Year of Publication

2013

Degree Name

Doctor of Philosophy (PhD)

Document Type

Doctoral Dissertation

College

Engineering

Department

Mechanical Engineering

First Advisor

Dr. I.S. Jawahir

Abstract

NiTi alloys have been the focus of Shape Memory Alloys (SMA) research and applications due their excellent ductility and shape memory properties, and these alloys have been extensively used in automotive, aerospace, and in biomedical applications.

The effects of machining on the surface integrity and the corresponding material and mechanical properties of alloys can be best studied by utilizing NiTi alloys as workpiece material since their physical and mechanical properties are highly microstructure dependent. However, due to very poor machining performance of NiTi shape memory alloys, no comprehensive or systematic investigation on this topic has been conducted by researchers as yet.

The current study makes a substantial and unique contribution to this area by making the first and significant contribution to studies on machining performance of NiTi shape memory alloys, and by achieving improved surface integrity and machining performance using cryogenic applications, which give significant reductions of tool-wear, cutting forces, and surface roughness. The influence of machining process conditions, including dry, MQL, preheated, cryogenic machining, and the effects of prefroze cryo machining on surface integrity characteristics such as microhardness, phase transformation, phase transformation temperature, depth of plastically deformed layer have been examined extensively, and unique findings have been obtained.

The effects of machining process conditions, in particular preheated and cryogenic machining conditions, on thermo-mechanical and shape memory characteristics were identified through thermal cycling and stress-strain tests.

For the first time, orthogonal cutting of NiTi shape memory alloys has been carried out in this study to investigate surface integrity comprehensively. Surface integrity and machining performance are compared for dry and prefroze cryogenic cooling conditions under a wide range of cutting speeds. Stress-induced martensitic phase transformation and deformation twinning were found in prefroze cryogenic and dry cutting conditions respectively.

The existing microstructure-based constitutive models were used and modified to predict machining-induced phase transformation and resulting volume fraction. The modified model was implemented in commercial FEM software (DEFORM-2D) as a customized user subroutine. The obtained results from simulation and orthogonal cutting tests were compared considering martensitic volume fraction during cutting with various cutting speeds. The model captured the experimental trend of volume fraction induced by various cutting speeds and process variables. Overall, FEM simulation of cutting process of NiTi was successfully presented.

Available for download on Thursday, June 25, 2020

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