CN103769451B - A kind of method of ultra-thin tubing minor radius bending forming - Google Patents

Abstract

The invention relates to a metal material plastic processing forming method. A method for bending and forming an ultra-thin pipe with a small radius, which is characterized in that it includes the following steps: 1). Using steel wire to tightly wrap the part of the pipe to be bent, the diameter of the steel wire is 1-3 times the wall thickness of the pipe, and the steel wire is wrapped The thickness of the pipe can be 1-4 times the wall thickness of the pipe according to the needs; 2). Put the steel wire wrapped pipe into the mandrel, and then use the conventional CNC bending equipment to perform CNC bending on the steel wire wrapped pipe; 3) .After the bending is completed, the wrapped steel wire is removed by a winding machine to obtain an ultra-thin pipe with a small bending radius. This method introduces an additional tensile stress to the pipe when it is bent and deformed by wrapping the pipe, so that the central layer of the pipe moves to the outside of the pipe, thereby reducing the tensile stress on the outside of the pipe and reducing the amount of thinning of the outer wall thickness of the pipe. Effectively alleviate the instability and wrinkling defects of the inner layer of the pipe during bending.

Description

A method of bending and forming ultra-thin pipe with small radius

technical field

The invention relates to a metal material plastic processing forming method, in particular to a new method for ultra-thin pipe material bending forming with small radius, and belongs to the technical field of material plastic processing.

Background technique

With the development trend of lightweight, high performance and low cost in the field of aerospace and automobile transportation, the requirements for bent pipe fittings are getting higher and higher, and the ratio of diameter to thickness (the ratio of diameter D to wall thickness t) is required to be larger, and the bending The radius (the ratio of the radius R ₀ of the pipe bending neutral layer to the pipe diameter D) is smaller and the forming accuracy is higher. The use of ultra-thin pipe fittings with a small curvature radius can not only reduce weight and achieve lightweight, but also make the layout of the piping system compact and save costs. However, with the increase of the diameter-thickness ratio of the pipe and the decrease of the bending radius, the deformation of the metal increases. Using the conventional bending pipe forming process, the wall thickness thinning rate and the ellipticity of the cross section of the small radius elbow will exceed Poor, the inner side produces waves and wrinkles, which seriously affects the quality of the elbow. At present, the traditional forming method of such small-radius curved pipe fittings is generally to use stamping sheet metal welding to prepare small bending radius elbows, and then weld and assemble them with straight pipes. However, the existence of a large number of welds not only leads to low production efficiency and high cost, but also the bending pipe must bear the pressure and impact of the internal flow medium when it is working, and the welds increase safety hazards and poor reliability.

The ultra-thin and small bending radius pipe fittings are integrally formed by seamless pipes, which not only saves space, but also ensures the reliability of the structure and the weight reduction of components. However, in order to ensure the bending quality of the pipe, the conventional pipe bending process must control the diameter-thickness ratio and relative bending radius of the pipe within a certain range. For example, when bending, roll bending and filler bending are used, the limit diameter-thickness ratio of pipe fittings is generally less than 30, and the minimum bending radius is also greater than 2d, which is far from meeting the requirements for forming ultra-thin pipe bends. The push bending process is suitable for bending short pipes with a diameter of 20mm-120mm, a wall thickness of 0.8mm-2mm, a bending angle of 15°-120°, and a bending radius of 1-2d. Since the pipe fittings all pass through the bending die, it is also not suitable for single-bend pipe fittings with long straight sections at both ends. At the same time, the external axial thrust intensifies the axial compressive stress on the inner side of the elbow. For thin-walled pipe fittings with a relatively large diameter and thickness, instability is prone to occur during the bending process of the pipe. Studies have shown that the diameter-thickness ratio of stainless steel 1.2d pipe fittings bent by the push bending process is generally less than 35.

At present, for the above-mentioned ultra-thin and small-radius curved pipes, in order to avoid or reduce the occurrence of forming defects, it is a common method to use solid fillers or flexible fillers as internal supports in engineering. Common solid fillers include ice, sand, rosin, or low-melting point alloys, etc., but these fillers are not only time-consuming to fill, but also have uncontrollable internal pressure in the support, and are likely to cause internal damage to the pipe, poor bending quality, and elongation of the low-melting point alloy. The rate is limited, and the bending radius cannot be too small. Flexible fillers usually refer to gas and liquid, and are generally pressure-filled with liquids such as water or oil. Due to the use of liquid as the internal filling medium, the surface quality of the formed parts is good, the inner side of the pipe wall has a small tendency to wrinkle, and the bending forming precision is high, especially suitable for bending pipes with variable curvature and large diameter-thickness ratio. One of the more typical applications is the liquid-filled shear bending method proposed by the Hydraulic Forming Engineering Center of Harbin Institute of Technology, which relies on shear deformation to generate material flow to achieve bending, and can form aluminum alloy and titanium alloy pipe joints with a relative bending radius of less than 0.5. However, using a liquid medium to provide internal pressure requires special equipment, which is difficult to implement, requires a large investment in equipment, and the forming mold is complicated and liquid sealing is difficult. The size of the formed fillet mainly depends on the internal pressure, and it is difficult to achieve precise quantitative control. In addition, the increase of internal pressure increases the normal compressive stress of the pipe wall, which will increase the friction between the pipe and the mold. When the forming fillet is small, the required forming internal pressure is very large, and the adverse effect of friction on the forming accuracy will be more prominent. Table 1 shows the limit diameter-to-thickness ratios that can be achieved by filling hydraulic bending of low-carbon steel and aluminum alloy thin-walled tubes at different bending radii. It can be seen that the limiting diameter-to-thickness ratio decreases gradually as the bending radius decreases. When the bending radius reaches (2~3)d, the limit diameter-thickness ratio of low-carbon steel and aluminum alloy pipe fittings cannot exceed 40.

Table 1 Achievable limit diameter-thickness ratio under different materials and bending radii

CNC bending is an advanced pipe plastic processing technology developed by combining traditional bending technology with machine tool industry and numerical control technology. The pipes of the three-dimensional space axis can be formed continuously at one time, and more than 30%-40% of the pipes outside the aircraft engine in developed countries in foreign countries are bent by CNC pipe bending machines. NC bending forming includes multiple sets of mold constraints to control the deformation behavior of pipe fittings collaboratively, which can better control defects such as section distortion and wrinkling, increase the bending forming limit of pipes, and is suitable for forming thin-walled pipes with small bending radius. Generally, the relative bending radius of CNC bending with special mold structure such as multi-ball mandrel and anti-wrinkle block can reach 1.5. However, with the increase of the diameter-to-thickness ratio of the thin-walled pipe, the minimum relative bending radius of the CNC pipe bend is also gradually increasing. Foreign aviation companies have mastered mature CNC pipe bending technology, and have made great breakthroughs in the maximum diameter-thickness ratio and the minimum relative bending radius. The limit diameter thickness that can be achieved by its CNC bending technology is shown in Table 2. It can be seen from the table that the diameter-to-thickness ratio of stainless steel thin-walled pipes can reach a maximum of 127, and the minimum bending radius can reach 2d. The maximum diameter-thickness ratio of aluminum alloy pipe fittings reaches 77, and the minimum bending radius can reach 1.7d. In recent years, with the rapid development of domestic CNC bending forming technology, the gap between the ability to bend large-diameter-thickness ratio thin-walled tubes and foreign countries has gradually narrowed, and many important research results have been achieved in the study of CNC bending forming laws and thin-walled tube bending theory. The results of numerical control bending experiments on stainless steel thin-walled tubes show that when the relative bending radius reaches 1.5, the maximum diameter-thickness ratio can reach 50. The results of numerical control bending experiments on aluminum alloy thin-walled tubes show that when the relative bending radius reaches 1.5-2, the maximum diameter-thickness ratio can reach 50; when the relative bending radius reaches 1, the maximum diameter-thickness ratio decreases to 38.

The above research data show that, using the existing bending forming process, the limit diameter-thickness ratio of the thin-walled tube that can be formed is about 100, and the relative bending radius is about 2.

Table 2 The ultimate diameter-thickness ratio that can be achieved by CNC bending abroad

In the traditional bending method, the pipe is bent under the action of an external moment M, as shown in Figure 3. The outer material of the bending deformation zone is elongated by tangential tension, and the inner material is shortened by tangential compression. Since the distribution of tangential stress σ _θ and strain ε _θ is continuous along the section of the pipe, when the bending process ends, the tension zone transitions to the compression zone, and there is a layer of fibers at the junction, whose length is equal to the original length of the tube blank , that is, the strain ε _θ =0 of the fiber layer. This fiber layer is called the strain neutral layer, and its position in the section can be expressed by the radius of curvature ρ. When the outer material of the bending deformation zone of the pipe is subject to tangential tensile elongation higher than its material elongation, the pipe will rupture, as shown in Figure 3b; when the inner material of the pipe is subjected to tangential compression and becomes unstable, the pipe Wrinkling occurs on the inside, as shown in Figure 3b.

Contents of the invention

The purpose of the present invention is to provide a method for bending and forming ultra-thin pipes with small radius. This method introduces an additional tensile stress to the pipes when the pipes are wrapped to achieve bending deformation, so that the central layer of the pipes moves to the outside of the pipes, thereby reducing the pressure of the pipes. The tensile stress on the outside is used to reduce the thinning of the outer wall thickness of the pipe, and at the same time, it can effectively alleviate the instability and wrinkling defects of the inner layer of the pipe during bending.

In order to achieve the above object, the technical solution adopted by the present invention is: a method for bending and forming an ultra-thin pipe with a small radius, which is characterized in that it includes the following steps:

1). Use steel wire 1 to tightly wrap the part C to be bent of pipe 2. The diameter of the steel wire (square steel wire is the side length) is 1-3 times the wall thickness of the pipe (that is, the wall thickness of the pipe: the diameter of the steel wire = 1: 1-3), the thickness of the steel wire wrapping can be 1-4 times the wall thickness of the pipe (original pipe) according to the needs, and the gap between the wrapped steel wire and the pipe is about 0.1mm (the difference between the outer diameter of the wrapped steel wire and the pipe around 0.1mm);

2). Put the inside of the steel wire-wrapped pipe into the mandrel (multi-ball mandrel) 3, and then use conventional numerical control bending equipment to perform numerical control bending on the steel wire-wrapped pipe [use static clamping block 4 and anti-wrinkle block 5 , and the movable clamp block 6 and the bending die 7 perform traditional numerical control bending on the pipe];

3). After the bending is completed, the wrapped steel wire is removed by a winding machine (removing the steel wire wrapped by the pipe after bending), and an ultra-thin pipe with a small bending radius can be obtained.

The steel wire 1 is a square steel wire (belt) or a round steel wire.

The pipe 2 is a pipe with a large diameter-to-thickness ratio. The diameter-thickness ratio refers to the ratio of the outer diameter of the pipe to the wall thickness.

Ultra-thin pipes (ultra-thin pipe fittings) are pipes (pipe fittings) with a diameter-to-thickness ratio greater than 50. The minimum bending radius is the outer diameter of the pipe whose bending radius is less than 1.5 times.

Using this method for pipe bending can reduce the diameter-thickness ratio of the entire pipe when it is bent. At the same time, when the steel wire wrapped in the outer layer of the pipe is bent and deformed, a friction force is introduced on the pipe, which reduces the tensile stress on the outside and the compressive stress on the inside when the pipe is bent, so that the stress neutral layer moves outward, which not only restrains the wall on the outside of the pipe Thickness and thinness can also effectively improve the instability and wrinkling of the inner side of the pipe, and increase the bending forming limit of the pipe.

The invention solves the problem that the existing method cannot realize small-radius bending by bending ultra-thin pipes with a large diameter-thickness ratio.

The beneficial effects of the invention are: the structure of the adopted equipment is simple, convenient, practical, novel in conception, and the steel wire wrapping pipe is simple and easy. Since the thickness of the steel wire is 1-4 times the wall thickness of the pipe, the ratio of diameter to thickness when the whole pipe is bent can be reduced. At the same time, when the steel wire wrapped by the outer layer of the pipe is bent and deformed, a frictional force on the pipe is introduced. This frictional force will prevent the elongation of the outer layer and the shrinkage of the inner layer when the pipe is bent, and generate tangential compressive stress on the outside of the pipe. Tangential elongation stress is generated on the inner side of the pipe, thereby reducing the drawing stress on the outer side and the compressive stress on the inner side when the pipe is bent, so that the stress neutral layer moves outward, which not only inhibits the thinning of the outer wall thickness of the pipe, but also effectively Improved destabilization wrinkling on the inside of the pipe. Therefore, compared with the traditional bending process, this process can effectively avoid defects and improve the bending forming limit of the pipe.

Description of drawings

Fig. 1 is a schematic diagram of the pipe material of the present invention after being wound and wrapped with steel wire.

Fig. 2 is a schematic diagram of CNC bending of a steel wire-wrapped pipe according to the present invention (a schematic diagram of bending and forming of a wrapped pipe).

Figure 3 is the force analysis and defect diagram during the traditional pipe bending process; a is the pipe bending force diagram; b is the pipe bending defect diagram.

Fig. 4 is a force analysis diagram during the bending process of the steel wire-wrapped pipe of the present invention.

In the figure: 1-steel wire, 2-pipe, 3-mandrel, 4-static clamping block, 5-anti-wrinkle block, 6-moving clamping block, 7-bending die; C in Figure 1 represents the part to be bent (bending Section); in Figure 2, W represents the rotation speed of the bending die; in Figure 4, F represents the support force of the pipe by the mandrel, f represents the friction force, M represents the bending moment, σ _θ represents the tangential stress, and ε _θ represents the tangential strain.

detailed description

Example 1:

A method for bending and forming an ultra-thin pipe with a small radius, comprising the steps of:

1). The diameter of the pipe 2 is 60mm, the thickness is 0.73mm (that is, the ratio of diameter to thickness is 82), and the length is 400mm. The material of the pipe 2 is pipe 201; the steel wire 1 is a round steel wire, and the diameter of the steel wire is 1.5mm. The specific wrapping layer is 1.5mm thick, the bending radius is 90mm, the length of the part C to be bent is 200mm, the outer diameter of the mandrel (multi-ball mandrel) 3 is 57.8mm, and the diameter of the ball contained is 57.4mm. The arrangement form is that three balls are connected in series on the mandrel through a hinge; as shown in Figure 1;

The steel wire 1 is used to tightly wrap the part C to be bent of the pipe 2, and the gap between the wrapped steel wire and the pipe is about 0.1mm (the outer diameter difference between the wrapped steel wire and the pipe is about 0.1mm);

2). Put the inside of the steel wire-wrapped pipe into the mandrel (multi-ball mandrel) 3, and then use conventional numerical control bending equipment to perform numerical control bending on the steel wire-wrapped pipe [use static clamping block 4 and anti-wrinkle block 5 , and the movable clamp block 6 and the bending die 7 perform traditional numerical control bending on the pipe];

3). After the bending is completed, the wrapped steel wire is removed by a winding machine (removing the steel wire wrapped by the pipe after bending), and an ultra-thin pipe with a small bending radius can be obtained.

The steel wire 1 is a round steel wire.

The structure of the mandrel 3 is shown in Fig. 2, and the mandrel has three core balls connected in series through a hinge, forming a multi-ball mandrel together. However, the biggest feature of the present invention is the steel wire wrapped outside the pipe, which can change the force characteristics of the bending deformation of the pipe and realize the small-radius bending of the ultra-thin pipe. The basic structure is shown in the three-dimensional CAD diagram of Fig. 1, and the force analysis of bending deformation is shown in Fig. Figure 4.

The inner side of the finally obtained ultra-thin elbow was smooth without any wrinkling phenomenon, and no defects such as cracks were found on the outer side.

Example 2:

A method for bending and forming an ultra-thin pipe with a small radius, comprising the steps of:

1). The diameter of the pipe 2 is 60mm, the thickness is 0.73mm (that is, the ratio of diameter to thickness is 82), the length is 400mm, and the material of the pipe 2 is 201; the steel wire 1 is square steel wire, and the square steel wire is equilateral 1.5× 1.5mm, the specific wrapping layer is 1.5mm thick, the bending radius is 90mm, the length of the part to be bent is 200mm, the outer diameter of the mandrel (multi-ball mandrel) 3 is 57.8mm, and the diameter of the ball contained is 57.4mm , the arrangement of the balls is that three balls are connected in series on the mandrel through a hinge; as shown in Figure 1;

The steel wire 1 is used to tightly wrap the part C to be bent of the pipe 2, and the gap between the wrapped steel wire and the pipe is about 0.1mm (the outer diameter difference between the wrapped steel wire and the pipe is about 0.1mm);

2). Put the inside of the steel wire-wrapped pipe into the mandrel (multi-ball mandrel) 3, and then use conventional numerical control bending equipment to perform numerical control bending on the steel wire-wrapped pipe [use static clamping block 4 and anti-wrinkle block 5 , and the movable clamp block 6 and the bending die 7 perform traditional numerical control bending on the pipe];

3). After the bending is completed, the wrapped steel wire is removed by a winding machine (removing the steel wire wrapped by the pipe after bending), and an ultra-thin pipe with a small bending radius can be obtained.

The steel wire 1 is a square steel wire.

The inner side of the finally obtained ultra-thin elbow was smooth without any wrinkling phenomenon, and no defects such as cracks were found on the outer side.

Claims (3)

1. a method for ultra-thin tubing minor radius bending forming, is characterized in that comprising the steps:

1). adopt steel wire (1) that the position to be bent (C) of tubing (2) is closely wound around parcel, gauge of wire is 1-3 times of tube wall thickness, the thickness of steel wire parcel can be the 1-4 of tube wall thickness doubly as required, and the gap between the steel wire of parcel and tubing is 0.1mm;

2). plug (3) is put in the tubing inside after wrapped wire, then adopts conventional numerical-control bending equipment to carry out numerical control bending forming to the tubing after wrapped wire;

3). after bending forming terminates, utilize wrapping machine to take off on the steel wire of parcel, the ultra-thin pipe fitting of small-bend radius can be obtained; Described ultra-thin pipe fitting is the pipe fitting that radius-thickness ratio is greater than 50, and small-bend radius is the tube outer diameter that bending radius is less than 1.5 times.

2. the method for a kind of ultra-thin tubing minor radius bending forming according to claim 1, is characterized in that: described steel wire (1) is square steel silk or round steel wire.

3. the method for a kind of ultra-thin tubing minor radius bending forming according to claim 1, is characterized in that: the tubing that described tubing (2) is large radius-thickness ratio.