The process analysis of finishing titanium alloy by four-axis machining center. Titanium alloy materials are widely used because of their small density, high specific strength, high temperature resistance and good oxidation resistance. However, the poor machining properties of titanium alloys have affected the widespread use of the material.
Titanium alloys add alloying elements to industrial pure titanium to increase the strength of titanium. There are three types of titanium alloys: a titanium alloy, b titanium alloy and a + b titanium alloy. The ab titanium alloy is composed of a and b dual phases. This type of alloy has a stable structure, good high temperature deformation properties, toughness and plasticity, and can be quenched and aged to strengthen the alloy. The performance characteristics of titanium alloy are mainly manifested in:
- 1 /) Higher specific strength. Titanium alloy has low density / (4.4kg / dm3 /) and light weight, but its specific strength is greater than ultra-high strength steel.
- 2 /) High thermal strength. Titanium alloy has good thermal stability, and its strength is about 10 times higher than that of aluminum alloy at 300 ～ 500 ℃.
- 3 /) Great chemical activity. Titanium can have a strong chemical reaction with oxygen, nitrogen, carbon monoxide, water vapor and other substances in the air, forming a hardened layer of TiC and TiN on the surface.
Poor thermal conductivity. The thermal conductivity of the titanium alloy is poor. The thermal conductivity of the titanium alloy TC4 at 200 ° C is 1 = 16.8 W / m · ° C, and the thermal conductivity is 0.036 calories / cm · sec · ° C.
Analysis of machining characteristics of titanium alloy
First, the thermal conductivity of titanium alloys is low, only 1/4 of steel, 1/13 of aluminum, and 1/25 of copper. Because the heat dissipation in the cutting area is slow, which is not conducive to the heat balance, the heat dissipation and cooling effect is very poor during the cutting process, and it is easy to form high temperatures in the cutting area. After the processing, the part deforms and rebounds greatly, resulting in increased cutting tool torque and fast edge wear Durability is reduced. Secondly, the low thermal conductivity of the titanium alloy makes it difficult for the cutting heat to radiate in a small area near the cutting blade, the rake face friction increases, the chip removal is not easy, the cutting heat is not easy to radiate, and the tool wear is accelerated. Finally, titanium alloy has high chemical activity, and it is easy to react with the tool material at high temperature, forming a coating and diffusion, resulting in sticking knife, burning knife, broken knife and other phenomena.
Features of titanium alloy processed by machining center
- The machining center can process multiple parts at the same time, improving production efficiency.
- Improve the machining accuracy of parts, and the product consistency is good. The machining center has a tool compensation function, which can obtain the machining accuracy of the machine tool itself.
- Has wide adaptability and greater flexibility. Such as arc processing, chamfering and transition fillet of this part.
- Multiple functions can be realized in one machine. The machining center can perform a series of processing such as milling, drilling, boring and tapping.
- Accurate cost calculation can be performed to control the production schedule.
- No special fixtures are needed, saving a lot of cost and shortening the production cycle.
- Greatly reduce the labor intensity of workers.
- Can perform multi-axis machining with UG and other machining software.
Tool material selection
Tool material selection should meet the following requirements:
- Enough hardness. The hardness of the tool must be much greater than the hardness of the titanium alloy.
- Sufficient strength and toughness. Since the cutting tool bears a large torque and cutting force when cutting titanium alloy, it must have sufficient strength and toughness.
- Sufficient wear resistance. Due to the good toughness of titanium alloys, the cutting edges should be sharp during machining, so the tool material must have sufficient wear resistance to reduce work hardening. This is the most important parameter for selecting titanium alloy cutting tools.
- The tool material has poor affinity with the titanium alloy. Due to the high chemical activity of titanium alloys, it is necessary to avoid the formation of alloying and diffusion of tool materials and titanium alloys to form alloys, causing sticking and burning.
Tests on domestic commonly used tool materials and foreign tool materials show that the use of high cobalt tools is ideal. The main role of cobalt can strengthen the secondary hardening effect, improve the red hardness and the hardness after heat treatment, and have high toughness and wear resistance. , Good heat dissipation.
Geometrical parameters of milling cutter
The processing characteristics of titanium alloy determine the geometric parameters of the tool and there is a big difference from the ordinary tool.
- The spiral angle β selects a smaller spiral elevation angle, the chip flute increases, the chip removal is easy, the heat dissipation is fast, and the cutting resistance during the cutting process is also reduced.
- The cutting edge is sharp when cutting, and the cutting is light and fast, avoiding excessive cutting heat generated by the titanium alloy, thereby avoiding secondary hardening.
The clearance angle α reduces the wear rate of the blade, which is conducive to heat dissipation, and the durability is also greatly improved.
Cutting parameter selection
Titanium alloy machining should choose a lower cutting speed, an appropriate large feed rate, a reasonable depth of cut and a finishing amount, and sufficient cooling.
- Cutting speed VcVc = 30 ～ 50m / min.
- The feed amount f takes a larger feed amount during roughing, and a moderate feed amount for finishing and semi-finishing.
- The cutting depth apap = 1 / 3d is suitable. The titanium alloy has good affinity and difficult chip removal. If the cutting depth is too large, it will cause tool sticking, burning and breaking.
- The finishing allowance αc is moderate. The hardened layer of titanium alloy surface is about 0.1 ~ 0.15mm. The allowance is too small. The cutting edge is cut on the hardened layer. The tool is easy to wear. The hardened layer should be avoided.
- It is best not to use chlorine-containing coolant for processing titanium alloys, to avoid the generation of toxic substances and hydrogen embrittlement, and to prevent high-temperature stress corrosion cracking of titanium alloys.
- Synthetic water-soluble emulsifier is selected, and coolant can also be used.
- When cutting, the coolant must be sufficient, the coolant circulation speed should be fast, the cutting fluid flow and pressure should be large, the processing center is equipped with special cooling nozzles, as long as pay attention to adjustments to achieve the desired effect.
Through the analysis of the characteristics of titanium alloys, the difficulties in the cutting process of titanium alloys are solved; through the preparation of correct and scientific processing techniques, costs can be reduced and production efficiency can be improved, and the following conclusions are drawn:
- The titanium alloy is processed by the machining center, which meets the requirements of complex shape and high precision, and can process multiple pieces at the same time, which improves the production efficiency.
- High cobalt tool material is an ideal machining tool for titanium alloy.
- Choose reasonable tool geometry parameters, cutting parameters, and coolant, which can extend the life of cutting tools and improve production efficiency.
- Arranging reasonable and scientific technological procedures is the best way to improve efficiency and save costs.