Determination of Cutting Forces and Process Stability in Micro-Milling of Ti6Al4V Alloy by Considering the Size-Effect Phenomenon

S.      M. Afazov; S.      M. Ratchev; J.      Segal

Abstract

This paper is focused on modelling and validation of cutting forces and process stability in micro-milling of Ti6Al4V alloy. The micro-milling cutting forces are determined by a model developed by the authors. The model consists of integrating predicted cutting forces from finite element (FE) model of orthogonal cutting described by non-liner equation with an uncut chip thickness model considering the full kinematics of the cutting tool including the run-out effect. Two material models are incorporated within the FE model of orthogonal cutting to represent the Ti6Al4V flow stress. Johnson Cook (JC) material model is employed to consider the plastic strain, strain rate and temperature effects of the flow stress during cutting. Size-effect (SE) material model is also employed where the flow stress is a function of the plastic strain, strain rate, temperature and strain gradient plasticity. The micro-milling cutting forces determined using both material models are compared with experimentally measured micro-milling cutting forces for 10 cases with cutting depth of 100 µm, feed rates in the range 0.3-12 µm/tooth and spindle speeds of 5, 25 and 50 krpm. It has been observed that the SE material model results with better correlation with the experimentally obtained micro-milling cutting forces for feed rates of 6 µm/tooth and smaller while the JC material model predicts more accurately the micro-milling cutting forces for feed rates of 12 µm/tooth. The micro-milling stability limits are determined for different cutting conditions using the predicted micro-milling cutting forces with the SE material model and incorporated into a chatter model developed by the authors. High depth of cuts and high material removal rate at stable micro-milling can be achieved at high spindle speeds in the ranges 41-46 krpm, 30.5-33 krpm and 24.5-25.5 krpm. Also refering to the obtained stability lobes, high depths of cut at stable cutting can be achieved at lower spindle speeds (e.g. 18 krpm and 15.5 krpm).

Keywords: Chatter, Ti6Al4V, size-effect, process stability, micro-milling, FEA, cutting forces, FE modelling, Modelling of Chatter