Study On Resistance Spot Welding Of AZ31 Magnesium Alloy And DC54D Steel

With the rapid development of manufacturing industry,The energy crisis caused by automobiles and the greenhouse gas emissions have gradually become a hot spot of social concern.Automotive lightweighting has become an important way to solve the energy crisis and reduce greenhouse gas emissions.Owing to excellent ultra-deep stamping performance The DC54D has become a mainstream cover in the automotive industry.The combination of AZ31 magnesium alloy and DC54D steel can achieve the advantages of both weight reduction and safety.At the same time,resistance spot welding is a method of material connection used in the field of vehicle manufacturing.Therefore,the research on resistance spot welding of AZ31 and DC54D is very important.This paper studies AZ31/DC54D resistance spot welding,AZ31/DC54Dresistance spot welding with Zn slice interlayer,and AZ31/DC54D resistance spot welding with the DC54D auxiliary plate.The macroscopic features,microstructure and mechanical properties of joint were studied.Besides,mechanism of nugget formation and growth during resistance spot welding and fracture failure behavior of spot welded joint were analyzed.The side of the magnesium alloy of AZ31 and DC54D spot welding joint can be divided into base material zone?BM?,heat affected zone?HAZ?and fusion zone?FZ?.The grains in the heat-affected zone are significantly coarser,with an average grain size of 12microns.The grain at the edge of the nugget zone is in the shape of columnar crystal.The center of the nugget zone is mainly composed of equiaxed crystals.Lots of precipitates including Mg₂Si and Al₁₁Mn₃Zn₂ is deposited on the grain boundary of the fusion zone.The nugget on one side of the DC54D steel is mainly formed from the center of the steel side,showing an elliptical shape.The steel side mainly consists of a parent material area and a heat affected zone,and the base material area is composed of equiaxed ferrite grains.The heat affected zone is a relatively coarse ferrite grain.The Fe-Al intermetallic compound and the magnesium-zinc eutectic structure are mainly formed at the joint.As the welding current and welding time increase,the fracture form of the joint changes from interface fracture to plug fracture,and the tensile shear strength increases and remains stable.When the welding current is 8 KA,the welding time is 14 cycles,and the electrode pressure is 2.5KN,the spot welding joint is peeled and fractured,and the tensile shear strength of the joint reaches a maximum value of 3.1KN.After the addition of a 20 micron,50 micron intermediate layer of zinc foil,the shape of the AZ31 and DC54D spot welded joints changed greatly,the diameter of the nugget increased slightly,and the thickness of the nugget decreased by 30%and 60,respectively.%.The nugget region of the joint is mainly composed of magnesium-zinc eutectic structure.The second phase of MgZn₂ is continuously distributed on the grain boundary of the nugget zone.The microhardness of the nugget zone is increased to 135HV,and the hardness value is increased by 28.6%.When the thickness of the intermediate layer zinc slice is 20 microns,the welding current is 8KA,the welding time is 14 cycles,and the electrode pressure is 2.5KN,the mechanical properties of the joint are improved,and the tensile shear strength reaches 3.6KN.18%higher than joints without Zn slice interlayer under the same welding process parameters.A three-layer resistance spot welded joint with a DC54D auxiliary plate added to the side of the AZ31 magnesium alloy was studied.Since the contact resistance on both sides is high,the heat input during the welding process increases.The diameter of the AZ31 nugget increases by an average of 30%.The grains inside the nugget are also significantly roughened,and the equiaxed grain size inside the nugget reaches 10microns.At the same time,microcracks,shrinkage cavities and shrinkage in the nugget area are also significantly increased.When the welding current is 7 KA,the welding time is 14 cycles,and the electrode pressure is 2.5KN,the tensile shear force of the three-layer spot welding joint reaches a maximum value of 4.9KN,which is 50%higher than the common AZ31 and DC54D spot welding heads under the same welding process parameters.