In recent years, the demand and the performance of aircraft have been increasing with the continuous development of the air cargo, passenger transport, and military industries [
1]. Airplanes with higher output efficiencies, better mobility, lower weights, and better fuel utilizations that are greener, more environmentally friendly, and safer are becoming increasingly favored by many, and these demands will continue to promote the application of new materials in aircraft. Titanium alloys are the best choice because of their light weights, high strengths, corrosion resistance, and high-temperature resistance. Titanium alloys have higher strengths than aluminum alloys. Titanium alloys can effectively reduce the weight of aircraft compared with steel [
2,
3,
4]. However, the excellent performance of the titanium alloys also causes difficulty in its machining. For example, when machining the Ti6Al4V alloy, which is most commonly used in the aviation field, at a high cutting speed, the maximum temperature between the chip and the tool can reach 1000 °C, and cold work hardening easily occurs during machining. This result causes several problems, such as difficulty in improving the material removal rate and insufficient tool life. Cutting titanium alloy with different material tools has attracted wide attention of scholars. As the main tool material used for cutting Ti6Al4V parts, cemented carbide has always been a popular research topic in the aviation industry.
In early research, scholars tried to use tools made from different materials to cut Ti6Al4V. Abbasi et al. used polycrystalline diamond (PCD) tools to mill Ti6Al4V; the results show that the homogeneity of the Ti6Al4V microstructure and the cutting temperature affect the performance of PCD tools. Uniform microstructures and lower cutting temperatures can prolong the cutting life of PCD tools [
5]. Su et al. used PCD tools and polycrystalline cubic boron nitride (PCBN) tools during high-speed milling of a titanium alloy. Analysis of a tool flank using an energy dispersive spectrometer (EDS) shows that PCD tools have longer cutting lives than PCBN tools. Especially during high-speed cutting, PCBN tools experience oxidation wear during machining, which seriously affects the tool life [
6]. Zareena et al. obtained a higher surface machining quality using single-crystal diamond tools for ultra-precision machining of titanium alloys. They found that using a perfluoropolyether (PFPE) coating on the substrate of single-crystal diamond (MCD) tools can effectively reduce material adhesion and friction and further improve the surface processing quality and tool life [
7]. Zhang et al. used Si3N4 ceramic tools to mill titanium alloys at high speeds and analyzed the machinability of the tool flank face wear. The results showed that tool failure occurred primarily because of wear. They also obtained a better surface processing quality, which could be used during machining of titanium alloys [
8].
Although scholars have fully analyzed the performance of different tool materials during the machining of titanium alloys, cemented carbide tools are still the primary cutting tools. According to chemical composition and use characteristics, cemented carbide can be divided into four categories: tungsten cobalt (WC+Co), tungsten titanium cobalt (WC+TiC+Co), rare metal carbide (WC+TiC+TaC/NbC+Co), and titanium carbide-based (WC+TiC+Ni+Mo) cemented carbide. Li et al. used titanium carbide-based cutting tools to cut titanium alloys, finding that diffusion occurred in the tool wear region, thereby reducing the cutting life [
9]. Li et al. observed the diffusion phenomenon when cutting Ti6Al4V with tungsten–cobalt–titanium-cemented carbide. The tool had serious adhesion, diffusion, and oxidation wear. Under high-temperature conditions, W and Co elements penetrate into Ti6Al4V, while Ti elements penetrate in the cemented carbide direction. Element diffusion reduces the tool hardness and the tool life, but tungsten–cobalt cemented carbide tools have lower diffusion rates than titanium-based cemented carbide tools. It can effectively ensure the wear resistance of the cutting tool [
10,
11,
12]. At the same time, a similar phenomenon was found in Yang’s research [
13]. Kuczmaszewski et al. analyzed the influence of the WC grain size of tungsten cemented carbide on the cutting performance when cutting a titanium alloy. Compared with coarse-grain and fine-grain bars, tools made of ultra-fine-grain bars have excellent wear resistance and better surface processing quality [
14]. Liu et al. analyzed the effect of tungsten–cobalt carbide on the cutting performance of Ti6Al4V under different flank wear and cooling conditions, and found that the element diffusion increased with the increase in flank wear. Compared with dry cutting and cutting under coolant conditions, a low-temperature auxiliary chip can effectively improve tool life and inhibit element diffusion [
15]. Olander et al. carried out Ti6Al4V turning test with tungsten–cobalt carbide inserts, and detected the wear area. It was found that the C element in WC grains was seriously lost, forming a W rich zone [
16]. Lindvall et al. also conducted Ti6Al4V turning test, and found no W rich area in the wear area, but found the existence of W element on the machined surface. At the same time, the C poor WC grain interacted with Co element to form Co
3W, and the Co rich area at the tool-chip contact interface formed TiCo
2, which can effectively delay the further occurrence of element diffusion [
17]. Wang et al. found that the addition of rare metal carbides, such as TaC, NbC, Cr3C2, and VC, in ultra-fine-grain cemented carbide materials can improve the density, inhibit abnormal grain growth, effectively improve the high-temperature hardness and strength of the tools, and reduce tool wear during titanium alloy cutting [
18]. Zhou et al. prepared ultra-fine gradient cemented carbide tools with different gradient layer thicknesses by adjusting the Co and Ti contents; they emphasized that Co
10Ti
3 substrate material performed best during high-speed titanium alloy milling [
19].
In this study, the influence of diffusion wear on the life of Ti6Al4V cutting tools is analyzed in detail, which provides guidance for the design of tool life extension. The solubilities of W, Co, and Ti elements at different temperatures were first obtained through calculations, and the trends of diffusion between elements as functions of temperature were analyzed. Second, an experiment was conducted to analyze the diffusion trends for the WC grain size and the Co element content at different temperatures and different holding times to provide guidance for the selection of cemented carbide materials to inhibit diffusion. Finally, solid carbide endmills were prepared for a cutting experiment to verify the diffusion trends, and the results can provide guidance for the selection of tool materials for Ti6Al4V cutting.