Research on single point incremental flanging of metal pipe with variable angle

In the process of single point progressive pipe flanging, due to the large radial forming force, it is easy to cause the defects such as wrinkling and cracking of the pipe, which reduces the forming quality and precision. A new technology is proposed to replace the spinning wheel with ball head metal rod and to flanging thin-walled metal pipe end in the form of variable angle. By changing the instantaneous contact state between tool head and forming part and deformation mechanism of material, forming force is reduced and forming quality is improved. In order to verify the feasibility of the forming process, the test platform is built, and the single point gradual angle flanging test of metal pipe end is carried out. The test results show that the use of ball head metal rod to flanging the pipe with variable angle can effectively reduce the initial radial forming force, the surface quality of the forming parts is good and the surface fluctuation is small. The results provide the test and technical basis for further analysis of the flanging process of single point progressive forming pipe.

SPIF is a kind of technology that introduces the idea of “layered manufacturing” into plastic forming process to complete the processing of sheet metal, and can form complex geometric components. This technology is a kind of molding free, flexible and green process, and has a broad application prospect in medical, aviation, automobile and other fields. It has been paid attention by scholars from all over the world.

The single point progressive forming technology is introduced into the metal pipe processing, which makes the pipe processing more flexible and can improve the forming performance and limit of the pipe. However, some defects are also produced. In the experiment, it is found that a large number of defects such as fracture, wrinkle, deformation and wear will occur in the process of the process [2,3]. It is found that the forming parts of the pipe will break The main reason for serious defects such as deformation is the large forming force. Therefore, it is an important step to study how to reduce the forming force of pipe. Some scholars have put forward the ultrasonic vibration single point progressive forming, local heating single point progressive forming technology, the purpose is to reduce the forming force by changing the grain size of the material from the material micro point of view. Cai Gai Qian studied the change of forming force after vibration. In order to collect the change of force in forming process, a multi-channel data acquisition device was designed. Jyhwen [5] and other experimental research were carried out based on the forming force model. According to the research results, a model which can effectively predict the forming force was proposed, which is convenient for the study of forming force. Pasierb[6] and so on carry on radial ultrasonic to the punch, which significantly reduces the load of the sheet metal during the drawing. The results of experiments show that ultrasonic vibration can effectively improve the formability and reduce the forming force. Duflo[8] by heating the forming area locally, cooling the surrounding area properly, improving the local deformation and improving the forming accuracy. The experimental analysis of shrivastava[9] is carried out from the material micro point of view. The forming force is reduced by controlling the temperature to improve the forming performance. The relationship between the heating up of sheet metal and forming force is obtained by bagudach[10]. In view of the problem that the radial force is large in the process of pipe flanging, the pipe rupture is caused by the introduction of ultrasonic vibration and local heating, which can effectively reduce the forming force and improve the forming performance. However, the ultrasonic vibration causes the material temperature to rise, which makes it difficult to consider both the forming efficiency and the forming performance simultaneously in the forming process. In order to improve the efficiency and increase the feed speed, the forming efficiency and the forming performance can be improved, The newly processed area is easy to recrystallize and cause stress concentration and fracture.

Some studies are carried out from the influence rules of influencing factors on forming force. Through experimental research and analysis, the influence rules of influencing factors on forming force are mastered, and then the optimal parameter combination is found to minimize the forming force. Chen[12] and other modeling models were established to study the shear force in the process of progressive forming of spinning wheel, and the influence of material thickness, spindle speed and feed amount of the spinning wheel on the shear pressure was studied. Duflou[13] studied the influence of different process parameters on forming force. Wen [14] studied the progressive forming of pipe end curl, and obtained the influence rules of various factors on the wall deformation, flatness and load. Through studying the influence rules of influencing factors on forming force, the optimal parameter combination can be found, which can reduce the forming force to some extent, but it can not fundamentally solve the problem of the parts rupture caused by the large forming force.
This paper studies how to reduce radial force and improve surface quality of forming parts from the angle of feed mode of forming tool. A new technology method of forming metal pipe end with ball head metal rod is proposed. In the forming process, the forming method of axial feeding of forming tool is adopted to conduct single point progressive flanging test of metal pipe with variable angle. In order to better reflect the advantages of this method in reducing radial force and improving surface quality, the radial feed flanging of pipe with rotating wheel set is compared with the method proposed in this paper. In order to verify the feasibility of the method, the forming parts with the target flanging height of 10 mm and the target flanging angle of 90 ° are obtained by experiments. The results show that the method can effectively reduce the radial force in the initial stage of the flanging process of metal pipes, improve the surface quality of the forming parts, and the material rebound can also be effectively restrained.

Characteristics and principle of flanging with variable angle

Characteristics of flanging process with variable angle

The processing method of single point progressive flanging of metal pipe with variable angle is to use ball head metal rod to flanging the pipe axially. It is mainly aimed at the problem that the radial force is large in the process of pipe feeding flanging by using forming tool, which leads to the pipe rupture. The flanging process of variable angle pipe is mainly made use of the difference between the metal rod and the area to be flanged, and then the axial feed is used to form the tool to realize the flanging purpose.
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Figure.1. The area and forming track of the pipe during forming
As shown in Fig.1, the forming area is one of the spiral tracks and the layer to be flanged at present. The contact area is the main stress area of the layer; In the formed area, the angle between the pipe wall and the spindle and the angle of the forming tool and the spindle are equal. The force between the two is mainly caused by the material rebound, and the pipe wall is no longer further squeezed by the forming tool. As shown in the enlarged drawing of the local part in Figure.1, the angle difference exists between the forming area and the formed area. The forming tool only flanges the forming area, and after the processing is completed, The angle of the tube wall in the forming area is equal to that of the formed area, and the forming tool begins to process the next layer. During the whole process, the contact area between forming tool and tube forming area is approximately a contact point. The point completes the process of gradual flanging of pipe with variable angle by taking the conical spiral line track. The cone angle is the angle of bell mouth in the current unformed area, which belongs to single point progressive forming (SPIF).

Forming principle of changing angle flanging

During the forming process, the spindle rotation is maintained, and the axial feed and angle change of the forming tool are controlled by the tool holder. After each axial feed is completed, the forming tool is returned, and the angle between the forming tool and the metal pipe wall is changed, and then the axial feed is completed. After the multiple angle changes and axial feeding of the forming tool, the process of flanging with variable angle is completed. The principle of flanging process of variable angle metal pipe is shown in Figure.2.
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Figure.2 forming process schematic diagram
After the forming process is completed, the ball head part of the forming tool is moved to the junction of flanging area and non deformation area, and then radial feed forming tool completes the pre forming angle calibration process. The principle of the shaping process is shown in Figure.3. V and N are the axial feed speed and spindle speed respectively, a is the rotation angle required before each axial feeding of the metal rod, l is the initial total length of the metal pipe, and R and R are the inner diameter and outer diameter of the pipe respectively.
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Fig.3 Schematic diagram of the shaping process

Simulation analysis

Establishment of finite element model

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Figure.4 simulation geometry model
The flanging process is simulated by ABAQUS. The 3D modeling is shown in Figure.4. The inner clamp is installed in the pipe to prevent the deformation of the pipe due to the radial force during the clamping and forming process of the three claw chuck, and the outer clamp replaces the three claw chuck; The diameter of the ball head and the cylinder part of the tool head are 8mm. Both the internal and external clamps and forming tools are assumed invariance in the forming process, and are set as analytical rigid bodies; The metal pipe is set as deformable entity with an inner diameter of 30mm and a thickness of 1mm; The friction coefficient is set to 0.1 between the forming tool and the pipe wall; In order to speed up the calculation speed, the mass scaling coefficient is set to 1000, and the whole analysis step adopts display and power.

Definition of material properties

In the gradual forming of pipe flanging, the thin-walled pipe is made of copper alloy pipe, and a piece of copper alloy pipe material is cut for tensile test. The material performance parameters are shown in Table 1, and the material stress-strain parameters are shown in Table 2.
Table 1 material parameters

Material name Red copper
Density g/mm3 8.9×10-3
Modulus of elasticity E (GPA) 107.9
Poisson’s ratio V 0.33

Table 2 material stress and strain parameters

Stress/Mpa Strain m/m
0 0
23 0.02
100 0.035
141 0.07
173 0.105
199 0.014
222 0.175
243 0.21
263 0.245
280 0.28
297 0.315

Comparison of simulation results of machining with rotating wheel and metal rod

In order to compare the advantages and disadvantages of flanging pipe by radial feed of rotating wheel (method 1) and using ball head metal rod axial feed to flanging (method 2), the two forming methods are simulated and analyzed respectively. The material properties and size of the pipe are set in the same way, and the grid division method and grid density are identical. The simulation 3D modeling of the two flanging methods is shown in Figure.5.
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Figure.5 three dimensional model of radial feed and metal rod axial feed of rotating wheel
The simulation results of the two methods are compared, as shown in Figure.6, where a) and b) are axial direction angles. In contrast, the surface of B) is more flat and the surface quality is better. A) the pipe end has sharp angle, which is at risk of crack in the experiment; c),d) from the radial direction perspective, the grid at the pipe end can see that the grid of C) has serious deformation, which indicates that the force in the flanging area is uneven during forming, which may affect the surface quality of the forming part in the experiment.
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Figure.6 cloud diagram of simulation results when the flanging angle is right angle
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Fig.7 Schematic diagram of rotary gear processing
The flanging process of pipe by radial feed of rotating wheel is shown in Fig.7. Figure.8 is the analysis of its geometric force. From the geometric relationship, the force fr=f can be obtained from the vertical and the pipe wall × Cosa and fr gradually decrease, that is, the pipe wall is subjected to the largest radial force in the initial stage. In order to compare the radial force size and surface quality of the two methods in the initial stage, the cloud graph of the results when the flanging angle of the pipe is 10 ° is compared. It can be seen from the results that the flare shape of the flanging simulation results of method 1 is uneven and the deformation of the pipe end is uneven, The flanging area of method 2 is smooth and even in deformation, as shown in Figure.9.
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Figure.8 geometric analysis of the force on the machining of the rotating wheel
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Figure.9 cloud diagram of simulation results at 10 ° flanging angle
In order to explore the advantages and disadvantages of the two methods, the radial stress of the pipe when the flanging is 10 ° is output from the results, in which the positive value indicates the tensile stress of the pipe in the X direction, and the negative value indicates the pressure stress of the pipe in the X direction. Fig.10a) is a cloud diagram of stress formed by method I, with a maximum tensile stress of 3.383 × 102pa, FIG. 10b) is the stress cloud diagram of method 2, with the maximum tensile stress of 1.863 × 102pa, it can be seen that the maximum radial tensile stress of method 2 is significantly lower than that of method 1.
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Figure.10 radial feed of rotating wheel and radial force of metal rod axial feed
The radial stress values of the two flanging methods are introduced into origin to obtain the stress change of the flanging region. The horizontal coordinate indicates the position of 13 stress values in the deformation area, which is pointed from the pipe end to the junction of flanging area and the non deformation area. It is expressed by Qi. It can be seen from Figure.11:
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Figure.11 radial feed of rotating wheel and radial stress of metal rod axial feed

  • 1) Method 2 is used to flanging the pipe, the radial stress is between -400pa and 350pa. Method I is used to flanging the pipe. The pipe is less under radial stress, and the radial stress range is between -150pa and 200Pa. The change of the force is more relaxed, which can effectively improve the forming performance and the quality of the forming parts.
  • 2) When using method 1 for flanging, Q1-Q6 is tensile stress, and from Q6 to Q13 is compressive stress; When using method 2 for flanging, Q1-Q8 is tensile stress, and Q8-Q13 is compressive stress, that is, method 1, material rebound occurs from Q6. In the results obtained by method 2, material rebound occurs from Q8, and the elasticity is smaller, which is conducive to improving the quality of forming parts. It can be concluded that method 2 is used to flanging the pipe, It can effectively restrain the rebound of the material and reduce the material resilience.
  • 3) The results show that the maximum tensile stress decreases 45% and the maximum pressure stress decreases by 66% when the method 2 is used for flanging. It is proved that method 2 can effectively reduce the radial force in the initial stage.

Experimental verification

The simulation results show that the metal rod can reduce the small-diameter force effectively, get the better surface quality forming parts, and can restrain the material rebound and effectively reduce the material resilience. In order to verify the advantages and feasibility of the method, an experimental platform is set up to verify the single point progressive flanging method of metal pipe with variable angle.

Platform of experimental device

In this experiment, the common lathe is used as the experimental platform for metal pipe flanging with variable angle. The thin-walled copper pipe is fixed on the main shaft by three claw chuck and core die. It is rotated in a circumferential direction along with the main shaft. The angle change between the ball head tool and the pipe wall is realized by the tool holder, as shown in Figure.12.
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Figure.12 platform of experimental device

Process of flanging with variable angle

Forming process: after the installation of the experimental device according to figure 12, the initial position of the metal rod contacts the inner wall of the pipe, and the initial angle between the two is 5 °. During the forming process, the tool is controlled by the tool holder to make axial feed motion. The first axial feed is 10 mm. After the axial feeding is completed, the flanging area of the pipe is processed as 5 °, and then it is fed 5.03mm in the axial direction, and the first one is completed; Then the angle between the forming tool and the pipe wall is increased to 10 °, and then the axial feed is carried out. The feed rate is 5.03mm, then the feed is 3.37mm in the axial direction, and the second pass is completed. At this time, the pipe is processed as 10 ° in the flanging area; According to this process, the metal pipe is flanged to 50 ° to complete the forming process of pipe flanging. The metal pipe rotates at a speed of 100r/min during the whole process. Fig.13 shows the feeding rate between two adjacent angles, in which green is the pipe wall, blue and red are forming tools, which are the first and rear positions of the two feeding processes, and the feed direction is v. The pipe is formed according to the arrow direction of the dotted line part; A is the flanging angle of the pipe feeding in the blue part, and the red part is the axial feeding process of the next angle, with the flanging angle of 5 ° and the total flanging angle of a + 5. Table 3 shows the corresponding axial feed required by the forming tool at each angle, calculated by formula (5).
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Figure.13 feed of forming tool
Table 3 corresponding feed rate of each tool angle/mm

Tool angle Feed rate/mm
5 10
10 5.03
15 3.37
20 2.55
25 1.77
30 1.67
35 1.52
40 1.37
45 1.23
50 1.14

From the geometric relationship in Figure.13, formula (1), (2), (3) and (4) can be obtained:
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From formula (1), (2), (3), (4), it can be obtained as follows:

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(5) Shaping process: after the forming process is completed, the forming tool is restored to the initial angle position, the ball head part of the forming tool is moved to the junction of flanging area and non deformation area, the spindle rotation is maintained, and then the forming tool is fed radially to the outer edge of the pipe end to complete the shaping process. The shaping process is shown in Figure.3.

Experimental results and analysis

Surface morphology of forming parts

Two methods were used to process the thin-walled copper tube with the flanging target height of 10 mm and the target angle of 90 ° respectively. The results of the forming parts obtained by the two methods were compared. In Figure.14, ① the surface of the forming part obtained by using the axial feed flanging of the ball head metal rod; ② ③ ④ the surface of the forming part obtained by radial feeding flanging of the rotary wheel. Compared with the surface of the forming part, it is clear that the surface of the forming part obtained by using the ball head metal rod to flanging at different angles is obviously smoother and the surface quality is better; The surface of the forming parts obtained by turning the rotating wheel with variable angle has obvious cracks, and the machining trace left by the rotating wheel tool is poor, which is consistent with the simulation results.
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Figure.14 forming results
In order to observe the micro surface morphology of the forming parts, the forming parts were sliced along the axial direction by the wire cutting machine, and the surface morphology was observed by vhx-5000 kehens 3D microscope.
The area of one piece of arbitrary interception observation results is 400μm×400μm. The surface morphology and the size of the fluctuation amplitude of the surface are output in the area of M. Figure.15 shows the surface morphology. Z axis is the thickness direction of the flanging area of the pipe, the X axis is the tangential direction of the circular path of the forming tool, and the Y axis is perpendicular to the circular path of the forming tool. In the figure, the long strip gully in the surface morphology of the forming part is the middle processing trace in the processing process. As shown in Figure.16, the fluctuation of z-axis decreases gradually along the y-axis direction, because the metal rod contacts the formed area continuously, it has the function of polishing and polishing the surface of the formed area, and effectively suppresses the material rebound, reduces the defects caused by the material rebound, and approaches the pipe end, The longer the contact time, the more obvious the effect is, so along the y-axis, the long-strip gully gradually becomes shallow.
In Figure.16, the fluctuation of the forming part in the z-axis direction is continuously changed, and the maximum fluctuation amplitude is maintained 2 μ Within m, the surface of the forming parts is relatively flat.
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Figure.15 observation results of surface morphology of forming parts
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Figure.16 surface waveform of forming parts

Thickness measurement of forming parts

The target flanging height of this experiment is 10 mm, and the final flanging height is about 11mm due to the influence of material flow during forming. To obtain the thickness reduction of the forming parts, a sampling point is marked every 1mm along the radial direction of the flanging area, and it is recorded as Si. Observe the thickness value of each sampling point with metallographic microscope, and select five observation results along the radial direction, as shown in Figure.17. The thickness values at each sampling point are introduced into origin to get the line diagram. As shown in Figure.18, there are differences in thinning of different areas of pipe. Among them, the thickness reduction of forming parts at 1-3mm is faster because the position is at the junction of flanging area and non deformation area, and the material flow is large; The thickness of the forming parts at 8-11mm is the fastest because the position is at the pipe end, the deformation of the material is large, and the metal rod adopts the axial feeding mode, which causes the material to flow from the pipe end to the pipe; The thickness of 4-8mm fluctuates slightly, but it is almost no thinning, because the material at the deformation direction of material flows to the middle, and secondly, the axial feeding process inhibits the material flow to the outer end of the circular tube, and there is more material accumulation at 8mm, which leads to local thickness increase; The definition of thinning rate is as follows:
(6) Where ψ In order to reduce the thinning rate, t is the initial thickness and t0 is the thickness of the deformed sheet. The thinnest part of the material is the outer edge of the pipe, and the thinning rate is about 48%. Compared with the research results [15], the thinning rate at the end of the pipe needs further improvement.
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Figure.17 thinning process of forming parts
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Figure.18 thickness change of forming parts

Feasibility and advantages of the process method

The profile of the forming part was obtained by scanning the slice with the kehens 3D profilometry. The angle between the contour lines measured in vision can be seen that the flanging angle near the junction of the deformation area and the undisturbed area is about 85.4 °, and the error with the target angle is 4%; The angle near the pipe end is about 73.9 °, and the deviation is large. It is mainly due to the large thinning rate of the outer end of the pipe, which leads to the reduction of the resilience of the material at the outer end of the pipe, which needs further exploration, as shown in Figure.19.
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Figure.19 scanning results of slice outline
It can be seen from the simulation results and experimental results that, compared with the use of rotating wheel set pipe flanging, the use of metal rod to flanging the pipe with variable angle can greatly reduce the radial force in the initial stage, effectively avoid the occurrence of surface cracks and machining traces of the formed parts, effectively improve the formability and forming quality of the pipe flanging, as shown in Figure.20. Overall, the method of metal pipe flanging with variable angle proposed in this paper reduces radial force and improves the formability of pipe flanging.
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Figure.20 forming parts

Conclusion

  • 1) The metal pipe is formed by changing angle flanging with ball head metal rod, which replaces radial feed with the axial feed of forming tool, which effectively reduces the radial force in the initial stage of flanging process, in which the maximum tensile stress is reduced by 45%, the maximum pressure stress decreases by 66%, and the risk of pipe pipe rupture caused by excessive radial force is reduced.
  • 2) The radial feed of the spinning wheel will leave the machining trace of the layer and affect the surface quality; During the process of variable angle forming, the metal rod contacts the formed area of pipe wall continuously after each angle change, and has polishing and grinding effect on the formed area, which can effectively eliminate the feed trace of layer, restrain the rebound of material, reduce the elasticity of the material, and improve the surface quality.
  • 3) Because the feeding mode of forming tool from the end of pipe to the inside of pipe in flanging process leads to the material flowing inward in the flanging area, which easily causes the thinning rate of the outermost end of the flanging area to be large, which leads to the reduction of the resilience of the material at the outer end of the pipe, which causes the difference between the flanging angle of the outer end of the pipe and the expectation. The problem needs to be explored in the next step.

Authors: Qiu Xu, Gao Xinqin, Hou Xiaoli, Yang Mingshun, Wang Zhanjun

Source: Network Arrangement – China Stub End Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

If you want to have more information about the article or you want to share your opinion with us, contact us at sales@steeljrv.com

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