1.Physical phenomena of titanium processing
The cutting force of titanium alloy is only slightly higher than that of steel with the same hardness.
Most titanium alloys have very low thermal conductivity, only 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 1 000 ℃ above, make the cutter blade quickly 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, leading to the decrease of the geometric precision of the parts and the phenomenon of work hardening, which seriously reduces the fatigue strength of the parts.
The elasticity of titanium alloys may be beneficial to the properties of parts, but the elastic deformation of the workpiece is an important cause of vibration in the cutting process.The cutting pressure causes the "elastic" workpiece to move away from the tool and bounce back so that the friction between the tool and the workpiece is greater than the cutting action.The friction process also produces heat, which aggravates the poor thermal conductivity of titanium alloys.
This problem becomes more serious when machining thin-walled or ring-shaped parts that are prone to deformation. It is not easy to process thin-walled parts of titanium alloy to the expected dimensional accuracy.Because as the workpiece material is pushed by the tool, the local deformation of the thin wall has exceeded the elastic range and produced plastic deformation, the strength and hardness of the cutting point of the material increased significantly.At this point, machining at the previously determined cutting speed becomes too high, further leading to sharp tool wear.
"Hot" is hard to process titanium alloy "culprit"!
2. Technical know-how of processing titanium alloy
Based on the understanding of the machining mechanism of titanium alloy and previous experience, the main technological know-how of processing titanium alloy is as follows:
(1) the blade of positive Angle geometry is adopted to reduce cutting force, cutting heat and workpiece deformation.
(2) keep constant feed to avoid the hardening of the workpiece, the cutting tool should always be in the feed state in the cutting process, milling radial feed a e should be 30% of the radius.
(3) the use of high pressure and large flow cutting fluid, to ensure the thermal stability of the process, to prevent high temperature caused by the workpiece surface denaturation and tool damage.
(4) keep the blade sharp, blunt tool is the cause of heat accumulation and wear, easy to lead to tool failure.
(5) machining in the softest state of titanium alloy as far as possible, because the material becomes more difficult to process after hardening, heat treatment increases the strength of the material and increases the wear of the blade.
(6) use a large tool tip radius or chamfering to cut as many blades as possible.This reduces the cutting force and heat at each point, preventing local damage.In the milling of titanium alloy, the cutting speed had the greatest influence on the tool life vc, followed by the radial feed (milling depth) ae.
3. Solve the titanium processing problem from the blade
The blade groove wear in titanium alloy machining is the local wear in the direction of cutting depth in the back and front, which is often caused by the hardening layer left in the early processing.The chemical reaction and diffusion between the cutting tool and the workpiece material at the processing temperature over 800℃ is also one of the causes of groove wear.During processing, the titanium molecules of the workpiece accumulate in front of the blade and are "welded" to the blade at high pressure and temperature, forming debris nodules.The carbide coating of the blade is carried away when the nodular chip is removed from the blade. Therefore, special blade materials and geometric shapes are required for titanium alloy processing.
4. Tool structure suitable for titanium processing
The focus of titanium alloy processing is heat, a large number of high pressure cutting fluid to the cutting edge in a timely and accurate injection, can quickly remove the heat.There are special milling cutters for titanium alloy processing on the market.