ABSTRACT
The effect of pre-ageing thermal conditions on the hardness, corrosion resistance and microstructural morphology of antimony modified Al-Si-Mg alloy has been studied. The thermal treatment employed were Double Thermal Ageing, T7 (DTAT) comprising of pre-ageing temperatures of 900C, 1050C and 120oC with variable pre-ageing time of 1-5 hrs respectively, along with Single Thermal Ageing (STAT). The result of the average HRF hardness values show that, the hardness of the treated alloy DTAT conducted gave better hardness value compared with those obtained in the convectional STAT. The DTAT treatment increased the hardness by 67.0% as compared to 21.9% obtained for STAT treatment. The X-Ray Diffractometry (XRD) revealed that Al3Mg7Si2, Al8FeMg3Si6, Al9Si and FeSi phases were present in the As-cast and Mg2Si, Al9Si, FeSi, Al3Si3 and Al12Mg17 phases were present in the treated alloy. For DTAT treatment, ageing at 900C has been observed to be the peak ageing temperature at 2hrs which gave a maximum value of 8.40HRF. Equally the alloys aged at 900C for 1hr and 1200C for 1hr gave hardness value of 7.80HRF and 7.70HRF respectively for what could be attributed to the very fine coherent intermetallic precipitates which impede dislocation movement. The corrosion characteristic of as-cast, STAT and DTAT were evaluated using weight loss and electrochemical potentiodynamics techniques. From the linear polarization data and curves, the corrosion rate of the treated alloy decreases at all ageing temperatures along with the ageing time. The Optical microscope (OPM) results of as-corroded samples revealed that the alloy suffers pitting/uniform corrosion. The corrosion behaviour of the DTAT samples in simulated sea water showed a higher improvement in corrosion resistance than the as-cast and conventional STAT alloy