Characterization of the after thixoforming microstructure of a 7075 aluminium alloy gear

Abstract

Among various methods of material preparation before thixoforming, applied to obtain globular microstructure, like cold or hot deformation, Equal Channel Angular Pressing ECAP allows to obtain the correct microstructure after heating to the semi-solid range. In the present study 7075 aluminium alloy was cold pressed using ECAP method in order to obtain the appropriate semi-solid range microstructure. The bar of 7075 alloy annealed at 400oC of diameter of 30 mm, was cold pressed using ECAP at a pressure of 800 MPa. Samples had only a small cracks and the hardness of 95 HV5. The characteristic temperature of the thixo-process was determined using DSC at 615°C which is slightly higher than that used in the literature. A series of a gear shape samples were thixo-formed from the alloy after ECAP using modified high pressure die casting. The globular microstructure with the globule’s size between 60μm -90μm was observed. The hardness was lower than after ECAP, close to 80 HV5. Additionally, T6 heat treatment was performed: supersaturation from 460°C and ageing for the period of 12 hours at 120°C. Transmission Electron Microscopy TEM enabled identification of η’ precipitates of size near 20 nm present in the alloy after the thixoforming and T6 treatment, responsible for a rise of hardness up to 150 HV5.