Study of burr formation and phase transformation during micro-milling of NiTi alloys

Abstract

Micro-milling can be defined as milling with end mills smaller than 1 mm of diameter. The top-down approach from milling to micro-milling is often used to define cutting conditions. Unfortunately geometries either for the active part or the overall shape are quite different from conventional tools, leading to inexistent problems at the macro-scale, such as a larger cutting edge radius to uncut chip thickness ratio leading to ploughing effect. Moreover, micro-milling can be used on particular material such as shape memory alloys in biomedical domain which are difficult to machine. This study focuses on burr formation during shoulder milling for two biocompatible NiTi alloys: a martensitic NiTi (shape memory effect) and an austenitic one (pseudo-elasticity effect). Design of experiment is used to highlight the influence of various parameters (cutting parameters and material phases) on the burr formation in micro-milling NiTi alloys. Burrs were observed and measured using confocal, optical and electronic microscopy and tend to be as large as shoulders dimensions. Material phase transformation was also examined. Analysis of variance emphasizes that the larger the feed per tooth and the smaller the width of cut are, the smaller the top burr is. Cutting strategy leads to different burr shape: up-milling burrs have a large curvature, whereas down-milling burrs are slightly bent. An affected layer of about 10 μm has been observed for the austenitic NiTi. The proposed experimental approach give the opportunity to study burr formation in micro-milling, the machinability of alloys or superelastic NiTi shape memory and a qualitative explanation of burr formation has been developed.