High-speed electric spark hole machining principle
High-speed electric spark hole machining is an emerging advanced manufacturing technology in recent years.
The principle is that a high-pressure working fluid is passed through the rotating hollow tubular electrode to wash away the machining debris while maintaining a high current density for continuous normal discharge. The rotation of the electrode can make the end face loss uniform and not be deflected by the reaction force of the high pressure and high speed working fluid. However, in the case of EDM small hole processing, especially deep hole processing, it is easy to leave the burr core and the removed fine particles of the workpiece material on the workpiece, hinder the high-speed circulation of the working fluid, and make the processing speed low and the processing unstable. As well as the increase of tool electrode loss, etc., these tiny particles trapped in the narrow processing gap often cause secondary discharge, which makes the process unstable and even short-circuit phenomenon, which makes processing difficult.
On the other hand, since the small-diameter tool electrode is liable to sway or vibrate during processing, it is more conducive to secondary discharge. It is precisely because of the existence of such secondary discharge caused by the electric discharge small hole processing that the thickness of the remelted layer is increased, which puts forward higher technical requirements for the processing technology of the high pressure turbine blade of the aeroengine. Through process test and verification, we can reasonably optimize the process parameters of EDM machining, which can reduce the remelting layer and improve the quality of film hole processing.
Analysis of processing technology of high vortex blade film hole
High-pressure turbine blades are key components in aero-engine turbines and operate in a very harsh environment. In the engine cycle, the high-pressure turbine blades are subjected to the impact of high-temperature and high-pressure gas generated after combustion. The effective cooling measures can ensure safe and reliable operation of the engine, prolong the service life of the engine, and reduce the cost of high-temperature materials. The performance of the engine depends to a large extent on the temperature of the turbine inlet, which is limited by the material and construction of the turbine blades. The turbine blades are thus continuously cooled so that they can operate safely and reliably beyond the melting point of the material at the permissible working ambient temperature.
The film cooling technology is one of the representative important structural improvements, which greatly improves the performance of the engine: air film cooling is to distribute n rows of cooling film holes in the blade height on the turbine blade leaves, and eject from the film holes. The cooling airflow forms a gas film on the surface of the blade body to block high temperature gas, thereby improving the high temperature resistance of the blade material. The processing quality of the cooling film hole directly affects the cooling effect of the high pressure turbine blade and affects the T life of the blade.
The processing method of the film cooling hole usually has a method such as laser processing or electric discharge machining (EDM). The former has a fast processing speed and low cost, but due to the laser melting effect, the roughness in the hole is not uniform, forming a thick remelted layer, and the actually measured hole diameter is smaller than the actual flow diameter of the hole; the latter has high processing precision. The roughness in the hole is uniform, and the remelted layer is thin, and the formed hole can be processed, but the processing time is long and the cost is high.
In order to obtain better quality of film hole processing, most of the high-vortex blade film holes in the aviation industry are processed by electric spark. In the EDM process of the film hole, there are the following problems: 1 the working fluid pressure is reduced, the matrix impurities, the residual surface of the inner cavity surface core, etc., which will lead to prolonged processing time, resulting in thickening of the remelted layer, and even Microcracks appear. 2 When the film is filled, the electrode and the original hole are completely overlapped, and secondary discharge is likely to occur, which may affect the surface state of the part. 3 At present, there is no mature processing technology in which the sharp corner of the slanting hole of the air film hole is rounded. It can be seen that the thickness of the remelted layer is a key factor affecting the quality of the film hole. The optimization of the process parameters can minimize the thickness of the remelted layer to improve the processing quality of the film hole.
The thickness of the remelted layer is the finger used to measure the quality of the film hole in the parameter design. It is the core factor of the parameter design. The basic idea is to make the processing quality of the film hole by selecting the best horizontal combination of all the parameters in the system. Preferably, the smaller the thickness of the remelted layer, the better the processing quality of the film hole. The analysis process of the remelted layer is actually a preferred problem. It uses the orthogonal table to measure the thickness of the remelted layer T as the evaluation index of the processing quality of the film hole, and uses statistical techniques to analyze and determine the optimal level combination. The greater the difference in the thickness of the remelted layer, the higher the level of influencing factors.
In the mechanical manufacturing industry, titanium alloy materials have characteristics that other metal materials cannot possess: high specific strength, high thermal strength, good corrosion resistance; alloy density is only 58% of steel, therefore, made of titanium alloy material Thin-walled shell structural parts will become common workpieces for defense products. Due to the thin wall of the workpiece, the machining is easy to deform, the radial clamping force causes the workpiece to be elastically deformed, the tool wear is fast, the processing size is unstable, the processing quality is not easy to be guaranteed, the workpiece rejection rate and the processing cost are high, and the workpiece processing deformation has been plagued. Technician and processing operator.
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