Performance evaluation of different materials as chills in sand casting of aluminium alloy

Thesis Overview

<p> </p><p>INTRODUCTION 1.1 Background to the study Metal casting is a shape forming process whereby molten metal is poured into a prepared mould and allowed to solidify such that the shape of the solidified object is determined by the shape of the mould cavity. Sand casting is a metal casting process characterized by using sand as the mould material (Ibhadode, 2001). Casting can be broadly divided into two main categories as expendable and nonexpendable mould casting. It can also be classified according to the mould material used to cast the metal such as sand casting, ceramic casting or metal mould casting and depending on the pouring methods as gravity casting, low pressure die casting and high pressure die casting (Navaneeth, 2009). Good mechanical properties are achieved in sand casting with the help of metallic insert in the mould known as chill (Mehr, 2012). Strong directional solidification is difficult to obtain in casting of intricate part made of aluminum alloys without the use of chills. The tendency for solidification to start throughout the metal makes proper feeding difficult. Chills must often be used to obtain satisfactory directional solidification (Chi-Yuan et al., 2006). Chills are metallic inserts moulded into the sand surface to promote high solidification rate in metal casting. Normally the metal in the mould cools at a certain rate relative to thickness of the casting. When the geometry of the moulding cavity prevents directional solidification from occurring naturally, a chill can be strategically placed to help promote it to obtain good mechanical properties. Chills are of two types, internal and external chills.<br>Chills are usually made from iron, aluminium or copper and can be machined or cast. The type of chill used depends on ease of manufacture and the desired thermal effects of the chill.<br>2<br>Its effectiveness depends on size, conductivity, thermal capacity and the thermal transfer across the molten metal alloy/chill interface. Chilling has been found to improve the soundness of a casting when measured by standard non-destructive testing techniques like radiography or dye penetration inspection, but the influence of microstructure and mechanical properties can be significant (David, 2011). 1.2 Statement of the Research Problem Normal solidification results a time to poor mechanical properties (hardness, tensile and impact strengths) due to the coarse grains structure after the casting has been produced from the sand mould. The undesired microstructure (coarse grains size and porosity) which leads to poor mechanical properties is due to low rate of solidification (Aran, 2007 and Navaneeth, 2009). The challenge here is to enhance the higher rate of heat removal (solidification). The need to investigate into some materials that can be used as chillers to assist in fast heat removal from the mould to increase solidification rate becomes very important. 1.3 The Present Research This research is on the performance evaluation of different material as chill in sand casting to increase solidification rate and to improve the mechanical and microstructural properties. 1.4 Aim and Objectives The aim of this study is to investigate the effects of different metallic materials as chills in sand casting of aluminium alloy. The specific objectives are:<br>i. to evaluate the effectiveness of different metallic materials such as copper, mild steel brass as chill in sand casting of aluminium alloys<br>ii. to evaluate the mechanical properties and microstructure of the chill cast aluminium alloys<br>iii. to correlate microstructure to mechanical properties of the cast alloys<br>3<br>1.5 Justification Machine components are usually of complex shape due to the nature of environment the part intended to perform its function. Casting of complicated shape is an integral part of engineering work carried out by foundry man. It is well known that aluminium alloys that freeze over a wide range of temperature are difficult to feed during solidification. The dispersed porosity caused by pasty mode of solidification can be effectively reduced by the use of chills. Most previous works have centered on the uses of metallic mode (direct-chill casting) to enhance solidification. Others have investigated on the placement of chill materials at one side from pouring base to enhance directional solidification. No work has been reported on placement of chill materials at regular intervals to increase solidification of aluminium alloys. This work will investigates the effectiveness of different metallic chills on the soundness of aluminium alloy in sand casting by placement of the chills at regular intervals so as to enhance directional solidification. 1.6 Scope of the Work The scope of this work covers;<br>i. Design of the sand mould by inserting the metallic chills at regular intervals.<br>ii. Casting of aluminium alloy.<br>iii. Evaluation of mechanical properties.<br>iv. Chemical composition and micro structural examinations of the cast alloys.<br>v. Structure – property correlation.</p><p>&nbsp;</p> <br><p></p>