Rapid Solidification of Al-7 wt.% Si Using The Twin-Roll Technique Maftah H. Alkathafi and Awanikumar P. Patil Abstract— The aim of this study is to understand the improvement in the properties of Al-7% Si with rapid solidification processing using the twin–roll technique. The process is based on two rolls rotating in opposite direction with 3900 revolution per minute (r.p.m.). The rapid solidification leads to a fine particle distribution of the second phase in the aluminium matrix. It is found in the metallographies test that the second phase undergoes coarsening on annealing and extent of coarsening increases with increasing annealing temperature (200, 300, 400 and 500 ). It is also found that the micro- hardness is increased with rapid solidification and decreased in subsequent annealing. The micro-hardness depends on the thickness of the ribbon where with lower thickness the micro-hardness increases because of the highest cooling rate. This is attributed to increasing the chilling effect on de- creasing thickness of ribbon. X-ray diffraction was employed and it was found that the lattice parameter of rapidly solidified sample was more than the slowly cooled one. It is attributed to entrapment of silicon in the aluminium matrix owing to rapid solidification. Keywords: Rapid Solidification, Twin-Roll Technique, Annealing, Al-7% Si. ——————————  —————————— 1 INTRODUCTION C ast Aluminium-Silicon alloys have been studied in detail due to their excellent combination of properties such as fluidi- ty, low coefficient of thermal expansion and high wear re- sistance. The Al-Si phase diagram is a simple eutectic system type; two metals are completely soluble in the liquid state but silicon is only partly soluble in the FCC structure of Al, while the solid solubility of Al in Si is negligible. The phase diagram for Al-Si is shown in Figure 1. The melting point of aluminium and silicon are 660.37 and 1414 respectively. The eutectic temperature is 577 with eutectic composition of 12.6 wt.% Si and the maximum solubility of Si in Al is 1.65 wt.%. Hypo- eutectic alloys form coarse dendrite grains of alpha-Al and large brittle flakes of Si. Such cast structure is brittle and ex- hibits poor ductility and also makes machining difficult. For this reason, Al-Si eutectic is modified with additives like Ce, Sr, No, Sb, CA, P, B, etc. to improve strength and ductility of the alloys [1],[2],[3]. A method called ‘Strain Induced Melt Ac- tivation’ (SIMA) is also used to obtain globular αAl grains [4]. As a result of this modification Al-Si alloys are finding a large number of applications in automobile industries [5]. In addition, rapid solidification of metals and alloys has become a field of extensive attention, since this method offers prospect of new or improved engineering materials with superior properties. All methods of rapid solidification produce struc- tures which otherwise can not be obtained economically by other manufacturing techniques like casting, rolling or forg- ing. In addition, the creation of certain shapes such as ribbons, wires, powder particles may be aspired to meet certain re- quirements. Rapid solidification of alloys leads to extension of solid solubility, decreasing grain size, and enhanced super- saturation of alloying elements [6]. The twin-roll technique is one of the techniques that are used for rapid solidification processes. The technique essentially consists of melting an alloy charge in a crucible and pouring the melt directly into a gap between a pair of rapidly counter-rotating rollers held together by special design. The resulting material emerges from the nip, the thickness of solidified material is a function of contact time (or residence time) with the roller surface, while the overall thickness of the solidified material is partial- ly controlled by roll gap. The products from various rapid quenching techniques come under the general classification of powder, flake, foil and wire. These products exhibit unique microstructure which may be amorphous (metallic glasses), microcrystalline or other structures. The present work deals with modification of Al-Si alloy micro-structure without the addition of a modifier by employing a twin roll technique. 2 EXPERIMENTAL PROCEDURE 2.1 Rapid Solidification Using The Twin - Roll technique The equipment that has been used in the work to produce rap- id solidified specimens by the twin-roll technique is shown in Figure 2. Double rolls are made of red copper and each has a diameter of 75mm and length 100mm. Copper is chosen be- cause it has a very high thermal conductivity and thus will be able to produce a greater chilling effect without artificial cool- ing arrangement. The pure Al and Al-7% Si were melted in ———————————————— • Maftah H. Alkathafi, Faculty of Engineering, Sirte University P.O.Box 565, Sirte-Libya, maftahok@hotmail.com • Awanikumar P. Patil,Department of Metallurgical and Materials Engi- neering,Visvesvaraya National Institute of Technology, Nagpur 440010 (India).) International Journal of Scientific & Engineering Research, Volume 4, Issue 11, November-2013 ISSN 2229-5518 1241 IJSER © 2013 http://www.ijser.org IJSER