Microscale laser shock peening: Experiment, modeling and spatially resolved material characterization

Laser shock peening (LSP) is an innovative process in which beneficial compressive residual stress is imparted into surface layer of metal parts by a series of laser pulses. Motivated by the rapid progress being made in microsystem design and fabrication, micro scale laser shock peening (muLSP, using laser spot size of a few microns) is a flexible and precise process that can potentially be applied to improve fatigue, strength, and reliability of metal structures in micro-devices. The previous results showed that the compressive residual stress and plastic deformation can still be generated by such a small beam size. While it is shown feasible, the muLSP poses new challenges ranging from lack of knowledge for interactions at this scale, less tolerance for approximations and simplifications in modeling, and insufficient methodology in post-process material characterization. In order to solve those challenges, a comprehensive model that accounts for laser induced breakdown, plasma expansion and shock wave evolution to predict the process of laser-material interactions was developed and the shock pressure was determined. The dynamic deformation of the target under the shock load was studied with hydrodynamics simulations that account for the effects of laser nonlinear absorption and plasma evolution. Single crystal aluminum and copper were chosen to systematically investigate the material response at micro scale. By choosing appropriate crystal orientation and shock condition, approximate 2-D and 3-D plastic deformation state can be achieved and samples post muLSP were experimentally characterized both macroscopically and microscopically. The new experimental methodologies were developed which is based on the X-ray microdiffraction technique coupled with sub-profiling and Fourier analysis, as well as electron backscatter diffraction (EBSD). The macroscopic quantities include spatially resolved stress and strain deviation, and the microscopic quantities include small lattice rotation fields and mosaic structures. The material response for different material and crystal orientation was investigated. The anisotropic property along different crystal direction was also studied. Small lattice rotation fields were simulated based on the single crystal plasticity theory incorporated in the 2-D and 3-D FEM analysis, which allows study of the length scale effect and the crystal anisotropic property for muLSP. This research has advanced the state of knowledge in muLSP technology and laid the foundation for applying muLSP to actual MEMS devices.