弯曲轴线中空零件充压镦形工艺及机理研究

Hollow component is a competitive structure of weighting reduction for auto industry. But there are many difficulties to hydroform such components, especially in the Advanced High Strength Steel. These problems limit the widely use of such component. Troubles in axial feeding were the primary cause of these problems. When axial feeding difficult to apply, bulging deformation could happen. Unfortunately, internal pressure is the only deformation driving force in bulging deformation, and need to reach several hundred MPa for plastic deformation happening. High pressure induced bursting defect, poor thickness distribution and low production efficiency. To overcome such difficulties, a now forming technology, named as hydro-upsetting is put forward. The main idea is tube blank could be upsetted under the internal pressure supported. In this forming technology, press machine provides the driving force for deformation, and the internal pressure only works for supporting. As a result, the pressure is decreased to tens of MPa. On the other hand, thickness thinning and bursting are removed because of thickening deformation happening during upsetting process. As a result, axial feeding does not need in hydro-upsetting technology. In this study, the forming limit and the main type of defects would be investigated deeply. At the same time, the instability mechanism and the defects forming mechanism would also be researched and implemented.

中空零件是实现汽车减重的先进结构形式，但其中弯曲轴线零件内高压成形难度仍然很大，制约了该类零件在车身上的广泛应用。原因是通过“膨胀”来成形中空零件时，塑形变形全部靠内压来驱动，存在对高压依赖严重、胀形区减薄壁厚均匀性不佳、生产效率低等行业难点问题。基于此本项目提出了充压镦形技术，成形思路与膨胀成形恰好相反。原理是借助内压的支撑，对管坯进行“截面压缩”来成形变截面中空零件。利用了法向力支撑下板壳面内压缩失稳应力提高的力学原理。变形类型的改变，解放了内压，冲压镦形中变形驱动力由压机提供，内压仅起辅助支撑作用，所需压力降至几十MPa。此外，压缩变形从根本上避免了膨胀变形模式下壁厚减薄和开裂问题。可解决当前弯曲轴线中空零件成形难度大阻碍车身轻量化的问题。本课题重点研究法向力支撑下管材压缩失稳机制，压缩成形极限、塑性变形规律及充压镦形的主要缺陷形式及形成机制，形成一种新的中空零件成形技术。

中空零件是实现汽车减重的先进结构形式，但内高压“膨胀”成形该类零件时，存在对高压依赖严重、胀形区减薄壁厚均匀性不佳、生产效率低等行业痛点问题，制约了该类零件在车身上的广泛应用。针对上述问题，本项目提出一种全新的管材件成形思路-充压镦形，其基于法向力支撑下板壳面内压缩失稳应力提高的力学原理，借助内压支撑进行“截面压缩”来成形变截面中空零件。建立充压镦形力学模型，明晰内压对管材压缩失稳和压缩极限的影响机制，系统揭示了各主要参数对成形极限、壁厚分布、成形精度的影响规律。.由塑性弯曲理论建立了内压作用下皱纹展平力学模型，预测波高波长比δ/L随内压的变化曲线，分析材料属性、波长及壁厚对皱纹展平的影响，提出了死皱的临界条件：壁厚t0在1~3mm范围内，皱纹弧长与波长的差值u小于1.3t0+0.63。.基于塑性全量理论，建立了应力应变解析模型，得出环向壁厚分布，揭示圆角处壁厚较小的原因，提出增大圆角半径可以改善圆角处壁厚分布，并给出提高壁厚均匀性的分模方式。该工艺从根本上避免膨胀变形模式下壁厚减薄和开裂问题。推导合模力计算公式，发现合模力与宽度、壁厚、摩擦系数及内压有关，而与圆角半径没有关系。.提出充压镦形中的两种失稳形式：翘曲失稳和压缩失稳，分别建立两者的临界内压理论模型，揭示不同的产生机理，指出前者临界应力比后者临界应力小，相同条件下翘曲失稳更易发生，进而获得充压镦形失稳抑制压力。得益于变形类型的改变，充压镦形中内压仅起辅助支撑作用，使得成形压力仅为传统“膨胀”工艺的20%~40%。.此外，发明垂向加载应力均匀法，能够有效控制高强钢弯曲管状构件的轴线回弹，这能消除95%的回弹量。由于塑性加工硬化和塑性变形量差异，存在临界压缩量。临界压缩量主要与材料硬化指数和弯曲半径有关。系统研究了压缩量与形状精度之间的关系，建立了工艺成形窗口。.综上，本项目形成一种新的中空零件成形技术，可解决当前弯曲轴线中空零件成形难度大阻碍车身轻量化的问题。