Mechanical and Metallurgical Studies on Thixoextruded Al-Si Alloys

Abstract

Thixoforming is a commonly known material processing technique that involves the fabrication of materials between solidus and liquidus temperatures. One of the prerequisites for this process is that the microstructure of the feedstock material must contain solid near-globular grains in a liquid matrix and a broad solidus-to-liquidus transition range. The aim of the present work is to study the application of Al-Si powder metallurgy (P/M) alloy in the thixoextrusion process and to evaluate the properties before and after extrusion. Al-Si P/M alloys were prepared for various Si compositions (1-7%) and Mg compositions (0.2-0.8%). The best among the Al-4Si and Al-4Si-0.6 Mg were selected based on hardness and density results. Differential thermal analysis revealed the solidus and liquidus temperatures to be 557 and 642 °C for Al-Si alloy and 564 and 636 °C for Al-4Si-0.6 Mg alloy, respectively. Compaction temperature and holding time studies have been carried out and the corresponding optimized parameters are found to be 550 °C and 60 min. Thixoextrusion was performed on the hot compacted preforms for three temperatures namely 560, 580 and 600 °C, respectively. An extrusion ratio of 4:1, strain rates of 0.1, 0.2 and 0.3 s−1 and die approach angle of 60° were used for the tests. Extrudates were divided into three regions namely front, middle and end parts to analyse its mechanical and metallurgical properties. Micrographic analysis revealed the presence of globular grains in the specimen. Hardness values showed a marginal increase from 45 to 60 HV for Al-4Si alloy and 42-80 HV for Al-4Si-0.6 Mg alloy.