Titanium alloys are widely used in aerospace, aerospace, mechanical electronics, biomedical and other industries due to their excellent comprehensive properties such as high specific strength, good corrosion resistance, high heat resistance, low temperature performance and light specific gravity. Therefore, it is increasingly important to improve the production efficiency of cutting titanium alloy materials and ensure the processing quality.
1. Titanium alloy cutting is difficult
Titanium alloy has the characteristics of high strength, high hardness and low density. For example, the most widely used titanium alloy Ti-6Al-4V (TC4) has a tensile strength of 900MPa, a hardness of 250? 375HB, and a density of 4.42 g / cm3, making the overall titanium alloy Structural parts are increasingly used in the aviation industry. Therefore, achieving high-efficiency and high-quality cutting of titanium alloy structural parts has become the key to aviation manufacturing. Compared with aluminum alloys, the difficulties of cutting are mainly reflected in the following aspects:

1. Large cutting force
The source of cutting force has two aspects: deformation and friction. One is the resistance caused by the elastic deformation and plastic deformation of the cutting layer metal, chips and metal on the surface of the workpiece; the second is the friction resistance between the tool and the chip and the surface of the workpiece. Therefore, the higher the hardness and strength of the metal material, the greater the cutting force required for the cutting process, the higher the cutting temperature, the faster the tool wear, so the relative machinability is also worse.

2. High cutting temperature
Titanium alloy has poor thermal conductivity and high specific heat capacity, and cannot transfer cutting heat from the cutting area through chips. About 80% of the heat generated during the cutting process is transferred to the tool, and the temperature of the cutting point is extremely high, which can reach about 1100 ~ 1200 ℃. Therefore, the high temperature heat of the cutting point of the tool during the titanium alloy cutting process is difficult to be quickly taken away by the chip, which accelerates the tool wear.

3. Easy to harden
0.15mm Due to the large chemical activity of titanium, it is easy to chemically react with oxygen and nitrogen in the air to form a hard and brittle skin at high cutting temperatures, resulting in a hardened layer on the cutting surface, the depth of which can reach 0.1? , Resulting in a substantial increase in the surface hardness and accelerated tool wear.
In order to improve the cutting efficiency and processing reliability of titanium alloy materials and obtain the best results of processing titanium alloy components, high-pressure cooling should be combined with the rational selection and design of tool materials, coatings, geometric angles and cutting amounts.
Second, the use of high-pressure cooling and lubrication is an effective measure to solve the problem of titanium alloy cutting

1. Characteristics of high-pressure cooling
In actual use, effective cooling of cutting tools and effective control of chips are prerequisites, requiring high pressure, sufficient flow rate, and precise alignment of the cooling lubricant with the contact area between the cutting edge and the chips. Using a high-pressure cooling system that directly penetrates the spindle / tool and drives multiple external nozzles to spray the external cutting tool to maintain the good running performance of the spindle, quickly cool the tool, workpiece and punching chips, improve the processing quality of the parts, and increase the service life of the tool. Improving the reliability of processing is conducive to the automation of the cutting process.

2. The advantages of high-pressure cooling

  • 2.1 Using high pressure cooling can improve chip breaking
    Accurate orientation and high-speed coolant jet with high pressure of 0 ~ 100MPa can remove chips from the surface of the indexing blade, which can play the role of cooling and early chip breaking, and achieve better chip control. At the same time, the effective cooling of the cutting edge reduces the sensitivity of the cutting edge to thermal fluctuations.
  • 2.2 High-pressure cooling can increase cutting speed
    When processing titanium alloys, tools that mechanically hold indexable sheets and solid carbide tools are generally used. Compared with conventional processing, when high-pressure cooling is used, the cutting tool temperature is increased, and the durability of the tool is improved. The cutting speed can be increased by up to 200%.
  • 2.3 High-pressure cooling can improve the surface quality
    The fluctuation of surface quality is due to the influence of temperature change and cutting force generated during machining. The traditional cooling method has a positive effect on improving the surface quality of the workpiece. The use of high-pressure cooling greatly affects the final finishing result.

3. High-pressure cooling is adopted, and the following points should be considered:

  • 3.1 Accurately coordinate the relationship between pressure, flow and nozzle aperture
    The size of the nozzle aperture should be selected to produce the highest pressure and the flow of cooling lubricant can be used optimally. According to Sandvik’s data, for example, using a 1mm-aperture nozzle on the tool requires a flow of 5L / min of cooling lubricant to maintain adequate pressure.
  • 3.2 Determination of the maximum safe coolant working pressure

It should be noted that the coolant pressure cannot exceed the maximum safe coolant working pressure that the tool can bear. When working, refer to the maximum safe coolant pressure of the tool series provided by the tool manufacturer.

3. the use of liquid nitrogen cooling is another effective measure to solve the problem of titanium alloy cutting

  • Characteristics of liquid nitrogen cooling
    Liquid nitrogen cooling cutting is to use a low boiling point (-196 ℃) medium, and use the characteristics of liquid nitrogen under pressure to send the nitrogen generated by the nitrogen generator into the cutting point in the form of liquid nitrogen instead of a large amount of oil. This type of dry cooling not only cools the tool, helps to break chips quickly and extend the life of the tool, but also has many economic, technical and ecological benefits of dry cutting.

Advantages of using liquid nitrogen cooling

  •  Guarantee accurate dimensional tolerance
    Using cold liquid can remove the heat generated during the cutting process, so that the workpiece being processed is kept at a constant temperature. This ensures precise dimensional tolerances.
  •  Improve tool life
    Liquid nitrogen also keeps the tools in an extremely cold state. Low temperature freezing changes the characteristics of ceramic tools, making them stronger, harder and tougher.
  • Improve the quality of the processed surface
    Cold nitrogen can quench the workpiece and increase the surface hardness of the workpiece.

Using liquid nitrogen cooling, the following points should be considered:

  • Need to consider suspending special solid particles in liquid nitrogen, combined with the speed of the gas hitting the tool and the workpiece, these particles will destroy the gas boundary layer, and the liquid nitrogen is sprayed onto the tool and the workpiece through a nozzle that can adjust its flow rate.