Differences In Thermophysical Properties Research Articles

Solidification microstructure selection maps for laser powder bed fusion of multicomponent alloys

Solidification Microstructure Selection (SMS) maps provide a simple yet effective approach to predict the non-equilibrium solidification microstructure and grain morphology during Additive Manufacturing. In this study, SMS maps have been created for the Inconel 625 (IN625) alloy processed by Laser Powder Bed Fusion (LPBF). Toward this end, theoretical solid growth models, a model of the Columnar to Equiaxed Transition (CET), interface response theory, thermal simulation results and computational thermodynamics are utilized. The predicted microstructures are compared both qualitatively and quantitatively to experimentally-obtained micrographs. The theoretical analysis was also compared to the earlier analytical calculation for Al-10Si-0.5Mg alloy to show how differences in thermophysical properties affect the microstructural predictions. The theoretical predictions are shown to be in good agreement with the experimental results in terms of the resulting microstructure and dendrite arm spacings. A discussion on the use of SMS maps, formed over a broad range of thermophysical conditions, to help guide industry in improving LPBF microstructure, is provided.

Comparative analysis of pulsed Nd:YAG laser welding of 304L and 904L stainless steel

The current study investigates the comparative study of similar pulsed Nd:YAG laser welding of 304L and 904L stainless steel. In the present study both austenitic stainless steel (304L) and super austenitic stainless steel (904L) are laser welded at 5 mm/s scanning speed, 25 Hz pulse frequency and 0.75 mm laser spot diameter. The ultimate tensile strength of both laser welded specimens were equal to the respective parent metals. The percent of ductility is more in 304L than 904L laser welded specimen comparatively. The weld bead shape of the two welded specimens are V shaped for 304L and H shaped for 904L at the same laser process parameters due to difference in thermophysical properties of the two materials.

Microstructure and Mechanical Properties of Dissimilar TC4 Titanium Alloy/304 Stainless Steel Joint Using Copper Filler Wire

TC4 titanium alloy was successfully welded to 304 stainless steel using gas tungsten arc welding with a CuSi3 filler wire. The Ti/weld transition zone in the joint consisted of β-Ti, Ti2Cu, AlCu2Ti, TiCu, and Ti5Si3 intermetallic compounds (IMCs) when welded with low current. As the welding current increased, more Fe dissolved into the weld pool, and massive ternary Ti-Cu-Fe and AlCu2Ti IMCs formed in the Ti/weld transition zone. The segregated Ti5Si3 phase disappeared, and complex Fe-Ti-Si-Cr IMCs formed outside the Ti/weld transition zone. The microhardness was much higher in the Ti/weld transition zone than that in the TC4 substrate and copper weld, reaching a maximum value of 619 HV. The highest tensile strength was 328 MPa when the welding current was 120 A and the traveling speed was 4 mm/s. All joints failed through the Ti/weld transition zone, and brittle cleavage features were present on the fractured surfaces. Due to the large difference in thermophysical properties between the two base metals, cracks were easily generated, which restricted the improvement in the tensile strength of the TC4/304 SS joint.

Ultrasonic measurement of sound velocity fluctuations in biological tissue due to ultrasonic heating and estimation of thermo-physical properties.

Tissue characterization in terms of the differences in thermo-physical properties of biological tissues was investigated in this study. The objective was to measure the ratio of variation in sound velocity due to ultrasonic heating and to derive the relational expression between the ratio and thermo-physical properties. The ratio of sound velocity variation before and after the temperature rise of tissue samples exposed to ultrasound was measured by ultrasonic pulse echo method. The thermo-physical properties were estimated for a tissue-mimicking material and porcine muscle and fat tissues due to theoretical expression. The transducer for heating had a resonance frequency of 3.2MHz, and the transducer for measurement of sound velocity variation had a resonance frequency of 5.2MHz. In the phantom study, the measured values of the temperature rise agreed with the values calculated by a finite element method (FEM). The estimated values of the temperature rise from the sound velocity variations of muscle and fat tissues were 0.36°C and 1.1°C, respectively. Also, the estimated values of thermo-physical properties agreed with the reference values within an error of 10%. The thermo-physical properties of the porcine tissues were measured by sound velocity variation due to ultrasonic heating within the safety regulations.

Experimental investigation of geyser boiling in a two-phase closed loop thermosyphon with high filling ratios

The geyser boiling instability in a two-phase closed loop thermosyphon (TPCLT) is experimentally investigated through flow visualization and simultaneous measurement of pressure and temperature fluctuations. Wide ranges of filling ratios of R134a fluid from 90% to 53%, and heat flux from 20 W cm−2 to 220 W cm−2 are examined. The Power Spectrum Density (PSD) method is applied to analyze the periodicity of geyser boiling, and the parameter Standard Deviation (SD) is used to characterize the oscillating amplitude. The effects of heat flux and filling ratio on the geyser boiling occurrence and the oscillation characteristics are discussed in detail. The results show that, the flow regimes experience the consecutive variation of single-phase flow, bubbly flow, churn flow, bubbly flow, and single-phase flow within each geyser boiling cycle, leading to the fluctuation of flow and heat transfer characteristics. The geyser boiling is more liable to occur in the conditions of higher filling ratio and moderate heat flux. The initiating heat flux for the onset of geyser boiling decreases with the increase of filling ratio, but the range of heat flux for the geyser boiling occurrence is narrow under the high filling ratio conditions. The frequency of geyser boiling increases with the increase of heat flux for a certain filling ratio. With the increase of filling ratio, the oscillating frequency firstly decreases and then increases. The minimum oscillating frequency occurs at the combination of filling ratio of 76% and heat flux of 90 W cm−2 under the experimental conditions in this work. Both the fluctuation amplitude of pressure and temperature increase with the increase of heat flux, while decrease with the increase of filling ratio. Compared to R134a under the same filling ratio of 76%, the water has higher heat flux for geyser boiling occurrence, smaller oscillating amplitude of pressure, and lower oscillating frequency due to the difference in thermophysical properties.

Devolatilization and Combustion of Coarse-Sized Coal Particles in Oxy–Fuel Conditions: Experimental and Modeling Studies

A generalized unsteady-state kinetic model, coupled with all modes of heat transfer, was developed to describe the combined coal devolatilization and the subsequent combustion of the residual char under oxy–fuel condition in both O2–CO2 and O2–N2 environments. Experiments were conducted to validate the model, which was also found to predict the experimental data published in the literature well. The effect of coal particle diameter, temperature of the reactor, and oxygen concentration on devolatilization time was investigated. Peaks in devolatilization and char combustion rates and particle center temperature were studied, and the effect of different parameters assessed. Higher reaction time was observed in an O2–CO2 environment compared to that in an O2–N2 environment due to lower particle temperatures resulting from endothermic gasification reaction and the difference in thermo-physical properties. Simulation studies were carried out to generate temperature, carbon, O2, CO, and CO2 contours to understan...

Parametric study of laser welding of copper to austenitic stainless steel

Welding of copper to stainless steel is challenging because of sharp difference in thermophysical properties of materials and the presence of miscibility gap in Fe-Cu system. The parametric study of continuous Yb:YAG laser welding between copper and austenitic stainless steel 316L has been performed. The influence of laser power, welding speed and beam offset from joint line on weld composition, microstructure and tensile properties was studied. The corrosion behaviour of the welds was evaluated in 0.1M NaCl with the potentiostatic pulse testing method, salt fog and immersion tests.In function of copper dilution in the melted zone, different types of microstructure were observed: homogenous solid solution (≥2 at.% Cu), copper-rich net between austenite cells (2-5 at.% Cu) and formation of Cu-rich droplets (> 20 at.% Cu). Selective corrosion of Cu-rich microstructures took place in the melted zone. Tensile properties of the welds were determined by the formation of heat-affected zone in solid copper, where ductile fracture took place.

Coating synthesis controlled by electron-beam heating

The methods of combined electron-beam treatment of parts made of steel with one- and two-layer coatings are studied experimentally. Ti-Ni, Ni-Al and Al-Ti systems were used as the examples in the experiments. The mathematical model is suggested for coating formation in the controlled regime of high temperature synthesis during high energy source motion along the preliminarily deposited layer of exothermic composition. The study takes into account the difference in thermophysical properties of the materials of coating and substrate, heat release from chemical reaction that leads to the coating properties formation and other factors. The realization of the synthesis depends on technological parameters. Various regimes of the treatment process are investigated numerically.

Phase interface effects in the total enthalpy-based lattice Boltzmann model for solid–liquid phase change

In this paper, phase interface effects, including the differences in thermophysical properties between solid and liquid phases and the numerical diffusion across phase interface, are investigated for the recently developed total enthalpy-based lattice Boltzmann model for solid–liquid phase change, which has high computational efficiency by avoiding iteration procedure and linear equation system solving. For the differences in thermophysical properties (thermal conductivity and specific heat) between solid and liquid phases, a novel reference specific heat is introduced to improve the total enthalpy-based lattice Boltzmann model, which makes the thermal conductivity and specific heat decoupled. Therefore, the differences in thermal conductivity and specific heat can be handled by the dimensionless relaxation time and equilibrium distribution function, respectively. As for the numerical diffusion across phase interface, it is revealed for the first time and found to be induced by solid–liquid phase change. To reduce such numerical diffusion, multiple-relaxation-time collision scheme is exploited, and a special value (one fourth) for the so-called “magic” parameter, a combination of two relaxation parameters, is found. Numerical tests show that the differences in thermophysical properties can be correctly handled and the numerical diffusion across phase interface can be dramatically reduced. Finally, theoretical analyses are carried out to offer insights into the roles of the reference specific heat and “magic” parameter in the treatments of phase interface effects.

Study of XeF2 fluorination potential against Rh2O3, RuO2, ZrO2, and U3O8 for use in reactive gas recycle of used nuclear fuel

Triuranium octoxide (U3O8) and stable surrogate oxides of selected fission product species representative of a Used Nuclear Fuel (UNF) matrix typical of Light Water Reactor (LWR) service were fluorinated using an alternate, solid-phase fluorinating agent, XeF2. This fluorination reaction formed volatile and non-volatile compounds and demonstrated the possibility of chemical and thermal separations of the UNF matrix based on this approach. A series of experiments was conducted at the milligram quantity scale using a Shimadzu DTG-60 TG/DTA for testing of all non-radioactive samples and a Netzsch STA 409 TGA for testing of all radioactive samples. The fluorination and subsequent volatilization potentials were analyzed by mixing excess fluorinating agent with an oxide form of the UNF material and then heating to elevated temperatures for analysis. Thermogravimetric and differential thermal analysis allowed reaction pathways to be analyzed and suggested windows both thermally and chemically for separations of these various components. The differences in thermophysical properties of these products can be utilized as a starting point to effectively separate, isolate, and collect product streams with different compositions for further processing. The study of these chemistries could be incorporated into advanced separations methods to provide another possible solution for the long-term sustainability of nuclear energy applications.

Effect of internal nozzle flow and thermo-physical properties on spray characteristics of methyl esters

In compression ignition engines, the quality of the spray atomization significantly affects the performance and emissions of the engine. The differences in thermo-physical properties of biodiesel have significant effect on both the internal nozzle flow and spray characteristics. In this study, the internal nozzle flow and spray characteristics of three major methyl esters found in various biodiesels, namely methyl oleate, methyl stearate, and methyl linoleate were studied as a representative of different biodiesels. A new hybrid spray model developed by coupling cavitation induced spray model with KHRT model in KIVA4 CFD code was used in this study. The model was validated against diesel spray characteristics obtained from the experiments conducted in house using constant volume spray chamber and good agreement was found. The internal flow simulations shows that methyl stearate cavitates the least followed by methyl oleate and linoleate. The spray simulations shows that spray tip penetration of methyl stearate is higher than other methyl esters and diesel because of its high viscosity and large droplet diameter. Methyl linoleate was found to atomize better than other esters and comparable to diesel. However at high ambient temperature, liquid length is highly dominated by both latent heat of vaporization and viscosity.

Role of zinc coating at liquid-vapor interface during laser material processing of zinc coated steel

In laser material processing, one of the major interests is characterizing interfacial phenomena induced by thermal phase changes of materials. The interfacial characteristics in the laser processing of multi-coated materials show different behaviors compared to those of single material processing. The difference in thermo-physical properties of the coated and primary materials induces the contrasting characteristics of multiple interfacial phenomena including temperature, recoil pressure, capillary force, and thermo capillary force. The influence of coating layer to the interfacial physics evolutions is difficult to be modeled mathematically when the laser beam penetrates the multi-coated material layer by layer. This paper addresses the role of the zinc coating at the liquid-vapor interface during the laser processing of zinc coated steel, as a representative case of multi-coated materials. Computational modules incorporating the zinc layers were established and selectively applied at the locations where the zinc coatings exist to investigate the interfacial phenomena. The level set method was integrated with the modules to track the evolution of the liquid-vapor interface in a self-consistent manner. The interfacial phenomena characteristics were estimated by a 3D mathematical simulation study. A reflective topography method was employed to validate the mathematical model and to supplement our understandings of the interfacial evolution.

Crack formation in chemically resistant glass-fiber reinforced plastic pipes

The behavior of chemically resistant glass-fiber reinforced plastic pipes with a sharp change in working medium temperature, i.e., thermal shock, is studied. It is shown that a marked difference in thermophysical properties of components and layers of a glass-fiber reinforced plastic pipe leads to occurrence under these conditions of a temperature drop, and as a consequence, thermal stresses. Prolonged existence of these stresses within layers of a glass-fiber reinforced plastic lining, weakly reinforced by comparison with a structural layer, may lead to crack formation due to a difference in thermal conductivity. An equation is proposed for calculating the time for existence of a temperature drop between the operating layers of glass- fiber reinforced plastic.

Crustal compositions exposed by impact craters in the Tyrrhena Terra region of Mars: Considerations for Noachian environments

Crater ejecta blankets distinguished by differences in thermophysical properties from the target material were investigated in Tyrrhena Terra, Mars. Approximately one third of craters exhibit spectral differences from target material in Mars Global Surveyor Thermal Emission Spectrometer (TES) data. Craters that exhibit these spectral differences from the target material do not strongly correlate with phyllosilicate exposures detected in near infrared data from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM). Shock processes cannot fully explain the observed spectral trends. Rather, the subtle spectral differences observed in TES data are likely due to differences in plagioclase and/or high-silica phase abundance relative to pyroxene abundance, with the crater ejecta exhibiting lower pyroxene abundance than target materials. These mineralogic trends are consistent with several scenarios, none of which can be ruled out from existing observations: 1) vertical variations in primary lithology (considered least likely), 2) subsurface alteration exposed by impact, 3) syn- or post-impact alteration associated with the impact process, such as devitrification of melt glass materials or alteration of highly shocked materials, and 4) light surface alteration that preferentially altered/removed plagioclase and/or high-silica glass relative to pyroxene and subsequent exposure of less-altered materials from the subsurface by impact. Scenario 2 is consistent with phyllosilicate detections; Scenarios 1, 3 and 4 are consistent with CRISM phyllosilicate detections in some impact craters if phyllosilicate mineral abundance is volumetrically small. Results from this work suggest that some signatures of alteration may be escaping detection by near infrared measurements; alteration environments would be best interpreted through a combination of near infrared and thermal infrared data sets, rather than near infrared detections alone.

Modeling of the transport phenomena in GMAW using argon–helium mixtures. Part I – The arc

This article presents a numerical investigation on the transient transport phenomena in the arc which include the arc plasma generation and interactions with moving droplets and workpiece for pure argon and three argon–helium mixtures (75% Ar + 25% He, 50% Ar + 50% He, and 25% Ar + 75% He) during the gas metal arc welding (GMAW) process. The results indicate that the arcs in various shielding gases behave very differently due to the significant differences in thermophysical properties, including the ionization potential and the electrical conductivity, thermal conductivity, specific heat, and viscosity at high temperatures. For the same welding power input, it was found the increase of helium content in the mixtures results in (1) the change of plasma arc shape from bell-like to cone-like and (2) the change of arc pressure distribution along the workpiece surface from Gaussian-like to flat-top with decreasing peak value. Detailed explanations to the physics of the very complex but interesting transport phenomena are given.

Measurement of Al Freezing-Point Temperature: Effect of Initiation Process

In CCT documents it is stated that “...for the freezing curves of the metallic fixed points, the maximum observed temperature on the plateau should be taken as the best approximation of the liquidus temperature. The fixed points should be realized with the inner and outer liquid-solid interfaces and extend past the maximum by 10 % to 20 % of the fraction frozen, to clearly establish the value of the maximum and the resolution of its determination.” Also, it is accepted that “...the inner interface is essentially static. It is the temperature of the inner liquid/solid interface that is measured by the thermometer.” The analysis of freezing curves obtained by the standard method of fixed-point realization shows that the parameters of the initial part of the freezing curve, the mean temperature value of which is usually taken as the liquidus temperature, depend on how the inner interface is initiated. Variations in the duration and intensity of initiation cause changes in the initial part of the freezing curve and in the resulting SPRT measurement. Moreover, the relation between the duration of the initial section of the plateau with a minor temperature change and the duration of its final section with a significant slope also depend on the initiation method used and on the furnace temperature. The effect of freezing initiation conditions on the measurement result is individual for each fixed point because of the differences in thermophysical properties of metals and in conditions of the heat transfer from the liquid–solid interface to the thermometer. Aluminum has a maximum value of the melting specific heat in comparison with other metals used in ITS-90 fixed points; in the present study, the effect of the intensity and duration of the inner liquid–solid interface initiation was investigated both experimentally and through calculation.

PURE NH3 AND CO2 HEAT TRANSFER AND PRESSURE DROP IN HORIZONTAL SMOOTH TUBES FOR NH3 TWO-STAGE AND NH3/CO2 CASCADE REFRIGERATION SYSTEMS: A REVIEW

This paper presents a review of differences and similarities of in-tube heat transfer and pressure drop between ammonia (NH3) and carbon dioxide (CO2) from the perspective of the design of heat exchangers for NH3 two-stage and CO2/NH3 cascade refrigeration systems. The focus is on differences in thermophysical properties and thus different characteristics of heat transfer and pressure drop. A brief summary of published literatures about CO2/NH3 cascade refrigeration systems is provided and literature review of available correlations and developed correlations are presented for flow boiling and condensation heat transfer and pressure drop. Because of large deviation of calculated values with exiting correlations from measured results, a new correlation to predict flow condensation heat transfer coefficients was developed based on experimental results for CO2 at -15°C. From comparison of measured and predicted values, it is shown that some correlations, previously published in open literature, can be used to calculate flow boiling heat transfer coefficients for NH3 at -20°C, if a flow pattern can be appropriately determined for a flow condition. Also, it is presented that existing correlations can predict well the heat transfer coefficients for CO2 flow boiling at -15 and -30°C. It is shown that some correlations can predict pressure drop relatively well for NH3 and CO2 two-phase flow. The NH3 and CO2 flow evaporation heat transfer and pressure drop characteristics at -40°C are compared with predicted values.

NH 3 in-tube condensation heat transfer and pressure drop in a smooth tube

This paper presents an overview of the issues and new results for in-tube condensation of ammonia in horizontal round tubes. A new empirical correlation is presented based on measured NH 3 in-tube condensation heat transfer and pressure drop by Komandiwirya et al. [Komandiwirya, H.B., Hrnjak, P.S., Newell, T.A., 2005. An experimental investigation of pressure drop and heat transfer in an in-tube condensation system of ammonia with and without miscible oil in smooth and enhanced tubes. ACRC CR-54, University of Illinois at Urbana-Champaign] in an 8.1 mm aluminum tube at a saturation temperature of 35 °C, and for a mass flux range of 20–270 kg m −2 s −1. Most correlations overpredict these measured NH 3 heat transfer coefficients, up to 300%. The reasons are attributed to difference in thermophysical properties of ammonia compared to other refrigerants used in generation and validation of the correlations. Based on the conventional correlations, thermophysical properties of ammonia, and measured heat transfer coefficients, a new correlation was developed which can predict most of the measured values within ±20%. Measured NH 3 pressure drop is shown and discussed. Two separated flow models are shown to predict the pressure drop relatively well at pressure drop higher than 1 kPa m −1, while a homogeneous model yields acceptable values at pressure drop less than 1 kPa m −1. The pressure drop mechanism and prediction accuracy are explained though the use of flow patterns.

Pool Boiling Heat Transfer in Microgravity. Experiments with CFC-113 and Water Utilizing a Drop Shaft Facility.

Microgravity pool boiling experiments were carried out utilizing the drop shaft at the Japan Microgravity Center, which enables control of microgravity to within one-ten-thousandth of terrestrial gravity for a period of 10 seconds. Vapor bubbles were generated on a Joule-heated stainless steel or ITO film backed with a thin alumina or glass substrate, which was immersed in a pool of subcooled or saturated liquid at atmospheric pressure. The CHF (critical heat flux) to CFC-113 was lowered, in microgravity, to one-fifth of that achieved under the normal terrestrial condition. In contrast, the CHF to water remained more than one-half that achieved under the terrestrial condition. This difference in the gravity dependence of CHF is presumably ascribable to the difference in the frequency of bubble detachment from the heater surface between CFC-113 and water in microgravity, which is dependent on the difference in thermophysical properties, particularly the surface tension and the heat of vaporization, between the two fluids.

Heat and metal transfer in gas metal arc welding using argon and helium

This article describes a theoretical investigation on the arc parameters and metal transfer in gas metal arc welding (GMAW) of mild steel using argon and helium shielding gases. Major differences in the predicted arc parameters were determined to be due to large differences in thermophysical properties. Various findings from the study include that an arc cannot be struck in a pure helium atmosphere without the assistance of metal vapor, that a strong electromagnetic cathode force affects the fluid flow and heat transfer in the helium arc, providing a possible explanation for the experimentally observed globular transfer mode and that the tapering of the electrode in an argon arc is caused by electron condensation on the side of the electrode.