Experimental investigation of the effects of super-elasticity on the machinability of NiTi alloys

Abstract

Machining of NiTi alloys is a challenging task owing to their inherent material properties and unique phase transformation-based behaviors. In this study, the effects of super-elasticity on the machinability of room-temperature austenitic NiTi alloy were investigated through turning experiments conducted at various cutting speeds under the non-preheating and preheating conditions. As a result, the workpiece exhibited an unavoidable super-elastic shape recovery and underwent a partial phase transformation because the temperature of the workpiece had not exceeded the martensite desist temperature (Md) during machining under the non-preheating condition. The super-elastic recovery of the workpiece resulted in the deterioration of the machinability, through the decrease in the dimension accuracy, increase in the cutting resistance, and shortening of the tool life. The deterioration of the machinability caused by super-elasticity was intensified with the increase in the cutting speed. However, an excessively low cutting speed of 10 m/min induced severe built-up edge deposition on the machined surface. When the workpiece was preheated to a temperature above the Md, the dimension accuracy was enhanced, the cutting resistance decreased and the tool life was prolonged owing to the elimination of the effects of the super-elasticity. Therefore, machining at moderate cutting speeds in the range of 25 to 50 m/min after preheating the workpiece to a temperature above the Md can improves the machinability of room-temperature austenitic NiTi alloy.