Statistical analysis of porosity in Al-9 wt% Si-3 wt% Cu-X alloy systems

A set of 72 solidification experiments was carried out on Al-9 wt% Si-3 wt% Cu-X alloys, varying the additives (namely, strontium, grain refiner and other alloying elements), hydrogen level and thermal parameters in order to obtain a statistical analysis of the resulting porosity in such alloy systems. For all the cases studied, it was found that in each case, the smaller sized pores (in terms of pore length or pore area) could be described by an exponential function, while the larger sized pores were distributed in an irregular fashion, small and large being distinguished by that limiting value of pore length or area at which the average value equalled the standard deviation. Two different approaches i.e. factorial and regression methods were utilized to quantify the importance of the parameters controlling the pore size and porosity volume fraction. Two main observations were made: (i) hydrogen is the strongest parameter enhancing porosity formation, with the hydrogen-grain refiner, strontium, strontium-titanium and solidus velocity-solidification time interactions being the other parameters that contribute significantly to porosity formation and increase in pore size; (ii) the grain refiner, hydrogen-phosphorus, strontium-magnesium and iron-phosphorus interaction parameters reduce the porosity, though in different magnitudes. The reliability of predicting the observed effects by the two methods has been discussed.