Titanium alloy has become one of the most important engineering materials in the machinery industry. It has been more and more widely used. Due to its high material strength. Titanium alloy has poor cutting performance due to its high chemical activity and low elastic modulus. It is a typical difficult-to-machine material. Combined with the technical information and experience in the field of titanium alloy parts processing, the fitter process methods in the processing of titanium alloy shell parts are summarized and studied.

The outstanding advantages of titanium alloys are light weight, high strength, good heat resistance and excellent corrosion resistance; but at the same time, it is a difficult-to-process material, which has a large friction coefficient, poor thermal conductivity, and heat is not easy to dissipate. It causes burns on the surface of the parts, especially when the fitter drills and taps due to the limited processing method of the parts, the processing is more difficult. This article takes the case as an example to explain the drilling and tapping of titanium alloy parts.

1 Analysis of titanium alloy shell parts
The shell part is an important structural part in a certain component. This part is the reference for installation and positioning of components, and at the same time it is also the main force-bearing part. There are many drilling and tapping requirements for parts, and the machining accuracy is high, and the material is titanium alloy.

2 Physical properties of titanium alloy
Titanium alloy has the advantages of small specific gravity, high specific strength and thermal strength, good thermal stability and corrosion resistance, etc., which can significantly reduce the weight of the product, improve the thrust-to-weight ratio and the heat resistance and reliability of the structure. So in aviation, aerospace, Petroleum, chemical, shipbuilding and other departments are widely used. The performance characteristics of titanium alloy are mainly manifested in:

  • (1) The thermal conductivity and thermal conductivity coefficient are small, and the cutting temperature is high. The average thermal conductivity of titanium alloy is half that of industrial pure titanium, and the thermal conductivity coefficients are l, 4 and 1/16 of iron and aluminum, respectively. Therefore, under the same cutting conditions, the cutting temperature of titanium alloy is more than l times higher than that of 45 steel. . (2) The cutting deformation coefficient is small, and the cutting force per unit area is large. The cutting deformation coefficient of the titanium alloy is less than 1 or close to l. After the titanium alloy chips are cut by the main cutting edge, they are immediately turned up to reduce the contact between the chips and the tool rake face. The pressure per unit area of ​​the tool is greater than that of ordinary steel Much bigger. (3) Great chemical activity. Titanium alloys are very easy to “affinity” with tool materials at high temperatures above 300, resulting in serious knife sticking.

3 Process analysis of drilling and tapping of shell parts
The reason why titanium alloy is difficult to process lies in the comprehensive effect of the mechanical, chemical and physical properties of the titanium alloy itself. Due to the low thermal conductivity of titanium alloy, the heat dissipation is slow, which is not conducive to heat balance. Especially in the drilling process, the heat dissipation and cooling effects are very poor. The high temperature is formed in the cutting area, and the springback is large after processing. At this time, the contact area between the tool tip and the workpiece increases, which increases the axial force of the tool tip, and the tool tip easily causes wear and damage. After the drill bit enters the workpiece, the radial elasticity of the titanium alloy is tightened and the thermal conductivity is poor, so that the drill bit is subject to large radial and axial forces. The ordinary drill bit is prone to processing difficulties and increased wear. Titanium alloy has high chemical activity, and is processed under high temperature and high pressure. It reacts with the tool material to form a cladding and diffusion to form an alloy, which causes sticking of the tool and difficult removal of chips. The bit is often bitten and the bit is twisted.
Titanium alloy material is used for internal thread tapping. It is the most difficult process in the titanium alloy cutting process, especially for the processing of small hole threads. And tap tapping is inseparable. The main reason is that the titanium alloy has a low thermal conductivity. During the tapping process, the cutting temperature is high, the chip sticking phenomenon of the chip is not easy to eliminate, and the cutting fluid is not easy to reach the cutting area, etc .; the processing surface is prone to stress concentration. , The surface quality of the processed thread is poor. In addition, the titanium alloy has a large rebound after processing, the hole wall squeezes the tap, and even covers the tooth shape. The tap cannot rotate and the tap is easily broken.

4 Processing plan for parts drilling and tapping
When drilling, select a special high-speed steel drill made of titanium alloy to increase the rigidity by thickening the drill core and reducing its length as much as possible. During processing, the drill bit retracts the knife after drilling into 1-1.5mm, and then retracts the knife after drilling to improve heat dissipation, lubrication conditions and reduce elastic force. The drill is not allowed to rotate in the hole without feeding, so as not to cause work hardening. In actual processing, the measured hole diameter is smaller than the diameter of the drill bit by about 0.1mm, which is one of the performance characteristics of the titanium alloy material. Therefore, a drill bit larger than the standard diameter by about 0.1mm can be used for drilling. You need to drill a hole in the center before drilling. There are two ways to improve the processing effect during tapping.

  • (1) Improve the structure of the tap to increase the rigidity of the tap and reduce the contact area with the processed material. The method is to increase the thickness of the cone core, reduce the friction between the chip and the rake face, select the taper angle of the tap as 8-10 degrees, increase the taper angle of the tap during processing, widen the chip tolerance slot, reduce the width of the blade, and use the jump Taps, etc. In addition, the spiral groove can increase the sharpness of the tap, reduce the cutting force, avoid the break of the tap, and improve the processing quality; it is conducive to chip removal, so that the titanium chips can be discharged outside the workpiece along the spiral groove, to avoid the tap jamming, and improve the processing conditions; The coolant can easily enter the cutting area, improving the advantages of tap cooling and lubrication performance. It was confirmed in the processing that satisfactory results can be obtained when tapping the threaded hole with M3 or more on the machining center using a machine tap with a spiral groove.
  • (2) To set tolerance requirements for the design of threaded bottom holes requires requirements for the tolerance of the diameter of threaded bottom holes. Under the premise of meeting the requirements of thread accuracy, the bottom hole of the thread is processed to the upper difference of the small diameter of the thread as much as possible. If necessary, the hole can be reamed to meet the tolerance requirements. When drilling titanium alloy materials, a drill bit about 0.1mm larger than the bottom hole diameter needs to be used to offset the amount of springback. At the same time, it is necessary to use a sharp drill to drill the bottom hole to reduce the surface hardening layer of the threaded bottom hole. When tapping a threaded hole in a machining center, a floating tapping jacket is needed to offset the coordination error between the spindle and feed, and chamfer the end of the bottom hole before tapping to enhance the accuracy of the tap entry. Slower speed can be selected during tapping to ensure smooth cutting.
  • (3) The coolant used for tapping and drilling is the same. Chlorine-containing coolant is not recommended. Because in the cutting process, the cutting fluid decomposes at high temperature to release hydrogen, which is absorbed by titanium and causes hydrogen embrittlement. At the same time, chlorine may also cause high-temperature stress corrosion cracking of titanium alloys. When drilling shallow holes, electrolytic cutting fluid can be used; when drilling deep holes, N32 mechanical oil plus kerosene is used, the ratio is 3: 1.5, and sulfurized cutting fluid can also be used. The preparation of titanium alloy processing coolant is suitable to extend the life of cutting tools; it is prohibited to use low-melting metals and their alloys (lead or zinc-based alloys, copper, tin and cadmium-based alloys) to make tools, fixtures or temporary fasteners. Tools, fixtures or other devices that come into contact with titanium alloys must be clean and free of dirt. After processing the surface of the titanium alloy, low-melting metals such as lead, zinc, copper, tin and cadmium are not allowed to remain. If there is any residue, it should be washed carefully with hydrofluoric acid and ferrous sulfate solution. When loading and unloading cleaned titanium alloy workpieces, contamination of grease and fingerprints must be prevented to avoid salt stress corrosion and fracture of the titanium alloy workpiece.

5 Conclusion
After his own research in actual production, he summarized the processing methods of drilling and tapping titanium alloy parts. And more reasonable process parameters, it is feasible to use the process method we have summarized to guide production. This process method can also be widely used in future parts.