The Research On Interface Microstructure And Propertis Of The Lap Joint Of Al/Mg Alloy By Friction Stir Welding

In the paper, rolled 6061 aluminum alloy and AZ31 B magnesium alloy with 2mm thickness were welded by friction stir lap welding. The effects of welding parameters(rotation speed, welding speed and the length of stir pin) on the interfacial microstructure and the performance of the joints were studies as well as the microstructure evolution during post-weld annealing treatment. Further, Zn interlayers were added in the interface of Al/Mg joints to discuss the effects on microstructure of interface and performance of the joints.The experimental results shows that: the selection of welding parameters for the quality of Al/Mg lap joints with or without Zn foil have great effect. It is able to obtain lap joints with fine appearance and defect-free under optimum welding parameters; otherwise, weld surfaces and the joints have flash groove, tunnel type hole and other weld defects. The analysis for the transition layer of Al/Mg lap joints show that transition layer is different from both sides of the parent materials due to mutual diffusion of Al and Mg elements. The transition layer is consisted of γ-Al12Mg17 and β-Al3Mg2 phase. When welding parameters is too low, it is easy to lead to unreliable joints; however, the welding parameters is too high which is easy to lead to the thickness of intermetallic compounds increased. Due to the brittleness of intermetallic compounds increased, the shear strength of the lap joints reduced sharply. In addition, the distribution of microhardness of perpendicular to the Al/Mg interface shows that the microhardness of the interface is higher than that of parent materials.In order to further understand growth behavior of intermetallic compounds of the Al/Mg interface, annealing treatment for the samples with 700rad/min and 50mm/min were carried out at different temperature and time. The results find the enrichment of Si and Fe elements near to Al matrix and the holes between enrichment region and β-Al3Mg2 phase layer. The β-Al3Mg2 phase layer grows faster than theγ-Al12Mg17 phase layer and the growth of both phases obeyed the parabolic rate law. The growth of β-Al3Mg2 phase layer is mainly controlled by volume diffusion. However, the growth of γ-Al12Mg17 phase layer is mainly controlled by volume diffusion and grain boundary diffusion under low temperature and is mainly controlled by volume diffusion under high temperature. The growth rate of β-Al3Mg2 phase is an order of magnitude greater than that of the γ-Al12Mg17 phase. The apparent activation energies for the growth of the β-Al3Mg2 and γ-Al12Mg17 phases are 69.3 and 113.5 kJ mol-1, respectively.The transition layer of Al/Mg lap joints with Zn foil is analyzed. When selected welding parameters is proper, the formation of Al-Mg intermetallics can be prevent effectively in the interface. Instead, the transition layer is consisted of Al rich phase, Al5Mg11Zn4 and Mg-Zn eutectic structure. However, when welding parameters is too low, Zn layer is not to spread completely in the interface; when welding parameters is too high, the reduction of diffusion layer with Zn lead to the formation of Al-Mg intermetallics. Although, the distribution of microhardness value of perpendicular to the Al/Zn/Mg interface show that microhardness value of diffusion layer is always high, it is lower than that of the Al/Mg interface, which is benefit to the transmission of power. Therefor, it can reduce the residual stress and limit crack propagation which improve the joint strength.