UCLA

The objective of this study is to significantly advance the fundamental understanding of laser interaction with metal matrix nanocomposites (MMNCs) and to overcome the fundamental limits of current laser processing techniques by tuning heat transfer and fluid flow using nanoparticles.

Ultrasonic assisted electrocodeposition was used to prepare MMNCs samples (e.g., Ni/Al2O3) for laser melting experiments. Microstructural study showed that uniform distribution and dispersion of nanoparticles were achieved. The effects of nanoparticles on the optical and thermophysical properties were studied experimentally and theoretically. The viscosity was greatly increased while the surface tension and thermal conductivity were slightly decreased by nanoparticles. The knowledge of these properties would provide valuable insights to fundamentally understand how laser interacts with MMNCs.

To understand the influences of the changes in the thermophysical properties on the laser melting process, an analytical model was developed and used to predict the melt pool flows. The study indicated that thermocapillary flows were tremendously suppressed because of the increased viscosity. As an emerging application of laser melting, laser polishing could significantly benefit from this phenomenon because it would result in improved surface finish. Systematic laser polishing experiments at various laser pulse energies were conducted on Ni/Al2O3 and pure Ni. The normalized surface roughness was decreased by nearly a factor of two with the help of Al2O3 nanoparticles. The proposed methodology of controlling heat transfer and fluid flow by nanoparticles successfully overcame the fundamental limit in laser polishing. Microstructural study on the laser processed region also revealed interesting features. By the addition of the Al2O3 nanoparticles, the laser melted depth was increased while the heat affected zone was, surprisingly, largely reduced. It would be of great significance if this phenomenon can be utilized to other manufacturing processes such as laser welding and laser additive manufacturing where a minimal HAZ is highly desired.

In summary, the work in this dissertation has significantly advanced the fundamental knowledge on how laser interacts with MMNCs. Under the guidance of the fundamental knowledge, some existing limits of laser melting have been successfully overcome, which will broaden its processing capability and application space.