Abstract
Synchrotron radiation (SR) and X-ray free electron laser (XFEL) are indispensable tools not only for the exploration of science but also for the evolution of industry. We used SR and XFEL to elucidate the mechanism and the effects of laser peening without coating (LPwC) which enhances the durability of metallic materials. X-ray diffraction (XRD) employing SR revealed that the residual stress (RS) in the top surface became compressive as the laser pulse irradiation density increased with appropriate overlapping of adjacent laser pulses. SR-based computed tomography (CT) was used to nondestructively reconstruct three-dimensional (3D) images of fatigue cracks in aluminum alloy, revealing that LPwC retarded crack propagation on the surface and inside of the sample. SR-based computed laminography (CL) was applied to friction stir welded (FSWed) aluminum alloy plates to visualize fatigue cracks propagating along the welds. The fatigue crack had complicated shape; however, it became a semi-ellipsoid once projected onto a plane perpendicular to the fatigue loading direction. Ultra-fast XRD using an XFEL was conducted to investigate the dynamic response of aluminum alloy to an impulsive pressure wave simulating the LPwC condition. The diffraction pattern changed from spotty to smooth, implying grain refinement or subgrain formation. Shifts in diffraction angles were also observed, coinciding with the pressure history of laser irradiation. The durations of the dynamic phenomena were less than 1 µs; it may be possible to use high-repetition lasers at frequencies greater than kHz to reduce LPwC processing times.
Highlights
Structural materials in actual service are subjected to external loads in corrosive environments.Defects or cracks almost always commence at the surface because of stress concentration attributable to bending, torsion, roughness, or pitting
The results of ultra-fast X-ray diffraction (XRD) using an X-ray free electron laser (XFEL) are described in Section 4; these revealed the dynamic responses of an aluminum alloy under an impulsive pressure wave that simulated the
It is difficult to apply computed tomography (CT) to laterally extended objects such as friction stir welded (FSWed) joints because X-ray attenuation drastically varies during the rotational scan
Summary
Structural materials in actual service are subjected to external loads in corrosive environments. In LPwC, laser pulses directly impinge on the surface, heat it, and evaporate the surface to create a laser-induced high-pressure plasma, which induces impulsive pressure waves that propagate internally to generate permanent strain and a beneficial compressive RS in the near-surface layer. CL was employed to reconstruct images of fatigue cracks propagating in friction stir welded (FSWed) joints of A6061 aluminum alloy [30]. A finite element method (FEM) based on continuum mechanics well-reproduced the RS state after LPwC, except that of the top surface subjected to a thermomechanical effect [5,25,46,47] These approaches do not deal directly with microscopic phenomena, such as dislocation, grain refinement, phase transformation, or precipitation; such data are essential when seeking to understand plasticity, material hardening, and RS stability. The results of ultra-fast XRD using an XFEL are described in Section 4; these revealed the dynamic responses of an aluminum alloy under an impulsive pressure wave that simulated the LPwC condition
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