Tool life and surface integrity in turning titanium alloy

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Abstract

Titanium alloys have been classified as “difficult-to-machine” materials. Most of the information on the behaviour of titanium alloys and the mechanism involved at the tool–chip interface available are based on the observations taken during the commercial production of titanium components in the aerospace industry. In the present work, uncoated cemented carbide tools were used for the turning of Ti–6Al–2Sn–4Zr–6Mo. The experiments were carried out under dry cutting condition. The cutting speeds selected in the experiment were 100, 75, 60 and 45 m min−1. The depth of cut was kept constant at 2.0 mm. The feed rates used in the experiment were 0.35 and 0.25 mm rev−1. Two types of insert were used in the experiments. Tool wear was measured under optical microscope and tool life for machining titanium alloy of Ti-6246, has been recorded. The results have shown that the inserts with finer grain size have a longer tool life. SEM analysis has been carried out on the worn tools to determine the tools wear mechanisms. Majority of the tool failure mechanisms was due to flank face wear and excessive chipping on the flank edge. The paper also details some aspects of workpiece surface integrity following roughing operation. The surface of titanium alloy is easily damaged during machining operation due to their poor machinability. The machined surface experience microstructure alteration and increment in microhardness on the top white layer of the surface. Machined surfaces have shown severe plastic deformation and hardening after prolonged machining time with worn tools, especially when machining under dry cutting condition.

Introduction

Titanium alloys are extremely difficult to machine material. The machinability of titanium and its alloys is generally considered to be poor owing to several inherent properties of materials. Titanium and titanium alloys have low thermal conductivity and high chemical reactivity with many cutting tool materials. Its low thermal conductivity increases the temperature at the cutting edge of the tool. Hence, on machining, the cutting tools wear off very rapidly due to high cutting temperature and strong adhesion between tool and workpiece material. Additionally, the low modulus of elasticity of titanium alloys and its high strength at elevated temperature further impair its machinability.

Many researchers have studied the machinability of titanium alloys in the past [1], [2], [3], [4], [5], [6]. They suggested that straight cemented carbides (WC–Co) with Co content of 6 wt.% and a WC grain size between 0.8 and 1.4 μm gave the optimum performance. The steel cutting grades or ‘P’ grades of ISO standard of cemented carbides are not suitable for machining titanium alloys due to their thermal properties and the higher wear rate of the mixed carbide grains relative to the WC grains [1], [4], [5]. Machining of titanium alloys at higher cutting speed will cause rapid chipping at the cutting edge which leads to catastrophic failure of the inserts [7]. A higher cutting speed also results in rapid cratering and/or plastic deformation of the cutting edge [1], [2], [5]. This is due to the temperature generated which tends to be concentrated at the cutting edge closer to the nose of the inserts. The heat affected zone is very small when cutting titanium alloys. The smaller heat affected area produced is as a result of the shorter chip/tool contact length. It is mainly for this reason that the cutting speeds are limited to about 45 m min−1 when using straight grade cemented carbides (WC–Co) [1], [6], [8].

Titanium alloys are generally used for a component, which require the greatest reliability, and therefore the surface integrity must be maintained. According to Field and Kahles [9], when machining any component it is essential to satisfy surface integrity requirements. However, during machining and grinding operations, the surface of titanium alloys is easily damaged because of their poor machinability. As far as the surface metallurgy of the machined component is concerned, the heat generated during cutting is a main source of damage, especially in grinding process. Possible surface and subsurface alterations include: plastic deformation, microcracking, phase transformations and residual stress effects. Several studies on surface integrity parameters have been carried out [10], [11], [12], [13]. When machining titanium alloys in an abusive manner an overheated white layer can be produced which resulted in a layer of being softer or harder than the base materials [9].

The aims of this work were to investigate tool wear and surface integrity effects when machining titanium alloy Ti–6Al–2Sn–4Zr–6Mo. This paper will explain various factors and parameters involved when machining titanium alloys with carbide tools.

Section snippets

Workpiece materials

The workpiece materials used in all the experiments was bar of an alpha–beta titanium alloy, Ti–6Al–2Sn–4Zr–6Mo. The nominal compositions of the alloys (in wt.%) are given in Table 1 [14]. The workpiece had a microstructure, which consisted of elongated alpha phase surrounded by fine, dark etching of beta matrix. Titanium alloy, Ti–6Al–2Sn–4Zr–6Mo is a widely used titanium alloy and offers high strength, depth hardenability and elevated temperature properties up to 450°C. The mechanical

Results and discussions

The results are divided into two sections. In the first section, tool life and tool wear data were presented, while in the second section, some workpiece surface integrity aspects are presented on the roughing operation.

Conclusions

The following conclusions are based on the results for turning tests with straight grade cemented carbide tools with chip breaker on titanium alloy Ti-6246 (Ti–6Al–2Sn–4Zr–6Mo):

  • 1.

    Straight grade cemented carbides are suitable for use in machining titanium alloy Ti-6246. The wear resistance and cutting edge strength of insert CNMG 120408-883 are superior to insert CNMG 120408-890 (finer grain size).

  • 2.

    The dominant wear mechanisms for cemented WC–Co tools are dissolution/diffusion and plucking at tool

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