Drilling
It is difficult to machine titanium alloys to drill, and tool burns and drill breaks often occur during the machining process. This is mainly caused by several reasons such as poor sharpening of the drill bit, delayed chip removal, poor cooling, and poor rigidity of the process system. Therefore, the following points must be paid attention to in titanium alloy drilling processing: Tool material: high-speed steel M42, B201 or cemented carbide. Reasonable drill sharpening: increase the top angle, reduce the front angle of the outer edge, increase the rear angle of the outer edge, and increase the inverted cone to 2 to 3 times that of the standard drill. Withdraw the knife frequently and remove the chips in time, paying attention to the shape and color of the chips. If the chips appear feathery or the color changes during the drilling process, it indicates that the drill is blunt, and the tool should be changed and sharpened in time.
Add enough cutting fluid: Soybean oil is generally used, and French OLTIP special oil for drilling and tapping can be added when necessary. Improve the rigidity of the process system: the drilling jig should be fixed on the worktable, the drilling jig guide should be close to the processing surface, and the short drill bit should be used as much as possible. There is also a problem worth noting: When manual feed is adopted, the drill must not stay in the hole, otherwise the drill blade will rub against the machined surface, causing work hardening and dulling the drill.
Reaming
The tool wear is not serious when machining titanium alloy reaming, and cemented carbide and high-speed steel reamers can be used. Commonly used in factories are W18Cr4V, M42, YW1, YG8, YG10HT, etc. When using cemented carbide reamer, the rigidity of the process system similar to drilling should be adopted to prevent the reamer from chipping. The main problem of titanium alloy reaming is that the reaming is not complete. The following measures can be taken: use oilstone to narrow the width of the reamer blade to prevent the blade from sticking to the hole wall, but ensure sufficient strength. Generally, the blade width is 0.1 ~0.15mm is better. The transition between the cutting edge and the calibration part should be a smooth arc, and it should be sharpened in time after being worn, and the size of the arc of each tooth should be the same. If necessary, the inverted cone of the calibration part can be enlarged. Reaming twice. The margin of coarse hinge is 0.1mm, and the margin of fine hinge is generally less than 0.05mm. The spindle speed is 60r/min. When the reaming is completed, the hand hinge cannot be reversed and exited, and the reaming machine should not stop and exit the reamer.
Tapping
Titanium alloy tapping, especially for small holes below M6mm, is quite difficult. Mainly because the chip is small, it is easy to bond with the blade and the workpiece, resulting in a large surface roughness value and a large torque. When tapping, improper selection and improper operation of taps can easily cause work hardening, extremely low processing efficiency and sometimes tap breaks. The solution is as follows:
Dongguan cnc processing plant preferentially selects a threaded jumper tap, the number of teeth should be less than the standard tap, generally 2 to 3 teeth. The cutting taper angle should be large, and the taper part is generally 3 to 4 thread lengths. In order to facilitate chip removal, a negative inclination angle can also be ground on the cutting cone. Try to choose short taps to increase the rigidity of the taps. The inverted taper part of the tap should be appropriately enlarged compared to the standard to reduce the friction between the tap and the workpiece. When processing the threaded bottom hole, first rough drill and then use the reaming drill to ream the hole to reduce the work hardening of the bottom hole. For threads with a pitch of 0.7～1.5mm, the size of the bottom hole can be machined to the upper difference of the standard thread bottom hole specified in the national standard and it is allowed to increase by 0.1mm.
If machining is not limited by the position of the screw hole and the shape of the workpiece, try to use machine tapping to avoid work hardening caused by uneven feed and stoppage in the manual tapping. Titanium alloy has become an ideal manufacturing material for aircraft and engines due to its high specific strength, good mechanical properties and good corrosion resistance. However, its poor machinability has restricted its application for a long time. With the development of processing technology, in recent years, titanium alloys have been widely used in the manufacture of parts such as the compressor section of aircraft engines, engine hoods, and exhaust devices, as well as the manufacture of structural frame parts such as aircraft girder frames. The titanium alloy parts of a new type of aero engine of our company account for about 11% of the total number of parts. This article is a summary of the knowledge of the cutting characteristics of titanium alloy materials and the specific characteristics shown in different processing methods and the experience summary of the technological measures that should be taken during the trial production of the new machine.
1 Machinability and general principles of titanium alloys
Titanium alloys are divided into a phase, b phase, and a+b phase according to the metal structure. TA, TB, and TC represent their grades and types, respectively. The materials used in a new engine of our company are TA and TC. General castings and forgings use TA series, and bar materials use TC series.
Features and machinability
Compared with general alloy steel, machining titanium alloy has the following advantages:
The specific strength becomes higher: the density of titanium alloy is only 4.5g/cm3, which is much smaller than iron, and its strength is similar to that of ordinary carbon steel.
Good mechanical properties: Titanium alloy has a melting point of 1660°C, higher than iron, and has higher thermal strength. It can work below 550°C and usually shows better toughness at low temperatures. Good corrosion resistance: A dense oxide film is easy to form on the surface of titanium alloy below 550°C, so it is not easy to be further oxidized. It has high corrosion resistance to the atmosphere, sea water, steam, and some acids, alkalis, and salt media. On the other hand, the machinability of titanium alloys is relatively poor. The main reason is: poor thermal conductivity, resulting in high cutting temperature, reducing tool durability. At temperatures above 600°C, a hard oxide layer is formed on the surface, which has a strong abrasion effect on the knife. Low plasticity and high hardness increase the shear angle, the contact length between the chip and the rake face is very small, the stress on the rake face is large, and the blade is prone to breakage. The elastic modulus is low, the elastic deformation is large, and the surface rebound of the workpiece near the flank face is large.