Prevention of macrodefects in squeeze casting of an Al-7 wt pct Si alloy

Abstract

The squeeze casting of an Al-7 wt pct Si alloy was carried out in order to investigate the conditions for the formation and the prevention of macrosegregation. The effects of process parameters such as applied pressure, die temperature, pouring temperature, delay time, degassing, and inoculation on the formation of macrosegregation were investigated, in correlation with the evolution of macrostructure and shrinkage defects. Three critical applied pressures were defined, based on the experimental results for the squeeze-cast Al-7 wt pct Si. The first is the critical applied pressure under which shrinkage defects form (PSC). The second is the critical applied pressure above which macrosegregates form (PMS). The third is the critical applied pressure above which and under which minor segregation forms. (Pm and PMS, respectively). With the concept of these three critical pressures, an experimental diagram describing the optimum process conditions was proposed for obtaining sound squeeze castings. It was concluded that sound castings without macrosegregation and shrinkage defects can only be obtained when the applied pressure is in the range where PSC < P<Pm (<PMS). Both degassing and inoculation treatments greatly enhanced the soundness of the castings. It was also found that the pouring temperature and the delay time should not exceed TD-critical and tD-critical, respectively, in order to achieve sound castings.