High Performance Magnesium Alloy Friction Stir Welding Process Optimization And Performance Study

The cast magnesium alloy AZ91D and deformation magnesium alloy ZK60-Gd between dissimilar and same kind of friction stir welding process was researched and analyzed, the influence on the organization morphology and properties of the joints of the rotational speed and the welding speed of the mixing head were also invertigated. The scope of the appropriate friction stir welding process parameters was respectively obtained, and the forming mechanism of stir pin in friction stir welding magnesium alloy and the reason of generation of various defects. The results showed that:(1) AZ91D alloy plates with a thickners of six millimeters were successfully friction stir welded (FSW), and when the rotational speed was between1000r/min and1400r/min and welding speed was between50mm/min and150mm/min, weld with good formation on the surface and without gas holes and tunnel-type defects inside was obtained. There was a huge difference between the welding area and the base metal zone, the grains at the nugget zone were fine recrystallized grains and the second phases were distributed dispersedly. The heat-affected zone was partly recrystallized organization, and the recrystallization grains growed around the grain boundaries of the original casting grain. Tensile experiments on the joints were conducted. It turned out that tensile strength of the joints was higher than that of the base metal zone, tensile fracture of the base metal zone was brittleness obviously, and when joints were in prefabricated gap, tensile fracture of the joints presented ductile fracture.(2) When the rotational speed was between900r/min and1300r/min and welding speed was between120mm/min and240mm/min, effective connectivity friction stir welding of magnesium alloy ZK60-Gd were produced; The grains at weld nugget zone were obviously refined. After FSW, high-temperature phase Mg-Zn-Gd were dispersed, bulky MgZn2phases were broken up and part or all of them were dissolved. The hardness value of the nugget zone was higher than that of the base metal zone. Transverse tensile tests showed that joint fracture appeared in the advance side of the heat-affected zone and the strength of the weld was lower than that of the parent material. Furthermore, the welding parameters didn’t have apparent effect on the hardness value of the nugget zone, and after aging treatment, the hardness value and strength of the weld were both slightly enhanced.(3) Magnesium alloy AZ91D and ZK60-Gd were successfully friction stir welded at the rotation rates from800to1400r/min and welding speed from50to300mm/min when magnesium alloy AZ91D was placed at the back side and ZK60-Gd was placed at the forward side. The hardness value of the nugget zone was obviously higher than that of the two kinds of parent material magnesium alloys, and the hardness values weren’t well-distributed and volatile. All the welds failed at parent material magnesium alloy AZ91D, which became the most vulnerable areas.(4) Plastic metal moved along the thickness direction of the joint and pump-extrusion was the principal character of joint plastic metal flow under the action of mixing needle thread. The rotational speed and welding speed had main impact on the quality of friction stir welding head of magnesium alloys. In the welding process, their effects were not independent, and they should properly match with each other, or high quality welds couldn’t be obtained.