An experimental study on microstructural, mechanical and machinability properties of as-cast and heat-treated al-7Si-mg alloy

Abstract

The automotive industry is constantly striving for targets for the development of lightweight materials for less fuel consumption and carbon emissions. For this purpose, lighter and higher-performance Al-Si alloys with high service life are preferred in mechanical systems to increase fuel efficiency. However, uncertainties regarding the mechanical and machining properties of these alloys continue in the industrial sector for sustainability in manufacturing. In particular, knowing these properties of Al-Si-Mg alloys used in engine blocks, engine covers, cylinder heads and pistons is important in terms of operating performance and service life in mechanical systems. In this study, Al-7Si-Mg alloy was manufactured by the permanent mould casting (PMC) method and the microstructural, mechanical and machining properties of this alloy in as-cast (ACst) and T6 heat-treated (HTed) states were investigated. Optical microscope (OM) and Scanning Electron Microscope (SEM) imaging processes were used to determine the phases in the microstructure of the alloy. The mechanical properties of the alloy were stated using tensile tests and the Brinell hardness measurement method. Milling tests were applied in the machinability tests, and these tests were realized with TiAlN coated carbide tools on the CNC milling machine using three different cutting speeds-CSs (V: 50, 80 and 110 m/min), feed rates-FRs (f: 0.08; 0.16 and 0.24 mm/rev) and constant depth of cut-DoC (1 mm). The microstructure of the ACst alloy consisted of α-Al, primary Si, coral-like eutectic Al-Si, acicular β-Fe (β-Al5FeSi) and script-like π-Fe (π-AlSiMgFe) intermetallic phases. After the heat treatment (HT), the present phases in the microstructure partially dissolved and clustered, and these phases coarsened and turned into a spherical morphology. As a result of the HT, the hardness, yield (YS) and tensile strength (TS) values of the alloy augmented, while the elongation to fracture (EF) was reduced. After milling tests, cutting force (F), surface roughness (Ra), BUL (Built-up layer) and BUE (Built-up edge) values in the ACst and HTed alloys decreased depending on the increase in CS, while they increased with the increase in FR. The results showed that HTed alloys should be subjected to machining operations with a combination of high cutting speed and low feed rate to achieve high mechanical service life and machined surface quality. It was determined that industrial practitioners could increase performance in manufacturing by considering this situation.