Fig. 6 (a) (Mg, Si)-rich phase, (b) EDS result of (Mg, Si)-rich particle, (c) SAED image of (Mg, Si)-rich particle, (b) EDS result of Al-Fe-Cr-Si phase
After FSW, the hardness and yield strength of as-welded SZ were declined despite the grain refine strengthening effect from fine grain. The low dislocation density largely determined the poor strength of as-welded SZ. Hu et al [23] found a lower geometrically necessary dislocations (GNDs) density of SZ in 2219 FSW joint. According to the TEM image from [24], the low dislocation density (ρ ) of SZ was also observed. The yield strength from dislocation (\({\sigma}_{d}\)) can be estimated using Equation (1)[25]:
\({\sigma}_{d}=M\alpha Gb\rho^{\frac{1}{2}}\) (1)
where M is Mean orientation factor of fcc Al, α is Constant of fcc Al, G is Shear modulus of 6061 alloy, b is Burgers vector of fcc Al [26]. According to Equation (1), the yield strength from dislocation in as-welded SZ was only approximately 14 MPa. After rolled to 2 mm, the yield strength from dislocation increased to approximately 143 MPa. Therefore, the hardness and strength of SZ was greatly improved after rolling process.