Monolithic And Dissimilar Friction Stir Welding Between 5052 Aluminum And AZ31 Magnesium Alloys

Friction stir welding is a solid-state joining technique and has achieved the monolithic welds of aluminum and magnesium alloys. Friction stir welding is a potential candidate for the joining of dissimilar welds between aluminum alloys and magnesium alloys due to the lower processing temperature. Microstructural evolution was complicated and fine equixed grains were obtained in stir zone. Therefore, it is essential to reveal the mechanism of the grain growth during friction stir welding process. In this paper, microstructure and mechanical properties of the monolithic and dissimilar weld between 5052 aluminum alloy and AZ31 magnesium alloy produced by friction stir welding were studied. Microstructural evolution during friction stir processing of AZ31 magnesium alloy was observed with the employment of stop action and"rapid quench"technique. The main conclusions are summarized as follows.(1) Effects of friction stir welding parameters on the microstructure and mechanical properties of 5052 aluminum alloy joints were investigated. Sound welds were achieved under a range of rotation rate 600 rpm-1500 rpm at a constant welding speed 150 mm/min. Fine equiaxed grains were obtained in stir zone and the minimum grain size was 6.3μm. The grain size of stir zone increased with the rotation speed increasing. The profile of hardness distribution presented a"W"shape and the maximum value of stir zone was equal to the base metal. Furthermore, the minimum value of hardness, about 52 HV, located at a distance of 3 mm from the weld center on the advancing side. Joints with its tensile strength of 236.2 MPa and elongation of 22.4% were achieved with a rotation speed of 600 rpm and a welding speed of 150 mm/min, which were 92.9% and 96.1% of the base material, respectively.(2) Friction stir welding of AZ31 magnesium alloy plate with its thickness of 6 mm was studied, and the tensile fracture mechanism of FSW joints was investigated. At a constant rotation rate of 1000rpm, defect-free joints could be obtained under a range of welding speed 60~300mm/min. Coarse grains in base material were changed into fine equiaxed grains in stir zone after FSW. The grain size of stir zone decreased with the welding speed increasing. Micro-hardness measurement showed that the hardness of stir zone was higher than that of the other zones. Joints with its tensile strength of 92.7% of the base material were achieved under the welding speed of 150 mm/min. Heat affected zone was the weakest zone in the whole weld, due to the inhomogeneous distribution of coarse grains and the lowest hardness in the heat affected zone. (3) Stop action and"rapid quench"technique was adopted to understand the microstructural evolution during friction stir processing of AZ31 magnesium alloy. The relationship between the processing time and grain size was investigated. Analysis of microstructure revealed that grain refinement occurred and the DRX grain structure was obtained in the stir zone. The minimum average grain size in the whole weld was about 1.46μm, which located near the keyhole, and the proportion of fine grain smaller than 1μm was up to about 80%. The number density of fine grain decreased and grain became coarser along the centerline of weld from the keyhole. The proportion of fine grain was reduced to 75% in location of 15 second and the average grain size was up to 2.06μm in 25 second. The experimental results also showed that the process of grain growth would be continued after the stirring of the tool.(4) Dissimilar friction stir welding between 5052 Al alloy and AZ31 Mg alloy with the plate thickness of 6 mm was investigated. Abrasion of shoulder was found during dissimilar friction stir welding and materials composed with intermetallic compound of Al12Mg17 and Al3Mg2 were found to envelope the rotational pin. Dissimilar joints were produced under the rotational speed of 600 rpm-800 rpm with welding speed of 40 mm/min and 60 mm/min and no cracks were found in the appearance of the joints. Sound weld was obtained at 600 rpm rotation speed and 40 mm/min welding speed. A simple bond interface was formed on the top of joint and an intermixed structure existed in the bottom of the joint. Complex flow pattern characterized by intercalation lamellaes was formed in the stir zone. Structure of Mg alloy presented different refined grains in different region of stir zone. The average grain size of Mg alloy was 5.4μm in stir zone near the Mg side and the minimum was about 2.8μm in stir zone near the Al side. Aluminum alloy was also refined in stir zone and the average grain size was about 3μm. Analysis of TEM and XRD revealed that intermetallic compound of Al12Mg17 was formed in stir zone. Microhardness measurement of the dissimilar welds presented an uneven distribution due to the complicated microstructure of the weld, and the maximum value of microhardness in the stir zone was twice higher than that of the base materials. The tensile fracture position located at the advancing side (aluminum side), where the hardness distribution of weld showed a sharp decrease from the stir zone to 5052 base material.