Temperature Gradient In Melt Research Articles

On the Role of ZrN Particles in the Microstructural Development in a Beta Titanium Alloy Processed by Laser Powder Bed Fusion.

Additive manufacturing of titanium alloys usually ends up with large columnar grains due to the steep thermal gradients within melt pools during solidification. In this study, ZrN particles were added into a beta titanium alloy, Ti-10V-2Fe-3Al, with the aim of promoting columnar-to-equiaxed grain transition during laser bed powder fusion (L-PBF). It was found that the addition of ZrN leads to the development of alternate layers of equiaxed grains and refined columnar grains, which is in sharp contrast to the dominant large columnar grains formed in the pure L-PBF-processed titanium alloy. An investigation on single laser melted tracks revealed that the sample with added ZrN showed fine equiaxed grains in the upper regions of solidified melt pools and columnar grains in the lower regions, whereas the solidified melt pools of the pure titanium alloy were dominated by large columnar grains due to epitaxial growth from the previous layer. The formation of equiaxed grains in the former sample is attributed to multiple factors including an increased gradient of liquidus temperature due to the solution of N and a reduced actual melt temperature gradient due to the melting of high-melting-point ZrN particles, which would have expanded constitutional undercooling, a grain growth restriction effect induced by the segregation of N along grain boundaries and the accumulation of unmelted ZrN particles in the upper regions of melt pools. The addition of ZrN also resulted in significant α precipitation, which showed strong variant selection and was found to be driven by laser reheating and the N solution in the matrix.

3D printing of fully recyclable continuous fiber self-reinforced composites utilizing supercooled polymer melts

A 3D printing approach for fully recyclable continuous fiber self-reinforced composites (CFSRCs) utilizing supercooled polymer melts was proposed. Continuous autologous fibers were added to the supercooled melt zone below the polymer matrix melting temperature avoiding fiber melting which successfully enlarged the processing temperature window. Through controlled supercooled melt conditions and heat transfer simulation, the supercooled melting temperature gradient control strategy for 3D printing CFSRCs were established. The processing temperature window enlarged from 2℃ to 30℃. Then the intrinsic connection among the printing temperature, crystallization behavior and mechanical properties of 3D printed CFSRCs were analysed. Furthermore, the interlaminar shear strength and transverse tensile property of CFSRCs were excellent than carbon fiber reinforced composites. Lastly, confirmation of the full recyclability of self-reinforced composites by mechanical recycling without mechanical property loss compared with virgin matrix. Fully recyclable CFSRCs based on 3D printing potentially establish a closed-loop recycling strategy for space applications.

Study on mechanical properties of co-injection self-reinforced single polymer composites based on micro-morphology under different molding parameters

Self-reinforced single polymer composites (SRCs), which are fabricated by combining the same type of polymer with different properties into one body, have high specific strength, no interfacial heterogeneity, and ease of recycling. To better understand the relationship between the micro-morphology and mechanical properties of SRCs, the co-injection molding process was used in this study to process SRCs parts with different molding parameters and obtain the co-injection self-reinforced single polymer composites parts(CI-SRCs parts). Further, the micro-morphology of CI-SRCs parts were observed by polarizing microscope (PLM), scanning electron microscope (SEM), differential scanning calorimetry (DSC) and wide Angle X-ray diffraction (WAXD). From the results, it was found that the tensile properties of CI-SRCs parts with different molding parameters were improved by up to 23.94% compared with the conventional parts. Through PLM observation, it is found that the section shape of CI-SRCs parts perpendicular to the flow direction shows a double ‘skin-core’ structure, and the area ratio of skin layer was higher than that of conventional parts, with a maximum increase of 68%. The low-temperature and low-speed environment were conducive to the formation of skin layer, and the tensile property of CI-SRCs parts were positively correlated with the area ratio of skin layer. SEM was carried out on the skin layer near the fusion position of the interface, and the highly oriented ‘shish-kebab’ structure was observed. The 1D-WAXD pattern analysis shows that the crystallinity of CI-SRCs parts were lower than that of conventional parts, with a maximum reduction of 19.32%. The crystallinity of CI-SRCs parts were positively correlated with melt temperature gradient, and its tensile properties were negatively correlated with the change of crystallinity. The 2D-WAXD pattern analysis shows that the molecular orientation of CI-SRCs parts were higher than conventional parts, with the maximum increase of 37.44%. Low temperature and low speed can improve the molecular orientation of CI-SRCs parts, and the change of molecular orientations were positively correlated with the tensile properties of CI-SRCs parts. By means of response surface method, the molecular orientation obtained was the decisive factor affecting the performance of CI-SRCs parts. Furthermore, by means of the least squares minimization program, the dimensionless equations among molding parameters, micro-morphologies and mechanical properties were established. The prediction of mechanical properties of CI-SRCs parts based on micro-morphologies were realized, providing theoretical support for the ‘adjustability’ of CI-SRCs parts properties.

Study on crystal-melt interface shape of sapphire crystal growth by the KY method

In this article, the influence of the flow field structure and temperature gradient of forefront interface on the shape of crystal-melt interface which may reflect the interface stability were analyzed through the method of numerical simulation by using CGSim software. In order to get a suitable interface shape and grow high-quality sapphire crystal, the heater arrangement should be adjusted during the KY process. The results indicate that the effect of Marangoni convection cannot be neglected at the last stage, the crystal-melt interface is governed by the flow field structure and the temperature gradient in melt at the crystal-melt interface. The phenomenon of shoulder concave appears at the stage of shoulder turning and interface inversion appears at the last stage during the crystal growth is discussed. Adjusting heater arrangement may effectively optimize the shape of crystal-melt interface.

Study on heat removal capability of calandria vault water from molten corium in calandria vessel during severe accident of a PHWR

Recent Fukushima nuclear accident has triggered further awareness amongst reactor designers regarding enhancing the safety measures in a nuclear reactor. It has become important to analyse the capability of decay heat removal in a reactor to avoid radioactivity releases to the environment. Such a study has been carried out for an Indian PHWR. In a hypothetical severe core damage accident in PHWR, multiple failure of the core cooling system may lead to collapse of pressure tubes and calandria tubes, which may ultimately relocate inside the calandria vessel forming a debris bed. Due to decay heat generation, the debris ultimately melts down forming a molten pool inside calandria vessel. Calandria vessel is surrounded by calandria vault water that acts as heat sink. In order to study the extent of heat transfer from molten pool to surrounding water under severe accident condition, an experiment was carried out wherein a simulant material was poured inside a simulated calandria vessel immersed in the simulated calandria vault water. The amount of melt and water present in calandria vault scaled proportionately with regard to an Indian 700MWe PHWR. Results show that as soon as the melt was poured in the vessel, a thick crust was formed on the inner calandria vessel, which reduced the heat transfer from the melt pool to vault water resulting high temperature gradient in melt. Even though the cylindrical vessel inner temperature was found to be very high, the water outside the vessel never boiled. When the cylindrical vessel was opened after the experiment, there was no gap observed between the vessel and crust. Numerical analysis was carried out to predict the temperature profile of the molten pool and the vessel which were in good agreement with experimental results. Results were compared for the crust growth rate and temperature profiles in the melt pool considering the decay heat and without decay heat. Results show that with no decay heat consideration, the crust thickness continuously increases with time and in case of decay heat generation, crust thickness is found to be a function of decay heat. The melt temperature is found to increase above a decay heat of 1%.

Thermodiffusion (thermomigration) and convection in molten semiconductor–metal layers

Thermodiffusion or thermomigration phenomenon and the presence of convection are investigated in a binary molten semiconductor–metal layer heated from top or bottom. These phenomena may be locally present in the zone melting temperature gradient (ZMTG) method used for the fabrication of semiconductor devices and solar cells. Thermodiffusion is studied using the linear nonequilibrium thermodynamics, whereas convection is studied by solving the transport equations numerically in a stationary molten cell. An expression is obtained to estimate thermodiffusion factor in stationary binary semiconductor–metal mixtures. The presence of convection in various cases is studied as well. In the absence of any experimental data on stationary semiconductor–metal mixtures, the prediction power of the developed expression is tested against the experimental data of the moving melting zone in the ZMTG process on mixtures comprised of silicon as a semiconductor and aluminum, gallium, and gold as metal dopants. The expression can only qualitatively predict the migration velocity of the melting zone, perhaps due to the inadequacy of the nonequilibrium thermodynamics to model this process, and the uncertainties in the experimental data due to the presence of both thermodiffusion and the gravity-induced convection in the molten zone.

The Microstructure of The Semi-Solid ZL201 Alloy Obtained by Near-Liquidus Electromagnetic Casting and the Fine Mechanism

Electromagnetic field was applied by ZL201 alloy melt when it was cast in near-liquidus temperature. The effects of electromagnetic stirring power and cooling condition on microstructure were studied. The result showed that, by using electromagnetic field at near-liquidus temperature, alloy microstructures were the fine and uniform non-dendritic grains. Moreover, with current strength increasing and cooling condition strengthening, the trend of the fine and uniform reinforced. When ZL201 alloy was cast at low superheat, the radius of the critical nucleus reduced. Under electromagnetic field, the melt temperature gradient decreased, and the formation of quasi-solid phase atom clusters in the melt was promoted. So the nucleation rate increased and microstructures were fine and uniform. Electromagnetic field made solute uniform, and the rapidity of nucleus growing up in all directions was almost at the same rate, therefore, the grain grew up near-spherical shape.

Antimony Concentration Limitation in Dislocation-Free CZ Silicon Crystals

The maximum antimony concentration can be incorporated in a Si(100) crystal using the Czochralski (CZ) crystal growth method was investigated. The antimony limitation was found to be about using a charge size of . It was limited by the occurrence of constitutional supercooling in the center of the crystal. A ripple or instability in the solid/melt interface was observed in the center of the crystal at the onset of constitutional supercooling. Cellular growth structure was observed when the ripple reached a critical magnitude of in the growth direction. When cellular growth occurred, cellular structure formed and dislocation networks were found in the boundaries of the cells. The melt temperature gradient at the solid/melt interface was calculated to be about .

Thermopower of Molten Aluminium in Ultrasonic Field

Temperature dependence on the liquid aluminium thermopower is reported for the range 933-1043 K. In order to highlight the ultrasonic wave effect on the thermoelectric power, the experiments were carried out under similarconditions with and without sonication. The lower values of the thermopower in sonication conditions are explained by the decrease in the melt temperature gradient and due to the specific effects of ultrasound propagation process (acoustic flow and acoustic cavitation). A functional relationship was found, Q = -A/T + BT, where A and B are positive constants. These two terms are regarded as electron-ion collisions and spatial redistribution of conduction electrons in the necessary temperature gradient. The effect of cavitation at high temperature is debated. A good agreement between the experimental data and theoretical models of the transport properties is achieved.

Noncontact measurement of parison thickness profiles affected by swell and sag in continuous extrusion blow molding

AbstractOptimization of final part thickness distributions is crucial in the extrusion blow molding process in order to minimize resin usage. Prediction of part thickness distributions from basic process and material parameters would be ideal. However, attempts to do so have been unsuccessful, largely because of the inability to predict parison thickness profiles. One must therefore resort to measurement of the parison thickness profile and estimation of the final part thickness distribution by computational methods. This paper describes a new technique for the noncontact estimation of parison thickness profiles in continuous extrusion blow molding. The method accounts for sag and requires no previous knowledge of rheological data. It can be employed on‐line for the purposes of process monitoring and control. The approach is based on the measurement of the parison length evolution with time during extrusion, the parison diameter profile, the flow rate, and the melt temperature gradient along the length of the parison. These parameters are utilized in conjunction with a theoretical approach that describes the extrusion of a parison under the effects of swell, sag, and extrusion into ambient conditions. Results are presented for three resins of various molecular weight distributions. The degree of sag is minimal at the top and bottom of the parison, and reaches a maximum near the center of the parison. Results are also presented to demonstrate the versatility of the method under other process conditions, such as varying flow rate, die temperature, and die gap.

Influence of Extruder Conditions on Orientation Distribution in LLDPE Blown Films

We describe the effect of extruder temperature profiles on melt temperature homogeneity and bubble stability, the effect of melt temperature gradient on the suface temperature variation around the bubble and the effect of surface temperature variation around the bubble gauge and molecular orientation distribution around the film

Influence of growth conditions on melt interface temperature oscillations in silicon czochralski growth

The gradient and oscillation amplitude of temperature are accurately measured in the melt and solid during silicon crystal growth by the Czochralski technique. Temperature oscillations near the growth interface are closely related to the growth of high quality silicon crystals. The oscillation amplitude decreases as the heater position is raised or the pulling rate is increased. The amplitude also decreases with a reduction of the crystal rotation rate but is independent of the crucible rotation rate. The amplitude depends on the melt temperature gradient near the growth interface and on the shape of the growth interface. These results should contribute to the fabrication technology of semiconductor substrates for very large scale integrated circuits (VLSI).

Mathematical modeling and parametric study of heat transfer in Bridgman-Stockbarger growth of crystals

A one-dimensional heat transfer model was developed that allows calculation of interface melt temperature gradient and interface locations in Bridgman-Stockbarger growth. A set of parameters appear naturally in this analysis. A parametric study was used to predict the effect of these parameters on temperature gradient and interface location. The validity of the model was tested by comparison with experimental results on the MnBi/Bi eutectic system. The agreement is fair.

The dependence of the melting temperatures of iron upon the choice of the interatomic potential

Summary The melting curve (melting temperature versus pressure) of iron is fundamental to an understanding of the physics of the Earth's core. Since experimental data are available for pressures a few orders of magnitude smaller than the core pressures, the problem is to find the correct ways of extrapolating them. Here we employ Ross’ melting criterion, a reformulation of Lindemann's law in the framework of statistical mechanics, because all the proposed empirical melting laws can be deduced from it; it also allows us to take into account the interatomic potential and the various crystal structures which iron may possess. The results of our calculations lead us to the conclusion that the melting temperatures of iron at the core pressures are extremely uncertain since no definite choice among possible potential functions is allowed. The melting temperature at the mantle–core boundary may range between 3500 and 4300 K, while at the inner core–outer core boundary it may range between 4500 and 7000K. The average melting temperature gradient is shown to be between 0.6 and 1.1 deg/km. The problem of establishing if the core is made of fcc or hcp iron is definitely secondary to that of the choice of the potential function.

Zonal melting as mechanism in evolution of the Earth's crust

The paper treats the problem of upward movement of a molten zone in the Earth's crust. Due to the melting temperature gradient of 3°C/sm, such a zone is in an unstable state: melting occurs at the upper boundary, where crystallization takes place at the base. During this process, the net thickness of the molten zone decreases, since additional heat is required to raise the temperature of the overlying rock to the melting point. The process continues until the thickness of the molten zone is reduced to zero or until it reaches a level above which open fractures exist. Based on thermodynamic considerations, estimates are given for the rate of upward movement and thinning: lifting the upper boundary of the molten zone from a depth of 100 km to 20 km requires a reduction in thickness by about 60 km or 30 km, in the continental or oceanic crust, respectively. The lifting time is estimated as 10–100 m. y. --H. Jahns.