1. Introduction
Titanium and its alloys are not only essential structural materials for aerospace applications such as aircraft, missiles, and rockets but have also found widespread use in mechanical, marine, biomedical, and chemical engineering fields. In environments where acidic media are present, both stainless steel valves and titanium valves are employed. However, titanium valves demonstrate a significantly longer service life due to their superior corrosion resistance and durability.
The addition of alloying elements to titanium results in the formation of titanium alloys, which greatly enhance the material’s strength, increasing the tensile strength (σb) from approximately 350–700 MPa to as high as 1200 MPa. This makes titanium alloys highly valuable in industrial applications. These alloys are typically 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 commonly used. However, the poor machinability of titanium alloys presents significant challenges in practical applications. Based on years of production experience, this study explores the relative machinability of titanium alloys and provides practical tool recommendations for readers.
2. Research on the Machinability of Titanium Alloys
When comparing the machinability of No. 45 steel (set at 100%), titanium alloys exhibit a machinability range of about 20–40%. Although this is worse than that of stainless steel, it is slightly better than that of high-temperature alloys. Within titanium alloys, β-type, (α+β)-type, and α-type alloys show progressively improved machinability, with pure titanium having the best performance. Generally, the higher the hardness of the material and the more alloying elements it contains, the worse its machinability. For instance, when the hardness is below HB 300, titanium tends to stick to the tool, while above HB 370, machining becomes extremely difficult. Therefore, it is recommended to maintain the hardness of titanium alloy between HB 300 and 370 for optimal machining conditions.
2.1 Study on the Cutting Mechanism of Titanium Alloys
(1) Influence of Gas Impurities
Gas impurities, particularly oxygen, hydrogen, and nitrogen, significantly affect the machinability of titanium alloys. As the concentration of these gases increases, the material becomes more brittle and harder, leading to increased tool wear. At temperatures above 600°C, titanium reacts strongly with oxygen, forming oxides that diffuse into the metal and create an embrittlement layer. This layer thickens over time, resulting in what is known as the “alpha layer,†further hardening the material. Similarly, hydrogen enhances embrittlement at high temperatures, while nitrogen forms brittle titanium nitride (TiN) at elevated temperatures, worsening machinability.
(2) Influence of Material Properties
From a machinability perspective, titanium alloys have several properties that make cutting challenging. Their low plasticity leads to minimal chip deformation during cutting, resulting in small contact areas between the chip and the tool. Additionally, their high strength limit and poor thermal conductivity (about 1/5 of iron and 1/10 of aluminum) lead to high cutting temperatures, reducing tool life and increasing wear. The high reactivity of titanium also causes welding and adhesion between the tool and the workpiece, further complicating the process.
(3) Influence of Carbon Content
The carbon content in titanium alloys directly affects machinability. When the carbon content exceeds 0.20%, hard carbides form, decreasing machinability. Conversely, lower carbon content improves the material’s ability to be machined effectively.
(4) Effect of Work Hardening
Work hardening is a key factor in the difficulty of machining titanium alloys. While the degree of work hardening is less severe than in stainless steel, it still contributes to surface hardening. During cutting, local high temperatures cause titanium to absorb oxygen and nitrogen, forming a hard and brittle layer on the surface. This layer can reach a depth of 0.1–0.15 mm and increase hardness by 20–30%.
2.2 Study on Relative Machinable Cutting Conditions of Titanium Alloy
(1) Tool Material
When machining titanium alloys, YT cemented carbide inserts should be avoided due to their affinity with titanium, leading to tool adhesion and chipping. Instead, YG-type inserts are commonly used despite their lower wear resistance. For roughing operations, YG8 is preferred, while YG3 is used for finishing. YG6X is often applied for general machining. Fine-grained tungsten-cobalt cemented carbide (YA6) has proven effective, offering better wear resistance and improved performance compared to YG6X. High-vanadium or high-cobalt high-speed steels (e.g., W12Cr4V4Mo and W2Mo9Cr4VCo8) are suitable for low-speed or complex surface cutting but are expensive and rare, so they should be used sparingly.
(2) Tool Geometry
The relief angle (α0) of the turning tool is critical in titanium machining. A relief angle of 15° is generally recommended to reduce tool wear and improve cutting efficiency. The rake angle (γ0) should be small (around 5°) to increase contact area and reduce heat concentration. A small radius (r = 0.5 mm) and positive rake angle (λ = +3°) are also beneficial. However, some studies suggest that a rake angle of 28–30° may optimize tool life, and increasing the tip radius can reduce tool chipping.
(3) Effect of Cutting Parameters on Temperature
Cutting speed (v) has the most significant impact on temperature, followed by feed rate (f), while depth of cut (ap) has the least effect. To maintain a cutting temperature of around 800°C, typical parameters are v = 40–60 m/min and f = 0.11–0.35 mm/r. High speeds can lead to excessive tool wear and surface hardening, so careful selection is essential.
(4) Effect of Cutting Parameters on Surface Roughness
Surface roughness is crucial for titanium parts due to their sensitivity to stress concentrations. To achieve Ra 1.6 μm, a feed rate of 0.16 mm/r is required. Increasing the feed rate to 0.25, 0.35, or 0.45 mm/r results in higher roughness values (Ra 3.2, 6.3, and 12.5 μm, respectively). Finishing operations require sharp tools and optimized parameters to ensure quality surfaces.
3. Titanium Alloy Cutting and Tool Design
3.1 Turning
Titanium alloys have a low elastic modulus (e.g., TC4: E = 110 GPa), making them prone to elastic deformation under cutting forces, which reduces workpiece accuracy. Therefore, the machining system must be rigid, and the workpiece must be securely clamped. The tool must be sharp to avoid vibration and friction, which can shorten tool life and reduce accuracy. Under normal conditions, no built-up edge forms, allowing for good surface quality. However, coolant is essential to reduce temperature and prevent fire hazards. Cutting parameters must be carefully selected to avoid overheating and surface hardening.
3.2 Drilling
Drilling titanium alloys is particularly challenging due to issues like drill breakage and burning. Proper tool selection, chip removal, and cooling are essential. Carbide drills (YG8) are preferred for larger diameters, while high-speed steel (M42 or B201) is suitable for smaller holes. Drill geometry, including apex angles and clearance angles, must be optimized for efficient cutting. Coolant, such as emulsions or specialized oils, is necessary to improve tool life and prevent work hardening. Careful feed control and periodic chip removal are also important to avoid tool damage and ensure successful drilling.
Titanium and its alloys are not only essential structural materials for aerospace applications such as aircraft, missiles, and rockets but have also found widespread use in mechanical, marine, biomedical, and chemical engineering fields. In environments where acidic media are present, both stainless steel valves and titanium valves are employed. However, titanium valves demonstrate a significantly longer service life due to their superior corrosion resistance and durability.
The addition of alloying elements to titanium results in the formation of titanium alloys, which greatly enhance the material’s strength, increasing the tensile strength (σb) from approximately 350–700 MPa to as high as 1200 MPa. This makes titanium alloys highly valuable in industrial applications. These alloys are typically 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 commonly used. However, the poor machinability of titanium alloys presents significant challenges in practical applications. Based on years of production experience, this study explores the relative machinability of titanium alloys and provides practical tool recommendations for readers.
2. Research on the Machinability of Titanium Alloys
When comparing the machinability of No. 45 steel (set at 100%), titanium alloys exhibit a machinability range of about 20–40%. Although this is worse than that of stainless steel, it is slightly better than that of high-temperature alloys. Within titanium alloys, β-type, (α+β)-type, and α-type alloys show progressively improved machinability, with pure titanium having the best performance. Generally, the higher the hardness of the material and the more alloying elements it contains, the worse its machinability. For instance, when the hardness is below HB 300, titanium tends to stick to the tool, while above HB 370, machining becomes extremely difficult. Therefore, it is recommended to maintain the hardness of titanium alloy between HB 300 and 370 for optimal machining conditions.
2.1 Study on the Cutting Mechanism of Titanium Alloys
(1) Influence of Gas Impurities
Gas impurities, particularly oxygen, hydrogen, and nitrogen, significantly affect the machinability of titanium alloys. As the concentration of these gases increases, the material becomes more brittle and harder, leading to increased tool wear. At temperatures above 600°C, titanium reacts strongly with oxygen, forming oxides that diffuse into the metal and create an embrittlement layer. This layer thickens over time, resulting in what is known as the “alpha layer,†further hardening the material. Similarly, hydrogen enhances embrittlement at high temperatures, while nitrogen forms brittle titanium nitride (TiN) at elevated temperatures, worsening machinability.
(2) Influence of Material Properties
From a machinability perspective, titanium alloys have several properties that make cutting challenging. Their low plasticity leads to minimal chip deformation during cutting, resulting in small contact areas between the chip and the tool. Additionally, their high strength limit and poor thermal conductivity (about 1/5 of iron and 1/10 of aluminum) lead to high cutting temperatures, reducing tool life and increasing wear. The high reactivity of titanium also causes welding and adhesion between the tool and the workpiece, further complicating the process.
(3) Influence of Carbon Content
The carbon content in titanium alloys directly affects machinability. When the carbon content exceeds 0.20%, hard carbides form, decreasing machinability. Conversely, lower carbon content improves the material’s ability to be machined effectively.
(4) Effect of Work Hardening
Work hardening is a key factor in the difficulty of machining titanium alloys. While the degree of work hardening is less severe than in stainless steel, it still contributes to surface hardening. During cutting, local high temperatures cause titanium to absorb oxygen and nitrogen, forming a hard and brittle layer on the surface. This layer can reach a depth of 0.1–0.15 mm and increase hardness by 20–30%.
2.2 Study on Relative Machinable Cutting Conditions of Titanium Alloy
(1) Tool Material
When machining titanium alloys, YT cemented carbide inserts should be avoided due to their affinity with titanium, leading to tool adhesion and chipping. Instead, YG-type inserts are commonly used despite their lower wear resistance. For roughing operations, YG8 is preferred, while YG3 is used for finishing. YG6X is often applied for general machining. Fine-grained tungsten-cobalt cemented carbide (YA6) has proven effective, offering better wear resistance and improved performance compared to YG6X. High-vanadium or high-cobalt high-speed steels (e.g., W12Cr4V4Mo and W2Mo9Cr4VCo8) are suitable for low-speed or complex surface cutting but are expensive and rare, so they should be used sparingly.
(2) Tool Geometry
The relief angle (α0) of the turning tool is critical in titanium machining. A relief angle of 15° is generally recommended to reduce tool wear and improve cutting efficiency. The rake angle (γ0) should be small (around 5°) to increase contact area and reduce heat concentration. A small radius (r = 0.5 mm) and positive rake angle (λ = +3°) are also beneficial. However, some studies suggest that a rake angle of 28–30° may optimize tool life, and increasing the tip radius can reduce tool chipping.
(3) Effect of Cutting Parameters on Temperature
Cutting speed (v) has the most significant impact on temperature, followed by feed rate (f), while depth of cut (ap) has the least effect. To maintain a cutting temperature of around 800°C, typical parameters are v = 40–60 m/min and f = 0.11–0.35 mm/r. High speeds can lead to excessive tool wear and surface hardening, so careful selection is essential.
(4) Effect of Cutting Parameters on Surface Roughness
Surface roughness is crucial for titanium parts due to their sensitivity to stress concentrations. To achieve Ra 1.6 μm, a feed rate of 0.16 mm/r is required. Increasing the feed rate to 0.25, 0.35, or 0.45 mm/r results in higher roughness values (Ra 3.2, 6.3, and 12.5 μm, respectively). Finishing operations require sharp tools and optimized parameters to ensure quality surfaces.
3. Titanium Alloy Cutting and Tool Design
3.1 Turning
Titanium alloys have a low elastic modulus (e.g., TC4: E = 110 GPa), making them prone to elastic deformation under cutting forces, which reduces workpiece accuracy. Therefore, the machining system must be rigid, and the workpiece must be securely clamped. The tool must be sharp to avoid vibration and friction, which can shorten tool life and reduce accuracy. Under normal conditions, no built-up edge forms, allowing for good surface quality. However, coolant is essential to reduce temperature and prevent fire hazards. Cutting parameters must be carefully selected to avoid overheating and surface hardening.
3.2 Drilling
Drilling titanium alloys is particularly challenging due to issues like drill breakage and burning. Proper tool selection, chip removal, and cooling are essential. Carbide drills (YG8) are preferred for larger diameters, while high-speed steel (M42 or B201) is suitable for smaller holes. Drill geometry, including apex angles and clearance angles, must be optimized for efficient cutting. Coolant, such as emulsions or specialized oils, is necessary to improve tool life and prevent work hardening. Careful feed control and periodic chip removal are also important to avoid tool damage and ensure successful drilling.
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