Using machining force feedback to quantify grain size in beta titanium

Daniel Suárez Fernández,M Jackson,P Crawforth,K Fox,B.P Wynne

doi:10.1016/j.mtla.2020.100856

Abstract

The fluctuating forces on the cutting tool generated during machining of β processed Ti-17 alloy are shown to contain sufficient information to enable measurement of β grain size to an equivalent accuracy of standard etching methods. Three orthogonal forces were gathered, cutting force tangential to the rotation, the force in the feed (radial) direction, and the normal force in the longitudinal axis. Each individual force produced a microstructure image with a high level of contrast but in some cases did not fully highlight all features as shown in the optical image of the equivalent area. By normalising and combining the three forces into a vector, followed by noise reduction, a high-resolution image with sufficient detail to undertake grain size measurements using the linear intercept was produced. The measured grain size differed by no more than 5% with respect the grain size measured in the etched micrograph. It is believed that the forces which have a higher proportion of elastic response in their total values, i.e., the feed and normal forces, produced the higher contrast images, indicating that elastic stresses produce the highest contrast between grains and plastic strains smear out the grain to grain variation.

Highlights

In the titanium manufacturing supply chain, when supplying material for the aerospace industry, extensive and tedious materials testing is performed at intermediate stages of manufacture to assess the quality of the material before being sent for further downstream processing
This work investigates the potential of measuring grain size in beta titanium alloys using the forces generated during machining, which if feasible could lead to a higher level of confidence in the quality of the final product as well as significant productivity improvements during manufacturing
This paper reports on the further development of the concepts presented above, describing how cutting force measurements during machining can be used to characterise microstructure, in particular grain size in polycrystalline material, as long as the machining interaction volume is less than the effective structural unit size

Summary

Introduction

In the titanium manufacturing supply chain, when supplying material for the aerospace industry, extensive and tedious materials testing is performed at intermediate stages of manufacture to assess the quality of the material before being sent for further downstream processing. These tests, such as chemical etching for grain size analysis, are extremely time and resource consuming, leading to only a few selected batches being selected for quality assessment. Finite element modelling (FEM) has shown that cogging of Ti-64 ingot generates strains as high as 2 near the surface, decreasing to about 1 in the centre, directly impacting β grain size distribution and the fraction of globularised α [1]. Crystallographic texture has been shown to vary throughout the cross-section of Ti-834 billet using both electron backscattered and neutron diffraction [2]

Methods

Findings

Discussion

Conclusion