Determination of the effect of Si content on microstructure, hardness and wear rate of surface-refined Al-Si alloys

摘要

Surface refining of Al-Si alloy was carried out using the 'Gas Tungsten Arc' (GTA) heat source and the effect of Si content on the microstructure, hardness and wear properties of the surface modified alloys was evaluated. In order to further improve its wear resistance, the Surface Refining Process (SRP) was employed in this study. In the SRP, the surface of the parent material is melted by a suitable heat source and the molten zone is allowed to solidify progressively. It can be noted from the literature that e-beam and laser are the only heat sources employed. Both these sources are highly expensive and their productivity is also low. The depth of the modified layer by e-beam or laser process is found to be inadequate for wear applications. In order to overcome the above shortcomings, in this study, the surface refining of Al-Si alloy was carried out by using an inexpensive, high productivity and commonly available GTA source. In this work, the effect of Si content on the properties is evaluated by varying the Si content from 4-16 wt%. The alloys were sand-cast in the form of bar(150x30x30 mm). The Surface melting was carried out with the following GTA parameters: current-150 A, travel Speed-2 mm/s, arc length-3 mm, tip angle-180°and electrode diameter-2.4 mm. The hardness was measured at different locations by using Vickers Hardness Tester by applying 100 gm load for 15 s and an average value was taken. The wear testing was conducted as per ASTM G99 standard under a dry sliding condition in air using Pin-on-Disc wear tester. In this study, it was noted that the typical as-cast microstructure of the Al-Si alloy illustrating the elongated morphology of the eutectic-Si has been completely refined, and the eutectic-Si is finely dispersed within the α-Al matrix. It is inferred that the microstructure was refined due to fast cooling to have a globular eutectic-Si dispersed within a fine-grained matrix in the GTA process. It was found that the depth of the modified layer is significantly higher than that obtained in the e-beam/laser process. The hardness of the modified layer was found to increase when the Si content is increased from 4-16 wt%. The wear rate is found to decrease with an increase in the Si content whereas the coefficient of friction tends to remain the same. The wear mechanism was found to be adhesive. Finally, the peak hardness of the modified layer increased significantly upon ageing. The observations are in agreement with that of previous studies.