楔横轧小断面收缩率轧件螺旋组织缺陷研究

Study on the spiral microstructure defect of workpiece with a small area reduction formed via cross wedge rolling

轧件发生局部变形是楔横轧的主要工艺特征,尤其小断面收缩率轧件轴向流动能力弱,内外变形差异显著导致楔横轧成形困难.除了容易产生心部破坏缺陷,在轧件表层一定范围内出现的螺旋组织缺陷,也会降低产品的机械性能.本文通过轧制实验,展示出轧件螺旋组织缺陷宏观上呈现为车削后在表层一定深度范围内沿展宽螺旋线分布的亮带,微观上由轧件表面折叠向内部延伸呈带状分布的组织形态.结合有限元数值模拟方法研究了缺陷产生的主要原因,发现由于成形区的金属发生沿展宽负向的金属流动,导致轧件形成沿展宽螺旋线分布的表面折叠和小轴向应变带.同时,螺旋带附近较大的径向压缩使轧件由表面向内部沿折叠裂纹方向组织具有方向性.采用对模具楔尖倒圆角局部改善金属沿负展宽方向的轴向流动,可以既消除表层螺旋组织缺陷,又避免轧件心部损伤风险,使成形质量满足使用要求.经实验验证,确定了模具楔尖圆角的最优取值.

Local deformation of workpieces is a main process characteristic of cross wedge rolling; thus,it is difficult to cross wedge rolling the workpiece with a small area reduction owing to the weak ability to flow in the axial direction and the significant deformation difference between the surface and interior. Besides the central damage of workpiece,a spiral microstructure defect in a certain depth of surface layer reduces the mechanical strength of products manufactured by cross wedge rolling. A rolling experiment is performed to reveal that the spiral microstructure defect appears macroscopically as a bright band along the stretching spiral line in a certain depth of the surface layer after machining and microscopically as a strip microstructure distribution that extends from the surface fold to the interior of the workpiece. The cause of the microstructure defect is investigated by a combination of rolling experiments with finite element analysis,and the metal in the forming zone flows along the negative stretching direction,resulting in the surface fold being distributed along the stretching spiral line and the small axial strain band on the formed surface. Meanwhile,large radial compression near the spiral band causes the directional distribution of the microstructure along the fold cracks from the surface to the interior of the workpiece. In addition,the fillet of the wedge tip is adopted to improve the local metal axial flow in the negative stretching direction,which can eliminate the spiral microstructure defects near the surface and avoid the risk of central damage to ensure that the forming quality in the center of workpiece meets the requirements. Finally,the optimal value of fillet on the wedge tip is determined via ex-perimental verification.