Titanium alloy is difficult to process the 4 major reasons and 6 major countermeasures
First, temperature concentration
The thermal conductivity of most titanium alloys is very low, only 1/7 of steel, 1/16 of aluminum, and 1/25 of copper. Therefore, the heat generated during cutting titanium alloy will not be quickly transferred to the workpiece or taken away by the chips, but will be concentrated in the cutting area.
The temperature generated at the tool tip can be as high as 1000°C, causing the cutting edge of the tool to wear rapidly, crack and produce chips, shortening the tool life.
The high temperatures generated during the cutting process also destroy the surface integrity of titanium alloy parts, resulting in a decrease in the geometric accuracy of the parts and work hardening, seriously reducing their fatigue strength.
Second, elastic deformation
The elastic modulus of titanium alloy is not very high. For example, the elastic modulus of TC4 is only 110Gpa, while the elastic modulus of 45 steel is 210Gpa, and the elastic modulus of stainless steel such as 303, 304, 316 is also around 200Gpa, so when processing titanium alloy, elasticity is easy to occur. Deformation.
This problem is even more severe when machining thin-walled or annular parts. It is not easy to process titanium alloy thin-walled parts to the expected dimensional accuracy. Because when the workpiece material is pushed away by the tool, the local deformation of the thin wall has exceeded the elastic range, resulting in plastic deformation, and the material strength and hardness of the cutting point increase significantly.
The cutting pressure causes the "elastic" workpiece to leave the tool and rebound, causing the friction between the tool and the workpiece to be greater than the cutting action. The friction process generates heat, exacerbating the problem of poor thermal conductivity of titanium alloys.
Third, the affinity of titanium alloys results in the formation of long, continuous chips during turning and drilling processes that can entangle tools and impede functionality. When the cutting depth is too large, it will cause sticking, burning, breakage, etc.
Of course, good affinity is also useful in other places. For example, in ion pumps, it is used as a titanium cathode plate. When titanium atoms are sputtered to the anode tube wall, it can adsorb gas, thereby generating ultra-high vacuum.
Fourth, vibration
The elasticity of titanium alloys may be beneficial to the performance of the part, but during the cutting process, the elastic deformation of the workpiece is an important cause of vibration.
The vibration generated by machining titanium alloy is about 10 times that of steel. Since cutting heat is concentrated on the cutting part, zigzag chips are generated and cause fluctuations in cutting power.
Countermeasures for difficult processing of titanium alloys
What are the countermeasures?
First, cooling
Coolant can be used to reduce the high temperature generated by the cutting process. Generally, non-soluble oil coolant is used for low-speed and heavy-duty cutting and shearing, and soluble cutting coolant is used for high-speed cutting or shearing.
In addition, low-temperature cutting methods can be used, using liquid nitrogen (-180°C) or liquid CO2 (-76°C) as the cutting fluid, which can reduce the temperature of the cutting zone. This method can reduce the main cutting force by 20% and the cutting temperature by more than 300°C. At the same time, the built-up edge disappears, the quality of the machined surface is improved, and the tool durability is increased by 2 to 3 times.
Second, choose the right tool
Great improvements can be achieved by choosing the right cutting tool.
Because heat must be removed through the cutting edge and coolant, rather than through chips like steel, a small portion of the cutting edge must withstand extremely high thermal and mechanical stresses. Use a sharp cutting edge to reduce cutting forces.
Additionally, cutting stress can be reduced by using ground and highly positive indexable inserts with polished grooves.
If desired, coated tools can also be used to minimize friction during chip evacuation by resisting the stickiness of the alloy and breaking up long chips, both of which help prevent heat build-up during machining.
Third, constant feed or increased feed rate
Titanium is susceptible to work hardening, i.e. as the material is cut, titanium becomes harder and therefore more prone to wear on the tool, constant feed ensures that work hardening is kept to a minimum.
Of course, if the machine allows, the feed rate can be increased, which means the tool spends less time in a specific area and therefore no more time for heat buildup and work hardening.
Fourth, reduce the cutting speed
For example, use 1/3 of the steel cutting speed or less for heat release control.
Fifth, replace the tool according to the process
Ceramic, titanium carbide and titanium nitride coated tools are used to machine titanium alloys and their lifespan will be shortened. Typically, hard steel tools are used to cut large amounts of titanium and high-speed carbide tools are used to cut small amounts of titanium.
Currently, ultrasonic machining is being developed with the goal of reducing tool contact time and extending tool life.
Sixth, use high-rigidity machine tools
Robust and durable machine tools are essential for successful machining of titanium alloys.
The ideal titanium milling machine must be rigid and the spindle must be able to operate at low speeds and high torque.
This helps absorb vibrations and reduce chatter during cutting, a common problem when machining titanium alloys.
Finally, machining equipment and cutting tools should be cleaned regularly to prevent debris buildup.
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