The objective of this paper was to study the induced and residual stresses when the holes were subjected to cold hole expansion (CHE) using ball and taper mandrel methods. Al 2024, EN 24 and spring steel alloys were selected for the base material, mandrels (both ball and tapered) and split sleeve respectively. The CHE was processed from 2 to 6%, in steps of 1% by both and ball and tapered mandrel using UTM at a constant strain rate of 0.2 mm/min. To determine the residual stresses (hoop stress and radial stress) around the hole, four strain gages - one in the radial direction and three in tangential direction were mounted on the polished surfaces of the specimens. The experimental results showed that for both ball and tapered mandrel methods, the induced radial and hoop stresses increased up to 5% of expansion and further reduced. Further, the tapered mandrel showed better performance than the ball mandrel method due to more uniform expansion (surface contact) in the former than the latter (line contact). Also, the residual stresses for the 6% expanded holes drastically reduced i.e. by about 35% at 2 mm and 70% at 4, 6 mm from hole edge, with respect to the induced stress. This shows that there is little effect of expansion above 5%. For all other levels of expansion, the reduction was between 5 and 15%.
Cold Hole Expansion (CHE), Al 2024, Induced Stress, Residual Stress, Ball & Taper Mandrel
