The specific strength of titanium alloy products is very high among metal structural materials. Its strength is equivalent to that of steel, but its weight is only 57% of steel. In addition, titanium alloy has the characteristics of small specific gravity, high thermal strength, good thermal stability and corrosion resistance, but titanium alloy materials are difficult to cut and have low processing efficiency. Therefore, how to overcome the difficulty and low efficiency of titanium alloy processing has always been a problem to be solved urgently.
Reasons for difficult titanium alloy processing
The thermal conductivity of titanium alloy is small, so the cutting temperature is very high when processing titanium alloy. Under the same conditions, the cutting temperature of processing TC4[i] is more than twice as high as that of No. 45 steel, and the heat generated during processing is difficult to pass through the workpiece. Release; the specific heat of titanium alloy is small, and the local temperature rises quickly during processing. Therefore, the temperature of the tool is very high, the tip of the tool wears sharply, and the service life is reduced.
The low modulus of elasticity of titanium alloy [ii] makes the machined surface easy to spring back, especially the processing spring back of thin-walled parts is more serious, it is easy to cause strong friction between the flank face and the machined surface, which will wear the tool and collapse. blade.
Titanium alloys are very chemically active and easily interact with oxygen, hydrogen, and nitrogen at high temperatures, increasing their strength and decreasing plasticity. The oxygen-rich layer formed during heating and forging makes machining difficult.
Principles of cutting processing of titanium alloy materials [1-3]
In the machining process, the selected tool material, cutting conditions and cutting time will all affect the efficiency and economics of titanium alloy cutting.
1. Choose reasonable tool materials
In view of the properties, processing methods, and processing technical conditions of titanium alloy materials, the tool materials should be selected reasonably. The tool material should be more commonly used, lower price, good wear resistance, high thermal hardness, and have sufficient toughness.
2. Improve cutting conditions
The rigidity of the machine tool-fixture-tool system is better. The clearance of each part of the machine tool should be adjusted well, and the radial runout of the spindle should be small. The clamping work of the fixture must be firm and rigid enough. The cutting part of the tool should be as short as possible, and the thickness of the cutting edge should be increased as much as possible when the chip capacity is sufficient to improve the strength and rigidity of the tool.
3. Carry out appropriate heat treatment on the processed material
Through heat treatment to change the properties and metallographic structure of titanium alloy materials [iii], to achieve the purpose of improving the machinability of the material.
4. Choose a reasonable cutting amount
The cutting speed should be low. Because the cutting speed has a great influence on the temperature of the cutting edge, the higher the cutting speed, the sharp increase in the temperature of the cutting edge, and the temperature of the cutting edge directly affects the life of the tool, so choose an appropriate cutting speed.
Turning titanium alloy products can easily obtain better surface roughness, and the work hardening is not serious, but the cutting temperature is high and the tool wears quickly. In view of these characteristics, the following measures are mainly taken in terms of tools and cutting parameters:
Tool material: YG6, YG8, YG10HT are selected according to the existing conditions of the factory.
Tool geometry parameters: suitable front and rear angles of the tool, tool tip rounding.
Low cutting speed, moderate feed rate, deep cutting depth, sufficient cooling, the tool tip cannot be higher than the center of the workpiece when turning the outer circle, otherwise it is easy to pierce the tool, and the tool will be biased when finishing turning and turning thin-walled parts. The angle should be large, generally 75-90 degrees.
Milling of titanium alloy products is more difficult than turning, because milling is intermittent cutting, and the chips are easy to bond with the cutting edge. When the sticky teeth cut into the workpiece again, the sticky chips are knocked off and a small piece of tool material is taken away. Chipping greatly reduces the durability of the tool.
Milling method: Climbing milling is generally used.
Tool material: high-speed steel M42.
Generally, the machining of alloy steel [iv] does not use down milling. Due to the influence of the gap between the screw and nut of the machine tool, during down milling, the milling cutter acts on the workpiece, and the component force in the feed direction is the same as the feed direction. Intermittent movement of the workpiece table, resulting in knife hitting. For down milling, the cutter tooth hits the crust at the beginning of the cut, causing the cutter to break. However, because up-milling chips vary from thin to thick, the tool is prone to dry friction with the workpiece during the initial cutting, which increases the sticking and chipping of the tool. In order to make titanium alloy milling smoothly, it should also be noted that the rake angle should be reduced and the relief angle should be increased compared to the general standard milling cutter. The milling speed should be low, and sharp tooth milling cutters should be used as much as possible to avoid the use of relief tooth milling cutters.
For the tapping of titanium alloy products, because of the small chips, it is easy to bond with the blade and the workpiece, resulting in large surface roughness and large torque. When tapping, improper selection and improper operation of the tap [v] can easily cause work hardening, the processing efficiency is extremely low, and sometimes the tap is broken.
It is necessary to use a jump-tooth tap with a wire in place first, and the number of teeth should be less than that of a 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 titanium alloy reaming, the tool wear is not serious, and cemented carbide and high-speed steel reamers can be used. When using cemented carbide reamer, the rigidity of the process system similar to that of drilling should be adopted to prevent the reamer from chipping. The main problem of titanium alloy reaming is that the finish of the reaming hole is not good. Whetstone must be used to narrow the width of the reamer blade to prevent the blade from sticking to the hole wall, but sufficient strength must be ensured. Generally, the blade width is 0.1～0.15mm As well.
The transition between the cutting edge and the calibration part should be a smooth arc, and it should be sharpened in time after wear, and the size of the arc of each tooth should be the same; if necessary, the inverted taper of the calibration part can be enlarged.
It is difficult to drill titanium alloys, and the phenomenon of burning tools and broken drills often occurs during the processing. 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, it is necessary to pay attention to reasonable drill sharpening in the titanium alloy drilling process, increase the vertex angle, reduce the front angle of the outer edge, increase the rear angle of the outer edge, and increase the inverted taper 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.
The drilling jig should be fixed on the worktable, and the guiding face of the drilling jig should be close to the processing surface. Use short drill bits as much as possible. Another noteworthy problem is that when manual feed is adopted, the drill should not advance or retreat in the hole, otherwise the drill blade will rub against the machined surface, causing work hardening and dulling the drill.
The common problems in grinding titanium alloy parts are sticky debris causing blockage of the grinding wheel and burns on the surface of the part. The reason is the poor thermal conductivity of the titanium alloy, which causes high temperature in the grinding zone, so that the titanium alloy and the abrasive are bonded, diffused and strongly chemically reacted. Sticky chips and blockage of the grinding wheel lead to a significant reduction in the grinding ratio. As a result of diffusion and chemical reactions, the workpiece is burned on the ground surface, resulting in a reduction in the fatigue strength of the parts, which is more obvious when grinding titanium alloy castings.
To solve this problem, the measures taken are:
Choose a suitable grinding wheel material: green silicon carbide TL. Slightly lower grinding wheel hardness: ZR1.
The cutting of titanium alloy materials must be controlled in terms of tool materials, cutting fluids, and machining process parameters in order to improve the overall efficiency of titanium alloy material processing.