Cast and Wrought Aluminium Bronzes - Properties, Processes and Structure PDF
Preview Cast and Wrought Aluminium Bronzes - Properties, Processes and Structure
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 1Carlton House Terrace London SW1YSDB,UK rOM Communicaions Ltd isawholly-owned subsidiary of 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 This paperback edition first published in 2008 by Maney Publishing Suite Ie,Joseph's Well Hanover Walk Leeds LS3 lAB, UK Allrights reserved. No part of this publication may bereproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying or otherwise, without the written consent of the copyright holder. Requests for such permission should be addressed to Maney Publishing, [email protected]. uk Statements in this book reflect those of the authors and not those of the Institute or publisher Typeset in the UKby Dorwyn Ltd, Rowlands Castle, Hants Printed and bound in the UKby The Charlesworth Group, Wakefield 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
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