Fracture analysis of Ti–6Al–4V bolts

Macrocracks were found along the axis of Ti–6Al–4V bolts during the process of thread rolling. Metallographic tests and EDAX analysis were carried out to determine the reason for the cracks. The microstructural morphology of the bolts is characterized by an equiaxed α + β structure and a basketweave structure in good regions and coarse single α phase in bad regions (the regions close to the cracks), respectively. The cracks initiated and propagated in the bad regions, leading to the cleavage fracture of the bolts. A higher content of oxygen was found in the bad regions than that in the good regions. As an α stabilizer, oxygen increased not only the α → β transformation temperature (β transus), but also the recrystallization temperature in the bad regions, resulting in the formation of coarse single α phase, and consequently, poor ductility and crack propagation resistance in bad regions.