Study On Resistance Spot Welding Of 16Mn Steel/6063-T6 Aluminum Alloy Dissimilar Materials

With low density, high specific strength, strong corrosion resistance and good processability, the aluminum alloy was regarded as the most ideal lightweight automotive material. Although the aluminum space frame(ASF) could reduce weight by 50%, the high cost and complex manufacturing process restrict its development and use. As a consequence, the introduction of aluminum alloy components to steel bodies and the effective joining between aluminum and steel have attracted more and more attention. Due to large difference of thermophysical properties and low solubility between aluminum and steel and the formation of brittle and hard Fe-Al intermetallic compounds on the interface, the weldability of steel and aluminum alloy is poor. The research about resistance spot welding between aluminum and steel was relatively less, and the mechanical properties of joints were unsatisfied so far. Therefore, the study possessed important theoretical and practical value.In this paper, 6063-T6 aluminum alloy with the thickness of 1.5mm and 16 Mn steel with the thickness of 1.0 mm were lap welded by resistance spot welding. The microstructures of 16Mn/6063-T6 dissimilar material resistance spot welded joints were investigated. The results indicated the welded joints were formed by wetting and spreading of molten aluminum alloy on the steel surface, which belonged to welding-brazing joint in essential. The microstructure had the characteristics of epitaxial solidification near the fusion line in the aluminum alloy nugget. The intermetallic compounds had a double layer structure, tongue-like Fe2Al5 near steel and needle-like Fe4Al13 near aluminum alloy. The intermetallic compound layers were the thickest in the center region, and became thinner toward the outer region. When the nugget diameters of the joints were lower than 5.8mm, the fracture occurred in interfacial failure mode. Otherwise, the joints fractured in button pullout mode.The influences of welding parameters on the microstructures and mechanical properties of welded joints were studied when F-type electrode caps were used. With the increase of welding current and welding time, the indentation depth and nugget diameter of joints increased, the tensile shear load increased firstly and then decreased. Under the condition of welding current of 9kA, welding time of 250 ms and electrode force of 2.5kN, the nugget diameter and tensile shear load of the welded joint were 5.35 mm and 2534 N, respectively. And the fracture occurred in interfacial fracture mode, and the intermetallic layer was the primary factor limiting tensile shear load of the joint.The influences of welding parameters on the microstructures and mechanical properties of welded joints were investigated when a flat electrode cap with the diameter of 10 mm close to steel and a spherical electrode cap with the diameter of 35 mm close to aluminum alloy were used. The results indicated that the influences of welding parameters on the nugget diameter and tensile shear load were consistent with those using F-type electrode caps. Under the condition of welding current of 15 kA, welding time of 250 ms and electrode force of 3.5kN, the nugget diameter and tensile shear load of the welded joint were 8.0mm and 3622.9N, respectively. The tensile shear load increased by 43% and the indentation ratio decreased greatly compared with those using F-type electrode caps.The influences of Cu, Zn, Si, Ti, Ni on microstructures and mechanical properties of the welded joints were discussed systematically. The results indicated that the elements Cu, Zn, Si and Ni could improve the wetting and spreading of liquid aluminum alloy on the steel’s surface and limit the growth of intermetallic compound layers. The elements Cu, Si, Ti and Ni could refine the grains and improve the microstructures of aluminum nuggets. The Cu promoted the formation of(Fe,Cu)2Al5 and decreased the brittleless of Fe-Al intermetallic copmpouds. After the appropriate amount of Cu, Zn, Si, Ti and Ni were added, the tensile shear loads of the welded joints were 2780 N, 2915 N, 2882 N, 2915 N and 2992 N, respectively, which increased by 24%, 30%, 29%, 30% and 31% compared with those without alloy element addition. As a consequence, the nugget alloying was an effective way to improve the mechanical properties of the resistance spot welded joints.