INVESTMENT CASTING –

AN INVESTMENT IN QUALITY

John Cotton

v1 Dec 2013

This work by Lucideon is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike

4.0 International License.

AN INVESTMENT IN QUALITY

Investment or ‘lost wax’ casting is a key process in the manufacture of high quality engineering components such as orthopaedic implants. The process may be applied to a wide range of metals and alloys and can be used to produce both large and small castings. The application of investment casting has seen significant growth in the last 5 years with estimates placing the current market size at $US 8.6 billion and whilst US remains the largest single producer, Asian markets account for approx 35% of this value.

This white paper examines the major issues involved with the investment casting process. The problems that can occur during pattern manufacture, shell moulding, de-waxing and casting are discussed and solutions to these problems are identified. The white paper also looks at some of the non-technical issues facing investment casting in 2011 and the future.

APPLICATIONS FOR INVESTMENT CASTING

Investment casting is used in applications where complexity of shape would otherwise require extensive and complex machining leading to an uneconomic process cost burden. The process is commonly used for manufacture of biomedical dental and orthopaedic implants such as hip prostheses, femoral stems, knee joint components, dental bridges and device enclosures, e.g. for pacemakers. In aerospace, the process is commonly used for manufacture of turbine components including blades, blisks, and nozzle guide vanes where internal structure is often defined by the use of removable ceramic cores, and for larger structural components in the airframe.

THE PROCESS

The process starts with a wax pattern (sometimes referred to as the investment) which closely matches the shape and size of the required casting. A ceramic mould or shell is formed around this pattern by repeatedly dipping it in a ceramic slurry and coating with ceramic powder (stucco). Once formed and hardened, the wax pattern is removed from the shell by melting, leaving a cavity of the same shape as the required casting. Molten metal is then poured into the cavity and allowed to solidify before the shell is removed from the casting surface. In practice several patterns are usually assembled into a ‘tree’ arrangement each connected to a pouring tube/runner system so that multiple components may be made from the casting pour.

The process is shown schematically in Fig 1.

Investment casting is a very reliable and consistent process which is able to produce high quality components for critical applications in aerospace and medical devices. A number of issues exist however, which affect both quality and yield, and many of these have their origins in specific parts of the process, these being:

- Pattern manufacture

- Shell Moulding

- Dewaxing

- Casting

Figure 1. Principles of the Investment Casting Process [Jones et al. J. Mater. Process. Technol. Vol. 135, 2003 p.259]

PATTERN MANUFACTURE

The wax patterns used in investment casting are usually made by low pressure injection moulding into metal dies. The patterns are subsequently assembled onto a casting tube and runner system which is itself composed of the same or similar type of wax. This assembly is called a tree. The use of metal moulds in the pattern making process ensures that the pattern accurately reflects the component shape; however, at the start of a moulding run (before the mould achieves its ambient operating temperature), undersized and, in some cases, distorted patterns may be produced. These distortions may not be immediately apparent on visual inspection.

Complex shaped wax patterns can be difficult to remove from the mould and the use of release agents

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on the mould surface is common practice. Excessive release fluids such as silicone oils or PTFE based aerosols however, can interfere with subsequent stages in the process or, can themselves cause surface defects in the pattern which ultimately transfer to the casting surface.

In addition the, usually manual, process of handling the moulded pattern during and after extraction from the mould, i.e. while the moulding is still soft, risks the introduction of surface imperfections on the pattern which, if undetected, will emerge as surface flaws in the finished casting. The risk of introducing surface flaws into the pattern also exists during the assembly of the pattern onto a tree. This assembly process is often carried out by hand and hot wax is used as an adhesive. Management of excess wax and cleaning of joints is critical to maintaining the shape of the runner system and ensuring high quality castings.

SHELL MOULDING

The shell moulding process involves application of multiple layers of bonded ceramic powder evenly onto the surface of the wax pattern. Typically this multi coating process will involve:

- A wash coat to remove any remaining release agent from the pattern surface

- A pre-treatment coat to modify the hydophilicity of the wax pattern surface – this is done to ensure that the pattern surface is fully wetted by the primary coat

- A primary coating of a fine ceramic suspension. This coating may contain a grain refiner such as cobalt aluminate (for Co-Cr alloy). The primary coating may be a single or double layer.

- A series of backing coatings containing coarser ceramic particles. These backing coats may contain 7 – 10 layers and be several mm thick.

- A final seal coat – containing fine grained ceramic.

Each of the ceramic containing layers also contains a ceramic binder such as a silicate solution or a sol. The process for introducing coarser ceramic particles into the outer coatings is usually as follows:

1. The pattern with its primary coat is dipped into a suspension containing fine ceramic powder and ceramic binder.

2. While still wet the pattern is coated in dry coarse powder – this may achieved by rotating the pattern in a powder cascade or in a fluidised bed of the coarse powder.

3. The coating is slowly dried before application of the next coat (a above)

The major investment casting issues associated with the formation of the shell mould relate to three areas.

1. The quality and uniformity of the primary coat. This is the part of the mould which is in direct contact with the cast metal so it must be of uniform texture and free of inclusions and contaminants.

2. Variation in the thermomechanical performance of the mould which may lead to accelerated solidification in critical areas of the casting. Such variation may be attributed to density or coat thickness variations which could result from raw material variation or drift in the properties of the dipping slurry.

3. Cracking in the outer layers of the shell – usually associated with the drying stages, weakens the shell mould and may lead to failure of the mould during de-wax or casting.

The design of the component tree should ensure that bridging between the outmost coatings of the patterns is avoided. Inter-component bridging alters the thermal characteristics of the shell leading to non-uniform and uncontrolled solidification after casting.

DE-WAXING

The removal of the wax, from the green (unfired) ceramic is achieved by melting it and allowing the molten wax to drain from the shell mould under gravity. However, the wax has a much higher thermal expansion coefficient (approx 10X) than the green ceramic and, if simply heated, would generate internal stresses which would rupture or distort the shell before the wax reached its melting point. To prevent this, the heating of the wax must be carried out as quickly as possible so the wax begins to melt and flow before it expands sufficiently to overstress the shell.

This process is commonly carried out in an autoclave where high pressure steam at ~8kg/cm-2 (8BAR) is injected into a sealed oven, rapidly raising the temperature of the shell and pattern to 170-175˚C [Brum 2009]. This is done for a specified time relating to the mould size and wax volume. Once the wax has drained out of the mould, the hollow ceramic shell is ready to be fired.

The major issues relating to the dewaxing stage are:

- Shell cracking due to uncontrolled wax expansion

- Incomplete removal of wax from the mould. This may not be completely combusted during the subsequent firing process and in that case will leave

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residues which become inclusions in the finished casting.

CASTING

Two approaches may be taken to mould preparation prior to casting:

1. The moulds may be fired to, or just above the casting temperature, allowed to cool and stored to await casting in a separate process.

2. The mould firing and casting process takes place sequentially – i.e. the moulds are not allowed to cool before casting takes place.

The choice between these two approaches is determined largely by the metals to be cast and hence ceramics used in the moulds.

For metals such as Co-Cr alloy, a typical shell mould would contain fused and amorphous silica bonded by an organic silicate. Firing of this composition converts some of the silica to critstobalite (– a crystalline form of silica which undergoes a rapid change in dimensions as it cools). Hence, when this material is fired and casting takes place as in 2 above, as the shell mould cools, it contracts around the solid casting and cracks – making its removal from the finished casting easier.

For more refractory alloys or batch casting processes, such as vacuum casting, zircon, zirconia, or alumina based shell compositions are commonly used. These are often fired according to approach 1 above.

The major quality limiting issues arising during the firing and casting process are:

- Cracking of the refractory shell during the firing cycle resulting in, at best, surface imperfections (veining) and, at worst, metal break out during casting. Cracks appearing at this stage are usually already present in the shell due to unrecognised damage earlier in the process.

- If there is residual wax present in the shell and if the firing atmosphere is not oxidising enough, incomplete combustion of the residual wax may result in a residue or surface deposit which may be trapped by the casting. This can lead to sub surface inclusions and surface imperfections.

- Inclusions – dust or ceramic particles which can enter the shell mould during kiln placement or in the turbulent environment of the kiln during firing.

Although some of the defects can be identified during the process, most flaws and quality issues are only

identified once the component has been cast, i.e. when the process energy and materials costs have been embedded in the component. Sub surface flaws such as porosity and inclusions are often only revealed during subsequent finish machining processes when even more investment has been made in the component.

IMPROVING THE PROCESS

It is possible to make improvements in the yield from the investment casting process, something which, of course, helps to reduce losses and enhance the cost effectiveness of the process. This can usually be achieved by taking an holistic approach to the process as a whole, recognising that improvements to individual stages may influence the performance in others.

PATTERN MAKING

Typical flaws seen at the pattern making stage are incomplete patterns – often due to inadequate venting of the die, and surface imperfections which could result from excess release agents or knit lines arising from uncontrolled flow of the wax in the die.

Distortion of the pattern during handling of the component from the die is also possible. Most of these flaws are revealed by visual inspection and gauging of the part after manufacture.

Assembly of the patterns onto a casting tree also represents a potential source of flaws as excess wax at the joins will lead to inconsistency in runner shape and influence metal flow during casting.

Improvements in the pattern making stage can be achieved by focussing on the critical areas. For example, Lucideon has helped clients to overcome pattern making problems through process evaluation consultancy, using FED optimisation of wax injection parameters, application of flow and thermomechanical modelling to tree design, and through the use of surface chemical analysis for forensic investigations and cleanliness evaluation.

SHELL MOULDING

The shell moulding process is a potential source of several types of flaws seen in investment castings. The most important part of the mould from the point-of-view of the quality of the castings produced is the primary coat which defines the casting surface. Any imperfections in this layer will be immediately transferred to the casting surface hence the quality and integrity of the primary coat is critical to the component quality and yield. The influence of imperfections in the

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back up coats is less significant but can also be the source of flaws in the cast component.

Primary coat imperfections can arise from lack of control of the pattern-coating interface often influenced by inadequate cleaning of release agent from the pattern/tree before dipping. The thickness and uniformity of the primary coat is controlled by the rheology of the primary coat suspension and quality checks to identify and rectify aging effects and density and viscosity changes are essential to maintain consistency of performance. Changes in the primary coat and back up coat suspensions may be due to gradual change in solids content of the suspension after repeat dippings, or deterioration (ageing) of the binder. The dipping process itself must be consistent time after time and, to this end, robot handling is the norm for this process. The coating process often takes place in close proximity to the stucco application area and airborne contamination of the primary dip coat can occur.

At Lucideon we have used our skills in process evaluation, raw material characterisation and QC testing, suspension characterisation and control, rheology assessment, including both viscosity and zeta potential measurement, to help clients improve the

performance of the shell moulding process. We have also used our extensive testing and analysis capability in the characterisation of shell structure, and measurement of thermomechanical properties. Surface chemical analysis techniques have also been used for forensic investigations and cleanliness measurements.

DE-WAXING

Removal of the wax pattern from within the shell mould without damaging the mould structure relies on management of the thermal expansion mismatch between the ceramic shell and the more expansive wax. In practice, this often requires the rapid melting and removal of the wax before sufficiently high tensile stresses can be generated to crack the shell. This may be achieved through the use of rapid acting autoclave where steam pressure assists in stabilising the wax/shell interface, or more recently microwave assisted heating, where the wax phase is preferentially heated.

Most autoclave shell cracking and damage occurs on the edges of components, such as trailing edges and sharp corners, where reduced shell build and high stress results in ceramic failure.

Figure 2. Showing the heat soaking through the shell from the start of the autoclave cycle for a period of 60 seconds.

Lucideon has helped clients understand and improve their de-wax processes by modelling of the dewax process to simulate the effect of process and tree

design changes on the thermo-mechanical performance of the shell. Characterisation of the shell moulds after dewax has also been used to

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identify cracks in the shell and the presence of residual wax in the shell cavity.

CASTING

This process consists of three elements; shell firing, metal melting, and metal pouring.

The shell firing process strengthens the mould and, in single fired systems, may develop appropriate ceramic phases such as cristobalite which assists in the removal of the shell mould after solidification of the casting. An additional aspect of the firing process is the combustion of any residual wax from the dewaxing stage in the shell prior to casting. If the conditions during firing are not sufficiently oxidising however, the wax residue may char and leave adherent residue on the mould surface causing inclusions and surface pitting in the castings.

The shell mould may crack during the firing process but go unnoticed, unless there is breakout of the metal during casting. In these cases the (closed) cracks result in features such as flash or veining on the casting surface.

Figure 3. Shell deformation after 60 seconds

Casting defects can arise from inclusions or slag in the metal prior to casting. This can arise from reaction of the metal with the melting crucible causing deterioration of the refractory lining. Refractory filters are often included in the pouring tubes of shell trees to minimise these inclusions.

Once the casting has cooled, the outer ceramic shell can be removed by water jet, vibratory hammers, vapour blasting or shot blasting leaving the complete metal mould in the shape of the original wax mould. If pre-formed ceramic cores are used, these need to be leached out with a caustic fluid or another relevant core removal method at this stage.

Lucideon has assisted clients at the casting stage through process evaluation and troubleshooting consultancy, including refractory analysis and testing, fault diagnosis and characterisation. We have also used thermomechanical modelling of the response of shells during casting and for both bake out and casting furnace design.

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TABLE 1. CAUSES AND REMEDIES FOR FLAWS APPEARING DURING THE INVESTMENT CASTING PROCESS

Process Stage Typical Flaw Potential Cause Remedy

Wax Pattern Incomplete patterns often with rounded surfaces at the unfilled areas

Insufficient venting of the die

Wax flow rate too high

Poor flow/fill of the die

Enhance venting in the critical areas

Modify the injection conditions

Modify gate positions to avoid opposing flow of wax

Shell Small pimples evenly distributed over the surface of the casting

Pinholes in the primary coat caused by:

Poor coverage of the wax pattern

Trapped air bubbles in the slurry – which may be due to early gelling of the slurry

Poor dispersion of the filler

Casting temperature too high

Check the wax pattern for contamination

Control the rheology of the primary coat slurry

Reduce the particle size of the filler in the slurry

Reduce the casting temperature

Shell General surface roughness of the casting

Poor coverage of the wax pattern

Use of an unsuitable washing agent

Erosion of the primary coat during de-waxing

Penetration of the primary coat by stucco grains

Drying of the primary coat before application of the stucco

Draining of the primary coat particularly from sharp edges

Primary coat slurry too thick

Ageing of the primary coat slurry

See above

Use a less aggressive washing agent to avoid etching of the wax surface. Limit the immersion time during washing

Use binders which are not attacked by steam in the dewax stage

Use a lower energy process for application of the stucco (eg fluid bed vs curtain)

Control temperature and humidity of the shell area

Modify the rheology of the slurry – use of appropriate wetting agents

Modify the viscosity of the slurry

QC checks on the pH and solids content of the slurry

Shell Scab – areas of surplus metal on the surface of the casting – sometimes with

Primary coat lifting from the mould due to poor adhesion to

Improve the keying between the primary and first back up coat by using a coarser, more

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refractory material embedded

the backing layers, moisture trapped between the primary and back up layers, or differential expansion stresses between the primary and secondary layers of the shell

angular stucco powder

Increase the drying time or improve the drying conditions for the primary and back up coats

Ensure the mould is dry before de-waxing

Matching of thermal expansion of primary and first back up coat

Shell Inclusions – shallow irregular depressions sometimes with refractory material embedded

Spalling of the primary coat

Poor fixing of the pattern to the tree resulting in gaps which are filled by the primary coat

Entrapment of slag in the casting

Metal/mould interaction

Improve cleaning of the pattern before dipping

Ensure correct drying of the primary coat

Matching of thermal expansion of primary and first back up coat

Control of the rheology of the slurries to avoid drying cracks

Ensure adequate filtration

Modify deoxidation practices

Redesign the runner system to reduce turbulence during casting

Minimise casting temperatures

Modify primary coat to improve its refractoriness

Some of the non-technical issues facing investment casting in 2011 and the future are:

- Periodic shortages in zircon, alumino-silicates and alumina leading to volatility in raw materials supply and price

- Cristobalite which is formed in some shells is subject to review by the European Carcinogens Directive and may come under increasing restrictions to its use and disposal

- Depending on its source, Zircon can exhibit radioactivity which prohibits the disposal of waste shell material in many countries

- Heavy metal containing materials such as cobalt aluminate are not able to be disposed in landfills in UK making disposal of shell waste increasingly difficult and expensive.

The shell material cannot currently be readily recycled or reused emphasising the need to minimise losses in the process.

CONCLUSION

The growth of investment casting, particularly in the medical devices industry, shows no sign of slowing. Process issues do exist however, which affect both quality and yield. In order to minimise these issues, and so improve yield and profitability, it is important to fully understand all parts of the investment casting

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process, so that problems can be easily identified and solved quickly. The investment casting process is one which lends itself to constant improvement, however an holistic approach, employing techniques such as those practised by Lucideon, is required to ensure that the interdependence of the different stages of the process

are managed thereby ensuring optimum quality and yield.

ABOUT LUCIDEON

Lucideon is a leading international provider of materials development, testing and assurance.

Through its offices and laboratories in the UK, US and the Far East, Lucideon provides materials and assurance expertise to clients in a wide range of sectors, including healthcare, construction, ceramics and power engineering.

The company aims to improve the competitive advantage and profitability of its clients by providing them with the expertise, accurate results and objective, innovative thinking that they need to optimise their materials, products, processes, systems and businesses.

ABOUT THE AUTHOR

JOHN COTTON - BUSINESS DEVELOPMENT MANAGER, AEROSPACE & DEFENCE

John is a Chartered Engineer who holds a Degree in Applied Physics and is a Fellow of the Institute of Mining Minerals and Materials (IOM3). John serves on the Ceramic Science Committee of IOM3 and is a member of Peer Review College for the Engineering and Physical Sciences Research Council (EPSRC). John also acts as a Technology Expert for Materials KTN.

With over forty years of experience in advanced materials – specialising in refractories and technical ceramics at Lucideon, John is an expert in all aspects of materials R&D and problem-solving. From identifying and solving production issues to advising on application design and performance, John has worked with manufacturers, systems integrators and end-users to make a real difference to their businesses.

John has contributed to several materials textbooks, composed a large number of papers and is a frequent presenter at conferences worldwide.

ADVANCED MATERIALS

Throughout his term at Lucideon John has worked with a range of advanced materials including both monolithic and composites for applications such as fuel cells, lightweight materials for airframe and sporting goods, as well as sensors, actuators, and high temperature and wear resistant components.

AEROSPACE AND DEFENCE

John's experience in aerospace and defence materials incorporates ceramic armour, lightweight and high temperature composites and coatings for thermal and corrosion management.

CERAMICS

John has been involved in a range of ceramic projects including the development of sinterable silicon nitride ceramics, evaluation of ceramic materials as electrochemical gas sensors, design and manufacture of ceramics for engine components, and design of dies and development of extrusion technology for the production of thin ceramic and metal powder tapes.

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