Lanterna Rally

Author: Yixiang Zhao

Publisher:

Published: 2012

Total Pages: 203

ISBN-13:

Laser shock peening (LSP) is a novel surface treatment process that generates deep compressive residual stresses and microstructural changes and thereby dramatically improves fatigue strength of critical metal aircraft engine parts. In the past, researchers have evaluated the mechanical effects of LSP experimentally through residual strain/stress measurements, microhardness measurements or fatigue life improvement. A number of microstructure characterizations have been done on variety laser shock peened materials. However, getting better view of how LSP brings about changes in the microstructure and establish quantitative relations between LSP parameters and residual strain/stress distributions, microstructure and texture evolution is still challenging. The present study was undertaken to develop a basic understanding of the effects of LSP on the residual strain/stress distributions, texture evolution and deformation microstructural changes in Ti-6Al-4V alloy. Scanning Electron Microscopy, Scanning Probe Microscopy, Conventional X-ray Diffraction, Synchrotron X-ray Diffraction, Electron BackScattered Diffraction, microhardness and nanoindentation have been used to characterize the laser shock peened Ti-6Al-4V alloy samples. The microstructure and surface modification of laser shock peened sample are outlined in terms of laser shock peening processing parameters. Naked laser peened samples show prominent evidence of surface melting and recasting. Little difference between the peened and virgin materials can be found in the taped laser peened samples surface microstructures. Depth-resolved characterization of the residual strains and stresses was achieved using high-energy synchrotron X-ray diffraction as well as by conventional X-ray diffraction. Compressive residual strain at peened surface and tensile residual strain in the interior of the sample are found in taped samples. Naked LSP-treated samples show tensile residual stresses at peened surfaces, then dramatically change to compressive within short depth. Multiple diffraction peaks in the synchrotron X-ray diffraction patterns were used to analyze the residual elastic strain and plastic strain distributions in the LSP-treated Ti-6Al-4V samples. Anisotropic elastic lattice strain response in the hexagonal close-packed alpha titanium was revealed by Williamson-Hall plots of the peak broadening data. The depth profiles of mean diffraction ring width in synchrotron X-ray diffraction and FWHM in conventional X-ray diffraction give evidence of anisotropic plastic strains in the laser peened Ti-6Al-4V samples. Furthermore, using the whole pattern fitting method the Structure-Texture-Microstructure-Phase-Stresss combined analysis was performed based on the synchrotron diffraction data. The evolution of maximum pole intensity values from surface to interior proves that laser shock peening can change the texture in the laser peened samples. The near-surface and through-the-depth changes in strain/stress, texture and microstructure in samples were correlated with the laser processing energy levels applied on the samples. Residual stress relaxation in LSP-treated Ti-6Al-4V alloy due to the sample sectioning was also studied using SXRD and CXRD and was found to be significant to small section widths (to about 8 mm), but not as significant at larger widths, though the sectioning was found to introduce complex gradients. Finally, the local property changes were examined using microhardness and nanoindentation and near-surface hardening due to LSP treatment was noted and related to the plastic strain generated by the process.