Effect of laser shock peening on residual stress, microstructure and fatigue behavior of ATI 718Plus alloy

Abstract

The effects of Laser Shock Peening (LSP) on the microstructure, residual stress, hardness, strength, and fatigue life of ATI 718 Plus (718Plus) alloy was investigated and the results are reported. Microstructure before and after LSP and after mechanical testing was characterized using scanning electron microscopy (SEM), electron backscatter diffraction (EBSD)/orientation imaging microscopy (OIM), transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS). LSP led to severe surface plastic deformation, which, in turn, led to a high magnitude of surface compressive residual stresses (∼−725MPa) and changes in the near-surface microstructure. This change was in the form of high dislocation density forming dislocation entanglements and slip bands and formation of a few near-surface sub-grains, which, in turn, caused high surface hardening (∼8.3GPa). In addition, a 16% increase in the yield strength (∼175MPa) and around 15% increase in the endurance limit (∼110MPa) occurred in corresponding tests at room temperature. The improvement in fatigue life was due to the shielding and resistance to crack initiation that the LSP treatment provided to the material through the near-surface microstructure, hardening and high compressive residual stress. This shielding also hindered crack propagation, lowering its rate to a third compared with baseline. Results of studies of stress relaxation with cycles showed how the LSP shield was affected by cycling and, in turn, the improvement in fatigue life of 718Plus. The results have demonstrated that LSP is a powerful surface engineering technique that can improve mechanical properties and fatigue performance of many important metallic materials.