Search

The laser welding (LW) process of highly reflective materials presents low thermal efficiency and poor stability. To solve the problem, the effects of subatmospheric environment on LW process, technological parameters in subatmospheric environment on weld formation and welding with sinusoidal modulation of laser power on the stability of LW process in subatmospheric environment were explored. The AZ31 magnesium (Mg) alloy was used as the test materials. The test result revealed that the weld penetration in subatmospheric environment can increase by more than ten times compared with that under normal pressure. After the keyhole depth greatly rises, significantly periodic local bulge is observed on the backwall surface of the keyhole and the position of the bulge shifts along the direction of the keyhole depth. Eventually, the hump-shaped surface morphology of the welded seam is formed; moreover, the weld width in local zones in the lower part of the welded seam remarkably grows. During LW in subatmospheric environment, the weld penetration can be further greatly increased through power modulation. Besides, power modulation can inhibit the occurrence of bulges in local zones on the backwall of the keyhole during LW in subatmospheric environment, thus further curbing the significant growth of the weld widths of hump-shaped welding beads and local zones in the lower part of welded seams. Finally, the mechanism of synchronously improving the thermal efficiency and stability of LW process of highly reflective materials through power modulation in subatmospheric environment was illustrated. This was conducted according to theoretical analysis of recoil pressure and observation results of dynamic behaviors of laser induced plasma clouds and keyholes in the molten pool through high speed photography.

Laser processing of pure Cu is often challenging because of Cu's very low absorptivity to most laser beams, and this difficulty is applicable to the recently developed laser powder bed fusion (LPBF) additive manufacturing (AM). How to realize high‐quality LPBF forming is urgently pursued to maximize its advanced manufacturing potential in directly making various Cu parts. Herein, two approaches have been applied, individually or simultaneously, to address the poor printability problem of the pure Cu: one is to conduct intentional oxidation treatment to the pure Cu powder to vary its surface structure and then enhance its laser absorptivity; and the other is to adopt laser remelting to counter its high thermal conductivity and provide extra liquid phase for better densification. Various research means were used to investigate mechanical performances, microstructural details, and electrical performance of the as‐printed Cu. Results show that a maximum relative density of ≈99.0% can be realized by using the two approaches simultaneously, while oxidation temperature and remelting times are the two important factors to be considered. The methodology developed in this study is expected to be applicable to other highly reflective materials as well. [ABSTRACT FROM AUTHOR]

The laser welding (LW) process of highly reflective materials presents low thermal efficiency and poor stability. To solve the problem, the effects of subatmospheric environment on LW process, technological parameters in subatmospheric environment on weld formation and welding with sinusoidal modulation of laser power on the stability of LW process in subatmospheric environment were explored. The AZ31 magnesium (Mg) alloy was used as the test materials. The test result revealed that the weld penetration in subatmospheric environment can increase by more than ten times compared with that under normal pressure. After the keyhole depth greatly rises, significantly periodic local bulge is observed on the backwall surface of the keyhole and the position of the bulge shifts along the direction of the keyhole depth. Eventually, the hump-shaped surface morphology of the welded seam is formed; moreover, the weld width in local zones in the lower part of the welded seam remarkably grows. During LW in subatmospheric environment, the weld penetration can be further greatly increased through power modulation. Besides, power modulation can inhibit the occurrence of bulges in local zones on the backwall of the keyhole during LW in subatmospheric environment, thus further curbing the significant growth of the weld widths of hump-shaped welding beads and local zones in the lower part of welded seams. Finally, the mechanism of synchronously improving the thermal efficiency and stability of LW process of highly reflective materials through power modulation in subatmospheric environment was illustrated. This was conducted according to theoretical analysis of recoil pressure and observation results of dynamic behaviors of laser induced plasma clouds and keyholes in the molten pool through high speed photography. [ABSTRACT FROM AUTHOR]

• Design of variable temperature BRDF measurement system up to 1000℃. • BRDF scanning measurements for hemispherical spaces of highly reflective surfaces. • BRDF measurements during two heating processes on a stainless-steel sample. Bidirectional Reflectance Distribution Function (BRDF) under variable temperature conditions is important for target identification and stealth in aerospace applications of highly reflective materials. In this paper, a variable temperature BRDF measurement system based on a robotic arm with an ingenious heater is designed. For BRDF measurements of highly reflective surfaces, a scanning strategy with variable angle intervals in continuous space is proposed to improve the resolution near the maximum BRDF and to reduce the measurement time. An ingenious heater is designed to be mounted on the robotic arm to move in space and heat a sample. The measurement uncertainty of the proposed variable temperature BRDF of highly reflective materials is discussed, which has been divided into the components of laser intensity fluctuations, scanning speed and heating stability. The effectiveness of the measurement system was verified by the variable temperature BRDF measurements during two heating processes on a highly reflective stainless-steel sample. [ABSTRACT FROM AUTHOR]

The Urban Heat Island (UHI) is a well-known phenomenon concerning an increasing percentage of the world’s population due to the growth rates of metropolitan areas. Given the health and economic implications of UHIs, several mitigation techniques are being evaluated and tested. In this study, we consider the use of highly reflective materials for urban surfaces, and we carried out numerical experiments using the Weather Research and Forecasting model coupled with the CHIMERE model in order to investigate the effects of these materials on the meteorology and air quality in the urban area of Milan (Italy). Results show that an increase in albedo from 0.2 to 0.7 for urban roofs, walls and streets leads to a decrease in UHI intensity by up to 2–3 °C and of the planetary boundary layer (PBL) height of about 500 m. However, the difference of PM10 and ozone between urban and surrounding areas increases by a factor of about 2, attributable to the reduction of PBL height and wind speed and to the increased reflected solar radiation that may enhance photochemical production during the daytime. Therefore, if anthropogenic emissions are held at the same levels, the potential benefit to the UHI in terms of thermal discomfort may have negative repercussions on air quality.