This paper shares details about Mitsubishi’s development of a TiAl alloy designed specifically for turbocharger applications (turbine wheels). While some information is dated, the research and development of this alloy was a significant step towards designing modern turbochargers.
A turbocharger is a device used by many gasoline and diesel power vehicles. It recycles pressurized gases coming from a vehicle’s exhaust manifold, using them to spin a turbine and create positive pressure (under load) in a motor. Engines utilizing this technology are more fuel efficient and environmentally friendly (a smaller displacement turbocharged motor can produce more power than a larger naturally aspirated motor). Due to increasing environmental regulations, optimization of turbocharger has been and continues to be a priority for many companies. This paper shares one of the first breakthroughs reducing the weight of rotating parts within a turbo: the easiest method of improving efficiency and turbo response. The breakthrough involves a TiAl alloy which is light, able to be precisely shaped, and able to handle the extreme temperatures within a turbo.
The TiAl turbine was constructed with a precision casting technique (LEVICAST) developed by Daido Steel Co. Ltd.. Hot isostatic pressing (combination of high heat and pressure applied at the same time) was used to ensure there were no casting defects. The result was a fully lamellar structure (thin, alternating layers of the two materials) ideal for the high temperatures experienced by the turbine.
The new alloy demonstrated a lower specific gravity (relative density) value than other materials traditionally used for such a high temperature application. The low relative density translates to good resistance to centrifugal stress (turbine spins at very high rpm). Strength attributes of the alloy are possibly connected to high Nb content. Another benefit of the new alloy was increased resistance to oxidation (corrosion) compared to previous TiAl alloys.
When it comes to assembling the turbo, consideration was taken about the differing expansion between the TiAl turbine and the steel shaft it connects to. Repeated heat cycles would compromise the joint without special measures. These measures included using a joining material with expansion properties similar to the TiAl turbine. Electron beam welding and brazing were used to assemble the unit. A stress free ultrasonic inspection system was used as a quality control check for the final assembly.
A 0.2 second improvement was seen in the time the new turbo took to accomplish a 50kPa pressure. The value seems small, but the acceleration difference can easily be felt by the vehicle’s operator. Tests conducted at 1000 degrees C (operating turbo temperatures are around 900 degrees C) demonstrated the new alloy’s temperature specific strength. Rigorous endurance tests were also performed, with no notable deformations or failures.
Discussion Traditional fabrication methods can continue to be used with this new alloy. It spans the previously problematic gap between heat resistant metal components and ceramic materials (great with heat but difficult to shape). While this particular paper looks at the development of a TD05 turbo from Mitsubishi, the strengths of this alloy make it ideal for many rotational applications.