Integral impeller is the core component of turbo engine and turbocharged engine. It is widely used in energy power, aerospace, petrochemical, metallurgy and other industries. It is a typical channel type complex part with standardized modeling. The machining means, machining accuracy and machining surface quality of its profile directly affect the aerodynamic performance and mechanical efficiency of the engine, and have a decisive impact on the engine performance. Integral impeller has become a typical difficult to machine part because of its complexity of curved surface and high machining accuracy.
With the increasing demand for turbine engines in the market, it is increasingly necessary to achieve efficient machining of integral impellers. At present, the common materials of integral impeller are aluminum alloy, titanium alloy, stainless steel, etc.
The complexity of impeller machining mainly lies in the complexity of blade surface modeling, which can be divided into ruled surface and non ruled surface according to the molding principle, and ruled surface can be divided into developable ruled surface and non developable ruled surface. Five axis NC milling is one of the commonly used methods for machining integral impeller with good flexibility, high machining efficiency and wide application. According to the different surface shapes of impellers, two kinds of methods are usually used for machining on CNC machine tools, namely, point milling method and side milling method.
Due to the complex shape of the integral impeller and the large distortion of the blade, the machining of the integral impeller is very easy to interfere, so the machining difficulty lies in the rough and finish machining of the runner and blade. In the process of NC machining of integral impeller, taper ball end milling cutter is often used to minimize over cutting and interference caused by the cutter, and the cutter can still have good rigidity when machining narrow runner.
In order to make the impeller meet the requirements of aerodynamics, the blade often adopts the structure of large twist angle and variable fillet at the root, which puts forward higher requirements for the processing of the impeller. The machining technical requirements of integral impeller include the requirements of size, shape, location, surface roughness and other geometric aspects, as well as the requirements of mechanical, physical and chemical properties. The impeller blade must have good surface quality, and the accuracy is generally concentrated on the blade surface, hub surface and blade root surface. The surface roughness value should be less than Ra0.8um.
In order to meet the above requirements, the following problems are often encountered in the processing of integral impeller:
① The integral impeller has narrow flow passage, relatively long blade and low stiffness, which belongs to thin-walled parts and is easily deformed during processing. In the process of machining, due to the role of its own structure and external cutting force, the inlet of the runner and the upper end of the blade will produce vibration lines. Sometimes, in order to avoid chatter marks, it is necessary to change the sharpness of the tool, which will cause burrs at the inlet, outlet and blade edges of the runner. In order to ensure the surface quality of the integral impeller, it is required that the cutter has enough rigidity, enough chip removal space and appropriate sharpness.
② The blade depth at the narrowest part of the runner is far more than the tool diameter, and the adjacent blade space is very small. The tool diameter is small during angle cleaning, and the tool is easy to break. The control of the cutting depth is also a key technology for machining.
③ The surface of the integral impeller is a free surface, with narrow flow passage, severe blade distortion, and a clear tendency to lean back. It is very easy to cause interference during processing, which is difficult to process. Some impellers have auxiliary blades. In order to avoid interference, the curved surface must be machined in sections. Therefore, it is difficult to ensure the consistency of the machined surface.
④ Considering that the overall impeller rotates at a high speed in actual work, with a speed of 30000-50000 revolutions, it is necessary to prevent vibration and reduce noise, so the requirements for dynamic balance are high, which improves the requirements for machine tools and tools. It is necessary to save the processing time, and ensure the stability and symmetry of cutting.
Considering the rigidity of machine tool, cutter, fixture and integral impeller, designing reasonable cutter structure and selecting appropriate manufacturing process can meet the manufacturing requirements of integral impeller.
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