Vol 60 • No. 9 • September 2014 Indian Foundry Journal 21 Introduction Aluminium alloys are widely used in automobile sector due to their outstanding combination of mechanical and physical properties, controlled thermal expansion coeficient and damping capacity [1] . Aluminium alloys are classiied according to their major alloying elements. The 4xxx group contains silicon as the main alloying element for ease of casting. Addition of Si increases the luidity of the melt, reduces the melting temperature, and decreases the shrinkage during solidiication. Aluminium-silicon alloys form a eutectic at 12.6 wt% silicon, the eutectic temperature is 577°C. Al-Si alloys are widely applied in the manufacturing of piston, cylinder heads, brake discs in industry due to excellent abrasion and corrosion resistance, low coeficient of thermal expansion and high strength-to-weight ratio [2-4] . The Al-Si cast alloy shows the dendritic structure. Many processes such as mechanical or electromagnetic stirring, the addition of grain reining elements and cooling slope are used to obtain equiaxed or globular structures [5] . Semi-solid metal processing (SSM) can be used to obtain globular microstructure [6-7] . Semi-solid metal processing (SSM) technology is a forming process to produce near-net- shape components, and has been developed at Massachusetts Institute of Technology (MIT) during the 1970s by Spencer and co-workers. It has received considerable interest in recent years due to its several advantages over the conventional casting technology [8] . The key factor of SSM is weaving of the semi-solid materials with a globular microstructure. SSM methods such as rheocasting, thixoforming, and strain- induced melt activation (SIMA) process can be used to produce non-dendritic structure, free from initial dendrite which normally forms during casting processes [9] . Among these, SIMA is a potential technology with the signiicant commercial advantages of simplicity and low equipment cost in the fabrication process. The process has been demonstrated to be applicable to most engineering alloy families, including aluminium, magnesium, copper Microstructure and Mechanical Properties of Al-7Si Alloy: Conventional and Modiied Strain -Induced Melt Activation (SIMA) Process : A Comparative Study D. Mandal 2 *, Chandan Choudhary 1 , S. K. Mitra 1 and Sukomal Ghosh 2 1 Metallurgical and Materials Engineering Department, National Institute of Technology, Durgapur-713209 2 CSIR-National Metallurgical Laboratory, Jamshedpur-831007 *Corresponding Author : E-mail : durbadal@nmlindia.org In the present investigation, Al-7Si alloy produced through conventional casting, conventional strain induced melt activation (SIMA) process and modiied SIMA process have been investigated and their microstructures, mechanical properties were compared. The basic Al- 7Si alloy was prepared through melting and casting route. Two different deformation processes i.e., forged, warm rolling (conventional) and directly warm rolling (modiied) were used to store energy in the alloy. After warm rolling, the samples were heated to a temperature above the solidus but below the liquidus and retained in the isothermal conditions at temperature of 580°C for 30 min. After the semi-solid heat treatment the samples were quenched in water at room temperature. The microstructure was observed under optical microscope and SEM. The mechanical properties were evaluated. The results indicate that primary α-Al and Al-Si eutectic are the main phases in the microstructure of Al-7Si alloy obtained with this casting and SIMA process. The results reveal that the microstructure of conventional cast Al-7Si alloy is fully dendritic in contrast to globular morphology of SIMA-processed samples. The tensile strength (UTS) and % of elongation of SIMA-processed samples are considerably higher than that of conventional cast samples. Microstructure and mechanical properties of conventional SIMA and modiied SIMA-processed samples are almost same. The tensile strength, % of elongation and hardness of Al-7Si alloy after SIMA process obviously increase, they reach 170 MPa, 25% and 86.0 (HV), respectively and have a signiicant improvement in respect of properties as compared to cast alloy. The eutectic silicon particles are spheroidised and uniformly distributed at the grain boundaries after SIMA treatment. Keywords : Al-7Si alloy, SIMA-Processed, Microstructure, Mechanical Properties. and ferrous alloys [10-11] . SIMA process generally consists of three separate stages. First, the alloy is prepared through melting and casting route and a typical dendritic microstructure is formed. Subsequently, a cold mechanical work would be carried out to generate enough mechanical