薄壁管数控弯曲过程中失稳起皱的主要影响因素

 针对薄壁管数控弯曲精确成形过程在多因素交互作用下可能发生失稳起皱这一复杂物理问题, 基于起皱能量准则和有限元方法相结合, 提出了预测该过程起皱发生的能量( 数值) 方法, 研究了影响薄壁管数控弯曲成形过程起皱发生的主要因素及影响机制。结果表明, 弯曲半径、相对管径、芯棒伸长量和摩擦因素是影响薄壁管数控弯曲精确成形过程起皱发生的主要因素, 而材料的应力强度系数和加工速度等对起皱发生影响较小。研究结果为薄壁管数控弯曲精确成形过程参数的确定和优化创造了条件。     

A Study on Multi-defect Constrained Bendability of Thin-walled Tube NC Bending Under Different Clearance

Thin-walled tube numerical control (NC) bending is a tri-nonlinear physical process with multi-defect and multi-die constraints. The clearance on each contact interface is the major factor to indicate the contact conditions. A three-dimensional-finite element (3D-FE) model is established to consider the realistic dynamic boundary conditions of multiple dies under ABAQUS/Explicit platform. Combined with experiment, numerical study on bending behavior and bendability under different clearance between tube and various dies is conducted in terms of wrinkling, wall thinning and cross section deformation. The results show that (1)with smaller clearance of tube-wiper die and tube-mandrel, the wrinkling can be restrained while the wall thinning It and cross-section deformation Id increase; while excessive small clearance blocks tube materials to flow past tangent point and causes piles up, the onset of wrinkling enhances It and Id. (2)Both It and Id decrease with smaller clearance of tube-pressure die; the wrinkling possibility rises with larger clearance on this interface if the mandrel’s freedom along Y-axis is opened; smaller clearance of tube-bend die prevents wrinkling while increases It, and the clearance on this interface has little effect on Id. (3)A modified Yoshida buckling test (YBT) is used to address the wrinkling mechanisms under normal constraints in tube bending: the smaller clearance may restrain wrinkling efficiently; the smaller wall thickness, the less critical clearance needed; the critical clearance for tube bending 38 mm×1 mm×57 mm (tube outer diameter×wall thickness×centerline bending radius) equals about 20% of initial wall thickness.     

Deformation Behavior of Medium-strength TA18 High-pressure Tubes During NC Bending with Different Bending Radii

To improve the forming quality and forming limit of the numerical control (NC) bending of high-pressure titanium alloy tubes, in this study, using three-dimensional (3D) finite element method, deformation behavior of medium-strength TA18 high-pressure tubes during NC bending with different bending radii is investigated. The results show that the cross-sectional deformation and the wall thickness variation during NC bending of TA18 tubes using a small bending radius (less than 2 times of tube outside diameter) are clearly different from that using a normal bending radius (between 2 and 4 times of tube outside diameter). For bending with a normal bending radius, with or without a mandrel, the distribution of the flattening in the bending area resembles a platform and an asymmetric parabola, respectively. For bending with a small bending radius, with or without a mandrel, the flattening both distributes like a parabola, but the former has a stable peak which deflects toward the initial bending section, and the latter has a more pronounced peak with a bending angle and deflects slightly toward the bending section. The wall thickness variations with a normal bending radius, with and without a mandrel, both resemble a platform when the bending angle exceeds a certain angle. For the bending with a small radius, the distribution of the wall thickness variation without a mandrel follows an approximate parabola which increases in value as the bending angle increases. If a mandrel is used, the thickening ratio increases from the initial bending section to the bending section.