Laser Induced Deformation and Microstructural Modification.

The laser forming process as it pertains to the work presented herein is limited to application at the macroscopic scale, while the crystallization process is utilized for thin film structural modification. However, both processes are analyzed within the framework of the co-dependent nature of temperature, stress, strain, deformation and microstructure. This dissertation seeks to investigate these issues via a series of experimental and numerical analyses of geometric, constraint and phase transformation based effects for the laser forming process as well as microstructural and shape memory performance effects stemming from laser induced thin film melting and solidification. The implications of utilizing scan paths in close proximity to free or clamped edges of laser formed parts are examined. Several operating regimes are identified that relate the properties of the work piece with distance thresholds under which the non-semi-infinite boundaries are "felt" in a thermal or mechanical sense. Exceeding these thresholds lead to perturbations in the symmetric stress and/or temperature fields leading to altered deformation behavior. An analytical approach is also developed revealing the asymmetric temperature fields that result from irradiation near insulated boundaries. Effects stemming from laser forming superelastic nickel-titanium (NiTi) SMA plates are addressed as well. Due to their thermo-mechanical behavior, SMA's exhibit a highly non-traditional constitutive response, whose nature is itself, a strong function of temperature. A novel approach to simulating this strongly coupled response is developed for capturing the coupled thermal, mechanical and thermodynamic behavior, as well as the final part geometries and volume fraction fields, and is validated experimentally. Pulsed, melt-mediated laser crystallization techniques as applied to shape memory NiTi thin films are also investigated. Grain size, grain size distribution, surface morphology, crystallographic orientation as well as resultant phases are characterized via a number of techniques including atomic force microscopy, electron microscopy and x-ray diffraction. Additionally, the effects of solidification rate on the resulting microstructure and phase(s) are also examined through laser irradiation of pre-heated films/substrates. In addition to characterizing aspects of the resulting microstructure, the thermodynamic, mechanical and shape memory response of laser processed films are also examined. The ability to produce functionally graded material responses is demonstrated via local process parameter control. Martensitic phase transformation temperatures of laser processed films are obtained via temperature controlled optical microscopy and x-ray diffraction. Moreover, a coupled nanoindentation/atomic force microscopy technique is employed to characterize both the superelastic and shape memory effects. Specifically, an analysis of recoverable energy through the stress induced phase transformation and recoverable strain through de-twinning and the subsequent thermally induced phase transformation were performed. Furthermore, a stress based mechanism explaining the observed changes in film response as a function of incident laser fluence is proposed and experimentally validated.