In order to investigate the effects of laser peening on high cycle fatigue properties of (α + β) type Ti-6Al-4V alloy, rotating bending fatigue tests were carried out. The laser peening conditions were a spot size of 0.8mm in diameter, a pulse energy of 200mJ and an irradiation pulse density of 38 pulse/mm 2. The fatigue properties were not improved with these conditions of laser peening ; even though the material was hardened to some extent and high level compressive residual stresses were induced in the near-surface layer. Because the reason that the fatigue properties were not improved was considered the notch effect of surface roughness introduced by laser peening, further fatigue tests were performed with laser-peened specimens followed by surface polishing. However, the fatigue lives of polished specimens were shorter than those of unpolished specimens, unexpectedly. Considering the peening effects such as surface roughness, hardness and residual stress, the reasons for arriving at the unexpected results would be the influence of the strain-induced transformation of β phase into α' martensite and a larger yield strength in tension than that in compression due to the HCP structure of α phase of titanium alloy.
Rotating bending loading and axial loading fatigue tests were conducted to investigate the effect of zirconia shot peening on fatigue properties of Ti-6Al-4V ELI alloy. Specimen was treated in three different shot peening conditions. As a result of the rotating bending fatigue tests, the fatigue lives of shot peened specimen were longer life than non-peened specimen. Fish eyes were observed in the fracture surface of the most shot peened specimen. And there was the facet of -Ti or the fine granular region near the crack initiation site of the fish eye. And then, fracture morphology of shot peened specimens is different by peening conditions. From the result of axial loading fatigue tests, it becomes obvious that the facet of -Ti was formed by action of tensile stress over 500MPa without compressive stress and that the fine granular region was formed by action of compressive stress. These results suggested that low level tensile residual stress existed at subsurface near the surface of the SP treated specimen, and the residual stress distribution decided on internal crack origin type between the facet of -Ti or the fine granular region.
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