5. Conclusions
The inhomogeneity of microstructure, hardness, tensile properties and FCG behavior of 7085Sc FSW weld joint was investigated and discussed. Based on the experimental results, the following major conclusions can be drawn:
(1) The recrystallization resistance of the 7085Sc alloy was improved by coherent Al3(Sc,Zr) particles, thus, sub-grains were remained in BM after solution treatment and the grain growth was inhibited in AS-HAZ during FSW process. Frictional heating and intense plastic deformation caused fined recrystallized grains formed in WNZ, the average grain size was 4 μm, smaller than that of BM (14 μm) and AS-HAZ (16 μm).
(2) FSW heat input induced dissolution of GPII and slightly growth of metastable phases 𝜂′ in AS-HAZ, by contrast, completely redissolution of hardening precipitates occurred in WNZ forming supersaturated solid solution which tend to decompose to form new reprecipitated GPII, 𝜂′ and 𝜂 phases after FSW. However, the heterogeneous precipitation of overgrowth 𝜂 phases were reduced by coherent Al3(Sc,Zr) particles. Owing to precipitation strengthening and fine grain strengthening, the tensile properties of WNZ (UTS: 330MPa and YS: 485MPa) were higher than that of AS-HAZ (UTS: 415MPa and YS:262MPa).
(3) The FCG rate of AS-HAZ was lower than BM in Paris regime because the FCG resistance in HAZ was improved by slight grew semi-coherent 𝜂′ as shear mechanism. However, the FCG rates of BM and AS-HAZ were close at high ΔK due to the similar grain structures. WNZ possessed the best fatigue endurance because of shear mechanism of reprecipitated coherent phases (GPII and 𝜂′) and bypass mechanism of 𝜂 phases with large size and low volume fraction. Moreover, fine recrystallized grains with random crystal orientation in WNZ also contributed to reduction of FCG rate. The combined effects of precipitates and grain structures induced frequent deflection, torsion and bifurcation of fatigue crack, resulting in significant reduction of FCG rate in WNZ.