Interlayer for resistance spot welding of aluminum / steel dissimilar metal and resistance spot welding method

Abstract

The invention discloses an intermediate layer for resistance spot welding of aluminum / steel dissimilar metals and a resistance spot welding method, the intermediate layer is a FeMnCuNi medium-entropy alloy, and the intermediate layer comprises the following components by atomic percent: 20-30% of Fe, 20-30% of Cu, 5-10% of Ni, 5-10% of Ni, and the balance of Fe and inevitable impurities. 20%-30% of Mn; 20%-30% of Cu; the sum of the atomic percentages of all the components is 100%. The method comprises the steps that (1) the aluminum alloy plate and the steel plate serve as welding base materials to be assembled in a lap joint mode, and the middle layer is added to the lap joint area; and (2) through resistance spot welding, a part of the middle layer and the base metal are melted through resistance heat at the lap joint interface, high entropy of nuggets is achieved through atomic diffusion and the high entropy effect, a welding joint with a solid solution as the main part is formed, and the aluminum / steel dissimilar metal composite structure is obtained. Compared with an existing aluminum / steel resistance spot welding technology, high entropy of nuggets is achieved by applying the high entropy effect, generation of brittle-hard phase intermetallic compounds in a welding joint is remarkably reduced, and a high-performance aluminum-steel composite structure is easily obtained.

Description

Technical Field

[0001] This invention relates to the field of aluminum / steel dissimilar metal joining technology, and more specifically to an intermediate layer and resistance spot welding method for aluminum / steel dissimilar metal resistance spot welding. Background Technology

[0002] In the production and use of automobiles and rail vehicles, reducing the weight of main components such as the car body can not only effectively save raw material consumption and improve energy utilization, but also reduce running resistance and increase operating speed. Therefore, it is necessary to design automobiles and rail vehicles with lightweight materials to meet practical needs. Aluminum alloys are lightweight alloys with low density and good corrosion resistance, making them ideal materials for achieving lightweighting. However, their strength is relatively low, and using them alone cannot meet the strength requirements of automobiles. High-strength steel, on the other hand, occupies an important position in structural components due to its extremely high strength and stiffness, good formability, and relatively low cost. Therefore, combining aluminum alloys and high-strength steel is one of the best combinations to balance lightweighting, manufacturing costs, and strength requirements.

[0003] Currently, industrial applications mostly use riveting, threaded connections, or adhesive bonding to join aluminum alloys and high-strength steel. However, these methods suffer from drawbacks such as poor appearance quality, inadequate sealing, and low production efficiency, resulting in less than ideal performance in the automotive and rail transportation sectors. Furthermore, the addition of rivets, screws, and other connectors increases the vehicle's weight, hindering overall vehicle lightweighting.

[0004] In this context, resistance spot welding has become the primary joining method in vehicle body manufacturing due to its low cost, high efficiency, high reliability, and ease of automation. However, aluminum alloys and high-strength steel have significant differences in their physicochemical properties and are also metallurgically immiscible. Direct resistance spot welding of aluminum alloys and high-strength steel will result in the formation of an excessively thick, brittle Fe-Al intermetallic compound layer at the interface, leading to a reduction in the mechanical properties of the joint.

[0005] Therefore, there is an urgent need to design an improved aluminum / steel resistance spot welding method to enhance joint performance. Summary of the Invention

[0006] In view of the above-mentioned shortcomings of the existing technology, the purpose of this invention is to provide an intermediate layer and a resistance spot welding method for resistance spot welding of dissimilar metals such as aluminum and steel, which solves the problem that when resistance spot welding aluminum alloy and high-strength steel is performed, an excessively thick brittle Fe-Al intermetallic compound layer is formed at the interface, resulting in a reduction in the mechanical properties of the joint.

[0007] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows:

[0008] An intermediate layer for resistance spot welding of dissimilar metals such as aluminum and steel, wherein the intermediate layer is a FeMnCuNi medium-entropy alloy, and its composition, by atomic percentage, includes: Fe: 20%–30%; Mn: 20%–30%; Cu: 20%–30%; Ni: 20%–30%, and the sum of the atomic percentages of each component is 100%.

[0009] The compositional design of the FeMnCuNi medium-entropy alloy intermediate layer in this invention is based on the following multi-dimensional considerations:

[0010] (1) Compared to high-entropy alloys, medium-entropy alloys, although having lower entropy values, are less stable. During welding, they can achieve in-situ high-entropy through atomic diffusion, thereby improving the mechanical properties of the welded joint. High-entropy alloys, while having very high entropy values, also have stable properties, resulting in a very high melting point and increased energy consumption. Furthermore, due to their inherent stability, high-entropy alloys are difficult to diffuse with the base material during resistance spot welding, thus affecting the quality of the welded joint. Therefore, this invention selects a medium-entropy alloy as the intermediate layer component.

[0011] (2) From the perspective of wettability, the intermediate layer material should contain a certain amount of Fe and Al elements to ensure the metallurgical compatibility between the intermediate layer and the base material. However, since the Al matrix has a low melting point and will melt in large quantities during the resistance spot welding process, the intermediate layer does not add Al elements, but only Fe elements.

[0012] (3) Mn and Cu are the main face-centered cubic (FCC) phase-forming elements, which can effectively improve the plasticity and toughness of medium entropy alloys (MEA);

[0013] (4) Ni acts as an austenite stabilizer, which can maintain the austenite in the weld even during rapid cooling; in addition, the high solubility of elements such as Al, Fe, and Mn in Ni can delay the formation of intermetallic compounds and produce supersaturated solid solutions at higher cooling rates.

[0014] Furthermore, the thickness of the intermediate layer is 0.3–1 mm.

[0015] Furthermore, the thickness of the intermediate layer is 0.5–0.7 mm.

[0016] A method for resistance spot welding of dissimilar metals such as aluminum and steel includes the following steps:

[0017] (1) The aluminum alloy plate and the steel plate are used as welding base materials and are assembled by overlapping, and the above-mentioned intermediate layer is added to the overlapping area.

[0018] (2) By resistance spot welding, the intermediate layer and the base material at the lap interface are melted by resistance heat. The high entropy of the molten nucleus is achieved by using atomic diffusion and high entropy effect. After forming a welded joint with solid solution as the main component, an aluminum / steel dissimilar metal composite structure is obtained.

[0019] The principle of this invention is as follows: During the resistance spot welding of dissimilar metals such as aluminum and steel, selective melting of a portion of the medium-entropy alloy interlayer and a portion of the base metal is achieved through precise control of the welding heat input. Specifically, the melting point of the interlayer is designed to be lower than that of the steel-side base metal but higher than that of the aluminum-side base metal. This characteristic ensures that during welding, the aluminum-side base metal completely melts while the steel-side base metal remains solid; the interlayer exhibits a partially melted state, forming a solid-liquid coexistence phase. This unique melting behavior produces multiple beneficial effects:

[0020] (1) Diffusion barrier effect: The partially melted intermediate layer acts as a physical barrier, effectively hindering the direct interdiffusion of Al and Fe elements. Its slow diffusion characteristics significantly reduce the interdiffusion rate between dissimilar metal elements, thereby inhibiting the excessive growth of the brittle intermetallic compound (IMC) layer.

[0021] (2) High-entropy effect: In the steel-side melt nucleus region, Fe elements diffuse from the steel-side base material into the melt nucleus region, resulting in an actual Fe content in the melt nucleus (64-71 at.%) that is significantly higher than the designed content (20-30 at.%) of the original medium-entropy alloy intermediate layer. Although this compositional change makes the melt nucleus no longer conform to the strict definition of a high-entropy alloy, the synergistic effect of multiple alloying elements (Fe, Mn, Cu, Ni, Al, etc.) still produces a significant entropy effect. This effect promotes the formation of a microstructure dominated by simple solid solutions in the melt nucleus region, rather than complex intermetallic compounds. Specifically, the high mixing entropy and Gibbs free energy of the multi-component system provide a thermodynamic driving force for the formation of solid solutions; this promotes the formation of solid solutions and inhibits the formation of brittle intermetallic compounds.

[0022] (3) Performance improvement: Through this "in-situ high entropy" mechanism, the content of brittle phase in the welded joint is significantly reduced, and the mechanical properties of the joint (especially the shear strength) are significantly improved, providing an effective way to achieve high-performance aluminum-steel dissimilar metal connection.

[0023] Furthermore, the resistance spot welding specifically involves: first applying a pre-pressure of 3-4 kN to ensure close adhesion between the base material and the intermediate layer; then applying a current of 3-5 kA for preheating for 80-120 ms; finally applying a current of 13-16 kA for welding for 250-350 ms; and maintaining the electrode pressure at 3-4 kN during the resistance spot welding process.

[0024] Furthermore, the aluminum alloy plate is a 6-series aluminum alloy, and the steel plate is an Fe-Mn series high-strength steel.

[0025] Furthermore, the aluminum alloy plate is a 6082-T6 aluminum alloy plate, and the steel plate is a DP590 steel plate.

[0026] Furthermore, before overlapping the base materials, use sandpaper to remove the oxide film on the surface of the aluminum alloy plate and steel plate, then clean and dry them for later use.

[0027] Compared with the prior art, the present invention has the following beneficial effects:

[0028] This invention adds a FeMnCuNi medium-entropy alloy interlayer of a specific proportion and thickness to the lap area of ​​the base materials and precisely controls the resistance spot welding process parameters to selectively melt part of the interlayer and the base material at the interface using resistance heat. Utilizing atomic diffusion and entropy effects, high-entropy melting of the weld nucleus is achieved, forming a weld joint dominated by solid solution. This effectively suppresses the formation of brittle intermetallic compounds and significantly improves the mechanical properties of the weld joint (e.g., shear strength reaches 7.935±0.3kN, an 88.2% improvement compared to the composite structure without the interlayer), ultimately obtaining a high-quality aluminum-steel composite structure. This structure has advantages such as convenient welding and assembly, wide applicability, and ease of promotion and application. Attached Figure Description

[0029] Figure 1 This is a schematic diagram of the aluminum / steel dissimilar metal resistance spot welding of the present invention;

[0030] Figure 2 This is a schematic diagram of the welding process in Embodiment 1 of the present invention; I1: preheating current; I2: welding current; T1: preheating time; T2: welding time; F: electrode pressure;

[0031] Figure 3 The following are scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS) images of different regions in the melt nugget of Embodiment 1 of the present invention: (a) is the region on the upper side of the melt nugget near the fusion line on the steel side; (b) is the region reaching or near the center of the melt nugget; and (c) is the region at the bottom of the melt nugget near the intermediate layer of the medium-entropy alloy.

[0032] Figure 4 The image shows the XRD pattern of the welded joint obtained in Example 1 of this invention. Detailed Implementation

[0033] The specific embodiments of the present invention will be described in further detail below with reference to specific examples.

[0034] The numerical ranges in this invention should be understood to also specifically disclose each intermediate value between the upper and lower limits of the range. Each smaller range between any stated value or intermediate value within a stated range, and any other stated value or intermediate value within said range, is also included in this invention. The upper and lower limits of these smaller ranges may be independently included or excluded from the range.

[0035] Unless otherwise stated, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. While only preferred methods and materials are described herein, any methods and materials similar or equivalent to those described herein may be used in the implementation or testing of this invention. All references to this specification are incorporated by way of citation to disclose and describe methods and / or materials associated with those references. In the event of any conflict with any incorporated reference, the content of this specification shall prevail. The terms “comprising,” “including,” “having,” “containing,” etc., as used herein are open-ended, meaning that they include but are not limited to.

[0036] Unless otherwise specified, the experimental methods used in this invention are all conventional methods.

[0037] Unless otherwise specified, all materials and reagents used in this invention can be purchased or synthesized by known methods.

[0038] In the quantitative experiments of this invention, each experiment was repeated three times, and the average value of the results was taken.

[0039] Existing resistance spot welding methods for aluminum / steel dissimilar metals have the following problems: an excessively thick intermetallic compound layer is easily formed at the lap interface, resulting in low mechanical properties of the welded joint. Therefore, this invention effectively solves the above problems and improves the mechanical properties of the aluminum / steel dissimilar metal composite structure by adding a FeMnCuNi medium-entropy alloy interlayer between the aluminum and steel base materials before resistance spot welding. The specific implementation method is as follows:

[0040] Example 1

[0041] like Figure 1 , Figure 2 As shown in the figure, this embodiment provides a method for resistance spot welding of dissimilar metals such as aluminum and steel. The specific steps are as follows:

[0042] (1) Base material pretreatment: DP590 steel plate (thickness 1.4mm, length × width 138mm × 30mm) and 6082-T6 aluminum alloy plate (thickness 2.5mm, length × width 138mm × 30mm) were used as welding base materials, and they were polished with 240# → 400# sandpaper to remove the oxide film on the surface of the base materials; then ultrasonically cleaned with alcohol for 10 minutes, and then dried with a hair dryer for later use;

[0043] (2) Overlap assembly: Place the 6082-T6 aluminum alloy plate horizontally on the lower electrode platform of the resistance spot welding machine (model DTBZ-80), then lay the FeMnCuNi medium-entropy alloy intermediate layer (thickness 0.5mm; diameter 30mm) flat on the upper surface of the aluminum alloy plate, and then overlap and cover the intermediate layer with the DP590 steel plate to form a "steel / intermediate layer / aluminum" sandwich structure; in the FeMnCuNi medium-entropy alloy, by atomic percentage, Fe: 23%; Mn: 26%; Cu: 27%; Ni: 24%.

[0044] (3) Electrode positioning: Align the P-type copper alloy electrode (end face diameter 8mm) vertically with the center of the overlap area, with the upper electrode contacting the upper surface of the DP590 steel plate and the lower electrode contacting the lower surface of the 6082-T6 aluminum alloy plate.

[0045] (4) Welding execution: Start the DTBZ-80 resistance spot welding machine, first apply a pre-pressure of 3.5kN to make the base material and the intermediate layer fit tightly together and eliminate the assembly gap; then apply a current of 4kA and preheat for 100ms; then apply a current of 15kA and weld for 300ms. After welding, hold the pressure for 1000ms and then remove the electrode to obtain the aluminum / steel dissimilar metal composite structure; the electrode pressure is kept at 3.5kN during the resistance spot welding process.

[0046] Figure 3 The following are scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS) images of different regions in the melt nugget of Embodiment 1 of the present invention: (a) is the region on the upper side of the melt nugget near the fusion line on the steel side; (b) is the region reaching or near the center of the melt nugget; and (c) is the region at the bottom of the melt nugget near the intermediate layer of the medium-entropy alloy.

[0047] Depend on Figure 3 (a) It can be seen that the region on the upper side of the fusion nucleus near the fusion line on the steel side is composed of columnar crystals and equiaxed crystals. The inventors analyzed that this region, being closest to the copper electrode, was the first to be rapidly cooled to a supercooled state by the cooling water, providing thermodynamic conditions for grain nucleation; at the same time, the surface energy of the semi-molten grains is low, which promotes the rapid non-spontaneous nucleation of the microstructure under these conditions, and directional growth along the cooling gradient direction of the joint (perpendicular to the fusion line), ultimately forming a microstructure characterized by the coexistence of columnar crystals and equiaxed crystals.

[0048] Depend on Figure 3 (b) It is known that as the solidification process proceeds, when the grains reach or approach the center of the melt nucleus, they no longer grow along a fixed direction, but grow freely in all directions, eventually forming a large number of equiaxed dendrites. The inventors believe that this is because the heat dissipation in the center region of the melt nucleus has lost its directionality (i.e., heat diffuses uniformly from the center of the melt nucleus to the surrounding area, and there is no longer a single dominant heat dissipation direction), which causes the temperature gradient to become gentler, and the grain growth loses its directional driving force, thus forming an isotropic equiaxed dendritic structure.

[0049] Depend on Figure 3 (c) It can be seen that the region near the bottom of the melt nucleus and the intermediate layer of the medium-entropy alloy exhibits a phenomenon of coexistence of columnar crystals and equiaxed crystals with distinct directions. The inventors believe that this is because the undercooling degree in this region is low and the compositional uniformity of the melt nucleus is high, which inhibits grain nucleation and mainly results in the growth of grains by attaching to primary grains. At the same time, due to the influence of the thermal conductivity of the intermediate layer material, the heat dissipation direction is mainly unidirectional and perpendicular to the interface, which promotes the growth of grains along the heat dissipation direction to form columnar crystals. However, due to compositional fluctuations or instantaneous changes in undercooling conditions in local micro-regions, a small number of equiaxed crystals will also form, ultimately exhibiting the characteristic of the coexistence of two crystal structures.

[0050] Figure 3 The energy dispersive spectroscopy (EDS) results for different regions are shown in Table 1.

[0051] Table 1 Figure 3 Energy dispersive X-ray spectroscopy (EDS) results (at%) for different regions of China

[0052]

[0053] As shown in Table 1, the dilution effect of the steel base material caused the Fe content in different regions to exceed the composition range of high-entropy alloys. Although this prevented the direct formation of high-entropy alloys in the weld nugget, in-situ high-entropy formation was achieved through atomic diffusion. This process promoted the formation of simple solid solutions of various metallic elements rather than complex intermetallic compounds. This is because the interdiffusion of multiple elements within the weld nugget generated a high Gibbs free energy, which promoted the formation of solid solutions within the spot weld nugget and inhibited the formation of compounds, especially brittle intermetallic compounds, thus ultimately achieving high-quality welding of aluminum and steel.

[0054] It should be noted that, regarding Table 1, due to the detection limits of light elements (such as H, Li, and B), overlap between characteristic peaks of some elements, errors in the background subtraction process, and biases caused by rounding of quantitative results, the total elemental content obtained from EDS analysis may not strictly add up to 100 at%. In this study, the EDS data have been normalized based on the detected elements.

[0055] Figure 4 The XRD pattern of the welded joint obtained in Embodiment 1 of the present invention is shown below. Figure 4 As can be seen, at the welded joint, in addition to the detection of α-Al and α-Fe, the diffraction peaks mainly indicate a single-phase solid solution structure of face-centered cubic (FCC) phase, without the formation of common brittle intermetallic compounds. Therefore, the resistance spot welding method of this invention can effectively suppress the formation of brittle intermetallic compounds.

[0056] Example 2

[0057] This embodiment provides a method for resistance spot welding of dissimilar metals, aluminum and steel, mainly the same as in Embodiment 1, except that in the FeMnCuNi medium-entropy alloy, the atomic percentages are: Fe: 20%; Mn: 26%; Cu: 30%; Ni: 24%. The thickness of the intermediate layer is 1 mm. The welding parameters are: welding current 16 kA, welding time 350 ms, and electrode pressure 3 kN during spot welding.

[0058] Example 3

[0059] This embodiment provides a method for resistance spot welding of dissimilar metals, aluminum and steel, mainly the same as in Embodiment 1, except that in the FeMnCuNi medium-entropy alloy, the atomic percentages are: Fe: 30%; Mn: 20%; Cu: 20%; Ni: 30%. The thickness of the intermediate layer is 0.3 mm. The welding parameters are: welding current 13 kA, welding time 250 ms, and electrode pressure 4 kN during spot welding.

[0060] Comparative Example 1

[0061] This comparative example provides a method for resistance spot welding of dissimilar metals of aluminum and steel, which is mainly the same as in Example 1. The difference is that instead of adding a FeMnCuNi medium-entropy alloy intermediate layer between the aluminum and steel dissimilar metals, the two base materials are directly overlapped.

[0062] Test Results

[0063] Tensile tests were conducted on the welded joints obtained by the welding methods in Examples 1-3 and Comparative Example 1 using an electronic universal testing machine (Sans-CMT5105). The tensile rate was set to 1 mm / min. To ensure the accuracy of the test and reduce random errors, three tensile samples were cut for each set of parameters, and the average value of the test results was taken. The test results are shown in Table 2.

[0064] Table 2 Comparison of Tensile Test Results

[0065] Example Shear strength Example 1 7.935±0.3kN Example 2 6.193±0.2kN Example 3 5.143±0.5kN Comparative Example 4.216±0.2kN

[0066] As shown in Table 2, the shear strength of the aluminum-steel dissimilar metal composite structure prepared by the resistance spot welding method of the present invention is significantly improved compared with the existing aluminum-steel composite structure without the addition of FeMnCuNi medium-entropy alloy intermediate layer. Among them, the shear strength of the welded joint of the aluminum-steel composite structure obtained in Example 1 is the highest, which is 7.935±0.3kN, which is 88.2% higher than that of the welded joint of the aluminum-steel composite structure without the addition of intermediate layer in Comparative Example 1, thus obtaining a higher quality welded joint.

[0067] In summary, using the FeMnCuNi medium-entropy alloy of this invention as an intermediate layer for resistance spot welding results in good compatibility with aluminum alloys and steel during welding, and the weld (melt nugget) has a single-phase face-centered cubic solid solution structure, significantly improving the overall mechanical properties of the joint.

[0068] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit the technical solutions. Those skilled in the art should understand that modifications or equivalent replacements of the technical solutions of the present invention that do not depart from the purpose and scope of the technical solutions of the present invention should be included in the scope of the claims of the present invention.

Claims

1. An intermediate layer for resistance spot welding of dissimilar metals such as aluminum and steel, characterized in that, The intermediate layer is a FeMnCuNi medium-entropy alloy, whose composition, by atomic percentage, includes: Fe: 20%~30%; Mn: 20%~30%; Cu: 20%~30%; Ni: 20%~30%, and the sum of the atomic percentages of each component is 100%.

2. The intermediate layer for resistance spot welding of dissimilar metals (aluminum / steel) according to claim 1, characterized in that, The thickness of the intermediate layer is 0.3~1mm.

3. The intermediate layer for resistance spot welding of dissimilar metals (aluminum / steel) according to claim 2, characterized in that, The thickness of the intermediate layer is 0.5~0.7mm.

4. A method for resistance spot welding of dissimilar metals such as aluminum and steel, characterized in that, Includes the following steps: (1) The aluminum alloy plate and the steel plate are used as welding base materials and are assembled by overlapping, and an intermediate layer as described in any one of claims 1 to 3 is added in the overlapping area; (2) By resistance spot welding, the intermediate layer and the base material at the lap interface are melted by resistance heat. The high entropy of the molten nucleus is achieved by using atomic diffusion and high entropy effect. After forming a welded joint with solid solution as the main component, an aluminum / steel dissimilar metal composite structure is obtained.

5. The method for resistance spot welding of dissimilar metals such as aluminum and steel according to claim 4, characterized in that, The resistance spot welding process is as follows: first, apply a pre-pressure of 3~4kN to ensure close contact between the base material and the intermediate layer; then, apply a current of 3~5kA and preheat for 80~120ms; finally, apply a current of 13~16kA and weld for 250~350ms; during the resistance spot welding process, maintain the electrode pressure at 3~4kN.

6. The method for resistance spot welding of dissimilar metals of aluminum and steel according to claim 4, characterized in that, The aluminum alloy plate is a 6-series aluminum alloy, and the steel plate is a Fe-Mn series high-strength steel.

7. The method for resistance spot welding of dissimilar metals of aluminum and steel according to claim 6, characterized in that, The aluminum alloy plate is a 6082-T6 aluminum alloy plate, and the steel plate is a DP590 steel plate.

8. The method for resistance spot welding of dissimilar metals of aluminum and steel according to claim 4, characterized in that, Before splicing the base materials, use sandpaper to remove the oxide film from the surface of the aluminum alloy plate and steel plate, then clean and dry them for later use.

Images