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.
Titanium Alloys Research SummaryAlthough suspension spring materials traditionally have been made of steel, metastable beta-titanium alloys are being considered as possible alternative materials. Among their benefi ts, metastable beta-titanium alloys offer high specifi c strength, low elastic moduli, high corrosion resistance, and manufacturability. However, the cost-effective substitution of metastable beta-titanium for steel will require that titanium suspension springs be manufactured using existing steel suspension fabrication equipment and techniques. These techniques involve coil winding followed by short-time aging and shot peening. This paper describes an evaluation of the effects of the steel suspension fabrication techniques on a prominent metastable beta-titanium alloy.
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