Three Key Points in Titanium Alloy Machining: Coating Selection & Cutting Parameters

Titanium alloy is widely used in aerospace, medical, automotive and other high-end manufacturing fields due to its excellent properties such as high specific strength, corrosion resistance and biocompatibility. However, its poor machinability—characterized by high cutting temperature, severe tool wear, and easy work hardening—poses great challenges to machining processes. To improve machining efficiency, reduce tool consumption and ensure workpiece quality, mastering the following three key points is essential, with a focus on coating selection and cutting parameter optimization.

Key Point 1: Understand the Machinability of Titanium Alloy

Before selecting coatings and setting cutting parameters, it is necessary to clarify the intrinsic characteristics of titanium alloy that affect machining, which is the basis for subsequent optimization:

• Low thermal conductivity: The thermal conductivity of titanium alloy is only 1/4~1/5 of that of steel. During cutting, most of the heat generated accumulates in the cutting zone (tool tip and workpiece contact area) instead of being dissipated through chips or workpieces, leading to extremely high local temperature (up to 800~1000℃), which accelerates tool wear and workpiece deformation.

• High chemical activity: At high temperatures, titanium alloy is easy to react with oxygen, nitrogen and carbon in the air to form hard and brittle compounds (such as TiO₂, TiN, TiC), which will increase cutting force and cause abrasive wear of tools. It may also bond with the tool material, resulting in adhesive wear.

• Work hardening tendency: Titanium alloy has a high yield strength and obvious work hardening effect. During cutting, the surface of the workpiece is prone to hardening layers (hardness can be increased by 20%~50%), which will scratch the tool and affect the surface quality of the subsequent machining.

Note: The P1 can be a comparison chart of thermal conductivity between titanium alloy and common metals, or a microscopic diagram of work hardening layer of titanium alloy after cutting.

Key Point 2: Rational Selection of Tool Coatings

Tool coatings play a crucial role in titanium alloy machining by reducing friction, isolating high temperature, improving chemical stability and enhancing wear resistance. The selection of coatings should be based on the type of titanium alloy (such as Ti-6Al-4V, pure titanium), machining method (milling, turning, drilling) and machining requirements (roughing, finishing). Common high-performance coatings for titanium alloy machining are as follows:

2.1 Titanium Nitride (TiN) Coating

TiN coating is a traditional hard coating with a hardness of about 2000~2500 HV and a low friction coefficient (0.4~0.6). It has good wear resistance and adhesion, and can effectively reduce adhesive wear between the tool and titanium alloy. However, its oxidation resistance is poor, and it will oxidize and fail when the temperature exceeds 500℃. It is suitable for low-speed roughing of pure titanium and low-alloy titanium, or machining scenarios with low cutting temperature.

2.2 Titanium Carbonitride (TiCN) Coating

TiCN coating is an improved version of TiN, with a hardness of 2500~3000 HV, higher wear resistance and thermal stability than TiN. The addition of carbon element enhances the coating's resistance to adhesive wear and abrasive wear, and its oxidation resistance temperature is increased to 600~650℃. It is suitable for medium-speed turning and milling of Ti-6Al-4V and other commonly used titanium alloys, and can balance machining efficiency and tool life.

2.3 Aluminum Titanium Nitride (AlTiN) Coating

AlTiN coating is a high-temperature resistant coating with excellent comprehensive performance, with a hardness of 3000~3500 HV and oxidation resistance temperature up to 800~900℃. The aluminum element in the coating forms a dense Al₂O₃ film at high temperature, which can effectively isolate the chemical reaction between titanium alloy and the tool substrate (such as carbide), and significantly reduce thermal wear and chemical wear. It is the preferred coating for high-speed finishing and semi-finishing of titanium alloy, especially suitable for high-temperature machining scenarios such as high-speed milling and deep-hole drilling.

2.4 Diamond-Like Carbon (DLC) Coating

DLC coating has an extremely low friction coefficient (0.1~0.2) and high hardness (1500~2500 HV), which can minimize the friction and adhesion between the tool and titanium alloy, and avoid work hardening caused by excessive cutting force. However, its thermal stability is poor (oxidation failure above 400℃) and it is brittle, so it is only suitable for low-speed, low-temperature finishing of pure titanium and soft titanium alloys (such as Ti-Gr2), and not for high-temperature roughing.

Note: The P2 can be a performance comparison table of different coatings (hardness, oxidation temperature, applicable scenario) or a physical diagram of coated tools for titanium alloy machining.

Key Point 3: Scientific Setting of Cutting Parameters

Cutting parameters (cutting speed, feed rate, depth of cut) directly affect cutting temperature, cutting force, tool wear and workpiece quality. For titanium alloy machining, the core principle of parameter setting is "low cutting speed, moderate feed rate, small depth of cut", so as to control cutting temperature and reduce work hardening. The following are the recommended parameters for common machining methods (taking Ti-6Al-4V, the most widely used titanium alloy, and carbide tools as examples):

3.1 Turning Parameters

• Cutting speed (vc): For roughing, the speed is 30~60 m/min; for finishing, it is 60~100 m/min. If using AlTiN coated tools, the speed can be appropriately increased to 80~120 m/min; for pure titanium, the speed should be reduced by 20%~30% to avoid excessive adhesion.

• Feed rate (f): The feed rate is 0.1~0.3 mm/r for roughing and 0.05~0.15 mm/r for finishing. Too high feed rate will increase cutting force and work hardening; too low feed rate will cause the tool to rub against the workpiece, accelerating wear.

• Depth of cut (ap): The depth of cut for roughing is 1~3 mm, and for finishing is 0.1~0.5 mm. It is not recommended to use a depth of cut less than 0.1 mm, because the tool will slide on the hardened layer of the workpiece, resulting in severe abrasive wear.

3.2 Milling Parameters

• Cutting speed (vc): For peripheral milling (roughing), the speed is 20~50 m/min; for finishing, it is 50~80 m/min. For face milling, the speed can be slightly higher, 40~70 m/min for roughing and 70~100 m/min for finishing. Coated tools can increase the speed by 10%~20%.

• Feed rate per tooth (fz): The feed rate per tooth is 0.05~0.15 mm/tooth for roughing and 0.02~0.08 mm/tooth for finishing. For end milling of thin-walled workpieces, the feed rate should be reduced to avoid workpiece deformation.

• Depth of cut (ap/ae): The axial depth of cut (ap) for roughing is 0.5~2 mm, and for finishing is 0.1~0.3 mm; the radial depth of cut (ae) is generally 50%~100% of the tool diameter.

3.3 Drilling Parameters

Drilling titanium alloy is prone to problems such as chip clogging, tool breakage and poor hole quality. The parameters should be set to facilitate chip removal:

• Cutting speed (vc): 10~30 m/min, which is lower than turning and milling, to reduce the temperature of the drill tip.

• Feed rate (f): 0.1~0.2 mm/r, ensuring that chips can be discharged smoothly without clogging the drill flute.

• Auxiliary measures: Use internal cooling drills to spray cutting fluid directly to the drill tip, which can effectively reduce temperature and flush chips; adopt intermittent drilling (drill in and out repeatedly) to avoid chip accumulation.

Note: The P3 can be a parameter setting diagram for turning/milling/drilling, or a curve diagram of the relationship between cutting speed and tool life.

Summary

The key to successful titanium alloy machining lies in three aspects: first, fully understanding the machinability characteristics of titanium alloy to target optimization; second, selecting the appropriate tool coating according to machining scenarios to improve tool wear resistance and high-temperature stability; third, setting scientific cutting parameters to control cutting temperature and reduce work hardening. In actual production, it is also necessary to match with high-quality cutting fluid (preferred for water-based cutting fluid with good cooling performance, or oil-based cutting fluid for low-speed machining) and reasonable tool geometry, so as to achieve the best machining effect.