Since the discovery of titanium in 1790, humans have been exploring for a century to obtain its extraordinary properties. In 1910, humans first produced titanium metal, but the road to the application of titanium alloys was long and arduous. It was not until 40 years later in 1951 that industrial production was finally realized.
Titanium alloys have the characteristics of high specific strength, corrosion resistance, high temperature resistance and fatigue resistance. The weight of titanium alloys of the same size is only 60% of that of steel, but it is stronger than alloy steel. Due to its good properties, titanium alloys have been increasingly widely used in aviation, aerospace, power generation equipment, nuclear energy, ships, chemicals and medical equipment.
Reasons for the difficulty in processing titanium alloys
The four characteristics of titanium alloys, such as low thermal conductivity, severe work hardening, high affinity with cutting tools and small plastic deformation, are the essential reasons for the difficulty in processing titanium alloys. Its cutting index is only equivalent to 20% of that of easy-to-cut steel.
Low thermal conductivity
The thermal conductivity of titanium alloy is only about 16% of that of 45# steel. The heat cannot be conducted away in time during processing, resulting in local high temperature of the cutting edge (the tip temperature during processing is more than 1 times that of 45# steel), which easily causes diffuse wear of the tool.
Severe work hardening
The work hardening phenomenon of titanium alloy is obvious, and the surface hardening layer is more serious than that of stainless steel, which will cause certain difficulties in subsequent processing, such as increased damage to the tool boundary.
High affinity with tools
Severe adhesion with titanium-containing cemented carbide.
Small plastic deformation
It is about 1/2 of the elastic modulus of 45 steel, so the elastic recovery is large and the friction is serious. At the same time, the workpiece is also prone to clamping deformation.
Technical tips for processing titanium alloy
Based on the understanding of the processing mechanism of titanium alloy and previous experience, the main process tips for processing titanium alloy are as follows:
(1) Use a blade with a positive angle geometry to reduce cutting force, cutting heat and workpiece deformation.
(2) Maintain a constant feed to avoid hardening of the workpiece. The tool should always be in feed during the cutting process. The radial cutting depth ae during milling should be 30% of the radius.
(3) Use high-pressure and high-flow cutting fluid to ensure the thermal stability of the machining process and prevent surface degeneration and tool damage due to excessive temperature.
(4) Keep the blade edge sharp. Blunt tools are the cause of heat accumulation and wear, which can easily lead to tool failure.
(5) Process titanium alloy in its softest state as much as possible, because the material becomes more difficult to process after hardening. Heat treatment increases the strength of the material and increases blade wear.
(6) Use a large tool tip radius or chamfer to cut in and put as much of the blade into the cutting as possible. This can reduce the cutting force and heat at each point and prevent local damage. When milling titanium alloy, the cutting speed has the greatest impact on tool life among all cutting parameters, followed by radial cutting depth.
Solve titanium processing problems by starting with the blade
The blade groove wear that occurs when titanium alloy is processed is local wear on the back and front along the cutting depth direction. It is often caused by the hardened layer left by the previous processing. The chemical reaction and diffusion between the tool and the workpiece material at a processing temperature of more than 800°C is also one of the reasons for the formation of groove wear. Because during the processing, the titanium molecules of the workpiece accumulate in front of the blade and "weld" to the blade under high pressure and high temperature to form a built-up edge. When the built-up edge is peeled off from the blade, the carbide coating of the blade is taken away. Therefore, titanium alloy processing requires special blade materials and geometries.
Tool structure suitable for titanium processing
The focus of titanium alloy processing is heat. A large amount of high-pressure cutting fluid must be sprayed onto the cutting edge in a timely and accurate manner to quickly remove the heat. There are unique milling cutter structures specifically for titanium alloy processing on the market.
Starting from the specific machining method
Turning
Titanium alloy products are easy to obtain good surface roughness when turning, 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, and YG10HT are selected according to the existing conditions of the factory.
Tool geometry parameters: appropriate tool front and rear angles, rounded tool tip.
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 get stuck. When finishing and turning thin-walled parts, the tool main deflection angle should be large, generally 75~90 degrees.
Milling
Milling of titanium alloy products is more difficult than turning, because milling is intermittent cutting, and the chips are easy to stick to the blade. When the sticky chip teeth cut into the workpiece again, the sticky chips are knocked off and take away a small piece of tool material, forming a broken edge, which greatly reduces the durability of the tool.
Milling method: generally use down milling.
Tool material: high-speed steel M42.
Generally, alloy steel processing does not use down milling. Due to the influence of the gap between the machine tool lead screw and the nut, when down milling, the milling cutter acts on the workpiece, and the force in the feed direction is the same as the feed direction, which can easily cause the workpiece table to have intermittent movement, causing the cutter to break. For down milling, the cutter teeth will hit the hard skin when they start to cut in, causing the cutter to break. However, since the chips of reverse milling are from thin to thick, the cutter is prone to dry friction with the workpiece when it first cuts in, which aggravates the sticking and chipping of the cutter. In order to smoothly mill titanium alloys, it should also be noted that the front angle should be reduced and the back 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, and the use of shovel-tooth milling cutters should be avoided.
Tapping
When tapping titanium alloy products, because the chips are small, they are easy to stick to the blade and the workpiece, resulting in a large roughness value and high torque on the machined surface. Improper selection and improper operation of taps during tapping can easily cause work hardening, extremely low processing efficiency, and occasional tap breakage.
It is necessary to give priority to the use of a tap with one thread in place. 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 threads long. To facilitate chip removal, a negative angle can also be ground on the cutting taper part. Try to use short taps to increase the rigidity of the taper. The reverse taper part of the taper should be appropriately larger than the standard one to reduce the friction between the tap and the workpiece.
Reaming
The tool wear is not serious when reaming titanium alloys, and both carbide and high-speed steel reamers can be used. When using carbide reamers, the process system rigidity similar to drilling should be adopted to prevent the reamer from chipping. The main problem when reaming titanium alloys is that the reaming finish is not good. The width of the reamer blade must be narrowed with an oilstone to prevent the blade from sticking to the hole wall, but sufficient strength must be ensured. Generally, the blade width is 0.1 to 0.15 mm.
The transition between the cutting edge and the calibration part should be a smooth arc. After wear, it should be repaired in time, and the size of the arc of each tooth should be consistent; if necessary, the calibration part of the back taper can be increased.
Drilling
Titanium alloy drilling is difficult, and the phenomenon of burning and breaking the drill often occurs during the processing. This is mainly due to several reasons such as poor drill bit grinding, untimely chip removal, poor cooling, and poor rigidity of the process system. Therefore, in the drilling of titanium alloys, attention should be paid to reasonable drill bit grinding, large top angle, reduced outer edge front angle, increased outer edge back angle, and the back taper should be increased to 2 to 3 times that of the standard drill bit. Frequently retract the tool and remove the chips in time, paying attention to the shape and color of the chips. If the chips appear feathery or change color during drilling, it means that the drill bit is blunt and the tool should be changed and sharpened in time.
The drill jig should be fixed on the workbench, the drill jig guide knife surface should be close to the processing surface, and try to use a short drill bit. Another issue worth noting is that when manual feeding is adopted, the drill bit should not move forward or backward in the hole, otherwise the drill blade will rub against the processing surface, causing work hardening and making the drill bit blunt.
Grinding
A common problem in grinding titanium alloy parts is that sticky chips cause grinding wheel blockage and part surface burns. The reason is that the thermal conductivity of titanium alloy is poor, which causes high temperature in the grinding area, so that the titanium alloy and the abrasive are bonded, diffused, and react strongly chemically. Sticky chips and grinding wheel blockage lead to a significant decrease in the grinding ratio. As a result of diffusion and chemical reaction, the surface of the workpiece is burned, resulting in a decrease in the fatigue strength of the parts, which is more obvious when grinding titanium alloy castings.
To solve this problem, the measures taken are:
Select suitable grinding wheel material: green silicon carbide TL. Slightly lower grinding wheel hardness: ZR1.
The cutting processing of titanium alloy materials must be controlled from the aspects of tool materials, cutting fluids, and processing parameters to improve the comprehensive efficiency of titanium alloy material processing.
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