Microstructure And Mechanical Properties Of Friction Stir Welded Magnesium And Copper Alloys

Six millimeters thick ZK60 alloy plates were successfully friction stir welded (FSW). The sound welded joints were achieved under a wide range of welding parameters (rotation rate of 600-800 rpm and traverse speed of 50-200 mm/min). After FSW, the coarse grains in ZK60 alloy were changed to fine equiaxed recrystallized grains at the nugget zone. Furthermore, the coarse MgZn2 particles were broken up and mostly dissolved into the matrix. The nugget zone exhibited relative weak basal texture compared to the parent material. For ZK60 magnesium alloy, the precipitation strengthening effect was more dominant than fine-grain and texture strengthening effect. Therefore, hardness value of the nugget zone was lower than those of the parent material and the fracture occurred at the nugget zone. At low heat input parameters (≤800 rpm), the grain sizes in the nugget zone and the tensile strengths and elongations of welded joints at different parameters were similar, but at high heat input parameter (1200 rpm), the grains in the nugget zone became large and the tensile strength of welded joint slightly decreased. After aging treatment, the precipitation of the fine MgZn2 particles made the hardness of the nugget zone be higher than that of the parent material with the heat affected zone having the lowest hardness, and the the strengths and elongations of welded joints were increased. Furthermore, all aged welded joints failed in the heat affected zone.Six millimeter thick ZK60-0.6Y/ZK60-1.1Y plate was successfully friction stir welded/processed (FSW/FSP) at a tool rotation rate of 800 rpm and a traverse speed of 100 mm/min. After FSW/FSP, coarse microstructure in ZK60-0.6Y/ZK60-1.1Y alloys changed into fine equiaxed recrystallized grains in the nugget zone. Furthermore, bulky ternary phases (I and W phases) were broken up and dispersed, and most of MgZn precipitates were dissolved. Micro-hardness measurement indicated that the hardness of the nugget zone was higher than that of the parent material and the lowest value of hardness was found in the heat affected zone. Transverse tensile tests showed that the strengths and elongation of the weld was only slightly lower than those of parent material with ultimate tensile strength of weld reaching 95% of the parent material. For FSP ZK60-1.1Y alloy, maximum superplasticity of 635% was achieved at 450°C and a relatively high strain rate of 3×10-3 s-1. By comparison, the as-extruded sample did not exhibit superplasticity. Grain boundary sliding was identified to be the primary deformation mechanism in the FSP ZK60-1.1Y by superplastic data analyses and surfacial morphology observations. Furthermore, the superplastic deformation kinetics of the FSP ZK60-1.1Y is significantly faster than that of equal channel agular pressed (ECAP) magnesium alloys under both as-ECAP and annealing conditions.Defect-free friction stir welds of 5 mm thick pure copper plates were produced in a relatively low heat input conditions. The characteristics of the microstructure and mechanical properties of the welded joints were investigated. Decreasing the tool rotation rate or increasing the traverse speed decreased the grain sizes of the nugget zone and increased the hardness and yield strength of the nugget zone."Onion ring", consisting of differently-sized grain bands, were observed at 400 rpm, but disappeared at 600-800 rpm. Variations of both micro-hardness and yield strength of the nugget zone with grain size followed the Hall-Petch relationship. The number fraction of high angle grain boundaries (grain boundary orientation angle >15°) in the nugget zone increased with increasing the rotation rate. A weak texture was observed at the contral zone of the nugget zone of FSW joints. The ultimate tensile strength of the welds under various FSW papameters was equivalent to that of the parent material. However, the yield strength and elongation of the welds were lower than those of parent material. With increasing the traverse speed or decreasing the rotation rate, the yield strength of the welds increased and elongation decreased. All the welds failed at the heat affected zons that had the lowest hardness values within the whole welds.Five mm thick brass plates were successfully friction stir welded at the rotation rates from 400 to 1000 rpm for a constant traverse speed of 100 mm/min. The nugget zone consisted of the incompletely and completely-recrystallized regions. This is attributed to the prohibition of a great amount of fineβphase particles. With increasing the rotation rates, the fraction of the non-recrystallized grains decreased and the grain size of the recrystallized grains increased. The hardness values in the nugget zone were higher than those in the parent material. Increasing the roration rate did not exert a noticeable effect on the strengths of the welds that reached up to ~99 and 80% of the parent material. The fracture occurred in the heat affected zone that had the lowest hardness.