Thermodynamic prediction of thixoformability in alloys based on the Al–Si–Cu and Al–Si–Cu–Mg systems

Most commercial semi-solid processing (of which thixoforming is one type) utilises the conventional casting alloys A356 and A357. There is, however, a demand to widen the range of alloys, including those with higher performance which tend to show poor characteristics for thixoforming. Thermodynamic calculation packages, such as MTDATA, provide a tool for predicting thixoformability. Here, the effects of compositional variations, in particular the effect of added copper on the thixoformability of alloy A356 and the effect of added silicon on the thixoformability of alloy 2014, have been investigated using MTDATA thermodynamic and phase equilibrium software combined with the MTAL database. Criteria for thixoformability are identified and a range of alloy compositions based on Al–Si–Cu and Al–Si–Cu–Mg evaluated in relation to these criteria. Compositions which satisfy these criteria include: 308 (Al–5.5Si–4.5Cu); 319 (Al–6Si–3.5Cu); 238 (Al–10Cu–4Si–0.3Mg); 355 (Al–5Si–1.3Cu–0.5Mg); 2014 based alloys Al–4.4Cu–0.5Mg–(4–6)Si; and a range of alloys (7.5 ⩽ Si + Cu ⩽ 9 and 1.5 ⩽ Si/Cu ⩽ 2.33) and alloys (9 < Si + Cu ⩽ 10 and Si/Cu = 1.5) based on the Al–Si–Cu–Mg system.