First, Material characteristics of titanium alloy.
The specific strength of titanium alloy products is very high in metal structural materials. Its strength is comparable to steel, but its weight is only 57% of that of rigid materials. In addition, titanium and its alloys have strong heat resistance, can still maintain good strength and stability in the atmosphere of 500 ℃, and the working temperature can even be higher in a short time. Titanium alloy has the characteristics of small specific gravity, high thermal strength, good thermal stability and corrosion resistance, but the material is difficult to cut and has low processing efficiency. So how to overcome the difficulty of processing titanium alloy and the difficulty of low efficiency has always been our problem.
Second, the cutting of titanium alloy
Titanium alloy products are easy to get good surface roughness, work hardening is not serious, but the cutting temperature is high, and the tool wears quickly. In response to these characteristics, the following measures are mainly taken in terms of tools and cutting parameters:
Cutter material: YG6, YG8, YG10HT are selected according to the existing conditions of the factory.
Tool geometric parameters: suitable front and back angles of tool and rounded 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 pierce the tool, and the main deviation of the tool when finishing and turning thin-walled parts The angle should be large, generally 75 ～ 90 °.
The 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 teeth of the viscous chips cut into the workpiece again, the viscous chips are hit and take away a small piece of tool material The chipping greatly reduces the durability of the tool. Metal processing WeChat, the content is good, it is worthy of attention. Therefore, three measures have been taken for titanium alloy milling:
Milling method: generally adopt down milling. Tool material: high-speed steel M42. Improve the rigidity of the process system in terms of workpiece clamping and equipment.
It should be pointed out here that the general alloy steel processing does not use down milling. Due to the influence of the clearance between the screw and the nut of the machine tool, the milling cutter acts on the workpiece during the down milling, and the component force and feed in the feed direction The direction is the same, it is easy to cause gap movement of the workpiece table, causing a knife. For down-milling, the cutter teeth hit the hard skin as soon as they cut in, causing the tool to break. However, due to the thin-to-thick cutting chips, the tool is prone to dry friction with the workpiece during initial cutting, which increases the sticky chips and chipping of the tool. The latter contradiction is more prominent in terms of titanium alloys.
In addition, in order to make titanium alloy milling smoothly, the following points should also be noted: Compared with the general standard milling cutter, the front angle should be reduced and the rear angle should be increased. ; Milling speed should be low. ; Try to use sharp tooth milling cutters, avoid using shovel tooth milling cutters; the tip should be smooth transition; use a lot of cutting fluid. ; In order to improve production efficiency, the milling depth and width can be appropriately increased. The milling depth is generally 1.5 to 3.0mm for rough machining and 0.2 to 0.5mm for finishing.
The common problems of grinding titanium alloy parts are sticky chips causing blockage of the grinding wheel and burns on the surface of the parts. The reason is that the thermal conductivity of the titanium alloy is poor, which causes high temperature in the grinding area, so that the titanium alloy and the abrasive are bonded, diffused, and have a strong chemical reaction. Sticky chips and grinding wheel clogging lead to a significant reduction in the grinding ratio. As a result of diffusion and chemical reactions, the workpiece is burned by the grinding surface, resulting in reduced fatigue strength of the part, which is more obvious when grinding titanium alloy castings.
To solve this problem, the measures taken are: select the appropriate grinding wheel material: green silicon carbide TL. Slightly lower grinding wheel hardness: ZR1. The coarser wheel size: 60. Slightly lower wheel speed: 10 ～ 20m / s. For a slightly smaller feed volume, fully cool with emulsion.
Titanium alloy drilling is more difficult, and burn-out and broken drills often occur during processing. This is mainly due to poor grinding of the drill bit, inadequate chip removal, poor cooling, and poor rigidity of the process system. Therefore, the following points must be noted in the drilling of titanium alloys: Tool material: high-speed steel M42, B201 or cemented carbide. Reasonable bit sharpening: increase the apex angle, reduce the front edge angle of the outer edge, increase the back angle of the outer edge, and increase the inverted cone to 2 to 3 times of the standard bit. Withdraw the tool frequently and remove the chips in time, paying attention to the shape and color of the chips. If the chips appear plume or color changes during drilling, it indicates that the drill bit is blunt, and the blade should be changed and sharpened in time.
Add enough cutting fluid: generally use soybean oil, if necessary, add French OLTIP drilling and tapping special oil. Improve the rigidity of the process system: the drill mold should be fixed on the workbench, the guide of the drill mold should be close to the processing surface, and the short drill bit should be used as much as possible. Another noteworthy problem is that when manual feed is used, the drill bit must not advance or retreat in the hole, otherwise the drill edge rubs against the machining surface, causing work hardening and dulling the drill bit.
Titanium alloy reaming tool is not serious, you can use carbide and high-speed steel reamer. Factory commonly used are W18Cr4V, M42, YW1, YG8, YG10HT and so on. When using cemented carbide reamer, the rigidity of the drilling process system should be adopted to prevent the reamer from collapsing. Metal processing WeChat, the content is good, it is worthy of attention. The main problem that occurs when reaming titanium alloy is not only reaming, but the following solutions can be taken: narrow the width of the blade band of the reamer with whetstone to prevent the blade band from sticking to the hole wall, but to ensure sufficient strength, the blade width is generally 0.1 ~ 0.15mm is better.
The transition between the cutting edge and the calibration part should be a smooth arc. After wear, it should be sharpened in time, and the arc of each tooth should be the same size; if necessary, the inverted cone of the calibration part can be increased; twice reaming. The coarse hinge allowance is 0.1mm, the fine hinge allowance is generally less than 0.05mm; the spindle speed is 60r / min; when the cutter is retracted, the hand hinge cannot be reversed to exit, and the machine hinge should not stop to exit the reamer.
Tapping of titanium alloy products, especially tapping of small holes below M6mm, is quite difficult. Mainly because the chips are small, it is easy to bond with the blade and the workpiece, resulting in a large surface roughness value and a large torque. Improper tap selection and improper operation during tapping can easily cause work hardening, extremely low processing efficiency and occasional breakage of the tap. The solution is as follows:
Preferably use a thread-in-place tap, the number of teeth should be less than the standard tap, generally 2 to 3 teeth. The cutting cone angle should be large, and the taper part is generally 3 to 4 thread lengths. In order to facilitate chip evacuation, a negative inclination angle can also be ground on the cutting cone. Try to use short taps to increase the rigidity of the taps. The inverted cone part of the tap should be properly 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 ream the hole to reduce the work hardening of the bottom hole. For threads with a pitch of 0.7 to 1.5mm, the size of the bottom hole can be processed to the upper difference of the standard thread bottom hole specified by the national standard and allowed to be increased by 0.1mm.