shown in this figure, the effect of SH on the forming load became stronger with a decrease in the final forming
velocity VL. The variation in the forming load due to the variation of SH could be ignored for VL exceeding 10
mm/s. This could be attributed to the temperature drop caused by an increase in contact time at the final forming
stage. The forming load reduced steadily for VL ranging from 1 mm/s to 5 mm/s, and became stable for VL
exceeding 10 mm/s. The forming load was minimum for SH of 48 mm and VL of 5.75 mm/s (condition C3). The
temperature and effective stress distributions in the workpiece at three selected conditions are shown in Fig. 8. The
temperature distribution for the condition C2 is the highest, given the fact that the time span of contact between the
workpiece and die is shortened when compared to the other conditions. However, the effective stress distribution
becomes lowest for the condition C3 due to the synergistic effects of increasing temperature and decreasing
effective strain rate around the final stroke.
On the basis of the information obtained from the DOE study, we performed the forging experiment of the
pulley upsetting process. The detailed experimental conditions are summarized in Table 3. The range of SH and VL
was adjusted considering the result of the DOE study, so as to reduce the total number of experiments. The
photographs of the forged product and the surface plot of the forming load variation with respect to the
experimental conditions are depicted in Fig. 9 and Fig. 10, respectively. As shown in Fig. 10, the tendency of the
forming load variation with respect to the design variables was similar to that predicted by the DOE study,
although the absolute values of measured forming load were about 20% - 30% higher than the simulation result.
The main reason for the observed difference is the inaccuracy of the in-house flow stress data used in the
simulations. More reliable material data obtained from carefully treated material test should be applied to improve
the simulation accuracy up to the acceptable level for quantitative evaluation.