Gradient microstructure and foreign-object-damaged fatigue properties of Ti6Al4V titanium alloy processed by the laser shock peening and subsequent shot peening

Abstract

A dislocation-gradient microstructure layer with large depth and high compressive residual stress (CRS) value on the surface of Ti6Al4V titanium alloy is processed by laser shock peening and subsequent shot peening (LSP + SP). The microstructure evolution and its foreign-object-damaged fatigue properties were investigated by scanning electron microscope, transmission electron microscope, X-ray diffraction, microhardness tests, finite element numerical simulation and vibration fatigue tests. Foreign object damage (FOD) was introduced through the indentations of hardness testers, which can lead to the increase of stress concentration and redistribution of surface residual stress around craters. However, the dislocation-gradient microstructure layer can prolong crack initiation and propagation lives by 184% and 45% for specimens with FOD, respectively. The median fatigue life of (LSP + SP) specimens with FOD even shows longer than that of untreated specimens without FOD. Furthermore, a synergistic strengthening mechanism, involving dislocation strengthening, grain refinement and CRS, was clearly revealed.