Effect Of Heat Treatment On Microstructure And Mechanical Properties Of AZ31B Magnesium Alloy TIG Welded Joint

Magnesium as an important light metal engineering material,has attracted extensive attention in aerospace,rail transit and military industries due to its excellent performance.TIG welding is a common welding method for magnesium alloys,but the welding quality of TIG welding joints is poor,the mechanical properties of the joints are generally low,Which is difficult to meet the requirements of welding engineering structure.This article takes AZ31B magnesium alloy TIG welded joint as the research object,solid solution treatment,solid solution and aging treatment test are performed respectively.The influence of heat treatment method and process parameters on microstructure,mechanical properties and corrosion performance of the welded joint are studied.The following main conclusions have been obtained:(1)The weld zone of unheated weld joints are distributed with fine equiaxed grain,consisting of a-Mg matrix and Mg17Al12 phases which distributed discontinuously along grain boundaries.The heat-affected zone distributes a small number of short strip Mg17Al12 phases along the grain boundaries,and the grains are coarse.The weld zone produced the highest hardness,while the heat affected zone has the lowest hardness.(2)After solution treatment,the Mg17Al12 phase in the joint dissolves into a-Mg matrix with different degree.At solution condition of 400?×8h,the second phase in the joint dissolves most,and the grain size is the most uniform.The solid solution joints and unheated joints have the same hardness distribution trend,both show a stepped distribution.With the increase of the solution time,the tensile strength of the joints increases first and then decreases.At solution condition of 420?×8h,the tensile strength of the joints reaches 201 MPa,which is 23%higher than unheated joints.The joints mainly show ductile fractures with tough-brittle mixed fracture morphology.(3)Aging temperature and aging time affect the distribution of the Mg17Al12 strengthening phase in the joint.With the increase of aging temperature and aging time,the distribution of the Mg17Al12 phase in the weld zone changes from granular to aggragated.The microhardness of weld zone,heat-affected zone and the tensile strength of the joints both showed the trend of increases first and then decreases.At aging condition of 200x8h,the hardness of each area of the joint reached the maximum;and the tensile strength of the joints reached maximum of 230MPa,which is 15%higher than solution condition with 420?×8h.With the extension of aging time,the joint has a characteristic of transition from ductile fracture to brittle fracture.(4)After different heat treated conditions joints soaked in 3.5%NaCl,the corrosion resistance of magnesium alloy base metal is the best,the weld zone is followed,and the corrosion resistance of the heat affected zone was the worst.The heat treatment process has a great influence on the corrosion resistance of AZ31B magnesium alloy welded joints.The corrosion resistance of the joint performance the best at the solution condition of 400?×8h.The corrosion resistance of the joint performance the worst at the aging condition of 200?×12h.Compared with weld zone of unheated joint,the self-corrosion current density of the joint treated with solution of 420?×8h was 2.82×10-6 A/cm2,which is reduced into one times compared with unheated joint,and the corrosion potential was increased;The self-corrosion current density of the joint treated with aging condition of 200?×12h increased by 2.2×10-6 A/cm2,and the corrosion potential decreased.