This paper reports on laser surface remelting experiments performed on a Zn-2wt.%Cu hypoperitectic alloy by employing a 5kW CW CO2 laser at scanning velocities between 6 and 1207mm/s. The growth velocities of the mi- ...This paper reports on laser surface remelting experiments performed on a Zn-2wt.%Cu hypoperitectic alloy by employing a 5kW CW CO2 laser at scanning velocities between 6 and 1207mm/s. The growth velocities of the mi- crostructures in the laser molten pool were accurately measured. The planar interface structure caused by the high velocity absolute stability was achieved at a growth velocity of 210 mm/s. An implicit expression of the critical solidification velocity for the cellular-planar transition was carried out by nonlinear stability analyses of the planar interface. The results showed a better agreement with the measured critical velocity than that predicted by M-S theory. Cell-free structures were observed throughout the whole molten pool at a scanning velocity of 652 mm/s and the calculated minimum temperature gradient in this molten pool was very close to the critical temperature gradient for high gradient absolute stability （HGAS） of the η phase. This indicates that HGAS was successfully achieved in the present experiments.展开更多
This study numerically investigates the influence of molten matter dynamics on the gasification and subsequent ignitability of an inclined thermoplastic specimen subjected to localized irradiation heat flux normal to ...This study numerically investigates the influence of molten matter dynamics on the gasification and subsequent ignitability of an inclined thermoplastic specimen subjected to localized irradiation heat flux normal to the surface. A thermoplastic material is modeled as a phase change material with predefined solidification and melting temperatures, respectively, and the gasification process is modeled by the Arrhenius law of molten matter. Gas phase kinetics is not considered for simplicity purposes;instead, the onset of ignition of polymer is estimated on the basis of the critical mass flux concept. According to the numerical results, as the inclination angle becomes steeper (toward the vertical angle), the estimated ignition delay becomes shorter, showing ignition is promoted, whereas it is turned to be difficult to occur when inclination angles are above the vertical angle (>90?) having a longer delay time for the onset of gasification. With careful observation, the thermal interaction between the hot molten matter and unmelted (cold) solid is found to play an important role in gasification. The formation of a bulge due to resolidification to suppress the dripping downstream could be the source to promote ignition. By contrast, the hot molten matter is enforced to detach from the unmelted solid and “freely fall-off” to prohibit ignition for inclination angles beyond 90?. This supports the notion that high-enthalpy caused by the external heating is simply lost because of dripping, and there is less chance of catching fire there.展开更多
基金Project supported by the National Natural Science Foundation of China （Grant Nos 50201012 and 50471065）.
文摘This paper reports on laser surface remelting experiments performed on a Zn-2wt.%Cu hypoperitectic alloy by employing a 5kW CW CO2 laser at scanning velocities between 6 and 1207mm/s. The growth velocities of the mi- crostructures in the laser molten pool were accurately measured. The planar interface structure caused by the high velocity absolute stability was achieved at a growth velocity of 210 mm/s. An implicit expression of the critical solidification velocity for the cellular-planar transition was carried out by nonlinear stability analyses of the planar interface. The results showed a better agreement with the measured critical velocity than that predicted by M-S theory. Cell-free structures were observed throughout the whole molten pool at a scanning velocity of 652 mm/s and the calculated minimum temperature gradient in this molten pool was very close to the critical temperature gradient for high gradient absolute stability （HGAS） of the η phase. This indicates that HGAS was successfully achieved in the present experiments.
文摘This study numerically investigates the influence of molten matter dynamics on the gasification and subsequent ignitability of an inclined thermoplastic specimen subjected to localized irradiation heat flux normal to the surface. A thermoplastic material is modeled as a phase change material with predefined solidification and melting temperatures, respectively, and the gasification process is modeled by the Arrhenius law of molten matter. Gas phase kinetics is not considered for simplicity purposes;instead, the onset of ignition of polymer is estimated on the basis of the critical mass flux concept. According to the numerical results, as the inclination angle becomes steeper (toward the vertical angle), the estimated ignition delay becomes shorter, showing ignition is promoted, whereas it is turned to be difficult to occur when inclination angles are above the vertical angle (>90?) having a longer delay time for the onset of gasification. With careful observation, the thermal interaction between the hot molten matter and unmelted (cold) solid is found to play an important role in gasification. The formation of a bulge due to resolidification to suppress the dripping downstream could be the source to promote ignition. By contrast, the hot molten matter is enforced to detach from the unmelted solid and “freely fall-off” to prohibit ignition for inclination angles beyond 90?. This supports the notion that high-enthalpy caused by the external heating is simply lost because of dripping, and there is less chance of catching fire there.
