Characterisation of the influence of minor elements on microstructure variations in a secondary cast aluminium piston alloy

Abstract

The influence of minor elements (Sr, Sb, Mn, Cr and Al-5Ti-B) on the microstructure and porosity in a secondary piston Al-Si alloy was investigated using SEM-EDX and micro-focus computed tomography (CT) tools. Porosity and intermetallic characteristics were measured in the 3D CT volumes as well as on 2D SEM images. The intermetallics occurring were identified using EDX and by cross-referencing with previous work. The volume fraction (Vf) of intermetallics increased with minor element additions except when 0.2%Sb was added. Sb decreased Vf but increased beta-Al5FeSi particles in the alloy. This effect was cancelled with addition of 0.2%Sb+0.53%Mn. Porosity increased with all additions except for 1%Cr, which resulted in fine compact intermetallics and reduced porosity. Addition of 0.53%Mn and 0.3%Mn+0.2%Cr also resulted in compact phases and low porosity. Porosity was highest in the Sr-modified alloys with 0.02%Sr giving higher porosity than 0.05%Sr. However, porosity reduced in the 0.02%Sr alloy with 0.53%Mn addition while addition of 0.53%Mn+0.05%Sr significantly increased the level of porosity and intermetallics.