Three-dimensional freeform bending technology has been an important technological innovation in plastic forming in recent years. It can realize precise moldless continuous bending forming of pipes, profiles, and wires. It is especially suitable for curved components with complex spatial shapes or complex curved components with continuously changing bending radii.
In China, the Nanjing University of Aeronautics and Astronautics team took the lead in carrying out systematic research and development work, making comprehensive progress in basic theories, key technologies, digital equipment, and major engineering applications, and achieving the goal of catching up with similar technologies in the world.
Research on the basic theory of 3D freeform bending of pipes
Deformation rules of 3D freeform bending materials
A free bending mechanics model considering axial thrust was established, and the movement law of the strain neutral layer and its influence on the inner and outer wall thickness distribution of the pipe was analyzed, as shown in Figures 1 and 2. Conclusion: The axial thrust causes the stress-strain neutral layer to move outward, which reduces the wall thickness reduction rate on the outside of the bend and improves the pipe forming quality.
Mechanism of defect formation in 3D freeform bending
The formation mechanism and distribution pattern of pipe cross-section distortion and instability wrinkling defects were studied, and a defect prediction model based on shell energy was established.
The elliptical distortion and distribution pattern of the free-bending cross-section of the pipe are shown in Figures 3 and 4. The cross-sectional distortion rate at the beginning of the bending part of the pipe shows a higher peak value; in the middle and end parts of the bending process, the cross-sectional distortion rate of the bent pipe is low. , that is, obvious cross-sectional distortion occurs in the transition section.
The wrinkling prediction model based on the shell energy principle is shown in Figure 5. The corrugated wrinkling phenomenon mainly occurs in the arc segment forming part.
Figure 5: Wrinkling prediction model based on shell energy principle
Profile bending and torsion mechanism
A theoretical analytical model of free bending and torsion of square cross-section profiles was established, and the stress and strain distribution rules on different faces of square cross-section profiles were clarified. Conclusion: The overall stress on the curved inner surface is in a downward trend; the equivalent stress on the curved outer surface, the twisted inner surface, and the twisted outer surface all show an overall upward trend; in the counterclockwise direction, the twisted outer surface, the curved outer surface and the twisted inner surface The equivalent strain distributions on all showed a trend of first decreasing and then increasing.
Mechanism of action of torsion on bending
A theoretical analysis model of the relationship between the cross-section torsion angle and the radius after bending rebound during free bending and torsion of flat elliptical cross-section profiles was established, and the influence of cross-sectional torsion on bending deformation during bending and torsion along the long and short axes of flat elliptical profiles was established.
6 Key technologies for 3D freeform bending
Key technologies 1#: Accurate analysis of pipe axis
A three-dimensional motion trajectory model of the freeform bending mold was established, and the bending arc segments were divided (Figure 6): the single bend was divided into transition section 1, intermediate arc section, and transition section 2, which represent eccentricity, dwell, and return.
The multi-bend is divided into several bending planes, and the movement direction of the bending die is determined based on the relative position of the planes. A method for optimizing the movement trajectory of the metal pipe freeform bending system is proposed to achieve precise planning from the geometric axis of the complex bending component to the movement trajectory of the forming die. see Figure 7.
Key technologies 2#: Accurate analysis of deformation curvature axis
This paper proposes a bending die motion control method that changes from bending sections to key point control, establishes a trajectory analytical model for continuously variable curvature configurations, and achieves a breakthrough in the freeform bending process from forming constant curvature components to forming continuously variable curvature components.
Key technologies 3#: Analysis of profile bending and twisting process
A bending direction reverse compensation algorithm coupled with cross-section torsion was proposed, and a trajectory analytical model suitable for the one-time forming of profile spiral components was established. Conclusion: Adding additional cross-section torsion angles during the profile bending forming process will cause periodic continuity of the bending direction of the component. Change.
Key technologies 4#: Process software system
The 3D freeform bending process software system has been independently developed to improve the free bending analysis efficiency. Figure 8 shows the six-axis freeform bending control system.
Key technologies 5#: The whole process of digitalization
Conducted research on freeform bending forming axis acquisition, computer-aided process analysis, and measurement feedback methods, built a digital forming platform, established the entire process from axis acquisition to comparative analysis, and realized efficient and high-precision digital processing of complex curved components. The process is as follows.
- Acquisition of axis: The accurate acquisition of the axis is the prerequisite for high-precision digital freeform bending forming. Professional software is used to extract and fit the bending axis and the coordinates of key points on the axis based on the geometric configuration of complex curved components, providing a digital model for subsequent process analysis and comparative analysis steps.
- Process analysis: Carry out freeform bending process analysis based on the axis point coordinate data obtained in the first step, convert the geometric information of the digital model into free bending process information, and obtain processing instructions that can be directly input into the free bending equipment.
- Forming test: Input processing instructions and carry out complex bending component forming tests.
- Scanning detection: Use professional scanning measurement equipment to obtain digital models of actual formed components.
- Comparative analysis: Compare the original digital model of the component with the digital model of the actual formed part in professional software, analyze the bending accuracy and bending quality of the actual formed part, and provide feedback to modify the forming process parameters for components that do not meet the requirements.
Key technologies 6#: Mold lining design optimization
To reduce scratches on the surface of the pipe, research on the lining design and material selection of the bending die and guide mechanism for freeform bending was carried out. By designing a new ceramic lining, the surface quality of the pipe was improved, the wear of the bending die was reduced, and the equipment was improved. life.
Study on friction and wear properties of zirconia/stainless steel.
Under the lubrication condition of 20# special lubricating oil, the friction coefficient between zirconia and stainless steel can reach 0.02.
Conclusion: Optimize the bending die material, effectively reduce the friction coefficient, and significantly improve the pipe forming quality.
Research on the lining design of bending die and guide mechanism.
The bending dies and guide mechanism lining made of zirconia material is shown in Figure 9. Wear problems such as furrows and spalling pits on the pipe surface are significantly reduced.
Engineering application of three-dimensional freeform bending forming of pipes
Aerospace
Complex conduits and structural parts required in the aerospace engineering field have extensive demand for three-dimensional freeform bending technology. Typical parts are oil, fuel, and gas pipelines, see Figure 16.
Cooperated to complete the trial production of special-shaped cross-section heat pipes for a certain type of spacecraft, as shown in Figure 19.
Automotive engineering field
The complex pipelines and profile components required by the automotive industry have extensive demand for three-dimensional freeform bending technology, including body frames, chassis, seat frames, A-pillars, and other parts.
In the automotive field, the free bending and torsion forming of high-strength aluminum alloy “eye-shaped” cross-section automobile anti-collision beam components have been completed, see Figure 20; the free bending and torsion forming of aluminum alloy asymmetric hexagonal cross-section roof rack components have been completed, see Figure 21; For the roof trunk outer frame using complex cross-section aluminum alloy profile components, the efficient and high-quality forming of two different cross-section components was successfully achieved at one time, as shown in Figure 22; the high-strength steel A-pillar products for new energy vehicles were completed Processing, see Figure 23.
Applications in other engineering fields
In addition, in the fields of rail transit, petrochemical engineering, nuclear energy engineering, architectural decoration, home decoration, etc., three-dimensional freeform bending of pipes is involved. The China Southern Airlines team completed the three-axis free bending process of the stainless steel seat back shape, see Figure 25; the spatial continuous variable curvature bending process for the outdoor landscape sculpture, see Figure 26; the stair handrail The processing of bent pipes with continuously variable curvature in space is shown in Figure 27. At present, a stable supply of multiple batches has been achieved and has been well received by demanders.
Conclusion
Three-dimensional free bending forming technology has the advantages of changing the bending radius without changing the bending die, achieving continuous changes in the bending radius of hollow components, realizing a variety of complex bending forms, high forming accuracy, and high forming quality. With the continuous deepening and improvement of relevant basic theoretical research, as well as the rapid development of numerical simulation technology, complex multi-axis control systems, reverse scanning systems for complex bent pipes, and other related technologies, three-dimensional free bending forming technology will play an important role in the field of manufacturing engineering in my country. Get important applications. ,
