Abstract:The drilling of copper using a dual-pulse femtosecond laser with wavelength of 800 nm, pulse duration of 120 fs and a variable pulse separation time (0.1-150 ps) is investigated theoretically. A one-dimensional two-temperature model with temperature-dependent material properties is considered, including dynamic optical properties and the thermal-physical properties. Rapid phase change and phase explosion models are incorporated to simulate the material ablation process. Numerical results show that under the same total laser fluence of 4 J/cm 2 , a dual-pulse femtosecond laser with a pulse separation time of 30-150 ps can increase the ablation depth, compared to the single pulse. The optimum pulse separation time is 85 ps. It is also demonstrated that a dual pulse with a suitable pulse separation time for different laser fluences can enhance the ablation rate by about 1.6 times.

Laser material processing has been demonstrated as an effective means for machining almost every solid material. The quality of laser machining depends on the processing parameters that dictate material ablation mechanisms. The understanding of the complex physics associated with ultrashort pulsed laser (USPL) material interaction and ablation has advanced significantly owing to a great many theoretical and experimental studies in the past 20 years. To date, USPLs have been considered as a novel tool for micro-and nano-machining of bulk or thin film materials and for internal modification of transparent materials via multi-photon absorption in a tiny focal volume. Moreover, USPL material processing is now gaining interest in other applications, such as in sensors, electronics and medical device industries.

Fundamental mechanisms of selective laser melting (SLM) of metal powders in additive manufacturing (AM) were investigated numerically and experimentally. A simplified 2D finite element model of multiphase fields was proposed to simulate the SLM process based on the conservation equations of mass, momentum and energy. Multiple dynamic physics/ phenomena considered in this work include heat transfer, solid/liquid and liquid/vapor phase changes, vapor pressure, surface tension, gravity, melt flow, gas flow, wetting and bonding of powder particles with the melt, and re-solidification. To deposit laser energy to the powder bed, the liquid/gas interface was tracked using a level set method. The numerical simulation was carried out using COMSOL Multyphysics®. To validate the proposed methodologies, an SLM experiment was performed for Ti6Al4V powders. It was shown that the simulation results of the cross-section shapes and the heights of re-solidified parts are in good agreement with the experimental measurements.

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Extended AbstractA 2D finite element model of multiphase fields was developed to investigate selective laser melting (SLM) of metal powders during additive manufacturing. The entire SLM process was simulated based on the three conservation equations of mass, momentum and energy. Multiple dynamic phenomena considered in this model include phase changes of solid/liquid and liquid/vapor, vapor pressure, surface tension, gravity, motion of molten liquid, and wetting of powders with liquid. To deposit laser energy to the mobile surfaces of powders, the liquid/vapor interface was tracked using a levelset method. All the physics were coupled, incorporated into and solved simultaneously in COMSOL Multyphysics®. For comparison, an SLM experiment was performed for Ti6Al4V powders.In the numerical analysis, the parameters of material and laser beams are the same as those used in the experiment. The average diameter (about 30 μm) of the Ti6Al4V powders is assumed for all the powder particles modeled. The power of the laser beam is 175 W, and the laser spot radius at e-2 is 35 μm. Three laser scanning speeds considered are 1,650 mm/s, 1,250 mm/s and 850 mm/s. For simplicity, two computational models are used. The first model (I) consists of three powders in the direction perpendicular to the laser scanning direction, and the other model (II) consists of seven powders along the laser scanning direction. The latter is to simulate the height of the re-solidified layer, while the former is to simulate the cross-section profile produced by a single line scanning.For the model I, the simulation results include temporal evolutions of the temperature distribution, flow velocity, and geometry profile of the melt pool. At the highest laser scanning speed (1,650 mm/s) studied here, the powders partially melt, especially the two side ones. In this case of high scanning speeds, an AM part could be fabricated, but its shape and quality strongly depends on the scanning speed. As a result of more laser energy deposited into the powders by reducing the scanning speed, on the other hand, the central powder could significantly melt from the top surface and quickly lose its spherical shape due to the surface tension and gravity that immediately pull the newly liquefied material into the melt flow. After that part of molten liquid of the central powder touches the boundary of the side powders, formation of the melt pool begins and its dynamics can be manifested by the flow velocity field of the molten liquid. The powders could fully melt, consolidate and finally form a semi-circular cylinder. The simulated height and width of the re-solidified part are about 31.5 μm and 83 μm for the scanning speed 1,250 mm/s and about 28 μm and 104 μm for the scanning speed 850 mm/s. The experimentally measurements are about 35.5 μm and 113.5 μm and about 26.8 μm and 131 μm, respectively. It should be noted that no porosity in the consolidated part is found in the slowest scanning speed case (850 mm/s). For the model II, the average height of the simulated re-s...