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This study has evaluated the effectiveness of metallic materials as chill in sand casting of aluminium alloy. Four plates of dimension 165mm x 80mm x10mm were cast using sand mould. Steel, copper and brass chills in form of cylindrical bar of geometry 7mm in diameter and 50mm long were inserted, side by side at regular intervals of 30mm in each sand mould and the last sample was left unchilled. Experimentation involved testing of mechanical properties and metallographic analysis of cast samples.
The results obtained revealed that the sample chilled with copper has the highest mechanical properties (ultimate tensile strength of 126.13MPa, hardness of 6.8Hv and impact strength of 23.5J).Also sample chilled with copper revealed evenly distributed microstructure which is due to the fast solidification rate of the casting due to the high thermal conductivity of copper. The brass chill sample displayed better mechanical properties (ultimate tensile strength of 115.8MPa, hardness of 5.7Hv and impact strength of 22.4J) than sample chilled with steel (ultimate tensile strength of 101.33MPa and hardness of 5.4Hv). However, the unchilled sample showed the lowest ultimate tensile strength of 70.67MPa, hardness of 4.2Hv and impact strength of 22.5J.
1.0 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.
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