Hard spot defects in aluminium-silicon cast alloys

Hard spots are one of the main machining defects in cast Al alloys. There are four types of "hard spots" in Al-Si-0.4Mg cast alloys: oxides, intermetallics, refractory particles and cold shots. The oxides in Al-Si-OAMg cast alloys include Al2MgO4, Al2O3 and MgO. SiO2 inclusions are entrained sand or refractory material. Fe-rich intermetallics are perhaps the most common hard spots in Al-Si alloys containing Fe and Mn. Other internietallics in Al-Si-0.4Mg alloy containing Ti or Sr are composed of TiAl3, Ti(AlSi)(2), Al2Si2Sr or Al4Si2Sr. Some intermetallic or non-metallic compounds originate from the use of grain refiners in the charges (added titanium boride and/or titanium carbide particles), or other nitrides, carbides or borides. Cold shots are the small droplets of cast alloy themselves formed during excessively violent mould filling. The machining damage is often exacerbated by these hard spots and associated oxides.