| To achieve high machining qualities in the process of nanosecond UV laser nucromachining of metals, it is necessary to find the conditions under which melting, redeposition, and unfavorable stress distribution can be avoided or reduced. Detailed understanding of this process at microscale is lacking in the literature. In this research, UV laser micromachining of metals is studied using a Q-switched Nd:YAG laser with 50ns pulse duration and 355nm wavelength. A 2D axisymmetric model based on the enthalpy method is developed taking into account laser intensity distribution, phase changes, plasma effects and nonlinear properties of metals. Temperature contour and cavity formation are successfully predicted and the optimal operation conditions for laser micromachining in open air are suggested. However, the problem of melting and unfavorable thermal stress is difficult to solve in open air. Preliminary experiments and simulations show that the combination of water-confined microscale laser shock processing (LSP) with laser micromachining can reduce the melting, redeposition and heat affected zone in laser micromachining of metals, and simultaneously introduce favorable residual stress distributions in the metal targets. Fundamental research is needed to predict the effects of microscale LSP. However, before this work, only large beam sized LSP (beam size >1mm) has been studied based on simple ID shock models, while a beam size of around 10 microns is needed in microscale LSP. In this research, detailed physics in microscale LSP is considered by modeling the expansion of the plasma as a laser supported combustion wave. The spatial expansion of the shock pressure is considered to account for the microscale beam size. This model reduces the arbitrariness in the existing 1D shock models and gives better predictions of experimental results. Various experimental methods are used to investigate the shock processed metal foils and metal thin films, and 2D/3D FEM simulations are carried out to study the LSP induced stress/strain distributions. It is found that substantial compressive residual stress distributions can be induced by microscale LSP, and the fatigue performance, microstructure and hardness of the shock treated samples are improved.{09}For the first time, the stress/strain field in shock treated copper thin films are measured with micron level spatial resolution using X-ray microdiffraction and instrumented nanoindentation. It is proved that microscale LSP of metal thin films is feasible and beneficial. This research lays the foundation for the integration of microscale LSP with laser micromachining and shows the potential applications of microscale LSP in MEMS. |
