SELECTION OF CUTTING TOOL MATERIAL FOR MACHINING TITANIUM ALLOYS

SCIENCE AND INNOVATION INTERNATIONAL SCIENTIFIC JOURNAL VOLUME 4 ISSUE 10 OCTOBER 2025 ISSN: 2181-3337 | SCIENTISTS.UZ 100 SELECTION OF CUTTING TOOL MATERIAL FOR MACHINING TITANIUM ALLOYS I.O. Kamolova1, R.Kh. Saydakhmedov2 Tashkent State Transport University, Department of Aviation Engineering, Assistant1 Tashkent State Transport University, Department of Aviation Engineering, Doctor of Technical Sciences, Professor2 https://doi.org/10.5281/zenodo.17518529 Abstract. The criteria for selecting tool materials for processing titanium alloys can vary and are interrelated. This article analyzes the main factors and criteria for selecting cutting tool materials for processing titanium alloys. The goal of this work is to determine the optimal tool material that provides high durability and efficiency in the cutting process. The practical significance of this research lies in justifying the selection of WC-group hard alloys with a cobalt content of 6-8% as the most suitable for processing titanium alloys. Keywords: cutting tool, titanium alloys, cemented carbides (WC–Cо), hardness, strength, wear resistance, work hardening, chemical affinity, plastic deformation, heat resistance. Introduction. In the aerospace industry, components made of various titanium alloys – such as VT22, VT3, VT5, VT6, and OT-4 – as well as complex alloyed steels with extremely high strength, heat resistance, and creep resistance, are being increasingly used [1]. The widespread application of titanium alloys is attributed to a combination of their physical, mechanical, and chemical properties, including low specific weight, high specific strength, heat resistance (up to 500 °C), corrosion resistance in air and seawater, and chemical inertness toward many organic and inorganic acids and alkalis [1, 2]. Although modern structural materials demonstrate improved long-term strength at elevated temperatures and maintain short-term strength without a significant loss of toughness, their enhanced fatigue and corrosion resistance – along with other properties that ensure reliable performance under severe mechanical and thermal loads, including cyclic stresses and aggressive environments – make titanium alloys particularly difficult to machine. Because of their unique combination of physical and mechanical characteristics, complex alloying systems, and phase-structural transformations, titanium alloys can only be effectively machined at considerably lower cutting speeds than conventional structural materials. Due to the low thermal conductivity of titanium, extremely high tool temperatures are generated during machining, particularly in the chip-tool contact zone. While in steel cutting the temperature in this area typically reaches 300-350 °C, under similar conditions the temperature during titanium alloy machining can rise to 1100-1200 °C. Such values far exceed the red-hardness limits of high-speed steels and cemented carbides [3]. In view of the above, the machining of titanium alloys and the selection of cutting tools, as well as cutting parameters, remain the main tasks in improving the productivity of mechanical processing of aircraft components. When developing a technological process based on the physical and mechanical characteristics of the material to be machined, it is first necessary to select an appropriate group of tool materials. At the same time, possible changes in the cutting process conditions – such as work hardening, tool wear, an increase in temperature in the cutting zone, and others should be taken into account. SCIENCE AND INNOVATION INTERNATIONAL SCIENTIFIC JOURNAL VOLUME 4 ISSUE 10 OCTOBER 2025 ISSN: 2181-3337 | SCIENTISTS.UZ 101 Therefore, this article considers the main requirements for selecting the material of the cutting tool when machining titanium alloys, as well as analyzes the key factors affecting its performance under changing cutting conditions. Methods of research. 1. Criterion – high hardness exceeding that of the workpiece material. The requirements imposed on tool materials are determined by the conditions in which the contact surfaces of the tool operate during the cutting process. In order to prevent deformation of the cutting edge during metal layer removal, the tool material must possess high hardness that significantly exceeds the hardness of the workpiece material – by more than 3-4 times [4]. Hardness is defined as the property of a material to resist the penetration of another, harder body into its surface. Hardness is indirectly related to the strength of the tool material. The ratio of the contact hardnesses of the tool and workpiece materials determines the wear resistance of the tool, especially under high-temperature conditions accompanying the cutting process, as well as during work hardening and the increase in hardness of the material being cut [5-6]. The wear of the cutting tool depends on the ratio of the hardnesses of the tool and the workpiece during the cutting process, when both the tool and the workpiece are heated and their hardness changes. The tool operates normally as long as its hardness significantly exceeds that of the workpiece. If, with an increase in the hardness of the tool material, its mechanical strength was preserved, then an increase in the ratio of the tool hardness to the workpiece hardness (H₁/H) would unambiguously indicate an improvement in the cutting properties of the tool material. However, an increase in the hardness H1 of the tool material is accompanied by greater brittleness of the cutting edge. When the hardness of the tool approaches that of the workpiece or only slightly exceeds it, the cutting edge becomes dull, making further operation impossible. Metal cutting is accompanied by the formation of a complex and interrelated set of kinematic and physico-chemical factors, among which plastic deformation and work hardening play a decisive role. During the cutting process, not only does the hardness of the working part of the cutting tool change, but also the hardness of the workpiece in the area where the chip is formed (work hardening) [7]. The main characteristic that determines the cutting properties of a tool material is primarily the optimal ratio of the hardness of the tool material to that of the workpiece material [4]: 𝐻1 Н≥ 3 − 4 (1) where H1 is the hardness of the tool material, and Н is the hardness of the workpiece material. The greater the H1/H ratio between the hardness of the tool and the workpiece materials, the lower the wear and the higher the tool life. A decrease in the ratio of contact hardness increases the rate of tool wear, which should be taken into account when selecting the grade of tool material. During cutting, the maximum microhardness of the boundary contact layer exceeds the microhardness of the original material by 2-2.5 times [8]. The cutting tool wears not due to the material’s initial properties, but due to the properties acquired during the cutting process as a result of work hardening and high temperatures [9]. By comparing the hardness of the tool material and the workpiece, it is possible to select a tool in advance for which the ratio of contact hardness and strength between the tool and the workpiece will be optimal. According to [10], tool wear mainly depends on the ratio of these parameters at the contact zone. The hardness of the tool material H1 is known or can be determined from reference data. The hardness of the workpiece material H SCIENCE AND INNOVATION INTERNATIONAL SCIENTIFIC JOURNAL VOLUME 4 ISSUE 10 OCTOBER 2025 ISSN: 2181-3337 | SCIENTISTS.UZ 102 should be determined taking into account the degree of work hardening N. Depending on the cutting conditions, for preliminary calculations when machining titanium alloys, the approximate values of N are: N=1.4 for turning and N=1.5 for milling [11]. For example, when turning the VT6 alloy with a hardness of HB=340, the required tool hardness should be selected considering the work hardening effect: H1=340 · 1,4 = 476, which corresponds to approximately 75-77 HRA. Table 1 presents a comparative summary of the properties of titanium alloys and tool materials widely used in mechanical engineering and the aerospace industry. Table 1. Comparative table of hardness and strength of titanium alloys and tool materials [3, 12–14]. Material Hardness HB, kgf/mm² Ultimate tensile strength σв, kgf/mm² Ultimate bending strength σв, kgf/mm² Titanium alloys [3, 13] VT3 260-320 95-115 - VT6 320-360 90-100 - VT5 250-269 70-95 - VT22 335 120-115 - OT-4 200-300 70-90 - VT8 310-350 105-120 - High-speed steels [3, 12] R18 227 237 353 R12 255 187 325 R9 255 200 320 R6M5 255 212 380 Cemented carbides [14] Hardness НRA, kgf/mm² Ultimate tensile strength σв, kgf/mm² Ultimate bending strength σв, kgf/mm² VK3 89-91 59 130-140 VK4 88 - 130 VK6 87-90 73 150-165 VK8 86.5-87.5 79 175-185 T5K10 - - 130 T14K8 89.5 - 115 T30K4 92 - 90 Superhard materials [3] Hardness НV Ultimate tensile strength σв, kgf/mm² Ultimate bending strength σв, kgf/mm² Composite 01 (Elbor RMK01) 3700 - 78,5 Composite 02 (Belbor) 2960 - - Diamond (A) 98700 МПа - 78,5 Diamond–carbide inserts 4000 30-40 80-85 2. Criterion – high mechanical strength. SCIENCE AND INNOVATION INTERNATIONAL SCIENTIFIC JOURNAL VOLUME 4 ISSUE 10 OCTOBER 2025 ISSN: 2181-3337 | SCIENTISTS.UZ 103 Another essential requirement is high mechanical strength. The cutting edge must be able to withstand high pressures without brittle fracture or plastic deformation. Chip formation occurs when the cutting tool applies a force to the material layer being removed that generates stresses exceeding the strength of the workpiece metal. In the regions of the material located farther from the cutting edge, the deformations (changes in shape) are small – up to about 10%. These are mainly elastic and weak plastic deformations that do not destroy the metal structure. However, the final deformation in the zone of contact between the material and the cutting edge can reach several hundred percent. According to [15], the deformation intensity may reach 200-300% or even higher. Results of microscopic studies [9, 16] have shown that the metal grains in the near-surface layer of the chip (close to the cutting edge) are compressed and elongated tens of times more than in the original structure – approximately by a factor of 20-25. Under such plastic deformation, the material near the cutting edge becomes significantly compacted under the action of normal stresses. A network of microcracks forms, which eventually combine into cracks of critical size. The elongated material fibers at the tip of the cutting edge rupture, and the crack length becomes comparable to the thickness of the layer being removed. As a result, the chip separates from the workpiece material. The greater the degree of plastic deformation (ε), the higher the material hardening – tensile strength (σв) and hardness (HB) increase – but ductility (δ) decreases. From Table 2 [17], it can be seen that as deformation increases, the strength properties of titanium alloys rise sharply, while their plastic properties decrease. The yield strength (σт) increases with deformation more rapidly than the ultimate tensile strength (σв); as the degree of deformation grows, the difference (σв – σт) decreases, thereby reducing the alloy’s ability to undergo further plastic deformation. The mechanical properties of titanium alloys change significantly during strain hardening: the ultimate tensile strength (σв) and the conventional yield strength (σ0,2) increase by about 1.5 times, hardness doubles, while the relative elongation (δ) decreases from 25% to 7.5%. At the same time, Young’s modulus (E) remains practically unchanged. Table 2. Properties of titanium alloy in the annealed state and after strain hardening [17] Titanium alloys σв , kgf/mm² σ0,2 , kgf/mm² δ, % Е·106, kgf/mm² НВ, kgf/mm² Annealed 56,2 47,1 25 1,18 88 Workhardened 84,4 79,4 7,5 1,08 250 As hardness and strength increase as a result of strain hardening, the toughness decreases, and the material becomes more prone to brittle fracture (cold brittleness) [18]. During deformation, impurities contribute to the appearance of the upper yield point (first stage). In the second stage, the strain hardening coefficient is significantly higher – approximately by an order of magnitude – than in the first stage. As deformation increases, the temperature in the cutting zone rises, which accelerates the onset of the third stage of hardening, up to the formation of martensite [17, 5]. During cutting, due to the work hardening of the material being machined, its initial hardness cannot fully characterize its machinability, especially since the degree of hardness change depends on the physical and mechanical properties of the metal, the cutting conditions, and the tool geometry [19]. Therefore, the cutting tool must possess high mechanical strength to maintain the SCIENCE AND INNOVATION INTERNATIONAL SCIENTIFIC JOURNAL VOLUME 4 ISSUE 10 OCTOBER 2025 ISSN: 2181-3337 | SCIENTISTS.UZ 104 shape of the cutting edge under heavy loads and to prevent brittle fracture. This requirement is especially critical when machining titanium alloys, which are characterized by significant plastic deformations in the cutting zone. 3. Criterion – no chemical affinity When selecting a tool material, it is necessary to be guided not only by its physical and mechanical properties but also by the chemical composition of these materials in relation to the workpiece material. In the theory of metal cutting [15], there is a rule stating that for effective machining, the tool and workpiece materials should not have chemical affinity. For example, when machining titanium alloys, it is recommended to use tools made of single-carbide cemented carbides (such as the VK group). Double-carbide and triple-carbide grades (TK group – WC-TiCCo, and TTK group – WC-TiC-TaC-Co) should not be used for machining titanium alloys due to the presence of titanium in their composition, which creates chemical affinity with the workpiece material. This results in chip adhesion to the cutting edge and causes edge chipping [20–22]. Hartung and Kramer [23] found that tool materials based on WC-Co cemented carbides and polycrystalline diamond are the most effective for machining titanium, since a stable reaction zone is formed between the tool and the chip. The carbon contained in WC-Co composites or diamond reacts with the titanium of the workpiece to form titanium carbide (TiC). This reaction layer exhibits high resistance to deformation at cutting temperatures and adheres strongly to both the tool and the chip. It quickly becomes saturated, which limits the diffusion of tool components to the surface, thereby reducing the wear rate. However, this apparently contradicts the fact that titanium carbide coatings applied to tools by chemical vapor deposition (CVD) are not effective in reducing wear. For example, in studies [23-25], coated cemented carbide tools (with coatings such as TiC, TiCN, TiN-TiC, Al2O3-TiC, TiN-Ti(C, N)-TiC, Al2O3, HfN and TiB2) exhibited a higher degree of wear compared to conventional uncoated WC-Co cemented carbides. Fig. 1. Standard free energies of formation (ΔG0f) for carbides under standard conditions [29]. Freeman [26] found that WC-Co grades demonstrate superior performance characteristics regardless of the prevailing wear mechanism. Dearnley, Grearson and Aucote [27, 28], carrying out many trials involving various tool materials in the continuous turning of Ti-6Al-4V, also confirmed the WC-Co grade carbides as the best choice. They suggested that those WC-Co alloys with Co contents of 6 wt% and a medium WC grain size (about 0.8 and 1.4 ~tm) gave the optimum SCIENCE AND INNOVATION INTERNATIONAL SCIENTIFIC JOURNAL VOLUME 4 ISSUE 10 OCTOBER 2025 ISSN: 2181-3337 | SCIENTISTS.UZ 105 performance. The authors [22, 29], in their studies on selecting coatings for tool materials intended for machining titanium alloys, used a thermodynamic criterion based on the evaluation of adhesive interaction between the tool and the workpiece. An analysis of the Gibbs free energies for the formation of carbides, nitrides, carbonitrides, carbo-oxynitrides, and oxides showed that, from a thermodynamic perspective, the most suitable tool coating is tungsten carbide (WC). This is due to its lowest free energy of formation, which indicates low adhesion to titanium alloys and, consequently, improved tool life during cutting (Fig. 1). Conclusion. As a result of the conducted analysis of the criteria for tool materials in the mechanical machining of titanium alloys, it has been determined that cemented carbides of the VK group (WCCo) best meet these requirements. They combine high hardness, mechanical strength, and heat resistance, and also exhibit a low level of chemical interaction with titanium. An optimal cobalt content in the range of 6-8% ensures the necessary combination of properties – sufficient binder strength while maintaining high hardness of the carbide phase – which guarantees tool durability and stability of cutting properties when machining titanium alloys. 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