Temperature Gradient Distribution Research Articles

Melt pool boundary extraction and its width prediction from infrared images in selective laser melting

Selective laser melting (SLM) involves periodic processes of rapid temperature rise and drop, so it is of great interest in evaluating the quality of the melted powders by real-time thermal information. In this study, a LumaSense MCS640 thermal imaging camera in coaxial system, which can track the location of melt pool, was used to real-time monitor the surface temperature during Ti-6Al-4V forming and predict an average melt width by extracting the boundary of the melt pool in each infrared image. The characteristics of the temperature gradient distribution around a melt pool instead of the temperature itself was analyzed, the boundary of a melt pool was determined then by the maximum (or minimum) temperature gradient point in space. Straight melt samples were obtained by a fixed laser scanning direction in this study, the extracted boundaries of the moving melt pool in the direction perpendicular to the laser scanning direction were compared with the measured melt width from a microscope. A best width divergency between the prediction and measurement around 5% is achieved.

Visualization Analysis of Interrelationship between Temperature Profile and Water Distribution inside the PEFC

Water management is very important for more spread of polymer electrolyte fuel cell (PEFC). Water distribution inside the PEFC must be kept proper and it is largely changed with the temperature profile change inside the PEFC. Therefore, the interrelationship between the temperature gradient and water distribution in the cathode gas diffusion layer (GDL) must be investigated in detail. In this report, X-ray computed tomography (CT) was used to visualize inside the cathode GDL. And thin film thermocouple (TFTC), which width is 30 µm and thick is 6 µm, has been fabricated by using micro electro mechanical system (MEMS) technology. With X-ray CT and TFTC, the interrelationship between temperature gradient in the direction of through-plane of the cathode GDL and water distribution inside the cathode GDL was investigated. From the results, we considered that temperature gradient may largely affect on water distribution and the PEFC performance.

Laser additive manufacturing of layered TiB2/Ti6Al4V multi-material parts: Understanding thermal behavior evolution

Selective laser melting (SLM) allows the fabrication of complex geometric shapes combined with improved functionalities and it has captured considerable attention for direct part production. While most studies conducted on the single materials, the multi-materials parts manufactured by this technology can offer more performance advantages of different materials to satisfy more demands in many engineering applications. Thus, TiB2/Ti6Al4V multi-material parts were fabricated to improve the properties of Ti6Al4V alloy components. In this study, a multilayer finite element model was proposed to investigate the complicated thermal behavior at the interface between the Ti6Al4V layer and TiB2 layer under varied process parameters. The temperature and temperature gradient distribution, remelting depth and liquid lifetime of the remolten pool at the as-fabricated Ti6Al4V layer during the SLM process of layered TiB2/Ti6Al4V multi-material parts were analyzed. The simulation results showed that the maximum temperature gradient was located at the interface. As the applied laser power increased from 300 W to 450 W, the maximum temperature gradient varied significantly, increasing from 24.920 °C/μm to 37.754 °C/μm while the maximum temperature gradient decreased slightly from 33.884 °C/μm to 31.478 °C/μm as the scan speed increased from 400 mm/s to 1000 mm/s. The interface temperature and liquid lifetime had an important influence on the wettability at the interface, which further impacted the metallurgical bonding at the interface. At the laser power of 400 W and the scan speed of 600 mm/s, a sound interfacial bonding could be achieved between the TiB2 layer and Ti6Al4V layer with a proper interface temperature of 2453 °C and liquid lifetime of 1.7 ms.

Layered Structure of Stably Stratified Turbulent Shear Flows

The data of numerical simulation of stably stratified turbulent Couette flows are analyzed for various values of the Richardson number. Two different methods were used: Direct Numerical Simulation (DNS) and Large Eddy Simulation (LES). It is shown that the flow contains large organized structures, along with chaotic turbulence, regardless of the simulation method. These structures appear as inclined layers in the temperature field with weakly stable stratification, separated by very thin layers with large temperature gradients. The existence of such layered structures in nature is indirectly confirmed by the analysis of field measurement data on the meteorological mast, where temperature gradient distribution histograms are found to be far from the normal distribution and similar to temperature gradient probability distributions obtained by numerical models data. The simulations indicate an increase of the turbulent Prandtl number with increasing of the gradient Richardson number. It is highly likely that the identified structures serve as effective barriers for vertical turbulent heat flux, without the blocking of momentum transfer. We proposed the hypothesis, that it is precisely these structures that serve as the physical mechanism for maintaining turbulence under supercritically stable stratification.

EXPERIMENTAL STUDY ON INTERNAL FORCE VARIATION OF STEEL-CONCRETE COMPOSITE BEAM UNDER FIRE

In practical engineering, most steel-concrete composite beams have axial constraints, which cause the composite beam to produce catenary action to continue bearing more external loads at large deformations. To study the mechanism of catenary action of steel-concrete composite beam under fire, two full-scale tests of steel-concrete composite beams were carried out to obtain the internal force variations with the time. The test designation, loading scheme and measurement contents are presented in this paper. The experimental phenomena and failure characteristics are also described. The test results show that the temperature gradient distribution of the composite beam section changes greatly under fire, and so it is easy to generate additional bending moment, which has an adverse effect on the ultimate bearing capacity of the composite beam. The failure mode of the composite beam is mainly the formation of obvious plastic hinge at the negative bending moment area at the beam ends, and the overall lateral instability occurs on the tensioned lower flange of the steel beam at the bottom of the beam, which is contrary to the common phenomenon that overall unstability of a steel beam is usually attributed to the compression flange buckling of a steel beam. The load ratio is one of the important parameters affecting the fire resistance of the composite beam. Through the actual measurement, the changing process of the internal force of the composite beam in fire is obtained. It is found that a larger load value leads to more obvious catenary action of the composite beam at the same time under the same conditions.

Linkage of meteorological parameters and anomalous radio propagation profile over Nigeria

The vertical distribution of refractivity gradient is important in determining anomalous propagation condition. Thirty five years of meteorological parameters, obtained from Era-Interim archive of European Centre for Medium-Range Weather Forecasts has been used to analyze and investigated surface meteorological data linked with refractivity gradient aloft altitude across Nigeria. Spatial distribution of surface temperature, relative humidity and refractivity gradient at 0.1 km, 0.5 km, 1.0 km and 1.5 km over Nigeria were plotted. Vertical distribution of temperature, relative humidity, refractivity and refractivity gradient were obtained for some locations across Nigeria. Similarly, spatial distribution of coefficient of determination between surface temperature, relative humidity and refractivity gradient at four different height were estimated. Linear regression were developed to investigate the relationship between surface data and refractivity gradient at different altitude. The result revealed existence of sub refractive, super refractive, and ducting across the country at 0.1 km and 0.5 km however occurrence of ducting and sub refractive disappeared at 1.0 km and 1.5 km. Likewise, the existence of temperature inversion was noticed between surface and 100 m across all the locations except in Lagos. Values of refractivity across the observed locations converged around 0.5 km. Through result of correlation coefficient and statistical parameters, significant linked have been established between surface data and refractivity gradient at different height.

Improving thermal conductivity of epoxy resin by filling boron nitride nanomaterials: A molecular dynamics investigation

Several kinds of inorganic nanofillers especially boron nitride (BN) are introduced to improve the thermal conducting properties of epoxy resin (EP). In this work, the influence of BN nanofillers on properties of EP composites is comprehensively investigated by using molecular dynamics (MD) technique. The thermal conductivity of BN/EP composites is calculated by reverse non-equilibrium molecular dynamics (RNEMD) method, then the effects of shapes (nanoparticle, nanotube and nanosheet), volume fractions and aspect ratios of BN nanofillers on the thermal properties are explored. To further elaborate the heat-conducting mechanism of EP composites, temperature gradient distribution and phononic density of states (PDOS) are analyzed. Moreover, the effect of BN volume fractions on the thermal conductivity of the composites is investigated by RNEMD simulation and Nielsen's theoretical model. The results indicate that the improving effectiveness of different BN nanofillers is ranked as nanosheet > nanotube > nanoparticle. This research provides new ideas for improving the thermal conductivity of composites.

Multiphysics Field Distribution Characteristics within the One-Cell Solid Oxide Fuel Cell Stack with Typical Interdigitated Flow Channels

Generally, the manufacturing technology of fuel cell units is considered to satisfy the current commercialization requirements. However, achieving a high-performance and durable stack design is still an obstacle in its commercialization. The solid oxide fuel cell (SOFC) stack is considered to have performance characteristics that are distinct from the proton exchange membrane fuel cell (PEMFC) stacks. Within the SOFC stack, vapor is produced on the anode side instead of the cathode side and high flow resistance within the fuel flow path is recommended. In this paper, a 3D multiphysics model for a one-cell SOFC stack with the interdigitated channels for fuel flow path and conventional paralleled line-type rib channels for air flow path is firstly developed to predict the multiphysics distribution details. The model consists of all the stack components and couples well the momentum, species, and energy conservation and the quasi-electrochemical equations. Through the developed model, we can get the working details within those SOFC stacks with the above interdigitated flow channel features, such as the fuel and air flow feeding qualities over the electrode surface, hydrogen and oxygen concentration distributions within the porous electrodes, temperature gradient distribution characteristics, and so on. The simulated result shows that the multiphysics field distribution characteristics within the SOFC and PEMFC stacks with interdigitated flow channels feature could be very different. The SOFC stack using the paralleled line-type rib channels for air flow path and adopting the interdigitated flow channels for the fuel flow path can be expected to have good collaborative performances in the multiphysics field. This design would have good potential application after being experimentally confirmed.

Analytical Modeling of In-Process Temperature in Powder Bed Additive Manufacturing Considering Laser Power Absorption, Latent Heat, Scanning Strategy, and Powder Packing.

Temperature distribution gradient in metal powder bed additive manufacturing (MPBAM) directly controls the mechanical properties and dimensional accuracy of the build part. Experimental approach and numerical modeling approach for temperature in MPBAM are limited by the restricted accessibility and high computational cost, respectively. Analytical models were reported with high computational efficiency, but the developed models employed a moving coordinate and semi-infinite medium assumption, which neglected the part dimensions, and thus reduced their usefulness in real applications. This paper investigates the in-process temperature in MPBAM through analytical modeling using a stationary coordinate with an origin at the part boundary (absolute coordinate). Analytical solutions are developed for temperature prediction of single-track scan and multi-track scans considering scanning strategy. Inconel 625 is chosen to test the proposed model. Laser power absorption is inversely identified with the prediction of molten pool dimensions. Latent heat is considered using the heat integration method. The molten pool evolution is investigated with respect to scanning time. The stabilized temperatures in the single-track scan and bidirectional scans are predicted under various process conditions. Close agreements are observed upon validation to the experimental values in the literature. Furthermore, a positive relationship between molten pool dimensions and powder packing porosity was observed through sensitivity analysis. With benefits of the absolute coordinate, and high computational efficiency, the presented model can predict the temperature for a dimensional part during MPBAM, which can be used to further investigate residual stress and distortion in real applications.

Formation mechanism and mechanical properties of titanium-doped NbC reinforced Ni-based composite coatings

Ni-based composite coatings reinforced with in-situ titanium-doped NbC particles were fabricated on Cr12MoV steel plate utilizing laser cladding technology. The microstructure and mechanical properties of the coatings were investigated, as well as the effect of different Nb/Ti mole ratio on the distribution characteristics and formation mechanism of titanium-doped NbC particles. The (Ti, Nb)C distributed homogeneously in the coatings. According to the EBSD results, the coating with Nb/Ti = 1:1 showed strong texture characteristics, and the grains size of the bottom was smaller due to faster heat dissipation. As lower surface energy and lattice misfits of TiC and NbC, (Ti, Nb)C was easily synthesized and the pre-formed TiC particles were the most favorable attachment sites. According to the analysis results of SEM, EPMA and TEM, when Nb/Ti = 1:1, (Ti, Nb)C was a multiple carbide with a NbC structure. In addition, the various morphologies of reinforcing phases were relevant to the solidification rates, temperature gradient and composition distribution. The micro-hardness and wear resistance of coating with Nb/Ti = 1:1 exhibited significantly improvement compared with other coatings.

Molten Steel Level Detection by Temperature Gradients With a Neural Network

Molten steel level is difficult to measure as a result of the high-temperature melt and the covering flux. For the measurement, in our previous work, a novel principle by using the temperature gradient was proposed, and a refractory sensor was inserted into the metallurgical container to sense the temperature gradients of the flux and the molten steel. However, variations of temperature gradient distributions are large when the fluctuation speed of the molten steel level is fast, causing difficulties in the detection of the flux-steel interface. For this issue, a neural network is introduced to learn the features of the temperature gradients around the flux-steel interface, and the convolution of the neural network is developed to detect the flux-steel interface from the temperature gradient distribution. The numerical data obtained from the heat transfer model is used to train and test the detection method. The detection method gives a good performance with the numerical data. But for actual on-site applications, noise in the temperature gradient distributions affects the reliability and accuracy of the detection results. To improve the reliability of the detection method in practice, lifting of the sensor at the wave crest of the flux level is adopted to ensure the large temperature gradients around the flux-steel interface. The statistics show that the detection errors of the flux-steel interface are within ±5 mm with a confidence of 98.3%.

Instrumental Evaluation of Dry Heat Loss of Footwear Under Different Activity Levels

An extreme microclimate inside footwear contributes to wear discomfort and performance impairment. A thermal foot manikin system was developed to evaluate the thermal insulation of footwear. The uniqueness of this foot manikin is that it utilizes a specially designed all-in-one flexible heating sheet with uneven distribution of resistive wires to achieve a non-uniform gradient temperature distribution along the foot surface. Three flexible heating sheets with different wire distributions were developed to simulate the local skin temperature for different activity levels, suggesting more realistic measures. The heated foot manikin was enclosed by a silicone ‘skin’ layer, which provides a soft texture. In addition, it was equipped with phalangeal joint and ankle joint for the ease of wearing various types of shoes. The local skin temperature and the power required to heat the foot manikin were recorded. Five types of footwear were compared; the results show that the leather sneaker provides the best thermal insulation performance, whereas the mesh shoe provides the worst thermal insulation performance in walking and running conditions. The findings also reveal that the toe portion is the coolest zone. This paper demonstrated that the measurements from the developed foot manikin are repeatable and versatile, which is useful for optimizing the thermal comfort of the footwear.

Temperature Field of Tool Engaged Cutting Zone for Milling of Titanium Alloy with Ball-End Milling.

When milling titanium alloy, the cutting temperature has a strong impact on the degree of tool wear and, in turn, tool life and the surface quality of the workpiece. The distribution of the temperature field on a tool’s rake face can be improved through the use of micro-textures, which help to reduce friction and, ultimately, wear on the tool. In this paper we present a new way to measure cutting temperature and examine heat distribution when milling titanium alloy with micro-textured ball-end milling tools. We first establish the heat flux density function for the contact area between the workpiece and the tool and then for the rest of the tool. Thermal stress simulation shows that adhesive wear tends to happen in the contact area and on the flank face, rather than at the tip of the tool, with the temperature distribution gradient for the rest of the tool being more uniform. The maximum value for thermal stress on the cutting edge was 2.0782 × 106 Pa. This decrease as you move away from the cutting edge along the contact area between the tool and the workpiece. Maximum deformation of the tool is also mainly concentrated at the principal contact point, with a value of 1.9445 × 10−9 m. This, too, decreases as you move away from the cutting edge and into the rest of the contact area. This research provides the basis for the optimization of tool structure and further investigation of the thermo-mechanical coupling behavior of micro-textured ball-end milling cutters when milling titanium alloy.

Microscopic damage behavior in carbon fiber reinforced plastic laminates for a high accuracy antenna in a satellite under cyclic thermal loading

For a high accuracy antenna in next radio astronomy satellite, a candidate material is carbon fiber reinforced plastics (CFRP), because negative longitudinal coefficient of thermal expansion (CTE) for unidirectional CFRP enables a laminate with 0 CTE through appropriate laminate design. This enables high structural accuracy under large temperature fluctuation like space. On the other hand, when the laminate is subjected to thermal cycles, cyclic thermal stress occurs and causes microscopic damages. In this study, we characterized damage progress in CFRP laminates and resultant variation in mechanical properties under cyclic thermal loading. Three types of matrices, such as polycyanate ester, polyimide and epoxy resin were used to prepare CFRP laminates. Specimens were subjected to thermal cycles from −197°C to 120°C. The test was periodically stopped for surface observation and flexural loading. Transverse cracks in 90° plies accumulated with thermal cycles, whereas flexural modulus remained constant. We also numerically evaluated temperature gradient and resultant thermal stress distribution during cooling by finite element analysis. The result indicates higher transverse stress appeared in the surface of the specimen and saturated to constant value which corresponded with the value calculated based on classical lamination theory.

Phase field simulation of microstructural evolution and thermomigration-induced phase segregation in Cu/Sn58Bi/Cu interconnects under isothermal aging and temperature gradient

Sn-Bi based alloys are widely used as thermal interfacial materials, solders in electronic packaging, thermal fuse and low temperature resistant performance electronic components. However, the inherent tendency of microstructure coarsening and phase segregation in Sn-Bi alloys under thermal loading can easily lead to the inhomogeneous mechanical and thermal response behavior, which can promote crack initiation and expansion near the interface between two major phases (i.e., Bi-rich and Sn-rich phases), and this has brought about serious reliability concern for the above applications and thus attracted increasing attention in recent years. In this study, a phase field model is developed to simulate the microstructural evolution and phase segregation behavior of the eutectic Sn58Bi solder in a line-type Cu/Sn58Bi/Cu interconnect under the condition of isothermal aging and temperature gradient respectively. Results show that the Bi-rich phase and Sn-rich phase distribute inhomogeneously in the Sn58Bi alloy matrix of the solder interconnect during thermal aging; the large size Bi-rich phase particles grow up at the expense of the small size ones, and the value of coarsening exponent n is 0.17. Under temperature gradient, Bi atoms migrate along the direction of the heat flux, and consequently a Bi-rich phase segregation layer forms on the cold end, while a Sn-rich phase area is left on the hot end. The relationship between the temperature gradient distribution and the microstructural characteristics is well revealed. Moreover, the phase segregation under temperature gradient induces the decrease of thermal conductivity, which in turn deteriorates the heat transfer performance of solder interconnects. In addition, comparing the coarsening behavior of the Bi-rich phase under temperature gradient with that during isothermal aging, it is clear that the temperature gradient leads to faster coarsening of the Bi-rich phase, and there is a linear relationship between the mean equivalent radius of the Bi-rich phase and nondimensional time. Finally, the study of thermomigration kinetics of the Bi-rich phase shows that the thickness of the Bi-rich phase segregation layer increases almost linearly with nondimensional time, which is consistent with the theoretical analysis.

Heat Transfer Modeling of a High-Temperature Porous-Medium Filled Solar Thermochemical Reactor for Hydrogen and Synthesis Gas Production

In this paper, heat transfer modeling of a high-temperature porous-medium filled solar thermochemical reactor for hydrogen and synthesis gas production is investigated. The numerical simulation is performed using a three-dimensional (3D) numerical model and surface-to-surface radiation model coupled to Rosseland approximation for radiation heat transfer. The effects of operating conditions and the porous structural parameters on the reactor thermal performance were investigated significantly. It was found that large axial temperature gradient and high-temperature distribution throughout the reactor were strongly dependent on the operating conditions. The inlet gas temperature has remarkable effects on the temperature distribution. The thermal performance of porous-medium filled solar thermochemical reactor could be improved by preheating the inlet gas up to 393.15 K. Moreover, a correlation was established between the protective gas inlet velocity and the porosity of porous media. The temperature difference decreased with the increase in the porosity of the inner cavity of the reactor. In contrast to the front and back parts of the inner cavity of the reactor, higher temperature distribution could be obtained in the porous region by increasing the average cell diameters of porous media.

Modeling of melting and resolidification of equiaxed microstructures in a temperature gradient

Melting and resolidification in a temperature gradient are simulated using a cellular automaton (CA) model for an Al–Cu alloy with initially equiaxed microstructures. The simulations exhibit liquid pockets appearing at grain boundaries and in the grain interior, as well as their coalescence and migration towards the high temperature direction due to temperature gradient zone melting. The simulated microstructures agree well with experimental micrographs. The liquid fraction and solute concentration distributions throughout the mushy zone are quantified; they compare reasonably well with analytical predictions. The simulations provide insight into the complex interactions among temperature gradient and solute (re)distribution during melting/resolidification.

Coupled Effects of Heating Method and Rate on the Measured Nonisothermal Austenization Temperature of Steel SUS420J1 in Heat Treatment

Steel SUS420J1, which is the key material of turbine blade, is generally treated by heat to improve the strength prior to use. And the austenization process at different heating rates would determine the depth and width of heat treatment. In this paper, the austenization temperatures in heat treatment with the heat from induction wire, infrared lamp, and laser are measured, respectively. The effect of heating rate on the austenization temperature has been investigated. The research results show that the measured austenization temperature increases with the heating rate. And this trend is specially enlarged in the heat treatment method with larger gradient of temperature distribution, e.g., laser. The calculated phase transformation threshold shows that negative linear relationship exists between the logarithmic heating rate and the logarithmic austenization threshold for both induction heating and infrared heating, while abnormal relationship exists for laser heating. Thermal finite element analysis (FEA) models are then developed to calculate the temperature distributions in these three heating methods, and the calculated results show that the nonuniform temperature distribution leads to the gap between the measured austenization temperature and that of the material, which also leads to the abnormal variation law of austenization threshold in laser heating. The measured austenization temperature in induction heating method is thought to be the closest to the actual austenization temperature of the material among these three methods. This paper provides a guide for choosing the proper parameters to heat the steel SUS420J1 in hardening.

Study of the Thermal Behavior of Wedge-Shaped Samples of HfB2–45 vol % SiC Ultra-High-Temperature Composite in a High-Enthalpy Air Flow

By spark plasma sintering, HfB2–45 vol % SiC ultra-high-temperature ceramic was prepared, from which wedge-shaped samples were cut. The behavior of the samples was examined in a flow of dissociated air produced by an induction plasmatron, where the surface temperature of the leading edges of the samples reached ~2700°C. The dependence of the temperature distribution gradient on the distance from the leading edges of the samples was experimentally investigated. For the samples after the experiments, the elemental and phase compositions were determined, and features of the surface microstructure in various regions of the sample and on its polished surface were studied.

Three-dimensional finite element temperature gradient analysis in concrete bridge girders subjected to environmental thermal loads

A thermal finite element study was carried out to investigate the temperature gradients in bridges with precast concrete girders that subjected to the daily and seasonal environmental variations of solar radiation and air temperature. The proposed finite element model was used to simulate the three-dimensional thermal behavior considering the extreme thermal loads of the region of Gaziantep, Turkey. Two construction stages were studied, the single girder and the girders-deck slab superstructure before surface topping. The results show that the maximum temperature gradient at the top surface was greater for the unsurfaced superstructure by about 6 °C. The predicted extreme temperature gradient distributions for the two construction stages were compared with the current American Association of State Highway and Transportation Officials (AASHTO) gradient model. The comparison showed that the current AASHTO temperature gradient distribution agreed well with the predicted gradient of the single girder, while a higher deviation was noticed when compared with the predicted gradient of the bridge superstructure. Moreover, the current AASHTO gradient model slightly overestimated the maximum predicted gradient of the single girder, while it underestimated the maximum gradient of the bridge superstructure.