1. Introduction
Titanium and its alloys are not only vital structural materials used in the manufacturing of aircraft, missiles, rockets, and other aerospace systems but have also found extensive applications in mechanical engineering, marine engineering, biomedical fields, and chemical industries. For instance, both stainless steel valves and titanium valves are commonly used in acidic environments. However, titanium valves demonstrate a longer service life due to their superior corrosion resistance and durability.
The addition of alloying elements to titanium significantly enhances its strength, with the ultimate tensile strength (σb) increasing from approximately 350 MPa to over 1200 MPa. This improvement makes titanium alloys highly valuable for industrial applications. Generally, titanium alloys are classified into three categories based on their microstructure: α-titanium alloys (denoted as TA), β-titanium alloys, and (α+β)-titanium alloys (denoted as TC). Among these, α and (α+β) titanium alloys are most frequently used. However, titanium alloys are known for their poor machinability, which presents significant challenges in practical applications. Based on years of production experience, this paper explores the machinability of titanium alloys and provides practical tools and recommendations for better machining performance.
2. Research on Machinability of Titanium Alloys
If the machinability of No. 45 steel is considered 100%, then that of titanium alloys ranges between 20% and 40%. While it is less favorable than that of stainless steel, it is slightly better than high-temperature alloys. In terms of machinability, β-type titanium alloys show the lowest performance, followed by (α+β) alloys, while α-type alloys exhibit the best machinability. Under normal conditions, the higher the material hardness and the more alloying elements present, the worse the machinability becomes. When the hardness is below HB300, severe built-up edge formation occurs, while above HB370, machining becomes extremely difficult. Therefore, it is recommended that the hardness of titanium alloy materials should be maintained between HB300 and HB370.
2.1 Cutting Mechanism of Titanium Alloys
(1) Influence of Gas Impurities
Gas impurities such as oxygen, hydrogen, and nitrogen significantly affect the machinability of titanium alloys. As the concentration of these gases increases, the machinability deteriorates. Titanium has a strong affinity for oxygen and can react with it at temperatures above 600°C. At temperatures exceeding 650°C, oxygen diffusion intensifies, forming an embrittlement layer that thickens over time. This leads to the formation of a "alpha layer," resulting in increased hardness and reduced machinability. Similarly, hydrogen diffuses into titanium at high temperatures, causing hydrogen embrittlement, while nitrogen reacts with titanium at elevated temperatures to form brittle titanium nitride (TiN). These impurities not only make the alloy brittle but also accelerate tool wear during machining. Additionally, they contribute to the formation of a harder oxide layer after forging or stamping, further reducing machinability.
(2) Influence of Material Properties
From a machining perspective, titanium's physical and mechanical properties are inherently unfavorable for cutting. Low plasticity, high strength, and low thermal conductivity all contribute to increased cutting forces and temperatures. The low chip shrinkage coefficient of titanium alloys (around 1 compared to 3 for carbon steel) results in small contact areas between the chip and the tool, leading to localized high pressure and temperature. Coupled with titanium’s high reactivity, this causes rapid tool wear and poor surface quality.
(3) Influence of Carbon Content
The machinability of titanium alloys is influenced by carbon content. When carbon content exceeds 0.20%, hard carbides form, reducing machinability. Conversely, lower carbon content improves machinability.
(4) Effect of Work Hardening
Work hardening is a key factor in the difficulty of machining titanium alloys. Although work hardening is less severe than in stainless steel, it still affects the surface hardness. During cutting, high temperatures cause titanium to absorb oxygen and nitrogen, forming a hard, brittle layer. This layer typically extends 0.1–0.15 mm deep and increases hardness by 20–30%.
2.2 Study on Relative Machinable Cutting Conditions
(1) Tool Material
YT cemented carbide inserts are unsuitable for titanium alloy machining due to their affinity with titanium, which leads to tool adhesion and chipping. YG-type inserts, despite their poor wear resistance, are commonly used. YG8 is suitable for roughing, while YG3 is used for finishing. YA6 cemented carbide, with improved wear resistance, is often preferred. High-vanadium and high-cobalt high-speed steels are ideal for low-speed or complex surface machining but are costly and rarely used.
(2) Tool Geometry
The relief angle (α0) of the turning tool is critical when machining titanium. A 15° relief angle is commonly used to balance tool strength and wear resistance. A smaller rake angle (γ0=5°) is recommended to reduce tool wear and improve heat dissipation. The tool tip radius is usually around 0.5 mm, and a positive rake angle (λ=+3°) helps enhance cutting performance.
(3) Effect of Cutting Parameters on Temperature
Increasing cutting speed (v) significantly raises the cutting temperature, while feed rate (f) has a lesser impact. Depth of cut (ap) has minimal effect. To maintain tool life, the cutting temperature should be kept around 800°C. Typical parameters include v=40–60 m/min and f=0.11–0.35 mm/r.
(4) Effect of Cutting Parameters on Surface Roughness
Surface roughness (Ra) is sensitive to cutting parameters. For Ra=1.6 μm, f=0.16 mm/r is optimal. Higher feed rates result in rougher surfaces. For finishing, YG cemented carbide inserts with γ0=10°, α0=15°, and r=0.5 mm are recommended. Increasing the tool tip radius and reducing feed can help achieve smoother surfaces.
3. Cutting and Tool Design for Titanium Alloys
3.1 Turning
Titanium alloys have low elastic modulus (e.g., TC4: E=110 GPa), making them prone to elastic deformation under cutting forces. This reduces workpiece accuracy, requiring a rigid machining system. Tools must be sharp to prevent vibration and friction. Built-up edges may form at low cutting speeds (1–5 mm/min), but they are uncommon under normal conditions. Coolant use is essential to reduce temperature, extend tool life, and avoid fire hazards.
3.2 Drilling
Drilling titanium alloys is challenging due to issues like drill breakage and poor chip evacuation. Proper tool selection and geometry are crucial. For holes larger than 5 mm, YG8 hard alloy drills are preferred, while high-speed steel drills (e.g., M42) are used for smaller diameters. Chip removal and coolant supply are essential to prevent overheating and tool wear.
Example: Drilling a TC4 titanium alloy workpiece using a molybdenum high-speed steel drill (D=6.35 mm, H=12.7 mm) with v=11.6 m/min and f=0.127 mm/r resulted in excellent performance, with 260 holes drilled per drill before dulling.
Titanium and its alloys are not only vital structural materials used in the manufacturing of aircraft, missiles, rockets, and other aerospace systems but have also found extensive applications in mechanical engineering, marine engineering, biomedical fields, and chemical industries. For instance, both stainless steel valves and titanium valves are commonly used in acidic environments. However, titanium valves demonstrate a longer service life due to their superior corrosion resistance and durability.
The addition of alloying elements to titanium significantly enhances its strength, with the ultimate tensile strength (σb) increasing from approximately 350 MPa to over 1200 MPa. This improvement makes titanium alloys highly valuable for industrial applications. Generally, titanium alloys are classified into three categories based on their microstructure: α-titanium alloys (denoted as TA), β-titanium alloys, and (α+β)-titanium alloys (denoted as TC). Among these, α and (α+β) titanium alloys are most frequently used. However, titanium alloys are known for their poor machinability, which presents significant challenges in practical applications. Based on years of production experience, this paper explores the machinability of titanium alloys and provides practical tools and recommendations for better machining performance.
2. Research on Machinability of Titanium Alloys
If the machinability of No. 45 steel is considered 100%, then that of titanium alloys ranges between 20% and 40%. While it is less favorable than that of stainless steel, it is slightly better than high-temperature alloys. In terms of machinability, β-type titanium alloys show the lowest performance, followed by (α+β) alloys, while α-type alloys exhibit the best machinability. Under normal conditions, the higher the material hardness and the more alloying elements present, the worse the machinability becomes. When the hardness is below HB300, severe built-up edge formation occurs, while above HB370, machining becomes extremely difficult. Therefore, it is recommended that the hardness of titanium alloy materials should be maintained between HB300 and HB370.
2.1 Cutting Mechanism of Titanium Alloys
(1) Influence of Gas Impurities
Gas impurities such as oxygen, hydrogen, and nitrogen significantly affect the machinability of titanium alloys. As the concentration of these gases increases, the machinability deteriorates. Titanium has a strong affinity for oxygen and can react with it at temperatures above 600°C. At temperatures exceeding 650°C, oxygen diffusion intensifies, forming an embrittlement layer that thickens over time. This leads to the formation of a "alpha layer," resulting in increased hardness and reduced machinability. Similarly, hydrogen diffuses into titanium at high temperatures, causing hydrogen embrittlement, while nitrogen reacts with titanium at elevated temperatures to form brittle titanium nitride (TiN). These impurities not only make the alloy brittle but also accelerate tool wear during machining. Additionally, they contribute to the formation of a harder oxide layer after forging or stamping, further reducing machinability.
(2) Influence of Material Properties
From a machining perspective, titanium's physical and mechanical properties are inherently unfavorable for cutting. Low plasticity, high strength, and low thermal conductivity all contribute to increased cutting forces and temperatures. The low chip shrinkage coefficient of titanium alloys (around 1 compared to 3 for carbon steel) results in small contact areas between the chip and the tool, leading to localized high pressure and temperature. Coupled with titanium’s high reactivity, this causes rapid tool wear and poor surface quality.
(3) Influence of Carbon Content
The machinability of titanium alloys is influenced by carbon content. When carbon content exceeds 0.20%, hard carbides form, reducing machinability. Conversely, lower carbon content improves machinability.
(4) Effect of Work Hardening
Work hardening is a key factor in the difficulty of machining titanium alloys. Although work hardening is less severe than in stainless steel, it still affects the surface hardness. During cutting, high temperatures cause titanium to absorb oxygen and nitrogen, forming a hard, brittle layer. This layer typically extends 0.1–0.15 mm deep and increases hardness by 20–30%.
2.2 Study on Relative Machinable Cutting Conditions
(1) Tool Material
YT cemented carbide inserts are unsuitable for titanium alloy machining due to their affinity with titanium, which leads to tool adhesion and chipping. YG-type inserts, despite their poor wear resistance, are commonly used. YG8 is suitable for roughing, while YG3 is used for finishing. YA6 cemented carbide, with improved wear resistance, is often preferred. High-vanadium and high-cobalt high-speed steels are ideal for low-speed or complex surface machining but are costly and rarely used.
(2) Tool Geometry
The relief angle (α0) of the turning tool is critical when machining titanium. A 15° relief angle is commonly used to balance tool strength and wear resistance. A smaller rake angle (γ0=5°) is recommended to reduce tool wear and improve heat dissipation. The tool tip radius is usually around 0.5 mm, and a positive rake angle (λ=+3°) helps enhance cutting performance.
(3) Effect of Cutting Parameters on Temperature
Increasing cutting speed (v) significantly raises the cutting temperature, while feed rate (f) has a lesser impact. Depth of cut (ap) has minimal effect. To maintain tool life, the cutting temperature should be kept around 800°C. Typical parameters include v=40–60 m/min and f=0.11–0.35 mm/r.
(4) Effect of Cutting Parameters on Surface Roughness
Surface roughness (Ra) is sensitive to cutting parameters. For Ra=1.6 μm, f=0.16 mm/r is optimal. Higher feed rates result in rougher surfaces. For finishing, YG cemented carbide inserts with γ0=10°, α0=15°, and r=0.5 mm are recommended. Increasing the tool tip radius and reducing feed can help achieve smoother surfaces.
3. Cutting and Tool Design for Titanium Alloys
3.1 Turning
Titanium alloys have low elastic modulus (e.g., TC4: E=110 GPa), making them prone to elastic deformation under cutting forces. This reduces workpiece accuracy, requiring a rigid machining system. Tools must be sharp to prevent vibration and friction. Built-up edges may form at low cutting speeds (1–5 mm/min), but they are uncommon under normal conditions. Coolant use is essential to reduce temperature, extend tool life, and avoid fire hazards.
3.2 Drilling
Drilling titanium alloys is challenging due to issues like drill breakage and poor chip evacuation. Proper tool selection and geometry are crucial. For holes larger than 5 mm, YG8 hard alloy drills are preferred, while high-speed steel drills (e.g., M42) are used for smaller diameters. Chip removal and coolant supply are essential to prevent overheating and tool wear.
Example: Drilling a TC4 titanium alloy workpiece using a molybdenum high-speed steel drill (D=6.35 mm, H=12.7 mm) with v=11.6 m/min and f=0.127 mm/r resulted in excellent performance, with 260 holes drilled per drill before dulling.
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