With the continuous improvement of lightweight requirements, the preparation of Mg/Al composite structures by welding is in urgent demand and has broad prospective applications in the industrial field. However, it is easy to form a large number of brittle intermetallic compounds when welding Mg/Al dissimilar alloys, and it is difficult to obtain high-quality welded joints. The solid-state welding method has the characteristics of low energy input and high efficiency, which can inhibit the formation of brittle intermetallic compounds and help to solve the problem of the poor strength of welded joints using Mg/Al dissimilar alloys in engineering applications. Based on the literature of ultrasonic welding, friction welding, diffusion welding, explosive welding, magnetic pulse welding, and resistance spot welding of Al/Mg in recent years, this paper summarized and prospected the research status of solid-state welding using Mg/Al dissimilar alloys from three aspects: the optimization of welding parameters, the addition of interlayers, and hybrid welding process.
Resistance spot-welding of dissimilar metals, medium manganese TRIP steel 7Mn and DP590, is carried out. The effects of single-pulse welding parameters and a double-pulse-tempering current on the quality characteristic parameters and mechanical properties of 7Mn/DP590 spot-welded joints are studied. The welding process parameters are optimized using the control variable method. The results show that the optimal process parameters under a single pulse are as follows: electrode pressure: 4.5 kN, welding current: 9 kA and welding time: 300 ms. The failure mode of the welding joint is partial pull-out failure (PF-TT). The welding parameters have great influence on the nugget diameter and thickness reduction. Expulsion, crack and shrinkage are displayed in the joint under high electrode pressure. Softening occurs in the heat-affected zone due to a strong halo effect in the single-pulse weld. The tempering zone on the DP590 side is 202.49 HV, which is the lowest hardness point, while the hardness of the nugget zone is 450 HV. The addition of the tempering current homogenizes the microstructure with different failure paths and eliminates the stress. The tensile shear force of the joint increases by 17.13 %. The 7Mn Steel/DP590 resistance spot-welding joint is from the fusion line to the center of the nugget, and the microstructure is composed of plane crystal, cellular crystal, dendritic crystal and columnar crystal, in turn. The nugget zone is composed of lath martensite and a small amount of residual austenite. Fine quasi-spherical and lamellar interbedded cementites are formed in the tempering zone of the DP590-side heat-affected zone.
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