| 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. They are regarded as ideal materials to realize lightweight and reutilization. The development of materials showed that the wide application of advanced materials depended not only on their properties but also on the progress of welding (joining) techniques. In recent years, welding of magnesium alloys has become a hot spot in welding field all over the world with the improvement of corrosion resistant property and the enlargement of application field of them. However, it is difficult to acquire reliable joint for magnesium alloy due to its unique physical and chemical properties. The weldability of magnesium alloys is the development foundation of welding technique and also the main content of theoretical studies on material welding. Hot cracking is one of the important aspects of the weldability of magnesium alloys, which affects the performance of joint badly. Therefore, it is of great theoretical significance and practical value to study the hot cracking of magnesium alloy weld metal deeply.In this paper, the MIG method is used to investigate the characteristic of weld microstructure and the effect of alloy elements and welding parameters on the hot cracking susceptibility of AZ31 magnesium alloy weld, and the solidification cracking mechanism is also researched systematically using the MIG method. The main conclusions are given as follows.(1) The welded AZ31B magnesium alloy joint mainly consists of weld zone, heat affected zone and base metal. The microstructure is mostly fine equiaxed grains in weld zone. The grains are bigger in the weld center while they are finer in adjoining to the fusion line. The phase composition of weld metal is mainlyα-Mg andβ-Al12Mg17 phases. When the welding wire W1 was used, there was no eutectic microstructure due to its low Al content, but a few granularβ-Al12Mg17 was precipitated in the grain boundaries. When the welding wire W2 was used, because of its higher Al content theβ-Al12Mg17 phases were precipitated not only in the grain boundaries but also inside theα-Mg. The prominent characteristic in HAZ of AZ31B is the more obvious coarsening ofα-Mg grain and the partial melting of grain boundaries because of welding heat effect. The microstructure of AZ31B base metal is approximatively equiaxed grains consisting mainly ofα-Mg phases.(2) The alloy elements have obvious effects on microstructures of AZ31 magnesium alloy weld. When Ce added to the magnesium alloy wire, the Al-Ce-Mn and Al-Ce appeared in the magnesium alloy weld because of electronegation. The Al-Ce-Mn and Al-Ce make for the heterogeneous nucleation, resulting in refiningα-Mg grains. The Al-Ce-Mn and Al-Ce at the grain boundaries can hinder the movement of grain boundaries and subgrain boundaries, and decrease the moving speed of them, which also can refine theα-Mg grains. The effect of Ce on the appearance of dendritic grain may be related to the Al content at forward position of solidification. With the increase of Ce content, the Al content will decrease due to the combination between Al and Ce, which will make the super-cooling region of composition narrower and promote the formation of dendritic grains. The welding wire Al content has an obvious effect on microstructure of AZ31 magnesium alloy weld. When the Al content was 2.730%, the weld microstructure was mainlyα-Mg, and the grain size was bigger. With the increase of the Al content, the amount ofβ-Mg17A112 in the weld increased and theα-Mg was refined. When the welding wire Al content was increased to 6.626%, theβ-Mg17A112 approximatively distributed along the grain boundaries and had a coarsening tendency.(3) The alloy elements and welding parameters have obvious effects on the hot cracking susceptibility of AZ31 magnesium alloy weld.a) The alloy elements have obvious effects on the hot cracking susceptibility of AZ31 magnesium alloy weld. Compared with the weld deposited with the welding wire without Ce addition, the weld with the welding wire containing 0.056%Ce has the lower K1 value, but with increasing the Ce content, the K1 value increases gradually. The Al content has the effect on the hot cracking susceptibility of AZ31 magnesium alloy weld metal, because of the concrescence effect of low melting point eutectic. When welding wire Al content was increased, the low melting point eutectic in the weld metal increased and a continuous liquid interspace formed. The concrescence effect of low melting point eutectic led to reducing the hot cracking susceptibility of weld metal.b) The welding parameters have the effect on the hot cracking susceptibility of AZ31 magnesium alloy weld. In the case of continuous welding, AZ31 magnesium alloys has a low hot cracking susceptibility. The welding current has not obvious effect on the hot cracking susceptibility of AZ31 magnesium alloy weld, but it has the effect on the weld appearance. The welding speed has an obvious effect on the hot cracking susceptibility of AZ31 magnesium alloy weld. With the increase of welding speed, the cracking susceptibility decreased obviously.(4) Solidification cracking of magnesium alloy weld was due to the fact that there were low melting point liquid film in the weld and it was subjected to tensile stress. It is effective to control the amount of low melting point eutectic through alloying and changing welding parameters, and decrease the tensile stress for improving the solidification cracking sensitivity of magnesium alloy weld metal.
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