Study On Diffusion Bonding And Post-weld Heat Treatment Of Extruded AZ31 Magnesium Alloy

Magnesium alloy has a good processing performance which make it widely used in the industry. However, the low melting point, high linear expansion coefficient of magnesium alloy make it difficult to obtain good welding joint by using the conventional welding technology. Diffusion bonding is an advanced welding method, which can effectively reduce the air holes, cracks and welding deformation produced by the conventional welding techniques. It is of important practical value and theoretical significance to study the diffusion bonding of magnesium alloy.The vacuum diffusion welding experiments of AZ31 magnesium alloy were carried out in the study. Before and after the welding experiment, specimens were made recrystallization annealing treatment. The effects of temperature and time on the shear strength, microstructure, interface composition, interface defects, fracture morphology and microhardness of the joints were studied by using the testing machine, electron microscope, energy spectrum, hardness tester and so on.It is important to refine the grain for diffusion bonding. The effects of different temperature and time on the microstructure of AZ31 magnesium were studied in the temperature range of 200-300℃ and in the time range of 15-60 min. The results show that heating temperature and holding time affect the grain size. It can rapid complete recrystallization nucleation, when the heating temperature is higher. The nucleation speed is slower, it takes a long time to get significant refining effect, when the temperature is lower. In the condition of 300℃×30min, the grain size of magnesium alloy refines to 11.09μm form 18.67μm and grain refinement proportion reaches 40.6%.Diffusion bonding experiments were carried out on the AZ31 magnesium alloys after the grain refinement pretreatment at relatively better annealing parameters. The effects of different temperature and time on the performance of joints were investigated under the condition of vacuum degree 15 Pa and pressure 10 MPa. It is found that the better diffusion parameters are 470℃×90min, the joint strength reaches 41.32 MPa. The analysis of the joint show that there are joint interface defects such as mechanical occlusion and micro void which lead the content of magnesium elements show downward trend. The joint fracture exhibits dimple-based mixed fracture, as well microhardness value reaches 72.8HV.In order to eliminate the residual stress of AZ31 magnesium alloy diffusion welding, improve the performance of the joint. The diffusion welding specimen of 470℃×90min was annealed in the temperature range of 310-390℃and in the time range of 1-5h. Under the condition of 350℃×4h the joint strength increases from 41.32 MPa to 49.29 MPa. Microscopic observation and EDS analysis show that magnesium further spread during the post-weld heat treatment. Compared to the untreated specimens, the joint interface defects decrease significantly. On the contrary,the number of fracture dimples and the hardness value increase significantly.