Titanium alloy microstructure fingerprint plots from in-process machining

D Suárez Fernández,M Jackson,P Crawforth,B.P Wynne

doi:10.1016/j.msea.2021.141074

Abstract

Titanium alloy components require several machining stages of forged billets which are supplied in a range of annealing conditions. Generally, the machining performance is influenced by the heat treatment and changes in billet microstructures are often overlooked by tool manufacturers and machinists. Due to the non-linear strain path during primary forging, titanium alloy billets are anisotropic in nature and require ex-situ non-destructive evaluation (NDE) during the manufacturing stages to ensure excellent service performance, particularly in safety-critical aerospace components. In this study, the local analysis of the fluctuations presented in the force response during face-turning operations is directly linked to the billet heat treatment condition and presented as microstructure fingerprint plots. The evolution of cutting forces in four different billet conditions of the alpha + beta titanium alloy Ti–6Al–2Sn–4Zr–6Mo (Ti-6246) was measured. The magnitude and fluctuations in force were directly correlated to microstructural features derived from the heat treatments. In addition, local spatial high-resolution synchronization of the cutting forces was used to determine the effects of microstructure from the heterogeneous upstream forging process and subsequent heat treatment. These rapidly produced microstructure fingerprint plots are an important development step for providing manufacturers with an in-process machining NDE method: this will help to qualify material upstream prior to expensive secondary forging or finish machining stages.

Highlights

Titanium alloys are widely used in the aerospace and biomedical industries because of their high strength-to-weight ratio combined with excellent corrosion resistance at elevated temperatures
The manufacturing rates of titanium alloy com ponents have subsequently increased, in order to try to meet this high demand. This is challenging because titanium alloys are considered very difficult to machine, owing to their low thermal conductivity and high strength, which typically results in high wear rates of cutting tools
It has been demonstrated for the first time that different Ti-6246 billet microstructures – as-forged, mill-annealed, beta-annealed and STA conditions – can be characterised through microstructure fingerprint plots directly from the cutting forces in the three spatial axes

Summary

Introduction

Titanium alloys are widely used in the aerospace and biomedical industries because of their high strength-to-weight ratio combined with excellent corrosion resistance at elevated temperatures. They are mechanically and corrosion compatible with carbon fibre-reinforced polymers (CFRPs) which are being increasingly used in commercial aircraft fuselage and wing structures, as reported by Halford [1]. The manufacturing rates of titanium alloy com ponents have subsequently increased, in order to try to meet this high demand This is challenging because titanium alloys are considered very difficult to machine, owing to their low thermal conductivity and high strength, which typically results in high wear rates of cutting tools. Cedergren et al [3], investigated the effect of Ti–6Al–4V (Ti-64) heat treatment on the chip formation and resultant machining forces

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