In the current study, an Al-Mg alloy refined with a nano-TiC/Ti grain refiner was successfully printed by cold metal transition wire arc additive manufacturing (CMT-WAAM). The effects of the nano-TiC/Ti grain refiner on the microstructure and mechanical properties of the CMT-WAAM-prepared Al-Mg alloy were studied. The results indicate that the nano-TiC/Ti grain refiner promoted a significant grain refinement effect and refined coarse columnar crystals into uniform fine equiaxed crystals. The average grain size of the CMT-WAAM Al-Mg alloy refined with the 0.6 wt.% nano-TiC/Ti grain refiner decreased from 53.06 μm to 25.32 μm (yz plane), and the average grain size in the xy plane decreased from 52.00 μm to 25.80 μm. The excellent grain refinement effect of the nano-TiC/Ti grain refiner was attributed to the presence of TiC particles at the grain, which acted as heterogeneous nucleation sites and improved the nucleation rate. Moreover, TiC particles located at the grain boundary inhibited the growth of the grain boundary. The mechanical properties, i.e., tensile and yield strength, were improved by 20-25%, and there was an increase in the microhardness (~10%) of the CMT-WAAM Al-Mg alloy refined with the 0.6 wt.% nano-TiC/Ti grain refiner.
This study aims to present the optimum technological parameters of hot forging process of aerospace bolt. Deformation Temperature and strain rate were determined according to relevant conditions for commercial bolt production. Deformation behavior and evolution of microstructure of Waspaloy were investigated using hot isothermal compression tests. The results show that the flow stresses show sufficient sensitivity to the deformation parameters and decreased with increase in deformation temperatures and decrease in strain rates. At high strain rates of 5-10 s-1. Flow curves exhibit a pronounced flow softening. At low strain rates of 0.01-0.1 s-1, the curves demonstrate a dynamic equilibrium. Constitute models of Waspaloy is established and the hot deformation energy is calculated as 446.824 kJ/mol. Microstructures showed that at low temperature (1000-1040°C), grain refinement of DRX is the main mechanism of microstructural evolution, while grain growth kinetics is the dominant mechanism at high temperature (1080-1120°C). DRX kinetics model of this superalloy is established. Also, constitutive and dynamic recrystallization models are brought into the Deform-3D software for validation. The results reveal that the simulation results match well with the experimental results. The study can provide guidance on optimizing technological parameters and predict the microstructure for commercial bolt forging.
In order to investigate the influence of meteorological factors on the variation characteristics of PM2.5 in Beijing. According to the survey of PM2.5 mass concentration in height of human respiration, humidity, the direction of the wind, wind speed and temperature. Using the methods of correlation analysis and nonlinear regression analysis, the effects of meteorological factors on the formation and variation of PM2.5 mass concentration in light and moderate air pollution days and heavy pollution were discussed respectively. The results showed that:① On mild to moderate pollution days, if the temperature was low, the daily average wind speed was low, the humidity was high, then the humidity was the decisive influencing factor of PM2.5 mass concentration; if the temperature, wind speed and humidity were all high, then the variation of PM2.5 mass concentration was influenced by the combined action of these three; when the temperature, humidity and wind speed were all low, then the PM2.5 mass concentration was mainly affected by the first two factors. This suggested that changes in the height of the human respiration PM2.5 mass concentrations were extremely sensitive to small changes in meteorological factors; ② During the process of air quality turning from good to heavily polluted, the accumulation of PM2.5 mass concentration was mainly due to the weak air turbulence, coupled with the high humidity, in addition, the northwest wind and northeast wind were larger during the daytime but the duration was shorter, while the southeast and southwest wind speed at night was lower with longer duration, which was conducive to pollutant accumulation;③ Short-term low amount of snow decreased the temperature and increased the air humidity, which not only could not reduce the PM2.5 mass concentration, but rather increased it by 72%, resulting in the jump phenomenon of particle concentration; ④ When the wind speed reached up to 2.0 m·s-1 and lasted for two hours, the local PM2.5 mass concentrations could be reduced to some extent, but it could not completely change the air quality situation. Only when the wind speed was greater than 3.5 m·s-1 and lasted for more than 4 hours, the fine particulate matter in the air could be quickly diffused and the air quality was changed from heavy pollution to excellent.
Global warming, fossil fuel shortage, and environment pollution are a growing concern on concentrated solar power (CSP) because of the largest amount of energy resource. Parabolic troughs and power towers are state-of-the-art commercial technologies. The primary drawbacks of current CSP technologies are low thermal efficiency and high investment cost. In the current study, a novel CSP system is proposed. This system integrates a solar parabolic trough power system and a solar tower power system. In this hybrid system the tower collectors with high concentration ratio generate high-temperature heat at 574 °C, and the trough collectors with a relative low concentration ratio generate mid-temperature heat at 390 °C. The mid-temperature heat from trough collectors generates steam up to 370 °C. The steam is then superheated and reheated by the high-temperature heat generated by the tower collectors. Compared with an individual solar trough plant, the temperatures of the primary and reheated steam are increased from individual trough plant’s 370 °C in the individual trough plant to 535 °C in the hybrid system, thus increasing the conversion efficiency from heat to power. Based on the simulation results, the annual thermal efficiency of the hybrid system can reach 15.84%, higher by 1.77 and 2.29 percentage points compared with those of the individual solar trough and tower plants. The electricity generation cost of the new system can be decreased by 7.5% to 12.4% compared with that of the individual trough or tower plants. The results obtained in the present study provide a new approach for utilizing solar energy more efficiently and more economically.
Mechanical performance of hot forging aviation bolt is strongly related to microstructures and deformation behaviors during hot processing. In order to determine the optimal technological parameters of Waspaloy alloy hot heading bolt, deformation behavior and microstructural evolution of Waspaloy were systematically studied. The experimental design (hot compression) is based on the actual production process parameters. The results from the strain-stress curves showed that there was an obvious flow softening with the decrease of temperature or the increase of strain rate, which can be attribute to the DRX mechanism. A dramatic yield drop was found with the temperature ranging from 1000°C to 1120°C at 1 s-1 due to the short-range ordering of phase forming elements (Ti and Al). Microstructures showed that high temperature and low strain rate promoted DRX behavior, but led to rapid grain growth, which was not conducive to the formation of uniform and fine DRX grains. DRX kinetics model of this superalloy was established based on the experimental results. Furthermore, the developed models were applied in the finite element simulation method (FEM). Combined with microstructure observation, “dead zone” of the bolt was prone to abnormal grain growth, resulting in mixed crystals and reducing the service life of the bolt. The calculated results in terms of grain sizes conform well to experimental ones, which implies a good application prospect of the theoretical calculation. With the two models, an optimization technology was put forward which could predict the microstructures in the aim of high fraction of DRX and fine DRX grains for Waspaloy during the bolt heading process. This study can provide guidance on commercial bolt heading process.
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