Fraunhofer researchers develop method for cost-effective titanium forming for car exhaust systems

17 October 2012

Fraunhofer researchers are presenting an economical titanium forming technology for car exhaust systems at the EuroBlech trade fair from 23 to 27 October in Hannover, Germany.

In the automotive industry, titanium has only been used for high-end vehicles and motor sport applications up until now. Due to the lack of cost-effective forming technologies for titanium, manifolds, exhaust pipes, catalytic converters and mufflers are currently primarily manufactured from high-alloy stainless steel. However, titanium has a great deal of potential, especially for mass production of exhaust systems. Titanium would not only be lighter—a total weight advantage of 40% can be achieved per component—but also more available. Titanium belongs to the ten most frequently occurring substances in the earth’s crust.

Efficient metal forming processes such as deep drawing or hydroforming can only be used with titanium in a very limited way.

Titanium tends to adhere to the forming tools. This leads to major damage which can cause components to fail in the worst case. This effect is amplified by the extremely high temperatures of up to 800 °C, at which titanium has to be formed.

—André Albert, group leader for media based forming technologies at the Fraunhofer Institute for Machine Tools and Forming Technology IWU

Schematic diagram depicting the forming process for titanium pipes within a process stage. Click to enlarge.

In collaboration with his colleagues at the Fraunhofer Institute for Surface Engineering and Thin Films IST, Albert developed a new technology for hydroforming titanium car exhaust systems at elevated temperatures. This new method enables forming to be undertaken in a single process stage.

Up until now, a minimum of three stages were necessary utilizing intermediate heat treatments which partially required processing at different locations. The scientists have developed a process and custom tool which can withstand temperatures of more than 800 °C.

Forming titanium at room temperatures leads to severe cold work hardening of the processed pipe. In order to prevent cracking, the metal requires frequent treatment by means of recrystallization processes. This leads to extremely complex multi-stage forming processes which are not economically viable in large-volume production of exhaust systems. This microstructural change can be avoided at extremely high temperatures.

—André Albert

The approximately 1.40 x 1.20 meter forming tool is manufactured from high-performance materials such as nickel-base alloys which remain stable at temperatures above 800 °C without oxidizing. A special coating just a few micrometers thick prevents titanium from adhering to the tool, which can lead to component cracking and severe damage to the surface.

At temperatures from approximately 500 °C, titanium exhibits a strong tendency to combine with oxygen and nitrogen from the surrounding atmosphere. For this reason, it is necessary to work with shielding gases at extremely high temperatures, such as argon, in order to prevent oxidization of the titanium. After extensive testing with various materials, we were able to develop the ideal coating for the special conditions encountered within the various temperature ranges.

—Martin Weber, expert for new tribological coatings at IST