Cast and Wrought Aluminium Bronzes - Properties, Processes and Structure

374 Pages·2000·21.55 MB·English

by  Meigh

CAST AND WROUGHT
ALUMINIUM
BRONZES
PROPERTIES, PROCESSES
AND STRUCTURE
Harry J Meigh CEng MIMech E
Book 697
First published in 2000 by
10M Communications Ltd
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London SW1YSDB,UK
rOM Communicaions Ltd
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The Institute of Materials
© Copper Development Association 2000
AllRights Reserved
The right of Harry JMeigh
to be identified as the author of this book
has been asserted in accordance with the
Copyright, Designs and Patents Act 1988
Sections 77 & 78
ISBN 978 1906540 20 3
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Part 2
MICROSTRUCTURE OF
ALUMINIUM BRONZES
INTRODUCTION TO PART 2
Alloy systems
In order to explain the development ofthe microstructure ofaluminium bronzes,
from those with the simplest to those with the most complex composition, it is
convenient to dividethem into the following three systems:
(1) The Binary System which consists ofonly two elements, copper and alumin-
ium, in varying proportions.
(2) Ternary Systems which consist of three elements: copper, aluminium and a
third element (iron, manganese, nickel, silicon etc.), also in varying
proportions.
(3) ComplexSystemswhich consist offour or more elements: copper, aluminium
and two or more other elements, likewisein varying proportions.
These systems willbe considered in the following chapters:
Chapter 11: Binary Systems
Chapter 12:Ternary Systems
Chapter 13: Copper/Aluminium/Nickel/Iron System
Chapter 14: Copper/Manganese/ Aluminium/Nickel/Iron System
Itwillbeseen that the ternary and complex systems are modifications ofthe basic
binary system. An understanding ofthe binary system is therefore essential to the
understanding ofthe more alloyed systems.
The structures of existing alloys are considered rather than their past develop-
ment. The rational for the composition ofthese alloys will, however, become evi-
dent, at least in part, from the effectsofthe various elements on mechanical and
corrosion properties.
The effect of heat treatment on microstructure will also be considered. This
should be considered in conjunction with Chapter 6 where the standard forms of
heat treatment are explained.
The structure ofaluminium bronzes isexplained in these chapters in a way that,
it ishoped, will be understandable to readers who may have little or no knowledge
ofmetallurgy.
Crystalline structure
As previously explained in Chapter 4, the structure ofa metal consists ofcrystals
bonded together. These crystals are themselves made up ofatoms or, more properly
speaking ofions. which were previously dispersed randomly in the liquid state, and
233
234 ALUMINIUM BRONZES
(a) body-centred cubic (b) face-centred cubic (c) hexagonal close-packed
Fig. 11.1 Three principal types ofspace lattices.
which assumed, on solidification, an orderly geometrical pattern, known as a 'space
lattice'. There are several types of space lattice but the three most common are as
follows and are illustrated in Fig. 11.1:
(a) body-centred cubic (bee), see Fig. Ll.La,
(b) face-centred cubic (fcc), see Fig. II.lb,
(d)hexagonal close-packed, see Fig.II.lc.
The space lattices shown in Fig. 11.1 are the simplest units and a crystal is made
up of a continuous series of these units in which adjoining units share a common
face. This assembly of space lattices constitutes therefore the structure of the crystal.
As we shall see in the following chapters, different chemical constituents of the
alloy may have different space lattices.
The type of space lattice has a bearing on the ease or difficulty with which a
wrought alloy can be worked and on the choice of working temperature. For
example, a face-centred cubic structure is far more malleable and ductile than a
hexagonal close..packed structure.
Growth 0/crystals
As a liquid metal approaches its solidification temperature, a number of 'nuclei' are
formed simultaneously in the melt, a 'nucleus' being a single unit of a given type of
space lattice. Other atoms then attach themselves to these nuclei, building up a
crystal of the same type of space lattice as the nucleus. The crystal initially grows
into a dendrite (see Fig. 11.2) which conforms to a rigid geometrical pattern.
Eventually the outward growth of the dendrite is impeded by other growing
dendrites in the vicinity. The crystal then grows in thickness as the liquid metal
remaining between the arms of the dendrite solidifies. This results in irregularly
shaped crystals. This growth process ofcrystals is illustrated in Fig. 11.1. The more
nuclei appear in the melt the sooner the growth of the crystal is halted by neigh-
235
MICROSTRUCTURE OF ALUMINIUM BRONZES
t
tIHEAT 01SS1PATION
ANO CRVSTAL GROWTH
Pig. ]].2 Early stage in the growth ofa dendrite."-'
Fig. 11.3 Formation ofcrystals or grains by dendritic growth-?
bouring crystals and therefore the smaller the 'grain' structure of the alloy. The
word 'grain' is effectivelyinterchangeable with the word crystal and is more com-
monly used when referring to the microstructure. The grain sizehas very import-
ant effectson alloy properties as willbe discussed later.
If the alloy cools slowly below the solidification temperature, the crystals keep
growing, but at each other's expense. Thus a sand casting willhave acoarser grain
than a diecasting and the thick sections ofa sand casting willhave a coarser grain
than its thin sections.
Chemical constitution of an aluminium bronze alloys
An aluminium bronze alloy in the liquid state consists of a solution of its various
elements in each other. It may also contain some intermetallic compounds which
236
ALUMINIUM BRONZES
have resulted from a chemical reaction between certain elements and which are also
in solution. On solidification, the liquid solutions become solid solutions, each crystal
being of a particular type of solution determined by its composition and space lattice
arrangement and known as a 'phase'. Aphase isnot necessarily a uniform solution,
however, because, at the top temperature of the solidification range, the metal solid-
ifying first will be richer in the higher melting point element, whereas the metal
solidifying last will contain a smaller proportion ofthat element. It follows that the
core ofany crystal will bericher in that element than its periphery. Thisisknown as
'coring'. Nevertheless, a phase has a given characteristic appearance under the mi-
croscopeand has certain specificproperties which affectthe properties ofthe alloy as
a whole. As we shall see in the subsequent chapters, an alloy may solidify into one or
more phases, depending on its composition.
As in the case of the solubility of liquids, metallic elements and compounds
become less soluble as the temperature falls in the solidstate. Thismay result, as we
shall see, in the gradual conversion ofone phase into a different phase which will
consist of a different solution and which may have a different space lattice struc-
ture. It will have a different characteristic appearance under the microscope and
different properties. Intermetallic compounds may also come out of solution as
precipitates. They then become visibleunder the microscope and will appear either
within a crystal or at the boundary between two crystals. They too constitute a
'phase' and their presence as precipitates in the structure will affect the properties of
the alloy in a different way than when they were in solution. Thus a 'phase' is a
constituent ofan alloy which exists as a distinct entity in the microstructure ofthe
alloy and which, in the case ofaluminium bronzes, consists in one or other ofthe
following:
(a) a solidsolution of one or more elements in another, or
(b) an intermetallic compound which has formed by chemical reaction and has
come out of solution before or after solidification.
(c) a combination oftwo or more individual phases to form duplex or complex
phases.
Heat treatment
If the alloy is re-heated, phase changes are reversed provided sufficient time is
allowed. By controlling the time ofexposure to a higher temperature and the rate of
cooling thereafter, the nature ofthe alloy, both in its grain size and phase constitu-
tion, can be adjusted to achieve a desired combination of properties. This is the
object ofheat treatment.
CONTENTS
Foreword xiii
Acknowledgements xiv
HISTORICAL NOTES xvii
Earliest aluminium bronze xvii
First systematic research into copper-aluminium alloys xviii
Addition of other alloying elements xxi
Inventors of the Tilting Process xxii
Leading contributors to the metallurgy ofaluminium bronze xxvii
Growing use ofaluminium bronze xxix
Part 1 Cast and Wrought AiuminiUID Bronzes:
Properties and production processes
1 ALUMINIUM BRONZES AND THEIR ALLOYING
ELEMENTS 3
The aluminium bronzes 3
Properties of aluminium bronzes
Effects of alloying elements 4
Aluminium - Iron - Nickel and Iron - Manganese - Silicon -
Lead- Impurities
2 PHYSICAL PROPERTIES 14
Melting ranges - Density - Thermal properties - Electrical and
magnetic properties - Blastic properties - Non-sparking properties
3 CAST ALUMINIUM BRONZES 24
A Cast alloys and their properties 24
Standard cast alloys 24
High strength alIoys -Medium strength alloys - Low magnetic
alloys
Factors affecting the properties ofcastings 27
Effect of alloy composition - Effect of impurities - Effect of
section thickness - Effect of heat treatment - Effect of operating
temperature
v
vi ALUMINIUMBRONZES
B Casting processes 43
Processes 43
Sand casting - Shell mould casting - Ceramic mould
casting - Diecasting orpermanent mould casting -
Centrifugal casting - Continuous and semi-continuous
casting - Choosing the most appropriate casting process
Applications and markets SO
4 MANUFACTURE AND DESIGN OF ALUMINIUM BRONZE
CASTINGS 53
A Manufacture of castings S3
The making of sound castings 53
Oxide inclusions - Shrinkage defects- Solidification range-
Gasporosity
Prevention ofdefects 56
Avoiding oxide inclusions - Directional solidification - Directional
solidification by astatic method - Avoidinggasporosity -
Blowing - Differential contraction and distortion
Quality control, testing and inspection 66
Importance ofquality control-'Methoding records- Pre-cast
quality control- Quality checks on castings
Design ofpatterns 68
B Design of castings 71
Introduction 71
Designing to avoid shrinkage defects 72
SimpliCity of shapes- Taper- Relationship of thin to thick
sections - Wall junctions and./illet radii- Isolated masses - Web
and ribs- Coredholes - Effect ofmachining allowance
Other design considerations 76
Fluidity and minimum wall thickness - Weight saving - Effects
of thickness on strength - Hot tears - Composite castings
Design ofcastings for processes other than sand casting 79
5 WROUGHT ALUMINIDM BRONZES 81
Wrought processes and products 81
Forging- Extruding - Rolling- Drawing- MiscellaneousProcesses
Wrought alloys: properties and applications 88
Composition andproperties
Single-phase alloys 92
Nature and working characteristics - Mechanical properties -
Corrosion resistance - Impact strength - Fatigue strength and
corrosionfatigue limits - Applications
CoNTENTS vii
Duplex (twin-phase) alloys 95
Nature and working characteristics - Mechanical properties -
Impact strength - Fatigue strength -
Applications and resistance to corrosion
Multi-phase alloys 98
Nature and working characteristics - Mechanical properties
at elevated temperature - Impact strength - Fatigue
strength - Torsion - Creepstrength - Applications - Temper
Factorsaffecting mechanical properties 106
Effects of composition - Effects of wrought process and of size and
shape ofproduct - Effects of hot and cold working
Heattreatment 107
6 HEAT TREATMENT OFALUMINIUM BRONZES 109
Forms of heat treatment 109
Annealing - Normalising - Quenching - Tempering and temper
anneal
Reasons for heat treatment 111
Relieving internal stresses - Increasing ductility - Increasing
hardness and tensile properties - Improving corrosion
resistance - Improving wear properties - Reducing magnetic
permeability
Heat treating different types ofalloys 113
Single-phase alloys - Duplex alloys - CuiAlINilFe type
complex alloys - CulMn/ AlIPelNi type complex alloys
7 WELDING AND FABRICATION (INCLUDING l\AETAILIC
SURFACING) 126
Welding applications 126
Welding characteristics 127
Aluminium-rich oxide film - Thermal conductivity and
expansion - Ductility dip
Choice ofwelding process 131
Tungsten-arc inert gas-shielded (TIG) process - Metal-arc inert
gas-shielded (MIG) process - Other electric arcprocesses - Electron
beam welding - Friction welding - Oxy.-acetylene gas welding
Weldingpractice: general 136
Weld procedure and welder approval- Cleanliness andfreedom
from grease - Selection offiller metal for TIG and MIG welding -
Selection of shielding gas - Current settings, voltJlgeand other
operating data - Fluxes