Thermodynamic predictions of wrought alloy compositions amenable to semi-solid processing

Semi-Solid Metal (SSM) Processing or Thixoforming is now a commercially successful manufacturing route producing millions of near net-shape parts per annum for the automotive industry. Currently, effort is directed towards developing wrought performance alloys that are amenable to the thixoforming process. Here, the 7000 series Al-Zn-Mg-Cu system has been studied by means of thermodynamic modelling to identify those compositions likely to be most suitable. The non-equilibrium solidification has been determined by using the Scheil equation and the liquid fraction/temperature relationship obtained. The results show the effect of chemical composition and Zn/Mg ratio on the slope of the liquid fraction/temperature curves. As the Zn+Mg+Cu content increases and the Zn/Mg ratio decreases, the slopes of these curves become less steep. The effect of chemical composition, Zn/Mg ratio and temperature on equilibrium phases is also considered. The phases in equilibrium at a given temperature are strongly affected by the Zn/Mg ratio and Cu content