ABSTRACT

The effect of chromium and manganese on the mechanical properties and
corrosion resistance of Al-Si-Fe alloy was investigated. Alloys of varying
percentages of Chromium and Manganese from 0.1 to 0.5% (0.1, 0.2, 0.3, 0.4,
and 0.5%) with the percentages of iron and silicon kept constant were sand cast
into cylindrical test bars of dimension 20mm by 300mm. The mechanical
properties (Tensile strength, Hardness and impact energy) of the as-cast and
age-hardened alloy samples were determined. Also the corrosion characteristics
of the two categories of alloys (as-cast and age-hardened) in 0.5M HCl solutions
at room temperature (280C) over period of 480hrs were investigated by weight
loss method. The results obtained showed an increase in the tensile properties
and hardness for the two different alloys with increased Cr and Mn addition.
However, the age-hardened samples have improved tensile strength, ductility,
hardness, impact energy and corrosion resistance than the as-cast. For example,
the highest tensile strength value obtained in the as-cast and age-hardened
conditions for Cr is 79.90N/mm2 and 100.44N/mm2, Mn is 77.34 N/mm2 and
98.18 N/mm2, and MnCr is 82.81 N/mm2 and 103.23 N/mm2 respectively. Lowest
corrosion rate in the as-cast and age-hardened conditions was at 0.5% (Cr, Mn,
MnCr) additions. However, it was also observed that the corrosion rate decrease
with increase in the number of days of exposure time for all the alloys. These
could be attributed to the corrosion products, formed which tends to shield up
corroding surface resulting to a decrease in corrosion rate of the samples
investigated.

 

 

TABLE OF CONTENTS

TITLE PAGE
Declaration – – – – – – – – ii
Certification – – – – – – – – – iii
Dedication – – – – – – – – – iv
Acknowledgement – – – – – – – – v
Abstract – – – – – – – – vi
Table of Contents – – – – – – – – vii
List of Tables – – – – – – – – – x
List of Figures – – – – – – – – xii
List of Plates – – – – – – – – – xiii
CHAPTER ONE
1.0 Introduction – – – – – – – 1
1.1 Aims and Objectives – – – – – – 3
1.2 Background Information / Justification of the Research – 4
1.3 Scope of the Study – – – – – – 5
1.4 Statement of the Problem/Limitations of the Study – – 5
1.5 Contribution to Knowledge – – – – – 6
CHAPTER TWO
2.0 Literature Review – – – – – – – 7
2.1 Al – Si – Fe alloy – – – – – – – 7
2.2 Ternary and multi-component alloys – – – – 8
2.2.1 Alloy modification – – – – – – – 11
viii
2.3 Properties of Aluminium and its alloys – – – – 11
2.3.1 Physical Properties – – – – – – – 11
2.3.2 Mechanical properties – – – – – – 11
2.3.3 Corrosion Properties – – – – – – 12
2.4 Heat treatment of aluminium and its alloys – – – 14
2.4.1 Precipitation Hardening – – – – – – 16
2. 5 Application of aluminium and its alloys – – – – 19
CHAPTER THREE
3.0 Materials and Methods – – – – – – 20
3.1 Materials – – – – – – – – 20
3.2 Equipments – – – – – – – 20
3.3 Methods – – – – – – – – 20
3.3.1 Heat treatment – – – – – – – 21
3.3.2 Corrosion test – – – – – – – 21
3.3.2.1Corrosion rate determination – – – – – 22
3.4 Microstructural Examination – – – – – 22
CHAPTER FOUR
4.0 Results and Discussion – – – – – – 24
4.1 Results – – – – – – – – 24
4.2 Discussion – – – – – – – – 24
4.2.1 Tensile Strength hardness for as-cast and age-hardened
Al-Si-Fe alloy with Cr, Mn, and MnCr additions – – – 24
4.2.2 Impact energy of the as-cast and age-hardened
ix
Al-Si-Fe alloy with Cr, Mn, and MnCr additions – – – 26
4.2.3 Corrosion Resistance of the as-cast and age-hardened Al-Si-Fe
alloy with Cr, Mn, and MnCr additions over the exposure time 26
4.2.4 Microstructural Interpretation of tensile strength and hardness
for the as-cast and age-hardened Al-Si-Fe alloy with Cr, Mn,
and MnCr additions through grain boundary phenomenon – 27
CHAPTER FIVE
5.0 Summary – – – – – – – – 42
5.1 Conclusion – – – – – – – – 42
5.2 Recommendations – – – – – – – 43
References – – – – – – – 45
Appendix A; micrographs (Plates) – – – – – 50
Appendix B; Graphs (Figures)- – – – – – 58

 

 

CHAPTER ONE

INTRODUCTION
Aluminium and its alloys are characterized by a relatively low
density (2.7g/cm3 as compared to 7.9g/cm3 for steel and 8.86g/cm3 for
copper), high electrical and thermal conductivities and resistance to
corrosion in some common environments such as atmosphere, water: and
salt water (William, 1997, Fontana and Greene, 1987, Micheal and James,
1993). These qualities makes aluminum alloys one of the most used non
ferrous alloys used in the production of automotives components,
construction materials, containers and packaging, marine, aviation,
aerospace and electrical industries (Allen, 1979). The good properties and
low cost of aluminium alloys have resulted in such increased use that in
1990 aluminium was the second most widely used metal i.e (Second only
to steel)(International Aluminium Institute, 2000, Kenneth, 1999) .
Based on these good mechanical properties, the alloys can be
forged, stamped, extruded and sand cast (Rajan et al, 1988). Based on
the fact that they can be forged to desired shapes at elevated
temperatures the can equally be solution treated and aged hardened to
obtain desired microstructures and mechanical properties (Nwajagu, 1994
and Rollason, 1964). However, the mechanical properties especially
strength of aluminum and its alloy can be enhanced by cold working and
alloying, although both processes tend to diminish resistance to corrosion
in some alloys (Micheal and James, 1993). Due to the numerous
2
applications of aluminium in a variety of corrosive environments, different
methods/processes have been investigated with the aim of increasing the
corrosion resistance of aluminium alloys (William, 1997).
The mechanical properties of aluminium alloys are improved by
heat treatment processes such as age-hardening and solution treatment
(Michael and James, 1993). Aluminum–silicon-iron alloy provides good
combination of cost, strength, and corrosion resistance, high fluidity and is
always free from hot shortness (Metals Hand Book, 1975). The
compositional specifications of the alloys rest mainly on the amount of
iron, silicon, chromium, manganese added as alloying elements in various
compositions. Chromium is generally noted for its improvement on
strength and resistance to corrosion (Metal Hand Book, 1979A).
Manganese is believed to increase toughness, hardenability and
counteraction of embrittlement and hot shortness. While iron increases the
strength, hardness and reduce tendency to hot cracking and silicon
improves the fluidity as well as the castability and some mechanical
properties of the cast alloys (Datsko, 1966).
Because of the combined excellent mechanical properties and
corrosion resistance, aluminium alloys have found wide applications in
aviation, automotive, marine Industries, etc. (Avner, 1974). Although the
effects of copper addition on the corrosion behavior of as-cast Al-Si-Fe
alloy in acidic media, have been studied by Yaro and Aigbodion, 2006, the
authors observed that the addition of Cu to Al-Si-Fe alloy increases its
3
susceptibility to corrosion attack in the two acidic media used (HCl and
HN03 ) up to 4% Cu addition. The rate of corrosion is higher in HCl than in
HN03 and the rate of corrosion of coupon in HCl decreased with time
(Yaro and Aigbodion, 2006). The Researchers also studied the effect of
Cu addition on the mechanical properties of Al-Si-Fe and observed that
addition of Cu increased the tensile strength and hardness up to 6% Cu
addition (Yaro et al, 2006).
1.1 AIMS AND OBJECTIVES
The aim of the research is to investigate the mechanical properties and
corrosion resistance of Al-Si-Fe alloy in 0.5M HCl solution in as-cast
condition at room temperature and compare the result with those obtained
when the alloys were age-hardened.
The specific aims and objectives of this research was to understand,
1. The individual and simultaneous effects of Cr and Mn addition with Al-
Si-Fe alloy on the mechanical properties (tensile properties, hardness
and impact strength) in the as-cast condition.
2. The individual and simultaneous effects of Cr and Mn with Al-Si-Fe
alloy and age-hardening treatment on the mechanical properties of Al-
Si-Fe alloy.
3. To determine the individual and simultaneous effects of concentration
of Cr, Mn, MnCr and time of exposure on the corrosion resistance of
Al-Si-Fe alloys in 0.5M HCl solution at 280C for 480 hrs in the as-cast
and age-hardened conditions.
4
4. To study the microstructural changes that occur as a result of Cr and
Mn additions to Al-Si-Fe alloy in as-cast and age-hardened condition
5. The correlation between the studied mechanical properties and
microstructures of all the alloys produced.
1.2 BACKGROUND INFORMATION / JUSTIFICATION OF THE RESEARCH
The successful development of aluminium castings in parts and
components applications requires, that the casting display a combination
of high strength and toughness in thin and thick sections. These properties
are determine by the strength and integrity of the microstructures. Though
years of research, development and experience, the microstructure
characteristics required to achieve these properties in casting have been
determined and expressed in a variety of ways. Ultimately, the quality of
the microstructures of an alloy determines the performances that can be
obtained. However, as reported by Odutola (2005), that all engineering
materials are chemically reactive. Hence, the corrosion characteristics of
as-cast and age-hardened alloys with individual and simultaneous
additions of Cr and Mn in 0.5M HCl solution at 280C over a period of 480
hrs. Based on the mechanical properties requirement and corrosion
resistance of Al-Si-Fe alloy in automobile industry as oil pan, flywheel and
rear-axle housing, crank cases, etc. In aerospace industry as tankages for
storage of liquid fuels and oxidizers, engines, airframes, propellers,
accessories, etc. This research work improved these properties
significantly through alloying and age-hardening treatment.
5
1.3 SCOPE OF THE STUDY
The study involves determination of the mechanical properties (tensile
properties, hardness, and impact strength) of Al-Si-Fe alloy with addition
of Cr and Mn in as-cast and age-hardened conditions using standard test
procedures. The corrosion test was also carried out on the as-cast and
age-hardened samples at room temperature (280C) using weight loss
method over a period of 480 hrs of exposure time.
1.4 STATEMENT OF THE PROBLEM / LIMITATIONS OF STUDY
There are other important mechanical properties of aluminium and its
alloys, but since the service condition is a major factor in selection of the
mechanical property to be tested for, other properties have to be
investigated. Based on this, the tensile strength, hardness and impact
properties were investigated. Despite various ways of evaluating corrosion
resistance of metal/alloys, the weight loss method of investigating
corrosion was used due to its simplicity and method of result
computations.
The HCl acid concentration used for the corrosion test was fixed at 0.5M
because it is the optimum concentration of most acids (Yawas, 2005).
While the use of the acid as an environment for the samples was as a
result of the fact that the alloys found area of applications most in
automobiles and aerospace industry. Corrosion due to HCl acid in
crude/oil represent a significant portion of the refining cost. In this unit,
corrosion comes primarily from chlorides. Chloride corrosion is caused by
6
hydrogen chloride, which is formed from hydrolysis of the chloride salt
contained in the crude. The released hydrogen chloride is relatively noncorrosive
in the vapor phase. However, below the dew point of water,
hydrogen chloride forms an acidic solution and becomes very corrosive to
many structural materials (Quraishi, 2000, 2003) in Yawas (2005).
However, some aircraft may be operated in an environment containing
ions chloride, sulphate and polluting dust (Odutola, 2005). These chlorides
combine with hydrogen gas in the atmosphere forming acid rain which
subsequently affects the aircraft components over a period of time. Hence,
the use of the HCl solution as the corrosion medium.
1.5 CONTRIBUTION TO KNOWLEDGE
So far, to the best of my knowledge, no previous work has been carried
out on the individual and simultaneous addition of Cr and Mn with Al-Si-Fe
alloy. Therefore, this research has been able to improve significantly and
simultaneously the mechanical properties and corrosion resistance of Al-
Si-Fe alloy by alloying and age-hardening treatment.

 

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