Research On Microstructure And Properties Of Friction Stir Welded Joints Of Super Hard 7050 Aluminum Alloy

It is very difficult to weld thick Al-Zn-Cu-Mg alloy by traditional welding method.The birth of friction stir welding technology has realized the welding of aluminum alloy between different systems,and solved the problem about poor quality of welded joints of aluminum alloy.Friction stir welding is a solid-phase bonding technology with great development prospects.In this paper,a 7050 aluminium alloy with with 12 mm thickness and poor weldability is taken as the research object.By changing the rotating speed and welding speed,,the defect-free welded joints are successfully obtained.The tensile strength and surface hardness distribution of the joints were measured by a tensile tester and a microhardness tester.The microstructures and properties of welded nugget zone,TMAZ,HAZ and BM as well as the distribution of second phase particles were investigated by metallographic microscope and scanning electron microscope,and the effects of heat treatment on the microstructures and properties of joints were analyzed.The interface between HAZ and TMAZ is the weak link of welded joints and the place where microhardness is the smallest.The hardness distribution on the weld surface is similar to "W" shape distribution,and the strength of the welded joints obtained under different welding processes is similar.The joints quality obtained by welding process parameter combination 300-100 is the highest,and the grain size and deformation degree of each region are very different.On the advance side,there is a very obvious boundary between the nugget zone and TMAZ.The location of tensile fracture was changed by heat treatment.Near the junction of nugget zone and TMAZ,the performance of the joints was the worst.Heat treatment can significantly improve the strength and hardness of welded joints.The quality of welded joints is the highest when the welding parameters are 200-50 after heat treatment.Compared with the metallographic structure before heat treatment,there is no obvious growth of grains.Although the second phase particles in each region grow up,the number of grains increases obviously.The combination of welding parameters of different processes shows the same rule.