1. Material properties of titanium alloys
The specific strength of titanium alloy products is very high among metal structural materials. Its strength is comparable to that of steel, but its weight is only 57% of that of rigid materials. In addition, titanium and its alloys have strong heat resistance, and can still maintain good strength and stability in the atmosphere of 500 ° C, 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. Therefore, how to overcome the difficulty of processing titanium alloys and the difficulty of low efficiency has always been our problem.
2. Machining of titanium alloys
1) Turning
Turning of titanium alloy products is easy to obtain better surface roughness, and 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: appropriate tool front and rear angles, tool tip rounding.
Low cutting speed, moderate feed rate, deep cutting depth, sufficient cooling, the tool tip should not be higher than the center of the workpiece when turning the outer circle, otherwise it will be easy to stick the tool, and the tool will be biased when finishing turning and turning thin-walled parts. The angle should be large, generally 75 to 90°.
2) Milling
Milling of titanium alloy products is more difficult than turning, because milling is intermittent cutting, and the chips are easy to bond with the blade. Chipping, greatly reducing 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: Climb milling is generally used. Tool material: high speed steel M42. Improve the rigidity of the process system in terms of workpiece clamping and equipment.
What needs to be pointed out here is: generally, the processing of alloy steel does not use climb milling. Due to the influence of the clearance between the screw and nut of the machine tool, when the milling cutter acts on the workpiece, the component force and feed in the feed direction In the same direction, it is easy to cause intermittent movement of the workpiece table, resulting in knife punching. For climb milling, the cutter teeth hit hard skin as soon as they cut in, causing the cutter to break. However, since the up-milling chips are from thin to thick, the tool is prone to dry friction with the workpiece during the initial cut, which increases the sticking and chipping of the tool. As far as titanium alloys are concerned, the latter contradiction is more prominent.
In addition, in order to make the titanium alloy milling smoothly, the following points should also be paid attention to: Compared with the general standard milling cutter, the rake angle should be reduced and the rear angle should be enlarged. ;The milling speed should be low. ; Use sharp-toothed milling cutters as much as possible, and avoid using spade-toothed milling cutters; the tool tip should be smoothly transferred; a large amount of cutting fluid should be used. ;In order to improve production efficiency, the milling depth and width can be appropriately increased. The milling depth is generally 1.5-3.0mm for rough machining and 0.2-0.5mm for finishing.
3)Grinding
Common problems with grinding titanium alloy parts are sticky chips that cause wheel clogging and burns on the surface of the part. The reason is that the thermal conductivity of titanium alloy is poor, which causes high temperature in the grinding area, so that titanium alloy and abrasive will bond, diffuse and have a strong chemical reaction. Sticky chips and blockage of the grinding wheel lead to a significant drop in the grinding ratio. As a result of diffusion and chemical reactions, the workpiece is burnt on the ground surface, resulting in a reduction in the fatigue strength of the part, which is more pronounced when grinding titanium alloy castings.
In order to solve this problem, the measures taken are: choose the appropriate grinding wheel material: green silicon carbide TL. Slightly lower wheel hardness: ZR1. Coarse grinding wheel grit: 60. Slightly lower grinding wheel speed: 10～20m/s. Slightly smaller feed, fully cooled with emulsion.
4)Drilling
Titanium alloy drilling is more difficult, and the phenomenon of knife burning and drill breaking often occurs during processing. This is mainly due to several reasons such as poor sharpening of the drill bit, untimely chip removal, poor cooling and poor rigidity of the process system. Therefore, the following points must be paid attention to 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 rake angle of the outer edge, increase the rear angle of the outer edge, and increase the reverse taper to 2 to 3 times that of the standard drill bit. Retract the tool frequently and remove the chips in time, pay attention to the shape and color of the chips. If the chips appear feathery or change in color during the drilling process, it indicates that the drill bit is blunt and should be replaced in time for sharpening.
Add enough cutting fluid: soybean oil is generally used, and French OLTIP special oil for drilling and tapping can be added if necessary. Improve the rigidity of the process system: the drill die should be fixed on the worktable, the drill die guide should be close to the processing surface, and the short drill bit should be used as much as possible. There is also a noteworthy problem: when manual feeding is adopted, the drill must not advance or retreat in the hole, otherwise the drill edge will rub the machined surface, causing work hardening and dulling the drill.
5)Reaming
When titanium alloy is reamed, the tool wear is not serious, and both cemented carbide and high-speed steel reamers can be used. Commonly used in factories are W18Cr4V, M42, YW1, YG8, YG10HT, etc. When using carbide reamers, the rigidity of the process system similar to drilling should be adopted to prevent the reamer from chipping. Metal processing WeChat, the content is good, it is worthy of attention. The main problem in titanium alloy reaming is that the reaming is not enough, and the following solutions can be taken: narrow the width of the reamer's margin with oilstone, so as to prevent the margin from sticking to the hole wall, but to ensure sufficient strength, generally the width of the blade 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 reground in time after wear, and the arc size of each tooth should be the same; if necessary, the calibration part can be increased to reverse taper; reamed twice. The rough reaming allowance is 0.1mm, and the fine reaming allowance is generally less than 0.05mm; the spindle speed is 60r/min; when the reaming is completed, the hand reaming cannot be reversed to exit, and the machine reaming should not stop to exit the reamer.
6)Tapping
It is quite difficult to tap titanium alloy products, especially small holes below M6mm. The main reason is that the chips are small and easy to bond with the cutting edge and the workpiece, resulting in a large surface roughness value and a large torque. Improper selection of taps and improper operation during tapping can easily cause work hardening, extremely low processing efficiency and sometimes tap breakage. The solution is as follows:
It is preferred to use a jumping tooth tap that is in place, and the number of teeth should be less than that of 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 use short taps to increase tap rigidity. The inverted taper part of the tap should be appropriately larger than the standard one to reduce the friction between the tap and the workpiece.
When processing threaded bottom holes, first rough drilling and then reaming holes to reduce the work hardening of bottom holes. For threads with a pitch of 0.7 to 1.5mm, the size of the bottom hole can be machined to the upper difference of the standard threaded bottom hole specified in the national standard, and an additional 0.1mm is allowed.
If it is not limited by the position of the screw hole and the shape of the workpiece, use machine tapping as much as possible to avoid work hardening caused by uneven feed and pause in the middle of manual tapping.