| Both aluminum alloy and magnesium alloy has some superior performance such as high specific stiffness and specific strength, high dimensional stability, and damping shock absorption and so on. So it has extremely important value in the fields of automotive, electronics and communications products, chemicals, machinery, aviation, and so on. However, the low melting point of magnesium, and the higher linear expansion coefficient and thermal conductivity makes it easy to form welding cracks, holes and other issues, which make the HAZ of the joint to be too broad, and easy to deform after welding, and other defects. Based on the characteristics of fusion welding of magnesium alloy, solid-phase connection has been becoming one of the hot topics of researches. Research shows that using the vacuum diffusion welding to connect the dissimilar metals of magnesium alloy and aluminum alloy can effectively reduce the above problems. By vacuum diffusion welding, this paper achieves connections of AZ91 magnesium alloy and 7075 aluminum alloy. The microstructure of the diffusion dissolution layer and the mechanical properties of joints are analyzed. Then the best diffusion welding process parameters are obtained.Firstly, the annealing recrystallization method is used to obtain fine-grained AZ91 magnesium alloy. So it can provide a better basis for the welding parent metal. The results show that the static recovery and recrystallization which occurs at the annealing process can significantly refine the grain, and the grain size firstly decreases and then increases with a rise of the annealing temperature and an extension of the annealing time. Then a annealing treatment is done on the condition of temperature of 300℃and holding time of 30 min to obtain the average grain size of 10.64μm recrystallization, and grain refinement proportion reached 44.8%.Secondly, the burnished and chemical-treated specimen of AZ91 magnesium alloy and 7075 aluminum alloy is put in the vacuum condition, and diffusion bonding is conducted under the condition of different welding temperature and holding time, then optical microscopy and micro hardness tester is used to analyze the organization structure and hardness of diffusion bonded joints of AZ91/7075 dissimilar metals. The results show that different areas of diffusion bonding interfacial transition zone have different micro-hardness values:The transition layer beside the aluminum has the highest micro-hardness achieving 317.5 HV; the micro-hardness of the transition layer beside the magnesium and at the middle is also higher than the base substrate. By a large number of shear properties tests of welded joints on different welding parameters, the different process parameters on the effect of joint performance are analyzed and researched. Then the author made the conclusion that a good diffusion bonded-joint is obtained when the pressure is 10MPa, heating temperature is 450-480℃and holding time is 60-120min, then got the best diffusion welding process parameter:pressure 10MPa, temperature 470℃and holding time 60min. In that parameter the highest shear strength value 49.8 MPa which is about 46.3%of AZ91 magnesium alloy is got.
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