The Effect of Active Phase of the Work Material on Machining Performance of a NiTi Shape Memory Alloy
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INTRODUCTION
NEAR-EQUIATOMIC NiTi shape memory alloys (SMAs) undergo a reversible martensitic phase transformation between a cubic (B2) austenite phase and a monoclinic (B19¢) martensite phase.[1] This first-order phase transformation gives SMAs their unique functional capabilities, exemplified by two behaviors: the shape memory effect (temperature-induced phase transformation) and superelasticity (stress-induced phase transformation).[1] Both the behaviors have been widely exploited in a range of applications in response to aerospace, biomedical, and industrial needs, among others.[2,3] However, one of the issues impeding greater acceptance of SMA technology is the need for improvements in manufacturing and fabrication techniques.[3] To develop bulk actuators, for instance, the major fabrication methods still include conventional machining processes including turning, milling, and drilling.[4] However, shape memory alloys are very sensitive to temperature changes near the transformation regime, and conventional NiTi alloys have transformation temperatures generally within ±100 C of room temperature. Consequently, the heat generated during the YUSUF KAYNAK, Assistant Professor, is with the Department of Mechanical Engineering, Faculty of Technology, Marmara University, Goztepe Campus, Kadikoy, 34722, Istanbul, Turkey. Contact e-mails: [email protected], [email protected] HALUK E. KARACA, Associate Professor, is with the Department of Mechanical Engineering, College of Engineering, University of Kentucky, Lexington, KY 40506. RONALD D. NOEBE, Materials Research Engineer, is with the Structures & Materials Division, NASA Glenn Research Center, Cleveland, OH 44135. I.S. JAWAHIR, Professor, is with the Institute for Sustainable Manufacturing (ISM), University of Kentucky, and also with Department of Mechanical Engineering, College of Engineering, University of Kentucky. Manuscript submitted April 30, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS A
cutting process could result in active transformation of the martensite phase to austenite, with each phase having different machining characteristics.[5] Thus, the machinability of NiTi alloys will change during the cutting process, and the machining process, in general, can be unpredictable. In addition, NiTi shape memory alloys are categorized as difficult-to-machine materials due to their high work hardening, relatively low thermal conductivity, superelasticity, and ductility.[6,7] Compared to other difficult-to-machine materials such as Inconel 718 and Ti-6Al-4V, studies on machining performance of NiTi shape memory alloys are very limited,[5] and more detailed research efforts focusing on understanding the fundamentals of conventional machining of this alloy are required. Previous studies[6,7] on machining of NiTi shape memory alloys show that the effects of cutting parameters on machining performance was relatively limited. Unfortunately, previous studies on the machining of shape memory alloys did not consider temperature as a factor, although the temperature
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