EFFECTS OF CHILLED AIR AND SOLID LUBRICANTS ON MACHINABILITY OF NICKEL TITANIUM SHAPE MEMORY ALLOY

Abstract

Nickel Titanium shape memory alloys with their unique property of super elasticity and shape memory effect are the frequent material of choice in medical and industrial engineering. These alloys are used in a diverse range of medical applications from surgical stents to orthodontic arch wires and eyeglass frames among others; in industrial engineering, significant utilisation of the material could be found in functional devices such as fasteners and sensors. However, the machinability of the material becomes a major challenge due to its high ductility, strong work hardening, and temperature sensitivity characteristics. Added to this is the tendency of Nickel Titanium alloys to transform from the martensite to the austenite phase at a certain heat level. Austenite phase has a much higher Young’s modulus whereas martensite is softer and more malleable. Cutting fluids are currently the common solution to this problem although there are concerns on their use in terms of health footprint and environmental effects. The purpose of this study was to develop an innovative, cost effective, and environmentally sustainable process as a viable alternative to conventional cutting fluids. This report presents a literature review on the current trends in Nickel Titanium alloy machinability, environment-friendly cutting fluids including the use of chilled air cooling, and solid lubricants in the machining process. A pilot study was conducted to further understand the role of the cooling and lubrication effect. In this initial research, a new approach of utilising chilled air systems to improve the machinability of Nickel Titanium alloys was investigated. The phase transformation temperature of Nickel Titanium was first ascertained with Differential Scanning Calorimetry (DSC). Micro-milling cutting tests were then performed using chilled air, minimum quantity lubricant (MQL), and chilled air applied concurrently with minimum quantity lubricant. Their impact on cutting force, tool wear, surface roughness and burr formation were experimentally studied to compare the effects of the different systems. Results showed that chilled air applied concurrently with minimum quantity lubricant produced lower cutting force, reduced tool wear, and presented improved surface finish on the machined parts due to the effective combination of lubricant and cooling action to the tool-work piece interface which resulted in lower friction. Another experiment was conducted to ascertain the machinability of Nickel Titanium with temperature control and optimising size effect. Based on micro X-ray Diffraction (XRD) analysis results using 0.6 mm x 18 mm spot size, the workpiece after machining processes was determined to be still at the martensite phase for all cutting conditions. The results of the simultaneous use of chilled air and minimum quantity lubricant (MQL) indicated its significant potential in improving microscale machinability of NiTi. Notable reductions were presented in cutting temperature, force, and flank wear while surface quality was enhanced and burr width minimised. In terms of size effect, ploughing-dominated cutting produced consistently undesirable results by way of increased specific cutting force, surface roughness, and burr size while at the same time inducing coating delamination. Milling in different phases was subsequently conducted at macroscale with the results pointing to chilled air significantly lowering workpiece temperature. This kept the workpiece in the martensite phase which subsequently reduced tool wear. By applying chilled air simultaneously with minimum quantity lubricant as well as using finer feed per tooth, surface roughness and tool wear improved by more than 50%. An important finding here is that the machining process was determined as able to alter the thermal gradient for phase transformation. Another test was conducted on the use of graphene and boron nitride nanoparticles to augment minimum quantity lubrication. Resul

Date of Award	1 Aug 2019
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Carl Diver (Supervisor) & Paul Mativenga (Supervisor)