Influence of surface liquid segregation on corrosion behavior of semi-solid metal high pressure die cast aluminium alloys

Abstract

The aim of SSM forming is to obtain a semi-solid structure (which is free of dendrites) with the solid present in a near spherical form. The nature of the SSM slurry (with solid spherical grains suspended in a liquid), makes it susceptible to liquid segregation during forming processes such as high pressure die casting (HPDC). Surface liquid segregation appears to be a characteristic of SSM forming - the degree of segregation is dependent on alloy composition, die design and processing parameters. One of the major advantages of SSM processing is that high strength wrought alloys such as 7075 and 2024 can be used to produce near-net shape products. Corrosion is a surface phenomenon, and since the surface compositions of the SSM-processed components and wrought components differ greatly, their corrosion properties are expected to differ. The purpose of this paper is to compare the corrosion behaviour (in chloride medium) of the surface layer of SSM-HPDC plates of alloy 7075-T6 and 2024-T6 with the wrought alloys 7075-T6 and 2024-T6. Potentiodynamic testing was performed in deaerated 3.5%NaCl solution. In separate tests, the open-circuit potential was monitored in aerated 3.5% NaCl for 30 minutes after immersion. The electrochemical tests show no significant difference in the pitting potential of the wrought alloys and that of the Cu-enriched surfaces of the SSM-HPDC alloys.