The Connection Mechanism Research On Friction Stir Spot Welding Without Keyhole Between Magnesium And Steel Dissimilar Alloys

Friction Stir Spot Welding (FSSW) based on friction stir welding (FSW) is a newly developed welding technology. With the advantage of low cost and low energy consumption, FSSW can completely replace the traditional spot welding method in automobile manufacturing. In particular, FSSW is superior in light alloy welding with simple process, high quality joint and clean working environment.In this paper, the self-designed FSSW without keyhole is employed. Based on this, a1mm thick DP600galvanized steel sheet to a3mm thick AZ31B magnesium alloy sheet are lapped by self-design FSSW without keyhole, and the steel sheet is placed on the magnesium sheet. Based on the research which is process research on friction stir spot welding without keyhole between magnesium and steel dissimilar alloys, under the condition of the optimal welding parameters (the plunge depth of the shoulder0.3mm, the pin diameter6.0mm, the rotating speed1200r/min). In order to research the degree of mixing and microstucture of lap joints, the whole spot welding process was divided into inserting (Ⅰ),the stirring and withdrawing (Ⅱ), withdrawing finished(Ⅲ) three phases. For the three stages of spot weld joints, in the welding nugget zone and thermo-mechanically affected zone and the heat affected zone, microstructure and principal component are researched.The fracture was analyzed by X-ray diffraction(XRD) and scanning electron microscope(SEM).By the temperature field simulation and thermocouple measurement, the temperature field distribution of different rotation speed were confirmed. And the jointing mechanism between DP600and AZ31was researched.Under the condition of the optimal welding parameters(the plunge depth of the shoulder0.3mm, the pin diameter6.0mm, the rotating speed1200r/min), then a sound joint with the strength of8.7kN was obtained.The latitudinal micro hardness distribution near the interface in the joint was an approximate "W"type.The maximum longitudinal microhardness emerged in the transition zone. The results show that the fracture type was brittle.The microstructure analysis showed that:welding nugget zone, the grain of steel became slender and flowed significantly. The dynamic recrystallization of steel and magnesium alloy occurred in the thermo-mechanically affected zone, the steel grain coarsening long, the magnesium alloy most coarse, but in the stirring pin withdrawing zone, the grain size of magnesium alloy was small and distribution uniform, which was completely converted into equiaxed grains. In the heat affected zone, steel grain was thicker and long, but also magnesium alloy grain more coarse. In addition, owing to the effect of stirring pin, steel is stirred into magnesium, steel like a "nails" embedded in the magnesium, which is the main reason for the better joint mechanical properties, the joint connects mechanically significantly.The joint main element distribution analysis showed that:(Ⅰ) stage, the interface between magnesium and steel has no diffusion.(Ⅱ),(Ⅲ) stage, the diffusion behavior occurs among the main elements in the nugget area.The temperature field of DP600/AZ31dissimilar metals friction stir spot welding was simulated by using ANSYS finite element software. The mixed heat source model was constructed, with the consideration of the variations of their physical properties with temperature.The actual temperature was measured by using thermocouple. Temperature of top surface and interface was acquired. The results showed that the calculated temperature was in agreement with the results of measurement.In summary, such magnesium-steel FSSW without keyhole has two binding methods for joints:the main is mechanical connection, followed by metallurgical combine.