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Novel Method for Casting High Quality Aluminum Turbocharger Impellers
Abstract:
A turbocharger essentially consists of a turbine and an impeller wheel connected on a common shaft. The turbocharger converts waste energy from the exhaust into compressed air, which is pushed into an engine to produce more power and torque, as well as improving the overall efficiency of the combustion process. The compression ratio for modern diesel engines can be up to 5:1, which can be only achieved using a complex impeller design and very high rotation speeds (up to 150,000 rpm for small impellers). The complex geometry and very high running speeds of impellers creates high stresses at locations such as blade roots and around the bore, and so impellers normally fail from fatigue. Therefore, it is vital to minimize defects while fabricating turbocharger impellers.
Current methods for producing aluminum turbocharger impellers are plaster casting or by forging + machining. However, both of these current methods have serious drawbacks. Plaster cast impellers tend to have both surface and interior defects that limit performance and life, while forged + machined impellers are expensive.
A new technique for producing high quality turbocharger impellers is semi-solid casting. This casting process injects metal that is approximately 50% solid and 50% liquid into a re-usable, hardened steel die using a state-of-the-art die casting machine. A combination of controlled die filling and a high intensification pressure produces high quality impellers - various tests are presented that demonstrate the castings are essentially free of porosity and other defects.
Semi-solid cast impellers have been in commercial production for about four years. The technology to produce the impellers, which is protected by a pending world-wide patent, is described in detail. Of interest is a special die design, which is required to cast the fourteen intricately-shaped impeller blades. Part of the steel die is ejected from the casting machine along with the cast impeller, and a special station is utilized outside of the machine to disassemble the die cartridge. After removing the casting, the cartridge is then reassembled and reinserted into the casting machine. Several steel cartridges are utilized to maximize productivity.
To optimize strength and fatigue resistance, the semi-solid cast impellers are fully heat treated to the T6 temper. Mechanical and fatigue properties of the semi-solid cast impellers are presented, and both are significantly better than impellers produced by conventional plaster casting. Semi-solid impellers have a comparable fatigue life to their forged + machined counterparts, although production costs are lower for semi-solid impellers.