Additive manufacturing (AM) of titanium alloys is a rapidly growing field due to an increase in design flexibility of parts. However, AM parts are highly anisotropic in material microstructure and ...mechanical behavior due to the change of the local processing conditions in the build-up process. This study follows a link chain model to investigate the relationships between process parameters, cooling rate, porosity and mechanical behavior. The aim of this work is to present a framework that is inspired by the three-link chain model. The framework combines theoretical, computational and experimental approaches. We demonstrate this by using an in-house thermal simulator to link predicted cooling rates with micrographs describing experimental shape descriptors to develop a relationship between solidification cooling rate and porosity geometry. Finally, representative volume elements from predicted porosity maps allow for a prediction of mechanical properties at localized areas. The capability of being able to predict mechanical behavior of titanium alloys is demonstrated for the directed energy deposition process.
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•A mechanistic model of wire arc additive manufacturing is validated experimentally.•3D transient model considers mass addition, heat transfer, fluid flow and free surface.•New ...results on cooling rates, solidification parameters, droplet impact & finger penetration.•Effects of power, welding speed, wire diameter and feed rate are evaluated.
Structure, properties and serviceability of components made by wire arc additive manufacturing (WAAM) depend on the process parameters such as arc power, travel speed, wire diameter and wire feed rate. However, the selection of appropriate processing conditions to fabricate defect free and structurally sound components by trial and error is expensive and time consuming. Here we develop, test and utilize a three-dimensional heat transfer and fluid flow model of WAAM to calculate temperature and velocity fields, deposit shape and size, cooling rates and solidification parameters. The calculated fusion zone geometries and cooling rates for various arc power and travel speed and thermal cycles considering convective flow of molten metal agreed well with the corresponding experimental data for H13 tool steel deposits. It was found that convection is the main mechanism of heat transfer inside the molten pool. Faster travel speed enhanced the cooling rate but reduced the ratio of temperature gradient to solidification growth rate indicating increased instability of plane front solidification of components. Higher deposition rates could be achieved by increasing the heat input, using thicker wires and rapid wire feeding.
Global urbanization drastically reshapes the land–atmosphere interactions, biogeochemical cycles, and ecosystems in the integrated earth system. Urban trees are used extensively for providing ...desirable ecosystem services, especially to mitigate elevated thermal stress in cities. Urban trees differ markedly from their natural counterparts in the physiology due to their close interactions with anthropogenic stressors. Here we present the cooling capacity of urban trees in response to thermal extremes in cities of the contiguous United States. The cooling capacity is quantified as the surface cooling rate, which is expressed as the negative ratio of land surface temperature (LST) changes to fractional tree cover (FTC) changes, i.e., –ΔLST/ΔFTC. Results show that the surface cooling rate is dominated by plant transpiration up to 1.336 °C per percentage of FTC in heat waves; its temperature dependence remarkably resembles the thermodynamic liquid-water-vapor equilibrium. Urban trees also exert pseudo cooling effect in cold waves in contrast to the anthropogenic heat emission. The average surface cooling rate in cold waves is 0.022 °C per percentage of FTC, which is much smaller than that in heat waves (0.202 °C per percentage of FTC). The enhanced cooling capacity of urban trees will enable their provision of better ecosystem services to the urban environment for projected future increase of extreme heat.
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•The surface cooling rate of urban trees during heat and cold waves is quantified.•Temperature dependence of tree cooling resembles the phase equilibrium of water.•The cooling of urban trees is dominated by plant transpiration in heat waves.•The synthetic cooling of cities is regulated by anthropogenic heat in cold waves.•Urban trees are ahead of the global change responses than their rural counterparts.
Medium- to long-chain alkanes can form upon cooling intermediate phases between isotropic liquid and solid crystalline, called rotator phases, where relative freedom of the molecules to rotate about ...their long axis is combined with long range translational order. Rotator phases are well documented experimentally but the mechanism of their formation at the molecular level is still not fully explained. In a previous work, we have shown that molecular dynamics simulations can produce rotator phases upon cooling of hexadecane S. Iliev et al., J. Col. Int. Sci., 2023, 638, 743. The aim of the current work is to develop a procedure to identify the specific ordered phase obtained in the simulations. The influence of the cooling rate on the freezing process of hexadecane (bulk and surfactant-interfaced to water) is tested as well. Several parameters are combined to quantify the degree of ordering and the type of phase in the studied systems. These are the tilt angle of the molecules with respect to the crystallite plane, the radial distribution function of the centre of mass of the molecules in the crystallite, the percentage of the gauche torsion angles in the molecules, the angle of the second principal axis of each molecule with respect to the x axis of the coordinate system, and estimates from Voronoi analysis. The results show that the systems form a rotator phase, which transitions gradually towards the thermodynamically most stable triclinic crystal, and the transformation progresses to different extent depending on the system. The influence of the cooling rate is related only to the size of the largest crystallite formed, the other parameters of the freezing process remain unaffected. The work also presents a robust procedure for obtaining and identifying different types of ordered phases in alkane-containing systems with thoroughly tested computational protocol and a comprehensive set of structural analyses. Several key characteristics are advanced, compared to previous research Ryckaert et al., Mol. Phys., 1989, 67, 957; Wentzel et al., J. Chem. Phys. 2011, 134, 224504, namely, a new methodology is proposed to compute the unit cell deformation parameter and azimuthal angle from MD simulation trajectories of the freezing process in alkane-containing systems. The suggested structural analysis, which is independent of the coordinate system, is applicable to any linear-chain system with polycrystalline structure.
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•The cooling rate effect on rotator phases formation in hexadecane is traced by MD.•Hexadecane exhibits multiple rotator phases upon freezing.•Slower cooling yields larger main crystallites in bulk or at an interface.•Thermodynamic and structural parameters quantify the degree of ordering.•The phase state of the systems is identified with a set of structural descriptors.
ABSTRACT
We analyse the average annual and seasonal air temperature conditions in the ‘local climate zones’ (LCZs) of Szeged, Hungary. The basis of our analysis is a 1‐year dataset from 2014 to 2015 ...for a 20‐station urban meteorological network. The network and its corresponding LCZ classes put temperature studies in Szeged into a new spatial framework to assess local climate and urban heat island (UHI) conditions. The stations were installed at locally representative sites using a Geographic Information System (GIS) method based on the standard surface parameters of the LCZ classification. The network was purposely designed to monitor thermal differences among LCZ classes in Szeged. We provide detailed site metadata for each of the monitoring stations used in the analysis. Our results show that the densely built‐up LCZ classes have higher annual and monthly mean and minimum air temperatures than structurally open and more vegetated classes, with nocturnal differences of >4 °C observed under calm, clear skies. Among select temperature indices measured in the urban LCZ classes, frost days, cooling degree‐days, and tropical nights differ markedly from the background rural LCZ classes. This difference suggests that local climatologies exist within Szeged, and that these have implications for thermal comfort, urban energy use, and urban agriculture. Finally, the evaluation of heating and cooling rates in Szeged shows an important role for LCZs in UHI analysis.
This paper provides insights into the effect of high thermal gradients and cooling rates on equiaxed grain nucleation and growth in conditions similar to those experienced during Additive ...Manufacturing (AM) processes. Bridgman type solidification is numerically simulated with columnar grains growing at a fixed pull rate under a user-imposed thermal gradient. Controlled inoculants of known nucleation undercooling were placed ahead of the growing columnar grains to allow quantitative analysis of nucleation events. At low thermal gradient and cooling rate only the inoculants with low nucleation undercooling were activated due to low melt undercooling driven by constitutional supercooling (CS). As the cooling rate is increased, for a given thermal gradient, a larger number of inoculants with higher nucleation undercoolings were activated. At higher cooling rates, thermal undercooling was generated by a lag in the growth rate of the solid-liquid (S–L) interface compared to the theoretical pull rate. Thus, thermal undercooling becomes dominant leading to the facilitation of nucleation on less potent substrates requiring higher undercooling. The results show a transition from solute-driven undercooling to cooling rate driven thermal undercooling which contributes to the undercooling that activates the nucleation events. Invoking the Interdependence model, it is also shown that the high cooling rate induced thermal undercooling reduces the size of the nucleation free zone substantially.
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•Instantaneous cooling rate is maximum with the modified pulse mode and Helium shielding gas by steeper temperature gradient.•The maximum instantaneous cooling rate is effective in laves formation ...temperature range.•Reduction in laves phase has been demonstrated.•Segregation of Nb decreases with enhanced cooling rates and the phenomenon need to be further probed for Mo.•Reduction in laves phase and alloying element segregation has been confirmed with dendrite arm spacing.
Inconel 718 (2mm thick) was welded using argon and helium gas shielded tungsten arc welding process with a filler metal. Both constant current and compound current pulse modes were applied and the cooling rates calculated. The dependence of Laves phase formation, dendrite arm spacing and niobium segregation ratios in fusion zone on the nature of shielding gases and current was studied. The maximum instantaneous weld cooling rate was achieved for the combination of Helium shielding gas and compound current pulse mode. This ultimately resulted in reduction of laves phase, segregation of niobium and dendrite arm spacing in the fusion zone.
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