Ti-6Al-4V合金の回転曲げ疲労特性に及ぼすレーザピーニング処理の影響

概要

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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<font size="-1">2</font>. 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.