Influence of Oxides on Porosity Formation in Sr-Treated Alloys

The aim of the present study has been to investigate the metallurgical parameters controlling the microstructural evolution and porosity formation in Al–Si–Mg and Al–Si–Cu alloys, through a study of the microstructural characteristics of directionally solidified A356 and 319 type alloys as a function of iron content, Sr addition (~250 ppm), and solidification rate. In the Sr-modified alloys, porosity formation is frequently associated with strontium oxides (particles or films), as well as β-Al5FeSi phase platelets. These oxides (with a stoichiometric composition close to Al2SrO3) are formed during melting, due to the high oxygen affinity of strontium, and are difficult to remove via degassing. The pore morphology (round or irregular) is determined by the form of the oxide, i.e. fine dispersed particles or thick films. Round pores are also observed surrounded by Al–Si eutectic regions. Aluminum oxide films trapped in the molten metal lead to the formation of coarser and deeper pores than those formed due to the strontium oxides. These pores can also link with each other and are characterized by the presence of solidified metal trapped within the aluminum oxide films, close to the periphery. The form of these pores is controlled by the amount of gases trapped within the pores during solidification.