Why do we think titanium alloy why do we think titanium alloy is a difficult material to work with? Because of the lack of deep understanding of its processing mechanism and phenomenon.
1. Physical phenomena 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 titanium alloy processing is much more complex than that of steel, so that titanium alloy processing is faced with great difficulties.
Most titanium alloys have very low thermal conductivity, 1/7 that of steel and 1/16 that of aluminum. Therefore, in the process of cutting of titanium alloy heat will not quickly passed to the artifact or taken away by chip, and agglomeration in cutting area, the temperature can be as high as 1000 ℃ above, make the cutter blade rapid wear, cracks, and generate the devolop tumor, rapid wear of the blade, and make the cutting area produces more heat and further shorten the life of cutting tools.
At the same time, the high temperature in the cutting process destroys the surface integrity of the titanium alloy parts, which leads to the decrease of the geometric precision of the parts and the phenomenon of work hardening which seriously reduces the fatigue strength.
The elasticity of titanium alloy may be beneficial to the performance of parts, but the elastic deformation of the workpiece is an important cause of vibration during the cutting process. The cutting pressure makes the "elastic" workpiece leave the tool and rebound, so that the friction between the tool and the workpiece is greater than the cutting effect. The friction process also produces heat, aggravating the problem of poor thermal conductivity of titanium alloy.
This problem is more serious when processing thin wall or ring or other easily deformed parts. It is not easy to process titanium alloy thin wall parts to the expected dimensional accuracy. As 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, the material strength and hardness of the cutting point increase obviously. At this point, according to the original set cutting speed machining becomes too high, further resulting in sharp tool wear.
"Heat" is the "culprit" of difficult titanium alloy processing!
2. Process know-how of titanium alloy
Based on the understanding of titanium alloy processing mechanism and the previous experience, the main technological know-how of titanium alloy processing is as follows:
(1) the use of positive Angle geometric shape of the blade, in order to reduce the cutting force, cutting heat and workpiece deformation.
(2) keep a constant feed to avoid hardening of the workpiece, in the cutting process, the tool should always be in the feed state, milling radial draft ae should be 30% of the radius.
(3) high pressure and large flow cutting fluid is used to ensure the thermal stability of the processing process and prevent the surface degeneration and tool damage of the workpiece due to high temperature.
(4) keep the edge of the blade sharp, blunt tool is the cause of heat accumulation and wear, easy to cause tool failure.
(5) as far as possible, the titanium alloy is processed in the softest state, because the hardened material becomes more difficult to process, and the heat treatment improves the strength of the material and increases the wear of the blade.
(6) use a large tip radius or chamfer to cut, as much as possible into the cutting edge. This reduces cutting force and heat at each point and prevents local damage. In the process of milling titanium alloy, the influence of cutting speed on tool life vc was the largest among all cutting parameters, followed by radial cutting load (milling depth) ae.
3. Solve the problem of titanium processing from the blade
The groove wear of titanium alloy is the local wear of the back and front along the direction of cutting depth. The chemical reaction and diffusion between the cutting tool and the workpiece material at the processing temperature of more than 800℃ is also one of the causes of groove wear. During processing, titanium molecules of the workpiece accumulate in front of the blade and "weld" to the blade under high pressure and high temperature, forming a chip tumor. When the debris is removed from the blade, the carbide coating of the blade is taken away. Therefore, the processing of titanium alloy requires special blade materials and geometric shapes.
4. Tool structure suitable for titanium processing
The focus of titanium alloy processing is heat, a large number of high-pressure cutting fluid to be timely and accurately sprayed on the cutting edge, can quickly remove the heat. There is a special structure of milling cutter for titanium alloy processing on the market.