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.