The processing and properties of aluminium-silicon alloys containing graphite

453 Pages·2017·14.88 MB·English

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The processing and
properties of
aluminium-silicon alloys
containing graphite
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the award of Doctor of Philosophy of Loughborough University.
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THE PROCESSING AND PROPERTIES OF
ALUMINIUM-SILICON ALLOYS
CONTAINING GRAPHITE
BY
P.R. GIBSON.
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ABSTRACT
THE PROCESSING AND PROPERTIES OF ALUMINIUM
-SI~ICON ALLOYS CONTAINING GRAPHITE
by P.R. GIBSON.
Cast aluminium-silicon alloys containing graphite particles are
reported to offer the potential for use as bearing and cylinder block
materials due to low friction and wear characteristics, good thermal
conductivity and resistance to corrosion. However, a fundamental problem
exists in dispersing graphite particles because liquid aluminium alloys
do not wet graphite and so particles are rejected by the melt.
The first section consists of a literature survey which examines
the potential for graphite aluminium alloys, and reviews various methods
for dispersing graphite particles. The potential for the use of recently
developed ~ethods of casting semi-solid alloys as a means of retaining
graphite particles in melts is then considered.
The second section reports on an investigation into the production
of aluminium-silicon alloys which contain graphite particles and an
evaluation of the mechanical and tribological properties of the composites
produced with various silicon and graphite contents. Processing semi
solid alloys produced an even dispersion of graphite particles without
segregation, agglomeration or rejection. Solidification under pressure
(squeeze casting) was used to optimise the mechanical properties of the
composites.
The evaluation of mechanical and tribological properties showed
that strength and ductility were reduced in alloys which contained
graphite. Wear tests sho,w ed'that graphite additions provided increased
.
load carrying capacity, reduced coefficient of friction, reduced steady
running temperature and reduced damage to m,a ti.n g components. There
was no evidence of a deteriorat~on in corros'{on from corrosion tests
and machinability tests showed that machinability was greatly improved
by the addition of graphite particles.
---------
-----
ACKNOWLEDGEMENTS
The author is deeply indebted to the following:
Professor R.J. Sury, Head of Department of Engineering
Production and Director of Research, for providing the
and the facilities necessary to carry out
o~portunity
this work.
Mr. A.J. Clegg and Dr. A.A. Das, Research Supervisors.
This work could not have been completed without their
construc.tive assistance, motivating influence and
encouragement~
All members of the technical staff in the Department of
Engineering Production who assisted with the experimental
work.
Dr. T.S. Eyre and the members of the Tribology Group at
,
BruneI University for their advice on tribological.
.
evaluatl.ons.
Finally, the author acknowledges the financial support of
the project relating to this thesis by the Science and
Engineering Research Council (Ref. GR/B/2536.0).
\
1
CONTENTS
Page
Introduction 2
?ECTION A - LITERATURE SURVEY AND REVIEW 4
CHAPTER 1 Bearing Materials 5
2 . Aluminium Alloy - Graphite Particulate
Composites 40
3 The Production and Casting of Aluminium
Alloys Containing Graphic Particles 63
4 Summary and Conclusions 72
5 Possible Areas for Investigation 75
SECTION B - EXPERIMENTAL WORK 79
Objectives. Equipment. Procedures
CHAPTER 6 Composite Alloy Production 81
7 Composite Alloy Evaluation 116
Results
8 Results Composite Alloy Production 148
9 Results Composite Alloy Evaluation 164
Observations of ResuLts
10 Observations - Composite Alloy Production 204
11 Observations - Composite Alloy Evaluation 212
Discussions
12 Discussion - Composite Alloy Production 226
13 Discussion - Composite Alloy Evaluation 241
14 Conclusions 259
15 Suggestions for Further Work 261
16 References 264
17 Appendices 279
18 Figures 324
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2
INTRODUCTION
The increase in the cost of energy is providing the automotive
industry with a strong impetus to produce lighter, more energy
efficient products. Aluminium alloy castings are playing an
increasingly important role in the drive to reduce weight and
provide bearing materials with good friction characteristics, high
thermal conductivity and resistance to corrosion. In particular,
aluminium-silicon alloys fulfil these requirements, in combination
with good casting characteristics'. However, a limitation exists
in their tribological performance in the form of a tendency to scuff
in conditions where lUbrication is sparse or intermittent.
The inclusion of evenly dispersed graphite particles in these
alloys has been reported to have improved tribological properties
by providing continuous solid lubrication. However, the production
of aluminium-silicon alloys containing dispersed graphite particles
presents difficulties because liquid alloys will not wet graphite,
with the result that particles tend to be rejected from liquid alloys.
Several methods have been proposed to overcome this limitation such
as injection onto an inert gas .. stream, and mechanical stirring.
These methods are slow and cumbersome and usually require an expensive
metallic coating on the particles to aid wetting.
The principal aim of this research was to utilise newly-developed
technology of processing partially'solid alloys as a means of dispersing
uncoated graphite particles in aluminium-silicon alloys. Having
evaluated the optimum conditions for the dispersion and retention
of particles in the alloys, the use of squeeze casting provided a