Selecting the Correct Ceramic Core in Investment Casting Applications

07/04/2014

The investment casting industry uses ceramic cores in a range of applications, due to their many advantageous properties such as strength, refractory qualities and stability in extreme conditions. Cores allow the creation of internal cavities that are too small or complex to be shelled during the investment casting process. However, if the best possible outcome is to be achieved, the ceramic core used in any application must be fashioned from the most appropriate materials – for each application, a particular type of ceramic will offer the best solution.

The type of alloy being cast is also crucial: ceramic cores can be used with a wide range of alloys, including nickel and cobalt based alloys, steels, aluminium and titanium, but the match between alloy and ceramic must be appropriate. The size and complexity of the item being cast will also influence the ceramic core selection, as will leachability – the ability of the core to be removed after processing.

The following are some of the frequently used ceramic core materials, with details of the casting processes and situations to which each is most suited.

>97% alumina

This material is particularly suitable for use in Equiax casting, a process in which ceramic moulds are pre-heated to a temperature just below that at which the alloy solidifies. Thus, when the alloy enters the mould, it quickly freezes and gives a uniform grain structure. Ceramic materials of >97% alumina can be used with a wide selection of alloys including those that are nickel and cobalt-based, and the ceramic takes the form of a single material with coarse particle size distribution. Ceramic materials of >97% alumina are not leachable by standard methods, but show great strength and refractory qualities.

>95% zircon

Another ceramic ideal for use in Equiax casting, >95% zircon is particularly suited to alloys containing high levels of cobalt. Cobalt reacts with cores that contain a high proportion of silica, making silica cores unsuitable for these applications. Ceramics that are >95% zircon have a coarse particle size and are suitable for the casting of large, chunky parts. They are non-reactive and can be removed mechanically or via water blasting.

>94% zirconia

Ceramics of >94% zirconia are used mainly in Equiax casting, and give excellent results when used with titanium alloys. They have a fine particle size and must be removed by knock out, sand blast or water jet.

100% fused silica

Another ceramic mainly used in Equiax casting, fused silica is good for use with most nickel and aluminium alloys. Fused silica ceramics are available with various particle size distributions, to permit the manufacture of a range of geometries, and they are highly leachable, including by water jet removal. Fused silica is stable, weak and crushable, and is resistant to hot tearing.

96% fused silica, 2% zircon, 2% alumina

This combination forms a ceramic material that is most often used in directionally solidified and single crystal casting, and with most DS and SX alloys. The combination of 96% fused silica, 2% zircon, 2% alumina is available in a range of forms, but generally has a fine particle size for use in the casting of small, complex parts. It is highly leachable and as such it is ideal for casting blind passageways and in small, intricate cores. This ceramic material shows great stability at high temperatures.

74% fused silica, 24% zircon, 2% alumina

The highly versatile blend of 74% fused silica, 24% zircon and 2% alumina can be used in Equiax, directionally solidified or single crystal casting, and with alloys that have high temperature pre-heats. It is often used with most nickel-based, DS and SX alloys. Available with various particle size distributions, it can thus be used in the manufacture of a range of geometries, and shows good leachability. The combination of 74% fused silica, 24% zircon, 2% alumina shows excellent stability at high temperatures and can be used in a wide range of core sizes and designs.

Additional processes

Further processes may be applied, after firing of the ceramic core itself, to enhance its performance during investment casting. Such processes include the impregnation of the core to improve its strength at room or high temperatures; the impregnation solutions most frequently used are colloidal silica, water soluble polymer and urea.

Another enhancement process added to the core when it has been fired, and another that can help to optimise the performance of ceramic cores in investment casting, is the coating of the core surface to prevent core/melt reaction, or to control gain growth. Alumina and cobalt aluminate coatings are often used.

Whichever post-firing process is undertaken, most materials are easily removed from the casting, either by chemical leaching or by the use of water jets.

Summary

Ceramic materials are widely used in investment casting applications across a wide range of industrial sectors, due to their many qualities including stability in the face of environmental extremes, strength and ease of removal once casting is complete. Modern ceramic materials combine high performance with cost effectiveness, and are ideal for complex casting processes. Modern processes also enable the adaptation of many of these materials to meet specific customer needs, while existing tooling can often be used to produce ceramic cores based on a wide variety of materials.

For further information about the use of ceramic cores in investment casting, and the varieties and forms of materials available, please visit our dedicated website.

Morgan Technical Ceramics,