In this project effects of Laser shock peening (LSP) on two aero engine alloys, IN718 and IN718 SPF were studied. The primary goal of the program was to secure required fundamental knowledge of e impact of LSP process parameters on these two aero engine alloys and thereby advance the science and application base of this process to other materials and parts. The research program designed accordingly includes the following key elements: 1) Developing LSP process parameters for typical Ni base aero engine alloys; (2) characterization of surface and sub-surface macro and micro residual strains/stresses a function of LSP process parameters (3) characterization of microstructural changes as a function of LSP process parameters and; (4) Study Thermal relaxation of residual stresses and understand the underlying kinetics.
Firstly, different LSP process parameters including: Power density, impact overlaps, ablative overlays and coverage were studied to impart deep compressive residual stresses to the near surface regions of peened coupons. A host of different techniques were then used to characterize distribution of residual stresses/strains, roughness, hardness, plastic strains and microstructure. Role of ablative layer was also investigated. Samples were peened using an ablative layer different ablative layers (black vinyl tape, aluminum tape) and without an ablative layer and compared in terms of topography, residual stress fields and microstructure.
Two different diffraction based techniques were used to characterize residual stress fields: conventional X-ray diffraction and Synchrotron X-ray diffraction (SXRD). Conventional X-ray coupled with electro polishing offers a fast means of analyzing residual stresses, while SXRD enables high resolution, non-destructive characterization of strains/stresses. Experiments showed that higher power density lead to compressive residual stresses which were higher in magnitude in near surface regions. There is a saturation power density, after which any increase in power density did not result in further increase in magnitude and depth residual stresses.
Plastic deformation introduced by LSP was characterized using a variety of tools including hardness measurements (micro hardness with a knoop’s indenter and nano indentation), analyzing full width half maximum of diffraction peaks, micro pillar compression tests and electron back scattered diffraction. Results indicated that LSP does not introduce high plastic strain in the material. This was evident in moderate increase in hardness (~20%), a small increase in FWHM of diffraction peaks and a small increase in low angle misorientations (obtained using EBSD). Using all these techniques it was determined that the plastic deformation introduced by peening extended 300- 500μm from the peened surface.
Microstructure of as-received and peened samples was studied using optical microscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Electron back scattered Diffraction (EBSD). Peened samples showed no grain refinement (unlike conventional shot peening) or change in chemistry. TEM revealed increase in dislocation density in near surface regions, which decreased as a function of distance. Thermal relaxation of both macro and micro residual strains/stresses and microstructure stability at selected temperatures as a function of time will be studied using the same tools and the kinetics of relaxation modeled from the data obtained.