Titanium compressor impellers
Modern commercial
vehicle turbochargers are subject to very high loads due to the wide range of applications
they are used in. In many cases where there are extreme loads, a compressor impeller
made of an aluminum alloy determines the service life of the turbocharger. In particular,
material fatigue can result from extreme loads, especially when the loads are cyclical
loads occurring at low frequencies. This phenomena is also known as low cycle fatigue
(LCF).
Various measures can be taken to increase the service life. For example, the circumferential
speed of the compressor impeller can be reduced by changing the aerodynamic design
of the impeller or by controlling or reducing the charging pressure. High-strength
aluminum, for example, that manufactured using the HIP technique, is often used
to reduce variations in the strength of the material. The high-end solution using
aluminum consists of milled compressor wheels which were also developed by BorgWarner
Turbo Systems.
The increasing number of engine applications with high cyclical loads as well as
the necessity to have higher charging pressures to remain within the stricter emission
regulations have made an additional innovation step beyond the moulded aluminum
compressor wheels necessary. In order to provide a customer with a technology that
can withstand these loads and also ensure a service life that is just as long as
it would be under standard conditions, BorgWarner Turbo Systems initiated a special
development program.
The demands on the compressor impeller stated above make it necessary to use higher
quality materials since the potential for improvement inherent in aluminum is just
not sufficient anymore. The engineers at BorgWarner Turbo Systems decided to use
a titanium alloy that is not only very hard, but also provides an excellent strength-to-thickness
ratio.
With the titanium compressor impeller, BorgWarner Turbo Systems is now able to offer
different compressor impeller technologies for special application requirements
and special cyclical loads. In this manner the customer can select between cast
or moulded aluminum compressor impellers or between cast or moulded titanium compressor
wheels depending on the area of application.
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