Effect of lead angle of hole expansion mandrel on surface integrity and fatigue performance of TC17 titanium alloy hole structure

Abstract

For the TC17 titanium alloy, the influence of the leading end angle of the mandrel on the expansion strengthening effect of the hole structure was studied, the surface integrity of the hole expansion strengthened under different process parameters was characterized, the high temperature and low cycle of the original and strengthened samples were tested. Fatigue life and the morphological characteristics of fatigue fracture are analyzed. The results show that the lead angle of the mandrel has a significant effect on the surface roughness after expansion. The uneven plastic flow of the metal on the surface of the hole wall during the expansion process leads to uneven residual stress distribution on the hole wall after expansion, and the residual stress amplitude at the exit end of the expansion is the largest, and the hole wall has a certain depth of residual compressive stress gradient field after expansion. When the interference of the core rod is constant, the fatigue life increases with the increase of the rear lead end angle of the mandrel. When the rear lead end angle is 8°, the median fatigue life gain after strengthening can reach 1.74 times, and the strengthening effect is the best, its minimum cycle life is 16331 times, which is higher than the longest cycle life of the original sample (13965 cycles). After strengthening, the origin of the cracks is changed from the multi-source type in the middle of the hole wall to the single-source crack initiation at the inlet end of the expansion.

Keywords

• hole expansion
• lead angle
• tc17 titanium alloy
• fatigue life
• residual stress