Few people realize what a turbine blade or turbine wheel is all about. The casting process of such a product begins with a wax model. However, a foundry can only reach this stage after around 10 weeks, following the receipt of drawings and 3D models. This is followed by the production of a shell mould, casting in a vacuum melting furnace, removal of the shell and, of course, a quality inspection of the finished cast piece. The journey from drawing to production in larger series is a matter of many months, and sometimes years.
Before production of a new cast piece itself, it is first necessary to carry out constructional processing of the wax modelling equipment. Model equipment production is carried out in cooperation with an external supplier on the basis of drawings and 3D models. Within these pre-production operations, in some cases it is also necessary to design preparations for the storage of wax models, measuring instruments, cooler moulds, etc.
Production of wax models and inflow systems
Once the model equipment (steel mould) is ready, production of a wax model can begin. Special wax is injected into the steel mould; this is carefully removed after cooling. The wax model must be handled with great care to prevent damage, which would cause deterioration of the final cast piece. It is not classic natural wax; it is a special synthetic product. Shrinkage of approximately 3% is expected for the wax we use. Therefore, the wax model must be slightly larger than the final cast piece,” says Jan Vojtěch, Manager of the DPL technology department.
Then, after the wax model is pressed, it must be attached to the prepared inflow system. In some cases, one wax model is glued to the inflow system; in other cases the inflow system may contain multiple wax models. It all depends on their shape and size. The inflow must be appropriately sized to ensure a perfect casting process. Usually, several sample series with different types of inflows are made. These tests are aimed at identifying the most suitable type of inflow system in order to provide maximum utilisation of liquid metal, while at the same time ensuring that the cast piece is optimally filled. Therefore, in the first phase, series with fewer pieces are made in order to eliminate possible scrapes caused by using an unsuitable inflow system,” Jan Vojtěch adds.
Manufacture of a ceramic shell mould
In the next stage of production, it is necessary to produce a ceramic shell mould. This can be formed by the gradual application of coating layers to the inflow system with wax models. Inflows with models are soaked in "wrapping slurry", onto which molochite spread is subsequently applied. And the process is repeated several times. The number of coating layers varies according to the shape and complexity of the piece which is to be cast. Usually there are 8 to 16 of these. “While producing the ceramic shells, many parameters must be observed, such as the density and viscosity of the slurries; and when drying the individual packages, one must maintain suitable moisture and temperature, as well as optimal air flow,” Jan Vojtěch explains. “Each layer requires about 4 to 6 hours to dry; the first layer may even require as much as 24 hours. Although the wrapping plant is automated, the first layer must be made manually to ensure the required quality,” Jan Vojtěch says.
After completion of the ceramic shell mould manufacturing process, the wax which is inside the shell must be melted. This process is carried out in a special device which works on the principle of preheated steam and high pressure.
Melting and casting
The next step in casting is the casting process itself. The ceramic shell mould is placed in a gas annealing furnace and annealed at a temperature of about 1,050 to 1,130°C for about 4 to 6 hours. “This burning process will reinforce the shell, but it will also eliminate heat shock, because the temperature difference between the shell and the molten alloy is minimised,” Jan Vojtěch says. Meanwhile, the material is melted in a vacuum furnace. Usually, this involves nickel or cobalt superalloys, which are characterised by high heat resistance and are therefore suitable for the most demanding castings, which have to operate at high temperatures.
The annealed shell mould is placed in one of the vacuum furnace's two chambers. Subsequently, the air is sucked out of the chamber and the material can be poured into the prepared mould. Thus, the material melting and casting process takes place in a vacuum environment, which ensures that the material is not contaminated with oxides. After casting, the shell is removed from the furnace and left to cool.
Removing the mould
In the last stage of casting, the shell mould must be removed. This is done mechanically, using a pneumatic hammer and then blasting. Smaller parts are blasted manually. The cast piece is then cut from the inflow system, and sent for quality inspection.
The first inspection includes a visual check. This is followed by special checks, such as FPI (UV lamp defectoscopy) and X-ray. In the case of sample series, a dimensional inspection must also be carried out, which is performed with a 3D measuring device. In some cases, a final co-operation must also be carried out, such as machining some parts of the cast pieces, heat treatment, or HIP, which is a special operation to remove the cast piece's internal porosity. If a cast piece meets all the requirements prescribed in the technical specifications, it can be sent for final inspection, where all the casting documentation is completed and a final inspection is performed before the piece is sent to the customer. The cast piece is then packed into a special box and shipped.
Casting development is a long process. The foundry may need several months for the journey from drawing to finished final product; it may even be more than a year. The complexity of the production process is best demonstrated by the number of operations that must be performed before the final cast piece can be sent to the customer, or before a new type of cast piece can be introduced into serial production.
How to check casting quality
In case you are interested in our precision casting products, make sure to visit our Product webpage Precision Casting Products and find out more about portfolio of products we offer. For more information, you can also contact our Sales Manager, Mr. Frank Jones.