Why is titanium alloy a difficult material to machine?

 

1. Physical phenomena of titanium machining

The cutting force of titanium alloy processing is slightly higher than that of steel with the same hardness. Still, the physical phenomenon of processing titanium alloy is much more complicated than that of processing steel, which makes titanium alloy processing face enormous difficulties.

The thermal conductivity of most titanium alloys is very low, only 1/7 of steel and 1/16 of aluminum. Therefore, the heat generated when cutting titanium alloys will not be quickly transferred to the workpiece or taken away by the chips. Still, it will accumulate in the cutting area, and the temperature generated can be as high as 1,000 °C or more, which will cause the tool's cutting edge to wear, chip, and crack rapidly. The formation of a built-up edge and the rapid appearance of a worn edge generate more heat in the cutting area, further shortening the tool's life. titanium machining

The high temperatures generated during the cutting process also destroy the surface integrity of the titanium alloy parts, resulting in a decrease in the geometric accuracy of the parts and work hardening that severely reduces their fatigue strength.

The elasticity of titanium alloys may be beneficial for part performance, but during cutting, the elastic deformation of the workpiece is an essential cause of vibration. The cutting pressure causes the "elastic" workpiece to move away from the tool and bounce, so the friction between the tool and the workpiece is greater than the cutting action. The friction process also generates heat, aggravating the problem of poor thermal conductivity of titanium alloys.

This problem is even more severe when machining thin-walled or ring-shaped, easily deformed parts. It is not easy to machine thin-walled titanium alloy parts to the expected dimensional accuracy. When the tool pushes the workpiece material away, the thin wall's local deformation exceeds the elastic range; plastic deformation occurs, and the material strength and hardness of the cutting point increase significantly. Machining at the previously determined cutting speed becomes too high, resulting in sharp tool wear.

"Hot" is the "culprit" that makes it challenging to process titanium alloys! 

 

2. Technological know-how for titanium CNC machining 

Based on understanding the processing mechanism of titanium alloys and adding experience, the primary process know-how for processing titanium alloys is as follows:

(1) Inserts with positive geometry are used to reduce the workpiece's cutting force, cutting heat, and deformation.

(2) Maintain a constant feed to avoid hardening of the workpiece. The tool should always be in the feed state during the cutting process, and the radial cutting amount should be 30% of the radius during milling.

(3) High-pressure and large-flow cutting fluid is used to ensure the thermal stability of the machining process and prevent workpiece surface degeneration and tool damage due to excessive temperature.

(4) Keep the blade edge sharp; blunt tools cause heat build-up and wear, quickly leading to tool failure.

(5) Machining in the softest state of the titanium alloy as much as possible because the material becomes more challenging to machine after hardening, and the heat treatment increases the material's strength and the wear of the insert.

(6) Use a large nose radius or chamfer to cut as much as possible into the cutting edge. This reduces cutting force and heat at every point and prevents local breakage. When milling titanium alloys, among the cutting parameters, the cutting speed has the most significant influence on the tool life vc, followed by the radial engagement (milling depth) ae.

 

3. Start with the blade to solve the titanium processing problem

The wear of the insert groove during the machining of titanium alloys is the local wear of the back and front in the direction of the depth of cut, which is often caused by the hardened layer left by the previous processing. The chemical reaction and diffusion of the tool and the workpiece material at a processing temperature of more than 800 °C are also reasons for the formation of groove wear. Because during the machining process, the titanium molecules of the workpiece accumulate in the front of the blade and are "welded" to the blade edge under high pressure and high temperature, forming a built-up edge. When the built-up edge peels off the cutting edge, it takes away the carbide coating of the insert, so titanium machining requires unique insert materials and geometries. cCustomprecision machining

 

4. Tool structure suitable for titanium machining

The focus of titanium alloy processing is heat, and a large amount of high-pressure cutting fluid must be sprayed on the cutting edge promptly and accurately to remove the heat quickly. There are unique configurations of milling cutters specifically for machining titanium.