Why Do We Think Titanium Alloy Is A Difficult Material To Machine?
Why do we think titanium alloy is a difficult material to machine? Because there is a lack of in-depth understanding of its processing mechanism and phenomena.
1. Physical phenomena of titanium processing
The cutting force when processing titanium alloy is only slightly higher than that of steel with the same hardness, but the physical phenomena of processing titanium alloy are much more complex than processing steel, making titanium alloy processing facing huge difficulties.
The thermal conductivity of most titanium alloys is very low, only 1/7 that of steel and 1/16 that of aluminum. Therefore, the heat generated during titanium alloy cutting will not be quickly transferred to the workpiece or taken away by the chips, but will be concentrated in the cutting area. The generated temperature can reach over 1000°C, causing rapid wear, cracking, and deformation of the cutting edge of the tool. Built-up edge is a rapidly wearing cutting edge that generates more heat in the cutting area, further shortening tool life.
The high temperature generated during the cutting process will also destroy the surface integrity of titanium alloy parts, leading to a decrease in the geometric accuracy of the parts and work hardening, seriously reducing their fatigue strength.
The elasticity of titanium alloys may be beneficial to part performance, but during the cutting process, the elastic deformation of the workpiece is an important cause of vibration. Cutting pressure causes the "elastic" workpiece to bounce away from the tool, causing friction between the tool and workpiece to exceed the cutting action. The friction process also generates heat, which exacerbates the problem of poor thermal conductivity of titanium alloys.
This problem is even more serious when processing thin-walled or annular parts that are prone to deformation. Machining titanium alloy thin-walled parts to the expected dimensional accuracy is not an easy task. Because when the workpiece material is pushed away by the tool, the local deformation of the thin wall has exceeded the elastic range and plastic deformation occurs, and the material strength and hardness of the cutting point increase significantly. At this time, the originally determined cutting speed becomes too high, further causing rapid tool wear.
"Heat" is the "culprit" that makes titanium alloys difficult to process!
2. Technological know-how for titanium alloy processing
On the basis of understanding the processing mechanism of titanium alloys and adding past experience, the main process know-how for processing titanium alloys is as follows:
(1) Use positive-angle geometry inserts to reduce cutting force, cutting heat and workpiece deformation.
(2) Maintain a constant feed rate to avoid hardening of the workpiece. The tool should always be in the feed state during the cutting process. During milling, the radial tool engagement ae should be 30% of the radius.
(3) Use high-pressure, high-flow cutting fluid to ensure the thermal stability of the machining process and prevent workpiece surface degeneration and tool damage caused by excessive temperature.
(4) Keep the blade sharp. Tool dullness is the result of heat accumulation and wear, which can easily lead to tool failure.
(5) Process titanium alloys in the softest state possible, because the material becomes more difficult to process after quenching, and heat treatment will increase the strength of the material and increase blade wear.
(6) Use a larger tool tip arc radius or chamfer to insert as much of the tool edge into cutting as possible. This reduces cutting force and heat at each point and prevents local breakage. When milling titanium alloy, among the cutting parameters, cutting speed has the greatest impact on tool life vc, followed by radial tool engagement amount (milling depth) ae.
3. Start with the blade to solve titanium processing problems
The flute wear that occurs when machining titanium alloys is local wear at the front and rear along the cutting depth direction. It is often caused by the hardened layer left by previous processing. The chemical reaction and diffusion between the tool and the workpiece material when the processing temperature exceeds 800°C is also one of the causes of groove wear. Because during the machining process, the titanium molecules of the workpiece accumulate in front of the blade and are "welded" to the blade under high pressure and high temperature, forming built-up edge. When built-up edge peels off the cutting edge, it takes away the insert's carbide coating, so titanium machining requires special insert materials and geometries.
4. Tool structure suitable for titanium processing
The focus of titanium alloy processing is heat treatment. A large amount of high-pressure cutting fluid must be sprayed on the cutting edge promptly and accurately to quickly remove heat. There are milling cutters on the market with unique structures specifically designed for titanium alloy processing.
