Cutting and processing of titanium
For the manufacture of structures and parts made of titanium alloys used various kinds of mechanical processing: grinding, turning, drilling, milling, polishing.
One of the important features in the machining of titanium and alloy is that it is necessary to provide resource, especially fatigue properties, are largely dependent on the qualities of the surface layer, which is formed during cold processing. Due to the low thermal conductivity etc. the specific properties of titanium, carrying out grinding as the final stage of processing is difficult. During grinding can very easily burn marks formed in the surface layer can occur defective structure and residual stresses stretching, which significantly affect the reduction in fatigue strength of the products. Therefore, grinding of titanium parts is necessarily performed at low speeds and, if necessary, can be replaced by cutting or grinding, low-speed methods. In the case of the use of grinding, it should be carried out with the use of strictly regulated regimes with a follow-up control surfaces for the presence of burn marks and be accompanied by improved qualities of parts by surface plastic deformation (PPD).
Due to the high strength properties of titanium are difficult to machining by cutting. It has a high ratio of yield strength to the rupture strength of approximately 0,85−0,95. For example, for steel this figure does not exceed 0.75. As a result, when machining titanium alloys, more effort is required, because of the low thermal conductivity results in a significant increase in temperature in the surface layers of the incision and hampers the cooling of the cutting area. Due to the strong adhesion of titanium is accumulated on the cutting edge, which significantly increases the friction force. In addition, the welding and titanium welding at points of contact of the surfaces leads to changes in the geometry of the tool. Such changes, changing the optimal configuration, entails a further increase efforts to handle, which, consequently, leads to a further increase in temperature at the point of contact and accelerated wear. Most of the increase in temperature in the working area affected by the cutting speed, to a lesser extent it depends on the feed force and the tool. The least influence on the temperature increase has a depth of carrying out of cutting.
Under the influence of high temperature at cutting oxidation of titanium chips and machined parts. This results in subsequent chip problem associated with its recycling and remelting. A similar process to the workpiece in the subsequent can lead to the deterioration of its operational characteristics.
The process of cold processing titanium alloys according to the complexity of 3−4 times more difficult than the processing of carbon steels, and 5−7 times — the processing of aluminium. According to information from the MMPP Salyut, titanium alloy VT5 and VT5−1 in comparison with carbon steel (0,45% C), have a coefficient of relative machinability of 0,35−0,48, and for alloys VT6, VT20 and VT22, this figure is even less, and is 0.22 and 0.26. It is recommended when machining to use a low cutting speed at low flow, using to cool a large amount of coolant. In the processing of titanium products used cutting tools from the most wear resistant high speed steel, preference is given to solid grades of alloys. But even if all the prescribed conditions for the cutting speed should be reduced at least 3−4 times in comparison with the processing of steel, which should provide acceptable tool life, it is especially important when working on CNC machines.
The temperature in the cutting zone and the cutting force can be reduced significantly by increasing the hydrogen content in the alloy by vacuum annealing and the corresponding machining. Conducting alloying of titanium alloys with hydrogen gives, ultimately, a significant reduction in temperature in the cutting zone, makes it possible to reduce the cutting force, increases the durability of carbide tools up to 10 times, depending on the nature of the alloy and the cutting mode. This method makes it possible to increase the processing speed in 2 times without quality loss, and to increase the force and depth when performing the cutting without reducing speed.
For machining parts made of alloys of titanium widely used technological processes, which allow to combine several operations into one through the use of multi-instrumental equipment. Most appropriate this sort of technological surgery on multioperational machine tools (machining centers). For example, for the production of power parts, forgings are used machines MA-655А, OP-17СМН, OP-27C; parts of type «bracket», «column», «body» slub-casting and forging — machines «Horizon», Me-12−250, MA-655А, sheet panels — machine VFZ-M8. On these machines in the processing of most parts implemented the principle of «maximum» completeness of processing in a single operation, which is achieved due to the serial processing of items from several sides on a single machine with multiple installed devices.
Because of the need for greater efforts for mechanical processing of titanium alloys are used, usually large machines (OP-7, OP-27, OP-9, VFZ-M8, etc.). Milling is the most time consuming process during the manufacture of the parts. A particularly large amount of such work necessary for the manufacture of load-bearing parts of the carcasses of the aircraft: ribs, frames, beams, spars, beams.
When milling of parts such as «traverse», «beam», «rib» is used several methods. 1) With special hydraulic or mechanical Cams on universal milling machines. 2) For copiers on koperna-hydraulic milling machines. 3) CNC type MA-655С5, FP-11, FP-14. 4) using three-axis CNC machine tools. Use: the special teams of cutters with a variable during the processing of the angle; contoured, concave and convex cutters radiation profile; end mills with a conclusion to the cylindrical workpiece surface plane of the table to the desired angle.
For the processing of aircraft materials in our country created many machines that are not inferior to world standards, and some of them have no analogues abroad. For example, machine WF-33 CNC (milling transpondernye three-axis) the simultaneous processing of three spindles panels, monorails, ribs, beams and other such parts for heavy and light aircraft.
Machine 2ФП-242 IN having two mobile portal and CNC (milling four-transpondernye) designed to handle the overall spars and panels with heavy and wide-body aircraft. Machine FRS-1, equipped with moving column, horizontal milling boring, 15-year-coordinate CNC — designed for treatment of butt surfaces of the wing center section and wide-body aircraft. Sgpm-320, a flexible manufacturing module, which includes lathe, CNC at-320, a store for 13 tools, the automatic manipulator for removal and installation of parts for CNC. Flexible industrial complex ALA-250, designed for the production of precision parts for the hull units.
To ensure the optimum cutting condition and high quality surface details, strict observance of geometrical parameters of tool from hard alloys and high speed steels. Cutters with plates of hard alloy VK8 used for turning of forgings. We recommend the following geometric parameters of the incisors during treatment in the gas-saturated crust: the main angle φ1 =45°, the auxiliary angle φ =14°, rake angle γ=0°; rear angle α = 12°.Under the following cutting conditions: feed s = 0,5 — 0,8 mm/Rev, cutting depth t of at least 2 mm, cutting speed v = 25 — 35 m/min, To conduct continuous finishing and semifinishing turning tools can be applied from solid alloys VK8, VK4, Wcbm, VK6, etc. when the cutting depth of 1−10 mm, the cutting speed is v = 40−100 mm/min and the feed should be s = 0,1−1 mm/Rev. Can also be applied tools of high speed steel (R9K5, R9M4K8, R6M5K5). For cutters made of high speed steel developed by the following geometry: nose radius r = 1 mm, clearance angle α = 10°, φ = 15°. Valid modes cutting in turning of titanium are achieved when the depth of cutting t = 0.5−3 mm, v = 24−30 m/min, s <0.2 mm.
Conduct milling operations of titanium hinders the buildup of titanium on the teeth of the cutter and the mowing. For the manufacture of the working surfaces of milling cutters are used in hard alloy VK8, ВК6М, VK4 and high-speed steels R6M5K5, R9K5, Р8МЗК6С, R9M4K8, R9K10. To conduct the milling of titanium with with the milling cutters with plates of alloy ВК6М it is recommended to use the following cutting conditions: t = 2 — 4 mm, v = 80 — 100 m/min, s =0,08−0,12 mm/tooth.
Conducting drilling of titanium makes it difficult to chip welding on the tool surface and tamping in the outlet groove of the drill that leads to increasing of cutting resistance and rapid wear of the cutting edge. To prevent this it is recommended that when carrying out deep drilling to periodically clean the tool from the chips. To drilling apply tools from high speed steels Р12Р9К5, R18F2, R9M4K8, R9K10, Р9Ф5, Ф2К8МЗ, R6M5K5 and carbide VK8. In this case we recommend the following parameters for the geometry of the drills: the angle of inclination of the spiral grooves 25−30, 2φ0 = 70−80°, 2φ = 120−130°, α = 12−15°, φ = 0−3°.
To improve performance when machining titanium alloys by cutting and extend the wear of used tool use liquids like coolant RZ-8. They belong to gallaitstraat cooling lubricant cooling. Cooling of the workpiece is carried out by copious irrigation. Application gallaitstraat liquids in the processing entails the formation of salt crusts on the surface of titanium components, which including heating and simultaneous action of stress can cause salt corrosion. To prevent this, after the treatment with the use of coolant RZ-8 the details are refined etched, during which removed the surface layer thickness up to 0.01 mm. During the Assembly operations the use of coolant RZ-8 is not allowed.
On the machinability of titanium alloys significantly affects their chemical and phase composition, type and parameters of the microstructure. The most difficult processing titanium semi-finished products and parts, with coarse lamellar structure. This kind of structure has shaped castings. In addition, of shaped castings of titanium have a gas-saturated crust on the surface, which strongly influences the tool wear.
Conducting grinding of titanium parts is difficult due to the high inclination of the contact setting during friction. The surface oxide film is easily destroyed during friction under the influence of specific loads. In the process of friction in places of contact surfaces is an active transfer of material from the workpiece to the tool («gripe»). Contribute to this as well, and other properties of titanium alloys: the lower the conductivity, the increase in elastic deformation at a relatively low module of elasticity. Due to the heat generated on the friction surface of the thickened oxide film, which in turn increases the strength of the surface layer.
When the processing of titanium parts used belt grinding and polishing abrasive wheels. For industrial alloys, the most common use of abrasive wheels, green silicon carbide, which has high hardness and fragility in the stability of physical and mechanical properties with a higher abrasive ability than black silicon carbide.
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