Temperature-dependent Thermophysical Parameters Research Articles

Sensitivity of a Process for Heating Thin Metal Film Described by the Dual-Phase Lag Equation with Temperature-Dependent Thermophysical Parameters to Perturbations of Lag Times

In the paper, an equation with two delay times (dual-phase lag Equation (DPLE)) in a version that takes into account the dependence of thermophysical parameters (volumetric specific heat and thermal conductivity) on temperature is considered. In particular, an analysis of the sensitivity of transient temperature field in relation to disturbances in delay times (the relaxation and thermalization times) is performed. The sensitivity model concerns the process of heating an ultrathin metal layer with a laser beam. First, the equation with two delay times in the case of temperature-dependent thermophysical parameters is presented. Next, the sensitivity equations with respect to delay times are derived using the direct method. The algorithms for solving the basic and sensitivity tasks are also briefly presented. At the stage of computations, an authorial program based on the implicit scheme of a finite-difference method is developed. In the final part of the paper, examples of numerical solutions (for layers made from gold and nickel) are presented. The research conducted here shows that disturbances in the temperature field are clearly visible and depend, on the one hand, on the thermophysical parameters of the material, and on the other hand, on the intensity of heating with an external heat source.

Atomistic-Continuum Study of an Ultrafast Melting Process Controlled by a Femtosecond Laser-Pulse Train.

In femtosecond laser fabrication, the laser-pulse train shows great promise in improving processing efficiency, quality, and precision. This research investigates the influence of pulse number, pulse interval, and pulse energy ratio on the lateral and longitudinal ultrafast melting process using an experiment and the molecular dynamics coupling two-temperature model (MD-TTM model), which incorporates temperature-dependent thermophysical parameters. The comparison of experimental and simulation results under single and double pulses proves the reliability of the MD-TTM model and indicates that as the pulse number increases, the melting threshold at the edge region of the laser spot decreases, resulting in a larger diameter of the melting region in the 2D lateral melting results. Using the same model, the lateral melting results of five pulses are simulated. Moreover, the longitudinal melting results are also predicted, and an increasing pulse number leads to a greater early-stage melting depth in the melting process. In the case of double femtosecond laser pulses, the pulse interval and pulse energy ratio also affect the early-stage melting depth, with the best enhancement observed with a 2 ps interval and a 3:7 energy ratio. However, pulse number, pulse energy ratio, and pulse interval do not affect the final melting depth with the same total energies. The findings mean that the phenomena of melting region can be flexibly manipulated through the laser-pulse train, which is expected to be applied to improve the structural precision and boundary quality.

Numerical Study of Heat Transfer in a Gun Barrel Made of Selected Steels

The results of numerical simulations of transient heat transfer in the barrel wall of a 35 mm caliber cannon for a single shot and the sequences of seven shots and sixty shots for chosen barrel steels are presented. It was assumed that the cannon barrel was made of one of the three types of steel: 38HMJ (1.8509), 30HN2MFA and DUPLEX (1.4462). To model the thermal phenomena in the barrel, the barrel wall material was assumed to be homogeneous and the inner surface of the barrel had no protective chromium or nitride layer. The calculations were made for temperature-dependent thermophysical parameters, i.e., thermal conductivity, specific heat and thermal expansion (in the range from RT up to 1000 °C) of the selected barrel steels. A barrel with a total length of 3150 mm was divided into 6 zones (i = 1, …, 6) and in each of them, the heat flux density was calculated as a function of time q˙i(t) on the inner surface of the barrel. Using lumped parameter methods, an internal ballistic code was developed to compute in each zone the heat transfer coefficient as a function of time hi(t) and bore gas temperature as a function of time Tg(t) to the cannon barrel for given ammunition parameters. A calculation time equaling 100 ms per single shot was assumed. The results of the calculations were obtained using FEM implemented in COMSOL Multiphysics ver. 5.6 software.

Modeling Thermal Behavior of Mass Concrete Structures at Early Age

In this study, a three-dimensional (3D) mathematical model based on conduction heat transfer in a solid is developed to predict the temperature profile of large concrete elements during the hydration process at early ages. Four different concrete mix designs are tested in the laboratory and on site. Heat of hydration and activation energy of each concrete mix is determined in the laboratory and temperature dependent thermo-physical parameters are considered in the simulations. The transient 3D conduction heat transfer equation with a heat generation source term is discretized by using finite volume method. The mathematical model is validated with temperature-time histories measured from four 1.8 m cube concrete blocks that are cast on site. Results obtained from the 3D model show that variable thermal properties are considered to accurately predict the temperature gradients. According to the analysis, the numerical model demonstrates that it can produce accurate predictions for temperature analysis within the 1.8 m large concrete elements.

Numerical Simulation and Experimental Study on Residual Stress in the Curved Surface Forming of 12CrNi2 Alloy Steel by Laser Melting Deposition.

The performance and service life of the nuclear emergency diesel engine shaft made of 12CrNi2 alloy steel is very important for the safety of nuclear power. Laser melting deposition (LMD) is a challenging camshaft-forming technology due to its high precision, rapid prototyping, and excellent parts performance. However, LMD is an unsteady process under the local action of laser, especially for curved surface forming, which is more likely to generate large residual stress on components, resulting in cracks and other defects. At present, the stress research on LMD curved surface forming is relatively insufficient. In the present paper, material parameter testing, high-temperature mechanical properties analysis, single-track sample preparation, and heat source checks are conducted. At the same time, the ABAQUS software and the DFLUX heat source subroutine are used to compile the curved double-ellipsoidal moving heat source, and the effects of the temperature-dependent thermophysical parameters and phase change latent heat on the temperature field are considered. A three-dimensional finite element model is established to analyze the thermal stress evolution and residual stress distribution of multi-track multi-layer on a curved surface by LMD, and the effect of the scanning method and interlayer cooling time on the residual stress of the formed components is studied. The results show that with the increase in temperature, the strength of the material reduces, and the fracture morphology of the material gradually transitions from ductile fracture to creep fracture. The material parameters provide a guarantee for the simulation, and the errors of the width and depth of the melt pool are 4% and 9.6%, respectively. The simulation and experiment fit well. After cooling, the maximum equivalent stress is 686 MPa, which appears at the junction of the substrate and the deposited layer. The larger residual stress is mainly concentrated in the lower part of the deposited layer, where the maximum circumferential stress and axial stress are the tensile stress. Compared with the axial parallel lap scanning method, the arc copying lap scanning method has a relatively smaller maximum thermal stress and residual stress after cooling. The residual stress in the deposited layer is increased to some extent with the increase in the interlayer cooling time.

Effect of Interactions Among Elements on Diffusion Process Associated with γ′ Coarsening in a Ni-Based Single-Crystal Superalloy

Coarsening of cuboidal γ′ precipitates and relevant diffusion process in Ni-based single-crystal superalloy CMSX-4 were investigated at 1000, 1020 and 1040 °C for specific times. The γ′ coarsening kinetics followed a cubic rate law with time and was presumably controlled by bulk diffusion of elements in γ matrix. The associated diffusion activation energy was experimentally determined to be about 300 kJ/mol when it is considered the temperature-dependent thermo-physical parameters in modified Lifshitz–Slyozov–Wagner theory. The influence of temperature on γ/γ′ microstructure is briefly discussed based on pseudo-binary [Ni]–[Al] phase diagram. Interactions among elements can effectively raise the local vacancy formation and vacancy–atom exchange barriers close to γ- and γ′-partitioning elements, respectively. Thus, it can significantly reduce the inter-coupling migrations of atoms during the macroscopic cross-diffusion process associated with γ′ coarsening of Ni-based superalloys.

Ab initio modeling of nonequilibrium electron-ion dynamics of iron in the warm dense matter regime

The spatiotemporal electron and ion relaxation dynamics of iron induced by femtosecond laser pulses was studied using a one-dimensional two-temperature model (1D-TTM) where electron and ion temperature-dependent thermophysical parameters such as specific heat ($C$), electron-phonon coupling ($G$), and thermal conductivity ($K$) were calculated with ab initio density-functional-theory (DFT) simulations. Based on the simulated time evolutions of electron and ion temperature distributions [${T}_{e}(x,t)$ and ${T}_{i}(x,t)$], the time evolution of x-ray absorption near-edge spectroscopy (XANES) was calculated and compared with experimental results reported by Fernandez-Pa\~nella et al., where the slope of XANES spectrum at the onset of absorption ($s$) was used due to its excellent sensitivity to the electron temperature. Our results indicate that the ion temperature dependence on $G$ and $C$, which is largely neglected in the past studies, is very important for studying the nonequilibrium electron-ion relaxation dynamics of iron in warm dense matter (WDM) conditions. It is also shown that the $1/s$ behavior becomes very sensitive to the thermal gradient profile, in other words, to the values of $K$ in a TTM simulation, for target thickness of about two to four times the mean free path of conduction electrons. Our approach based on 1D-TTM and XANES simulations can be used to determine the optimal combination of target geometry and laser fluence for a given target material, which will enable us to tightly constrain the thermophysical parameters under electron-ion nonequilibrium WDM conditions.

Thermal dynamic behavior during selective laser melting of K418 superalloy: numerical simulation and experimental verification

During selective laser melting (SLM) of K418 powder, the influence of the process parameters, such as laser power P and scanning speed v, on the dynamic thermal behavior and morphology of the melted tracks was investigated numerically. A 3D finite difference method was established to predict the dynamic thermal behavior and flow mechanism of K418 powder irradiated by a Gaussian laser beam. A three-dimensional randomly packed powder bed composed of spherical particles was established by discrete element method. The powder particle information including particle size distribution and packing density were taken into account. The volume shrinkage and temperature-dependent thermophysical parameters such as thermal conductivity, specific heat, and other physical properties were also considered. The volume of fluid method was applied to reconstruct the free surface of the molten pool during SLM. The geometrical features, continuity boundaries, and irregularities of the molten pool were proved to be largely determined by the laser energy density. The numerical results are in good agreement with the experiments, which prove to be reasonable and effective. The results provide us some in-depth insight into the complex physical behavior during SLM and guide the optimization of process parameters.

Laser surface remelting of WC–12Co coating: finite element simulations and experimental analyses

A three-dimensional finite element model taking into account interfacial topography, porosity of plasma sprayed coatings, temperature dependent thermophysical parameters and phase change has been developed to simulate multipass laser remelting process. Temperature evolution, temperature gradient, melting pool shape and dimensions are simulated. The laser remelting experiments are carried out on the plasma sprayed WC–12Co coatings. The simulated results are in good agreement with the experimental measurements. After laser remelting, a denser and more homogenous coating is obtained. The microhardness of the coating is significantly enhanced owing to the dispersion strengthening, the fine grain strengthening and the solution strengthening, which is increased by three times compared with that of the plasma sprayed coatings.

Laser thermal effect on silicon nitride ceramic based on thermo-chemical reaction with temperature-dependent thermo-physical parameters

In this study, a two-dimensional thermo-chemical reaction model with temperature-dependent thermo-physical parameters on Si3N4 with 10ns laser was developed to investigate the ablated size, volume and surface morphology after single pulse. For model parameters, thermal conductivity and heat capacity of β-Si3N4 were obtained from first-principles calculations. Thermal-chemical reaction rate was fitted by collision theory, and then, reaction element length was deduced using the relationship between reaction rate and temperature distribution. Furthermore, plasma absorption related to energy loss was approximated as a function of electron concentration in Si3N4. It turned out that theoretical ablated volume and radius increased and then remained constant with increasing laser energy, and the maximum ablated depth was not in the center of the ablated zone. Moreover, the surface maximum temperature of Si3N4 was verified to be above 3000K within pulse duration, and it was much higher than its thermal decomposition temperature of 1800K, which indicated that Si3N4 was not ablated directly above the thermal decomposition temperature. Meanwhile, the single pulse ablation of Si3N4 was performed at different powers using a TEM00 10ns pulse Nd:YAG laser to validate the model. The model showed a satisfactory consistence between the experimental data and numerical predictions, presenting a new modeling technology that may significantly increase the accuracy of the predicated results for laser ablation of materials undergoing thermo-chemical reactions.

Three-dimensional analysis and design of petal-like mould for continuous casting of steels

Understanding the shrinkage behaviour of a steel billet is very important for designing the continuous casting mould, and conversely, a well designed mould is beneficial when matching the shrinkage behaviour of the billet. The shrinkage behaviour of a billet is hard to measure in situ during continuous casting but can be calculated by numerical simulation. A three-dimensional finite element model has been built to simulate the thermal and stress fields of the billet in the mould. The dynamic thermal boundary condition, the effect of ferrostatic pressure and the temperature dependent thermophysical parameters have been considered in the model. The shrinkage of billet when considering ferrostatic pressure is on average 0·08 mm smaller than when not considering ferrostatic pressure. The temperature and stress distributions are analysed in the present paper, and based on this analysis, a novel petal-like mould was designed and its taper determined. The designed mould has been tested in industrial practice showing better lifetime and billet quality.

A new variant of temporary temperarure field correction method

The paper presents the possibilities of numerical solution of the non-linear boundary-initial problem described by the Fourier equation. In particular, the equation con- taining the temperature-dependent thermophysical parameters (volumetric specific heat and thermal conductivity) is considered. The problem presented in this paper is connected with the artificial linearization of the task discussed (at the stage of numerical computations), in other words, the new numerical procedure which allows one to remodel the solution ob- tained for linear problem at the time level t t + � to the other solution corresponding to non- linear one. The procedure discussed can be a very effective supplement for different vari- ants of the boundary element method which, as a rule, requires a linear form of the energy equation. In the final part the examples of numerical simulations and the conclusions can be found.

Effects of processing parameters on the temperature field of selective laser melting metal powder

Based on temperature-dependent thermophysical parameters and nonlinear phase transformation from powder to liquid, a three-dimensional transient thermal finite model of selective laser melting process is developed. Commercial ANSYS software is used to simulate the distribution of the temperature field in selective laser melting. The simulation results show that a lower scan speed, higher laser power, and a lower scan interval enable much higher maximum temperature in the powder bed and wider scan track width. The scan modes also have great influence on the temperature field. Temperature in the powder bed experiences fluctuant variation with transverse scan mode in contrast with moderate variation with lengthwise scan mode. The simulated results are compared with the experimental results and good agreement is found in scan track width.

Experimental and theoretical investigation of cumulative laser irradiation effects in YBCO thin film pulsed laser deposition

Compositional and morphological modifications of superconducting YBCO targets under prolonged laser irradiation were investigated by EDS and SEM. Rotated and fixed targets were ablated at different process parameters - laser fluence and oxygen environment. Optical emission spectroscopy was employed for investigation of the evolution of the laser-induced plasma plume. The deposited film thickness distribution over the substrate surface area was examined through the film absorbance. The results can be consistently explained by assuming the existence of an additional effective ablation threshold imposed by the modified surface relief. A one-dimensional theoretical model of the laser beam-target interaction was developed involving temperature-dependent thermophysical parameters and the laser-induced plasma absorption. Reasonable agreement between the experimental and theoretical dependencies of the eroded depth per shot on the pulse energy is found. The model results show a possible influence of the post-pulse evaporation on the target surface stoichiometric deviation found in the experiments.