Tool wear characteristics of binderless CBN tools used in high-speed milling of titanium alloys

Abstract

Titanium alloys are difficult-to-cut materials, and the performance of conventional tools is poor when used to machine them. In this paper, a new tool material, which is binderless cubic boron nitride (BCBN), is used for high-speed milling of a widely used titanium alloy Ti–6Al–4V. The performance and the wear mechanism of the tool have been investigated when slot milling this alloy. This type of tool manifests longer tool life at high cutting speeds. Analyses based on the SEM and EDX suggest that adhesion of workpiece, attrition and diffusion–dissolution are the main wear mechanisms of the BCBN tool when used in high-speed milling of Ti–6Al–4V.

Introduction

Titanium alloys have been widely used in the aerospace, biomedical, automotive and petroleum industries because of their good strength-to-weight ratio and superior corrosion resistance. However, it is very difficult to machine them due to their poor machinability. In 1955, Siekmann [1] pointed out that “machining of titanium and its alloys would always be a problem, no matter what techniques are employed to transform this metal into chips”. The poor machinability of titanium and its alloys have led many large companies (for example Rolls-Royce and General Electrics) to invest much in developing techniques to minimize machining cost [2].

Among all titanium alloys, Ti–6Al–4V is most widely used; so it has been chosen as the workpiece material in this study. When machining Ti–6Al–4V, conventional tools wear rapidly because the poor thermal conductivity of titanium alloys results in higher temperature closer to the cutting edge during machining and there exists strong adhesion between the tool and workpiece material [3]. In addition, titanium alloys are generally difficult to machine at cutting speed of over 30 m/min with high-speed steel (HSS) tools, and over 60 m/min with cemented tungsten carbide (WC) tools, resulting in very low productivity [4]. Since the performance of conventional tools is poor when machining Ti–6Al–4V, and with the evolution of a number of new cutting tool materials, advanced tool materials, such as cubic boron nitride (CBN) and polycrystalline diamond, are being considered to achieve high-speed milling. Some of the ultra-hard materials, such as polycrystalline diamond and CBN have been used in machining of titanium alloys [3], [5], [6], [7], [8]. Both polycrystalline diamond and CBN are currently very expensive; in addition, they are highly reactive with titanium alloys, and consequently are not suitable for machining titanium alloys. Zareena [9] had carried out an extensive study that shows that the binderless CBN (BCBN) inserts have a remarkably longer tool life than conventional CBN inserts under all cutting conditions (up to 400 m/min); however, the wear mechanism of BCBN tool is still unknown. In this project, BCBN inserts are selected to investigate its wear characteristics when high-speed milling Ti–6Al–4V.