Experimental study on dieless drawing of Nickel–Titanium alloy

Abstract

The effect of a dieless drawing process on commercial grade Nickel–Titanium rods, of 5 mm diameter, was investigated by varying the established critical process parameters of temperature, cooling rate, drawing velocity, and heating/cooling velocity. The rods were successfully dieless drawn with a maximum steady state reduction in cross-sectional area of 54%. The thermal and mechanical loading profiles of the rod during processing, and the resulting changes in microstructure and hardness, have been investigated. Uniform levels of stress and strain resulted in uniform reduction of the rod cross-sectional area. The grain structure was highly deformed in the drawing direction and increased porosity was observed as a result of the process. The longitudinal section hardness of the rod was significantly reduced as a result of the dieless drawing process. Any failures that arose were due to discontinuities within the material microstructure caused by a high necking rate, shorter exposure time to the process temperature and low heating and cooling rates. A uniform oxidation layer was observed on the surface of the processed rods as a result of processing in atmospheric conditions. This oxidation layer has the potential to aid in the lubrication of subsequent cold working operations of the dieless drawn rods. Coupling the thermomechanical effects of the dieless drawing process with a cold drawing processing step has the potential to produce a NiTi wire in fewer passes, and therefore at a reduced cost.