Thermal Finite Element Simulation Research Articles

Advancing efficiency and reliability in thermal analysis of laser powder-bed fusion

In laser based powder-bed fusion of metals (PBF-LB/M), parts are fabricated by melting layers of powder using a high-intensity laser beam. During this process, the material is exposed to rapid cooling rates and intense thermal gradients, which are the underlying causes of residual stress formation and development of a unique microstructure in these components. Therefore, understanding the heat transfer phenomenon and reliably representing exposed temperature profiles in simulation frameworks are prerequisites for studying the microstructure and residual stress development during the PBF-LB/M process. This work employs a combination of experimental measurements and model development to study this phenomenon. Thermal properties of Hastelloy X were measured in the as-deposited state and used to setup finite element (FE) thermal simulations of the PBF-LB/M process. In addition, in-situ temperature evolutions near the laser tracks were measured by instrumenting thin-wall structures with K-type thermocouples in a two-stage fabrication process. The gathered data was used to calibrate uncertain modelling parameters, and ultimately, the simulation framework could closely represent the measured temperature histories. To address the high computational cost of FE thermal simulations, an adaptive-local/global multiscale modelling approach was proposed, which substantially reduced computation times without compromising the accuracy of the results. The modelling files and scripts are available in github.

A close look at temperature profiles during laser powder bed fusion using operando X-ray diffraction and finite element simulations

In laser powder bed fusion (LPBF), complex components are manufactured layer-by-layer via scanning the cross-sections of a 3D CAD model using a high intensity laser. Throughout this process, the material is exposed to temperature profiles that significantly differ from conventional manufacturing methods, and result in development of a unique and inhomogeneous microstructure and high levels of residual stresses in additively fabricated parts. The large temperature gradients and rapid cooling rates around the moving laser spot, and the overall heterogeneity of the temperature field need to be better understood in order to optimize the process parameters for increased production quality. In this study, operando X-ray diffraction (XRD) was employed to measure and compare temperature histories on the laser path under various processing conditions for Hastelloy X. Finite element thermal simulations were validated based on the acquired XRD data and then used as a supplementary tool to discuss the cooling behaviour and thermal heterogeneities across the geometry. The increase in the deposited energy density was qualitatively linked with higher temperature levels and slower cooling rates during LPBF. The melt-pool lengths showed strong sensitivity to the laser power and little variation with the scanning speed. Furthermore, even for a single set of parameters, large variations in the temperature field within the build were observed such that the cross-section edges located at higher build layers were exposed to markedly higher temperature levels.

Design and simulation of 1.5 T conduction-cooled superconducting magnet with flip-over capability

The Compact Torus Injection (CTI) system is a plasma physics device used to create and study compact torus plasmas, which are dense and highly energetic structures with potential applications in fusion energy research and space propulsion. The Experimental Advanced Superconducting Tokamak compact torus injection (EAST-CTI) system is a device used for plasma heating and fueling in fusion experiments. In this paper, a 1.5 T cryocooled superconducting magnet was developed for the EAST-CTI system, which requests the magnet system can work vertically or horizontally to meet different working conditions. The design of the superconducting magnet and cryogenic system is based on the analysis of the thermal and mechanical characteristics of the magnet system. Thermal and mechanical finite element simulation were used to support the design. The 1.5 T magnet superconducting magnet was manufactured according to the design and simulation works. After 35 h cooling, the superconducting magnet stabilized at 3.87 K, and successfully provided a horizontal magnetic field environment for the experiments on EAST-CTI system.

Predicting Maximum Effective Temperatures and Thermal Gradients for Steel I-Girder in Canadian Climate Regions

The constant fluctuation of thermal loads on steel members, especially during construction, causes non-uniform distributions of temperatures, resulting in possible constructional and structural defects leading to unfavorable thermally induced responses and potential safety risks. The Canadian Highway Bridge Design Code (CHBDC) provides one thermal gradient variation profile without accounting for the differences in the geometrical parameters of the steel members and the variations in the climate regions of Canada. Therefore, in this study, three-dimensional finite element (FE) thermal simulations were conducted to investigate the maximum effective temperatures and positive vertical thermal gradients for different Canadian climate regions. Parametric studies were performed to conduct the FE thermal analysis using the thermal model validated in ANSYS. The comprehensive study results showed that Canada could be divided into two main zones for vertical thermal gradient calculations, meaning that one stationary thermal gradient profile cannot be applicable to all climate regions of Canada, as recommended by the CHBDC. Based on the FE thermal analysis results, empirical formulas as a function of the significant parameters were proposed to predict the maximum effective temperature and thermal gradient variations of the steel I-girder. The predicted maximum effective temperature and thermal gradient variation values were found to be highly correlated with the FE values with coefficients of determination R2 of approximately 0.97 and 0.98, respectively.

Study on the Solder Joint Reliability of New Diamond Chip Resistors for Power Devices

New diamond chip resistors have been used in high-power devices widely due to excellent heat dissipation and high-frequency performance. However, systematic research about their solder joint reliability is rare. In this paper, a related study was conducted by combining methods between numerical analysis and laboratory reliability tests. In detail, the shape simulation and thermal cycling finite element simulation for solder joints with different volumes were carried out. The optimized solder volume was 0.05 mm3, and the maximum thermal cycling stress under the optimized shape was 38.9 MPa. In addition, the thermal cycling tests with current and high temperature storage tests were carried out for the optimized solder joint, which showed good agreement with the simulation results, clarified the growth and evolution law of intermetallic compound at the interconnection interface, and proved the optimized solder joint had great anti-electromigration, temperature cycling and high temperature storage reliability. In this work, an optimized solder joint structure of a diamond chip resistor with high reliability was finally obtained, as well as providing considerable reliability data for the new type of diamond chip resistors, which would boost the development of power devices.

Optimization of a phase change material enhanced low-concentration photovoltaic/thermal module

This paper presents a simulation and optimization study of a nanoparticle-enhanced phase change material (PCM) assisted low-concentration photovoltaic/thermal (LCPV/T) module. An optical, electrical, and thermal model coupling finite element simulation is developed, with the thickness of the PCM, the mass fraction of nanoparticles in the heat transfer fluid and PCM, and the velocity of the heat transfer fluid as optimization parameters. To improve the efficiency of the optimization process, a response surface methodology is used to investigate the polynomial relationship between the parameters and module performance. The significance level and average error of each polynomial are found to be lower than 0.05 and 2.5%, respectively, indicating their statistical significance and accurate prediction of module performance. The fitting polynomials are used to optimize and analyze the module, and the results show maximum values of output thermal energy, output thermal exergy, output electrical energy/exergy, total output energy, and total output exergy of 573.11 W, 13.05 W, 110.40 W, 869.93 W, and 121.08 W, respectively. Comparison with the previously designed module revealed that the newly designed module demonstrated better electrical output performance, providing valuable guidance for the energy supply of zero-carbon buildings.

A macroscale thermal simulation strategy with track-scale resolution for laser powder bed fusion

Numerical modelling is an effective tool to investigate the rapid temperature cycling in laser powder bed fusion. However, the high computational cost limits the modelling of part-scale components due to the significantly different scales between the highly local energy input region and the global field response region. This study presents a macroscale finite element thermal simulation strategy with a track-scale resolution by combining the recently developed characteristic time-based heat input (CTI) model with a currently proposed adaptive mesh re-mapping method. Due to the uniformly distributed power density along the scan direction in the CTI model, the optimized mesh strategy allows one element to capture the temperature solutions over the majority of a scan track except at the two ends. The adaptive mesh re-mapping method is used to map simulated results from a previous scan track model to a current scan track model with a new mesh configuration. This strategy has been implemented for a Ti-6Al-4V part with the dimensions of 20×40×3mm3. The predicted temperature histories agree well with the in-situ measurements at the points in both the printed part and powder bed. Three case studies demonstrate that our strategy can improve computational efficiency between 6 and 30 times.

A Hybrid-Excitation Synchronous Motor with a Change in Polarity

The hybrid-excitation permanent magnet (HEPM) motor is a synchronous motor characterized by a rotor that includes both permanent magnets and excitation windings. Thus, an active change in the magnetic flux by means of the excitation current is possible. In particular, the hybrid-excited rotor studied in this paper can be used to change the number of poles of the rotor itself. At the same time, the stator winding is also designed to perform a polarity change. The change in polarity allows users to obtain different torque versus speed characteristics with the same motor. In particular, a configuration with a lower pole number exhibits low torque at high speeds, while a configuration with a higher pole number produces high torque at low speeds. In this way, a single HEPM motor behaves like two different machines, extending the usual operating speed range of synchronous motors. In this paper, an HEPM rotor configuration is designed, and its performance is analyzed through finite element electromagnetic and thermal simulations.

Experimental and Finite Element Analysis of Butt Weld of Double-layer Steel Plates

This paper aims to study the weldability of butt weld of double-layer steel plates. Experiments for three groups of specimens subjected to various parameters are first conducted to obtain angular deformation, residual stress and temperature data. Then, 3-D finite element (FE) thermal simulation of butt welding is carried out to obtain the load-displacement curves under different included angles and weld length. Moreover, the butt welded steel plate welds were parametric analyzed for the stress concentration factor and the formula for the stress concentration factor was fitted according to the data. The analytical results have shown that double-layer butt welded steel plates with better performance compared to single-layer butt welded steel plates, and the proposed stress concentration factor formula predicts well and thus has certain reference value.

Simulation-guided variable laser power design for melt pool depth control in directed energy deposition

Melt pool control is essential in metal additive manufacturing processes since the melt pool geometry directly affects the geometric accuracy and material properties of the fabricated part. While online control of 2D melt pool size has been successfully achieved, controlling the melt pool depth, which cannot be directly monitored during the process, is still an outstanding issue. In this study, a simulation-guided process design framework is developed which enables the possibility of defining a time-series laser power profile that achieves a desired melt pool depth across an arbitrary geometry using just one simulation. This is accomplished by implementing a proportional-integral controller in a thermal finite element simulation that utilizes the laser power to control the melt pool depth. The resulting laser power profile from the simulation can then be used in a real deposition process. Example cases with uniform and site-specific melt pool depth control using the developed method are presented. The results show that melt pool depth can be successfully controlled within the simulation and the developed method can be used to design a laser power profile which can be implemented in an arbitrary part to enhance geometric accuracy and control material properties within a part.

Invisible Thermoplasmonic Indium Tin Oxide Nanoparticle Ink for Anti-counterfeiting Applications.

In this study, we present a thermoplasmonic transparent ink based on a colloidal dispersion of indium tin oxide (ITO) nanoparticles, which can offer several advantages as anti-counterfeiting technology. The custom ink could be directly printed on several substrates, and it is transparent under visible light but is able to generate heat by absorption of NIR radiation. Dynamic temperature mapping of the printed motifs was performed by using a thermal camera while irradiating the samples with an IR lamp. The printed samples presented fine features (in the order of 75 μm) and high thermal resolution (of about 250 μm). The findings are supported by thermal finite-element simulations, which also allow us to explore the effect of different substrate characteristics on the thermal readout. Finally, we built a demonstrator comprising a QR Code invisible to the naked eye, which became visible in thermal images under NIR radiation. The high transparency of the printed ink and the high speed of the thermal reading (figures appear/disappear in less than 1 s) offer an extremely promising strategy toward low-cost, scalable production of photothermally active invisible labels.

Effect of Finite Element Mesh Size and Time-Increment on Predicting Part-Scale Temperature for Powder Bed Fusion Process

Simulating powder bed fusion processes (PBF) can reveal temperature evolution in transient mode. Accurate temperature prediction using finite element (FE) method demands both mesh and timeincrements to be very small; thus, requiring a high computational cost. To avoid this, in part-scale simulation, coarse meshes representing multiple powder layers added at once, are usually used which results in fast solving of FE models. Powder layers and time increments are lumped in such a configuration, which results in a deviation of the temperature history. This research proposes a methodology to predict the nodal temperature (NT) due to the combined effect of space and time lumping for part-scale FE thermal simulation for PBF processes. It shows its effects in predicting both the local temperature history and the average far-field temperature.

Comparative Evaluation of the Two Multilevel Concepts With Full ZVS Operation Employing WBG Devices for Use in 1500-V PV Systems

A multi-level boost topology based on a flying capacitor is compared in this work with the hybrid, multi-level, partial power processing topology already presented in the literature for use as a highly efficient and compact dc/dc stage in 1500-V, two-stage, grid-connected PV systems. Different variations of the resonant stage of the hybrid topology are analyzed and compared in terms of losses, volume and complexity. Full multivariable optimization is conducted for the multi-level boost topology with different number of levels and for all the analyzed variations of the resonant stage of the hybrid topology. Appropriately rated new classes of 650-V and 900-V WBG devices are included in the optimization algorithm. It is concluded that due to the dominant conduction losses and gap in the voltage rating of the commercially available WBG devices between 650V and 200V, increase of the number of levels over 4 leads to the deteriorated loss and volume performances. All the analysis related to the resonant stage of the hybrid topology is confirmed by detailed and comprehensive measurements of two prototypes of 4kW of nominal and 7kW of maximal power. Additionally, all the loss models applied in the optimization algorithms presented in this paper are confirmed by detailed measurements, thermal calibration and finite element thermal simulations of the wide bandgap devices and inductor designs employed in this work.

Multiscale simulation of powder-bed fusion processing of metallic alloys

We present a computational framework for the simulations of powder-bed fusion of metallic alloys, which combines: (1) CalPhaD calculations of temperature-dependent alloy properties and phase diagrams, (2) macroscale finite element (FE) thermal simulations of the material addition and fusion, and (3) microscopic phase-field (PF) simulations of solidification in the melt pool. The methodology is applied to simulate the selective laser melting (SLM) of an Inconel 718 alloy using realistic processing parameters. We discuss the effect of temperature-dependent properties and the importance of accounting for different properties between the powder bed and the dense material in the macroscale thermal simulations. Using a two-dimensional longitudinal slice of the thermal field calculated via FE simulations, we perform an appropriately-converged PF solidification simulation at the scale of the entire melt pool, resulting in a calculation with over one billion grid points, yet performed on a single cluster node with eight graphics processing units (GPUs). These microscale simulations provide new insight into the grain texture selection via polycrystalline growth competition under realistic SLM conditions, with a level of detail down to individual dendrites.

QUBIC VI: Cryogenic half wave plate rotator, design and performance

Setting an upper limit or detection of B-mode polarization imprinted by gravitational waves from Inflation is one goal of modern large angular scale cosmic microwave background (CMB) experiments around the world. A great effort is being made in the deployment of many ground-based, balloon-borne and satellite experiments, using different methods to separate this faint polarized component from the incoming radiation. QUBIC exploits one of the most widely-used techniques to extract the input Stokes parameters, consisting in a rotating half-wave plate (HWP) and a linear polarizer to separate and modulate polarization components. QUBIC uses a step-by-step rotating HWP, with 15° steps, combined with a 0.4°s-1 azimuth sky scan speed. The rotation is driven by a stepper motor mounted on the cryostat outer shell to avoid heat load at internal cryogenic stages. The design of this optical element is an engineering challenge due to its large 370 mm diameter and the 8 K operation temperature that are unique features of the QUBIC experiment. We present the design for a modulator mechanism for up to 370 mm, and the first optical tests by using the prototype of QUBIC HWP (180 mm diameter). The tests and results presented in this work show that the QUBIC HWP rotator can achieve a precision of 0.15° in position by using the stepper motor and custom-made optical encoder. The rotation induces <5.0 mW (95% C.L) of power load on the 4 K stage, resulting in no thermal issues on this stage during measurements. We measure a temperature settle-down characteristic time of 28 s after a rotation through a 15° step, compatible with the scanning strategy, and we estimate a maximum temperature gradient within the HWP of ≤ 10 mK. This was calculated by setting up finite element thermal simulations that include the temperature profiles measured during the rotator operations. We report polarization modulation measurements performed at 150 GHz, showing a polarization efficiency >99% (68% C.L.) and a median cross-polarization χPol of 0.12%, with 71% of detectors showing a χPol + 2σ upper limit <1%, measured using selected detectors that had the best signal-to-noise ratio.

Partitioning of primary shear zone heat in face milling

The outcome of this paper allows calculating the fraction of heat generated in the primary shear zone that is transferred to the workpiece in face milling. The proposed approach is based on a sequentially coupled analysis of the heat partitioning in the cutting edge normal plane and in the reference plane. The latter, for the first time, allows to systematically take into account the removal of heated workpiece material by subsequent cutting tool engagements. The generated heat is related to the uncut chip thickness. Utilizing Weiner's approach, the heat flux density distribution is determined which serves as input for a three-dimensional thermal finite element simulation that is validated experimentally by temperature measurements.

On accurate time integration for temperature evolutions in additive manufacturing

AbstractWe investigate two numerical challenges in thermal finite element simulations of laser powder bed fusion (LPBF) processes. First, we compare the behavior of first‐ and second‐order implicit time‐stepping schemes on a fixed domain. While both methods yield comparable accuracies in the pre‐asymptotic regime, the second‐order method eventually outperforms the first‐order method. However, the oscillations present in the pre‐asymptotic range of the second‐order method can render it less suitable for simulating LPBF processes. Then, we consider sudden domain extensions resulting from subsequently adding new layers of material with ambient temperature. We model this extension on the continuous level in an energy conservative manner. The discontinuities introduced here reduce the convergence order for both time‐stepping schemes to 0.75. First and second order accuracy could only be achieved by strongly grading the time‐steps towards the domain expansion.

Integrated Electromagnetic-Thermal Approach to Simulate a GaN-Based Monolithic Half-Bridge for Automotive DC-DC Converter

New technological and packaging solutions are more and more being employed for power semiconductor switches in an automotive environment, especially the SiC- and GaN-based ones. In this framework, new front-end and back-end solutions have been developed, and many more are in the design stage. New and more integrated power devices are useful to guarantee the performances in electric vehicles, in terms of thermal management, size reduction, and low power losses. In this paper, a GaN-based system in package solution is simulated to assess the structure temperature submitted to a Joule heating power loss. The monolithic package solution involves a half-bridge topology, as well as a driver logic. A novel integrated electromagnetic and thermal method, based on finite element simulations, is proposed in this work. More specifically, dynamic electric power losses of the copper interconnections are computed in the first simulation stage, by an electromagnetic model. In the second stage, the obtained losses’ geometrical map is imported in the finite element thermal simulation, and it is considered as the input. Hence, the temperature distribution of the package’s copper traces is computed. The simulation model verifies the proper design of copper traces. The obtained temperature swing avoids any thermal-related reliability bottleneck.

Evaluation of PIRs Post-Fire Pull-Out Strength in Concrete Exposed to ISO 834-1 Fire.

Post-installed rebars (PIRs) using mortar can offer bond strength at ambient temperature equal or higher to that of cast-in place rebars. However, high temperatures have the effect of weakening the bond, typically governed by the chemical and physical properties of the mortar which is often sensitive to temperature increase. Therefore, the behavior of PIRs in a fire situation becomes vulnerable. Moreover, after exposure of PIRs to high temperature, the heat transfer continues during the post-fire phase, which might endanger the construction after a fire event. In order to evaluate the evolution of the pull-out capacity during fire, Pinoteau et al. have developed the bond resistance integration method (Pinoteau’s RIM) to predict the bond resistance value of a rebar subjected to various temperatures in accordance with the fire exposure curves. Therefore, accurate temperature profiles during the post-fire phase are needed to ensure a correct calculation of the post-fire behavior of the PIR connection. This paper presents 3D finite element thermal simulations of PIRs in concrete exposed to ISO 834-1 fire conditions then cooled with ambient air. Numerical thermal profiles are then compared to the experimental results (i.e., post-fire pull-out tests). The proposed model provides guidelines for conducting numerical simulations to determine the thermal entry data necessary for predicting thermal profiles in PIRs during heating and cooling phases. Then, the post-fire pull-out capacity of PIRs in concrete is calculated using Pinoteau’s RIM, and compared to experimental post-fire pull-out results.

Leaf Vein-Inspired Bionic Design Method for Heat Exchanger Infilled with Graded Lattice Structure

Due to its excellent performance and high design freedom, the lattice structure has shown excellent capabilities and considerable potential in aerospace and other fields. This paper proposes a method to map the biometric model to the lattice structure. Taking leaf veins as bionic objects, they are used to generate a bionic design with a gradient lattice structure to improve the performance of a heat exchanger. In order to achieve the above goals, this article also proposes a leaf vein model and a mapping method that combine the leaf vein model with the lattice structure. A series of transient thermal finite element simulations was conducted to evaluate and compare the heat dissipation performance of different designs. The analysis results show that the combination of the bionic design and the lattice structure effectively improves the heat dissipation performance of the lattice structure heat exchanger. The results indicate that the application of bionic design in lattice structure design has feasibility and predictable potential.