Cryogenic Treatment Of Microstructure And Mechanical Properties Of Az31 Magnesium Alloy Tig Welding Head

Many of the structures of magnesium alloy in the practical applicationinevitably involves the connection technology, but its physical characteristics(low melting point, high coefficient of linear expansion and thermalconductivity) require a large amount of heat input when welding, resulting inpoor weldability, easily in the welding process making heat-affected zone toobroad, coarse grains and welding defects such as porosity, slag and crack in theweld zone. After welding deformation will decrease the mechanical property ofwelding joint, the soften of welding joint is one of technical problems, whichconstraints its wide use.In this paper TIG welded joints of7mm thick AZ31magnesium alloy wereput into the liquid nitrogen for cryogenic treatment to analy how themicrostructure and mechanical property of magnesium alloy welding jointschange for different cryogenic processing time and numbers. Using opticalmicroscopy, X-ray diffraction, microhardness tester, tensile testing machine andscanning electron microscopy the microstructure of magnesium alloy weldingjoint, phase composition in the weld zone, the mechanical property and tensilefracture morphology of welding joint were observed before and after deepcryogenic treatment.The test shows that cryogenic treatment contributes to make the structurein the weld zone of AZ31magnesium alloy refine, the brittle second phaseparticles (β-Mg17A112) precipitate from the matrix and distribute on the matrixand in the grain boundary, with the extension of time grain has been furtherrefined, more second phase particles precipitate and their distribution moreuniform. The change of grain size in weld zone after cryogenic treatment twiceis not significant, compared to the first cryogenic treatment the precipitation ofsecond phase particles obviously increase.only after cryogenic treatment for2hand4h twice. X-ray diffraction analysis shows that there are two kinds ofdiffraction peaks (α-Mg and β-Mg17A112) in the weld zone before and after the cryogenic treatment and β-Mg17A112diffraction peak is weak. Deep cryogenictreatment does not make the main crystal orientation change, but somediffraction peaks of α-Mg phase change significantly, indicating that somegrains rotate after cryogenic treatment. The microhardness value, tensilestrength and fractured elongation of magnesium alloy TIG welding joint aftercryogenic treatment improve to some extent, and especially after cryogenictreatment for8h the overall mechanical property of welding joint improves mostsignificantly, tensile strength of its joint increases from212.4MPa to246.6Mpa,its strength coefficient from83.7%to97.1%and the elongation rate from5.93%to9.32%. SEM of the fracture shows that cryogenic treatment can improve thetoughness of magnesium alloy welding joint and with the time prolonging thenumber of dimples first increase, then tend to be stable; while the number ofcryogenic treatment increase, plastic of the welding joint somewhat reduces inthe same time.