In recent years, the requirements for higher strength and lighter weight of automobile and aircraft parts have become higher and higher. Therefore, the proportion of CNC machining of these parts using titanium alloy materials is also increasing. Airframe components and jet engine compressor components are also increasingly using titanium alloys. In Boeing’s B787-8 jetliner, the amount of titanium alloy used per aircraft has increased to about 10 tons. This kind of titanium alloy material is generally called “difficult-to-cut material” with poor machinability. CNC machining cutting tools for processing such materials must also be able to adapt to the unique processing conditions and processing methods of difficult-to-cut materials.
At present, the industrial method for the production of titanium metal is the Kraul method, and the product is titanium sponge. The traditional process of making titanium is to melt and cast sponge titanium into an ingot, and then process it into titanium material. Titanium adopts plastic processing, the size of adding soil is not limited, and it can be mass-produced, but the yield rate is low, and a large amount of scraps and residues are generated during the processing.
In view of the above shortcomings of titanium plastic processing, powder metallurgy technology has been developed in recent years. The powder metallurgy process of titanium is the same as ordinary powder metallurgy, but the sintering must be carried out under vacuum. It is suitable for the production of large-volume, small-sized parts, and is especially suitable for the production of complex parts. This method hardly needs to be processed again, and the yield rate is high. It can make full use of titanium waste as a raw material, and can reduce production costs, but it cannot produce large-size titanium parts. Titanium alloy is to add alloying elements to industrial pure titanium to increase the strength of titanium.
Titanium alloys can be divided into three types: α titanium alloy, β titanium alloy and α-β titanium alloy. α-β titanium alloy is composed of α and β dual phases. This kind of alloy has stable structure, high temperature deformation performance, toughness, and plasticity. It can be quenched and aging treated to strengthen the alloy. The main performance characteristics of titanium alloy are:
- High specific strength. Titanium alloy has low density (4.4kg/dm3) and light weight, but its specific strength is greater than that of ultra-high-strength steel.
- High thermal strength. Titanium alloy has good thermal stability, and its strength is about 10 times higher than aluminum alloy under the condition of 300～500℃.
- High chemical activity. Titanium can react strongly with oxygen, nitrogen, carbon monoxide, water vapor and other substances in the air to form a hardened layer of TiC and TiN on the surface.
- Poor thermal conductivity. Titanium alloy has poor thermal conductivity. The thermal conductivity of titanium alloy TC4 at 200℃ is l=16.8W/m•℃, and the thermal conductivity is 0.036 cal/cm•sec•℃.
Selection of tool materials
The selection of tool materials should meet the following requirements:
- Sufficient hardness. The hardness of the tool must be much greater than the hardness of the titanium alloy.
- Sufficient strength and toughness. Since the tool bears great 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 alloy, the cutting edge must be sharp during processing, so the tool material must have sufficient wear resistance, so as to reduce work hardening. This is the most important parameter when choosing a tool for machining titanium alloys.
- The affinity of tool materials and titanium alloys is poor. Due to the high chemical activity of titanium alloy, it is necessary to prevent the tool material and titanium alloy from forming an alloy by dissolving and diffusing, causing sticking and burning.
Accuracy, conditions and correct cutting parameters
Tests on domestic commonly used tool materials and foreign tool materials show that the effect of using high-cobalt tools is ideal. The main function of cobalt can strengthen the secondary hardening effect, increase the hardness and the hardness after heat treatment, and have high toughness and wear resistance. , Good heat dissipation.
Geometric parameters of milling cutter
The machining characteristics of titanium alloy determine that the geometric parameters of the tool are quite different from those of ordinary tools.
Helix angle β Choose a smaller helix angle, the chip flutes will be enlarged, chip evacuation is easy, heat dissipation is fast, and the cutting resistance in the cutting process is also reduced.
When the rake angle γ is cutting, the cutting edge is sharp and the cutting is brisk, avoiding excessive cutting heat from the titanium alloy, thereby avoiding secondary hardening.
The clearance angle α reduces the wear rate of the blade, which is beneficial to heat dissipation, and the durability is also greatly improved.
Cutting parameter selection
Titanium alloy CNC machining should choose lower cutting speed, appropriately large feed rate, reasonable depth of cut and finishing amount, and sufficient cooling.
Cutting speed Vc: Vc=30～50m/min
Feed rate: Take a larger feed rate for rough machining, and a moderate feed rate for finishing and semi-finishing.
Cutting depth ap: ap=1/3d is appropriate, titanium alloy has good affinity, chip removal is difficult, and cutting depth is too large, which will cause tool sticking, burning, and fracture.
The finishing allowance αc is about 0.1～0.15mm on the surface hardened layer of titanium alloy. If the allowance is too small, the cutting edge is cut on the hardened layer, and the tool is easy to wear. The hardened layer processing should be avoided, but the cutting allowance should not be too large.
Titanium alloy processing method
Minimize the temperature rise of the tool tip.
- The cutting speed should not be too high (40～60m/min)
Excessive cutting speed will generate a lot of cutting heat, resulting in reduced tool life. Therefore, excessive cutting speeds should be avoided.
- Shorten the contact time between the tool and the workpiece
The longer the contact time between the tool and the workpiece, the more heat is generated, which will reduce the tool life. The larger the tool diameter, the longer the contact time, so within the allowable range, small diameter tools should be used as much as possible.
- It is not advisable to increase the cutting width
The larger the cutting width, the longer the contact time, which will increase the heat generation. Therefore, it is not advisable to increase the cutting width during processing, but to increase the processing efficiency by increasing the cutting length. The use of long-edged tools is effective for roughing. Shoulder milling with a small cutting width can reduce the cutting heat and make it possible to increase the cutting speed.
- Use 45° entering angle tool
As long as the shape of the workpiece allows, tools with an entering angle of 45° should be used as much as possible to reduce chips and extend tool life.
- Make full use of cutting fluid to improve cooling effect
Make full use of cutting fluids, especially ultra-high pressure cutting fluids above 15MPa, to reduce the tool tip temperature, improve the chip treatment effect, and prevent the buildup of buildup. This can increase the cutting speed and productivity.
Improve tool rigidity
By shortening the overhang of the tool and increasing the diameter of the tool, the rigidity of the tool can be improved and the deflection of the tool can be reduced. Plunge milling is also very effective in applications where a larger overhang must be used due to the shape of the workpiece.
Use of serrated cutting edge tools
Since the serrated cutting edge can reduce the contact width, the heat generation can be reduced. In addition, the cutting fluid can reach the surface to be cut from between the serrations, so the serrated cutting edge tool is very effective for the processing of titanium alloys.
The various properties of titanium make it an attractive material for parts, but many of its properties also affect its machinability. Titanium has an excellent strength-to-weight ratio, and its density is usually only 60% of steel. Titanium has a lower coefficient of elasticity than steel, so it has a harder texture and better deflection. The corrosion resistance of titanium is also better than that of stainless steel, and its thermal conductivity is low. These properties mean that titanium will produce higher and more concentrated cutting forces during processing. It is prone to vibration and causes chattering during cutting; moreover, it easily reacts with the cutting tool material during cutting, thereby increasing the wear of the crescent crater. In addition, its thermal conductivity is poor. Since the heat is mainly concentrated in the cutting area, the tool for processing titanium must have high thermal hardness.
Stability is the key to success
Some CNC machining workshops find that titanium is difficult to process effectively, but this view does not represent the development trend of modern processing methods and tools. The reason for the difficulty is partly because titanium metal processing is an emerging technology and lacks experience to learn from. In addition, difficulties are usually related to expectations and the operator’s experience. In particular, some people have become accustomed to the processing of materials such as cast iron or low-alloy steel. The processing requirements for these materials are generally very low. In contrast, machining titanium seems to be more difficult because the same tools and the same speed cannot be used during machining, and the tool life is also different. Even compared with some stainless steels, the difficulty of processing titanium is still higher. Of course, we can say that processing titanium must take different cutting speeds and feed rates and certain preventive measures. In fact, compared with most materials, titanium is also a completely processable material. As long as the titanium workpiece is stable, the clamping is firm, the machine is selected correctly, the power is appropriate, the working condition is good, and the spindle is equipped with a short tool overhang, all problems will be solved-as long as the cutting tool is correct.
Vibration and heat must be considered
The non-ideal environment also contains other factors, one of which is that most machine tools are currently equipped with IS040 spindles. If the machine tool is used intensively, it will not be able to maintain a new tool state for a long time. In addition, if the structure of the part is complex, it is usually not easy to clamp effectively. Of course, the challenge does not stop there. The cutting process must sometimes be used for full slot milling, side cutting or contour milling, all of which may (but not necessarily) generate vibration and create poor cutting conditions. It is important that when setting up the machine, attention must always be paid to improving stability to avoid vibration trends. Vibration can cause the blade to shatter, damage the blade and produce unpredictable and inconsistent results. One improvement measure is to use multi-level clamping to bring the part closer to the spindle to help counteract vibration.
Advantages and disadvantages of titanium alloy
Advantages of titanium alloy:
- light weight;
- high specific strength;
- resistance to metal fatigue;
- stable chemical properties.
Disadvantages of titanium alloy:
- Poor machinability;
- The strength decreases when the temperature exceeds 400℃ (therefore, it is used in the low temperature part of the engine).
Plastic processing of titanium and titanium alloys has the characteristics of large deformation resistance; normal temperature plasticity difference, high yield limit and strength limit ratio, large springback, sensitivity to notches, easy to bond with the mold during deformation, and easy to absorb harmful gas when heated, etc. Plastic processing is more difficult than steel and copper.
Therefore, the processing technology of titanium and titanium alloys must take these characteristics into consideration.
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