Mathematical modelling and development of response surfaces to predict and analyze the salt fog corrosion resistance of friction stir welded AA7075-T651 alloy joints: effect of retrogression and reaging

AA7075 is a heat treatable high strength aluminium alloy that finds applications in aerospace sector for structural parts and frames. This alloy has good mechanical properties. However, its inferior corrosion resistance and non-weldability limits its applications. In this investigation, the 10 mm thick plates of AA7075-T651 aluminium alloy were welded using friction stir welding (FSW) and subjected to the post weld heat treatment (PWHT) of retrogression and reaging (RRA). The corrosion resistance of stir zone (SZ) of joints was determined using a test of salt fog corrosion in unwelded, as-welded (AW) and RRA condition. The experimental matrix was designed using response surface methodology (RSM) by utilizing the design expert software. The joints were fabricated, heat treated and tested as per the experimental runs. The corrosion rate prediction (CRP) models were formulated using the regression methodology and validated using analysis of variance (ANOVA). The salt fog corrosion parameters were optimized to minimize the corrosion rate of SZ of joints. The SZ of joints showed lower corrosion rate of 01351 mm/year, 0.8756 mm/year and 0.6956 mm/year for unwelded, AW and RRA-treated condition when subjected to the salt fog corrosion environment of pH value of 7, spraying time of 72 h and Cl ion concentration of 0.6 Mol/lit. The AW and RRA-treated joints showed inferior corrosion resistance than unwelded base metal. The RRA-treated joints showed 20.55% reduction in corrosion rate of SZ of joints than AW joints.