Effect of precipitate size distribution on hardness of aluminium 6063 alloy

In this study, the effects of precipitate size and size distribution on the hardness of Al 6063 alloy were examined. Al 6063 samples were subjected to a solution treatment at 530 degrees C for 4 hours and quenched in water followed by storing in freezer (-18 degrees C) to prevent natural ageing. Ageing treatment was done at 190 degrees C for 135, 180, 225, 270 and 315 minutes. Precipitates distributed in the matrix were identified as Fe-Si-rich and Fe-Si-Mg-rich precipitates by performing Scanning Electron Microscope/Energy Dispersive Spectroscopy (SEM/EDS) analysis. Number of precipitates, average size and the area covered by precipitates were calculated by Image J software based on the precipitates observed in SEM images. For all heating profiles, precipitate size was less than 3.2 mu m. Maximum hardness of 143.90 HV was achieved for 270-minutes ageing time. A significant decrease in hardness was evident when the particles were coarsening above 1.5 mu m, possibly due to overageing for ageing time beyond 270 min.