Abstract This investigation has considered the influence of heat treatment, environment and residual stress level on the response of titanium alloys to mechanical surface treatment. It has been shown that the high cycle fatigue benefits to be gained by mechanical surface treatment will depend upon all of these variables. For example, shot peening of Ti–6Al–7Nb a/b solution treated, water quenched and aged condition results in a substantial increase in the high cycle fatigue performance in laboratory air. In contrast, shot peening of Ti–6Al–7Nb a/b solution treated, air cooled and aged condition has only a relatively small impact on the high cycle fatigue performance. This study also shows that high cycle fatigue crack nucleation of mechanically surface treated titanium alloys within the low stress, high cycle regime is associated with subsurface fatigue crack nucleation. Further, that this phenomenon may be related to the presence of a process-induced residual tensile stress necessarily present below the mechanically treated surface and required to balance the outer process-induced residual compressive stresses.
The present investigation was performed on the well known (α+β) titanium alloy Ti-6Al-4V having a typical commercially available mill annealed microstructure. In order to establish optimum conditions with regard to fatigue performance, shot peening and roller-burnishing were performed using a wide variation in Almen intensity and rolling force, respectively. Additional polishing treatments were performed to reduce process-induced roughnesses and microcracks in order to find out if the fatigue behavior can be further improved.
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