Fatigue strength of Ti6Al4V titanium alloy machined under flood and cryogenic conditions

Ti6Al4V titanium alloy wrought round bars characterized by a fine equiaxed microstructure were machined using two different lubrication strategies, namely traditional waterbased cutting fluid and liquid nitrogen. Surface integrity after machining was assessed in terms of (i) sub-surface severe plastic deformation layer, (ii) surface topography, and (iii) residual axial stresses as a function of the distance from the machined surface. Finally, the machined specimens were subjected to fatigue tests on a four-point rotating bending machine. For the two sets of specimens, the Wöhler curves were derived to determine their fatigue strength using the modified staircase method. No remarkable difference in terms of surface topography was found between the flood and cryogenic machined specimens, but the latter were characterized by higher and deeper compressive residual stresses and higher severe plastic deformation layer beneath the machined surface. Despite that, it is shown that the fatigue limit of the flood and cryogenic machined specimens is very similar, as well as crack initiation sites have the same location. An explanation of that can be found in the very fine microstructure of the as-received material, which prevails over the other characteristics induced by any machining strategy.