Study On The Microstructure And Mechanical Properties Of Friction Spot Welded Dissimilar Aluminum Alloys

Friction spot welding(FSpW) was a new solid-state welding process developed by Helmholtz-Zentrum Geesthacht(former GKSS) in Germany as an alternative joining method of the fixed friction stir spot welding, melting spot welding and riveting. This method can be widely used in aerospace, vehicle and shipping industries. In this study, the joints of 5052-H112 and 2024-T4, 7075-T6 and 2024-T3 aluminum alloys with 2 mm thickness welded by friction spot welding were studied to analyse the influence of different welding parameters on joint microstructure, defects, hardness profile, mechanical properties and fracture mode.Firstly, the joints of 5052-H112 and 2024-T4 aluminum alloys welded by friction spot welding have been studied. No matter which one was set as the upper sheet, the joint can be divided into stir zone, thermo-mechanically affected zone, heat affected zone and base metal. Hook became more smooth with the increase of heat input. When 2024-T4 was set as the upper sheet, voids occurred in hook, and the hardness profile of the upper sheets all present W-shape and the hardness profile of 5052-H112 is like a ?bowl?. While when 5052-H112 was set as the upper sheet, the hardness profiles of the upper sheets are all in bowl-shape, and the hardness profiles of 2024-T4 present W-shape. The shear tensile load is higher when rotation speed is lower. When 2024-T4 was set as the upper sheet, the failure modes of joints were shear fracture and plug type fracture. While when 5052-H112 was set as the upper sheet, shear fracture and plug-shear fracture were observed.Secondly, the joints of 7075-T6 and 2024-T3 aluminum alloys welded by friction spot welding have been studied. When 2024-T3 was set as the upper sheet, both sheets contain stir zone, thermo-mechanically affected zone, heat affected zone and base metal. However, it is quite different when 7075-T6 was the upper sheet, because no stir zone and thermo-mechanically affected zone were observed in the lower 2024-T3 sheet. When 2024-T3 was the upper sheet, the voids are smaller under lower rotation speed, and the voids tend to expand with the increase of rotation speed and welding time. In contrast, when 7075-T6 was the upper sheet, voids still exist, but the voids tend to shrink with the increase of rotation speed and welding time. When 2024-T3 was the the upper sheet, the hardness profiles of both sheets are in W-shape. When 7075-T6 was set as the upper sheet, the hardness profile of the upper sheets present W-shape, while the hardness profile of lower 2024-T3 is in bowl-shape. When 2024-T3 was set as the upper sheet, higher shear tensile load can be obtained under lower rotation speed, and it decreased with the the increase of rotation speed and welding time. The shear tensile strength of the joints with 7075-T6 as the upper sheet is lower than that of the joints with 2024-T3 as the upper sheet. When 2024-T3 was set as the upper sheet, shear fracture and plug-shear fracture were observed. And when 7075-T6 was the upper sheet, the failure mode is shear fracture.