Titanium alloy materials have the advantages of small density, high hardness and specific strength, and strong corrosion resistance, and are widely used in the aviation and aerospace fields. However, its poor processing performance greatly restricts the processing quality and production efficiency of titanium alloy parts, especially in drilling and tapping. The article mainly describes the performance characteristics of titanium alloys, analyzes the factors that affect the drilling of titanium alloy holes, and proposes corresponding measures to improve the drilling of titanium alloy holes.
Keywords: titanium alloy; hole machining; machining technology
1 Titanium alloy performance characteristics
Titanium alloy is an alloy composed of titanium added with other elements. Titanium has two kinds of heterogeneous crystals: below 882 ℃ is close-packed hexagonal structure α titanium, above 882 ℃ is body-centered cubic β titanium. Titanium alloy elements can be divided into three categories according to their influence on the phase transition temperature: α titanium alloy, β titanium alloy and α + β titanium alloy. Due to the stable microstructure of the alloy, the high temperature deformation properties, toughness, and plasticity are good, and it is an important raw material for the aviation industry. The characteristics of titanium alloy are mainly manifested in:
- High strength, high thermal strength, good corrosion resistance
The density of titanium alloy is generally around 4.5g / cm3, which is only 60% of steel, but the specific strength (strength / density) of titanium alloy is much greater 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, it still has a high specific strength, which is about 3 to 4 times that of the aluminum alloy. Titanium alloys are particularly resistant to stress corrosion, and the dense oxide film formed on the surface has excellent corrosion resistance to organic objects such as acids, alkalis, chlorides, and chlorine. Therefore, titanium alloys are used in aircraft engine components, skeletons, skins, fasteners and landing gear.
- Poor thermal conductivity
The thermal conductivity of titanium λ = 15.24W / (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 worse, and high temperatures are easily formed in the cutting area.
- Titanium and its alloys have high chemical activity
Titanium and its alloys can chemically react with 0, N, H, C0, CO2, water vapor, etc. in the air, forming a hardened layer of TiC and TiN on the surface of the titanium alloy, which increases brittleness and reduces plasticity; processing under high temperature and high pressure It reacts with the tool material to form a cladding 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 processing quality of titanium alloy holes. By understanding the performance characteristics of titanium alloy, analyzing its processing characteristics, and combining the problems that occur in processing, the difficulty of processing titanium alloy is mainly reflected in the following points.
- During the drilling process, high temperature is easy to form in the drilling area; coupled with the small contact area between the drill bit and the rake face, the tool tip stress is large, the chip is not easy to discharge, the drilling amount is not easy to control, and it is easy to cause parts deformation and Burning knife phenomenon.
- The titanium alloy has a small modulus of elasticity, and the titanium alloy produces a large deformation under the action of drilling force when drilling; when the drilling is completed, the processed surface produces a large rebound, causing the processed parts Super poor, rough surface.
- Due to the high chemical activity of titanium and titanium alloys, the large affinity of titanium alloys, coupled with the effect of high temperature and high pressure, it is easy to produce sticking phenomenon during drilling, and the chips are not easy to be discharged when they are squeezed in the groove of the drill bit, resulting in chip adhesion , Even the phenomenon of twisting the drill bit.
In view of the above-mentioned problems of titanium alloy holes in processing, it is necessary to improve the processing methods in titanium alloy processing, and improve the quality and processing efficiency of titanium alloy processing holes by improving tools.
- 3 Measures to improve the processing quality of titanium alloy holes
Because titanium alloys are prone to high-temperature regions and ablated tools during hole machining, they have built-up edges, parts are out of tolerance, and rough surfaces are processed. In hole machining, the quality and efficiency of holes can be improved from the following aspects .
Choose the right drill and tool
- Through the analysis of the drill test of different materials, the ultra-fine-grained carbide drill bit has sharp edges and little wear when processing titanium alloy, and the efficiency has been significantly improved. It is an ideal cutting tool material. If there is no condition, you can choose high speed Steel M42, B201 or cemented carbide drill bit.
- Designing four guide blades on the drill bit can increase the cross-sectional moment of inertia of the drill bit and increase rigidity. Its durability is about 3 times higher than that of the standard drill bit. At the same time, the expansion of the hole is reduced due to the stable guidance. For example, the expansion of the hole of the four guide blades of Ф3mm is only 0.03mm, while the standard drill is 0.06mm.
- Adopt “S” shape or “X” shape to grind the cutting edge of the drill bit, the length of the cutting edge is 0.08 ～ 0.1 bit diameter, and ensure the symmetry of the cutting edge is less than 0.05mm. Both types of transverse edges can form a second cutting edge, which plays the role of chip separation and reduces the axial force during drilling.
- Select a suitable gun drill: when drilling a deep hole with a titanium alloy length-diameter ratio greater than 5, when the hole diameter is less than or equal to 30mm, a cemented carbide gun drill is generally used; when the hole diameter is greater than 30mm, a carbide drill or Spray suction drill, etc.
Choose the right cutting fluid
Choose a cooling method suitable for titanium alloy processing, effective cooling, can extend the life of cutting tools and improve processing efficiency.
- Electrolytic cutting fluid can be used when drilling shallow holes. Its components are triethanolamine, sebacic acid, boric acid, sodium nitrite, glycerin and water.
- When drilling deep holes, water may form steam bubbles on the cutting edge at high temperature, which is prone to build up chips and make the drilling unstable, so it is not suitable to use water-based cutting fluid. Soybean oil is generally used. If necessary, oil and additives for drilling and tapping can be added as cutting fluid. After being atomized by a high-pressure air oil mist atomizer, the cooling air mist is directly sent to the cutting area through the through-hole channel for cooling and lubrication, and the chips are easy Discharge the drill bit to get better cooling and lubrication effect.
Use reasonable processing methods
The performance characteristics of titanium alloys require that titanium alloys adopt appropriate methods during drilling. Generally low speed and moderate feed rate are used; when processing deep holes, the tool should be withdrawn frequently and the chips should be removed in time, paying attention to the shape and color of the chips. If the chips appear plume or the color changes during drilling, it indicates that the drill bit is blunt, and the blade should be changed in time.
In order to improve the rigidity of the process system, the drilling die should be fixed on the workbench, and the guiding of the drilling die should be close to the processing surface, and the short drill bit should be used as much as possible; when manual feed is adopted, the drill bit should not advance or retreat in the hole, otherwise the drilling edge will be rubbed. The surface causes work hardening and dulls the drill bit, affecting hole quality and processing efficiency.
Titanium alloy materials are widely used in aviation 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 are proposed in the hole processing area to improve the processing quality of the hole and the processing efficiency.