The Study On MIG Welding Procedure, Microstructure And Property Of AZ31B Magnesium Alloys

Magnesium alloys have recently attracted great attention owing to their unique properties such as low density,high specific strength and specific stiffness,good castability and machinability,excellent thermal conductivity and electromagnetic shielding efficiency, and recyclable characteristics。Magnesium alloys have a broad application prospect in automobile,motorcycle and aviation in dustries.Rare earths(RE),as an important class of alloying elements to magnesium-based alloys,are often added in Mg matrix to enhance the high temperature properties,corrorsion-resistance and casting characteristics of alloys.A variety of useful role of RE makes the RE become increasingly widely used in magnesium alloys.The application of any advanced material not only depends on its own properties,but also relies on the technologies of material manufacturing,such as welding plays an important role in exploiting the new fields of applications.Because of the special physical properties of magnesium alloys,their poor weldability has seriously hindered the extensive use of magnesium alloy structures.However,up to now,the information on welding metallurgy, weldability and welding process of magnesium alloys is still limited,and many welding theoretics and technologies of magnesium alloys have not been solved.It has become one of main problems for welding of magnesium alloys.In this paper,the effect of welding parameter on weld formation and welding joint of magnesium alloys(AZ31B) is researched using MIG methods.Besides,the structure and mechanical properties of the welding joints are analysed by adding aluminum and ceriumthe into magnesium alloy weiding wire.The main conclusions are given as follows:1.Because of the restictions of welding wire diameter,DC MIG welding cannot achieve a good coordination of weld formation and spray droplet transition.When the pulsed MIG welding is used,the penetration increases with the average current increasing,and with the increase in pulse frequency,penetration depth increases gradually.2.When MIG welding is used,the welding seam structure of magnesium alloy(AZ31B) is small equiaxed grains,and there are some intermittent structures distributing along the grain boundaries which was mainlyα-Mg andβ-Al₍₁₂₎Mg₍₁₇₎.The sticks areβ-A112Mg17 and the laminate structures are eutectic structure.Because of the heat generated in welding making the grain coarsening,there are someβ-A112Mg17 seperated out of the grain boundaries.3.The hardness of the weld zone and the heat affected zone gradually decreases,and the structure grows coarsen when the average current is increasing.When the pulse frequency increases to 4 Hz from 2 Hz,the grain size grows significantly,the hardness gradually decreases and the grain size and hardness of the heat affected zone are little changed.4.Added cerum into the welding wire,the structure of the weld zone becomes non-uniform.The equiaxed distributes over the weld center and the columnar dendrite distributes near the fusion-line.Besides,the grain size gradually increases with the increase of cerium.Some of the acicular Al₍₁₁₎Ce₃ separated out of welding seam distributes in grains,and others distributes along the grain boundaries.Besides,there are some lump Al₍₁₁₎Ce₃ separated out of grains at the same time.The structure of weld zone may be coarser than the structure of the heat affected zone,especially the columnar dendrites near the fusion-line are coarser.5.As the content of cerium increasing,the strength of welding joints drop to 155 MPa from 225 MPa,and the elongation drop to 6.2%from 22.6%.The fracture of the welding joint happened in columnar zone.Adding the RE and the cerium into the welding wire makes the performance of the welding seam descent.6.The strength of welding joints increases and the tenacity drops as the content of aluminum increasing.The structure of the welding seam is non-uniform columnar dendrite and equiaxed grains when the content of aluminum is low.As the content of aluminum rises to 5.36%,the structure of welding seam is equiaxed grains.