Recycling-oriented design of the Al-Zn-Mg-Ca alloys

Though recycling has made advance under immense changes in new techniques, the consumption of primary aluminum remains very high that is mostly advocated by demand of premium wrought alloys, including Al-Zn-Mg, intolerant to Fe and Si impurities of recycling origin. Recently, several works on Al-Ca-Fe-Si alloys showed that calcium may bind the impurities into finely shaped ternary phases Al₁₀CaFe₂ and Al₂CaSi₂. We find it very promising to develop new Al-Zn-Mg-Ca-Fe-Si alloys with a structure containing the foregoing ternary phases instead of Al₃Fe and Mg₂Si. In this work we studied six compositions based on Al-8%Zn-3%Mg alloy separately and jointly alloyed by 1-2%Ca, 0,5%Fe and 0,5%Si. CALPHAD calculations showed that the alloys may contain up to six intermetallic phases Al₄Ca, Mg₂Si, A₄Ca, Al₂CaSi₂, Al₂Mg₃Zn₃ and MgZn₂ along with (Al). Moreover, the non-equilibrium solidification always ends at approximately 480 ^oC while the equilibrium solidus is in the 535-560 ^oC range depending on the alloying content. Having the consistent results on thermal analysis, we proposed the homogenization treatment of cast samples including the first step at 450 ^oC for dissolving of the non-equilibrium Zn- and Mg-rich eutectic and the second step at 520 ^oC for tuning the shape of the Ca- and Fe-rich particles up to their spheroidization. Microstructural observations of as-cast samples showed that 1-2%Ca alloying leads to formation of the (Al, Zn)₄Ca phase, while the addition of 0.5%Fe and 0.5%Si favors the formation of the Mg₂Si and Al₈Fe₂Si phase. Joint alloying with Ca, Fe and Si brings a far more complicated structure included mostly Ca-rich phases (Al, Zn)₄Ca, Al₁₀CaFe₂ and Al₂CaSi₂. The latter has a needle-like morphology, especially coarse at 2%Ca. Hence, after heat treatment the Al-8%Zn-3%Mg-1%Ca-0.5%Fe-0.5%Si alloy showed a better response in spheroidizing of intermetallics. Complex investigation of microstructure of heat-treated samples and hardening showed that the (Al, Zn)₄Ca phase brings a decrease in effective Zn content in (Al) and weakening of ageing response (170-180 HV in T6) in comparison to Al-8%Zn-3%Mg (~200 HV in T6). A quite similar result was demonstrated on Al-8%Zn-3%Mg-0.5%Fe-0.5%Si (185 HV in T6) due to formation of insoluble Mg₂Si phase along with lowering of the effective Mg solubility. On the contrary, joint alloying with Ca, Fe, and Si provides appropriate strengthening (195 HV in T6) probably due to decrease in amount of (Al, Zn)₄Ca and binding of Ca with Fe and Si-bearing phases. In the end we tried out hot and cold rolling (95% total reduction) and showed very good performance of the Al-8%Zn-3%Mg-1%Ca-0.5%Fe-0.5%Si. Though the composition still needs to be tuned, the results show a good promise to progress a currently static research in recyclability of Al-Zn-Mg alloys. Joint alloying with Ca, Fe and Si brings an option in using Fe- and Si-rich aluminum scrap or technically pure primary aluminum.