Melt Pool Temperature Research Articles

Thermal experiment study on the self-luminous properties of metallic iron during solid–liquid phase transition

According to the thermal experiment study of the self-luminescence spectrum of industrially pure iron (DT8) at a heating and cooling rate of 5 °C/min, at the moment of 0 (800 °C), DT8 had no significant self-luminescence characteristics in the wavelength range of 200–1100 nm. At the same time, the first-order differentiation of the spectral intensity versus time was equal to zero at the beginning and end moments of the solid-liquid phase transition. When the solid-liquid melt temperatures are equal (i.e. 1531 °C), there are two sharp peaks at wavelengths of 589.1 nm and 766.7 nm of the self-luminous spectrum in the pure liquid state compared to the self-luminous spectrum in the pure solid state. The intensity of the spectrum decreases significantly as the metallic iron changes from the pure solid phase to the pure liquid phase, with a reduction rate greater than 20%. The appearance of the fitted peaks at 589.1 nm and 769 nm was found by XPS fitting analysis to be a significant feature of the difference in the auto-luminescence spectra of pure solid phase and pure liquid phase metallic iron at a constant melt pool temperature. After the transformation of pure solid phase into pure liquid phase, the decay rate of the self-luminous spectrum in the long-wave direction at wavelengths greater than 762 nm is greater than that in the short-wave direction at wavelengths less than 625 nm.

Temperature measurement in laser–metal processing using optical sensing: Radiometric calibration and validation of a multispectral camera

In laser–metal processing such as Laser-based Powder Bed Fusion of Metals (PBF-LB/M), the quality of the final product is significantly influenced by the thermal behaviour of the melt pool. Accurate temperature measurements are challenging due to high temperature gradients and rapid cooling. This study introduces Multispectral Imaging (MSI) as a highly effective thermometry technique to address these demanding conditions. MSI captures multiple bands of radiation, enabling the simultaneous determination of absolute temperature and surface emissivity. This capability is crucial for ensuring high-quality outcomes in laser–metal processing such as PBF-LB/M, contingent upon robust radiometric calibration.To enhance reliability, two radiometric calibration models were developed: an empirical model based on linear sensor data correlations and an analytical model leveraging sensor behaviour. An efficient calibration was found for both models across a range of signal-to-noise ratios in the black body calibrator, where the analytical model even performs robust under a high noise level.Additionally, the temperature accuracy in the solid-state case was tested with a thermo-mechanical simulator. Results showed that the empirical and analytical models achieved mean relative errors of 2.0% and 1.6%, respectively, outperforming the state-of-the-art. Further, laser irradiation experiments highlighted the analytical model’s ability to accurately determine the emissivity decrease during the solid-to-liquid transition, allowing for accurate melt pool temperature estimations. Conversely, the empirical model struggled to estimate temperature effectively in the liquid phase.This study not only proposed a robust methodology of MSI in temperature measurement but also confirmed the accuracy and reliability of the system, broadening the scope for further investigation into the intricate thermal dynamics involved in laser–metal interactions.

Assessing melt pool temperature independence from hardness via photodiode-based plank thermometry (PDPT)

Directed Energy Deposition (DED) has emerged as a promising metal additive manufacturing technique due to its ability to create components comparable or superior to traditional fabrication. Although industrial adoption of this technology is underway, there exists a considerable need to improve the predictability of the components it manufactures to promote wider adoption of this technology. Currently, melt pool temperature is the primary metric for build monitoring and control. However, stronger correlations must be established between the process parameters, including melt pool temperature, and resulting material outcomes to determine if melt pool control is a sufficient metric to control part quality. The goal of this research is to establish correlations between laser power (W), scanning speed (mm/s), powder flow rate (g/min), average melt pool temperature (C), and hardness. The average melt pool temperature was measured through Photodiode-based Planck thermometry (PDPT) with dual on-axis photodiodes. To obtain the correlation between these factors, 54 samples of Inconel 718 were printed, cross-sectioned, and tested using a Vickers diamond indenter. The resulting correlations exhibited extremely low R2 values and validated the findings of previous studies in that process parameters are exceedingly poor predictors of component properties. Average melt pool temperature lacked a significant correlation with hardness, instead process scanning speed demonstrated a more significant impact on the final material hardness. Since hardness is largely dictated by average grain size which in turn is controlled primarily by solidification rate, solidification rate should be the focus of future work to control material properties. The results of this study indicate that melt pool thermal control is not a sufficient metric to control part quality, and a solution is needed that can oversee the build process in such a way that solidification rate can be monitored and controlled in addition or instead of melt pool temperature. Future work must be done to apply the PDPT methodology to real time calibrated indirect measurement of melt pool solidification rates to improve prediction and control of final part mechanical properties.

Numerical simulation and analytical modelling of temperature and morphology of melt pool in electron beam powder bed fusion of copper

Numerical simulation and analytical modelling of temperature and morphology of melt pool in electron beam powder bed fusion of copper

Investigations on the Heat Balance of the Melt Pool during PBF-LB/M under Various Process Gases

During the powder bed fusion of metals using a laser beam (PBF-LB/M), an inert atmosphere is maintained in the build chamber to avoid reactions of the liquid metal with ambient air leading to the creation of oxides or nitrides, which alter the mechanical properties of the processed part. A continuous gas flow is guided over the process zone to remove spatters and fumes. This flow induces a convective heat transfer from the molten metal to the gas, which, depending on the level of the heat flow, may alter the melt pool dimensions by influencing the cooling rate. The present work investigated these phenomena with single-line trials, both experimentally and numerically. For this reason, a smoothed-particle hydrodynamics model was utilized to investigate the temperatures of the melt pool, cooling rates, and the integral heat balance with various gas atmospheres. In parallel, an on-axis pyrometer was set up on an experimental PBF-LB/M machine to capture the surface emissions of the melt pool. The atmosphere in the simulations and experiments was varied between argon, helium, and two mixtures thereof. The results showed a slight increase in the cooling rates with an increasing fraction of helium in the process gas. Consistently, a slight decrease in the melt pool temperatures and dimensions was found.

Influence of Laser-Based Powder Bed Fusion of Metals Process Parameters on the Formation of Defects in Al-Zn-Mg-Cu Alloy Using Path Analysis.

High-strength aluminium alloys are prone to porosity and cracking during laser-based powder bed fusion of metals (PBF-LB/M) due to the complex solidification behaviour, thus limiting the preparation of high-quality aluminium alloys. In order to effectively reduce the defect formation, this study investigated the influence mechanism of different process parameters on the formation of porosity and cracks in Al-Zn-Mg-Cu alloys in the PBF-LB/M process by combining experimental and numerical simulation. The degree of influence of the process parameters on the temperature field and the temperature field on the defect formation was also quantified using path analysis. The results show that modulation of the process parameters can effectively reduce the formation of cracks and pores, although it is difficult to eliminate them. The melt pool temperature has a significant effect on the formation of porosity, and the temperature gradient has a significant effect on the formation of cracks. The degree of influence of laser power on the melt pool temperature and temperature gradient was greater than that of scanning speed, with values of 0.980 and 0.989, respectively. Therefore, the priority of modulating the laser power is higher than that of scanning speed in order to reduce the formation of defects more effectively.

Closed-Loop Control of Melt Pool Temperature during Laser Metal Deposition.

Laser metal deposition (LMD) is a technology for the production of near-net-shape components. It is necessary to control the manufacturing process to obtain good geometrical accuracy and metallurgical properties. In the present study, a closed-loop control method of melt pool temperature for the deposition of small Ti6Al4V blocks in open environment was proposed. Based on the developed melt pool temperature sensor and deposition height sensor, a closed-loop control system and proportional-integral (PI) controller were developed and tested. The results show that with a PI temperature controller, the melt pool temperature tends to the desired value and remains stable. Compared to the deposition block without the controller, a flatter surface and no oxidation phenomenon are obtained with the controller.

Understanding coaxial photodiode-based multispectral pyrometer measurements at the overhang regions in laser powder bed fusion for part qualification

Coaxial photodiode monitoring sensors provide a digital signature at the melt pool level in laser powder bed fusion (L-PBF), facilitating faster part qualification. However, current signatures are plagued by significant noise and signal variation and show counterintuitive trends such as a decrease in measured temperature near overhang surfaces. This paper investigates the behavior of coaxial photodiode-based melt pool monitoring (PD-MPM) thermal measurements at overhang regions by conducting a combination of experiments and multiphysics simulations. High-speed in situ synchrotron X-ray imaging is coupled with a coaxial photodiode system to enable comparison of the observed melt pool phenomena and monitoring signals during double-track AlSi10Mg experiments. A multiphysics model is developed to simulate the melt pool dynamics and concurrent sensor signals throughout the process. We propose a surrogate model that clarifies the correlation between sensor signals and melt pool temperature. Both experimental and simulation results emphasize the significant impacts of laser energy and keyhole formation on solid-liquid interface discontinuities and PD-MPM signals. Results reveal that a boiling region with a relatively smaller projected area, as near an overhang, can lead to a decrease in the measured melt pool temperature, even when the true peak temperature remains constant, meaning that PD-MPM temperature measurements cannot be used to estimate absolute melt pool temperature. Consequently, in overhang regions, interaction between the melt pool and underlying powder results in an abruptly deforming melt pool and a pronounced decrease in both individual photodiode intensities and overall measured melt pool temperature. This work illustrates how to correctly interpret the coaxial photodiode monitoring signals in L-PBF by uncovering the intricate dynamics within the melt pool, particularly in challenging overhang regions, and pave the foundation for data-driven part qualification.

Directed energy deposition with a graded multi-material compatibility interface enables deposition of W on Cu

Multi-material Directed Energy Deposition of new high-performance components is limited by phase incompatibilities between metals and by the composition variations which complicate parameter optimization. In this work, a methodology of incorporation of a graded Multi-Material Compatibility Interface (MMCI) was employed to allow deposition of tungsten layers (melting at 3410 °C) on a much less refractory metal, copper (boiling at 2562 °C). The manufacturing of parts combining tungsten and copper is of interest for components designed to dissipate heat in extreme environments, such as the divertor for tokamak fusion reactors. A succession of miscible filler metals rationally selected from thermodynamics were printed by laser and powder injection, with optimization of energy and material inputs for each layer, supported by material characterization and physical numerical analysis of the process phenomenology. 90 %wt. tungsten layer deposition on a copper initial substrate is achieved using an MMCI with 6 intermediate layers and 1.6 mm total thickness. The results reveal the importance of controlling the dilution ratio to achieve stable depositions of multi-material with compositional gradients, with a progressive increase in melt pool temperatures. The numerical analysis highlighted the important role of the dissolution and mixing of partially melted refractory powders, and the selective vaporization of volatile elements.

MPTP-Net: melt pool temperature profile network for thermal field modeling in beam shaping of laser powder bed fusion

MPTP-Net: melt pool temperature profile network for thermal field modeling in beam shaping of laser powder bed fusion

Dynamic evolution of temperature field, flow field, and solidification behavior during multilayer multitrack laser cladding

During multilayer multitrack laser cladding, secondary melting and solidification in the interlayer lap zone have a significant impact on the mechanical properties of cladding coatings. However, mathematical modeling of the melt flow and solidification behavior of multilayer, multitrack laser cladding interlayer lap zones is lacking. In this paper, a 3D finite element model was established to investigate the mass and heat transfer processes in both lap and non-lap zones during the laser cladding of multilayer multitrack Fe-Cr-based alloys on 45# steel surface. A dynamic grid is used to track the free surface of the melt pool, while the enthalpy method is used to simulate the solid-liquid phase transition. The change in thermophysical properties with temperature, as well as the change in laser energy and alloy powder flow density in the lap zone, were taken into consideration. The influence of the number of cladding layers on the heat and mass transfer in the molten pool, remelting, and solidification in the lap zone was investigated, along with its intrinsic mechanism. The results show that the model can accurately predict the size and microstructural change law of coating layers, including the grain size of the lap region between layers. The melt pool temperature and grain size increase with the number of cladding layers, while the convection and cooling velocities decrease. In addition, the grain size in the lap zone of the cladding is larger than that in the nearby non-lap zone. Using solidification parameters extracted from the solidification front of the molten pool, changes in grain size and morphology were predicted for both the interlayer lap and non-lap zones. The numerical calculation results were consistent with the experimental measurements.

Unveiling gas–liquid metal reactions in metal additive manufacturing: High-fidelity modeling validated with experiments

Gases in the atmosphere inevitably react with the melt pool during metal additive manufacturing (AM). Oxygen is particularly reactive, and excessive uncontrolled oxidation is detrimental, so most machines purge the chamber with inert gases, which can minimize but not eliminate such reactions. Alternatively, some users exploit the gas–liquid metal reactivity as an opportunity to introduce beneficial precipitates into the melt pool (“reactive AM”). However, the gas–liquid metal reaction and mechanisms in both scenarios remain unclear. Experimental works hitherto provide different explanations to the same phenomena. Therefore, this work seeks to clarify the mass transfer process of oxygen during metal AM through high-fidelity modeling by considering the competition between diffusion and chemical reaction, suboxide evaporation, and the influence of the vapor plume. The simulation results, validated with experiments, provide consolidated insights into the oxygen evolution behavior during metal AM. Counterintuitively, higher melt pool temperatures do not necessarily lead to greater oxidation rates during processing. The melt pool has regions of high and low oxygen gains due to temperature-dependent reaction regimes, with concurrent oxygen loss from evaporation of metal suboxides. Thus, the net oxygen flux varies for different materials, and the oxygen content cumulatively changes as multiple tracks are scanned. Overall, this work provides useful guidance to the AM community that seeks to ameliorate or exploit the inevitable gas–liquid interaction in metal AM. Cost-saving measures may be possible for determining the purity of shielding gas used in AM, and physics-guided measures can be taken to limit or control gas–liquid metal reactions.

Correction: In-process closed-loop control for stabilising the melt pool temperature in selective laser melting

Correction: In-process closed-loop control for stabilising the melt pool temperature in selective laser melting

Data-driven prediction of inter-layer process condition variations in laser powder bed fusion

The pressing problem of laser powder bed fusion of metals (PBF-LB/M) is the inability to predict and mitigate undesired process conditions in a build. This study aims to develop an efficient and geometric-agnostic prediction tool for the variations of the average meltpool temperature of each layer given a laser scanning path. The approach is a data-driven model based on defined physics-based features and measurement data from a two-color coaxial photodiode system. Although the coaxial photodiode system cannot measure absolute meltpool temperature, it can measure relative changes in the meltpool temperature if the process is performed in the shallow or no-keyhole regimes, the focus of this study. Once trained, the model can predict meltpool temperature variations before the start of the build process for a part geometry which had not yet been printed. The results show that variations in the layer average meltpool temperature can be predicted with the average coefficient of determination (r2) score of 0.65. Furthermore, the method shows the significance of considering the effect of surrounding parts on the average meltpool temperature profile of an observed part. For part geometries investigated in this study, the model shows the information from at least 40 previous layers is needed to enable sufficient and stabilized prediction performance.

Optimizing directed energy deposition of polymers through melt pool temperature control: impact on physical properties of polyamide 12 parts

The method of Directed Energy Deposition of polymers (DED-LB/P) was extended to allow control over the melt pool temperature using a pyrometer. DED-LB/P was used to build test specimen of polyamide 12 (PA12), orthogonal and parallel to the long side. Samples prepared under temperature control show superior mechanical properties over those generated without. The temperature of the melt pool allows to tune the quality of the built part. A too low temperature leads to a porous part on account of insufficient powder fusion, and a too high temperature leads to holes by formation of volatiles. The mechanical properties can be related to the porosity, the molecular mass of PA12 did not change substantially, the distribution width however increased with temperature. The best processing conditions were at 220 °C leading to a build part with a porosity of 0.6%, a Youngs modulus of 550 MPa and a fracture-strain of 15% with an ultimate strength of almost 28 MPa.

Numerical Simulation of the Hydrogen-Based Directly Reduced Iron Melting Process

In the context of carbon reduction and emission reduction, the new process of electric arc furnace (EAF) steelmaking based on direct hydrogen reduction is an important potential method for the green and sustainable development of the steel industry. Within an electric furnace for the hydrogen-based direct reduction of iron, after hydrogen-based directly reduced iron (HDRI) is produced through a shaft furnace, HDRI is melted or smelted in an EAF to form final products such as high-purity iron or high-end special steel. As smelting proceeds in the electric furnace, it is easy for pieces of HDRI to bond to each other and become larger pieces; they may even form an “iceberg”, and this phenomenon may then worsen the smelting working conditions. Therefore, the melting of HDRI is the key to affecting the smelting cycle and energy consumption of EAFs. In this study, based on the basic characteristics of HDRI, we established an HDRI melting model using COMSOL Multiphysics 6.0 and studied the HDRI melting process, utilizing pellets with a radius of 8 mm. The results of our simulation show that the HDRI melting process can be divided into three different stages: generating a solidified steel layer, melting the solidified steel layer, and melting HDRI bodies. Moreover, multiple HDRI processes are prone to bonding in the melting process. Increasing the spacing between pieces of HDRI and increasing the preheating temperature used on the HDRI can effectively reduce the aforementioned bonding phenomenon. When the melting pool temperature is 1873 K, increasing the spacing of HDRI to 10 mm and increasing the initial HDRI temperature to 973 K was shown to effectively reduce or eliminate the bonding phenomenon among pieces of HDRI. In addition, with the increase in the melting pool temperature, the time required for melting within the three stages of the HDRI melting process shortened, and the melting speed was accelerated. With the increase in the temperature used to preheat the HDRI, the duration of the solidified steel layer’s existence was also shortened, but this had no significant impact on the time required for the complete melting of HDRI. This study provides a theoretical basis for the optimization of the HDRI process within EAFs.

MeltpoolGAN: Melt pool prediction from path-level thermal history

In metal laser powder bed fusion, a laser beam rapidly scans through a thin layer of powder. The laser heats up and liquefies the powder forming a melt pool. The melt pool characteristics affect the material property and physical performance of the part. Physical simulations of melt pool dynamics are computationally expensive and are often limited to a short laser scan. Alternatively, Finite Element (FE) simulations for part-scale thermal history are less computationally intensive but do not capture the detailed melt pool temperature profile and geometry in general. Accurate modeling of the high spatial and temporal thermal gradients inside and near the melt pool is essential to many applications of the thermal history including design and optimization of the laser scan path, material microstructure characterization, and deformation prediction.In this work, we developed MeltpoolGAN, the first conditional Generative Adversarial Network (cGAN) to predict the melt pool image based on the simulated thermal history along the laser scan path. The integration of generative deep learning and thermal history simulation achieves melt pool-level accuracy while significantly reducing the physical and computational complexity. The data pair of thermal history and melt pool images for neural network training and validation are obtained through Contact-Aware Path Level (CAPL) thermal simulation and a co-axial melt pool monitoring system developed by NIST, respectively. The predicted melt pool morphology is validated against experimentally acquired melt pool images through geometric characteristics, including the length and width, of the melt pool.

Enhanced quasi-static and dynamic tensile properties of stainless steel 316L produced by laser aided additive manufacturing in controlled argon environment

In this paper, laser aided additive manufacturing (LAAM) of stainless steel 316L (SS 316L) in a controlled argon environment was investigated. The controlled argon environment can significantly reduce the oxidation and fundamentally change the process conditions. It was found that after inhibiting oxidation, the surface of the deposited SS 316L showed remarkably high reflectance (71%) to laser radiation, which minimised heat accumulation. Subsequently, stable melt pool temperature and high cooling rate (4.1 × 104 K/s) were also observed. Furthermore, nano-sized oxides were found to be nucleated owing to excessively low internal oxygen content (220 ppm). The unique process condition resulted in enhanced strengthening effect due to cellular structure refinement and dispersed nano-sized oxides. Consequently, a desirable balance of high strength and ductility was achieved for the as-deposited materials under both quasi-static tension (strain rate 10−3 s−1, yield stress 560.0 MPa, ultimate tensile strength 717.2 MPa, and elongation 64.0%) and dynamic tension (strain rate 3150 s−1, maximum flow stress 1241 MPa, and elongation 37.5%).

The thermal history of the directed energy deposition process monitored by pyrometer and camera

Abstract In the process of directed energy deposition (DED), the local thermal history of a component can change significantly owing to its complex heat transfer characteristics. The main objective of this study is to elucidate the distribution of the thermal history of a static scroll plate by monitoring its manufacturing process through a pyrometer and camera and to provide insights into the factors affecting the variation of its thermal history from the point of view of the part shape and scanning strategy. The melt pool area and temperature were extracted using a camera and pyrometer as the thermal features under study, and the thermal features were mapped in three dimensions. This mapping strategy provides a comprehensive visualization of the distribution of thermal features in parts with equal and variable cross-section structures. In the case of an equal cross-section structure, three factors, printing time, scan vector length, and probability density, are selected to analyze the main factors affecting the distribution of thermal features. In the case of the variable cross-section structure, the effects of the layer printing time, cumulative layer printing time, and probability density on the distribution of thermal features were analyzed. By calculating the correlation between the thermal features of the monitored signals and the factors mentioned above, it was found that the factors affecting the thermal distribution of the parts vary in different structures. In equal cross-section structures, the probability density plays a pivotal role, while in variable cross-section structures, the cumulative layer printing time has the most significant influence on the thermal distribution. The above studies have provided a better understanding of the thermal processes of DED technology, paving the way for the control and optimization of DED manufacturing.

Determining the Effects of Inter-Layer Time Interval in Powder-Fed Laser-Directed Energy Deposition on the Microstructure of Inconel 718 via In Situ Thermal Monitoring.

Cylindrical Inconel 718 specimens were fabricated via a blown-powder, laser-directed energy deposition (DED-L) additive manufacturing (AM) process equipped with a dual thermal monitoring system to learn key process-structure relationships. Thermographic inspection of the heat affected zone (HAZ) and melt pool was performed with different layer-to-layer time intervals of ~0 s, 5 s, and 10 s, using an infrared camera and dual-wavelength pyrometer, respectively. Maximum melt pool temperatures were found to increase with layer number within a substrate affected zone (SAZ), and then asymptotically decrease. As the layer-to-layer time interval increased the HAZ temperature responses became more repetitive, indicating a desirable approach for achieving a more homogeneous microstructure along the height of a part. Microstructural variations in grain size and the coexistence of specific precipitate phases and Laves phases persisted among the investigated samples despite the employed standard heat treatment. This indicates that the effectiveness of any post DED-L heat treatment depends significantly on the initial, as-printed microstructure. Overall, this study demonstrates the importance of part size, part number per build, and time intervals on DED-L process parameter selection and post-process heat treatments for achieving better quality control.