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Titanium is a commonly used material in various critical applications such as aerospace and biomedical applications. In this article, for the first time in the literature, development and implementation of a novel plasma boronizing process on Tungsten Carbide (WC) cutting tools is introduced. Plasma boronizing on WC tools is performed with gas combination of 10% BF3, 40% Argon and 50% H2 at different temperatures and durations. Performance enhancements of plasma boronized WC tools on Titanium (Ti–6Al–4V) machining are investigated under various cutting conditions. It is found that new plasma boronizing of WC is a very cost effective solution for significantly increasing tool life in Titanium machining.40965
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2010 CIRP.
1. Introduction
Titanium alloys are widely used in advanced engineering applications such as aerospace and biomedical industries due to their excellent corrosion resistance, high strength-to-weight ratio, high strength at elevated temperatures and biological compat-ibility. However, titanium alloys are regarded as extremely difficult to cut. Low thermal conductivity of titanium alloys induces a high amount of heat generation during the cutting operation that causes severe tool wear [1]. In addition, the high chemical affinity of titanium with majority of tool materials at high temperatures gives rise to a strong adhesion of the workpiece to the tool surface, thus leading to chipping and premature tool failure [2]. Due to the stated problems, high tooling costs occur eventually; therefore creating a cost effective cutting tool performance enhancement through coating or tool material comes into prominence. Successful results can be achieved by using tool materials such as cubic boron nitride (CBN) and polycrystalline diamond (PCD) [3–6]; however the high cost of these tools decreases their use in industrial applications. Most coating materials have a tendency to interact with Ti–6Al–4V that provokes build-up edge formation [7]. Early removal of some coatings is a problem in titanium machining. One of the reasons behind the removal of coatings is thought to be thermal expansion coefficient difference between the substrate and the coating layers. Due to this when a high temperature gradient exists, thermal cracking occurs [8]. Some grades of uncoated tungsten carbide tools can be readily used in machining of Ti–6Al–4V [4] and WC tools have lower cost compared to coated tools.
When the current situation is considered, it is observed that there is a need for an economical method of enhancing the WC tool
* Corresponding author.
performance in titanium machining. Plasma boronizing of WC tools is found to be promising method in this area of interest. In this study by employing a novel approach, new boron plasma implemented WC inserts have been produced and the machining tests on Ti–6Al–4V alloy have been carried out. Uncoated WC inserts have been plasma boronized at different conditions. Effects of plasma boronizing conditions are investigated on orthogonal machining and face milling operations. It is found that plasma boronizing process under certain conditions decreases the tool wear significantly and improves tool life almost triple.
2. Plasma and plasma boronizing
Plasmas consist of ionized gases, positive–negative ions and electrons in addition to neutral species. Not only there are fully ionized gases with a 100% ionization degree but also very low ionized gases with ionization degree as low as 10 4 to 10 6.
2.1. Glow discharge plasma mechanism
If the potential energy difference is sufficiently high between two electrodes placed in a gas mixture (Fig. 1), the latter is going to break down into positive ions and electrons, giving rise to a gas discharge. The gas breakdown mechanism can be explained as follows; a few electrons are emitted from the electrodes. If a sufficient potential energy difference is applied between two electrodes, the electric field in front of the cathode accelerates the electrons and the electrons collide with the gas atoms. The most crucial collisions are the inelastic collisions, and this leads to excitation and ionization. The excitation collisions, which then lead to de-excitations with the emission of radiation, called glow discharge. The ionization collisions create new electrons and ions, and afterwards these electrons and ions are accelerated by the electric field toward the cathode, where new electrons by ion