Sawing performance of titanium and titanium alloys
Titanium is an allotrope with a melting point of 1720°C. When it is lower than 882°C, it has a close-packed hexagonal lattice structure, which is called α titanium alloy; when it is above 882°C, it has a body-centered cubic lattice structure, which is called β titanium. There are three types of matrix structures in titanium alloys at room temperature. Titanium alloys are also divided into the following three categories: α titanium alloy, β titanium alloy, and α + β titanium alloy. The national standard represents TA, TB and TC respectively.
α titanium alloy has good machinability, followed by a+β titanium alloy, and β titanium alloy is inferior. Even if it is the same type of titanium alloy, due to its different tensile strength and composition, its cutting performance is also different. The following table is a part of the universal tensile strength and hardness table of titanium and titanium alloys.
|Model||Material composition||Tensile Strength/Mpa||Hardness (HB)|
|TA1||Industrial pure titanium||343|
|TA2||Industrial pure titanium||441|
|TA3||Industrial pure titanium||540|
|TC2||Ti-4Al-1 5Mn||687||HRB 60-70|
Although commercial pure titanium cuts similar to austenitic stainless steel, most titanium alloys are not easy to machine. When the hardness of the titanium alloy is greater than HB350, cutting is particularly difficult, and when the hardness is less than HB300, it is prone to sticking and cutting is difficult. But the hardness of titanium alloy is only one aspect that is difficult to cut. The key lies in the influence of the combination of chemical, physical and mechanical properties of titanium alloy on its machinability. Titanium alloy has the following cutting characteristics:
(1) Small deformation coefficient: This is a significant feature of titanium alloy cutting processing, and the deformation coefficient is less than or close to 1. The sliding friction distance of the chips on the rake face is greatly increased, which accelerates the wear of the band saw blade.
(2) High cutting temperature: Because the thermal conductivity of titanium alloy is very small (only equivalent to 1/5 to 1/7 of 45 steel), the contact length between the chip and the rake face is extremely short, and the heat generated during cutting is not easy to transfer It is concentrated in a small area near the cutting area and the cutting edge, and the cutting temperature is very high. Under the same cutting conditions, the cutting temperature can be more than twice as high as when cutting No. 45 steel.
The following table is a comparison table of the relative cutting performance of titanium alloy and other materials:
|Material||Cutting coefficient (%)|
|Q245 or 45 steel||100|
|Low alloy steel (annealed)||60|
|High alloy steel (annealed)||50|
|Austenitic stainless steel||40|
|Commercial pure titanium (TA3)||43|
|Titanium alloy TC4||25|
(3) The cutting force per unit area is large: the contact length between the chip and the rake face is extremely short, and the cutting force per unit contact area is greatly increased, which is likely to cause chipping. At the same time, due to the small elastic modulus of titanium alloy, it is prone to bending deformation under the action of radial force during processing, causing vibration, increasing band saw blade wear and affecting the accuracy of parts. Therefore, the process system is required to have good rigidity.
(4) Chilling phenomenon is serious: due to the high chemical activity of titanium, it is easy to absorb oxygen and nitrogen in the air to form a hard and brittle skin at high cutting temperatures; at the same time, plastic deformation during cutting will also cause surface hardening . Chilling phenomenon not only reduces the fatigue strength of the parts, but also aggravates the wear of the band saw blade, which is a very important feature when cutting titanium alloys.
(5) The tooth tip of the band saw blade is easy to wear: After the blank is processed by stamping, forging, hot rolling and other methods, it will form a hard and brittle uneven skin, which is very easy to cause chipping, making it difficult to remove the hard skin in the processing of titanium alloys. Process. In addition, due to the strong chemical affinity of titanium alloy to the material of the band saw blade tooth tip, the band saw blade is prone to adhesive wear under the conditions of high cutting temperature and high cutting force per unit area. When sawing titanium alloy, sometimes the abrasion of the rake face is even more serious than that of the flank face. In the sawing process, due to the low thermal conductivity of the titanium alloy material and the extremely short contact length between the chip and the rake face, the heat generated during cutting is not easily transferred, and it is concentrated in a small range near the cutting deformation zone and the cutting edge , During processing, the cutting edge will produce high cutting temperature, which will greatly shorten the life of the band saw blade. For the titanium alloy TC4, the cutting temperature is the key factor affecting the life of the band saw blade, not the size of the cutting force, under the conditions of the strength of the tooth tip and back strength of the band saw blade and the power of the sawing machine. The following table is a summary table of the material characteristics of titanium alloys and the influence of these characteristics on cutting.
|Material characteristics||Impact on cutting|
|Maintain strength even at high temperatures||High cutting force and high temperature|
|Thin chips, concentrated resistance||Large cutting force per unit area on the cutting edge|
|Low thermal conductivity||High red hardness is required for the tooth tip material|
|Resistance to periodic chip formation changes||Easy to cause vibration|
|Chemical affinity||Adhesive wear|
|High carbide content||Produce abrasive wear|
The following table shows the hardness of several typical titanium and titanium alloys and the cutting difficulty coefficient relative to TC4.
|Material type||Grade||Hardness||Cutting difficulty factor|
|Commercial pure titanium||TA2||HB 150-200||170|
|a titanium alloy||TA7||HRC 31-34||115|
|a+β titanium alloy||TC4 (annealed)||HRC 32-36||100|
|Ti185 (high-strength alloy)||HRC 34-38||97|
|TC4 (aging)||HRC 39-41||90|
|β titanium alloy||TB6 (aging)||HRC 35-42||56|
|Ti-13V-11Cr-3AI (annealed)||HRC 39-41||53|
|Ti-5AI-5Mo 5V-3Cr (aging)||HRC 36-44||51|
Different materials choose different saws
|Material type||Recommended band saw blade type||Band saw features|
|Commercial pure titanium and a titanium alloy||Bimetal band saw blade such as MTCUT, DTCUT||The former is a tooth-grinding band saw blade, which can reduce the friction of chips on the rake face, and the latter is a band saw blade with high and low teeth, which can reduce cutting force, high tool accuracy, and good back strength.|
|General purpose toothed carbide band saw blade CB-MP||Hard alloy tooth tip material, good red hardness, high tool precision.|
|a+β titanium alloy||Bimetal band saw blade such as MTCUT, giant bimetal band saw blade||Used for occasionally inefficient sawing of materials less than 100mm in diameter and less than 36HRC in hardness.|
|CB-MP toothed carbide band saw blade||The reason is the same as above, this kind of titanium alloy can be sawed efficiently.|
|CB-PRO segmented carbide band saw blade||Compared with CB-MP, the cutting force is further reduced (this series is recommended for sawing medium and large titanium alloy materials above 400mm).|
|β titanium alloy||CB-PRO segmented carbide band saw blade||This material is the most difficult to saw, and only band saw blade products that significantly reduce the cutting force are more suitable.|
|CB-X925 special tooth segmented carbide band saw blade||In fact, this saw blade is mainly developed to cut large superalloys, but the cutting difficulty of this material is too high. Therefore, it can also be applied.|
If a CB-MP carbide band saw blade is used, the rough sawing parameters of several typical titanium alloys are as follows (TC and TB use the same hardness, in order to explain that although the hardness is the same, the difficulty coefficient of different types of titanium alloys Also different):
If you switch to a bimetal band saw blade, the line speed and efficiency will be reduced accordingly, and if you switch to the CB-PRO and CB-X925 with lower cutting force, the cutting efficiency can be appropriately improved. Even if it is the same material, if the hardness difference is significant, such as 35HRC and 41HRC, then the parameters need to be adjusted appropriately.
|Material designation||hardness||Linear speed (m/min)||Cutting efficiency (cm2/min)|
If the sawing machine has a stepped variable speed or the saw band speed cannot meet the requirements, the lower saw band speed is used, and the cutting efficiency is reduced accordingly.
When sawing this material, if there is no crusty, there is no need to run in too much. When converting from running-in to standard sawing parameters, the speed of the saw band should be increased first, and then the feed rate should be increased. Avoid vibration during the entire adjustment process.
Use a long saw blade to process the hard leather first, so that you can save the amount of band saw.
Choose extreme pressure cutting oil or 10~15% extreme pressure emulsion. In addition, the supply of cutting fluid must be sufficient to play its role. If the cutting fluid supply of some machine tools is insufficient or the pressure is too low, consider modifying the cooling system.
The machine tool is in good working condition and has sufficient cutting power.
The position of the wire brush should be adjusted properly to eliminate the titanium chips in the tooth part of the saw belt.
The spacing of the guide arms should be adjusted appropriately to increase cutting stability and system rigidity.
The gap between the saw band and the back of the guide wheel is adjusted appropriately to maintain the life of the saw band.