Titanium is considered to be a “rare metal”. The main advantages of titanium alloys are their excellent anti-rust properties, and no allergic and immune reactions will occur when they are in contact with the human body for a long time. Therefore, titanium alloys are used in many medical equipment, such as prosthetic joints. In cutting tools, it is often used as a high-grade handle material. Because titanium is too stable, it requires a very complicated process to make titanium products, and most of them are made of pure titanium. It is difficult to integrate titanium with other metals and coatings, so the reason for the expensive production is that the production cost is too high. , Technical processing is difficult!
Titanium is difficult to machine. In fact, titanium alloy material is not hard, but very “sticky”. If the speed is too low, the cutting will not move, and if the temperature is too high and the titanium is too high, oxidation will occur, and ordinary processing manufacturers cannot complete the processing of titanium alloy products. For example, it is even more difficult to use titanium to make glasses. The frame of the frame is very thin and requires a lot of fine processing, which is a big challenge for titanium material processing. In addition, titanium alloy frames also involve titanium alloy welding technology, which is more difficult.
Titanium alloy has good properties such as high strength, corrosion resistance, high temperature resistance, fatigue resistance, etc., and has been more and more widely used in aviation, aerospace, nuclear energy, medical equipment and other fields. However, titanium alloy is also a difficult-to-machine material because of its low thermal conductivity, severe work hardening, high affinity with tools, and small plastic deformation, and its machinability index is only 20% of that of free-cutting steel. Therefore, when machining titanium alloys, special tools need to be selected to ensure efficiency.
“Why is the cost of titanium alloy CNC machining parts so high.” You can understand the reasons according to the following points:
1. Material cost is expensive. At present, the titanium alloy material in our market is very expensive,
2. Physical phenomenon of titanium processing
The cutting force of titanium alloy processing is only slightly higher than that of steel with the same hardness, but the physical phenomenon of processing titanium alloy is much more complicated than that of processing steel, which makes titanium alloy processing face huge 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 in the process of cutting titanium alloys will not be quickly transferred to the workpiece or taken away by the chips, but will accumulate in the cutting area, and the temperature generated can be as high as 1 000 °C or more, which will cause the cutting edge of the tool to rapidly wear, chip and crack. The formation of built-up edge, the rapid appearance of a worn edge, in turn generates more heat in the cutting area, further shortening the life of the tool.
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 important cause of vibration. The cutting pressure causes the “elastic” workpiece to move away from the tool and bounce so that 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 serious when machining thin-walled or ring-shaped parts that are easily deformed. It is not an easy task to machine thin-walled titanium alloy 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 and plastic deformation occurs, and the material strength and hardness of the cutting point increase significantly. At this point, machining at the previously determined cutting speed becomes too high, further resulting in sharp tool wear.
“Hot” is the “culprit” that is difficult to process titanium alloys!
Titanium alloy CNC machining method
1. Process know-how for machining titanium alloys
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) Inserts with positive geometry are used to reduce cutting force, cutting heat and deformation of the workpiece.
(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 ae 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 are the cause of heat build-up and wear, which can easily lead to tool failure.
(5) Machining in the softest state of the titanium alloy as much as possible, because the material becomes more difficult to machine after hardening, and the heat treatment increases the strength of the material and increases 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 greatest influence on the tool life vc, followed by the radial engagement (milling depth) ae.
2. Solve the titanium processing problem from the blade
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 one of the 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 special insert materials and geometries.
3. 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 in a timely and accurate manner to quickly remove the heat. There are unique configurations of milling cutters on the market specifically for machining titanium.