Über die Autorin bzw. den Autor

Dr Hasse Fredriksson is professor in Casting of Metals at the Royal Institute of Technology in Stockholm, Sweden. He is the leader of a very active research group since more than three decades. Dr Fredriksson is internationally known. He is author or co-author to more than 200 scientific papers. He has organized many Summer schools and international conferences in his field, the last one in June 2005.

Dr Ulla Åkerlind is a physicist (research field molecular physics) with long experience of undergraduate teaching at the Department of Physics at the University of Stockholm. She has co-operated in a set of text books in basic physics for the Swedish 'gymnasium' and produced learning aids for the undergraduate university level in physics.

Von der hinteren Coverseite

This text covers most aspects of casting. It deals with the principles behind modern casting with applications incorporated for both ferrous and non-ferrous metals. The text consists of 11 chapters:

• Chapters 1-2 presents a short introductory survey methods and equipment.
• Chapters 3-6 describe the theoretical basis behind modern casting processes in foundries and cost houses.
• Chapters 7-11 have a more practical approach to the casting of various metals. The principles derived in the earlier chapters are applied to teach the reader how various problems can be designed in an optimal way, according to the present knowledge of casting science. Due to the importance of steel and iron-based alloys a good deal of the content is devoted to these alloys but the principles and phenomena are general.

Most chapters contain several solved examples and at the end of chapters 3-11 aa selection of exercises is given. The answers to all of the exercises are listed at the end of the book.

An accompanying 'Guide to exercises' provides complete solutions to all of the exercises, step by step. The solutions in the Guide are designed to encourage the students to work on their own, i.e. the idea is help to self-help to achieve increased understanding of the topic. The Guide can be downloaded in PDF format from www.wiley.com.

This extensive book offer materials for several courses on different aspects of the subject with the option of varying the level of the courses. It is appropriate for undergraduate as well as masters students in materials technology, as well as for students in mechanical engineering with some knowledge of materials processing. This text can be used by PhD students and researcher in material science who are working with problems connected to casting processing.

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Materials Processing During Casting

John Wiley & Sons

Chapter One

1.1 Introduction 1 1.1.1 History of Casting 1 1.1.2 Industrial Component Casting Processes 1

1.2 Casting of Components 1 1.2.1 Production of Moulds 1 1.2.2 Metal Melt Pressure on Moulds and Cores 4 1.2.3 Casting in Nonrecurrent Moulds 5 1.2.4 Casting in Permanent Moulds 8 1.2.5 Thixomoulding 11

1.1 INTRODUCTION

1.1.1 History of Casting

As early as 4000 years BC the art of forming metals by casting was known. The process of casting has not really changed during the following millennia, for example during the Bronze Age (from about 2000 BC to 400-500 BC), during the Iron Age (from about 1100-400 BC to the Viking Age 800-1050 AD), during the entire Middle Ages and the Renaissance up until the middle of the Nineteenth century. Complete castings were prepared and used directly without any further plastic forming.

Figures 1.1, 1.2 and 1.3 show some very old castings.

In addition to improving the known methods of production and refining of cast metals, new casting methods were invented during the Nineteenth century. Not only were components produced but also raw materials, such as billets, blooms and slabs. The material qualities were improved by plastic forming, forging and rolling. An inferior primary casting result cannot be compensated for or repaired later in the production process.

Steel billets, blooms and slabs were initially produced by the aid of ingot casting and, from the middle of the Twentieth century onwards, also by the aid of continuous casting. Development has now gone on for more than 150 years and this trend is likely to continue. New methods are currently being developed, which involve the production of cast components that are in size as close to the final dimensions as possible.

1.1.2 Industrial Component Casting Processes

As a preparation for a casting process the metal is initially rendered molten in an oven. The melt is transferred to a so-called ladle, which is a metal container lined on the inside with fireproof brick. The melt will then solidify for further refining in the production chain. This is performed by transferring the melt from the ladle into a mould of sand or a water-chilled, so-called chill-mould of metal. The metal melt is then allowed to solidify in the mould or chill-mould.

This chapter is a review of the most common and most important industrial processes of component casting. The problems associated with the various methods are discussed briefly when the methods are described. These problems are general and will be extensively analysed in later chapters.

In Chapter 2 the methods used in cast houses will be described. The methods used in foundries to produce components will be discussed below.

1.2 CASTING OF COMPONENTS

1.2.1 Production of Moulds

A cast-metal component or a casting is an object that has been produced by solidification of a melt in a mould. The mould contains a hollow space, the mould cavity, which in every detail has a shape identical to that of the component.

In order to produce the planned component, a reproduction of it is made of wood, plastic, metal or other suitable material. This reproduction is called a pattern. During the production of the mould, the pattern is usually placed in a mould frame, which is called a flask or moulding box. The flask is then filled with a moulding mixture which is compacted (by machine) or rammed (with a hand tool). The moulding mixture normally consists of sand, a binder and water.

When the compaction of the flask is finished the pattern is stripped (removed) from it. The procedure is illustrated in Figures 1.4 (a-d). The component to be produced is, in this case, a tube.

Stage 1: Production of a Mould for the Manufacture of a Steel Tube

The cavity between the flask wall and the pattern is then filled with the mould paste and rammed by hand or compacted in a machine. The excess mould paste is removed from the upper surface, and the lower part of the future mould is ready. The upper one is made in the same way.

Components due to be cast are seldom solid. They normally contain cavities, which must influence the design of the mould. The cavities in the component correspond to sand bodies, so-called cores, of the same shape as the cavities. The sand bodies are prepared in a special core box, the inside of which has the form of the core. The core box, which is filled and rammed with fireproof so-called core sand, is divided into two halves to facilitate the stripping. The cores normally obtain enough strength during the baking process in an oven or hardening of a plastic binder. Figures 1.4 (e) and 1.4 (f) illustrate the production process of a core, corresponding to the cavity of a tube.

Stage 2: Production of the Core in what will become the Steel Tube

When the mould is ready for casting the complete cores are placed in their proper positions. Since the fireproof sand of the cores has a somewhat different composition than that of the mould, one can usually distinguish between core sand and mould sand.

A necessary condition for a successful mould is that it must contain not only cavities, which exactly correspond to the shape of the desired cast-metal component, but also channels for supply of the metal melt. These are called casting gates or gating system [Figure 1.4 (c)]. Other cavities, so-called feeders, which serve as reservoirs for the melt during the casting process, are also required [Figures 1.4 (c) and 1.4 (g)]. Their purpose is to compensate for the solidification shrinkage in the metal. Without feeders the complete cast-metal component would contain undesired pores or cavities, so-called pipes. This phenomenon will be discussed in Chapter 10. When the casting gate and feeders have been added to the mould, it is ready for use.

Stage 3: Casting of a Steel Tube

The casting process is illustrated in Figures 1.4 (g), 1.4 (h), and 1.4 (i).

1.2.2 Metal Melt Pressure on Moulds and Cores

During casting, moulds and cores are exposed to vigorous strain due to the high temperature of the melt and the pressure that the melt exerts on the surfaces of the mould and cores.

To prevent a break-through, calculations of the expected pressure on the mould walls, the lifting capacity of the upper part of the mould and the buoyancy forces on cores, which are completely or partly surrounded by melt, must be performed. These calculations are the basis for different strengthening procedures such as varying compaction weighting in different parts of the mould, locking of the cores in the mould and compaction weighting on or cramping of the upper part of the mould.

The laws, which are the basis of the calculations, are given below. The wording of the laws has been adapted to the special casting applications.

The laws given on above are valid for static systems. During casting the melt is moving and dynamic forces have to be added. These forces are difficult to estimate. The solution of the problem is usually practical. The calculations are made as if the system were static and the resulting values are increased by 25-50 %.

An example will illustrate the procedure. The pressure forces are comparatively large and the moulds have to be designed in such a way that they can resist these forces without appreciable deformation.

1.2.3 Casting in Nonrecurrent Moulds

Sand Mould Casting

Sand moulding is the most common of all casting methods. It can be used to make castings with masses of the magnitude 0.1 kg up to [10.sup.5] kg or more. It can be used for single castings as well as for large-scale casting. In the latter case moulding machines are used. A good example is in the manufacture of engine blocks.

In sand moulding an impression is made of a pattern of the component to be cast. There are two alternative sand-moulding methods, namely hand moulding and large-scale machine moulding. Hand moulding is the old proven method where the mould is built up by hand with the aid of wooden patterns as described earlier. This method has been transformed into a large-scale machine method where the mould halves are shaken and pressed together in machines. It needs to be possible to divide the mould into two or several parts. Large-scale moulds get a more homogeneous hardness, and thus also a better dimensional accuracy, than do hand-made moulds.

The advantages and disadvantages of sand mould casting are listed in Table 1.1.

The disadvantages of the sand moulding method have been minimized lately by use of high-pressure forming, i.e. the sand is compacted under the influence of a high pressure. The method can be regarded as a development of the machine moulding method of sand moulds. Normal mould machines work at pressures up to 4 [10.sup.6] Pa (4 kp/ [cm.sup.2]) while the high-pressure machines work at pressures up to (10-20) [10.sup.6] Pa (10-20 kp/[cm.sup.2]). The higher pressure offers a better mould stability, which results in a better measure of precision than that given by the low-pressure machines. Development in sand foundries proceeds more and more towards the use of high-pressure technologies.

Shell Mould Casting

The shell mould casting method implies that a dry mixture of fine-grained sand and a resin binder is spread out over a hot so-called brim plate, which covers half the mould. The resin binder melts and sticks to the sand grains, forming a shell of 6-10 mm thickness close to the pattern. The shell is hardened in an oven before it is removed from the plate with the pattern. The method is illustrated in Figure 1.5.

Two shell halves are made. After hardening they are glued together. Before casting, the mould is placed into a container filled with sand, gravel or other material, which gives increased stability to the mould during the casting process.

A shell with a smooth surface and a good transmission ability for gases is obtained with this method, which can be used for most casting metals. The advantages and disadvantages are given in Table 1.2.

Precision Casting or Shaw Process

In the Shaw process, a parted mould is made of fireproof material with silicic acid as a binding agent. The mould is heated in a furnace to about 1000C. The method gives roughly the same measure of precision as the shell mould casting method but is profitable to use for small series and single castings because the pattern of the mould can be made of wood or gypsum. The Shaw process is especially convenient for steel.

Investment Casting

Investment casting is also a precision method for component casting. In this method, a mould of refractory material is built on a wax copy of the component to be cast. An older name of the method is the 'lost wax melting casting' process. The method is illustrated in Figures 1.6 (a-f).

In investment casting a wax pattern of the component has to be made. The wax pattern is then dipped in a mixture of a ceramic material and silicic acid, which serves as a binding agent. When the mould shell is thick enough it is dried and the wax is melted or burnt away. Then the mould is burnt and the casting can be performed.

Investment casting can be used for all casting metals. The mass of the casting is generally 1-300 g with maximum masses up to 100 kg or more. The advantages and disadvantages are listed in Table 1.3.

Investment casting offers very good dimensional accuracy. With the proper heat treatment after casting the component acquires the same strength values for stretch and fracture limits as do forged or rolled materials.

The investment casting method and the Shaw method are complementary to each other in a way. The Shaw method is used when the casting is too big for investment casting or when the series is too small to be profitable with the investment casting method.

1.2.4 Casting in Permanent Moulds

Gravity Die Casting

In gravity die casting permanent moulds are used. Such a mould is made of cast iron or some special steel alloy with a good resistance to high temperatures (the opposite property is called thermal fatigue).

The gravity die casting method is often used for casting zinc and aluminium alloys. It is difficult to cast metals with high melting points due to the wear and tear on the mould, which is caused by thermal fatigue.

Cores of steel or sand can be used. It is also possible to introduce details of materials other than the cast metal, for example, bearing bushings and magnets. The advantages and disadvantages of the method are listed in Table 1.4.

Due to the high mould cost, series of less than 1000 components are not profitable. In these cases, another casting method must be chosen. There is also an upper limit, which is set by the thermal fatigue of the mould. In aluminium casting the maximum number of components is around 40 000.

High-Pressure Die Casting

The molten metal is forced into the mould at high pressure as indicated by the name of the process. The method is described in Figure 1.7.

The permanent mould is made of steel and the mould halves are kept together by a strong hydraulic press. The method can only be used for metals with low melting points, for example zinc, aluminium and magnesium alloys.

The mechanical properties of the components are good with this method, better than with the gravity die casting method. However, weak zones may occur in the material due to turbulence in the melt during the mould filling.

Due to high machine and mould costs, the high-pressure die casting method will be profitable only if the number of cast components exceeds 5000 to 10 000. The method is useful for production of large series of components, for example in the car industry.

The 'life time' of the high-pressure die casting machine varies from about 8000 castings for brass to 800 000 castings for zinc alloys.

The advantages and disadvantages of the method are listed in Table 1.5.

Low-Pressure Die Casting

The principle of this method is illustrated in Figure 1.8. Contrary to the high-pressure die casting machine, the low-pressure casting machine contains no pushing device and no piston. Nor is it necessary to apply the high pressure, required in high-pressure die casting, at the end of the casting process.

Air, or another gas, is introduced into the space above the melt. The gas exerts pressure on the melt and causes it to rise comparatively slowly in the central channel and move into the mould. The mould is kept heated to prevent solidification too early in the process. This is a great advantage when small components, with tiny protruding parts, are to be cast. In this way it is possible to prevent them from solidifying earlier than other parts of the mould. This is one of the most important advantages of this casting method.

The walls of the component to be cast can be made rather thin. The low melt flow gives little turbulence in the melt during the mould filling and very little entrapment of air and oxides. When the casting has solidified the pressure is lowered and the remaining melt in the central channel sinks back into the oven.

A list of the advantages and disadvantages of the method is given in Table 1.6.

Squeeze Casting

Squeeze casting is a casting method that is a combination of casting and forging. It is described in Figures 1.9 (a-c).

When the mould has been filled the melt is exposed to a high pressure and starts to solidify. The pressure is present during the whole solidification process so that pore formation, which causes plastic deformation, is prevented and the mechanical properties of the castings are strongly improved as compared to conventional casting.

Centrifugal Casting

In centrifugal casting centrifugal force is used in addition to gravitational force. The former is used partly to transport the melt to the mould cavity and exert a condensing pressure on it and partly, in certain cases, to increase the pressure, thus allowing thinner details to be cast and making surface details of the metal-cast components more prominent.

(Continues...)

Excerpted from Materials Processing During Castingby Hasse Fredriksson Ulla Akerlind Copyright © 2006 by John Wiley & Sons, Ltd. Excerpted by permission.
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