Effect Of Pre-welding Treatment On The Quality Of ZM6 Magnesium TIG Welding Joint

With the development of aerospace and auto industry,the demands for welding of magnesium alloy and connecting of structural component are increasing rapidly.But it is easy to crack during the welding operation which has limited the application of magnesium alloy.A kind of TIG technology was used to weld ZM6 alloy.The influence of pretreatment to the microstructure,mechanical property and welding crack of welded joints were researched in this paper.Before conditioning treatment,the welded joints of ZM6 alloy are composed by welding zone,heat-affected zone and base material zone.The fine grain in the welding zone is arranged uniformly.The grain in heat-affected zone is narrow and uniform.The grain in base material zone is the biggest.The precipitated phase of welded joints is composed by?-Mg and Mg₁₂Nd mainly.There are several kinds of cracks appear in welding joints such as crystalline crack,liquation crack and mixed crack combined by crystalline crack and liquation crack.The major characteristic of the grain in crystalline crack is distributed along crystal boundary,transgranular crack is the major style of the grain in liquation crack,the trace of ruptured liquefied silk can be seen obviously.Compared with the untreated specimen,the variation of the welding zone and base material zone at the ZM6 alloy welding joint with pretreatment is not obvious.The range of heat-affected zone is narrow relatively.Some performance tests have showed that tensile strength will increase from 135 MPa to 140 MPa when the preheat temperature increases from 100?to 200?.When the temperature rise up to 100?,the characteristics of the crack are large,plentiful and dispersed.When the temperature rise up to 200?,the crack size will significantly reduce.The length of the crack will be shorter and the strike will be uniformed.All of the characteristic above belong to crystalline crack,which means that preheat treatment can improve the crack resistance ability of the basal body.After the solution treatment and aging treatment,the test specimen does not appear any form of crack before welding.The tensile strength of the solution treatment specimen is 185 MPa,which has reach 87.7%of the mother board tensile strength,the microhardness has been promoted and the tensile fracture belongs to ductile-to-brittle mixed type fracture.The tensile strength of the ageing treatment specimen is 205 MPa,which has reach 89.1%of the mother board tensile strength,the microhardness is the best among all of the specimen,the tensile fracture belongs to brittle fracture.