Study On Microstructure And Mechanical Property Of DP780/AZ31B Lap Joint During Conductive Laser Welding

Based on the idea of automobile lightweight,the lap joint of DP780 dual phase steel（upper plate）and AZ31B magnesium alloy（lower plate）was studied during conductive laser welding,aiming at the problem of being difficult to realize reliable connection between steel and magnesium.The weld surface morphology and interface microstructure were studied by the processing control,such as changing the welding process parameters,couplinging ultrasonic and adding transition layer under the help of scanning electron microscope equipped with energy dispersion spectrum,electron backscattering diffraction,transmission electron microscope and X-ray diffraction.Then,the interface hardness distribution and joint tensile shear strength were explored.In addition,the interface connection mechanism was clarified during conductive laser lap welding.Firstly,the conductive laser lap welding of DP780 galvanized dual phase steel and AZ31B magnesium alloy was carried out by optimizing the laser power from 2.7 k W to 3.0 k W and welding speed from 5 mm/s to 7 mm/s.When the welding heat input was moderate,the surface morphology of the lap weld seam was sound.Various phases,such as the continuous and dense Mg Zn phase,Mg Zn₂ phase and（α-Mg+Mg Zn）eutectic structure were generated at the interface of the lap joint.The hardness near the interface was between 50-70 HV,and the change was not significant.Moreover,the tensile shear strength of the lap joint was mainly 140-170 N/mm.The maximum joint strength could reach 216 N/mm at the laser power of 2.8 k W and the welding speed of5 mm/s.Secondly,the ultrasonic assisted conductive laser lap welding was employed to weld DP780 galvanized dual phase steel and AZ31B magnesium alloy.A small amount of pit defects was produced on the weld seam under the effect of ultrasonic vibration and laser power.On the one hand,the ultrasonic could refine the interface structure and make it evenly distributed.On the other hand,it was conducive to the spread of the reaction layer at the interface of steel and magnesium.As the ultrasonic amplitude increased from 10%to 90%,the joint strength increased from 170 N/mm to 281 N/mm at the laser powser of 2.0 k W.When the ultrasonic amplitude was 50%and the laser power was 2.3 k W,the joint strength reached 290 N/mm,which was 34.2%higher than the maximum strength of lap welded joint without ultrasonic.Finally,the laser lap welding was carried out between DP780 dual phase steel（not galvanized）and AZ31B magnesium alloy by adding Ni foil or coupling ultrasonic simultaneously.In case of only adding Ni foil,the weld seam was sound at the laser power of 2.5 k W or 2.7 k W.During welding,the Ni foil in the center of the interface was melted,which reacted with magnesium alloy to form Al Ni phase,Mg₂Ni phase,Mg Ni₂ phase and（α-Mg+Mg₂Ni）eutectic structure.Moreover,the molten Ni foil diffused along the interface of steel and magnesium to form an enrichment zone of Ni element.The microhardness of the interface reaction layer reach 228 HV and the joint strength reach 188 N/mm as a contribution of these microstructure characteristics.While,in case of adding Ni foil and ultrasonic,the lap joint could be obtained at the laser power of 2.3 k W,however,the strength was not high,since the metallurgical reaction was insufficient as a result of the lower heat input failed to fuse the Ni foil.On the whole,the lap joint strength could be improved to a certain extent by adding Ni foil or coupling ultrasonic during conductive laser welding.