Surface modification of A390 hypereutectic Al-Si cast alloys using friction stir processing

In the present investigation, surface modification of A390 hypereutectic cast Al-Si alloys using friction stir processing (FSP) was conducted. The effect of the tool rotational and traverse speeds as well as the number of passes on the microstructural, mechanical and tribological characteristics of the modified surfaces was investigated. The results showed that FSP significantly improved the microstructure of the as-cast A390 Al-Si alloy by eliminating the porosity and refining both the alpha-Al grains and the Si particulates. The as-cast A390 alloy exhibited mean size and aspect ratio of Si particulates of about 59 +/- 24 mu m and 3.56 +/- 1.9, respectively. FSP significantly reduced both the mean size and aspect ratio of the Si particulates. The mean size of the Si particles increases with increasing the tool rotational and/or reducing the tool traverse speeds, but reduced by increasing the number of passes. Samples of friction stir (FS) processed at 1200 rpm, 20 mm/min and three passes exhibited the minimum mean size (4.39 +/- 1.9 mu m) and aspect ratio (1.18 +/- 0.4) of the Si particulates. The FS-processed regions exhibited less scattered and higher hardness values than the as-cast A390 alloy. The as-cast A390 alloy exhibited highly scattered hardness values between 62.5 and 94.6 VHN. Samples FS-processed at 1200 rpm, 20 mm/min and three passes exhibited the maximum hardness values between 114.66 and 119.34 VHN. The mean hardness of the stirred zones increases with increasing the tool traverse speed and the number of passes, while decreases with increasing the tool rotational speed. The FS-processed samples exhibited lower wear rates and coefficient of frictions than the as-cast A390 alloy. Both the wear rates and the coefficient of frictions were found to be reduced by reducing the tool rotational speed and/or increasing the tool traverse speed. Increasing the number of passes reduces the wear rate as well as the coefficient of friction. (C) 2013 Elsevier B.V. All rights reserved.