ABSTRACT
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.
TABLE OF CONTENTS
TITLE PAGE i DECLARATION ii CERTIFICATION iii DEDICATION iv ACKNOWLEDGEMENTS v ABSTRACT vi TABLE OF CONTENTS vii LIST OF FIGURES xi LIST OF TABLES xii LIST OF PLATES xiii LIST OF APPENDICES xiv CHAPTER ONE 1.0 INTRODUCTION 1 1.1Background to The Research Study 1 1.2 Statement of The Research Problem 2
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1.3 The Present Research 2 1.4 Aim and Objectives 3 1.5 Justification 3 1.6 Scope of the Work 4 CHAPTER TWO 2.0 LITERATURE REVIEW 5 2.1 Introduction 5 2.2 Sand Casting Process 8 2.2.1 Sand Selection 9 2.2.2 Parting line 9 2.2.3 Pattern Making 10 2.2.4 Core Making 12 2.2.5 Moulding 12 2.2.6 Melting and Pouring 13 2.2.7 The Start of Solidification 14 2.2.8 Riser Design 15 2.2.9 Cleaning 16 2.2.10 Quality Problems in Sand Casting Process 16
2.3 Application of Chills 17
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2.3.1 Types of Chills 18 2.3.2 Volumetric heat capacity (VHC) of chills 19 2.4 Literature Review of previous works 20 CHAPTER THREE 3.0 MATERIALS AND METHODOLOGY 25 3.1 Materials 25 3.2 Equipment 25 3.3 Experimental Procedures 26 3.3.1 Casting of Alloys 25 3.3.2 Tensile Strength Test 28 3.3.3 Hardness Test 29 3.3.4 Impact Strength Test 29 3.3.5 Metallographic Examination 30 CHAPTER FOUR 4.0 EXPERIMENTAL RESULTS 30 4.1 Results 31
4.2 Mechanical Properties of Aluminium Alloy 32
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4.3 Microstructural Evolution of Aluminium Alloy 32 4.4 Volumetric Heat Capacity (VHC) of the Investigated Chills 32 CHAPTER FIVE 5.0 DISCUSSION OF EXPERIMENTAL RESULTS 33 5.1 Influence of Chill Materials on Solidification of Aluminium Alloy 33 5.2 Influence of Chill Materials on Ultimate Tensile Strength of Aluminium Alloy 34 5.3 Influence of Chill Materials on Hardness Value of Aluminium Alloy 36 5.4 Influence of Chill Materials on Impact Strength of Aluminium Alloy 37 5.5 Influence of Chill Materials on Microstructure of Aluminium Alloy 38 CHAPTER SIX 6.0 SUMMARY, CONCLUSION AND RECOMMENDATIONS 41 6.1 Summary 41 6.2 Conclusion 41 6.3 Recommendations 42 REFERENCES 43 APPENDIX 48
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CHAPTER ONE
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.
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.
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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:
i. to evaluate the effectiveness of different metallic materials such as copper, mild steel brass as chill in sand casting of aluminium alloys
ii. to evaluate the mechanical properties and microstructure of the chill cast aluminium alloys
iii. to correlate microstructure to mechanical properties of the cast alloys
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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;
i. Design of the sand mould by inserting the metallic chills at regular intervals.
ii. Casting of aluminium alloy.
iii. Evaluation of mechanical properties.
iv. Chemical composition and micro structural examinations of the cast alloys.
v. Structure – property correlation.
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