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-process includes heat treatments and solution treatments, which are defined by all the OEM compressor wheel manufacturers, and create a much more robust wheel. It is these post-processes which increase the cost of the wheels but produce a wheel which will withstand the operating conditions of a turbocharger.
As the wheel spins up to full speed the pressure of the air increases and therefore the load on each blade increases. If the compressor wheel has been produced from weak cast materials the blade strength is not as strong as it should be. As the compressor wheel continues to spin at high speeds the blades will begin to bend backwards. This completely changes the compressor efficiency and it means the wheels do not perform as they are designed to do.
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.
Low cost, low quality compressor wheels can look very similar to OEM quality alternatives. To achieve OEM quality, a significant level of investment and knowledge is required to understand the importance of post processes and how to achieve them through manufacturing.
Melett castings are produced the same way as the OEMs so we can guarantee the correct strength of casting. This does add to the cost of our compressor wheels, but provides our customers with peace of mind that they are strong and durable.
Understanding Fatigue & Overspeeding Issues
As turbochargers evolve, operating conditions have altered significantly. Turbochargers are spinning faster than ever before and the centrifugal force on the compressor wheel is absolutely phenomenal at top speed. If the compressor wheel material is not as strong as it should be, the centrifugal force will cause the material to flow away from the centre. At these speeds the natural grains of the surface material are unable to cope. This can cause what is known as the ‘orange peel’ effect which is often associated with overspeeding.
Overspeeding is a term used for when a turbocharger is operating well above its normal operating limits. There are many causes of overspeeding such as inconsistent air flow to the turbo, worn injectors or engine modifications.
When a compressor wheel has been weakened, it will begin to show signs of fatigue. For example, if you bend a piece of Aluminium backwards and forwards enough times eventually it will break. In terms of the compressor wheel fatigue, the blades are exposed to a continuous cycle of positive and negative stress caused by the wheel spinning fast and then slow. As the compressor wheel reaches full speed, the blades bend back and then as it slows down they bend back into position, repeat this over many repetitions suction is created which creates negative stress pulling the blades even further in.
Eventually the continuous stress will become too much and the blades will break and cause the turbocharger to fail. Therefore, if the strength of the material is a lot lower than it should be the compressor wheel will break from fatigue a lot earlier than if a stronger wheel had been used. Using a stronger compressor wheel will ensure the blades do not move as far and reduce the risk of fatigue and failure.
Often overspeeding is disregarded as the mode of turbo failure as other symptoms can occur as a result of overspeeding. Whatever the cause of the overspeeding and fatigue, the quality of the compressor wheel is unlikely to be considered. If a low quality compressor wheel was used in a previous repair or during remanufacture, the life of the turbocharger is automatically reduced and likely to fail again; causing frustration to the owner of the car and an otherwise perfectly good turbo has failed because of the compressor wheel.
When it comes to compressor wheels, a quality wheel does come at more of a premium but quality matters to our customers and they understand what processes Melett undertake to ensure our compressor wheels are manufactured above and beyond OEM specifications.