Research On Microstructure And Properties Of Die Casting AlSi7MnMg Alloy Oscillating Laser Welded Joint

As technology develops rapidly and people demand lightweight new energy vehicles that are energy-efficient and environmentally friendly,aluminum(Al)alloys play an increasingly important role in human production and life due to their excellent properties.In particular,die-cast Al alloys have been widely used in new energy vehicles due to their excellent casting performance,high specific strength,and corrosion resistance.This study focuses on the Al Si7 Mn Mg die-cast alloy,which is difficult to weld due to its high hydrogen content and shrinkage porosity,in addition to the welding difficulties typically encountered with profile Al alloys such as weld collapse,porosity,and cracking.In order to obtain a high-quality lap joint weld of the Al Si7 Mn Mg die-cast alloy,this study uses oscillating laser welding technology to investigate the influence of oscillation frequency and amplitude on weld morphology,macro-defects,microstructure,and mechanical properties.The mechanism of the formation of fine equiaxed grains by changing the heat input distribution and temperature gradient direction of the weld pool by the linear oscillation mode of the weld is expounded.The study identifies the optimal oscillating laser welding process window and explains the superiority of oscillating laser welding over conventional laser welding in the lap joint welding of die-cast Al alloy plates.The main conclusions are as follows:(1)Compared with conventional laser welding,oscillating laser welding can effectively improve the macro-defects of the weld,especially the porosity rate.When the oscillation frequency or amplitude increases,the number of pores in the weld is significantly suppressed.Through the calculation of the Reynolds number in the oscillating laser welding melt pool,it is found that the porosity defects in the weld are negatively correlated with the Reynolds number.In addition,the oscillating laser turns the concave corner weld of the conventional laser into a convex corner weld,increasing the depth of the weld penetration and improving the weld collapse defects of the conventional laser weld.(2)Oscillating laser welding increases the width of the melt pool,thereby reducing the unit heat input of the melt pool and achieving the goal of refining the grain size in the weld zone.When the oscillation frequency or amplitude of the laser is too large,reaching 5 mm / 200 Hz or 7 mm / 100 Hz,the weld undergoes remelting welding,and the bottom of the melt pool shows layering.In addition,due to the stirring effect of the oscillating laser,the fluidity of the melt pool is enhanced,leading to the transformation of the eutectic silicon phase from continuous distribution at the grain boundaries to dispersed distribution on the α-Al matrix.(3)Oscillating laser welding significantly improves the tensile-shear performance of the welded joint.When the oscillation frequency is 100 Hz and the amplitude is 7mm,the maximum tensile-shear load and displacement of the oscillating laser welded joint are increased by 70% and 125%,respectively,compared with the conventional laser sample.Moreover,the fracture mode of the oscillating laser welded joint is mostly ductile fracture,which is superior to the quasi-cleavage fracture of the conventional laser welded joint.