Cutting and processing titanium
Relevance
Various types of machining are used: structures and parts made of titanium alloys: grinding, turning, drilling, milling, polishing.
Due to the low thermal conductivity and other specific properties of titanium, grinding as the final stage of processing is difficult. During grinding, burns can very easily form, defective structures and residual stresses, stretching can occur in the surface layer, which significantly affect the reduction in the fatigue strength of products. Therefore, grinding of titanium parts is necessarily carried out at low speeds and, if necessary, can be replaced by processing with special blades. Grinding should be carried out using strictly regulated modes and subsequent control of the surface of the parts for the presence of burns and be accompanied by an improvement in the quality of the surface of the part due to hardening by plastic deformation (PPD).
Comparative analysis
The process of cold working of titanium alloys is 3−4 times more laborious than the processing of carbon steels, and 5−7 times more difficult than the processing of aluminum. According to MMPP Salyut, titanium alloys VT5 and VT5−1, compared with carbon steel (with 0.45% C), have a relative machinability coefficient of 0.35−0.48, and as for alloys VT6, VT20 and VT22, this the indicator is even less and amounts to 0.22−0.26. It is recommended to use a low cutting speed using a large amount of coolant. Cutting tools are used from the most wear-resistant high-speed steel, preference is given to hard grades of alloys. The cutting speed should be reduced by at least 3−4 times compared to steel machining, this is especially important when working on CNC machines.
Milling
Due to the need to apply great effort during the machining of titanium alloys, large machines are usually used (FP-7, FP-27, FP-9, VFZ-M8, etc.). Milling is the most time-consuming process during the manufacture of parts. A particularly large amount of such work falls on the manufacture of power parts of aircraft frames: ribs, frames, beams, spars, traverses.
Power details
Milling parts such as «traverse», «beams», «rib» requires the use of several methods. 1) The use of special hydraulic or mechanical copiers on universal milling machines. 2) By copiers on copy-milling hydraulic machines. 3) On CNC machines like MA-655S5, FP-11, FP-14. 4) With the help of three-coordinate CNC machines. It is advisable to use: special prefabricated cutters with a variable angle during processing; shaped concave and convex cutters of the radiation profile; end mills with leading to the cylindrical surface of the part of the plane of the table at the required angle.
Machine tools
Cutting and processing of aviation materials is carried out on machine tools, some of which have no analogues abroad. For example, the VF-33 CNC machine (longitudinal milling three-spindle three-coordinate), the purpose of which is the simultaneous processing of panels, monorails, ribs, beams and other such parts for heavy and light aircraft by three spindles.
The machine 2FP-242 V, which has two movable portals and CNC (longitudinal milling three-spindle four-coordinate) is designed for processing overall spars and panels of heavy and wide-body aircraft. Machine FRS-1, equipped with a movable column, horizontal milling and boring, 15-coordinate CNC — designed for processing the butt surfaces of the center section and the wing of wide-body aircraft. SGPM-320, a flexible production module, which includes a lathe, CNC AT-320, a magazine for 13 tools, an automatic manipulator for removing and installing parts of CNC machines. Flexible production complex ALK-250, created for the production of precision parts for the body of hydraulic units.
Tools
To ensure optimal cutting conditions and high surface quality of parts, it is necessary to strictly adhere to the geometric parameters of tools made of hard alloys and high-speed steels. Forged blanks are sharpened with cutters with VK8 hard alloy plates. The following geometric parameters of the cutters are recommended during processing on a gas-saturated crust: the main angle in the plan φ1 =45°, the auxiliary angle in the plan φ =14°, the rake angle γ=0°; clearance angle α = 12°. At the following cutting conditions: feed s = 0.5 — 0.8 mm/rev, cutting depth t not less than 2 mm, cutting speed v = 25 — 35 m/min. Finishing and semi-finishing continuous turning is carried out using tools made of hard alloys VK8, VK4, VKbm, VK6, etc. To a 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. High-speed steel tools (R9K5, R9M4K8, R6M5K5) can also be used. For cutters made of high-speed steel, the following geometric configuration has been developed: tip radius r = 1 mm, clearance angle α = 10°, φ = 15°. Permissible cutting conditions when turning titanium are achieved at a cutting depth t = 0.5−3 mm, v = 24−30 m/min, s <0.2 mm.
Carbide
The working surfaces of the cutters are made of hard alloys VK8, VK6M, VK4 and high-speed steels R6M5K5, R9K5, R8MZK6S, R9M4K8, R9K10. For milling titanium using milling cutters with VK6M alloy inserts, it is recommended to use the following cutting mode: t = 2−4 mm, v = 80−100 m/min, s = 0.08−0.12 mm/tooth.
drilling
Drilling titanium is difficult due to chips sticking to the working surface of the tool and its stuffing into the retracting grooves of the drill, which leads to an increase in cutting resistance and rapid wear of the cutting edge. In order to prevent this, it is recommended to periodically clean the tool from chips during deep drilling. Drills made of high-speed steel R12R9K5, R18F2, R9M4K8, R9K10, R9F5, F2K8MZ, R6M5K5 and hard alloy VK8 are used. In this case, the following drill geometry parameters are recommended: for the helical groove angle of 25−30, 2φ0 = 70−80°, 2φ = 120−130°, α = 12−15°, φ = 0−3°.
coolant
In order to increase productivity in the processing of titanium alloys by cutting, liquids of the RZ SOZH-8 type are used. They belong to the galloid-containing lubricating-cooling. Cooling of the workpieces is carried out by the method of abundant irrigation. The use of liquids containing halides during machining entails the formation of a salt crust on the surface of titanium parts, which, due to heating and the simultaneous action of stress, can cause salt corrosion. To prevent this, after processing with the use of RZ SOZH-8, the parts are subjected to ennobling etching, during which a surface layer up to 0.01 mm thick is removed. During assembly operations, the use of RZ SOZH-8 is not allowed.
Grinding
The machinability of titanium alloys is significantly affected by their chemical and phase composition, type and parameters of the microstructure. The most difficult is the processing of titanium semi-finished products and parts with a rough lamellar structure. This kind of structure is present in shaped castings. In addition, shaped titanium castings have a gas-saturated crust on the surface, which greatly affects tool wear.
Grinding difficulties
The grinding of titanium parts is difficult due to the high tendency of contact setting during friction. The oxide surface film is easily destroyed during friction under the action of specific loads. In the process of friction at the points of contact of the surfaces, there is an active transfer of material from the workpiece to the tool («seizure»). Other properties of titanium alloys also contribute to this: lower thermal conductivity, increased elastic deformation at a relatively low modulus of elasticity. Due to the release of heat on the rubbing surface, the oxide film thickens, which in turn increases the strength of the surface layer.
Grinding belt and circles
When grinding parts made of titanium, belt grinding and grinding with abrasive wheels are used. For industrial alloys, the most common use of abrasive wheels is made of green silicon carbide, which has high hardness and brittleness with stable physical and mechanical properties with higher abrasive abilities than black silicon carbide.
Provider
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