Coldworking process for titanium?
Q:How does the coldworking process for titanium work, what has to be done to make cold working possible?
A:Generally, titanium alloys are marked by poor drawability, which is susceptible of improving by hot forming. Usually, both titanium blanks and dies are warmed up. As a result lower spring-back occurs and consequently it is possible to obtain better dimensional accuracy of the drawn-parts.
Unfortunately, elevated temperature processing must be used under special conditions (vacuum or protective atmosphere) in order to avoid gas diffusion into titanium because of its reactive nature. Titanium’s susceptibility to oxygen, nitrogen, and hydrogen causes titanium to become more brittle.
Subsequently, cold working methods are more often encountered in small industrial processing that hot ones.
Titanium can be shaped by cutting, bending, stamping, spinning and hydraulic bulging. The sheets applying for the drawn-parts should have the proper plastic properties (annealed state) and microstructure.
CP titanium grade 1 has the best drawability. The harder CP titanium grades require greater corner radius and higher forces for shaping. Additionally, forming speed should be kept as low as possible, thus good results are obtained on hydraulic presses. Sometimes, during cold stamping processes, it is necessary to apply intermediate annealing. In order to remove internal stresses the final products must undergo stress relief annealing. Both intermediate and stress relief annealing should be carried out with the protective atmosphere.
The galling tendency of titanium is another important problem during stamping. Galling marks the surface of the titanium drawn-parts causing local scratches and dents. The „build-ups” phenomenon can be limited or even completely eliminated by the application of technological lubricants and antiadhesive coatings on the tools.
Additionally, lubrication limits direct contact between the deformed material and the tool, so eliminates the possibility of “build-ups” creation on the tool – the main reason of adhesive wear.