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How to efficiently process titanium alloy integral impellers?

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CNC machining technology plays a vital role in impeller manufacturing. Impellers are mechanical parts composed of complex curved surfaces. They are the core components that impact the performance of power machinery such as small gas turbine engines, especially in fluid machinery such as pumps, compressors, and turbines.

Usually, the impellers of small gas turbine engines are integral, and the shape of their blades is the most difficult curved surface to form in CNC machining, which has always been a major challenge for engineers and technicians. With the in-depth study of the forces on the impeller in the fluid, the blades have evolved from the initial radial straight blades to twisted blades with backward bends and forward inclinations. In addition, the blades of the new impellers are long and thin, which further increases the difficulty of machining and puts higher requirements on CNC machining technology.

Through CNC machining technology, engineers can achieve higher precision and complex shape machining, effectively responding to the challenges in impeller manufacturing.

The main difficulties include the following aspects:

1. Complex geometry: The shape of the impeller is usually complex, including features such as curved surfaces, narrow grooves, and variable cross-section blades. The complex geometric shape makes traditional processing methods difficult to carry out, and high-precision CNC equipment and multi-axis linkage processing technology are required.

2. High precision requirements: The processing of impellers requires high precision to ensure balance and performance. Dimensional errors, surface roughness, and form and position tolerances all have a direct impact on the performance of the impeller, which places high demands on processing equipment and processes.

3. Difficulty in material processing: Common materials for impellers include stainless steel, titanium alloys, nickel-based alloys, etc. These materials have high strength, corrosion resistance, and high temperature resistance, but they are also difficult to cut and process, which can easily lead to tool wear, and increase processing difficulty and cost.

4. Complex programming and process design: Due to the complex shape of the impeller, professional programming technology is required to generate CNC machining programs. The optimization of machining paths, tool selection, and setting of cutting parameters are crucial to machining efficiency and quality.

5. Difficult to clamp and position: The clamping and positioning of the impeller during processing is a challenge, especially for impellers with thin blades and irregular shapes. How to stably fix the workpiece without introducing deformation is a technical difficulty.

6. Inspection and quality control: The inspection of impellers is complex and requires high-precision measuring equipment such as a coordinate measuring machine (CMM) to inspect the blade surface and key dimensions. In addition, the dynamic balance inspection of the impeller is also a key step to ensure its operational stability.

To accurately process the impeller with complex shapes, high-performance four- and five-axis CNC machining machines are now widely used to complete the processing of impellers. However, high-performance four- and five-axis CNC machining machines are generally expensive and have high processing costs.

In addition, the processing cycle of the integral impeller is long and the efficiency is low, which often causes the processing cost to remain high. How to improve the processing efficiency of the integral impeller and reduce the manufacturing cost is the focus of research by engineers and technicians in the impeller processing industry.

Titanium alloys are widely used in the manufacture of impellers due to their excellent mechanical properties, but titanium alloys are difficult to process in mechanical processing. It is difficult to use titanium alloys for the CNC machining of new integral impellers, and their machining quality has a decisive influence on engine performance. Therefore, how to efficiently develop high-quality titanium alloy integral impellers is a problem worthy of in-depth research.

Generally speaking, the processing difficulty of titanium alloys mainly includes the following points:

1. High strength and low thermal conductivity: Titanium alloys have high strength and high hardness, but low thermal conductivity. This means that the cutting heat is not easily conducted away from the cutting area, but is concentrated on the cutting tool and workpiece surface, causing the tool temperature to rise rapidly, aggravating the wear and damage of the tool.

2. High chemical activity: Under high-temperature conditions, titanium alloys are prone to chemical reactions with tool materials (such as carbides), forming built-up edges or adhering to the tool, further aggravating the wear of the tool. This phenomenon is called “sticking tool” or “chemical wear”.

3. Low elastic modulus: The low elastic modulus of titanium alloys makes the workpiece easy to deform and vibrate during the cutting process, affecting the processing accuracy and surface quality. This elastic deformation may also cause unstable contact between the tool and the workpiece, increasing the risk of tool breakage.

4. Large and unstable cutting force: When cutting titanium alloys, the chips are prone to curling and uneven fracture, resulting in unstable cutting force. The fluctuation of cutting force not only affects the processing accuracy but also easily causes tool vibration and fracture.

5. Obvious hardening trend: Titanium alloys are prone to industrial hardening during the cutting process, that is, the cutting surface becomes harder. This hardened layer will increase the difficulty of subsequent processing, have a greater impact on the tool, and further affect the processing efficiency and tool life.

6. High requirements for tool materials and coatings: Due to the cutting characteristics of titanium alloys, tool materials with high wear resistance, high heat hardness, and strong adhesion resistance (such as PCD, CBN tools) and special coatings (such as TiAlN coatings) are required. The selection and optimization of tools become very critical and also increase the processing cost.

Analysis of machining difficulties

The overall impeller CNC machining accuracy requirements for this case are relatively high, with a dimensional accuracy of IT6 and a surface roughness of Ra1.6. The blade surface is a free-form surface with a large curvature variation. The impeller flow channel is narrow, and it is very easy for the machining tool to collide with the adjacent blades. The blades are long, exceeding 50mm, and the blade thickness is thin, with the thickest part only 2.8mm. It is very easy to cause vibration during CNC machining, affecting the surface machining quality. As shown in Figure 1.

Figure1

Figure 1 Titanium alloy integral impeller blade

The impeller material is titanium alloy TC4, which has poor cutting performance. At the same time, the elastic modulus of titanium alloy is small (about 1/2 of 45# steel). In the process of CNC machining, a small cutting force will produce large deformation, which is prone to sticking, letting the knife go, and gnawing the knife, making it difficult to ensure the surface quality and geometric accuracy of the workpiece. The processing of titanium alloy materials often uses lower cutting parameters, and its processing efficiency is generally very low. The CNC machining efficiency of the integral impeller made of titanium alloy is lower than that of other materials.

The CNC machining of the impeller is generally divided into three steps: blade rough grooving, blade rough machining, and blade finishing. Blade rough grooving mainly removes most of the material between the blades, and blade finishing ensures the shape and surface quality of the blade. Since the blades of this impeller are in a cantilever state, the blades are not rigid enough during finishing, and elastic deformation is easy to occur, which puts the blades in a very unstable cutting state during finishing milling.

a. This impeller blade is a free-form surface, and point milling must be used during finishing. The cutting angle of the spot milling tool changes at any time, causing the cutting force to change frequently and the direction is uncertain. It is easy to cause the blade to produce violent forced vibration under the random excitation force of the tool, leaving serious vibration marks on the blade, affecting the surface processing quality;

b. The blade rigidity is insufficient during finishing so a small cutting force during the spot milling process will cause the blade to give up the tool, affecting the accuracy of the blade profile. Reasonable determination of the blade finishing allowance, ensuring the blade rigidity, and selecting appropriate finishing tool parameters to reduce the cutting force are the main ways to solve the unstable CNC cutting state.

The difficulty of the overall impeller CNC machining in this case lies in the milling process. Generally, the milling process accounts for more than 80% of the total CNC machining time. At the same time, the milling quality directly affects the performance of the impeller. How to improve the CNC milling efficiency and blade forming quality is the focus of this article.

Key points of the multi-axis CNC milling process

The CNC milling process of the whole impeller is generally: rough grooving of the impeller, removing the large allowance; semi-finishing milling of the blade, rough sweeping of the hub surface; finishing milling of the blade, and finishing of the hub.

To improve the CNC machining efficiency and machining quality of the impeller, combined with the characteristics of the impeller mentioned in this article and our accumulated experience in impeller machining, the actual arrangement of the milling process is: rough grooving of the impeller; finishing milling of the blade, and finishing of the impeller hub surface.

Compared with the previous arrangement, this arrangement removes the semi-finishing milling of the blade and the rough sweeping of the hub surface, theoretically saving 1/3 of the milling processing time.

The material utilization rate of the impeller to be processed in this article is only 8.5%, among which the rough grooving of the impeller is the main process for removing materials, which can remove about 1/3 of the material. Because the rough grooving process removes a large amount of material, the internal stress of the material is released, causing stress deformation, and affecting the geometric dimensions of the impeller. Actual machining measurements show that the size of the hole after the impeller is slotted changes by about 0.02mm.

To ensure strict dimensional accuracy and reduce the influence of stress deformation, the reasonable arrangement of the milling process must be considered. To obtain strict dimensional accuracy, a fine turning sequence is arranged between the rough and fine milling of the impeller to avoid dimensional changes caused by stress release and deformation of the impeller. Actual processing shows that this arrangement can eliminate the stress deformation caused by rough grooving of the impeller and obtain strict dimensional accuracy.

The focus of the rough grooving of the impeller is CNC machining efficiency and cost. Generally, low-end four-axis linkage CNC machine tools are used as much as possible to achieve rough grooving. In addition, choosing the appropriate fine milling cutting allowance during rough grooving, trying to strengthen the rigidity of the blades, and determining the appropriate cutting amount is of great significance to the difficulty and quality of fine machining.

This paper greatly improves the efficiency of rough grooving of the impeller by improving the rough grooving path and process parameters, so that the state of the impeller after rough grooving is more suitable for fine machining. During fine machining, suitable machining tools and cutting parameters are selected to improve the CNC machining efficiency and machining quality of the overall titanium alloy impeller under high speed and high feed.

Rough machining tool position planning

Rough machining aims to quickly remove the blank allowance, and its focus is on the machining efficiency in CNC machining. Try to make the tool work with a large feed rate and the largest possible cutting depth to remove as much material as possible in a shorter time. Rough machining does not require high surface quality. In CNC machining, large-sized tools should be selected as much as possible when grooving to improve cutting performance and efficiency. In addition, since the impeller cancels semi-finishing in the process arrangement, and considering the unstable cutting state during the impeller finishing, this puts forward strict requirements for the state of the impeller after rough grooving suitable for finishing.

First, the allowance of the blade after rough machining should not be too small. Since the elastic modulus of titanium alloy is small the hardness is relatively high, and the rigidity of the blade is poor, the allowance after rough grooving cannot be too small. In the process of CNC machining, on the one hand, it will affect the forming accuracy and surface quality of the blade, and even the dimensional accuracy; on the other hand, it will shorten the tool life. At the same time, it should not be too large, otherwise it is difficult to obtain high-quality surface roughness. According to experience, the allowance is generally between 2 and 4mm.

Secondly, the allowance after rough grooving must be uniform to avoid sudden changes in the allowance on the blade, otherwise, during CNC machining and finishing, due to inconsistent deformation of the blade, transition marks will be produced, affecting the surface quality.

Thirdly, the allowance on the blade surface should be made into a tower shape as much as possible, that is, the allowance gradually increases from the top of the blade to the root of the blade to enhance the rigidity of the blade. Blades with tower-shaped allowances have strong rigidity and can reduce the vibration caused by finishing.

According to the above requirements for the overall impeller grooving, the following CNC machining grooving path planning is introduced:

a. Press the impeller horizontally on the machine tool, which is suitable for impellers with no overlapping and no distortion of the blades from the axial view, otherwise the groove will not completely remove the blank allowance, and the purpose of removing the allowance between the flow channels cannot be achieved;

b. Perform profiling milling on the blades offset according to the allowance. This method has a uniform allowance, but in the CNC machining grooving process, when milling the offset blades, there is always half a circle of the entire tool (ball head and side edge) participating in the cutting, which makes the cutting feed rate unable to increase and affects the tool life;

Figure 2

c. Given the shortcomings of the above two slotting methods, this paper proposes to establish the tool path layer by layer from top to bottom according to the direction of the impeller flow channel and realize the reserve on the blade by controlling the processing area between the flow channels of each layer. At the same time, the reserve on the blade can be made into a tower shape according to the different processing areas, thereby ensuring the rigidity of the blade. Figure 2 shows the situation after the simulation of CNC machining is completed.

Table1

 

This paper adopts the method of grooving layer by layer along the flow channel. Actual CNC machining shows that this method is highly efficient, the blade margin is uniform after grooving, and the blade has sufficient rigidity. In addition, this method requires a shorter blade length, which can prevent excessively long blades from participating in cutting, and the cutting force is small. There is less tool interference, good chip removal, and sufficient cooling.

Blade finishing and hub surface finishing

Due to the existence of tool gnawing during ramp milling in the CNC machining of titanium alloy, the cutting feed rate cannot be very high. The cutting conditions during ramp milling are extremely unstable, and the tool is prone to chipping due to the almost zero linear cutting speed near the center. Actual CNC machining shows that in the presence of ramp milling, the tool life of upward milling is better than that of downward milling. The center of the upward milling tool does not participate in cutting, and the tool is not prone to tool gnawing and chipping, so upward milling should be used. Figure 3 is a schematic diagram of the tool gnawing situation.

Figure 3

To avoid frequent tool lifting during the whole process of gnawing and rough grooving, the upper layers of the flow channel are parallel to the XY plane in the process of establishing the tool path, and the equal height milling tool path is adopted. The lower layers of the tool path are moved from bottom to top, and the upward milling tool path is adopted. By post-processing the tool path, the rough grooving of the impeller is finally realized on a low-end four-axis linkage CNC machining machine. Figure 4 shows the impeller grooving process and the actual product.

Figure 4

Blade finishing and hub surface finishing are the key parts of impeller CNC machining. Their machining accuracy and surface quality have a great impact on the performance of the impeller. UG’s variable axis contour milling is used to program blade finishing. The cutting method is down milling, the machining step length of each layer is 0.3mm, and the surface roughness is 0.01mm. Figure 5 shows the blade finishing tool path trajectory.

Figure 5

The hub surface finishing bottom sweeping adopts the ZIG-ZAG bidirectional reciprocating cutting method, which makes the tool path bidirectional reciprocating CNC machining along the flow channel direction. The characteristic of this cutting method is that the forward milling and reverse milling are alternately performed during the cutting process, and its processing efficiency is high. Figure 6 shows the tool path trajectory of the hub surface finishing bottom sweeping.

Figure 6

Selection of impeller finishing tools and cutting parameters

In actual CNC machining, due to the unstable cutting state in finishing milling, the reasonable selection of tool parameters and the matching of machining parameters have an important impact on the machining efficiency and surface quality of titanium alloy impellers. An important way to solve the unstable cutting state during finishing is to select a suitable finishing tool. According to the processing characteristics of titanium alloy and our experience in titanium alloy processing, the finishing tool material is generally selected from fine-grained cemented carbide YL10.2.

The selection of the back angle of titanium alloy machining tools is the most critical. Properly increasing the back angle can improve the durability of the tool, but at the same time, the phenomenon of chipping will occur. Generally, 8°~10° is selected. The contact area between the front cutting edge and the material is small, and the wear of the front cutting edge is small. A smaller front angle should be selected, and the front angle value should be within 10°. The helix angle has a greater impact on the tool front angle. Increasing the helix angle can increase the actual cutting front angle of the tool, which can avoid reducing the tool strength due to excessive front angle, and can reduce cutting force and increase tool life.

According to experience, the helix angle of the titanium alloy tool should be between 30° and 40°. Actual processing shows that the contour of the impeller finishing tool has an important influence on the forming quality of the blade. If the actual profile of the tool is significantly different from the theoretical profile, the tool profile error will be reflected on the blade during finishing, affecting the forming quality of the blade profile. Therefore, the profile of the impeller finishing tool must be strictly controlled.

Another factor that has an important impact on the quality and efficiency of CNC machining is the matching of cutting parameters and the selection of machine tools. This paper uses the Hammer C40U five-axis CNC machine tool to process the impeller, with a speed of 10000rin/min and a feed rate of 900mm/min. Finally, a titanium alloy integral impeller with good surface quality is efficiently processed. Figure 7 shows the actual product after processing on the Hammer C40U.

Figure 7

Conclusion

In the process of CNC machining, the machining of impellers is mainly carried out on expensive multi-axis high-performance CNC machine tools. How to reduce costs and improve efficiency and quality is the focus of research by engineering and technical personnel. This paper combines the machining process of a titanium alloy integral impeller, adjusts the process flow, and proposes a new rough machining tool path planning, which realizes the CNC machining of the impeller at high speed and high feed on a five-axis high-speed milling machine. Actual machining shows that this scheme can complete the CNC machining of titanium alloy integral impellers with high efficiency and high quality.

a. This paper focuses on the tool position planning in the rough grooving stage. The advantages and disadvantages of the three rough grooving methods are analyzed and compared, and a CNC machining method for grooving layer by layer from top to bottom along the flow channel direction is proposed. This method has the advantages of high grooving efficiency, uniform reserve, good rigidity of the blades after grooving, and is more suitable for fine machining;

b. In the fine machining of the impeller, reasonable CNC machining tool parameters and cutting parameters are selected, and finally, the Hammer C40U five-axis high-speed machining center is used to achieve high-efficiency and high-quality CNC machining with high speed and high feed.

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