Mechanical properties and structure of butt joints of 6082T6 alloy plate made by double-sided friction stir welding

Comparative studies results of the structure of mechanical and corrosion properties of butt joints of a pressed profile with a thickness of 20 mm made of aluminum alloy 6082T6, which have been obtained by double-sided friction stir welding (FSW) in air and in water, are presented. It is shown that an increase in the cooling intensity of the metal of the stir zone and the heat-affected zone of the joint has practically no effect on the temporary resistance of welded joints. At the same time, the strength coefficient of the joints is at the level of 0.73–0.74. The use of FSW on a plate made of 6082T6 alloy in water ensures the formation of a stir zone with an average grain size of 4.3–4.7 microns, and with FSW in air, the average grain size in the joint is 6.5–6.8 microns. Both in the case of FSW in air and in the case of FSW of a pressed plate made of 6082T6 alloy in water, the fracture site of the welded joint during tensile tests originates along the thermo-mechanical impact zone (TMIZ) and then develops in the heat-affected zone. When performing FSW in water, the length of the heat-affected zone decreases by about 1.4–1.8 times compared to FSW in the air. The hardness of the stir zone, both after FSW in air and after FSW of alloy 6082T6 in water, is lower than the hardness of the base metal (108 HV). At the same time, the hardness of the stir zone during FSW in water is 90–95 HV, and during FSW in air is 80-83 HV. The fracture of the metal of the stir zone during tensile testing both after welding in air and in water, and after additional ageing at 170 oC for 12 hours, has a pronounced viscous nature with the presence of specific pits on the surface.