Abstract: Titanium alloys are more and more widely used in aeroengines due to their superior performance, but their poor machining performance restricts their practical application in aeroengines. Among them, the thin-walled cylindrical titanium alloy parts are the most difficult to process. In the past, the mechanical processing method was easy to cause parts deformation, and the size requirements of the parts drawings could not be guaranteed. In order to ensure the quality of parts, the processing method of chemical milling is used, and the performance of chemical milling fixtures directly affects the quality of chemical milling of parts. Through in-depth analysis of part design drawings and its chemical milling process, a set of scientific and feasible structural design methods of titanium alloy chemical milling fixtures were developed. It solves the problems that the main material of the chemical milling fixture is not corroded in the bath containing nitric acid and hydrofluoric acid during the chemical milling process, and the chemical milling fixture can drive the parts to rotate at a uniform speed. The successful application of the titanium alloy chemical milling fixture provides some experience for the design of chemical milling fixtures of such parts in the future.
Keywords: titanium alloy; chemical milling; fixture; anti-corrosion
Among them, the thin-walled cylindrical titanium alloy parts are the most difficult to process. In the past, mechanical processing methods were used to easily cause parts deformation, and the size requirements of parts drawings could not be guaranteed. In order to ensure the quality of parts, the processing method of chemical milling is used, and the performance of chemical milling fixtures directly affects the quality of chemical milling of parts. Through in-depth analysis of part design drawings and its chemical milling process, a set of scientific and feasible structural design methods of titanium alloy chemical milling fixtures were developed. It solves the problems that the main material of the chemical milling fixture is not corroded in the bath containing nitric acid and hydrofluoric acid during the chemical milling process, and the chemical milling fixture can drive the parts to rotate at a uniform speed, thereby meeting the requirements of the chemical milling of the part In turn, the chemical milling quality of the parts is guaranteed.
1 Titanium alloy with its superior performance. It is more and more widely used in aero engines, but its poor machining performance restricts its practical application in aero engines. Among them, the thin-walled cylindrical titanium alloy parts are the most difficult to process. In the past, the mechanical processing method was easy to cause parts deformation, and the size requirements of the parts drawings could not be guaranteed. In order to ensure the quality of the parts, the processing method of chemical milling is proposed. This will inevitably use fixtures to ensure the requirements of the chemical milling process. The performance of the fixture directly affects whether the quality of the chemical milling of the parts is qualified.
- 1.1 Structure before part milling
Parts are cylinders formed by sheet metal rolling and welding before chemical milling (see Figure 1). Among them, 6-φ11 is the installation fixing hole and the fixture. The fixture drawings will be bored after the design and manufacture of the chemical milling fixture. - 1.2 The structure after parts are milled. After the part is milled, it is a cylinder with a complicated mounting base and reinforcing ribs (see Figure 2).
- 1.3 Research content. Design a reasonable chemical milling fixture and apply it to the chemical milling of parts to meet the requirements of the chemical milling process.
2 Basic structure and working principle of chemical milling fixture. Chemical milling fixture is an important process equipment to ensure that chemical milling can be carried out smoothly, and the performance of the fixture directly affects whether the quality of the chemical milling of parts can be qualified. It is required that the chemical milling fixture must satisfy that the main material is not corroded in the bath containing nitric acid and hydrofluoric acid and can drive the parts to rotate at a uniform speed. After investigation, it is found that there is no such kind of chemical milling fixture in China at present. This requires that after in-depth analysis of the part design drawings and its chemical milling process, the design plan of the chemical milling fixture is proposed and the design plan is optimized.
- 2.1 Basic structure of chemical milling fixture. After several design improvements, the chemical milling fixture finally used a spoke composite structure, which has the characteristics of lightness, high strength and good corrosion resistance. The chemical milling fixture is mainly composed of No1 (fixed frame), No2 (movable central axis), No3 (washer), No4 (lock nut) and fasteners (see Figure 3). Among them, No1 (fixed frame) and No2 (active central axis) are the key parts of the chemical milling fixture, and its structure directly affects the success or failure of the chemical milling fixture. The specific structures of No1 (fixed frame) and No2 (movable center axis) are analyzed below: The fixed frame adopts a spoke-welded composite structure, mainly composed of No1 (fixed rod 1), No2 (fixed rod 2), No3 (positioning block 1 ), No4 (positioning block 2), No5 (reinforced triangle), No6 (central shaft sleeve), No7 (reinforced ring), No8 (counterhead cross self-tapping screw) (see Figure 4). The main body is welded with 0Cr17Ni2Mo2 (SUS316) which has good acid resistance. The anti-corrosion layer is planned to adopt the following schemes and requirements: the overall tooling can be hot-dipped PVC adhesive or glass fiber reinforced plastic anti-corrosion layer, with a thickness of 3 to 4 mm; the overall tooling can be hot-dipped PVC adhesive. The positioning block is installed before the first dipping is not cured, the screws are fixed, and the middle through-hole is closed to prevent glue from entering, and then the entire liner is cured. The tooling is lined with glass fiber reinforced plastic. In order to increase the adhesion of the anti-corrosion layer, No3 (positioning block 1) and No4 (positioning block 2) end faces both adopt a groove structure. The active shaft also uses a composite structure, mainly composed of No1 (sleeve), No2 (sleeve), No3 (screw), No4 (tube), No5 (tube), and No6 (pin) (see Figure 5). The internal use of acid-resistant material 0Cr17Ni2Mo2 (SUS316), the external use of PP material (PP material, scientific name polypropylene, good acid resistance, good welding performance; has good mechanical processing properties, widely used.), The joint uses PP hot melt Anticorrosive form of welding.
- 2.2 Working principle of chemical milling fixture. First install the parts to be milled on the surface of the milling fixture No1 (fixed frame) and fix it. After the workpiece back, lengthened part and fixing bolts are coated with self-drying PVC glue, lift it to the chemical milling equipment and wear No2 ( The active axis) is connected to the device (see Figure 1). At this point, the rotation mechanism of the chemical milling equipment drives the No. 2 (active central axis) to rotate, thereby driving the parts to rotate.
3 Manufacturing and application of chemical milling fixture
- 3.1 Manufacture of chemical milling fixture
1) Material aspect. 0Cr17Ni2Mo2 (SUS316) is more acid-resistant than 0Cr18Ni9 (SUS304), but SUS316 is expensive and SUS304 is used instead. The material does not have much impact on the tooling process, only the time in the acid solution is slightly reduced. The PP material is made of industrial grade PP material, which has good welding performance and good acid corrosion resistance. 2) Welding. Fixing frame welding adopts positioning fixture fixed welding, and the method of correcting the fixing rod after welding ensures the welding quality. PP material welding adopts the method of welding the PP electrode through a hot fan (more than 500 degrees air), which also guarantees the welding quality. 3) Anti-corrosion coating. Through the actual comparison of hot-dip PVC glue and glass fiber reinforced plastic as anticorrosion layer, the glass fiber reinforced plastic method is finally adopted: that is, resin and fiber cloth (polyester cloth) are used to wrap on the surface of the part, about 10 layers for 3-4mm, fixed time For 24 hours. This method has the characteristics of lining any profile, good adhesion and high strength. By comprehensively considering the use status of the tooling, polyester cloth + fiber cloth and 3311 resin glue are used for lining.
3.2 Application of chemical milling fixture. After on-site use, the chemical milling fixture is fully matched with chemical milling equipment and parts. In the chemical milling process, the chemical milling fixture drives the parts to rotate freely, and has strong corrosion resistance, which ensures the successful chemical milling of the parts.
in conclusion. Through in-depth analysis of part design drawings and its chemical milling process, a set of scientific and feasible structural design methods of titanium alloy chemical milling fixtures were developed. It solves the problems that the main material of the chemical milling fixture is not corroded in the bath containing nitric acid and hydrofluoric acid during the chemical milling process, and the chemical milling fixture can drive the parts to rotate at a uniform speed. The successful application of the titanium alloy chemical milling fixture provides some experience for the design of chemical milling fixtures of such parts in the future.
