Study On Weldability Of Magnesium Alloys (AZ91D And AZ31B)

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) technique. In recent years, welding of magnesium alloys is becoming 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 study on material welding. No systematic research on weldability of magnesium alloy was found up to now, which has been one of the main problems to restrict the development of welding technique and affect the welding quality of magnesium alloys. Therefore, it is of great theoretical significance and practical value to study the weldability of magnesium alloys and reveal their weldability characteristics.In this paper, the weldability of magnesium alloys (AZ91D and AZ31B) is systematically researched using TIG and MIG methods and the main weldability problems are explored in detail. The main conclusions are given as follows:(1) The welded magnesium alloy joints manly consist 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 adjoining the fusion line. The weld microstructure was mainlyα-Mg andβ-Al12Mg17 distributing along theα-Mg grain boundaries. The mass fraction ofα-Al12Mg17 has an increased tendency with increasing Al content in the weld.(a) The average grain size in the weld zone of TIG welded AZ91D is not homogeneous. Under the condition of non-equilibrium solidification, in the grain boundaries of weld metal with 9.8wt.%Al there exists a layer eutectic structure composed ofα-Mg andβ-Al12Mg17 and in theα-Mg grains someβ-Al12Mg17 phases precipitate.(b) The average grain size in the weld zone of MIG welded AZ91D is also not homogeneous. Compared with TIG weld metal, there exists eutectic or divorced eutectic structure composed ofα-Mg andβ-Al12Mg17 in the grain boundaries. The amount ofβ-Al12Mg17 precipitated fromα-Mg decreased.(c) The microstructural characteristics of weld metal for MIG welded AZ31B are the decreased amount of β-Al12Mg17 and significant coarsening ofα-Mg grain. When the weld Al content decreased to 3.2wt.%, there was no eutectic structure, but a few granularβ-Al12Mg17 was precipitated in the grain boundaries.(d) HAZ microstructure of AZ91D magnesium alloy has a tendency of grain coarsening. The grain boundary in PFZ partly melted and its width increased. PFZ microstructure mainly consists ofα-Mg andβ-Al12Mg17 distributing along the grain boundary, which has the characteristic of divorced eutectic structure. Compared with HAZ of AZ91D magnesium alloy, the prominent characteristic in HAZ of AZ31B is a narrower PFZ and the more obvious grain coarsening ofα-Mg.(e) Microstructure of AZ91D magnesium alloy is composed of dendriticα-Mg and eutectic structure (α-Mg andβ-Al12Mg17) that distributes in interdendrites.(2) Welding current and welding speed have obvious effects on microstructure and mechanical properties of magnesium alloy weld metal.(a) The welding current has an obvious effect on microstructures of weld and HAZ for MIG welded magnesium alloys (AZ91D and AZ31B). With the increase of the welding current, in the weld the outstanding characteristic is grain coarsening, in HAZ the general tendency is grain coarsening with wide grain boundary in the PFZ and the mass fraction ofβ-Al12Mg17 in weld and PFZ has an increased tendency.(b) The welding speed has an obvious effect on microstructures of weld and HAZ for MIG welded magnesium alloys (AZ91D and AZ31B). With increasing the welding speed, in the weld the prominent characteristic is grain refining and in HAZ the general tendency is grain refining with narrow grain boundary in PFZ.(c) The welding current has some effects on microstructures of weld and HAZ for TIG welded AZ91D magnesium alloy. When the welding current increased from 80A to 130A, the average grain size of the weld metal increased from 22.8μm to 32μm, the width of PFZ increased from 120μm to 210μm and the mass fraction ofβ-Al12Mg17 in the weld had an increased tendency.(d) The weld Al content has obvious effects on mechanical properties of AZ91D magnesium alloy weld. The tensile strength and elongation changed from 192MPa and 4.9% to 215MPa and 7.9%, with decreasing weld Al contents from 9.8wt.% to 6.9wt.%. The magnesium alloy weld had a characteristic of intergranular fracture. The tensile dimple had a large proportion in the fracture surface of weld with 6.9wt.% Al and only a few cleavage pattern was observed. The magnesium alloy weld with 9.8wt.% Al had a mixed fracture of dimple and cleavage.(3) Defects such as solidification cracking in welds, liquation cracking in HAZ and weld pore easily formed during welding of magnesium alloys due to physical, chemical and mechanical properties of magnesium alloys.(a) Magnesium alloy weld had a high cracking sensitivity. Cracks were mainly distributed in the center line and the end of the weld and belonged to solidification cracking. It was due to the fact that there were low melting point liquid film in the weld and it was subjected to tensile stress. It was effective to control the amount of low melting point eutectic and decrease the tensile stress to improve the solidification cracking sensitivity of magnesium alloy weld.(b) Cracks in HAZ of magnesium alloy belonged to liquation cracking. With the increase of welding speed (300mm/min-400mm/min), welding heat input and the tensile stress decreased, and the liquation cracking sensitivity decreased. The liquation cracking was not found in the MIG welded AZ31B joint. It was attributed to the smaller Al content in AZ31B than that in AZ91D.(c) The magnesium weld had high hydrogen gas pore sensitivity. The pores were classified as isolated pore, porosity, chain pore, dispersed pore and pore in fusion zone. Since the solubility of hydrogen in the weld pool of magnesium alloy decreased with the decrease of temperature, the supersaturated hydrogen aggregated and grew up. The low density of magnesium alloy caused the decreased rising velocity of pores and the high thermal conductivity of it led to the increased solidification speed.(d) Since the liquid contraction and solidification contraction are larger than solid contraction, shrinkage cavity easily forms in magnesium alloy weld. Stress concentration will lead to forming microcrack at the tip of the shrinkage cavity.(4) It was found, based on the FEM analysis, that with increasing welding current, welding thermal cycle peak temperature increased, cooling speed decreased, isothermal range increased and tensile stress in the welded joint increased. When the welding speed was increased, the isothermal range decreased, the temperature gradient along the weld cross section became larger and tensile stress in the welded joint had a decreased tendency. These results are consistent with the results obtained from experiments of effect of welding current and welding speed on weld microstructure and hot crack sensitivity.