AL-STEEL WELDED JOINT

Abstract

Welded joint (10) connecting an aluminum workpiece (12) and a steel workpiece (14) by means of welding, wherein the welded joint (10) comprises: an aluminum weld nib (44) contained in an aluminum workpiece (12), and an intermetallic layer (46) arranged between the aluminum weld nib (44) and a mating surface (28) of the steel workpiece (14), wherein the intermetallic layer (46) provides a weld bonding surface (34) of the welded joint (10) which adjoins and is connected to the mating surface (28) of the steel workpiece (14), characterized by an annular bead (48) of aluminide particles extending upwards and radially inwards from the weld bonding surface (34) of the welded joint (10) into the aluminum weld nib (44), wherein the annular bead (48) of aluminide particles comprises nickel aluminide particles and / or iron aluminide particles includewherein a notch base (38) is arranged around the weld joint (10) at a butt interface (16) of the aluminium workpiece (12) and the steel workpiece (14), wherein the notch base (38) comprises a notch base opening (40) and a notch base slot (42), wherein the notch base opening (40) is a gap of limited radial extent which separates a mating surface (22) of the aluminium workpiece (12) and the mating surface (28) of the steel workpiece (14), and wherein the notch base slot (42) is arranged radially within the notch base opening (40) directly adjacent to the weld joint (10) and furthermore is a non-connected, hydraulically sealed interface between the mating surfaces (22, 28) of the aluminium workpiece (12) and the steel workpiece (14).

Description

REFERENCE TO RELATED REGISTRATIONS

[0001] This application claims priority over preliminary US application No. 62 / 319,864, filed on April 8, 2016. The entire contents of the aforementioned preliminary application are incorporated herein by reference. TECHNICAL AREA

[0002] The technical field of this disclosure generally relates to a welded joint that joins an aluminium workpiece and a steel workpiece together in a weld zone. INTRODUCTION

[0003] Resistance spot welding is a process used in a number of industries to join two or more metal workpieces. The automotive industry, for example, frequently uses resistance spot welding to join metal workpieces during the manufacture of structural frame elements of the vehicle (e.g., body sides and cross members) and vehicle transverse elements (e.g., vehicle doors, hoods, trunk lids, and tailgates), among other applications. A number of spot welds are often formed at various points around a circumferential edge of the metal workpieces or within a specific joining area to ensure the part is structurally sound.While spot welding has typically been practiced to join certain similarly composed metal workpieces—such as steel to steel and aluminum to aluminum—the desire to incorporate lighter materials into vehicle body structures has sparked interest in joining steel and aluminum workpieces using resistance spot welding. This desire to resistance spot weld such dissimilar metal workpieces is not limited to the automotive industry; in fact, it extends to other industries, including aerospace, marine, railway, and building construction.

[0004] Resistance spot welding relies on the resistance to the flow of an electric current through overlapping metal workpieces and across their butt joint surface(s) to generate heat. To perform such a welding process, a set of opposing welding electrodes is pressed in an facing orientation against opposite sides of the workpiece stack, which typically comprises two or three metal workpieces arranged in a stacked configuration. An electric current is then passed through the metal workpieces from one welding electrode to the other. The resistance to the flow of this electric current generates heat within the metal workpieces and at their butt joint surface(s).When the workpiece stack comprises an aluminum workpiece and an adjacent overlapping steel workpiece, the heat generated at the interface and within the bulk material of these dissimilar metal workpieces induces an aluminum weld pool in the aluminum workpiece, causing it to expand. This aluminum weld pool then wets the adjacent mating surface of the steel workpiece and, after the current is cut off, solidifies into a weld joint, joining the two workpieces together.

[0005] In practice, spot welding an aluminum workpiece to a steel workpiece presents a challenge, as several properties of these two metals can adversely affect the strength—particularly the peel strength and transverse stress strength—of the weld. For one, the aluminum workpiece typically contains a mechanically robust, electrically insulating, self-healing, and heat-resistant surface oxide layer. This surface oxide layer typically consists of aluminum oxide compounds, but it can also include other metal oxide compositions, including magnesium oxide if the aluminum workpiece is made from a magnesium-containing aluminum alloy.Due to its properties, the heat-resistant surface oxide layer tends to remain intact at the butt joint interface, where it not only restricts the ability of the aluminum weld pool to wet the steel workpiece but also represents a source of defects near the interface within the growing weld pool. Furthermore, the insulating nature of the surface oxide layer increases the electrical contact resistance of the aluminum workpiece—specifically at its mating surface and at its contact point with the electrode—making it difficult to effectively control and concentrate the heat within the aluminum workpiece.

[0006] Furthermore, in many cases, remnants of the heat-resistant surface oxide layer and an organic intermediate layer—such as an uncured but still curable adhesive, sealant, sound-absorbing layer, etc.—present between the aluminum and steel workpieces at the butt joint can interact to form a more durable composite material. Specifically, it is thought that residual materials from the organic layer—which may include carbon ash, filler particles (e.g., silicon dioxide, rubber, etc.), and other derived materials—ultimately combine with a residual oxide film to form a composite residual film that is more resistant to mechanical disruption and dispersal during current flow compared to the original heat-resistant surface oxide layer.The formation of a more robust, compound residual film causes fragments of this film to remain grouped and aggregated at and along the butt interface in a more destructive manner compared to cases where no organic interlayer is present between the aluminum and steel workpieces. For example, the compound residual film can block the diffusion of iron into the aluminum weld pool, leading to excessive thickening of the hard and brittle Fe-Al intermetallic layer. Additionally, the compound residual film can provide a pre-defined crack path along the interface between the weld and the steel workpiece. Each of these events can weaken the weld.

[0007] Aside from the challenges posed by the heat-resistant surface oxide layer of the aluminum workpiece, whether in conjunction with the organic interlayer or not, the aluminum and steel workpieces possess different properties that can adversely affect the strength and characteristics of the weld. Specifically, aluminum has a relatively low melting point (~600°C) and relatively low electrical and thermal resistance, while steel has a relatively high melting point (~1500°C) and relatively high electrical and thermal resistance. As a consequence of these differences in material properties, most of the heat generated during current flow is generated within the steel workpiece, resulting in a thermal imbalance between the steel workpiece (higher temperature) and the aluminum workpiece (lower temperature).The combination of the thermal imbalance created during current flow and the high thermal conductivity of the aluminum workpiece means that immediately after the electric current stops flowing, a situation arises in which heat is not distributed symmetrically away from the weld zone. Instead, the heat is conducted from the hotter steel workpiece through the aluminum workpiece towards the welding electrode on the other side of the aluminum workpiece, creating a steep thermal gradient in that direction.

[0008] It is assumed that the development of a steep thermal gradient between the steel workpiece and the welding electrode on the other side of the aluminum workpiece weakens the resulting weld in several ways. First, because the steel workpiece retains heat for a longer time than the aluminum workpiece after the electric current has ceased, the aluminum weld pool solidifies in a directed manner, starting from the area adjacent to the colder welding electrode (which is often water-cooled) associated with the aluminum workpiece and spreading towards the mating surface of the steel workpiece.A hardening front of this type tends to carry or drive defects—such as gas porosity, shrinkage voids, and microcracking—toward and along the interface between the weld and the steel workpiece, where residual oxide film or composite film defects are already present. Secondly, the sustained elevated temperature in the steel workpiece promotes the growth of a hard and brittle intermetallic Fe-Al layer within the weld and adjacent to the mating surface of the steel workpiece. The presence of a distribution of weld defects, along with excessive growth of the intermetallic Fe-Al layer at the interface, tends to reduce the peel strength and transverse stress strength of the weld.

[0009] Given the challenges mentioned above, previous attempts to spot weld an aluminum workpiece to a steel workpiece used a welding plan that specified higher currents, longer welding times, or both (compared to spot welding steel to steel) in an attempt to achieve and maintain an adequate weld area. Such efforts were largely unsuccessful in a manufacturing environment and tended to damage the welding electrodes. Assuming that previous spot welding efforts were not particularly successful, mechanical fasteners, including drive rivets and flow-drill screws, were primarily used instead. However, mechanical fasteners require more time to install and have high consumable costs compared to spot welding.They also add weight to the vehicle – weight that is avoided when the joint is made by spot welding – thus offsetting some of the weight savings initially achieved by using an aluminum workpiece. Advances in spot welding that would make it easier to join aluminum and steel workpieces would therefore be a welcome addition to the technology. SUMMARY

[0010] According to one embodiment of the present disclosure, a weld joint connecting an aluminum workpiece and a steel workpiece may comprise an aluminum weld bead, an intermetallic layer, and an annular bead of aluminide particles. The aluminum weld bead is contained within an aluminum workpiece, and the intermetallic layer is arranged between the aluminum weld bead and a mating surface of the steel workpiece. The intermetallic layer provides a weld bonding surface of the weld joint, which adjoins and is bonded to the mating surface of the steel workpiece. The annular bead of aluminide particles extends upward and radially inward from the weld bonding surface of the weld joint into the aluminum weld bead. The annular bead of aluminide particles may comprise nickel aluminide particles and / or iron aluminide particles.

[0011] The weld joint of the aforementioned embodiment may have additional features. For example, the ring bead of aluminide particles may comprise nickel aluminide particles and iron aluminide particles. As another example, the ring bead of aluminide particles may comprise nickel aluminide particles containing 20 at.% to 50 at.% nickel and / or iron aluminide particles containing 20 at.% to 50 at.% iron. Such particles include NiAl3 particles, NiMnAl particles, FeAl3 particles, Fe2Al5 particles, and / or FeMnAl particles.

[0012] Other properties of the weld joint can also be further defined. For example, the aluminum weld bead of the weld joint can be conically radially inward extending away from the weld junction. Additionally, the weld junction can have a surface area ranging from 4πt to 20πt, where "t" is the thickness of the aluminum workpiece in a weld zone containing the weld joint prior to its formation. According to yet another example, the intermetallic layer can further comprise intermetallic iron-aluminum compounds and can have a thickness across the weld junction ranging from 1 μm to 5 μm.

[0013] A weld joint connecting an aluminum workpiece and a steel workpiece, and having a weld junction adjacent to and connected with a mating surface of the steel workpiece, may, according to another embodiment of the present disclosure, comprise an aluminum weld bead, an intermetallic layer, and an annular bead of aluminide particles. The aluminum weld bead is contained within the aluminum workpiece and extends conically radially inward as it extends away from the weld junction of the aluminum workpiece. The intermetallic layer is located between the aluminum weld bead and the mating surface of the steel workpiece and accordingly provides the weld junction of the weld joint. The intermetallic layer may comprise intermetallic iron-aluminum compounds.The annular bead of aluminide particles extends upwards from the weld bonding surface of the weld joint and radially inwards into the aluminum weld nib from a notch root surrounding the weld joint at a butt interface between the aluminum and steel workpieces. The annular bead of aluminide particles comprises nickel aluminide particles and / or iron aluminide particles.

[0014] The weld joint of the aforementioned embodiment may have additional features. For example, the ring bead of aluminide particles may comprise nickel aluminide particles with 20 at.% to 50 at.% nickel and / or iron aluminide particles with 20 at.% to 50 at.% iron. As another example, the ring bead may comprise NiAl3 particles, NiMnAl particles, FeAl3 particles, Fe2Al5 particles, and / or FeMnAl particles. Furthermore, the aluminum workpiece may have a base aluminum substrate consisting of an aluminum alloy containing at least 85 wt.% aluminum. According to yet another example, the intermetallic layer, which comprises iron-aluminum intermetallic compositions, may have a thickness across the weld bond area of ​​the joint ranging from 1 μm to 5 μm. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Fig. Figure 1 is a cross-sectional view of a weld joint that connects an aluminium workpiece (top) and a steel workpiece (bottom) in a weld zone;

[0016] Fig. 2 is an enlarged cross-sectional view of the left side of the in Fig. 1 shown weld joint, which has an embodiment of an annular bead made of aluminide particles;

[0017] Fig. Figure 3 is an enlarged cross-sectional view of the left side of the [unclear text]. Fig. 1 shown weld joint, which has a different embodiment of an annular bead made of aluminide particles;

[0018] Fig. Figure 4 is a micrograph of a cross-section of a weld joint which, according to an embodiment of the disclosure, has an annular bead of nickel aluminide particles;

[0019] Fig. Figure 5 is an enlarged view of the photomicrograph of Fig. 4, which is focused on the left side of the weld joint and the ring bead of nickel aluminide particles;

[0020] Fig. 6 is an enlarged view of the microphotograph of Fig. 4, which is focused on the right side of the weld joint and the ring bead of nickel aluminide particles;

[0021] Fig. Figure 7 is an enlarged view of the photomicrograph of Fig. 4, which is focused on the center of the weld joint and the intermetallic layer that separates the aluminium weld lens of the weld joint from the mating surface of the steel workpiece;

[0022] Fig. Figure 8 is a microphotograph of a section of a weld joint which, according to an embodiment of the disclosure, has an annular bead of nickel aluminide particles;

[0023] Fig. Figure 9 is an enlarged view of the photomicrograph of Fig. 8, which is focused on the left side of the weld joint and the ring bead of nickel aluminide particles;

[0024] Fig. Figure 10 is an enlarged view of the photomicrograph of Fig. 8, which is focused on the right side of the weld joint and the ring bead of nickel aluminide particles;

[0025] Fig. Figure 11 is an enlarged view of the photomicrograph of Fig. 8, which is focused on the center of the weld joint and the intermetallic layer that separates the aluminium weld lens of the weld joint from the mating surface of the steel workpiece;

[0026] Fig. 12 is a microphotograph of a section of a weld joint which, according to an embodiment of the disclosure, has an annular bead of iron aluminide particles;

[0027] Fig. Figure 13 is an enlarged view of the photomicrograph of Fig. 12, which is focused on the left side of the weld joint and the ring bead of iron aluminide particles;

[0028] Fig. Figure 14 is an enlarged view of the photomicrograph of Fig. 12, which is focused on the right side of the weld joint and the ring bead of iron aluminide particles;

[0029] Fig. Figure 15 is an enlarged view of the photomicrograph of Fig. 12, which is focused on the center of the weld joint and the intermetallic layer that separates the aluminium weld lens of the weld joint from the mating surface of the steel workpiece;

[0030] Fig. 16 is a microphotograph of a section of a weld joint which, according to an embodiment of the disclosure, has an annular bead of iron aluminide particles;

[0031] Fig. Figure 17 is an enlarged view of the photomicrograph of Fig. 16, which is focused on the left side of the weld joint and the ring bead of iron aluminide particles;

[0032] Fig. 18 is an enlarged view of the microphotograph of Fig. 16, which is focused on the right side of the weld joint and the ring bead of iron aluminide particles; and

[0033] Fig. Figure 19 is an enlarged view of the microphotograph of Fig. 16, which is focused on the center of the weld joint and the intermetallic layer that separates the aluminium weld lens of the weld joint from the mating surface of the steel workpiece. DETAILED DESCRIPTION

[0034] Spot welding an aluminum workpiece to a steel workpiece presents several significant challenges. As discussed above, several of the identified challenges that complicate the ability to spot weld these dissimilar metals relate to the susceptibility of the weld to degradation at the interface between the joint and the mating surface of the steel workpiece due to welding irregularities, such as porosity, residual oxide fragments, and the presence of a hard and brittle intermetallic layer. These challenges are particularly problematic when an organic intermediate material is present between the two workpieces at the butt joint, as the organic material can interact with the residual oxide film to form a mechanically more robust composite residual film. Here, a weld joint is described.10 reveals which of the accumulation of irregularities for the weld joint at and along a bonding interface of the weld joint 10 and at least partially counteracts the stress on the steel workpiece, and additionally protects the bond interface against crack propagation under applied force. The protection against crack propagation offered is noteworthy because the primary cause of failure of an interfacial connection under applied force is often rapid crack growth through the intermetallic layer, which is exacerbated by welding irregularities near the interface.

[0035] Now on Fig. 1. Referring to this, the welded joint 10 (in cross-section) shown, which is an aluminium workpiece 12 and a steel workpiece 14 at a collision interface 16 the two overlapping workpieces 12 ,14 connects them. The welded joint 10 is in a welding zone 18 integrated, which are characterized by the aluminium and steel workpiece 12 , 14 extends. The cross-sectional size and shape of the weld zone 18 is determined by the extent of the weld joint 10 at the bonding interface of the compound 10 and the steel workpiece 14 defined as in Fig. 1 is shown. In the welding zone 18 The aluminum workpiece 12 a thickness 120 and in a similar way the steel workpiece 14 a thickness 140 up. The thickness 120 of the aluminum workpiece 120 preferably ranges from 0.3 mm to 6.0 mm or narrower from 0.5 mm to 3.0 mm, and the thickness 140 The thickness of the steel workpiece preferably ranges from 0.3 mm to 6.00 mm or, more narrowly, from 0.6 mm to 2.5 mm. 120 , 140of the aluminum and steel workpiece 12 , 14 can outside the welding zone 18 due to the indentations in the workpieces 12 , 14 through the welding surfaces of opposing welding electrodes at the welding zone 18 will be left behind, larger than the thick ones 120 , 140 within the welding zone 18 be.

[0036] In some cases, at least one additional workpiece can be added to the adjacent pair of aluminum and steel workpieces joined by welding. 12 , 14 be present, which are in Fig. Figure 1 shows examples. For instance, an additional third workpiece (not shown) can be attached adjacent to the aluminum workpiece. 12 or to the steel workpiece 14 opposite to the impact interface 16The additional third workpiece can be arranged. The additional third workpiece can be made of either aluminum or steel and is adjacent to the workpiece. 12 , 14 arranged with the same base metal composition; that is, an additional aluminum workpiece adjacent to the aluminum workpiece 12 It may be arranged to provide an aluminium-aluminium-steel stack, or that an additional steel workpiece is adjacent to the steel workpiece. 14It can be arranged to provide an aluminum-steel-steel stack. According to yet another example, two additional workpieces (also not shown) can be present to provide any one aluminum-aluminum-steel-steel stack, an aluminum-aluminum-aluminum-steel stack, or an aluminum-steel-steel-steel stack. With respect to their composition and thickness, the additional aluminum workpiece and / or steel workpiece, if present, can correspond to its adjacent aluminum workpiece. 12 or steel workpiece 14 be the same or different from these, which at their collision interface 16 through the welded joint 10 are joined together by means of welding.

[0037] The aluminum workpiece 12 includes a base aluminum substrate 20, which consists of unalloyed aluminum or an aluminum alloy containing at least 85 wt.% aluminum. Some noteworthy aluminum alloys that form the base aluminum substrate 20 The possible alloys are an aluminum-magnesium alloy, an aluminum-silicon alloy, an aluminum-magnesium-silicon alloy, or an aluminum-zinc alloy. The aluminum workpiece 12 has a fitting area 22 and a reverse side 24 up, how best in Fig. 2– Fig. 3 is shown. Each of the fitting surfaces 22 and the reverse 24 is due to the base aluminum substrate 20 or a surface layer that limits the base aluminum substrate 20 overlapping, although the fitting area 22 not in the welding zone 18 is present because it occurs during the formation of the weld joint 10is destroyed. The surface layer, if present, can then be a natural, heat-resistant oxide layer comprising aluminum oxide compounds and possibly other oxide compounds, and / or an oxide layer formed due to the action of elevated temperatures on the aluminum substrate. 20 generated during manufacturing, e.g. mill scale, or alternatively, it may be a layer of zinc, tin, or a metal oxide conversion coating comprising oxides of titanium, zirconium, chromium, or silicon, as described in US Disclosure No. 2014 / 0360986.

[0038] The base aluminum substrate 20 It can be in a forged or cast form. For example, the base aluminum substrate can be... 20consisting of a sheet layer, an extruded part, a forging, or another machined article of a forged aluminum alloy of the 4xxx, 5xxx, 6xxx, or 7xxx series. According to another embodiment, the base aluminum substrate may consist of a sheet layer, an extruded part, a forging, or another machined article of a forged aluminum alloy of the 4xxx, 5xxx, 6xxx, or 7xxx series. 20 consisting of an aluminum alloy casting from the 4xx.x, 5xx.x, 6xx.x, or 7xx.x series. Some more specialized types of aluminum alloys, which form the base aluminum substrate. 20 The alloys that can be formed include, but are not limited to, the AA5754 and AA5182 aluminum-magnesium alloys, the AA6111 and AA6022 aluminum-magnesium-silicon alloys, the AA7003 and AA7055 aluminum-zinc alloys, and the Al-10Si-Mg aluminum casting alloy. The base aluminum substrate 20It can also be used in a variety of hardening processes, including annealing (O), cold hardening (H), and solution heat treatment (T), if desired. The term "aluminium workpiece," as used herein, therefore includes unalloyed aluminum and a wide variety of aluminum alloys, whether coated or uncoated, in various forms suitable for spot welding, including forged sheet metal parts, extruded parts, etc., and also castings.

[0039] The steel workpiece 14 includes a base steel substrate 26 any of a wide variety of strengths and grades, which is either coated or uncoated. The base steel substrate 26It can be hot-rolled or cold-rolled, and it can be made of a type of steel, such as soft unalloyed steel, non-porous steel, heat-treatable steel, high-strength low-alloy steel (HSLA steel), dual-phase steel (DP steel), complex-phase steel (CP steel), martensitic steel (MART steel), transformation-induced plasticity steel (TRIP steel), twist-induced plasticity steel (TWIP steel), and boron steel, for example, when the steel workpiece 14 Press-hardened steel (PHS) is included. Preferred compositions of the base steel substrate 26However, they include soft unalloyed steel, dual-phase steel, and boron steel, which are used in the production of press-hardened steel. These three types of steel have maximum tensile strengths ranging from 150 MPa to 350 MPa, 500 MPa to 1100 MPa, and 1200 MPa to 1800 MPa, respectively. Therefore, the term "steel workpiece," as used here, encompasses a wide variety of coated and uncoated steels.

[0040] The steel workpiece 14 has a fitting area 28 and a reverse side 30 up, how best in Fig. 2– Fig. 3 is shown. Each of the fitting surfaces 28 and the reverse 30 is through the base steel substrate 26 or a surface layer that limits the base steel substrate 26 superimposed. The fitting surface 28 of the steel workpiece 14 is not affected by the weld joint 10destroyed, as is the case with the pass area 22 of the aluminum workpiece 12 This is the case, and therefore it extends through the welding zone. 18 through it. The steel workpiece 12 If coated, it may then have a zinc-based alloy layer consisting of a zinc-nickel alloy layer or a zinc-iron alloy layer (i.e., heat-treated after zinc plating). Such zinc-based alloy layers may contain anywhere between 1 wt.% and 25 wt.% nickel or iron, with the majority of the remainder being zinc, or more narrowly, anywhere between 5 wt.% and 15 wt.% nickel or iron, with the majority of the remainder being zinc. In such cases, where the steel workpiece 14 a surface layer before the formation of the weld joint 10 exhibits that the surface layer is typically outside the welding zone 18intact, since the formation of the weld joint 10 the surface layer at the mating surface 28 destroyed.

[0041] The welded joint 10 is in the aluminum workpiece 12 It contains and is influenced by a heat-affected zone. 32 surrounded. Unlike conventional spot welding practices that join similarly composed workpieces (e.g., aluminum to aluminum and steel to steel), the weld joint welds 10 the workpieces 12 , 14 not by melting them together. The weld joint 10 destroys the pass area 28 of the steel workpiece 14certainly not within the welding zone and does not extend through its original position, and it certainly does not solidify out of a weld pool of mixed materials which contains essentially equal molten material proportions from each of the two workpieces. 12 , 14 instead, the weld joint is shown here. 10 assuming the relatively significant differences in the melting points of the two workpieces 12 , 14 by melting and resolidifying a section of the aluminum workpiece adjacent to the mating surface 28 of the steel workpiece 14 obtained. The weld joint 10 therefore extends from the pass area 28 of the steel workpiece 14 towards the back 24 of the aluminum workpiece 12 up to a penetration depth ranging from 20% to 100% of the thickness 120 of the aluminum workpiece12 within the welding zone 18 sufficient. The planar diameter of the weld joint is sufficient. 10 (i.e., the diameter of the largest section of the weld joint) 10 (in top view) depends on the thickness 140 of the steel workpiece 14 ab. In most cases, the planar diameter of the weld joint is sufficient. 10 however, from 4 mm to 16 mm or narrower from 5 mm to 10 mm.

[0042] The welded joint 10 is formed by a weld bonding surface 34 and a workpiece circumferential surface 36 limited. The weld bonding area 34 borders the pass area 28 of the steel workpiece 14 and is connected to it. According to a preferred embodiment, the weld bonding surface has 34 a surface area ranging from 4πt to 20πt, where the variable “t” is the thickness 140 of the aluminum workpiece 14within the welding zone 18 before the formation of the weld joint 10 is. The workpiece circumferential surface 36 extends in a narrowing manner from the weld bonding surface 34 away and forms the boundary of the weld joint 10 within the aluminum workpiece 12 The workpiece circumferential surface 36 The weld joint is being broken 10 from the surrounding heat-affected zone 32 inside the aluminum workpiece 12 , and it limits itself in cases where the welded joint 10 the aluminum workpiece 12 completely penetrates, to a section of a back 24 of the aluminum workpiece 12 on, like here in Fig. 1 is shown.

[0043] Outside the weld joint 10 and around this is at the impact interface 16 of the aluminum and steel workpiece 12 ,14 a notch 38 arranged. This is best in Fig. 2– Fig. 3 shown. The notch base 38 includes a notch base opening 40 and a notched base slot 42 The notch base opening 40 is a gap of limited radial extent, which defines the mating surface 22 of the aluminum workpiece 12 and the fitting area 28 of the steel workpiece 14 outside the welding zone 18 This gap is formed as a result of the high clamping force exerted on the aligned sections of the back. 24 of the aluminum workpiece 12 and the reverse 30 of the steel workpiece 14 either directly or indirectly (e.g. through an additional aluminum and / or steel workpiece) through the opposing welding electrodes during the formation of the weld joint 10 is exerted. The notch base slot42 is radial within the notch base opening 40 and just outside the welding zone 18 directly adjacent to the weld joint 10 arranged. The notch base slot 42 is a non-connected, hydraulically sealed interface between the mating surfaces 22 , 28 of the aluminum and steel workpiece 12 , 14 .

[0044] An organic intermediate material layer (not shown) can also be placed outside the weld joint. 10 and around these between the passables 22 , 28 of the aluminum and steel workpiece 12 , 14 be arranged, although the notch base 38is typically free of such material. The organic intermediate layer can be an adhesive layer comprising a structural thermoset adhesive matrix. The structural thermoset adhesive matrix can be any curable structural adhesive, including, for example, a heat-curing epoxy resin or a heat-curing polyurethane. Some specific examples of heat-curing structural adhesives that can be used as an adhesive matrix include DOW Betamate 1486, Henkel Terokal 5089, and Uniseal 2343, all of which are commercially available. Additionally, the adhesive layer may further include optional filler particles, such as silicon dioxide particles, distributed throughout the thermoset adhesive matrix to modify the viscosity profile and other properties of the adhesive for manufacturing processes.According to other embodiments, the organic intermediate layer can be a sealant or a sound-absorbing material. The organic intermediate layer preferably has [a feature] at the butt joint interface. 16 a thickness of 0.1 mm to 2.0 mm or narrower from 0.2 mm to 1.00 mm.

[0045] The welded joint 10 consists of three essential compositional components: (1) an aluminum welding lens 44 , (2) an intermetallic layer 46 and (3) a ring ridge 48 from aluminide particles. The aluminide particles are preferably selected from the group consisting of nickel aluminide particles, iron aluminide particles, and a combination thereof. These three components 44 , 46 , 48 the weld joint 10 are generated when resistance spot welding is performed with a source of diffusible nickel and / or iron at the butt joint interface. 16This is practiced as explained in more detail below. The welded joint 10 may exhibit a porosity or void content that is centrally located within the connection 10 either at the weld bonding surface 34 or from the weld bonding surface 34 is arranged away from the surface. Additionally, virtually no fragments of the residual oxide film can remain at the weld bonding surface. 34 the weld joint 10 This combination of centrally located porosity and the absence of fragments of a residual oxide film and / or fragments of a composite residual film at the weld bonding surface. 34 contributes to good strength – especially good peel and transverse stress strength – of the welded joint 10 , by creating a clean bonding interface between the weld bonding surface 34 the connection 10 and the adjacent impact interface28 of the steel workpiece 14 is created, in particular to increase the size of the welded joint 10 around.

[0046] The aluminum welding lens 44 consists of re-hardened aluminum material, which is produced from a weld pool of the aluminum workpiece. 12 originates from, and forms the largest part of the weld joint 10 both on a volumetric basis and on a weight percent basis. The aluminum welding lens 44 is from the pass area 28 of the steel workpiece 14 through the intermetallic layer 46 separated and runs conically radially inwards when it moves away from the weld bonding surface 34 the weld joint 10 extends away. The intermetallic layer 46 represents the weld bonding surface 34 the weld joint 10ready and is produced by a metallurgical reaction between molten aluminum (which is formed during welding) and iron from the steel workpiece 14 formed to create intermetallic iron-aluminum compounds (Fe-Al compounds). In fact, the intermetallic layer can 46 It includes intermetallic FeAl3 compounds, intermetallic Fe2Al5 compounds and possibly other intermetallic iron-aluminum compounds, and it typically has a thickness across the weld bonding surface. 34 on, ranging from 1 μm to 5 μm. With regard to its comparable physical and mechanical properties, the intermetallic layer 46 harder, more brittle and less tough than the aluminum welding lens 44 .

[0047] The ring ridge 48 aluminide particles extend from the weld bonding surface 34 upwards either along the workpiece circumference36 or within these, so that they are separated from the notch base 38 radially inwards into the aluminum welding lens 44 extends into the ring. The aluminide particles that are in the ring ridge 48 The particles contained in the weld pool can be selected from the group consisting of nickel aluminide particles, iron aluminide particles, and a combination thereof. The nickel aluminide particles are rich in nickel and also contain aluminum and, optionally, other alloying elements such as iron and / or manganese, each present in lower concentrations than nickel. In fact, some of the nickel in the nickel aluminide particles can be replaced by iron present in the aluminum weld pool. 108 is dissolved. Particles are rich in nickel if they contain at least 20 atm% nickel, such as between 20 atm% and 50 atm% nickel. Some examples of nickel aluminide particles found in the ring bead 48The particles that can be found are NiAl3 or NiMnAl. The iron aluminide particles are rich in iron and also contain aluminum, as well as optionally other alloying elements such as nickel and / or manganese, each present in lower concentrations than iron. In some cases, the nickel aluminide particles can even transform into iron aluminide particles if sufficient nickel is replaced by iron in the aluminum weld pool. 108 is dissolved. Particles are rich in iron if they contain at least 20 atm% iron, such as between 20 atm% and 50 atm% iron. Some examples of iron aluminide particles found in the ring bead 48 Particles of FeAl3, Fe2Al5, or FeMnAl may be found. Of course, other types of nickel and / or iron aluminide particles may also be present in the ring bead, either additionally or instead of the specific particles listed above. 48can be found.

[0048] The aluminide particles, which form the ring bead 48 These particles can form from the migration of nickel into the aluminum weld pool to form nickel aluminide particles and / or from the migration of iron into the aluminum weld pool to form iron aluminide particles during the formation of the weld joint. 10 originate as described in more detail below. In some embodiments, particularly when the aluminide particles consist of nickel aluminide particles, the ring bead can 48 a discreet barrier 50 form as they are idealized in Fig. 2 is shown. In other embodiments, particularly when the aluminide particles are formed from iron aluminide particles, the ring bead can 48 a distribution band 52 form, as it is idealized in Fig. Figure 3 shows the distribution band. 52is a less dense accumulation of aluminide particles than the discrete barrier 50 , but fulfills the same basic function.

[0049] It is assumed that the ring ridge 48 aluminide particles increase the strength – especially the peel strength and transverse stress strength – of the weld joint 10 improved in at least two ways. First, if the aluminum workpiece 12 originally a heat-resistant surface oxide layer over its mating surface 22 exhibits, which is often the case, the process for generating the ring bulge removes 48 from aluminide particles the fragments of the residual oxide film and / or the composite residual film which are located at the impact interface 16 They may be present, from the weld bonding surface 34 Specifically, the introduction of aluminide particles into the aluminum welding lens occurs. 44This occurs when the aluminum weld pool reacts with nickel and / or iron originating from a source of diffusible nickel and / or iron located at the impact interface. 16A source of diffusible nickel and / or iron refers to any material from which nickel and / or iron can be released into the aluminum weld pool to form aluminide particles. Therefore, a source of diffusible nickel and / or iron is a material containing dilute and weakly bound nickel and / or iron that can be readily released from this source to migrate into the aluminum weld pool. Sources with tightly bound nickel and / or iron do not release nickel and / or iron into the aluminum weld pool, while sources with high concentrations of nickel and / or iron are likely to form a cohesive aluminide layer, which prematurely terminates nickel and / or iron migration into the weld pool.

[0050] The migration of diffusible nickel and / or iron from the source of these metals leads to an accumulation of aluminide particles in the ring bead. 48 This migration of nickel and / or iron into the aluminum weld pool – and the simultaneous conversion of these migrating metals into aluminide particles – causes fragments of the residual oxide film and / or fragments of composite residual film to detach from the mating surface. 28 of the steel workpiece 14 are carried away, and keep them away from the welding surface. 34 the subsequently formed weld joint 10 removed. The removal of fragments of the residual oxide film and / or fragments of the compound residual film from the weld bonding surface. 34 is considered advantageous, as mentioned above, since defects of this type near the interface tend to compromise the integrity of the bond between the welded joint 10 and the fitting area28 of the steel workpiece 14 to disturb. The fragments of the residual oxide film and / or the fragments of the compound residual film are much less harmful when they are contained within the volume of the aluminum welding lens. 44 away from the weld bonding surface 34 condition.

[0051] Secondly, the ring ridge provides protection. 48 from aluminide particles, once it is created, the weld bonding surface 34 the weld joint 10 opposite cracks that extend from the notch 38 can spread. The ring ridge 48 Made from aluminide particles, it reliably acts as a hard and strong barrier to crack propagation. If a crack occurs at the notch root... 38 it forms and spreads inwards into the weld joint 10 As the crack begins to spread, it will follow a non-preferred path into the volume of the aluminum weld lens. 44deflected inwards. This deflection of the crack propagation impairs the weld joint. 10 probably less, since the aluminum welding lens 44 softer and tougher than the intermetallic layer 46 is, which is along the weld bonding surface 34 the weld joint 10 is arranged. The ring ridge 48 aluminide particles therefore protect the section of the weld joint 10 (i.e., the intermetallic layer) 46 ), which is most susceptible to crack propagation and therefore most likely to be the cause of failure of the weld interface 10 is when it is subjected to a force.

[0052] By removing the fragments of the residual oxide film and / or the fragments of the compound residual film from the weld bonding surface 34 and the shielding of the intermetallic layer 46from cracks that extend from the surrounding notch base 38 They can spread and consistently achieve good strength properties in the welded joint. 10 can be achieved. To demonstrate, show Fig. 4– Fig. 7 and Fig. 8– Fig. 11. A micrograph of a weld joint showing an annular bead 48 consists of nickel aluminide particles (as a discrete barrier) 50 ), and Fig. 12– Fig. 15 as well Fig. 16– Fig. 19 show a microphotograph of a weld joint that has an annular bead. 48 consists of iron aluminide particles (as a distribution band) 52 As can be seen, no more than a negligible amount of fragments of the residual oxide film or fragments of the composite residual film, if any, are present at the weld bonding surface. 34 the weld joint 10 found, and the ring ridge 48aluminide particles extend from the weld bonding surface 34 upwards to prevent cracks from gaining access to the thin intermetallic layer 46 attained. When subjected to a force during standard peel or transverse stress tests, these welded joints exhibited 10 to button pull defects (i.e., a failure of the aluminum workpiece), which is a sign of a strong weld joint 10 is, in contrast to a failure at the interface, which is a sign of a weak weld joint 10 is.

[0053] The welded joint 10 can be formed by a resistance spot welding process, which provides a source of diffusible nickel and / or iron at the butt joint interface. 16 of the aluminum and steel workpiece 12 , 14 in the welding zone 18 used during the current flow. First, the aluminum workpiece is12 and the steel workpiece 14 initially assembled in an overlapping manner, by placing the workpieces 12 , 14 are aligned and fixed together in such a way that the fitting surface 22 of the aluminum workpiece 12 and the fitting area 28 of the steel workpiece 14 each other with or without the presence of an organic intermediate material layer (e.g., an adhesive or a sealant) to form the impact interface 16 to produce. Furthermore, in one embodiment, the mating surface can be 28 of the steel workpiece 14The source of diffusible nickel and / or iron has one of a number of suitable shapes. In a preferred implementation of this embodiment, the source of diffusible nickel and / or iron is a zinc-nickel surface coating (as the source of diffusible nickel) or a zinc-iron surface coating (as the source of diffusible iron) which coats the base steel substrate. 26 covered and the fitting area 28 of the steel workpiece 14 limited. According to another embodiment, an organic intermediate material layer between the mating surfaces can be used. 22 , 28 of the aluminum and steel workpiece 12 , 14is arranged, for example, as a heat-curable adhesive layer, which has a source of diffusible nickel and / or iron in the form of reactive metal alloy particles, which include as a component the metal elements that can react with molten aluminum to form aluminide particles.

[0054] The term “butt interface” is used extensively in this disclosure and is intended to distinguish between cases of direct and indirect contact between the mating surfaces. 22 , 28 the adjacent aluminium and steel workpieces 12 , 14 to include the fitting surfaces 22 , 28 They are in direct contact with each other when they are physically adjacent and not separated by a discrete organic intermediate layer. The mating surfaces 22 , 28They are in indirect contact with each other when separated by a discrete organic intermediate layer – and therefore do not experience the type of physical contact at the interface found in direct contact – and yet are in sufficiently close proximity that resistance spot welding can still be practiced. Indirect contact between the mating surfaces 22 , 28 of the aluminum and steel workpiece 12 , 14 This typically occurs when an optional organic intermediate material layer is present between the mating surfaces. 22 , 28 is applied before spot welding.

[0055] Once the aluminum and steel workpiece 12 , 14When mounted in a stacked configuration, a pair of opposing and mutually oriented welding electrodes conducts an electric current through the aluminum and steel workpieces. 12 , 14 , while a clamping force is applied to the workpieces 12 , 14 The welding electrodes are preferably water-cooled and made of a suitable copper alloy or another electrically and thermally conductive material. The electric current exchanged between the welding electrodes crosses the butt joint. 16 , which are between the adjacent aluminium and steel workpieces 12 , 14The resistance to the flow of electric current, preferably a direct current with a constant or pulsed current level ranging from 5 kA to 50 kA, and lasting for a total duration of 50 ms to 2500 ms, heats the steel workpiece. 14 with higher thermal and electrical resistance. The heat generated by the flow of the electric current melts the aluminum workpiece. 12 and creates an aluminum weld pool only within the aluminum workpiece 12 The aluminum weld pool penetrates the heat-resistant surface oxide layer of the aluminum workpiece. 12 , if present, and wets the mating surface 28 of the steel workpiece 14 .

[0056] During the time during which the aluminum weld pool coats the mating surface 28 of the steel workpiece 14When wetted, the diffusible nickel and / or aluminum reacts with the molten aluminum to form nickel aluminide and / or iron aluminide particles. The formation of these aluminide particles contributes to the formation of the heat-resistant surface oxide layer at the mating surface. 22 of the aluminum workpiece 12 to destroy it, if present. Eventually, the aluminide particles migrate upwards into the hotter sections of the aluminum weld pool and are trapped by the section of the weld pool with lower penetration and narrowing circumference. The aluminide particles can become embedded in the annular bead. 48They accumulate and settle there because they are trapped in a low-temperature area and therefore subject to less thermal degradation compared to the center of the aluminum weld pool. As they migrate upwards into the aluminum weld pool, the aluminide particles can detach fragments of the residual oxide film and / or fragments of the composite residual film from the still intact mating surface. 28 of the steel workpiece 14 The migration of aluminide particles can also have a similar effect on the sources of porosity to some extent, although the electric current flow through the workpieces 12 , 14can be regulated in a manner which drives the porosity towards the center of the aluminium weld pool according to any one of a variety of techniques, including, for example, the welding plan disclosed in US patent application No. 14 / 883,249.

[0057] The aluminum weld pool solidifies to form the weld joint. 10 , which the aluminium and steel workpiece 12 , 14 at the termination of the electric current flow between the welding electrodes in the welding zone 18 connects them. The aluminide particles are formed in the ring bead at this time. 48 fixed, and consequently the fragments of the residual oxide film and / or the fragments of the compound residual film are fixed by the aluminide particles that are in the volume of the aluminum welding lens. 44 are included, from the weld bonding surface 34 the weld joint 10carried away. After the current flow between the welding electrodes has ceased and the aluminum weld pool has solidified, the welding electrodes are withdrawn, and the clamping force against the aluminum and steel workpiece is released. 12 , 14 The force exerted by the set of welding electrodes is released. On one or both of the back sides 24 , 30 of the aluminum and steel workpiece 12 , 14 can a depression of the contact surface (on each surface) 24 , 30 in Fig. (1 shown) are left behind as a result of the forced contact exerted by the welding electrodes. Variations of the described process are of course possible, including with regard to the structure and composition of the source of diffusible nickel and / or iron located at the impact interface. 16 is located, without being limited to these.

[0058] The foregoing description of preferred exemplary embodiments and specific examples is merely descriptive; it is not intended to limit the scope of the claims that follow. Each of the terms used in the appended claims should retain its original and customary meaning unless otherwise specifically and unambiguously indicated in the description. QUOTES INCLUDED IN THE DESCRIPTION

[0059] This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited patent literature

[0060] US 2014 / 0360986

[0037]

Claims

[1] Welded joint which joins an aluminium workpiece and a steel workpiece together by means of a weld, wherein the welded joint comprises: an aluminum welding lens that is contained within an aluminum workpiece, an intermetallic layer arranged between the aluminium weld lens and a mating surface of the steel workpiece, wherein the intermetallic layer provides a weld bonding surface of the weld joint which adjoins and is bonded to the mating surface of the steel workpiece; and an annular bead of aluminide particles extending upwards and radially inwards from the weld bonding surface of the weld joint into the aluminium weld lens, wherein the annular bead of aluminide particles comprises nickel aluminide particles and / or iron aluminide particles. [2] Welded joint according to claim 1, wherein the ring bead of aluminide particles comprises nickel aluminide particles and iron aluminide particles. [3] Welded joint according to claim 1, wherein the ring bead of aluminide particles comprises nickel aluminide particles with 20 at% to 50 at% nickel and / or iron aluminide particles with 20 at% to 50 at% iron. [4] Welded joint according to claim 3, wherein the ring bead of aluminide particles comprises NiAl3 particles, NiMnAl particles, FeAl3 particles, Fe2Al5 particles and / or FeMnAl particles. [5] Welded joint according to claim 1, wherein the aluminium welding lens of the weld extends conically radially inwards as the aluminium welding lens extends away from the welding bonding surface of the welded joint. [6] Welded joint according to claim 1, wherein the weld bonding surface has a surface area ranging from 4 πt to 20 πt, where “t” is a thickness of the aluminium workpiece in a weld zone containing the weld joint prior to the formation of the weld joint. [7] Welded joint according to claim 1, wherein the intermetallic layer comprises intermetallic iron-aluminium compositions and has a thickness over the weld bonding surface ranging from 1 μm to 5 μm. [8] Welded joint connecting an aluminium workpiece and a steel workpiece, wherein the welded joint has a weld bonding surface adjacent to and connected with a mating surface of the steel workpiece, the welded joint comprising: an aluminum welding lens contained in the aluminum workpiece, wherein the aluminum welding lens extends conically radially inwards as the aluminum welding lens extends away from the weld bonding surface of the weld joint; an intermetallic layer arranged between the aluminium weld lens and the mating surface of the steel workpiece, wherein the intermetallic layer provides the weld bonding surface of the weld joint and comprises intermetallic iron-aluminium joints; and an annular bead of aluminide particles extending radially inwards from the weld bonding surface of the weld joint upwards and from a notch root surrounding the weld joint at a butt interface of the aluminium and steel workpiece into the aluminium weld lens, wherein the annular bead of aluminide particles comprises nickel aluminide particles and / or iron aluminide particles. [9] Welded joint according to claim 8, wherein the ring bead of aluminide particles comprises nickel aluminide particles with 20 at% to 50 at% nickel and / or iron aluminide particles with 20 at% to 50 at% iron. [10] Welded joint according to claim 8, wherein the ring bead of aluminide particles comprises NiAl3 particles, NiMnAl particles, FeAl3 particles, Fe2Al5 particles and / or FeMnAl particles.