Collaborative Research: Feasibility and Fundamentals of Femtosecond-Laser Shock Peening Without Protective Coating in Air Environment

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Laser induced, localized shock waves can generate compressive stresses in a metal's surface that improve the hardness and fatigue of the part without altering its bulk material properties. This process, nanosecond laser shock peening, requires a protective coating to combat the heat affect zone, and a confining medium to promote the shock waves. The research will investigate the ability of a femtosecond laser shock peening process, which has higher energy density, to achieve the same result without the need for the protective coating and confinement layer. Eliminating the coating and confinement layer will significantly reduce setup complexity and manufacturing costs. Knowledge gained on the fundamental physical mechanisms occurring during femtosecond laser peening will be leveraged to realize a higher resolution process capable of processing a broader range of materials than currently possible by nanosecond laser peening. This advanced manufacturing capability will contribute to the competitiveness of the US by finding application in the development of progressive precision instrumentation, 3D-printing technologies, biomedical implants, and micro-mechanical systems. In addition to providing graduate students with advanced manufacturing expertise, the research results will be integrated into both course modules and educational outreach activities aimed at underrepresented minorities to further encourage more qualified people into the manufacturing work force. The research objective is to understand the fundamental mechanisms and limitations of femtosecond laser shock peening without a protective coating in an air environment. To achieve this both process related non-equilibrium electron dynamics, and super-high strain-rate deformation mechanisms will be investigated. A unique in-situ measurement method will be used to capture the plasma dynamics occurring during femtosecond laser shock peening of stainless steel 304 and aluminum 6061 samples. The microstructure evolution will be characterized to study the deformation dynamics under ultrahigh pressure and stain rate, while the surface morphology, residual stress, surface hardness, and fatigue will be measured to test the effectiveness of the process. Hybrid numerical models, validated by experiments, will be utilized to further understand the underlying mechanisms. A Fokker-Planck model will focus on revealing non-equilibrium electron dynamics. Outputs from this model will be incorporated into a hydrodynamic model to predict the resultant plasma formation and shock wave generation. The shock wave information will be integrated into a finite element model to assist in predicting the deformation process of metals subjected to super-high strain-rate impacts.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.