Research On Plasma-MIG Hybrid Welding Process Of AZ31B Magnesium Alloy Profiles

The wide-hollow profile of AZ31B magnesium alloy was welded by Plasma-MIG hybrid welding with 1.6mm diameter of filler wire. The filler was made by hot extrusion way whose composition was same to base material.The welding process was stable. High quality surfaces with continuous beads were obtained and the ripples of weld beads were uniform. In this paper, The effects of weld parameters, including MIG voltage, plasma current and welding rate, on weld bead geometry size and appearance were analyzed under the condition of different types of plasma current. The welding joints welded by unique parameters were selected to investigate their microstructure and mechanical property. The profiles without design-groove were welded directly, which would result in incomplete penetration or incomplete fusion near the weld root. The weld formation whose joint adopted design-groove was good. The tensile strength of the joint was 237.04MPa, which was up to 98.77% of that of the base metal.The experiment results showed that the range of adjustable welding parameters is very narrow if the plasma-MIG hybrid welding process was adopted. The influences of parameters on the weld formation were mainly on the width and reinforcement. If the plasma current was too little, the joints will not be fused or not welded. If the current of DC-Plasma was too large, leakage was occured. Simultaneously, the current of variable polarity plasma became too large would broaden the weld width but had little effect on depth of penetration. The MIG voltage is mainly affect the weld width.The grain size did not grow significantly in HAZ compared with the base metal. Precipitated phase was coarse obviously. Granulated eutectics were distributed along the grain boundary. The grain in fusion zone near the fuse line were fine and then change to columnar crystal toward to the weld center. The microstructure was characterized by equiaxed dendritic crystal like snowflakes in the weld seam zone, where size of grain was finer and more uniform. The grain in weld seam zone of joint-design was much finer and no columnar crysta existed near the bond line.The weld seam is mainly composed of a-Mg solid solution. There was a small amount of β-Mg17Al12 intermetallic compounds dispersed in the a-Mg matrix. The average microhardness in weld seam zone is slightly higher than that in base metal, and the microhardness in heat-affected zone is lower than that in base metal. The mechanical properties near weld zone was weakest in the joint. Tensile test confirm that the fracture was mixed with ductile and brittle, and the ductile mode was priority to the brittle mode.