Traditionally, compressor wheels are produced from aluminium, which is naturally a very weak cast material. Aluminium is the preferred material for compressor wheels as it is a relatively simple and inexpensive process to cast the compressor wheels, however, to create a stronger wheel post process treatments are essential.
The post production processes are defined by the OEM compressor wheel manufacturers and include heat and solution treatments, to create a more robust wheel. It is these post processes which increase the cost of a compressor wheel, however they are necessary to produce a strong wheel which will withstand the operating conditions of a turbocharger.
Effects of Weak Cast Material
- If the compressor wheel has been produced from weak cast materials, the blade will start to bend as the pressure of the air and the load on each blade increases;
- As the wheel continues to spin at high speeds the blades will continuously bend backwards and forwards;
- This completely changes the compressor map and the compressor efficiency and it means the wheels do not perform like they are designed to.
Aluminium is very flexible, so whilst it might bend at top speed, as the wheel slows down the blades will return to the original position. The wheel may appear to look fine, however if you compare the performance of a lower quality compressor wheel against a high quality wheel, you will find as it hits maximum speed the lower quality compressor wheel will lose efficiency and eventually fail. This is all down to the strength of the casting. It is very difficult to visually identify which post casting processes have been used and the strength of the compressor wheel.