EFFECTS OF RECASTING ON THE MICROSTRUCTURAL AND

EFFECTS OF RECASTING ON THE MICROSTRUCTURAL AND MECHANICAL

PROPERTIES OF ALUMINUM BASED ALLOY

Owuor, Jared Odhiambo

A thesis submitted to the Graduate School in partial fulfillment for the requirements of Master of Science D e g r e e in Engineering Systems and Management of Egerton

U n i v e rs i ty .

July, 2013

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ABSTRACT

Investigation of the effects of re-casting and multiple T6 heat-treatment on the microstructural evolution and mechanical property variations in the Al-7Si-0.3Mg alloy

(Alloy A356), with satisfactory results in terms of the improvements in the microstructure and mechanical behaviour. The tests were done on twelve sample-sets given different thermal t r e a t m en t s ; six casting operations a n d equal number o f T6 heat-treatment. The samples were then taken through metallographic tests on light optical microscope and subsequent mechanical analyses such as Brinell hardness, Charpy-impact, tensile and creep tests. These tests and analyses were conducted to establish the influence of the thermal treatments on A356 cast aluminum alloy.

A prominent feature of the investigations w a s the improved metallurgical and mechanical properties f o l l o w i n g re-casting and subsequent heat treatments. It was observed that re-casting allowed for redistribution of phases in the Al-7Si-0.3Mg system. In addition, s t e p -wise, re-casting and heat-treatment caused formation of many new sites for grain nucleation and growth (nucleation) and hence recrystallization of fine grained crystals as is evidenced by the secondary dendritic arm spacing, average grain size number and light optical microscope observations. The net effect of this on the mechanical properties is the enhanced tensile, plastic flow and creep properties. The casting and recasting processes coupled with the multiple heat-treatments were seen to have improved the mechanical properties that included creep life, tensile properties and plastic flow as noted from hardness tests. However, the ductility of these alloys was reduced. In conclusion, holding the composition of A356 alloy constant, the mechanical and metallurgical behaviour gets modified by multiple casting and thermal treatments.

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TABLE OF CONTENTS

DECLARATION AND RECOMMENDATION .................

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DEDICATION ........................................................................


ACKNOWLEDGEMENT ..................................................... Ошибка! Закладка не определена.

ABSTRACT ................................................................................................................................. 2

TABLE OF CONTENTS .............................................................................................................. 3

LIST OF TABLES .................................................................. Ошибка! Закладка не определена.

LIST OF FIGURES ................................................................


LIST OF ABBREVIATIONS ................................................


LIST OF SYMBOLS AND NOTATION .............................


CHAPTER ONE: INTRODUCTION ...................................


1.1 Background information ................................................

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1.2 Statement of Problem ....................................................


1.3 Objective .....................................................................


1.3.1 Main Objective ......................................................


1.3.2 Specific Objectives ................................................


1.4 Research Questions ....................................................


1.5 Justification .................................................................. Ошибка! Закладка не определена.

1.6 Scope and Limitations ................................................


CHAPTER TWO: LITERATURE REVIEW .....................


2.1 Microstructure of Al-Si Casting Alloys ........................ Ошибка! Закладка не определена.

2.2 Mechanical Properties ...................................................


CHAPTER THREE: METHODOLOGY ............................


3.1 Alloy Preparation ...........................................................


3.2 Melting .....................................................................


3.2.1Melting and Pouring during alloy preparation ............


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3.3 Permanent Mould Castings ............................................


3.3.1 Fabrication of the Permanent Mould .....................


3.3.2 Melting and Pouring of permanent mould casting


3.4 Sectioning of the Casting and Heat Treatment ..............


3.4.1 Sectioning .............................................................. Ошибка! Закладка не определена.

3.4.2 T6 Heat Treatment ................................................. Ошибка! Закладка не определена.

3.5 Metallography ................................................................ Ошибка! Закладка не определена.

3.6 Tensile Testing .............................................................. Ошибка! Закладка не определена.

3.7 Creep Testing .................................................................


3.8 Brinell Hardness Testing ...............................................


CHAPTER FOUR: RESULTS AND DISCUSSION .....


4.1 Results .....................................................................


4.1.1

Effects of Recasting and Heat-Treatment on the Microstructure of Recycled Al-

7Si-0.3Mg Alloys .................................................


4.1.2 Effects of Recasting and Heat-Treatment on the Mechanical Behaviour of Recycled Al-

7Si-0.3Mg Alloys ..................................................


4.1.2 Effects of Recasting and Heat-Treatment on the Mechanical Behaviour of

Recycled Al-7Si-0.3Mg Alloys ..........................


4.1.3

Effects of Recasting and Heat-Treatment on the Creep Behaviour of Recycled

A356 Alloys ........................................................


4.2 Discussion ..................................................................... Ошибка! Закладка не определена.

CHAPTER FIVE: CONCLUSIONS AND RECOMMENDATIONS .... Ошибка! Закладка не определена.

5.1 Conclusions ...................................................................


5.2 Recommendations .........................................................


REFERENCES .......................................................................


APPENDICES .........................................................................


Appendix A: Test Certificates .............................................


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