Diffusion Bonding Of Extruded AZ31Magnesium Alloy/7075Aluminum Alloy

Magnesium alloy has some characteristics such as low density, high specific strength, high specific stiffness, good shock absorption, poor corrosion resistance; Aluminum alloy has some performances such as low density, high specific strength, good plasticity, excellent processing performance, good electrical conductivity, thermal conductivity and corrosion resistance. Bonding of Mg and Al can not only combine their excellent performance, but also reduce the structural weight, save material, expand the application fields, such as automobile, aerospace, transportation, electronic products, etc. However, the mutual solubility and the linear expansion coefficient of aluminum and magnesium are different, which is easy to form the oxide film, so that the reliable connection of magnesium and aluminum is difficult to achieve. Using general fusion welding method makes it easy to form wide HAZ of the joint, welding cracks, holes, and other defects. As an advanced solid-phase welding technology, diffusion welding can effectively avoid weld defects in the fusion welding process, and has been widely used in the field of dissimilar materials connection. Therefore, the research on diffusion welding of magnesium alloy/aluminum alloy dissimilar materials has profound practical significance.In order to obtain excellent welding joint, recrystallization annealing is used for grain refinement of AZ31magnesium alloy and7075aluminum alloy before the diffusion welding. In the condition of annealing temperature300℃and holding time of30min the grain size of AZ31magnesium alloy decreases to12.35μm from21.15μm, and grain refinement proportion reached41.61%; The grain size of7075aluminum alloy decreases to minimum in the condition of annealing temperature350℃and holding time of60min.AZ31magnesium alloy and7075aluminum alloy which are annealed at the optimal parameters are placed in a vacuum environment for diffusion bonding under different welding temperature and holding time. The diffusion bonding effect, grain size and the width of diffusion layer of the sample under the different technological parameters is investigated. The elemental distributions at joint interface were characterized by EDS. Shear strength and microhardness of the interfacial transition zones were measured to evaluate the mechanical performance of the joints.The test results indicated that these three kind of intermetallic compounds Mg2Al3、MgAl and Mg17Al12are formed in the interfacial transition zone. Due to the formation of brittle intermetallic compound, the hardness of interface transition zone is higher than that of the matrix, the hardness value of interface transition zone reaches maximum, about249.6HV near the Al side. In welding stress and the vacuum degree of a certain case, the welding temperature correlates with holding time, which mutual influence the structure and performance of welded joint. The welding temperature is higher or the longer the holding time, the bigger grain growth, the wider diffusion layer. when the sample is welded under the temperature of470℃and holding time for60min, the joint shear strength of sample reached the maximum, about38.41MPa.By comparing the welding effect of the sample treated by recrystallization annealing treatment with that of untreatment before welding, it is found that the diffusion bonding joint performance of the sample after recrystallization annealing treatment is better, the grain size of the matrix is smaller, and the width of the diffusion layer is narrower, the organization is more dense, the joint shear strength is obviously improved.