Effects of cobalt and solidification cooling rate on intermetallic phases and tensile properties of a -Cu, -Zn, -Fe containing Al-Si alloy

Tensile properties of recycled Al-Si alloys are reduced in the presence of high iron (Fe) content. Until nowadays, the addition of manganese (Mn) is the only way permitting to transform the deleterious beta-Al5FeSi phase. Alternatively, for the first time, the effects of cobalt (Co) and solidification cooling rate on the neutralization of the beta-Al5FeSi phase are reported. The Al5FeSi phase changes into the Al-5(Fe,Co)Si phase. In the present study, recycled Al-7 wt%Si alloys are investigated. Their compositions were established guided by some typical scrap compositions (in wt%) having small impurities such as iron (0.6% Fe), zinc (0.25% Zn), and copper (0.35% Cu). The attempt is made in order to enhance the mechanical strength of the Al-7%Si-0.6%Fe-0.35%Cu-0.25%Zn alloy by means of modification with Co. Two directionally solidified (DS) alloy castings are generated, which are the Al-7%Si-0.6%Fe-0.35%Cu-0.25%Zn and Al-7%Si-0.6%Fe-0.35%Cu-0.25%Zn-0.5%Co alloys, in which samples solidified at different cooling rates have been generated. Tensile strength and ductility are reported for both tested alloys and their respective samples. The strength increases owing to the addition of Co, which is promising if small alloying scraps are considered in the production of Fe-rich Al-Si alloys. The presence of Co induces mechanical strengthening by the formation of higher fractions of intermetallics. The ductility, however, showed some loss with almost 25% of decrease caused by Co.