This paper presents a multi-phase flow model for the nanosecond laser ablation of aluminum alloy at a low fluence based on finite volume method, considering gravity, recoil pressure, buoyancy and surface tension to describe vaporization. Actual morphology of ablation crater was measured by a laser scanning confocal microscope to verify the model. Results show that vaporization is the main ablation mechanism for 100ns laser ablation at low fluences, and the peak temperature is only 50% of critical temperature. Both the experimental and calculated crater have a wall-like bulge around the rim, as a result of impact of recoil pressure and resolidification of pushed liquid metal. The calculated depth and diameter of crater are in good agreement with the corresponding experimental measurement indicating the feasibility of the model.
Ultra-narrow gap laser welding is a novel method for thick high strength aluminum alloy plate for its lower heat input, less deformation and higher efficiency. To obtain a perfect welding quality, it is vital to control the more complex droplet transfer behavior under the influence of ultra-narrow gap groove. This paper reports the effects of gap width of groove on droplet transfer behavior in ultra-narrow gap laser welding of 7A52 aluminum alloy plates by a high speed camera, using an ER 5356 filler wire. The results showed that the gap width had directly effects on droplet transfer mode and droplet shape. The droplet transfer modes were, in order, both-sidewall transfer, single-sidewall transfer, globular droplet transfer and bridging transfer, with different droplet shape and transition period, as the gap width increased from 2 mm to 3.5mm. The effect of gap width on lack of fusion was also studied to analyze the cause for lack of fusion at the bottom and on the sidewall of groove. Finally, with a 2.5 mm U-type parallel groove, a single-pass joint with no lack of fusion and other macro welding defects was successfully obtained in a single-sidewall transfer mode.
The picosecond laser has ultrashort pulse and superstrong peak power, which make it being focused on and applied in the micro-nanoscale fabrication field. Silicone elastomer PDMS is a typical antifouling material which can desorb defacement, using picosecond laser etching the surface through the way of galvanometer scanning in order to obtain a surface with micro-nano texture. The article studied the relationship between process parameters such as the power density, the scanning rate and the appearance of etched groove respectively, especially the width and depth of the groove. The results show that : for single marking, with the raise of the laser power density I, the depth of the groove increases, the inclination angle of the side wall is reduced. In another time, with the increase of the scanning rate v ,the depth of the groove decreases gradually and the surface morphology cannot be seen clearly. For multiple marking, the depth of the groove shown a falling slope from big to small with the increase of marking number. Finally,we got a path to optimize the process parameters to obtain a surface with micro-nano structures. After testing the surface contact angle, we found that the surface contact angle increased from 113° to 152°,which reached the level of superhydrophobic surface.
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