Research On A New Process Of Filler Push Bending For Very Thin Metal Tubes

The thin-walled, especially very thin-walled(wall thickness<0.5mm, outer diameter≥20mm) tubes can meet the demands for product lightweight, low consumption and so on, and can also reduce the production cost. Therefore such tubes are widely used in aviation, aerospace and automobile industries and other fields. During the tube bending process, it is easy to appear for the defects such as instability wrinkling at the intrados of the tube, thickness reduction even rupture at the extrados and cross-section distortion, which is more difficult to be solved especially for thin-walled, very thin-walled tube. A new process of ball filler push bending based on traditional push bending is proposed in this paper. Then, using very thin-walled TU1 copper tube as research object, the experimental methods, theoretical analysis and finite element simulation on the new process are carried out. It has been studied systematically on process parameters effecting on tube surface quality, thickness distribution and ovality. And the proposed ball filler push bending process is compared with rosin filler by experimental method. Finally, based on the above research, feasibility of push bending for very thin-walled 1Cr18Ni9Ti stainless steel tube is primitively discussed. The main contents and results are as follows:Firstly, the push bending die and the hydraulic system are designed. The pressure transmission characteristic of ball filler and the stress state of the tube during bending process are analyzed. And then the relationship between sizes of ball filler and pressure transmission properties have been obtained. Moreover, the non-uniformly pressure transmission characteristic of ball filler has also been verified by later experiments.The finite element simulation is carried out for push bending process for very thin-walled TU1 tube with the outer diameter 30mm and thickness 0.5mm. The results show that the internal pressure and friction condition have great effects on surface quality at the intrados of tube. It is likely to cause wrinkle at the intrados with internal pressure less than 2MPa or greater than lOMPa, and it is beneficial for stably forming of the intrados with less friction coefficient. In addition, the wrinkle at the intrados first occurs on the transition zone from deformed zone to non-deformed zone, and material is prone to be accumulated in the region 30-50mm far from the pushing end. What is more, the non-uniform distribution of internal pressure helps to improve the wall thickness distribution, which makes the wall thickness distribute and stress and strain change more uniform. The ball filler and rosin filler push bending experiments have been investigated for very thin-walled TU1 and 1Cr18Ni9Ti tubes respectively. The results indicate that size of ball filler, back pressure and lubrication condition have remarkable impacts on the surface quality and wall thickness distribution of bent tubes:with the larger size of ball filler, the positive pressure on the tube wall is greater, which causes to increase trend of wrinkle at the intrados. The wrinkle can be effectively inhibited with moderate size of ball filler and proper back pressure and good lubrication condition. The wall thickness of the tube will be increased with increasing back pressure or increasing friction coefficient.The cross-section distortion of bent tube is very small by using ball filler push bending process, with the ovality less than 1.5%. The quality of the internal surface can be greatly improved and the punch force can be significantly reduced by using inner set hose. The wall thickness distribution of bent tubes by using ball filler is more uniformly than those by using rosin filler. And the rosin filler can not be effectively used for 1Cr18Ni9Ti tube bending because of its insufficient strength.