Evaluation of Machining-Induced Chatter and Part Quality in TiAl Alloys Turning Processes by Means of Harmonics Analysis

Abstract

Titanium alloys have been reported as potential materials for aeronautical and automotive applications due to their interesting mechanical properties, combined with their low density. The manufacturing processes developed for these alloys require finishing machining operations to improve the surface quality of the parts and to meet the desired geometrical tolerances. Nevertheless, titanium aluminides exhibit extremely low machinability in comparison to traditional titanium alloys. The combination of the low thermal diffusivity of these materials and the high chemical affinity and friction coefficient with the cutting tools accelerate tool wear phenomena and lead to a deterioration of the part surface quality. Moreover, the mechanical properties of titanium aluminides contribute to increase the cutting forces which generates tool repulsion resulting in undesirable vibration or chatter phenomena. In this paper, the machining suitability of the turning process of Ti48Al2Cr2Nb titanium aluminide has been evaluated based on the analysis of chatter phenomena and the inspection of the surface roughness and roundness tolerance of the machined part. Experimental turning tests have been carried out by varying the main parameters of the process, cutting speed, feed rate and tool geometry, with the objective of determining the best cutting combination. For this purpose, a harmonic analysis methodology of the roundness profile based on the application of the discrete Fourier transform (DFT) has been employed. This technique has made it possible to isolate the vibration-induced machining effects from the lower frequency defects generated by part bending and to relate them to the surface quality and geometrical accuracy of the machined part.