Casting for the Home Workshop E-Book

Henry Tindell

• Herausgeber: Crowood
• Kategorie: Fachliteratur
• Sprache: Englisch

Veröffentlichungsjahr: 2018

Beschreibung

The techniques of casting are of crucial importance in our day-to-day lives, being used in the manufacture of diverse products ranging from dental implants and hip replacement joints, through bicycle frames and car engine parts, to the most exquisite items of sculpture and jewellery. Nevertheless, the prospect of casting can seem daunting to the home metalworker. Casting for the Home Workshop aims to demystify the craft and make it accessible to all. Topics covered include the history of casting; tools, materials and equipment; techniques; the home foundry and post-casting operations.

Leseprobe

CROWOOD METALWORKING GUIDES

CASTING FOR THE HOME WORKSHOP

HENRY TINDELL AND DAVE COOPER

THE CROWOOD PRESS

First published in 2018 by

The Crowood Press Ltd

Ramsbury, Marlborough

Wiltshire SN8 2HR

www.crowood.com

This e-book first published in 2018

© The Crowood Press 2018

All rights reserved. This e-book is copyright material and must not be copied, reproduced, transferred, distributed, leased, licensed or publicly performed or used in any way except as specifically permitted in writing by the publishers, as allowed under the terms and conditions under which it was purchased or as strictly permitted by applicable copyright law. Any unauthorised distribution or use of thistext may be a direct infringement of the author’s and publisher’s rights, and those responsible may be liable in law accordingly.

British Library Cataloguing-in-Publication Data

A catalogue record for this book is available from the British Library.

ISBN 978 1 78500 354 7

Dedication

To Richard, Laura and Henry;

Alistair and Lucy;

in memory of Susan;

and Jenny

Safety is of the utmost importance in every aspect of metalworking. The practical workshop procedures and the tools and equipment used in metalworking are potentially dangerous. Tools should be used in strict accordance with the manufacturer’s recommended procedures and current health and safety regulations. The author and publisher cannot accept responsibility for any accident or injury caused by following the advice given in this book.

Contents

Acknowledgements

Preface

Introduction – The History of Casting

1Fundamentals of the Casting Process

2Setting up a Home Workshop Foundry

3Methods of Casting

4Design for Castings

5Materials for Castings

6Post-Casting Processes

7Case Studies, Processes and Projects

Appendix I Safety Aspects

Appendix II Useful Data

Appendix III Glossary

Bibliography

Suppliers

Postscript

Index

Acknowledgements

Firstly, thanks are due to all at the Crowood Press, including our editors without whom this book would never have begun and certainly never been completed. And Joanne Hulse for again turning a scrawled and scrambled script into an orderly typed submission. We acknowledge the many friends and colleagues not mentioned who have freely contributed their knowledge and thoughts on casting topics. Peter Whitehead of the Whitehead Foundry for his casting wisdom and freedom to explore and photograph his works; Chas King and colleagues at Nortest for their expertise and assistance with NDT over the years; Ross Nolan and colleagues at Exova for help and guidance in a modern Materials Test House; Stephen Foster of Engineering and Foundry Supplies, Colne, for valuable advice and help with casting supplies; Ben Hale of the Cast Metals Association for the cast iron micrographs, supplied from the Thomas Dudley Foundry, Dudley, W.Midlands; Les Hall, master patternmaker for recalling a fascinating life in industry and home workshop; The Whitechapel Bell Foundry for insights into a remarkable casting history; Matthew McGillicuddy of PI Castings, Altringham, for illustations of their precision investment castings; Manchester University, Joule and John Rylands libraries for their extensive resources and help; Peter Gilmartin of Gilwoods Fabricators for for the encouragement (DC); and colleagues and friends at Hick Hargreaves and Edwards, Bolton (HAT).

Finally, we are indebted to Professor John Campbell for his invaluable advice, general comments, and first-hand knowledge of the Liberty Bell examination – as well as his fundamental theory of bifilms – proving, in the twenty-first century, as important for sound castings as the discovery, in the twentieth century, of dislocations was for plasticity in metals.

Despite this substantial support, the resulting contents, and possibly not uncontroversial opinions, remain those of the authors. In the continually developing technology of casting, it is merely to be hoped that this is satisfactory at the present, until superseded by the inevitable developments of the future – or to mis-quote the old aphorism, ‘where there’s life, there’s casting. . . .’

Preface

‘What have the Romans ever done for us. . . ?’ was the plaintive cry from the famous Monty Python film. So amusing, as the Romans’ contribution to a wild Britain were legion – no roads were built for 1,000 years after their departure in 400AD – perhaps we could adapt this rather adroit approach to posit, ‘What have Castings ever done for us. . . ?’

For castings have become so familiar in our modern world as to have been rendered invisible. This is particularly unfortunate as the once omnipresent local ferrous (iron), or non-ferrous, foundry is rapidly becoming an endangered species in the developed West where, apparently, the virtual world has displaced the need for actually manufacturing things, other than the primary purpose of simply juggling binary digits off into the ether. . . .

But are metal castings really so ubiquitous? Perhaps we need to remind ourselves of their existence and a little of how our modern world is dependent upon, and has been fashioned by, ‘Castings’. A few random examples:

◆ Tiny dental implants, from investment castings

◆ Exotic jewellery, in precious metals

◆ Replacement hip joints, in cobalt-chromium alloys

◆ High-value camera bodies, in magnesium alloys

◆ Lightweight bicycle frame parts, in aluminium alloys

◆ Car engine parts; crankshafts in ductile cast iron, carburettor bodies in zinc alloy, high pressure die castings

◆ Front-end jet engine turbine blades, single crystal casts of nickel base alloys

◆ Machine tools, from the watchmaker’s bench-top lathe, to massive vertical boring machines, in grey cast iron – providing machines to make machines, to make machines, to make parts.…

The list is nigh on inexhaustibly long, indeed a more sensible approach could be to compile a list of areas of human activity not dependent upon castings.

Even from this brief list, it is apparent that castings are employed where it is vital to use a precisely chosen alloy, often viable only through the casting route. Also from this list is the hint of the many and varied processes that have been developed to produce castings, from the basic sand casting – described in some detail for the aspirant amateur foundry worker in the home workshop – to the near net shape techniques such as investment (or lost-wax) casting.

The introduction explores the historical perspective, for no other manufacturing process can compete with an ancestry of some 5,000 years – with the oldest surviving manufacturing business in the UK, the Whitechapel Bell Foundry, established in 1570. And yet we are only just, in the last few decades, coming to terms with a sound theoretical basis for understanding some fundamentals of the casting process, from bifilm theory, see Bibliography, Campbell, (2011).

Chapter 1 ‘Fundamentals of the Casting Process’ sets this within the well-established theoretical background of the complete process, from melting, pouring and casting; with a closer look at the liquid flows through the gating system, into the solidification and cooling of the final cast product. How and why defects are inherent in virtually all castings is found in Chapter 1, and their elimination pursued. Practical aspects of subsequent defect correction and testing is included in Chapter 6, ‘Post-Casting Process’.

Chapter 3, ‘Methods of Casting’, Chapter 4, ‘Design of Castings’, and Chapter 5, ‘Materials for Casting’, provide a sense of how castings are manufactured. This is preceded by Chapter 2, ‘Setting Up a Home Workshop Foundry’, which considers its practicalities, limitations, and prospects – for this is an undertaking not to be underestimated.

This rather mechanistic outline should not obscure the truly dramatic nature of the whole casting process. For it is great creative work, involving huge elemental forces as the original molten charge is raised to super-heated temperature – white hot – ready for pouring from the crucible. This liquid can have fluidity akin to that of water (for cast irons) and is tipped into the mould in-gate, sending a violent stream of onrushing fluid into the casting mould cavity. Within seconds the mould is filled, flowing up the risers as solidification erupts through the casting from mould wall inwards, as vast numbers of tiny tree-like, dendritic, crystals form and crash around in the microscopic trauma of alloy formation. The solidified metal’s cooling phase can be hardly less dramatic as the temperature races down and different structures are wrought, until finally set at the near-ambient temperature of microstructural completion.

Photo 1 Whitechapel Bell Foundry, front entrance, Whitechapel Road, London E1.

Photo 2 Bell Foundry, goods inwards, Plumbers Row, London E1.

And this is just the metallurgical aspect of the business, albeit responsible for the strength and toughness of the casting – to the naked eye only the gross features of this process are apparent, as surface finish and, hopefully, lack of surface faults such as laps, tears, cavities, porosity, shrinkage and distortion.

When some grasp of these complexities of metal casting is attained, it should hardly be surprising that it is such a challenging business – but what could be more rewarding, notwithstanding the myriad obstacles, than the creation of a fine and everlasting casting?

HAT and DC Cheadle Hulme November 2016

Postscript: On 2 December 2016, the Whitechapel Bell Foundry announced – as reported by the Economist magazine in its ‘Obituary’ published on Christmas Eve 2016 – that it too would be joining the ranks of the world’s great foundries in the sky.

Introduction – The History of Casting

Detail of the earliest cast objects is, at best, rather imprecise but evidence of the pre-historical world is growing steadily. What is certain, however, is that casting was preceded by the Wertime Pyrotechnology period, where around 10,000 years ago fire was being exploited by primitive humans for heat-treating stone, one of the earliest materials utilized by man. Expertise with handling fire was useful in the manipulations of stone such as by carving; then the development of plaster from burning of lime; then to the firing of clay for the development of pottery.

The earliest metal objects were produced from native copper, as found in copper ore deposits, by wrought methods such as hammering with stone tools. This was sufficient to produce simple cutting tools and decorative artefacts made by beating to profile, possibly (but as yet unproven) with the use of fire-annealing to alleviate the effects of work hardening. This activity is associated with the Neolithic period, when ceramic work (production of pottery) become widespread, pre-dating the origins of metal casting (seeTable 1).

The Chalcolithic Period is the name given to the time when metals were developed in a more technological manner, essentially the start of the casting era, with its continuous thread to the present day. At 5000–3000BC, it immediately proceeds the Bronze Age (3000–1500BC). These are not precise periods of history but merely serve as a shorthand guide, with extensive overlap and mismatch across the world of metallurgical progress.

Although claims have been made for various early cast objects, such as the fine bronze anklets from lost-wax castings of 4500BC from Southeast Asia, it seems that the earliest sure evidence relates to the remarkable finds of the Nahal Mishmar Treasure, from a cave in the Jordanian Desert, discovered in 1961. More than 400 objects, including castings of copper and bronze made by the lost-wax process, can now be seen in the Metropolitan Museum of Art, New York. Fortuitously, these items were found wrapped in straw mat, hidden inside the cave, enabling Carbon-14 dating to show that they originate from 3500BC, or earlier. This region of the Levant was known to have a highly developed agriculture with the extensive use of tools by farmers in Israel and Jordan and an advanced civilization, producing fine terracotta work, sculptures and fine wall paintings. It has been suggested that this treasure was hurriedly hidden having been removed from a nearby shrine at Ein Gedi, near the Dead Sea in Israel.

The Nahal Mishmar findings support the theories that casting began in the cradle of civilization of the Middle East, around modern day Iraq, Iran and the Eastern Mediterranean, about 6,000 years ago. The first castings were produced from the melting of copper ore, followed by alloying of copper (Cu) with arsenic (As) and tin (Sn). Cu – 4 to 12% As items were identified at Nahal Mishmar and contemporary finds). Early workers surely found that Cu was not an easy material to cast, with sluggish fluidity, and made for tools too soft to hold a decent cutting edge. The discovery of alloying with As and Sn provided a greatly improved result, with improved castability, and a useful improvement in hardness permitting a cutting tool edge of practical use. This led to the onset of the Bronze Age, largely in recognition of the spreading of this technology across the known world.

BRONZE AGE

This enabled the use of fires of sufficient heat to melt these bronzes, greatly assisted by the alloying with As and Sn, which reduces the melting temperature. Original moulds were undoubtedly made from stone, with recesses carved into the stone faces into which the liquid metal was poured – skills in handling stone having been developed over the many millennia preceding the first casting of metals. Various stone ‘patterns’ have been found, some with a simple recess for use as open moulds, accepting the molten metal, to produce a free surface solidification method of casting. These stones could also be fashioned on several sides, providing a multi-tool approach, to make the most efficient use of stones such as steatite – convenient for carrying perhaps? Where a stone is used to form the upper half of a mould, as we would expect to produce a closed mould, these are known to archeologists as bivalves.

◆

5000-3000Bc

Copper-based experimental work during Chalcolithic Period; Middle East.

◆

3500BC

Early documented finds of castings; Nahal Mishmar treasures, Jordan, Middle East.

◆

3500-2500BC

Lost-wax casting origins; Middle East.

◆

3000-1500BC

Bronze Age, origins; Middle East.

◆

1500BC

Iron Age (wrought iron); Middle East.

◆

645BC

Sand moulding; China, Far East.

◆

600BC

Cast iron first produced; China.

◆

225BC

Colossus of Rhodes – giant bronze casting destroyed; Greece.

◆

1225AD

Great Buddha of Kamakura cast, 120 tons; Japan.

◆

1400AD

Great Bell of Beijing cast, 46 tons, (still sounds!); China.

◆

1500

Sand moulding origins in the West; France.

◆

1570

Whitechapel Bell Foundry established; London, England.

◆

1709

Coke used to fuel furnaces for cast iron; Coalbrookdale, England.

◆

1735

Great Bell of Kremlin, 193 tons(!), but cracked; Moscow, Russia.

◆

1752

Liberty Bell, Philadelphia, USA; cast in London; recast 2002, London.

◆

1779

Cast iron first structure, Ironbridge; Shropshire, England.

◆

1863

Metallography invented, Henry Sorby; Sheffield, England.

◆

1884

Aluminium produced by electrolytic refining.

◆

1895

Eros, first aluminium public statue; London.

◆

1913

Stainless steel invented; Germany and England.

◆

1924

18/8 stainless steel by W.H. Hatfield; Sheffield.

◆

1948

Ductile cast iron (SG iron) developed; USA.

◆

1965

Scanning electron microscope (SEM) invented; Cambridge, England.

◆

1970-present

Many new casting processes and theories developed; worldwide.

Table 1 A brief history of metal casting.

The early fires, providing rudimentary furnaces, used charcoal as fuel and indeed (in the West) this persisted until the industrial era before more efficient fuels of coal and coke were exploited. Similarly, with the use of forced draught, originally simply pumped by human power, blowing through pipes. This was still being observed in the Medieval period by the Conquistadors during their pillaging of Latin American gold and silver casting treasures.

Besides direct casting into stone moulds, it is now considered that the lost-wax process was also well developed in the Bronze Age. While there is clear evidence of early casting in the Middle East, it was thought that suppliers of tin, for instance, originated from Afghanistan and Cornwall, England. However, it now seems that stream deposits of these elements (Cu and Sn) were available in Mesopotamia (Iraq), Egypt and Anatolia (Turkey). Items such as the small silver-bronze statue of a stag, from the Hittite civilization of 2000at Alaca Höyük (Ankara, Turkey), were clearly made by the lost-wax process, although direct evidence of moulds and equipment have generally eluded researchers. However, in 1972 at Gussage, Dorset, England, an extensive excavation uncovered invaluable evidence of an Iron Age foundry, from around 1000BC. This provided several thousand fragments of ceramic moulds from the lost-wax process, with many bronze products for horses and chariots, such as bridle bits, showing an advanced industry of lost-wax casting was well established in England at least 3,000 years ago.

The earliest bronzes were copper-arsenic alloys, containing Cu–4% As; but up to 12 per cent As was also used, throughout the Middle East and as far west as Great Britain. This alloy can often be recognized as arsenical copper by a silver coloured surface, resulting from the inverse segregation of the low melting point, As-rich, phase appearing on the cast surface. This can confuse these with silver or silver-plated work. The phenomenon is similar to that of tin sweat, as observed in the tin bronzes (Cu-Sn). The subsequent discovery of the tin bronze alloys may have originated from ores containing both elements, Cu and Sn, in the natural state.

It should be remembered that the following ages, such as Iron Age, added another layer of technology, and bronze has continued to the present day as a familiar material of choice for casting. This is not only for the many and varied statuary and decorative items, but also critical engineering products. Often chosen for the excellent corrosion resistance in marine applications, it is almost exclusively used for the wonderful bells still being produced by specialized foundries such as the Whitechapel Bell Foundry, able to trace a continuous casting history from 1420 to the present day (see Chapter 4). We now move to the geographical spread of the early casting practice.

FAR EAST

The origins of casting in China, India and Southeast Asia are thought to be around 2000BC, some 1,000 years after the Middle East. It was once assumed that knowledge had slowly diffused towards China, but a study of the dates when Chinese foundry sites began indicates a progress westwards rather than the expected eastwards development profile. It is perfectly possible that casting developed independently in China, where it was certainly the only significant method for producing metal artefacts before ~500BC.

As the detail work of early Chinese bronze artefacts was so extensive it was originally assumed lost-wax was the predominant technique. However, more recent finds at Anyang provided unused ceramic piece-moulds. This method could deploy a number of piece-moulds to cast separate parts of a large statue often in delicately thin hollow sections. These were eventually joined together either by direct casting-on of the adjacent piece, or by a sophisticated stitching method, somewhat analogous to that of the present day for the joining of cracked cast iron vehicle parts, such as a cylinder block considered a problematical fusion weld repair. This enabled some impressively gigantic religious effigies to be constructed, in the manner of later works such as the Great Buddha of Kamakura, Japan. Built in 1252, the Great Buddha comprised some 120 tons of bronze, the alloy being Cu – 9% Sn, 20% Pb; which still survives, despite its location in an earthquake zone.

The early alloys used in China tended to be leaded tin-bronzes (Cu – Pb, Sn), used for improving castability through their greater fluidity, enabling thin sections to be safely reproduced. While this could be considered a problem with leakage at moulding joints producing flash on the mould line it could be used to advantage, either requiring only a simple operation to remove the flash, or it could be incorporated into the design, for instance along folds in the subject’s clothes. Interestingly, a reproduction bronze sword was recently cast by an Oxford academic, using a bifold mould to produce an item with joint-line flash that could be readily removed with minimal post-cast work.

When an early bronze casting site, c. 2000BC Thailand, was excavated it revealed evidence of the lost-wax process, from the unused ceramic bivalve mould found buried with the foundry man, having one half placed in each hand.

SOUTH AMERICA

While there appears to have been no casting activities in North America by the indigenous population before the eighteenth century colonization from Europe, and the subsequent explosive growth of casting to support the Industrial Age, the same is not true in Latin America.

Copper was discovered, exploited and exhausted long before the European explorers set foot there in the Middle Ages. By that time their metalworking expertise had reached an impressively high level, and castings had been produced not only in the copper-base alloys (bronzes) but also in the precious metals of gold (Au) and Silver (Ag). Tumbaga being an alloy commonly used containing, in varying amounts, gold, silver and copper. Following Tumbaga casting, the item was ‘pickled’ in an acid that attacked the copper and silver, leaving a gold-rich layer on the surface – effectively a gold plating. Termed mis en couleur (depletion gilding), this produced a gold appearance that drove the European explorers to distraction, and the wholesale plunder of these priceless artefacts.

AFRICA

Figure 1 The Grayer-Anderson Cat, currently residing in the British Museum. (Author’s sketch)

While the early casting history of Egypt can be more closely linked to the developments as noted above in the Middle East, present knowledge of West African casting begins from the Middle Ages onwards.

Perhaps it is the predominance of blacksmithing and small-scale forging that has obscured the origins of castings from the western and central part of the continent, but we begin with the most notable Benin Bronzes, made by the lost-wax process. These are indeed very fine examples of the intricate and highly controlled casting art. Several fabulous bronze statues, such as the head of Queen Iyoba from the Medieval period (sixteenth century), were made for the Benin Court in Nigeria. Discovered in the Benin expedition of 1897, a particularly fine example can now be seen in the British Museum in London.

In Nigeria and Ghana, zinc was alloyed with copper, producing Cu-Zn (brass) and, once again, the lost-wax process was used. Ghana is known for its gold weights, in fact these are normally brass with sufficient zinc to produce a golden colouration. Also developed was a clever arrangement of crucible and mould contained within a unified structure, based on the lost-wax process. When charged with Cu and Zn, and moulding de-waxed, it could be heated to liquid and inverted to fill the mould, while isolated from the atmosphere. This forms the approach preferred in present times in the pursuit of the defect-free casting.

Zinc refineries recently discovered that operated in the seventeenth century at Zawar, India, are thought to have supplied the long-range trade routes to West Africa, as one source of raw material.

Figure 2 Bronze investment casting of Queen Iyoba, Benin tribe, Nigeria, fifteenth century, presently in the British Museum. (Author’s sketch)

Figure 3 West African warrior casting, c 1800now in the British Museum. (Author’s sketch)

CAST IRON

Remarkably, for such an important material, cast iron was not produced in the western world until some 1,000 years after its introduction in China, around 600BC. The early Chinese cast iron is known to be high in phosphorus (P) and sulphur (S), the latter arising from melting in coal-fired furnaces. This composition results in exceptional fluidity with a melting point near of that of bronze and, like the Far Eastern bronzes, it is noted for the thin sections and fine replication of castings – contemporary cast iron being used not only for common artefacts but also decorative and statuary works.

While cast iron may well have appeared in Europe in the Medieval period, it was generally regarded as a source for producing wrought iron, the precursor to our ubiquitous modern-day ‘mild steel’. This was obtained by a long process of refining the cast iron to remove almost all the free carbon responsible for its lack of ductility and toughness. These properties were, even in early times, recognized as important in forged product, free from the sudden failure characteristic of brittle materials, aka cast iron.

Rather ironically, until the nineteenth century invention of the processing of iron ore in the Bessemer Converter, the route to manufacturing steel involved the aforementioned removal of carbon from cast iron to make the ultra-low and ductile wrought iron – only to add back small controlled amounts of carbon to make steel. Such was the means to produce the first steels in Europe – the Huntsman cast steels of 1740. Typically, cast irons contain around 2–4% C and steels 0.1–1% C; carbon being an extremely powerful element when combined with iron, influencing steel behaviour. This ranges from the lower strength, highly ductile, mild steel (< 0.2% C), to the high strength and low ductility, heat treatable, high C (~1% C) steels.

Photo 3 Ironbridge, Coalbrookdale, Shropshire, UK.

It was not until Abraham Derby began producing cast iron in volume during the eighteenth century at Coalbrookdale in Shropshire that the use of cast iron in the western world took off. Testament to this pioneering work is the famous Iron Bridge, the western world’s first major cast iron structure, built in 1779 to span the River Severn gorge, and still standing today. Darby was fortunate in this iron processing, having used coke as furnace fuel, the local iron ores contained sufficient manganese (Mn) to combine with the excess sulphur (S) from the coking coal. This produces benign MnS inclusions, thereby scavenging the S and allowing the great productivity gains of coke as a fuel, which soon replaced the use of charcoal as it was less efficient and of dwindling supply.

The excellent ‘castability’ of cast iron enables its use in a very wide range of applications, and it proved its worth with the rapidly growing need for heavy machinery to power the Industrial Revolution. The explosive growth of industry was well under way in Great Britain as the nineteenth century dawned and water power was steadily displaced by steam. However, the inherent lack of ductility of cast iron, making it unsuitable to accommodate tensile stresses for fear of sudden failure on overload, was recognized by the more knowledgeable Victorian engineers, deploying it only in compression loading for safety. It was not understood until the advent of modern physical metallurgy, in the late nineteenth century, that the problem with these cast irons lay in the manner in which the carbon was formed as microstructural long graphite flakes. These render the structure brittle, as will be explained inChapter 5. Means were later developed to overcome this problem, for designs involving tensile loading, by converting the graphite into non-damaging microstructural ’rosettes’ as ’Spheroidal Graphite’, or Ductile Cast Irons – arriving in the late 1940s for some extremely demanding applications such as motor vehicle crankshafts.

Photo 4 Ironbridge, detail of cast iron bridge, built 1779.

Following Ironbridge – notably successful as it was designed to maximize the use of cast iron members in compression, thus avoiding significant tension loading – architects found many uses for cast iron as a structural material, including the United States Capitol Building dome, painted to resemble masonry. Unlike bronze, of course, cast iron usually requires a protective coating such as painting to avoid corrosion; or copper-plating, in the case of the great staircase casting of the Chicago Stock Exchange. The effect of corrosion in cast iron can be deliberately harnessed by allowing a light rusting, then applying a hydrogen reduction process to produce a surface oxide of Fe3 O4, a durable, velvety finish suitable for attractive interior works.

PRODUCTS

The production of smaller items, from hand tools to domestic artefacts and decorative work, has been practised, as we have seen, since the earliest days of casting. Larger items present particular problems of scale, solved in the manufacture of massive statuary by making them as an assembly of parts, as described earlier. However, particular products, such as bells and cannons are required to be produced as a single item, and naturally even larger examples were sought for more ambitious designs.

BELLS

Large bells have, since antiquity to the present day, presented problems and opportunities for foundries. Even today, long established processes are in use in the developed world to satisfy the niche demand for high-class bells – still remarkably prominent in our consciousness – such as the sound of Big Ben ringing out from the Palace of Westminster, London (cast by Whitechapel Bell Foundry, see Chapter 4, ‘In Memorium – The Whitechapel Bell Foundry’).

The sheer size of these large bells is awe-inspiring, such as the Kremlin’s Great Bell, cast in 1735 at almost 200 tons, although it is cracked and non-functioning. In contrast, the Great Bell of Beijing, at more than 46 tonnes, of a Cu – 15% S, 1% Pb, bronze, is said to still be capable of generating a deafening 120dB, audible at 12 miles (20km). Cast around 1440AD, in the Ming Dynasty, it contrasts in shape, material and composition with Western European bells, typically Cu – 25% Sn. The Liberty Bell of Philadelphia, USA, and its recently made replica (both from Whitechapel), is a Cu – 23% Sn alloy and at 5 tons relatively small compared to the above bells. However, after pouring at 1,100°C it still required a full week to cool to room temperature! Although bronze is by far the most popular material, white cast iron bells have been cast in China and Russia; and cast steel bells became a product of Sheffield, England, after Huntsman’s work in the eighteenth century.

GUNS

The military use of large cannons meant that considerable effort was deployed in the development of castings to satisfy the desire for ever-grander weapons. Even the great Leonardo da Vinci had to abandon his grandiose project to produce a massive equestrian statue when its colossal consumption of bronze was redirected to the Florentines’ armoury projects.

Early guns were made from multiple parts, and used a variety of materials, or were cast with large cores that proved difficult to extract before they damaged the casting during cooling. Vannoccio Biringuccio was a pre-eminent metallurgist of the early Renaissance Period. His seminal work De la Pirotechnica, published in Venice posthumously in 1540, provided extensive information on the casting of guns and bells. Considered to mark the beginning of materials science and its technical literature, it pre-dated the next important metallurgical study, of De Re Metallica by Georgius Agricola, printed in 1556. These works contained information on the copper-base alloys of bronze and brass, mining, refining and the explosives used in the operation of the guns. The latter work remained so influential in Central Europe that it was eventually translated into English in London in 1912 by a prominent mining engineer, one Herbert Hoover, later to become the president of the USA.

In 1715, the Swiss engineer Johan Maritz developed a sufficiently powerful boring machine to enable gun barrels to be cast solid, without a core. Machining the bore conveniently removed the central planes of weakness and segregation, significantly improving the end result. Further work by the likes of Krupp in Germany and Armstrong in England continued the development of gun barrel machining concurrent with an improvement in casting technology that eventually discarded the bronzes in favour of the higher strength steels of the early twentieth century. Of historical interest are the great national armouries, such as the Royal Brass Foundry, part of the Woolwich Arsenal of south London, which was closed in the 1990s, but the detailed records of gun manufacture from the previous two centuries are still available in its archive.

Photo 5 Bronze statue – Nelson Mandela, College Green, Westminster, London.

SCULPTURES

Statues have been produced since the earliest times by casting, preceded by plaster and other non-cast materials, although many of the large cast metal statues have been destroyed, often through the need to reuse valuable raw material. Surviving examples are now more commonly found buried sub-sea or excavated from burial sites, including Egyptian tombs. Even early statuary was often constructed not by castings but used copper sheet, formed and supported by an internal framework. Ancient Egyptian statues, for instance, can be difficult to categorize as wrought or cast, due to the effects of centuries of weathering. Some Greek cast bronzes, such as the Charioteer of Delphi, have survived and can be found in museums around the world often, it seems, on the opposite side of the world to their place of origin.

Photo 6 Bronze statue – Winston Churchill, Westminster.

While bronze has been, and continues to be, the most popular material for cast statues, lead and silver have also been used extensively, albeit for smaller work. It is unlikely that gold has been used extensively, for obvious reasons. Although many examples appear to be gold, it is usually as a result of surface treatment, of which there are many varieties. Studies of the famous Roman bronze horses in Venice demonstrated that a bronze with a lower tin content was preferred for successful gilding by amalgam. A coppery colour of the, low Sn, base metal bronze is therefore an indication that a gilding process has been undertaken.

There have been some enormous statues produced since ancient times, perhaps most notably the Colossus of Rhodes that was more than 30m (100ft) tall, destroyed by earthquake in 224BC, which lay in pieces until being sold for scrap in the seventh centuryAD!

Although famous landmarks, such as the Statue of Liberty in New York, has the appearance of a copper casting, it is actually an enormous and complex structure clothed in wrought copper sheet. By contrast, the much smaller, but well-loved statue of Eros, residing amidst the swirling traffic in London’s Piccadilly Circus, was the world’s first aluminium public statue, commissioned in 1893. Its most recent significant repair, in 1986, produced valuable insights into the method and materials of construction, as some over-exuberant swinging on its flying leg had resulted in fracture. The base material is pure aluminium, or as near as this was possible in the 1890s. Earlier repair work had infilled the hollow body and limbs with an Al-12% Si alloy, in an effort to improve a not particularly strong structure. However, it was found that this later additional core had barely joined to the original material, thereby adding little to the mechanical strength. Numerous problems were associated with the TIG/GTAW (seeChapter 6) repair, corroded internal supports and solidification cracking of the weld, as is familiar in fullscale casting. Careful rebuilding and NDT (seeChapter 6 ‘Non-Destructive Testing’) gradually restored the piece, and provided the mechanical strength necessary to withstand again the rigors of environmental pollution and unprotected display in central London.

Photo 7 Aluminium statue – Eros, Piccadilly Circus, London.

1Fundamentals of the Casting Process

LATENT HEAT

The ‘states of matter’ are controlled by temperature and pressure, thus at room temperature (RT) and one atmosphere pressure this is known as STP – standard temperature and pressure. Almost all common metals are solids. Only a couple of the metals are liquid at RT, but this includes mercury (Hg), widely deployed in sealed glass tubes as thermometers.

Figure 4 Metal states: solid to liquid.

Adding sufficient heat to a solid body causes it to change state from solid to liquid, but requires extra heat energy at the boiling point, known as latent heat to effect this change, without temperature rise, as in Figure 4. For common solid materials, such as metals and ice, this is a reversible process as these changes of state can proceed either way, by adding or removing heat energy.

A similar process is involved to effect change from liquid to gas, but the latent heats are considerable larger. Similarly, from gas to plasma, except that this change requires extremely large quantities of heat, or electrical excitation, to reach the plasma through an ionization process, as described later.

STATES OF MATTER

The four fundamental states of matter are familiar in everyday life, although not necessarily always recognized as such.

Solids

Figure 5 The basic states of matter and phase transformations.

Figure 6 X-ray diffraction patterns – crystal and liquid.

Figure 5 shows these states as solid, liquid, gas and plasma. In fact, these all play their part, directly or indirectly, in the business of casting. Put simply, the solid state is the casting’s end product, being formed fundamentally as a structure comprising (normally) an extremely large number of minute grains formed from regular, crystallographic, arrays of the individual building blocks of molecules, as in Figure 6. These grains of highly ordered crystallographic structures possess what is known as long range order, in that each grain is (almost) perfectly composed in a regular array. The imperfections can be ignored at present, but are briefly discussed later to explain why the theoretical strength of most metals is not achieved. Nevertheless, it is self-evident that metals can be extremely strong, thereby providing the practical materials utilized in the products of the modern world, so familiar that we take them for granted. Importantly, the difference in strength and properties can, within each alloy system, be very considerably changed during the casting process.

For non-engineering castings, such as statues or many household artefacts, the mechanical properties of the casting are usually not of particular concern, and success is measured by producing a cosmetically acceptable solid structure, if sufficiently free from surface defects. However, for items that are required to withstand the rigours of engineering applications, such as those found throughout our motor vehicles, the process of casting requires considerable knowledge and understanding to provide the necessary mechanical properties. And these properties are, firstly, controlled by the casting process as the molten metal flows from furnace to mould and eventually solidifies into the final shape. This is where an understanding of the fundamentals, from fluid flow to nucleation in the solidification process, leading to a suitable microstructure, are appropriate. This sequence also determines the casting defects, entrapped or escaping from the casting, which will also determine its suitability. These aspects are reviewed later and form the difference between these modern-day castings and their predecessors in the several millennia of casting manufacture until the dawn of the ‘scientific era’. For our purposes, this was around the late nineteenth century, as the physical sciences steadily eroded the subject’s status as a ‘black art’, a consequence of a lack of understanding that included the behaviour of the four states of matter. This enabled castings to play their part in the enormous technological advances of the twentieth century. It becomes clear that the apparently simple process of casting benefits from this fundamental understanding of liquids and solidification to achieve the full potential of strong (metallic) materials. This is explored further in the next section on the control of structure.

Liquids

In contrast to solids, where in metals there exists a long-range crystallographic structure providing a settled overall shape, in a liquid this structure is no longer regularly defined. Figure 6 shows a molecular arrangement of loosely held atoms free to swim around and adopt the shape of a vessel, or disperse across a surface in the absence of containment. Thus, the familiar filling of a jug with water, that could have been produced by melting the solid (e.g. blocks of ice) with sufficient heat to reach the melting point and then overcome the latent heat of fusion.

Liquid

Density (lb/in3)

Kinematic Viscosity (in2/sec)

Water

36.1

1.6

Aluminium

86.6

2.0

Cast Iron

220.0

1.6

Steel

254.0

1.4

Magnesium

57.8

1.3

Copper

288.0

0.6

Table 2 Kinmatic velocities for molten metals.

The characteristics of liquids, such as water, are relevant to the foundry worker as some molten metals have a similar fluidity to water, as seen later. In fact, cast iron has a viscosity index identical to that of water, Table 2, enabling the use of transparent models of moulds and gating systems, to observe water flow in the analysis of casting processes.

Figure 7 Surface tension: (a) the impact of a water drop, and (b) in a container.

Liquids can be regarded as incompressible fluids, comprising mobile atoms held together by intermolecular bonds that allow filling of a vessel – but retains, under gravity, a distinct free surface with an associated surface energy. Surface tension (see Figure 7) is a characteristic of the liquid, as seen from the meniscus, surface profile, made with the wall of the container. Water, for example, exhibits a distinctively different profile to mercury.

Photo 8 Matterhorn, Switzerland; showing fracture, crevasses and slow movement of glaciers down the mountain.

Photo 9 High altitude glacial movement, Tête Blanche, Swiss Alps.

Liquids have a ready facility to flow, an important feature shared with gases, used in pouring or otherwise transferring molten metal into the casting mould. The manner of this flow is understood in the study of fluid dynamics. Whilst this is a deep field of knowledge, a general grasp of its basic features enables the foundry worker to produce gating systems suitable for advanced projects. Note that whilst flow is a readily observable feature of liquids (and gases), it can also occur in solids under sufficient pressure, as seen from the effects of creep (Photos 8 and 9). In metals, such as lead piping at ambient temperatures, it can take many years; or observed as the flow of ice in the slow movement of glaciers as they progress at the stately rate of metres per year down the high mountain ranges.

Figure 8 Turbulent flow in pipes.

Figure 9 The effect of increased flow rate on a liquid.

Figure 10 Laminar flow in pipe velocity gradient.

Laminar conditions, as Figure 9, are familiar when observing water flowing from a tap, initially as a low rate proceeding in an orderly, smooth flow. As the flow rate increases this pattern changes to irregular, essentially chaotic, turbulent conditions. The Reynolds number also details the onset of boundary layer failure, as noted above. The flow along a passage, normally either a pipe or similar for modelling purposes, has a velocity profile (seeFigure 10) that is important in casting as the sides of the mould provide a resistance to flow, with the fluid velocity increasing to maximum at the centre. The curves in the runner system and mould also strongly affect flow into the mould, and are dealt with in detail in gating and mould design (seeChapter 4, ‘Design for Castings’).

Figure 11 Fluidity test for casting metal.

The flow characteristics of each metal alloy for casting is an important factor determining successful filling of the mould, and a practical test, shown in Figure 11, provides a measure of how the liquid metal will, in practice, flow down a standardized spiral tube.

While cast iron has a fluidity equal to water, and largely for this reason has been regarded as ‘God’s gift to the foundry man’ (Campbell, 2011), the molten casting alloys often encountered, range from this ideal to far less accommodating slurries. Clearly, this has an important impact on how gating systems and mouldings are designed. Similarly, it would be preferred to have laminar flow and very low Reynolds numbers, but this can mean that the flow approaches stagnant, slow rate conditions. These are just one of the foundry worker’s constant concerns – producing misruns.

Figure 12 Bifilm theory – rupture and folding leading to trapped oxide film defect site. (Based on Campbell, 2011)

Furthermore, greater fluidity, achieved by increasing the temperature at pouring, is not generally metallurgically satisfactory, impacting on the microstructure formed from solidification (see this chapter, ‘Control of Structure’). The process of transferring the liquid metal into the mould has long been regarded as important, but it is only within the past couple of decades that the fundamental source of significant defects, of vital importance in critical engineering castings, has been established in the theory of bifilms as propounded by leading casting experts (Campbell, 2011). Simply put, this involves the entrapment of the extremely thin surface films, ever present on the casting fluid. This is familiar in conventional processes open to the atmosphere, bifilms finding their way into the bulk of the melt. They can form around liquid laps during turbulent flows, or around inclusions/foreign matter, as Figure 12. Once trapped in the fluid flow, then into the mould, these minute films can provide the source of familiar defects, such as the microporosity, that has previously been attributed to other mechanisms.

Figure 13 The Cosworth process casting unit (schematic).

These bifilm defects are now considered responsible for many of the minute internal flaws understood to occur even in apparently high-integrity ingots that subsequently are formed into wrought product, such as steel plates or forgings. The fact that these flaws are often below the threshold for detection by commercial NDT methods has led to the common assumption that such small defects do not exist (see Chapter 6). Methods to overcome such problems have been developed in recent years, such as the Cosworth Process (see Chapter 3), where the casting fluid is pumped from below the free surface up into the gating system and mould, with special precautions, to promote laminar flow and produce a significantly improved final casting microstructure, see Figure 13.

Gases