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Wallace G.,Aluminum Complex Components Inc. | Jackson A.P.,Aluminum Complex Components Inc. | Midson S.P.,Midson Group Inc.
SAE International Journal of Materials and Manufacturing | Year: 2010

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. urrent methods for producing aluminum turbocharger impellers are plaster casting or by forging + machining. owever, 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. 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 astings are essentially free of porosity and other defects. emi-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. o 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. © 2010 SAE International.

Jackson A.P.,Aluminum Complex Components Inc. | Wallace G.R.,Aluminum Complex Components Inc. | Midson S.P.,Midson Group Inc.
Advanced Materials and Processes | Year: 2010

A thixocasting semi-solid casting process was chosen to produce the aluminum turbocharger impellers. A real-time controlled die casting machine casted the impellers, and the semisolid slugs were injected into a reusable, hardened steel die. The runner system of the impeller was designed to strip away the surface oxide from the semi-solid slug. After the die was filled, the plunger continued to pressurize the casting at more than 1000 bar to feed solidification shrinkage. The combination of controlled die filling and a high intensification pressure eliminated porosity from the castings. Careful choice of die design, casting parameters, and die and plunger lubricants avoided detrimental blistering during T6 heat treating. A functional test measured the fatigue life of actual impellers, and results show that the fatigue life of the semi-solid impellers is similar to that of the forged and machined parts.

Wallace G.,Aluminum Complex Components Inc. | Jackson A.P.,Aluminum Complex Components Inc. | Midson S.P.,Midson Group Inc. | Zhu Q.,Cummins
Transactions of Nonferrous Metals Society of China (English Edition) | Year: 2010

The thixocasting process was chosen to produce the impellers as it is capable of producing castings with extremely high internal quality. Slugs cut from the DC cast bars are re-heated to the semi-solid casting temperature, and a specially-designed runner and gating system is used to prevent oxide from the surface of the slugs from becoming incorporated into the impeller. The technology used to produce the semi-solid impellers is described in detail. The semi-solid cast impellers, produced from an Al-Si-Mg-Cu alloy, are heat treated to the T6 temper. Results from testing are presented demonstrating that the impellers are free of porosity and other internal defects. Both mechanical property and fatigue data are presented showing that the semi-solid impellers have better properties than impellers produced by conventional casting and similar properties to forged and machined impellers. A short study is also described which identified suitable processing parameters to minimize hot tearing in the complex-shaped turbocharger impellers semi-solid cast from alloy 201. The surfaces of the impellers were examined using penetrant testing, and the results of modifying processing parameters on the propensity for surface cracking are presented. An aging study was performed to identify optimum mechanical strength. © 2010 The Nonferrous Metals Society of China.

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