Study On Laser Welding Process Of LZ91 Magnesium Lithium Alloy /TC4 Titanium Alloy Dissimilar Materials

The composite structure formed by the combination of titanium alloy and magnesium-lithium alloy can realize the complementary properties of the alloy,strengthen the strength of magnesium-lithium alloy,improve the corrosion resistance,and has a great potential for development in aerospace,deep sea operations and other fields.Due to the great difference in thermophysical properties between magnesium-lithium alloy and titanium alloy,the mutual solubility of the two alloys in the melting state is low,so the traditional welding methods have the problems of poor joint forming and low bonding strength.In this paper,two kinds of alloys were welded by laser butt welding,and the macroscopic morphology,microstructure and mechanical properties of LZ91/TC4 joint were studied by different technological methods.The following research results were obtained:Firstly,the effects of laser power,welding speed and spot offset on weld forming effect and mechanical properties of the joint were studied.The experimental result show that when the laser power in 1200 W?1400 W,the welding speed in 40 mm/s?50 mm/s,and the spot offset is between 0?-0.2 mm,forming a good weld and surface,melting magnesium lithium alloy wetting spreading at the side of the titanium alloy,the weld area distribution more granular material containing titanium white,and the tiny Mg₁₇Al₁₂ intermetallic compound,microhardness of weld area than magnesium lithium alloy material has increased 14.05 HV;The Mg-Li alloy was in a state of overheating for a long time in the welding process,leading to abnormal grain growth in the heat-affected zone,so the tensile sample broke in the heat-affected zone.From the fracture morphology,it can be seen that there is a little Mg-Li alloy residual on the side of the titanium alloy,and the bonding is firm,and the tensile strength of the joint is 90.4 MPa.Secondly,COMSOL software was used to simulate the temperature field distribution on the weld surface and cross section to observe the heat transfer process under different spot offset.It was found that when the spot shifted 0.2 mm or no shift to the base metal of Mg-Li alloy,more TC4 base metal was melted and the flow range of molten pool was wide,and more Mg-Al compounds were found at the interface.When the spot shift is greater than 0.2 mm towards the base metal of Mg-Li alloy,the Mg₁₇Al₁₂ phase in the weld decreases due to the decrease of the melting amount of TC4 base metal.The light spot is migrated to the titanium alloy base metal,the energy accumulates on the surface of TC4,and the thermal conductivity of TC4 is low,so the high temperature molten pool causes the oxide reaction layer to appear at the base metal junction.Finally,laser scanning welding with circular scanning path was used to suppress the pores in the welding seam and refine the grain structure at the welding seam.The experimental results show that when the scanning frequency is lower than 50 Hz,the inflection point of energy accumulation exists,and the surface of the weld has porosity defects,and there is basically no diffusion between the two base metals.When the scanning frequency is greater than 100 Hz,regular wavy lines appear on the weld surface and the content of Al element increases.At this time,the tensile strength of the joint varies from 135 MPa to 160 MPa.When the scanning amplitude is lower than 1.0 mm,the weld surface appears serious sag,and when the scanning amplitude is higher than 1.6 mm,the surface of titanium alloy base material appears slight oxidation.When the scanning amplitude is changed,the tensile strength of the joint fluctuates greatly.When the scanning amplitude is 1.4 mm,the tensile strength reaches the maximum,reaching 119 MPa.