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Analysis of titanium alloy hole processing technology
Jul 11, 2018


Key words: titanium alloy; hole processing; processing technology

1 Titanium alloy performance characteristics

Titanium alloys are alloys based on titanium and added to other elements. Titanium has two isomorphous crystals: below 882 ° C is a close-packed hexagonal structure α titanium, and above 882 ° C is a body-centered cubic β titanium. Titanium alloy elements can be classified into three types according to their influence on the phase transition temperature: an α titanium alloy, a β titanium alloy, and an α + β titanium alloy. Due to the stability of the alloy structure, high temperature deformation performance, toughness and plasticity are important raw materials for the aviation industry. The characteristics of titanium alloy are mainly manifested in:

1.1 High strength, high thermal strength and good corrosion resistance

The density of titanium alloy is generally about 4.5g/cm3, which is only 60% of steel, but the specific strength (strength/density) of titanium alloy is much larger than other metal structural materials. Secondly, the titanium alloy has good thermal stability and can maintain the required strength at moderate temperatures. At 300 °C to 500 °C, there is still a high specific strength, which is about 3-4 times that of the aluminum alloy. Titanium alloy is particularly resistant to stress corrosion, and the dense oxide film formed on the surface has excellent corrosion resistance to organic substances such as acid, alkali, chloride and chlorine. Therefore, the use of titanium alloys in aircraft engine components, skeletons, skins, fasteners and landing gear.

1.2 poor thermal conductivity

The thermal conductivity of titanium λ = 15.24 W / (mK) is about 1/4 of nickel, 1 / 5 of iron, 1 / 14 of aluminum, and the thermal conductivity of various titanium alloys is about 50% lower than that of titanium. Therefore, the heat dissipation is slow, which is not conducive to heat balance, especially in the drilling process, the heat dissipation and cooling effects are even worse, and the high temperature is easily formed in the cutting zone.

1.3 Titanium and its alloys have high chemical activity

Titanium and its alloys can react with 0, N, H, C0, CO2, water vapor, etc. in the air to form TiC and TiN hardened layers on the surface of titanium alloy, which makes the brittleness increase and the plasticity decreases. Processing under high temperature and high pressure It reacts with the tool material to form a solution and diffuse into an alloy, which is not conducive to cutting.

2 Problems in the processing of titanium alloy holes

There are many factors that affect the quality of titanium alloy hole processing. By understanding the performance characteristics of titanium alloys, analyzing their processing characteristics, combined with the problems in processing, the difficulty of titanium alloy processing is mainly reflected in the following points.

(1) During the drilling process, it is easy to form high temperature in the drilling area; in addition, the contact area between the drill bit and the rake face is small, the tool tip stress is large, the chip is not easy to discharge, the drilling amount is difficult to control, and the part deformation is easy. Burning knife phenomenon.

(2) The elastic modulus of titanium alloy is small, and the titanium alloy produces large deformation under the action of drilling force during drilling; when the drilling is completed, the surface of the machined surface is greatly rebounded, causing the processed parts. Out of tolerance, rough surface.

(3) Due to the high chemical activity of titanium and titanium alloys, the affinity of titanium alloy is large, coupled with the high temperature and high pressure, it is easy to produce sticky knife phenomenon during drilling. The chips are not easily discharged in the groove of the drill bit, resulting in the accumulation of built-up edge of the tool. There are even phenomena such as twisting the drill bit.

In view of the above problems in the processing of titanium alloy holes in Yishan, the processing method needs to be improved in the processing of titanium alloy, and the quality and processing efficiency of the processing holes of titanium alloy are improved by improving the tool.

3 Measures to improve the quality of titanium alloy hole processing

Due to the high temperature area, ablation tool, the built-up edge of the titanium alloy during the hole processing, the parts are out of tolerance, the processing surface is rough, etc., in the processing of the hole, the quality and processing efficiency of the hole can be improved from the following aspects: .

3.1 Choosing the right drill bit and tool

(1) According to the test of different materials, the ultra-fine grain hard alloy drill bit has sharp edge and low wear when processing titanium alloy, and the efficiency is significantly improved. It is an ideal cutting tool material. If there is no condition, you can choose high speed. Steel M42, B201 or carbide drills.

(2) Designing four guide blades on the drill bit can increase the section moment of inertia of the drill bit and increase the rigidity, and its durability is about 3 times higher than that of the standard drill bit. At the same time, since the guiding stability reduces the amount of hole expansion, for example, the four guide blades of the Ф3 mm have a hole expansion of only 0.03 mm, and the standard drill bit is 0.06 mm.

(3) Use the "S" shape or "X" shape to grind the chisel chisel edge. The length of the chisel edge is 0.08~0.1 bit diameter, and the symmetry of the chisel edge is less than 0.05mm. Both forms of the chisel edge form a second cutting edge that acts as a chip and reduces the axial force during drilling.

(4) Choosing a suitable gun drill: When drilling a deep hole with a titanium alloy aspect ratio of more than 5, when the hole diameter is less than or equal to 30 mm, a hard alloy gun drill is generally used; when the hole diameter is larger than 30 mm, a carbide drill bit or Spray suction drills, etc.

3.2 Choosing the right cutting fluid

The cooling method suitable for titanium alloy processing is used for effective cooling, which can extend the life of the cutting tool and improve the processing efficiency.

(1) Electrolytic cutting fluid can be used in drilling shallow holes, and its composition is triethanolamine, azelaic acid, boric acid, sodium nitrite, glycerin and water.

(2) When drilling deep holes, water may form vapor bubbles on the cutting edge at high temperatures, which may cause built-up edge and make the drilling unstable. It is not suitable to use water-based cutting fluid. Soybean oil is generally used. If necessary, drilling oil and additives for drilling and tapping can be added as cutting fluid. After atomization by high-pressure air oil mister, the through-hole channel directly sends the cooling gas mist to the cutting area for cooling and lubrication, and the chip is easy to cut. Drain the drill bit for better cooling and lubrication.

3.3 Adopt reasonable processing methods

The performance characteristics of titanium alloys require that the titanium alloy be processed in a suitable manner during processing. Generally use low speed, moderate feed; when machining deep holes, retreat the knife and remove the chips in time, pay attention to the shape and color of the chips. For example, when the cuttings are feathered or the color changes during the drilling process, it indicates that the drill bit is blunt, and the cutting edge should be changed in time.

In order to improve the rigidity of the process system, the drill should be fixed on the workbench. The drill guide should be close to the machined surface, and the short drill bit should be used as much as possible. When manual feed is adopted, the drill bit must not enter the hole, otherwise the drill edge is processed. The surface causes work hardening, which makes the drill bit dull, affecting the quality of the hole and the processing efficiency.

4 Conclusion

Titanium alloy materials are widely used in the aerospace and aerospace fields. However, its poor processing performance restricts the processing quality and production efficiency of titanium alloy parts, especially in drilling and tapping. Therefore, reasonable processing methods, processing methods and processing tools can be proposed in the hole processing to improve the processing quality of the holes and improve the processing efficiency.