Effects of primary phase morphology on mechanical properties of Al-Si-Mg-Fe alloy in semi-solid slurry casting process R. BURAPA 1 , S. JANUDOM 2 , T. CHUCHEEP 2 , R. CANYOOK 2 , J. WANNASIN 2 1. Department of Industrial Engineering, Faculty of Engineering, Rajamangala University of Technology Srivijaya, Songkhla 90000, Thailand; 2. Department of Mining and Materials Engineering, Faculty of Engineering, Prince of Songkla University, Songkhla 90112, Thailand Received 15 May 2010 ; accepted 25 June 2010 Abstract：The gas induced semi-solid (GISS) process was developed to create semi-solid slurry with fine and uniform globular structure. The combination of local rapid heat extraction and vigorous agitation by the injection of fine inert gas bubbles through a graphite diffuser in molten metal held at a temperature above its liquidus temperature changes the morphology of primary α(Al) from coarse dendritic to rosette-like and finally to fine globular. The GISS process produced semi-solid slurry at low solid fractions and then formed the slurry by a squeeze casting process to produce casting parts. The effects of primary phase morphology on the mechanical properties of Al-Si-Mg-Fe alloy were investigated. The results show that the ultimate tensile strength and elongation are affected by the shape factor and particle size of the primary α(Al). Key words: gas induced semi-solid (GISS); microstructure; mechanical properties; semi-solid slurry casting; rheocasting 1 Introduction Semi-solid metal (SSM) processing has been continuously developed to produce metal parts with uniform microstructure and improved mechanical properties in various applications. There are two major types for SSM processing: thixocasting and rheocasting. Thixocasting is a process that starts from a special billet that is carefully reheated to a semi-solid temperature range until the SSM has about 50% liquid fraction and then it is formed into parts. Rheocasting is a process that uses molten metal from the start and then produces it into semi-solid slurry by a special method. When it has about 50% solid fraction, the SSM is then formed into products[1−4]. In general, semi-solid forming is performed at high solid fractions (40%−60%), so it has the limitations such as high viscosity, which tends to be difficult to pour semi-sold into the hole of the shot sleeve. Therefore, the semi-solid metal casting with low solid fraction slurry is easy to produce casting products since only minor machine modifications are needed. At low solid fractions, the slurry has good fluidity, so it can flow easily and requires low pressures to form into products. In addition, semi-solid casting at low solid fractions behaves similar to that of liquid casting and can be poured into a mold using lower pressures. The process leads to lower energy consumption and cost savings of casting machines comparing with a semi-solid forming process at high solid fractions[2,5−7]. The semi-solid slurry casting at low solid fractions has been investigated in recent years. HAGA and KAPRANOS[5] studied two kinds of simple rheocasting process at low solid fractions. One process used a cooling slope and the other used low superheat casting. The results of this study show that the low solid fraction casting gives excellent mechanical properties to the products especially the percent elongation, which is caused by the small size of the primary α(Al). YURKO et al[8] reported that the uses of low solid fractions for Al-Si-Mg, Al-Si-Cu and Al-Cu were achieved by the SSR process. The process has been applied to the industry and proved that it could decrease the process cycle time and increase machine tool life. WANNASIN et al[9] studied the semi-solid die casting ADC10 alloy with low solid fraction of about 10% using the GISS process. The results show that the GISS process can improve the mechanical properties comparing with liquid die casting. However, the Corresponding author: J.WANNASIN; Tel: +66-74-287-312; E-mail: jessada.w@psu.ac.th, jessada@alum.mit.edu Trans. Nonferrous Met. Soc. China 20(2010) s857-s861