Simplified titanium forming for car exhaust production
17 October 2012
Fraunhofer researchers will be presenting an economical forming technology for titanium car exhaust systems at the EuroBlech trade fair (October 23-27, Hannover, Germany).
Efficient metal forming processes such as deep drawing or hydroforming can only be used in a very limited way with titanium as it tends to adhere to the forming tools. The problem is exacerbated by the high temperatures (800°C) at which titanium has to be formed.
In collaboration with his colleagues at the Fraunhofer Institute for Surface Engineering and Thin Films IST in Braunschweig, Germany, André Albert, group leader for media based forming technologies has 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 utilising intermediate heat treatments which required processing at different locations. A process and custom tool has been developed which can withstand temperatures of over 800°C.
“Forming titanium at room temperatures leads to severe cold work hardening of the processed pipe," explains Albert. "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."
The 1.40m x 1.20m forming tool is manufactured from high-performance materials such as nickel-base alloys which remain stable at temperatures over 800°C without oxidising. Meanwhile, a special coating, just a few micrometers thick, prevents the titanium from adhering to the tool.
Martin Weber, a tribological coatings expert at IST says: “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 oxidisation. After extensive testing with various materials, we were able to develop the ideal coating for the special conditions encountered within the various temperature ranges."
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