The analysis of variance on process parameters affecting the microstructures of semi-solid Al-Zn-Mg alloy billet by cooling plate method

The cooling plate method is a simple and effective technique for preparing thixoforming feedstock. In this study, a feedstock with rosette-type structure was prepared by the cooling plate method. To determine the major factors and to ensure reproducibility via the cooling plate method for thixoforming billets statistically, the analysis of variance (ANOVA) technique and the Taguchi method were employed. The ANOVA was used to define the effect of each parameter, including the pouring temperature, the cooling plate angle, the holding time after pouring and the mold temperature. Among the factors analyzed, the pouring temperature had the most significant effect, particularly on the grain size. In addition, the plate angle and holding time were varied to reduce the effect of pouring temperature. The optimum conditions to achieve the desired microstructure using this method involved the use of a low pouring temperature (superheat (Delta T) value of less than 20 A degrees C) and holding of the temperature of the cooling plate below the liquidus temperature. It was also found that the morphology of the grains quickly became globular when the billet was reheated to the liquid/solid temperature.