Preparation and Thermal Stability of Ultra Fine-grained Commercially Pure Titanium Wire

Co-authors DUCHEK Michal, HODEK Josef, DLOUHÝ Jaromír, KÖVÉR Michal
Collaboration: Slovak University of Technology in Bratislava, Faculty of Materials Science and Technology in Trnava, Paulínska 16, 917 24 Trnava, Slovak Republic

Processes based on severe plastic deformation (SPD) that are capable of producing microstructures with sizes of the order of nanometres are gaining importance these days. Their typical limitation is the small volume of material processed. One of available ways to enhancing the productivity is to combine the CONFORM continuous extrusion process with the ECAP method. This paper describes initial experience with this combined process in the CONFORM 315i machine, which is equipped with a specially-designed forming die chamber. In the pilot experiment, the influence of CONFORM set-up parameters on the resulting microstructure of Ti wire upon the first pass was examined. The feedstock was a titanium bar (CP – Ti grade 2) with 10 mm diameter. The crucial parameter of the process is the temperature of the die chamber. It was purposefully varied from the initial temperature of 500°C to the final one of 350°C. An important aspect is cooling, as it takes place immediately upon the exit from the die chamber. Specimens taken from the product were examined using EBSD and their average grain size was measured. The finest grain with the size of 1.4 µm was achieved with the die chamber temperature of 350°C. The strength of the titanium bar upon the first pass was 693 MPa. The yield strength increased from 354 MPa to 620 MPa. On the other hand, the impact toughness declined from 64.2 Jcm-2 to 27.5 Jcm-2. The thermal stability of processed Ti wire was also studied. The deformed UFG material and initial coarse grained titanium were then annealed at a range of temperature (400-600 °C) for up to 6 h.The microstructure evolution and the grain growth behavior were investigated by electron back scattered diffraction (EBDS) technique. The grain growth kinetics was characterized by calculating the grain growth activation energy Q and the time exponent n, based on the experimental results for both materials with different grain size. The value of Q for UFG titanium was found to be lower than the activation energy for raw titanium. Obtained values of Q will be comparing with value reported for coarse grained pure titanium and also with value of self diffusion activation energy of titanium.