超塑性材料通过锥形模口的稳态拔丝和挤压工艺的流变力学研究(英文)

RHEO-MECHANICAL APPROACH TO THE STEADY STATE DRA-WING AND EXTRUSION PROCESSES OF SUPERPLASTIC MATERIALS THROUGH TAPERED DIES

本文对圆柱体和宽条板分别通过锥形模口的稳态超塑性拔丝和挤压进行了流变力学分析。变形是粘性的,并在达到稳态以前经过应力的极大值。随十字头速度增大和温度降低,达到稳态需要的应变增大。应力-应变率关系对试件尺寸的相依性表明表面摩擦是重要的。和应变率一起的流变应力的相依性,能够造成均质物体超塑性加工期间的一些困难。在低应变率情况下,由于有效摩擦区仅占试件体积很小的一部分,并且慢速率下砧面与试件间存在着滑动,所以表观应力随试件径高比的增大是很小的。可是,随应变率的增大,较快的滑动导致较陡的倾斜面。在高应变率及因锥形模口具有锐角而横截面缩减大的情况下,可发生拉伸不稳定性。但是,在材料的进口截面上施以适当的推力,有助于避免这种不希望有的情况。

The steady-state superplastic drawing and the extrusion of a cylindrical body and awide strip, respectively, through a tapered die have been treated rheomechanically. Thedeformation is viscous and goes through a maximum in stress before attaining steadystate. The strain needed for reaching the steady state increases with increasing cross-head speed and decreasing temperature. The dependence of the stress-strain rate relation on specimen dimensions indicatesthat surface friction is important. Together with the strain-rate dependence of the flowstress this can create difficulties during the superplastic forming of objects possessinguniform properties. At low strain rates, the increases in observed stress with diameter toheight ratio of specimen is small because the friction affected zone is a small fractionof the volume of the specimen and the sliding between the anvil and the specimen is at aslow rate. With increasing strain rates, however, faster sliding leads to steeper slopes.At high strain rate and large reduction of cross-sectional area with steep angle of thetapered die, tensile instability may occur. However, by adding an appropriate amount ofpushing force on the entry section of the material, it may help to avoid this undesirablesituation.