空洞敏感材料超塑胀形过程的压力—时间曲线优化设计

压力－时间曲线在超塑胀形工艺参数的选择和工艺过程优化设计中至关重要。利用刚粘塑性有限元技术对含有细微空洞超塑性材料的胀形过程进行了数值模拟，并采用最大等效应变速率恒定的压力控制策略对胀形过程压力－时间曲线进行了优化设计。以半球壳和圆筒形零件为例，对自由胀形和充模胀形两种成形方式给出了压力－时间曲线设计实例，并对计算结果进行了讨论。本计算模型可推广至其他超塑性成形问题。

On Improving Control of Strain Rate in Superplastic Bulging Process

In order to shorten forming time and to achieve uniform thickness of parts in superplastic bulging of void-sensitive materials, it is well known that maximum effective strain rate should be kept within a very small range. To satisfy this well known requirement, we have to obtain the optimum design of pressure-time curve. We take into consideration effects of both material anisotropy and strain hardening. Four-node quadrilateral isoparametric element is adopted and the direct iteration method is used for generating the initial guess velocity. Also, arctangent function frictional model is selected to eliminate the sudden change of direction of the frictional stress at the neutral point and to maintain continuity along the whole boundary. Two examples of designing pressure-time curve using FEM (finite element method) code SBSS2D (superplastic bulging simulation system 2D) are given for free bulging of hemisphere and die bulging of cylinder parts respectively and the optimization results are presented. From the results, we find that the maximum effective strain rate remains within the specific error norm in the whole bulging process, which proves the effectiveness of our method for maintaining the maximum effective strain rate within a very small range.