Mechanical and microstructural properties of a nickel-titanium alloy at elevated temperatures relevant to the dieless drawing process

The traditional process of metallic wire and rod production pulls a wire/rod through a die to reduce its diameter in successive steps. The considerable die wear at the tool-die interface coupled with the additional lubrication and pre-cleaning costs add considerably to the overall cost of the process. The current research focused on a process known as dieless drawing; an innovative, non-contact metal forming process in which the cross-sectional area of the work piece was reduced through plastic deformation using elevated temperature and tensile forces. The effect of dieless drawing on Nickel Titanium (NiTi) rods was investigated by varying the critical process parameters of temperature, cooling rate, drawing velocity, and heater/cooler velocity. The rods were dieless drawn with a maximum steady state reduction in cross-sectional area of up to 54 % per pass. The thermal and mechanical loading profiles of the rod during processing were investigated. The resultant rod grain structure was observed to be highly deformed in the drawing direction. Hardness, DSC, EBSD and tensile testing results have shown the processed NiTi rod material to be attractive for shape memory and superelastic applications, as well as an improvement in its suitability for subsequent cold working. High temperature tensile tests of NiTi specimens determined the material true stress-true strain within a controlled atmosphere and a temperature range 700 – 900 oC at cross head velocities of 6 – 15 mm/min. NiTi exhibits very large strains at these test conditions and demonstrates a peak stress during yielding of the material between 1 – 5 % strain. The true stress-true strain data was used to define a simplified model of the material at elevated temperatures within an FE model of the dieless drawing process, which accurately simulated the thermal behaviour of the process and reduction in area of the workpiece.