4.1. Effect of Sc on microstructure and mechanical properties
Significant recrystallization suppression effect of Sc addition in aluminium alloys has been previously shown to arise from the precipitation of second Al3(Sc,Zr) particles formed during homogenization thermal treatment [16, 29]. In 7085Sc alloy, Al3(Sc,Zr) particles formed in the matrix and inhibited recrystallization during hot-rolled deformation as well as solution treatment. Therefore, noticeable sub-grains remained in BM and produced strengthening (Fig. 6 and Fig. 8a). Owing to sub-grains strengthening and precipitation strengthening of GPII zones as well as 𝜂′ phases, the UTS of 7085Sc-T6 increased to 593 MPa which was higher than AA7085-T6 reported in Ref. [30].
The welding behaviors of 7085Sc alloy was also affected by Al3(Sc,Zr) particles. It is known that grain growth and dissolution/coarsening of hardening phases tend to occur when suffered high FSW heat input, resulting significant softening in welded joint of heat-treatable strengthening Al alloys [24, 31]. For the studied 7085Sc FSWed joint, sub-grains in AS-HAZ did not grew during FSW because the migration of grain boundaries were retarded by Al3(Sc,Zr) particles (Fig. 6 and Fig. 7). Therefore, the presence of Al3(Sc,Zr) particles had significant benefit to decrease the strengthening degradation in HAZ. The softening bands in HAZ of 7085Sc FSWed joint could be mainly attributed to dissolution of coherent GPII and growth of semi-coherent metastable 𝜂′ phases (Fig. 9), similar to the previous reports [32, 33].
As compared with HAZ, WNZ showed higher hardness and better tensile properties (Fig. 3 and Fig. 4). One reason would be the high strengthening of finer grain structure. It is known that high friction heat input during FSW process would induce recrystallization and grain growth in WNZ. For the 7085Sc alloy, grain growth was restrained in WNZ due to high recrystallization resistance of the thermally stable Al3(Sc,Zr) particles (Fig. 7). Another main reason would be the strengthening of reprecipitated hardening phases. It has estimated that the temperature in WNZ would be above 500°C [34], redissolution of precipitated phases would occur in this condition and a supersaturated solid solution would form in WNZ during subsequent air-cooling process. Similar to the slow quenching process of Al-Zn-Mg-Cu alloys [35], the supersaturated solid solution in WNZ tended to decompose after welding, forming overgrowth quench-induced equilibrium phases and nanoscale precipitates. It has been proved that 𝜂 equilibrium phases as the result of heterogeneous precipitation appear to be mostly nucleated on pre-existing incoherent Cr or Zr-containing dispersoids in slow quench processing of Al-Zn-Mg-Cu alloys [21, 36]. For 7085Sc alloy, Zr-containing dispersoids in the matrix were mainly Al3(Sc,Zr), thus, the decomposition process of the supersaturated solid solution in WNZ would be associated with Al3(Sc,Zr) particles. As TEM observation showed that just a few Al3(Sc,Zr) particles coarsened and lost coherency when suffered the complicated thermo-mechanical input of FSW, more coherent Al3(Sc,Zr) particles retained in WNZ after welding (Fig. 10). Because coherent particles were able to reduce heterogeneous precipitation of overgrowth 𝜂 phases by preventing the spread of voids [37], only a small amount of incoherent Al3(Sc,Zr) particles acted as nucleate of 𝜂 equilibrium phases. Therefore, the formation of 𝜂 equilibrium phases in WNZ was restrained, instead, many Zn and Mg solutes in the solid solution tended to reprecipitate in form of 𝜂′ and GPII as observed in Ref. [38]. Consequently, mass of homogeneous fine hardening precipitates could be obtained in WNZ, resulting improvement of hardness, YS and UTS values.