Microstructure And Mechanical Properties Of AZ31B And 7075 Friction Stir Welding Joints

Aluminum alloy and magnesium alloy are of low density, high intensity and good machinability, thus being widely utilized in manufacturing industry of aerospace, electrical appliances, high-speed trains, automobiles and ships. Friction stir welding (hereinafter referred to as FSW) is applied in solid state bonding of low melting point metal and its alloys, with no gas shielding, filler wire and no arc light radiation, dust and spatter. The welding joints are of good mechanical property and with little welding defects of stomata, porosity and cracks. Therefore, FSW is fitted to being used in practice and promoting automation.FSW butt welding of the 10mm AZ31B magnesium alloy and 7075 aluminum alloy are studied in the thesis. The analysis of metallographic microstructure, micro-hardness and mechanical properties of welding joints under different welding parameters is carried out. The law of FSW is probed into by analyzing the welding quality, structure and mechanical property with changes of plunge depth of shoulder. The major conclusions are as follows:(1)FSW butt welding of 10mm AZ31B magnesium alloy and 10mm 7075 aluminum alloy are achieved successfully with good morphology, microstructure and property.(2) When the plunge depth of shoulder is increased to 1.5mm, better results of AZ31B magnesium alloy FSW can be achieved with finer, smoother welding surface and a more continuous corrugated structure. The trend can be enhanced by increasing the plunge depth of shoulder when the ratio of rotational speed and welding velocity reaches 28.(3) The micro-hardness of cross section of AZ31B mg alloy FSW joint presents a’W" shape, spreading symmetrically along the axis. Hardness of the BMZ is 70HV, and the highest hardness of the welding seam is 67HV which lies in the retreating side of WNZ.(4) When increased from 0.5mm to lmm, plunge depth of shoulder makes little influence on joint tensile strength and elongation. When the plunge depth of shoulder is increased from lmm to 1.5mm, joint tensile strength and elongation are enhanced dramatically. When plunge depth of shoulder reaches 1.5mm, joint tensile strength and elongation are maximized to 213MPa and 6.87%, with an increase of 17.7% and 29.4% compared to 0.5mm plunge depth of shoulder.(5) Dynamic recrystallization of WNZ structure is formed in 7075 aluminum alloy butt joints with fine equiaxed grains. Grains at the advancing side of TMAZ are longer than those at the retreating side, and all the grains in TMAZ are irregular and uneven. Grains in the HAZ undergo the same change.(6)Analysis of the second phase particles of 7075 aluminum alloy attests that: with the increase of plunge depth of shoulder, the second phase particles in WNZ are distributed more finely and evenly when compared to those in BMZ. The second phase particles at TMAZ are lengthened and coarsened, while those in HAZ increases.