Effect of bulge formation on strain inhomogeneity in axi-symmetric metal drawing of light reductions

Abstract

Bulge formation occurs in drawing with light reductions and large die angles. Reports concerning its effect on strain inhomogeneity in axi-symmetric drawing are limited. In this study, bulge formation is characterized by the occurrence of outward radial velocities in the finite element analysis. Strain distributions, including axial, radial, circumferential, shear, and effective strains, were analyzed to explain the effect of bulge formation on strain inhomogeneity. Hardness tests for the drawing experiment were conducted to verify the findings of the FE simulation. Optical microscopy of the microstructures produced by drawing under both a near homogeneous condition and a bulging condition was performed. The results indicate that bulging causes a peak in the axial strain distribution near the drawn workpiece's surface, leading to a slight fall-off of effective strain on the surface. The peak is caused by the excessive redundant deformation of a local reversion from compressive to tensile axial strain. The strain peak is greater when drawing with light reductions and large die angles and becomes more noticeable with small strain-hardening exponents or large friction factors. Bulge formation also causes fibrous flow lines near the surface of the drawn workpiece.

Piercing and surface-crack defects in combination extrusion of fasteners

“…high level of extrusion ratio. Both phenomena indicate that the forming defects occurred in combination extrusion are attributed to insufficient deformation level, which is also a primary factor causing the strain inhomogeneity of workpiece [9] in the forming processes. The low forming level also is an indication that the hydrostatic pressure is low, and the tendency of crack initiation in the workpiece is not suppressed.…”