Rheological behavior, microstructure and hardness of A356 aluminum alloy in semisolid state using backward extrusion process S. A. SADOUGH 1,2 , M. R. RAHMANI 1,2 , V. POUYAFAR 1,2 1. Center of Excellence in Termoelasticity, Amir Kabir University of Technology, Tehran, Iran; 2. Department of Mechanical Engineering, Islamic Azad University Science and Research Branch, Tehran, Iran Received 13 May 2010; accepted 25 June 2010 Abstract: The influence of temperature on the flow behavior and rheological characteristics of an A356 alloy in the semi-solid state was investigated using backward extrusion process. Experiments were performed at 5 temperatures and 4 different wall thicknesses. Viscosities were determined using the force-displacement graphs obtained form back extrusion tests. As observed experimentally, at a constant temperature, the increase of shear rate results in the decrease of alloy viscosity exponentially. Raising the temperature increases the liquid fraction hence reduces the semi-solid alloy viscosity. Metallographic and image analyses show that, because of low forming speed, liquid has time to escape from solid phase forward the sample wall. This condition is the main reason for the segregation phenomenon seen in the base and walls. Vickers hardness test on samples reveals that the hardness increases with the decrease of temperature and wall thickness. Key words: aluminum alloy A356; backward extrusion; semisolid forming; rheology 1 Introduction About 40 years ago, SPENCER et al[1] noticed that stirring a metallic alloy in the semi-solid state (SSS) produced non-dendritic microstructures with alluring rheological properties. The technological potential of this phenomenon was quickly recognized. Semi-solid forming (SSF) processes, also called thixoforming, gained a broad applications in the aerospace, transport, military and automotive industry[2]. Appropriate viscosity values depend on the establishment of a specific microstructure, typically non-dendritic, which is a precondition for the success of all semi-solid processing technologies. Perfectly, this microstructure should be composed of equiaxed particles of the primary phase α(Al) well insulated by a layer of eutectic liquid. These features can be obtained by a number of methods, such as: 1) mechanical or electromagnetic stirring in the liquid state[3]; 2) thermo-mechanical treatments consisting of plastic deformation followed by heating up to a chosen semi-solid temperature[4]; 3) cooling slope[5]. Along the way to industrialization, it is necessary to obtain the alloy behavior in all processing conditions. Rheological behavior in semi-solid state, and the impact of raw material characteristics and different parameters of forming process have to be carefully investigated. Rheological investigations are carried out mainly by rotational viscometry[6−7] and simple compression between parallel plates[8−9] and back extrusion tests[10−11]. Each method has its advantages and disadvantages. In this study, back extrusion method was chosen to investigate the temperature effects on rheological behavior of aluminum A356 samples. The influence of temperature on the microstructure and hardness of samples was also investigated. 2 Experimental 2.1 Apparatus and reagents The reference material was an A356 alloy. There was not any information about the thermal and mechanical history of the material; however, that was not necessary either; because aluminum ingots were remelted before casting by cooling slope. Solidification and melting temperature of this alloy are 555 °C and 618 °C, respectively. In this study, cooling slope treatment was employed in order to obtain a non-dendritic microstructure. The cooling slope parameters chosen in this study were pouring length of 30 cm, slope angle of 60° and pouring temperature of 640 °C. Aluminum melt Corresponding author: M. R. RAHMANI; Tel: +98-936-1544065; E-mail: rahmani_m_r@yahoo.com Trans. Nonferrous Met. Soc. China 20(2010) s906-s910