Microstructure and Mechanical Properties of Al-Si Alloy Processed by Multi-Directional Forging

Abstract

In this present work, the effect of multi-directional room temperature forging on the mechanical and wear characteristics of Al-7.3Si and Al-12.1Si alloys with varying cumulative strains was investigated. Severe plastic deformation via multidirectional forging technique can be used to modify the mechanical properties. MDF can be applied to a relatively large sample that can be used for industrial applications. In the MDF process, strain per pass can be controlled by maintaining the height-to-width ratio, and thus, a wide range of metals and alloys can be deformed at room temperature. The Al Si alloy ingots were melted in a furnace and then poured into a preheated metallic die to produce a cast sample of desired shape and dimension. Al-7.3Si alloys were machined to dimensions of 30 mm x 30 mm x 24 mm for MDF processing with an equivalent strain of 0.22. Similarly, Al-12.1Si alloys were machined to dimensions of 30 mm x 30 mm x 23 mm for MDF processing with an equivalent strain of 0.27. The machined samples were then subjected to solution heat treatment before being forged in order to change the morphology of shape-edged eutectic silicon of the as-cast sample. MDF successfully processed the Al-7.3Si and Al-12.1Si alloys for two and three cycles, respectively, with cumulative strains of 1.3 and 2.43 at room temperature. Optical microscopy, field emission scanning electron microscopy, and X-ray diffractometer were used to characterise the effect of MDF processing on Al-7.3Si and Al-12.1Si alloys. Microstructural observations showed that the coarse eutectic silicon particles were effectively broken into finer particles and uniformly redistributed. With increasing MDF cycles, the silicon particles fragmented into finer particles. According to XRD results, the peak broadening in the XRD pattern of MDF-processed sample is due to the combined effect of crystallite size and micro-strain. The hardness and tensile strength of MDF samples have significantly increased. The hardness and tensile strength of the as-cast Al-7.3Si alloy increased to 60% and 149%, respectively, after two cycles of MDF. Similarly, the hardness and tensile strength of the as-cast Al-12.1Si alloy increased to 50% and 98%, respectively, after three cycles. Scratch test was conducted to measure the scratch resistance of unprocessed and MDF-processed samples. Tests were performed at ambient temperature under a progressive load of 2 15 N over a 5 mm scratch distance at a speed of 1 mm/min. Dry sliding wear tests were iii performed on a tribometer (pin-on-disc) under varying sliding speeds and loads at ambient temperatures. MDF-processed materials exhibited better scratch and wear resistance compared to as-cast materials. Furthermore, scratch and wear resistance increased with an increasing number of MDF cycles. Al-7.3Si alloy with two cycles and Al-12.1Si alloy with two and three cycles have shown maximum wear and scratch resistance. After MDF process, the wear mechanisms in both alloys shifted from adhesive and delamination wear to a combination of abrasive and a lesser amount of adhesion wear. Moreover, the degree of delamination significantly decreased after MDF process. Improvements in the mechanical and wear properties of MDF-processed samples can be attributed to the refinement and uniform distribution of eutectic silicon particles, as well as strain hardening of the aluminium phase.