| During laser tube bending, a laser beam is used to produce non-uniform thermal strain leading to permanent bending without hard tooling or external forces. The advantages of laser tube bending include flexibility especially in multi-axial bending, no spring-back, and no wall thinning at the extrados. However, laser tube bending is currently impractical to be applied due to small bending generated at each laser scan. The advent of the high power direct diode lasers with a rectangular beam has provided potential to improve bending efficiency but requires a new set of problems to be addressed. In this thesis, four scanning schemes for tube bending, including point-source circumferential scanning, pulsed line-source axial procession, line-source axial scanning without and with water cooling are numerically investigated based on a coupled thermo-mechanical model using finite element method and compared in terms of bending deformation, input energy, cycle time and characteristics of deformed tubes. The influences of important parameters such as beam coverage, scanning velocity and cooling offset on the deformation are also investigated.;Laser cleaning is a process in which surface contaminants are ablatively removed by a pulsed laser without damaging the substrate surface. Due to its selectivity, controllability, environmental-friendliness and flexibility, laser cleaning is a promising alternative in microelectronic cleaning, art restoration and other applications. The removal of copper oxide from copper, the most widely utilized metal in the microelectronic field, is investigated. The chemical distribution and film thickness of copper oxide are characterized using X-ray photoelectron spectroscopy and ellipsometry. The effects of pulsing parameters such as laser intensity, pulse number and repetition rate on the successful laser cleaning are experimentally and numerically investigated, where a two-dimensional axial symmetric model is utilized to more accurately model laser ablative cleaning of copper oxide.;In restoration of stone artwork, it is important to preserve the aesthetic value of stone. Discoloration of the stone, say, marble, after cleaning hinders the potential of laser stone cleaning. The mechanism of discoloration is proposed from the viewpoint of thermochemical reactions. The reduction of iron oxides by graphite in an encrustation plays a key role in determining the surface color of cleaned stone. The kinetics of the thermochemical reactions are analyzed with a laser ablative cleaning model including reaction heats. The thermochemical reactions are also verified indirectly with experiments. The effects of fluence levels on the discoloration of marble cleaned using 355nm laser wavelength are comparatively studied. The effects of thin liquid layer covering the encrustation are also experimentally and numerically investigated. A two-dimensional axial symmetric model is proposed to simulate changes in temperature, liquid volumetric fraction and vapor pressure in the irradiated encrustation considering conservations of mass, momentum, and energy. Three liquids with different thermodynamic properties, that is, distilled water, ethanol and acetone, are considered in the thesis, along with their effect on the surface color of cleaned marble. The chemical components of marble surfaces are characterized through surface enhanced Raman spectroscopy. The surface color is measured with a chromameter using a 1976 CIE L*a*b* color system. |
