impeller-5-axis-machining

Step-by-step science popularization of large-diameter integral impeller

This paper proposes a new blade electrochemical machining method based on the past research, namely the step-by-step numerical control electrochemical machining method...

 

The overall structure of fans, compressors and turbines is a trend in the development of aero engines. The impeller is one of the main parts of aero engine compressor components. With the improvement of engine performance and the improvement of design methods, its blades are usually multi-disciplinary based on aerodynamic and heat transfer analysis, static strength analysis, vibration analysis, and life analysis. The optimized design of, the shape becomes more and more complex, most of which are free-form surfaces, so the processing becomes very difficult. The range of impeller diameter of aero engines varies greatly due to different purposes. From the perspective of processing, impellers with a diameter greater than 500 mm are usually called large-diameter impellers. In the past, my country's aero engine development was mainly on small and medium-sized engines, such as Kunlun 0 and Taihang 0 engine models. Therefore, the processing and research on the overall impeller was also concentrated on the small and medium-diameter overall impeller. With the implementation of China's large aircraft project, the quality range Large engines with a capacity of 10 000 to 20 000 kg are under development. Therefore, studying the processing technology of large-diameter integral impellers is of great significance to the development of large engines and large aircraft in China. In the past 20 years of research, domestic and foreign researchers have used the cathode design of the overall impeller electrochemical machining[ 1-3], process parameter selection[ 4-5], processing programming[ 6], and technical means[ 7-11] Accumulated a certain amount of experience. At present, the electrochemical machining methods of blades mainly include forming electrode processing method and numerical control generation method. Among the generation method, there are single-step processing method[12], cathode swing method[13] and leaf back modification method[14]. . This paper proposes a new blade electrochemical machining method based on the past research, namely the step-by-step numerical control electrochemical machining method, and discusses the machining process, machining path calculation and cathode design and other issues.

 

1 Analysis of electrochemical machining process of integral impeller blade profile

 

1.1 Improvement of processing plan

 

To

 

The integral impeller of an aero engine is composed of blades and a hub. The characteristic is that the blades and the hub are combined together. The large-diameter integral impeller has a large outer diameter, usually between 500 mm and 2500 mm, as shown in Figure 1. It has been tested to process large-diameter integral impellers with the blade back modification method. It first processes the inter-leaf passages based on the blade basin, and then finishes the blade back. In terms of processing principles, this processing plan is feasible, but from the experimental results, the main problems are: ¹ In the blade back finishing, the allowance for one pass is small, the number of passes is large, and the processing efficiency is low; º Processing based on leaf pots will still cause overcutting of leaf roots and low processing accuracy. Combining the characteristics of the large-diameter integral impeller on the basis of the original processing technology, a process plan for the stepwise processing of integral impeller blades is proposed.

 

The step-by-step numerical control electrochemical machining of blades is to divide the machining into multiple processes, and the processing area of ​​each process is appropriately divided through technical means such as processing simulation, so that the shape after the previous process meets the next electrochemical machining process. Processing requirements to achieve continuous blade processing. The blade processing is divided into steps

 

In order to process the leaf pot, leaf back and leaf root 3 processes, its advantages are: 

 

(1) Since the leaf pot and leaf back are processed separately, the machining accuracy of the leaf pot and leaf back can be improved by correcting the movement trajectory and error compensation measures.

 

(2) It is possible to plan the distribution of the blade root machining allowance in the leaf pot and leaf back processing, so that it can meet the leaf root processing conditions and improve the blade root processing accuracy.

 

1.2 Leaf pot and leaf back processing plan

 

In step-by-step processing, the processing sequence of leaf pots, leaf backs and leaf roots is related to the smooth progress of the process. Therefore, it is necessary to analyze what processing conditions exist in each process to determine the processing sequence. In the processing of leaf pots and leaf backs, there are two types of processing sequences: first leaf pots, then leaf backs, and leaf pots behind first leaves.

 

Option 1: Process the leaf pots behind the leaves first. In this scheme, the outlets on both sides of the cathode are semi-open when the blade back is processed, and the electrolyte is sprayed from the middle of the cathode bottom plate to both sides. Since the conditions on both sides are symmetrical, the pressure at the outlets on both sides is basically equal, and the processing plane is The electrolyte supply is sufficient and the processing conditions are good. When processing the leaf pot, the left side of the cathode is half open, and the right side is open. The conditions at the outlets on both sides are different, resulting in different pressures on both sides.

 

Option 2: Process the leaf pot first and then process the leaf back. In this scheme, the processing conditions of the leaf pot are basically the same as the processing conditions of the leaf back in scheme 1. When the blade back is processed, the left side of the cathode is open and the right side is half-open. Since the minimum inclination angle at the back of the blade is about 30b, it affects the pressure at the cathode outlet and thus the flow velocity.

 

 

Option 1 When processing the blade back, the outlet pressures on both sides of the cathode bottom plate are basically equal, and the processing conditions are good. Although there is a certain difference in the outlet pressure on both sides of the leaf pot when processing the leaf pot, it is found through experiments that as long as the liquid outlet on the bottom plate is offset to the left to reduce the flow rate difference on both sides, it can also be processed. Option 2 When processing the blade back, the pressure at the outlets on both sides of the cathode bottom plate is quite different, which cannot be adjusted by offsetting the position of the liquid outlet. Most of the electrolyte flows away from the open side, and it is easy to process short circuits on the side close to the blade back. . Therefore, scheme 1 is preferred as the processing scheme of leaf pot and leaf back.

1. 3 leaf root processing plan

 

The shape of the leaf root is a cylindrical surface, and the actual shape of the leaf root left after the leaf back and leaf pot is processed by the step-by-step method is related to the processing track of the leaf pot and leaf back and the shape of the cathode. According to the distribution characteristics of the machining allowance of the leaf root after the leaf pot and the leaf back are processed, the options for leaf root processing include forming cathode processing and numerical control generating processing.

 

Option 1: Forming cathode processing. The main considerations for processing blade roots with shaped cathodes are the margin distribution after processing the leaf back and the leaf pot, the design of the shaped cathode, and the cathode feeding method. Although the movement of forming cathode is simple, the tooling design is more complicated.

 

Option 2: Numerical control development processing. It uses a flat cathode to move from top to bottom along the inter-leaf passage to remove the leaf root margin. In the development process, the distribution of the machining allowance and the movement trajectory of the cathode must be considered. In actual machining, it is also necessary to divide the processing area and correct the angle of the blade basin and the back cathode/blade to make the blade root machining allowance. It is distributed in an inverted triangle, with a large margin in the middle and a small amount on both sides, so that the process of blade root processing can be easily realized.

 

Among the alternatives for blade root processing, the cathode profile of the formed cathode is difficult to process, and a set of sealed fixtures is required. The cathode needs to be rotated to enter the inter-leaf passage, while the NC-generated cathode is simple, but the motion trajectory calculation is relatively complicated. , Considering the above factors, the CNC generative processing program is preferred, namely program 2.

 

2 Leaf back and leaf pot processing

 

2.1 Motion trajectory calculation

 

To

 

The leaf pot and the leaf back are processed by a step-by-step method, and the processing motion trajectories are calculated based on the leaf pot and the back of the leaf respectively. The calculation process of the motion trajectory is as follows: First, use the leaf back and leaf pot contour data extracted from the geometric model as the basic data for calculation, fit the data points, and then discretize the curve. Since the shapes of leaf pots and leaf backs are similar, the calculation methods of their processing motion trajectories are the same. The following takes leaf pots as an example to illustrate the calculation process of motion trajectories.

 

2.2 Cathode design

 

Cathode design is the key to electrochemical machining. In the division of the processing area, the shape of the leaf back and the leaf pot has been preliminarily designed, but because the leaf pot and the leaf back are processed separately, the size of the cathode bottom surface is about half smaller than that of the leaf pot and the leaf back processed at the same time. Therefore, the flow field design is more difficult, and it needs the aid of CAD, electrochemical machining process simulation, motion simulation and flow field simulation software to complete.

 

The leaf pot and leaf back cathode are composed of a cover plate and a bottom plate. Compared with a cathode processed at the same time as the leaf pot and leaf back, the cathode has the following improvements.

 

(1) The angle between the main blade and the mounting edge of the bottom plate is changed to 90b, which is convenient for machining tool setting and compensation of machining errors; the blade root margin distribution is changed by correcting the angle between the front blade and the main blade; The included angle of the front edge is determined according to the size of the processing area.

 

(2) The fluid replenishment port on the cathode bottom plate is extended forward to strengthen the fluid replenishment; the fluid outlet on the bottom surface adopts an asymmetrical design to meet the pressure conditions of the leaf basin and leaf back.

 

 

(3) The overcut caused by blade back processing is no longer improved by correcting the inclination of the cover plate, but is solved by a motion compensation method, creating conditions for the design of the cathode flow field for blade back processing with small cross-sectional dimensions.

 

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