Microstructure And Mechanical Properties Of Friction Stir Welded 7A04 High Strength Aluminum Alloy

7A04 aluminum alloy which was developed in China, has been widely used in aerospace and automobile manufacturing and other industrial fields due to its high strength and good age-hardenable effect. However, the 7xxx aluminum alloys are difficult to be joined by conventional fusion welding techniques because the welding defects formed in the fusion zone may decrease the mechanical properties of the joint seriously. Friction stir welding(FSW), as a solid-state metal joining technique, has been proved to be a much more effective way in producing sound weld joints of low-melting-point metals such as aluminum and magnesium. In this paper, 7A04 plates with a thickness of 3mm were welded by FSW. Microstructure, precipitates, mechanical properties and fracture morphologies of the joints were investigated. The post-welded heat treatment(PWHT) and corrosion property were also investigated.The experimental results show that 7A04 aluminum alloy has a good FSW weld-ability. The joint is composed of four areas: nugget zone(NZ), thermo-mechanically affected zone(TMAZ), heat affected zone(HAZ) and base material(BM). The microstructure of NZ consists of fine equiaxed grains due to dynamic recrystallization with the precipitates dissolveing into the matrix. Since plastic deformation takes place in the TMAZ during FSW, the grains in the TMAZ are deformed and elongated. Both grains and precipitates in HAZ grow coarse. The average grain size of NZ decreases with welding speed increasing. So the same change tendency as the rotation speed decrease. The hardness curve is similar to W shape and the hardness of NZ is lower than that of BM and the lowest hardness locates at HAZ. When ω/v ranges in certain value, the hardness of joint didn’t change much. The hardness of NZ decreases with welding speed decreasing. The ultimate tensile strength(UTS) reaches its peak value(454.2MPa), at a welding speed of 120mm/min, which is 95% of the parental material. The elongation of the joint is 3.97% under this parameter. When ω/v decreases, the joint tensile strength increases first and then decreased.The post weld heat treatment test of joint indicates that different PWHT processing has different influence on the welding joint. Artificial aging treatment(AA), solution heat treatment(ST) and solution heat treatment followed by artificial aging(T6) were given to specimens to improve the mechanical properties of the 7A04-T6 FSW joints. It is found that the fine-equiaxed grains in NZ are stable after AA. The precipitates are coarsened and produce over aging effect, which reduce the mechanical property. After ST, most second phases dissolve in the matrix and the α-Al grains become coarse. Second phases precipitate in the α-Al matrix uniformly after T6 treatment. As a result, the mechanical property of joint increased. Effect of ST and T6 on the microstructure and mechanical properties of 7A04-O aluminum alloy FSW joint was investigated. The results show that after ST most second phases dissolve in the coarsened matrix. After T6, the density of precipitates increased. The Vickers hardness test shows that PWHT makes the hardness distribution of the 7A04-O aluminum alloy FSW joint more evenly. The tensile test results indicate that the tensile strength of the joint increases from 228 MPa(in as-welded condition) to 440 MPa and 528 MPa after ST and T6 treatment respectively, but the elongation decreases to a certain extent. SEM fracture morphologies indicate that the joint fails through ductile fracture for the as-welded and ST states and a mixture mode of brittle fracture and ductile fracture for the T6 state.The results of corrosion behavior investigation show that the BM has a better corrosion resistance property than that of FSW joint. The result of open circuit potential(OCP) shows that the open circuit potential of NZ and TMAZ+HAZ is lower than BM. The polarization test results indicate self corrosion potential of BM is higher than those of different zones in FSW joint. The corrosion current density of FSW joint is higher than BM. Results of electrochemical impedance spectroscopy(EIS) show that the Nyquist map of both BM and FSW joint are consist of two parts: high frequency capacitive arc and low frequency impedance arc. The radius of high frequency capacitive arc in BM is bigger than NZ. Both results of natural salt spray test and salt immersion test show that the HAZ and TMAZ are most susceptible to corrosion. It is found that the initial corrosion occurred in HAZ and TMAZ. As the extension of corrosion time, the corrosion products increase. The EDS and XRD analysis of the corrosion products indicate the main products are Al2O3, Al(OH)3 and Al Cl3. The corrosion weigh increases with corrosion time prolonging, the parent metal corrosion is a process of gradually slow down, and the joint is the process of accelerated corrosion.