Effect of stirring on oxide skins and porosity in A356 aluminum alloy

摘要

An experimental technique to investigate the effect of stirring on oxide skins and porosity in aluminum (A356) alloys was developed. Oxide skins become submerged in the melt during pouring or stirring and serve as sites where one or more pores may form during solidification. Although these oxide skins often elude detection, there is evidence that they increase in numbers with stirring, strontium addition and pouring. In this research, oxide skins were deliberately introduced into A356 melts via stirring, which created a vortex in the liquid. Samples were solidified under partial vacuum conditions to enhance the sizes of pores and make them visible by radiography. In most cases, an increase in time of stirring led to an increase in the number of pores, which indicates that porosity in A356 alloys is controlled by the content of inclusions in the melt, as well as the effectiveness of the melt fluxing and degassing treatment. The results suggest that models of microporosity formation should include the likelihood of preexisting intrusions or cavities on surfaces of nonwetting inclusions, where the pores form. In addition to the effect of stirring time, Sr-modification was found to increase the number of pores in samples that were fluxed and degassed before solidification under a partial vacuum.