| In this thesis,the processing of gas tungsten arc(TIG)welding and Plasma arc welding of magnesium alloy AZ31 plates with 5.0 thickness were studied.The microstructure and mechanical properties of joint regional was investigated through detection of tensile strength,HV hardness,X-ray inspection on the joints.Meanwhile,the characteristics of magnesium alloy AZ31 welding were further analyzed through the differences between the joints of TIG welding and Plasma arc welding.The results show that:the best standard of joints with good surface and have no defect was when the current was 140A and the speed was 100mm/min.The HV hardness of base metal was 60,grains in the heat-affected zone are coarse,the HV hardness was higher than base metal,can reach to 66,while the grains in the welded zone are finer,HV hardness was 50, lower than base metal.The tensile strength of welded joints can reach up to 83%of the strength of base metal.Tensile fracture of joints usually occurs in the HAZ,which shows that HAZ is the weak zone in the joints.The morphology of fracture is ductile and brittle mode.It's found that Mg-AZ31 5.0mm-thick plates can besuccessfully welded by Plasma arc welding,the best standard of joints with good surface and have no defect was when the current was 100A and the speed was 100mm/min.The HV hardness of base metal was 60, grains in the heat-affected zone are coarse,the HV hardness was higher than base metal,can reach to 74,while the grains in the welded zone are finer,HV hardness was 54.The tensile strength of welded joints can reach up to 89%of the strength of base metal.Tensile fracture of joints usually occurs in the WZ.The fractographs are similar in weld joint for with and without fluxes.The common welded defects in the joints are oxide slag,porosity,cracks,etc.The occurrence of weld porosity and cracks could be reduced by removing the oxide film,dirt and oil completely before welding,strengthening the protection for welding pool,etc.
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