation of the oxide layers 28 during current flow. This reduces the contact resistance of the washer 18 and allows the molten aluminum weld pool 44, which transforms into the load-bearing weld 48, to be generated with less heat input. Furthermore, the reduced heat input required to generate the molten aluminum weld pool 44, combined with the early removal of the refractory oxide film layer 28, particularly the oxide layer 28 that comes into contact with the mating surface 32 of the steel workpiece 12, the limited thickness 181 of the aluminum washer 18, and the composition of the washer 18, which contains elements (e.g.,Si and / or Mn) may be present, which inhibit the growth of Fe-Al intermetallic compounds. The susceptibility of the load-bearing weld 48 to interface fractures under load is thus reduced as much as possible. To this end, the high strength of the load-bearing weld 48 and the wide integrity of the interface 50 between the joint 48 and the mating surface 28 of the steel workpiece 12 enable the load-bearing weld 48 to protect a subsequently formed weld, as described below, between the steel workpiece 12 and an automotive-grade aluminum workpiece 14.

[0028] After the aluminum disc 18 has been pre-welded to the steel substrate 12, the workpiece stack 10 is assembled by positioning the aluminum workpiece 14 against the aluminum disc 18 such that the aluminum disc 18 is arranged between the aluminum workpiece 14 and the steel workpiece 12, as shown in Fig. Figure 2 shows that the workpiece stack 10 can be held together with any suitable clamping device. In this particular embodiment, a mating surface 56 of the aluminum workpiece 14 is brought into contact with the first facial surface 20 of the aluminum disc 18. In this context, while the aluminum disc 18 is sandwiched between the two workpieces 12, 14, the mating surface 56 of the aluminum workpiece 14 overlaps and faces the mating surface 32 of the steel workpiece 12. Additionally, the workpiece stack 10, in its assembled state, has a first side 101 and a second side 103. The first side 101 of the workpiece stack 10 is provided by an aluminum workpiece surface 101' and the second side 103 by a steel workpiece surface 103'. The two sides 101, 103 of the workpiece stack 10 are designed for a pair of welding electrodes at a welding point 58 ( Fig. 3) accessible, which is spanned by the aluminium disc 18.

[0029] The steel workpiece 12 and the aluminum workpiece 14 are then welded together through the aluminum disc 18 by resistance spot welding. For this purpose, with reference to Fig. 3. The workpiece stack 10 is clamped between a first welding electrode 60 and an opposing second welding electrode 62. A welding surface 64 of the first welding electrode 60 is pressed against the aluminum workpiece surface 101', which forms the first side 101 of the workpiece stack 10, and a welding surface 66 of the second welding electrode 62 is pressed against the steel workpiece surface 103', which forms the second side 103 of the stack 10. The welding surfaces 64 and 66 are effectively aligned at the weld point 58. In the Fig. In the “2T” workpiece stack configuration shown in Figures 1-5, in which the stack 10 includes only the opposing and adjacent steel and aluminum workpieces 12, 14 (with respect to the number of workpieces) that sandwich the aluminum disc 18, an outer surface 68 of the aluminum workpiece 14 forms the aluminum workpiece surface 101' and the outer surface 42 of the steel workpiece 12 forms the steel workpiece surface 103'. However, in other embodiments that include one or more additional aluminum workpieces and / or one or more additional steel workpieces, an additional aluminum workpiece that overlaps and is positioned outside the aluminum workpiece 14 can form the aluminum workpiece surface 101', and an additional steel workpiece that is located outside the steel workpiece 12 can, separately or simultaneously, form the steel workpiece surface 103'.

[0030] The first and second welding electrodes 60, 62 used for spot welding the workpiece stack 10 can be the same as the first and second welding electrodes 34, 36 previously used for pre-welding the aluminum disc 18 to the steel workpiece 12, or they can be different welding electrodes. Preferably, the same electrodes are used for both pre-welding and welding to increase efficiency. As with the welding electrodes 34, 36 used for pre-welding, each of the weld surfaces 64, 66 of the first and second welding electrodes 60, 62 can have a variety of weld geometries. In a specific implementation, as shown here in Fig. As shown in Figure 2, each of the weld surfaces 64, 66 of the first and second welding electrodes 60, 62 can include a series of two to six ribs projecting outwards from a base surface of the weld 64, 66, as disclosed in one of US 2013 / 0 200 048 A1, US 2015 / 0 083 694 A1, US 2017 / 0 225 262 A1 and US 2018 / 0 234 302 A1. The welding surface 64 of the first welding electrode 60 can have a diameter ranging from 6 mm to 20 mm or narrower from 8 mm to 15 mm and be convexly curved, and the welding surface 66 of the second welding electrode 62 can have a diameter ranging from 3 mm to 16 mm or narrower from 4 mm to 8 mm and can also be convexly curved. Each of the first and second welding electrodes 60, 62 (as well as the welding electrodes 34, 36) can be made of a copper alloy (e.g. CuAr, CuCr, CuCrZr), a tungsten-copper composite, or a dispersion-reinforced copper material such as copper with an aluminum oxide dispersion.

[0031] After the welding surfaces 64, 66 of the first and second welding electrodes 60, 62 have been pressed against their respective sides 101, 103 of the workpiece stack 10, a second electric current is passed between the welding surfaces 64, 66 and through the steel workpiece 12, the aluminum workpiece 14 and the intermediate aluminum disc 18, as shown in Fig. Figure 3 illustrates this. The second electric current can be constant or pulsed over time, or a combination of both, and typically has a current level between 5 kA rms (root mean squared) and 50 kA rms, lasting from 50 ms to 5000 ms, or more precisely, from 200 ms to 2000 ms. As some specific examples, the schedule of the second electric current may be of the type of multi-stage welding schedule described in US 10,058,949 B2 and US 2017 / 0106466 A1, or any other welding schedule suitable for the workpiece stack 10. Since the second electric current flows between the first and second weld areas 64, 66 of the first and second welding electrodes 60, 62, the thermally and electrically resistant steel workpiece 12 heats up quite rapidly.This heat is transferred through the aluminum disc 18 to the aluminum workpiece 14, causing the aluminum workpiece 14 and the load-bearing weld 48 to melt. The melting of the aluminum workpiece 14 and the load-bearing weld 48 produces a molten aluminum weld pool 70.

[0032] The molten aluminum weld pool 70 grows and is contained within the aluminum workpiece 14 and the load-bearing weld 48 of the aluminum disc 18. The aluminum weld pool 70 consumes an inner section of the interface 50 between the load-bearing weld 48 and the mating surface 32 of the steel workpiece 12 and wets a corresponding exposed section 72 of the mating surface 32. The molten aluminum weld pool 70 penetrates completely through the aluminum disc 18 within the load-bearing weld 48 and penetrates the aluminum workpiece 14 to a depth between 10% and 100%, or more precisely, between 20% and 80% of the thickness 141 of the aluminum workpiece 14.Thus, the aluminum melt pool 70 is surrounded by an unconsumed section 48' of the load-bearing weld 48, which in turn has reduced its original interface 50 to a peripheral interface 50' with the mating surface 32 of the steel workpiece 12, surrounding the exposed section 72 of the mating surface 32. The enclosure of the aluminum melt pool 70 by the unconsumed section 48' of the load-bearing weld 48 is supported by the ability of the aluminum disc 18 to deform easily between the steel and aluminum workpieces 12, 14 due to its relatively low strength. The passage of the second electric current between the welding surfaces 64, 66 of the first and second welding electrodes 60, 62 is finally terminated, allowing the molten aluminium weld pool 70 to solidify into a weld seam 74, which connects the aluminium workpiece 14 to the steel workpiece 12 through the aluminium disc 18, as in . Fig. 4 shown.

[0033] The weld 74 forms a connection interface 76 with the exposed section 72 of the mating interface 32 of the steel workpiece 12. The connection interface 76 of the weld 74, which extends from the steel workpiece 12 across the aluminum disc 18 to the aluminum workpiece 14, is thus surrounded by the peripheral connection interface 50' of the unconsumed portion 48' of the load-bearing weld 48. The weld 74 includes an aluminum weld nugget 78 and an intermetallic Fe-Al layer 80. The aluminum weld nugget 78 consists of reworked aluminum from the load-bearing weld 48 of the aluminum disc 18 and the aluminum workpiece 14.The aluminum weld nugget 78 is surrounded by the unconsumed section 48' of the load-bearing weld 48 within the aluminum disc 18 and extends further into the aluminum workpiece 14 to a distance ranging from 10% to 100%, or more narrowly from 20% to 80%, of the thickness 141 of the aluminum workpiece 14. The intermetallic Fe-Al layer 80 is located between the aluminum weld nugget 78 and the exposed section 72 of the mating surface 32 of the steel workpiece 12 and is adjacent to the interface 76 of the weld 74. The intermetallic Fe-Al layer 80 may contain FeAl3 compounds, Fe2Al compounds, and possibly other intermetallic Fe-Al compounds, as described above, and typically has a thickness of 0.5 µm to 10 µm. The weld 74 has a diameter 741 at the connection interface 76, which is smaller than the diameter 481 of the load-bearing weld 48 at its connection interface 50 or 50'.The diameter 741 of the weld seam 74 can, for example, range from 3 mm to 12 mm or narrower from 4 mm to 8 mm at the connection interface 76.

[0034] After completion of the spot welding and formation of the weld seam 74 to join the steel and aluminum workpieces 12, 14 as described above, the clamping force applied by the first and second welding electrodes 60, 62 is released, and the electrodes 60, 62 are withdrawn from their respective sides 101, 103 of the workpiece stack 10. The workpiece stack 10 can now be moved relative to the welding gun (not shown) that holds the welding electrodes 60, 62, so that the first and second welding electrodes 60, 62 are positioned in a facial orientation at another welding point 58, which may or may not include an intermediate aluminum disc 18, and spot welding is performed at this point. Alternatively, instead of spot welding at another welding point 58, the workpiece stack 10 can be moved away from the welding gun to make room for another workpiece stack 10.After the stack of workpieces 10 has been spot-welded at all of its intended welding positions 58, the stack 10 can be further processed if desired, which may include painting, shaping and / or attaching additional components to the stack of workpieces 10 or attaching the stack 10 to a larger manufactured article.

[0035] The formation of the weld seam 74 by the load-bearing weld seam 48 of the aluminum disc 18 creates a notch root 82 between the aluminum disc 18 and the aluminum workpiece 14, as best shown in Fig. Figure 5 illustrates this. As shown, the notch root 82 borders and surrounds the weld 74 and is displaced away from the mating surface 32 of the steel workpiece 12. The notch root 82 includes a notch root opening 84 and a notch root slot 86. The notch root opening 84 is a gap of limited radial extent that separates the mating surface 56 of the aluminum workpiece 14 and the first facial surface 20 of the aluminum disc 18 outside the weld 74. This gap is created as a result of the high clamping pressure exerted on aligned sections of the first and second sides 101, 103 of the workpiece stack 10 by the opposing first and second welding electrodes 60, 62 during the passage of the second electric current. The notch root slot 86 is located radially inward from the notch root opening 84 and is directly adjacent to the weld 74.The notch base slot 86 is an unbound, hydraulically sealed interface between the contact surfaces 56, 20 of the aluminium workpiece 14 and the aluminium disc 18.

[0036] A notch root 88 is also formed between the aluminum disk 18 and the steel workpiece 12. This notch root 88 is located laterally outside the notch root 82 formed between the aluminum disk 18 and the aluminum workpiece 14 and is therefore further away from a centerline (extending parallel to the thickness 181 of the disk 18) of the weld 74 than the notch root 82 formed between the aluminum disk 18 and the aluminum workpiece 14. In particular, the notch root 88 formed between the aluminum disk 18 and the steel workpiece 12 is adjacent to and surrounds the unconsumed section 48' of the load-bearing weld 48. The notch root 88 includes a notch root slot 90 and a notch root opening 92.The notch base opening 92 is a gap of limited radial extent that separates the second facial surface 22 of the aluminum disc 18 and the mating surface 32 of the steel workpiece 12, and the notch base slot 90 is located radially inward from the notch base opening 92 directly adjacent to the load-bearing weld 74. The notch base slot 90 is an unbound, hydraulically sealed interface between the contact surfaces 22, 32 of the aluminum disc 18 and the steel workpiece 12.

[0037] The peripheral connection interface 50', established between the unconsumed section 48' of the load-bearing weld 48 and the mating surface 32 of the steel workpiece 12, isolates the internally closed connection interface 76, established between the weld 74 and the exposed section 72 of the connection interface 32. This protects the connection interface 76 of the weld 74 and its adjacent intermetallic Fe-Al layer 80 from cracking and crack propagation when the connection 76 is subjected to stress by the application of forces to the steel and aluminum workpieces 12, 14.In fact, when force is applied to the workpieces 12, 14, the resulting stress tends to be redirected to the notch root 82, which surrounds the relatively tough aluminum weld nugget 78 of the weld 76. Crack formation and propagation are more strongly quenched there and are less damaging compared to the intermetallic Fe-Al layer 80. The weld 74 is thus better equipped to withstand the load, since the notch root 82 is formed away from the mating surface 32 of the steel workpiece 32 and the stress can be dissipated there.In fact, the notch root 82 is located closest to the center of the weld 74 and, in this embodiment, is the only one that borders and surrounds the weld 74, since the peripheral interface 50' of the unused section 48' of the load-bearing weld 48 essentially precludes the formation of a notch root between the aluminum disk 18 and the steel workpiece 12 within the load-bearing weld 48. Due to its location near the weld 74, the notch root 82 formed between the aluminum disk 18 and the aluminum workpiece 14 experiences a larger proportion of the stress that arises when forces are exerted on the steel and aluminum workpieces 12, 14.

[0038] The practice described in the present disclosure relating to resistance spot welding of a stack of workpieces which includes an intermediate aluminium filler piece 16 between the adjacent opposing steel and aluminium workpieces 12, 14, is not necessarily limited to those associated with the Fig. The embodiment shown and described in Figures 1-5. For example, the aluminum filler 16 is not limited to the aluminum washer 18, and even when the aluminum washer 18 is used, the adhesion of the washer 18 to the steel workpiece 12 by pre-welding is not the only way to create a notch root that is displaced away from the mating surface 32 of the steel workpiece 12, adjoins and surrounds the aluminum weld nugget 78 of the weld 74, and receives a reverse stress when the joint 74 is loaded, as described above. In the following discussion of alternative embodiments, reference numerals corresponding to those used in the description of the previous embodiment are used to identify identical elements with the same functionality. For this purpose, the description of aspects of the embodiment described in the Fig. The embodiments shown in Figures 1-5, described above, apply equally to aspects of the following embodiments, which are identified by corresponding reference numbers, unless expressly stated otherwise. Only the essential differences in the Fig. The alternative embodiments shown in Figures 6-9 are explained in detail below.

[0039] With reference to Fig. Figure 6 shows a stack of workpieces 210 comprising a steel workpiece 212 and an aluminum workpiece 214, as described above, but here the aluminum filler 216 is in the form of an aluminum coating 294 applied to the mating surface 232 of the steel workpiece 212. The aluminum coating 294 can consist of unalloyed aluminum or an aluminum alloy containing at least 85 wt.% aluminum, including one of the various aluminum grades that can form the aluminum substrate of the aluminum workpiece 214. The aluminum coating 294 is applied to the mating surface 232 of the steel workpiece 212 in such a way that the coating 294 metallurgically bonds with the mating surface 232. The aluminium coating 294, which includes a second facial surface 222 that forms a connecting surface 296 with the mating surface 232 of the steel workpiece 212, typically has a thickness in the range of 0.05 mm to 1.0 mm.Opposite the second facial surface 222, the aluminum coating 294 has a first facial surface 220 provided by a refractory oxide layer 228 that covers a mass section 230 of the coating 294. While the preferred aluminum grades for the aluminum coating 294 are more expansive than the preferred aluminum grades for the pre-welding embodiment described above, primarily because pre-welding is not practiced, the use of unalloyed aluminum, an aluminum-manganese alloy containing up to 1.8 wt.% manganese (e.g., 3xxx series wrought aluminum alloy), and an aluminum-silicon alloy containing up to 15 wt.% silicon (e.g.,4xxx series wrought aluminum alloys) are still preferred due to their low strength, relatively thin refractory oxide layer 228, relatively low transition resistance and their Si and / or Mn content, which inhibits the growth of intermetallic Fe-Al compounds.

[0040] The aluminum coating 294 can be applied to the mating surface 232 of the steel workpiece 212 using various techniques. For example, the aluminum coating 294 can be deposited by thermal spraying, in which a powder or wire of the desired aluminum composition is heated to a semi-molten state and small droplets of the semi-molten aluminum are accelerated over a defined area into the mating surface 232. These small droplets accumulate upon impact with the mating surface 232, thus depositing the aluminum coating 294 to the desired thickness. As another coating option, the aluminum coating 294 can be applied to the mating surface 232 of the steel workpiece 212 by cold spraying, in which a powder of the desired aluminum composition is accelerated over a defined area into the mating surface at a speed of 500 m / s to 1000 m / s.Upon contact with the mating surface 232, the powder particles deform plastically and accumulate to form the aluminum coating 294. Other coating methods, such as magnetron sputtering or hot-dip galvanizing, can also be used to form the aluminum coating 294 onto the mating surface 232 of the steel workpiece 212. Since the aluminum coating 294 has been deposited and grown onto the mating surface 232 of the steel workpiece 212, no intermetallic Fe-Al compounds form at the interface 296 between the aluminum coating 294 and the mating surface 232 of the steel workpiece 212.

[0041] The aluminum coating 294 functions similarly to the load-bearing weld 50, 150 described in the preceding embodiments. In particular, as in Fig. Figure 7 shows the weld 274 formed through the aluminum coating 294, while an inner section of the coating 294 and its interface 296 with the mating surface 232 of the steel workpiece 212 are consumed. The weld 274 forms an interface 276 with the exposed section 272 of the mating surface 232 of the steel workpiece 212, which is surrounded by the peripheral interface 296' formed between the unconsumed section 294' of the aluminum coating 294 and the mating surface 232. The formation of the weld 274 through the aluminum coating 294 creates a notch root 282 between the aluminum coating 294 and the aluminum workpiece 214.In fact, as shown, the notch root 282 surrounds the weld 274 and is pushed away from the mating surface 232 of the steel workpiece 212, and includes a notch root opening 284 that separates the first facial surface 220 of the aluminum coating 290 and the mating surface 256 of the aluminum tool 214, as well as a notch root slot 286. In this embodiment, no notch root is formed between the aluminum coating 294 and the steel workpiece 12, mainly because of the good adhesion between the unconsumed section 294' of the aluminum coating 294 and the mating surface 232 of the steel workpiece 212 immediately outside the weld 274 at the peripheral joint interface 296'. The unconsumed section 294' of the aluminium coating 294 insulates the interface 276 of the weld 274 and protects the intermetallic Fe-Al layer 280 of the joint 274 from crack-inducing stress as previously described.

[0042] With reference to Fig. Figure 8 shows a stack of workpieces 310 comprising a steel workpiece 312, an aluminum workpiece 314, and an aluminum filler piece 316 in the form of an aluminum disc 318. In this embodiment, however, the aluminum disc 318 is not pre-welded to the steel workpiece 312, but rather bonded together by a cured adhesive layer 398' ( Fig. 9) bonded to the mating surface 332 of the steel workpiece 312. The cured adhesive layer 398' is cured by curing a curable adhesive layer 398 ( Fig. 8) obtained after forming the weld seam 374. The curable adhesive layer 398 can comprise a thermosetting polymer adhesive such as thermosetting epoxy or thermosetting polyurethane, which may optionally include filler particles, such as silicon dioxide particles, distributed throughout the polymer adhesive to modify the viscosity profile of the adhesive layer for manufacturing processes. Some specific examples of a thermosetting polymer adhesive are DOW Betamate 1486, Henkel Terokal 5089, and Uniseal 2343, all of which are commercially available. The thickness of the curable adhesive layer 398 can range from 0.1 mm to 2.0 mm or, narrower, from 0.2 mm to 1.0 mm when applied and prior to welding the steel and aluminum workpieces 312, 314.

[0043] To prepare the workpiece stack 310, the curable adhesive layer 398 ( Fig. 8) The aluminum disc 18 is applied to the mating surface 332 of the steel workpiece 312, and the aluminum disc 18 is applied to the curable adhesive layer 398, bringing the second facial surface 320 of the disc 18 into contact with the curable adhesive layer 398. Then, during the formation of the weld seam 374, the curable adhesive layer 398 is driven laterally by the clamping pressure of the first and second welding electrodes 360, 362, and any remaining adhesive material at the same interface is thermally decomposed. The weld seam 374 is formed by the aluminum disc 318 and forms a connection interface 376 with the exposed section 372 of the mating interface 332 of the steel workpiece 312.After the weld 374 is formed, the surrounding section of the curable adhesive layer 398, which remains between the second facial surface 322 of the aluminium disc 318 and the mating surface 332 of the steel workpiece 312, is cured in an oven or other heating device to produce the cured adhesive layer 398'.

[0044] The formation of the weld seam 374 creates, as before, the notch root 382 between the aluminum disc 318 and the aluminum workpiece 314, and also creates a notch root 388 between the aluminum disc 318 and the steel workpiece 312, since the curable adhesive layer 398 is ejected from the area where the weld seam 374 is formed and thermally decomposes. The notch root 388 between the aluminum disc 318 and the steel workpiece 312 includes a notch root opening 390 and a notch root slot 392, as before, in conjunction with the Fig.The pre-weld formation described in Sections 1-5 is similar. Here, however, the notch root 388 is adjacent to and surrounds the weld 374, but due to the conical shape of the weld 374, it is located laterally outside and further away from the weld 374 than the notch root 382 between the aluminum disc 318 and the aluminum workpiece 314. The notch root 388 formed between the aluminum disc 318 and the steel workpiece 312 is not too problematic – and in fact, for the reasons described above, the stress can still be redirected to the other notch root 382 – because the cured adhesive layer 398' surrounds the weld 374 and, in particular, the interface 376 formed between the weld 374 and the exposed section 372 of the mating surface 332 of the steel workpiece 312.The adhesive bond between the second facial surface 322 of the aluminium disc 318 and the mating surface 332 of the steel workpiece outside the weld 374 and the notch root 388 created between the aluminium disc 318 and the steel workpiece 312 is sufficient to protect and keep intact the notch root 388 located at the base of the weld 374, while the stress is transferred to the other notch root 382.

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