Melt Pool Research Articles

Investigation of thermal and fluid dynamics behaviors of melt pool during laser direct metal deposition on inclined substrates

The laser direct metal deposition (DMD) technique offers an efficient solution for repairing and manufacturing structures with variable curvatures, such as impeller blades. However, how the non-vertical laser irradiation on curved surfaces impacts the thermal and fluid dynamics of the melt pool still remains unclear. In the current research, a high-fidelity multi-physics numerical model is developed to explore the mechanisms of melt pool development and the effects of inclined angle and laser spot diameter on morphological evolution of deposition layers during DMD under non-vertical irradiation on inclined substrates. Results indicate that as the inclination angle increases, deposited track fluctuations become evident, with a critical angle from 40° to 42.5° identified for hump formation. A smaller laser spot diameter increases the likelihood of humps and irregularities, while a larger spot diameter expands the deposition layer and enhances melt pool wettability. This study is expected to provide significant theoretical guidance for selecting and applying process parameters for the fabrication of curved structures by DMD.

Mechanical Performance of Laser Powder Bed Fused Ti-6Al-4V: The Influence of Filter Condition and Part Location

Deteriorated filter condition in laser powder bed fusion (L-PBF) systems may negatively impact shield gas flow, causing inadequate spatter particle/plume removal, leading to laser beam attenuation and reduction in melt pool depth, and potentially causing more frequent formation of volumetric defects. This work investigated the effects of filter condition and part location on the micro-/defect-structure and mechanical behavior, including tensile and fatigue, of Ti-6Al-4V parts fabricated by L-PBF. Interestingly, within the manufacturer recommended service intervals, no specific effect of filter condition could be observed on the micro-/defect-structure or the mechanical behavior of the fabricated parts. However, the parts’ defect-structures were affected by their location, with ones located near the center of the build plate having less porosity than the ones located away. Although these defects did not affect the tensile properties, they frequently observed to initiate fatigue cracks; the critical defects sizes were often in the range of a few tens of micrometers. Therefore, their sensitivity to location resulted in the location dependence of the fatigue behavior.

Minimization of melt-pool field variables fluctuation during selective laser melting of Ti6Al4V alloy through computational investigation

Abstract The optimization of selective laser melting (SLM) process parameters for a new material through experiments is a time-consuming and challenging process. Computational approaches, on the other hand, offer an economical and relatively faster approach to effectively predict the influences of process factors on the behaviors of the field variables of SLM process. In this work, multiphysics models built using COMSOL software were used to carry out optimization of SLM-Ti6Al4V processes through a single-level setup method followed by a parametric sweep optimization (PSO) approach. The simulated results of the melt pool field variables obtained from both approaches were compared. In the PSO approach, the melt pool velocity was found to have 14.3% higher flow and 78.8% reduction in the transient velocity fluctuation amplitude within the melt pool region. The average transient temperature of the melt pool region was found to have 5.9% increase and 36.4% reduction in the average fluctuation amplitude along the solidus and peak points, respectively. On the other hand, the associated temperature gradient was found to have a fluctuation amplitude reduction of 15.3% at the maximum side of the melt pool region. Finally, the optimal solutions of the melt pool field variables obtained from the PSO were compared with published data to verified the approach. The reductions in temperature and thermal gradient results were found by 18.3% and 28.5% respectively in the melt pool region of the current SLM-Ti6Al4V process and, hence, validating the predictions of the PSO technique.

Physics-guided spatiotemporal approach for melt pool size prediction in laser powder bed fusion

ABSTRACT This study first proposes a physics-guided spatiotemporal graph. The melt pool temporal feature nodes and spatial feature nodes are extracted from melt pool images based on spatiotemporal correlations. These feature points are treated as graph nodes, and the edge weights in the graph are calculated based on an analytical heat transfer model. It supplements the physical information of accumulated heat. Based on this, the graph convolutional network (GCN) is used to extract spatiotemporal information of heat transfer, and the long short-term memory (LSTM) is used to capture the evolution patterns of the melt pool. Thus, a melt pool spatiotemporal feature extraction model is constructed for melt pool size prediction. The model achieves prediction errors within 12% for the melt pool area and 8.5% for the melt pool perimeter. Subsequently, interpretability analysis indicates that the physics-guided approach enhances the model's learning ability for the cumulative contribution of heat from neighboring tracks.

In-Situ visual reconstruction of strut profiles in pulsed wire arc additive manufacturing of lattice structures

ABSTRACT The wire arc additive manufacturing (WAAM) process often results in geometric inaccuracies in the produced struts, highlighting the need for in-situ monitoring. Industrial cameras struggle to capture accurate images due to interference from arc light and metal radiation. This study introduces a real-time visual reconstruction method that leverages regions of interest (ROI) to enhance profile accuracy. Initially, the melt pool is analyzed and segmented from a series of frames to establish an initial strut profile. This profile then serves as the ROI for segmenting the solidified weld bead regions, yielding a more precise strut profile. Comparative analysis shows that the proposed method achieves a high Intersection Over Union (IOU) score of 0.911 and an Average Symmetric Surface Distance (ASD) error of 0.355 mm, demonstrating both accuracy and robustness. Additionally, the inclination angle and diameter are extracted from the reconstructed profiles.

Investigating Melt Pool Dimensions in Laser Powder Bed Fusion of Nitinol: An Analytical Approach

Nitinol (NiTi) has gained popularity across various industries due to its shape memory and superelastic properties. Recently, additive manufacturing (AM) has been increasingly utilized to produce NiTi components. This study focuses on single‐track nitinol samples fabricated via powder bed fusion using laser beam (PBF‐LB). Investigating the effects of laser power and scanning speed on melt pool dimensions reveals that melt pool width increases linearly with laser power and decreases logarithmically with scanning speed. However, melt pool depth exhibits outliers that deviate from these trends. Three analytical models are evaluated to predict melt pool dimensions, generally aligning with experimental trends. Notably, the Eagar–Tsai model delivers the most accurate predictions for melt pool width, with a mean absolute error of less than 10%, while the Gladush–Smurov model is more reliable for melt pool depth predictions, showing a mean absolute error under 20%. In contrast, the Rosenthal equation yields less reliable results for both dimensions. This suggests that a combined approach utilizing the strengths of both the Eagar–Tsai and Gladush–Smurov models may provide the most accurate predictions for the melt pool profile of NiTi in PBF‐LB.

Spatter detection and tracking in high-speed video observations of laser powder bed fusion

Purpose In laser powder bed fusion (L-PBF) additive manufacturing, spatter particles are ejected from the melt pool and can be detrimental to material performance and powder recycling. Quantifying spatter generation with respect to processing conditions is a step toward mitigating spatter and better understanding the phenomenon. This paper reveals process insights of spatter phenomena by automatically annotating spatter particles in high-speed video observations using machine learning. Design/methodology/approach A high-speed camera was used to observe the L-PBF process while varying laser power, laser scan speed and scan strategy on a constant geometry on an EOSM290 using Ti-6Al-4V powder. Two separate convolutional neural networks were trained to segment and track spatter particles in captured high-speed videos for spatter characterization. Findings Spatter generation and ejection angle significantly differ between keyhole and conduction mode melting. High laser powers lead to large ejections at the beginning of scan lines. Slow and fast build rates produce more spatter than moderate build rates at constant energy density. Scan strategies with more scan vectors lead to more spatter. The presence of powder significantly increases the amount of spatter generated during the process. Originality/value With the ability to automatically annotate a large volume of high-speed video data sets with high accuracy, an experimental design of observed parameter changes reveals quantitively stark changes in spatter morphology that can aid process development to mitigate spatter occurrence and impacts on material performance.

Effect of Processing Parameters on Recrystallization During Hot Isostatic Pressing of Stellite-6 Fabricated Using Laser Powder Bed Fusion Technique

Crack-free Stellite-6 alloy was fabricated using the laser powder bed fusion technique equipped with a heating module as the first attempt. Single tracks were printed with a build plate heated to 400 °C to identify the processing window. Based on the melt pool dimensions, two combinations (sample A: 300 W/750 mm/s and sample B: 275 W/1000 mm/s) were identified to print the cubes. The as-printed microstructure comprised FCC-Co dendrites with M7C3 in the interdendritic region. W-rich M6C particles were found in the overlapping regions between the melt pools, matching the Scheil simulations. However, gas pores were observed due to the higher nitrogen and oxygen content of the feedstock requiring hot isostatic pressing (HIP) at 1250 °C and 150 MPa for 2 h. Sample A was partially recrystallized with slightly coarsened M7C3, while sample B underwent complete recrystallization followed by grain growth along with higher coarsening of the M7C3 after HIP. The varying recrystallization behavior can be attributed to the difference in residual stresses and grain aspect ratio in the as-built condition dictated by laser power and scanning speed. The microhardness after HIP was slightly higher than its wrought counterpart, indicating no severe impact of post-processing on the properties of Stellite-6 alloy.

Electron Beam Additive Manufacturing of SS316L with a Stochastic Scan Strategy: Microstructure, Texture Evolution, and Mechanical Properties

This study examines the microstructure, crystallographic texture evolution, and mechanical properties of stainless steel 316L fabricated through electron beam melting using a stochastic scan strategy at a preheat temperature of 1123 K. X-ray diffraction confirmed the presence of a pure austenitic phase in the fabricated material. Equiaxed cellular structures were observed in the center of the melt pool regions and elongated cellular structures observed at the melt pool overlap regions. A finite element-based numerical model was employed to estimate the thermal gradients and solidification rates within the melt pool of an electron beam spot. Microstructural analysis indicated a generation of columnar grains from the bottom to the top of the build owing to high thermal gradients. A crystallographic texture investigation showed a generation of strong <110> fiber texture along the build direction of the material and reported that the stress distributions within the melt pool led to a strong crystallographic texture driven by the stress evolution observed from thermokinetic computational modelling of the electron beam-melting process. Mechanical properties were assessed using profilometry-based indentation plastometry, demonstrating strain hardening at a high temperature of 773 K.

Improvement in an Analytical Approach for Modeling the Melting Process in Single-Screw Extruders

Most single-screw extruders used in the plastics processing industry are plasticizing extruders, designed to melt solid pellets or powders within the screw channel during processing. In many cases, the efficiency of the melting process acts as the primary throughput-limiting factor. If the material melts too late in the process, it may not be sufficiently mixed, resulting in substandard product quality. Accurate prediction of the melting process is therefore essential for efficient and cost-effective machine design. A practical method for engineers is the modeling of the melting process using mathematical–physical models that can be solved without complex numerical methods. These models enable rapid calculations while still providing sufficient predictive accuracy. This study revisits the modified Tadmor model by Potente, which describes the melting process and predicts the delay-zone length, extending from the hopper front edge to the point of melt pool formation. Based on extensive experimental investigations, this model is adapted by redefining the flow temperatures at the phase boundary and accounting for surface porosity at the beginning of the melting zone. Additionally, the effect of variable solid bed dynamics on model accuracy is examined. Significant model improvements were achieved by accounting for reduced heat flow into the solid bed due to the porous surface structure in the solid conveying zone, along with a new assumption for the flow temperature at the phase boundary between the solid bed and melt film.

Human-in-the-loop Multi-objective Bayesian Optimization for Directed Energy Deposition with in-situ monitoring

Directed Energy Deposition (DED) is a free-form metal additive manufacturing process characterized as toolless, flexible, and energy-efficient compared to traditional processes. However, it is a complex system with a highly dynamic nature that presents challenges for modeling and optimization due to its multiphysics and multiscale characteristics. Additionally, multiple factors such as different machine setups and materials require extensive testing through single-track depositions, which can be time and resource-intensive. Single-track experiments are the foundation for establishing optimal initial parameters and comprehensively characterizing bead geometry, ensuring the accuracy and efficiency of computer-aided design and process quality validation. We digitized a DED setup using the Robot Operating System (ROS 2) and employed a thermal camera for real-time monitoring and evaluation to streamline the experimentation process. With the laser power and velocity as inputs, we optimized the dimensions and stability of the melt pool and evaluated different objective functions and approaches using a Response Surface Model (RSM). The three-objective approach achieved better rewards in all iterations and, when implemented in a real setup, allowed to reduce the number of experiments and shorten setup time. Our approach can minimize waste, increase the quality and reliability of DED, and enhance and simplify human-process interaction by leveraging the collaboration between human knowledge and model predictions.

Simulation of melt pool dynamics including vaporization using the particle finite element method

Simulation of melt pool dynamics including vaporization using the particle finite element method

Study on Mitigation of Interfacial Intermetallic Compounds by Applying Alternating Magnetic Field in Laser-Directed Energy Deposition of Ti6Al4V/AA2024 Dissimilar Materials

Brittle intermetallic compounds (IMCs) at the interface of dissimilar materials can seriously affect the mechanical properties of the dissimilar components. Introducing external assisted fields in the fabrication of dissimilar components is a potential solution to this problem. In this study, an alternating magnetic field (AMF) was introduced for the first time in the additive manufacturing of Ti6Al4V/AA2024 dissimilar alloy components by laser-directed energy deposition (L-DED). The effect of the AMF on the interfacial IMCs’ distribution was studied. The results indicate that the contents of the IMCs were different for different magnetic flux densities and frequencies, and the lowest content was obtained with a magnetic flux density of 10 mT at a frequency of 40 Hz. When an appropriate AMF was applied, the IMC layer was no longer continuous at the interface, and the thickness was notably decreased. In addition, the influence of the AMF on the temperature distribution and fluid flow in the melt pool was analyzed through numerical simulation. The simulation results indicate that the effect of the AMF on the temperature of the melt pool was not significant, but it changed the flow pattern inside the melt pool. The two vortices inside the cross-section that formed when the AMF was applied caused different orientations of club-shaped IMCs inside the deposition layer. A sudden change in the streamline direction at the bottom of the longitudinal cross-section of the melt pool can affect the formation of the IMC layer at the interface of dissimilar materials, resulting in inconsistent thickness and even gaps. This work provides a useful guidance for regulating IMCs at dissimilar material interfaces.

Area-based composition predictions of materials fabricated using simultaneous wire-powder-directed energy deposition

Functionally graded materials are an emergent method for designing components with programmable site-specific material properties. These materials are typically fabricated using metal additive manufacturing tools by simultaneously feeding multiple wire and/or powder feedstocks at various rates to achieve spatial composition change. The wire-powder-directed energy deposition (WP-DED) technique is of particular interest for many functionally graded material applications by balancing the low raw materials cost of wire with the high resolution of powder. However, feeding wire and powder are inherently different processes since all extruded wire enters the melt pool, while much of the blown powder is scattered, which makes determining the composition of the build challenging. In this study, we devise a simple area-based measurement method for estimating the composition of WP-DED structures. WP-DED single beads are printed using 309L stainless steel wire and commercially pure Fe powder at five wire feed rates (0.5, 0.75, 1.00, 1.25, 1.50 mm/mm) and five powder feed rates (2, 4, 6, 8, 10 rpm). Characteristic defects including interface gaps and macrosegregation (lack of mixing) tendencies are examined. High powder feed rates (8, 10 rpm) result in interface gaps at all wire feed rates, but smooth deposition and complete mixing is achieved at low powder feed rates, particularly with lower wire feed rates as well. The area-based composition measurement method is within ±20% of energy dispersive x-ray spectroscopy measurements for all samples, showing its effectiveness as a rapid composition estimate for WP-DED materials development.

Probing rapid solidification pathways in refractory complex concentrated alloys via multimodal synchrotron X-ray imaging and melt pool-scale simulation

AbstractRefractory complex concentrated alloys (RCCAs) show potential as the next-generation structural materials due to their superior strength in extreme environments. However, RCCAs processed by metal additive manufacturing (AM) typically suffer from process-related challenges surrounding laser material interaction defects and microstructure control. Multimodal in situ techniques (synchrotron X-ray imaging and diffraction and infrared imaging) and melt pool-level simulations were employed to understand rapid solidification pathways in two representative RCCAs: (i) multi-phase BCC + HCP Ti0.4Zr0.4Nb0.1Ta0.1 and (ii) single-phase BCC Ti0.486V0.375Cr0.111Ta0.028. As expected, laser material interaction defects followed similar systematic trends in process parameter space for both alloys. Additionally, both alloys formed a single-phase (BCC) microstructure after rapid solidification processing. However, significant differences in microstructure selection between these alloys were discovered, where Ti0.4Zr0.4Nb0.1Ta0.1 showed a mixture of equiaxed and columnar grains, while Ti0.486V0.375Cr0.111Ta0.028 was dominated by columnar growth. These behaviors were well described by the influence of undercooling effects on columnar-to-equiaxed transition (CET). Distinct microstructure formation in each alloy was verified through CET predictions via analytical melt pool simulations, which showed a ~ 5 × increase degrees in undercooling for Ti0.4Zr0.4Nb0.1Ta0.1 compared to Ti0.486V0.375Cr0.111Ta0.028. Overall, these results show that microstructure control based on modulating the freezing range must be balanced with process considerations which resist defect formation, such as solidification crack formation in RCCAs. Graphical abstract

Tailoring the microstructure and mechanical properties of wire arc additively manufactured Ti6Al4V alloy by in-situ microalloying with modified nanocarbon

Carbon alloying is a validated strategy for enhancing the mechanical properties of titanium alloys prepared by wire arc additive manufacturing (WAAM). However, achieving uniform carbon distribution, particularly nanocarbon, in the melt pool via the brushing method is challenging, which limits the improvement of mechanical properties. In this paper, nanocarbon was modified by nitric acid hydrothermal treatment and added into the Ti64 melt pool during the WAAM. The distribution of modified nanocarbon in the melt pool was characterized, and the microstructure and mechanical properties of the nanocarbon-alloyed titanium alloy deposits were studied. The findings revealed that the surface of modified nanocarbon generated oxygen-containing functional groups, which enhanced its dispersion in the melt pool. The addition of 0.1–0.3 wt% modified nanocarbon induced no pores in the deposits compared to unmodified ones. The tensile strength of the deposited alloys was continually enhanced with increasing modified nanocarbon content, while the elongation had a peak value. The Ti64 with 0.1 wt% nanocarbon exhibited a balanced comprehensive performance with an ultimate tensile strength (UTS) of 981 MPa coupled with an elongation of 8 %. The achievement of the balanced mechanical performances was attributed to the refinement of α-Ti and solid solution strengthening of nanocarbon. When 0.3 wt% nanocarbon was added, the UTS increased to 1012 MPa but the elongation sharply decreased to 4.5 % due to the precipitation of TiC.

Systematic Investigation into Laser Powder Bed Fusion of Ti-5553 through Single-track and Multi-layer Studies for Tailored Manufacturing Solutions

Laser Powder Bed Fusion (LPBF) is recognized as an appealing fabrication process for producing metallic parts with customized properties. In the current research, a comprehensive approach is employed to systematically correlate single-track and multi-layer fabrication, aiming to generate a reliable process map, assess the effect of process parameters on the properties of LPBF-made Ti-5553, and guide the manufacturing of tailored structures. Based on single-track morphology, melt pool geometry, and multi-layer density, 30 combinations of laser power and scanning speed were categorized into three groups to identify the desirable process parameters. The investigation of single-tracks and multi-layers reveals that deeper melt pools, created with higher energy input, result in a more elongated grain structure, higher α phase content, increased strength and hardness, and reduced ductility. It is observed that achieving higher ductility involves a slight decrease in strength. Specifically, a substantial increase of ∼65% in ductility occurs with only a ∼3.5% reduction in strength. Also, it is found that the volumetric energy density (VED) alone is not sufficient as a design parameter, and the significant process parameters (e.g., laser power and scanning speed) should be considered, as two samples with the same VED yield different properties.

Surface condition driven fatigue performance of laser powder bed fusion H13 steel

Surface variation is an inherent imperfection in the laser powder bed fusion (LPBF) processed samples, comprising of primary roughness (PRR) from the melt pool solidification and secondary roughness (SER) from adhered powder particles. Components under cyclic loading are sensitive to surface defects, making surface finishing crucial for fatigue-bearing parts. However, this undermines LPBF’s advantage of creating high-performance and geometrically complex components. This study examines the effect of surface roughness on fatigue crack initiation in LPBF H13 steel, a high-strength alloy with a tensile strength of 1800 MPa. The focus is on scenarios where contour scanning strategies are not employed. Vertically deposited samples were subjected to three surface conditions: as-built (AB) with PRR + SER, half-polished (HP) with only PRR, and well-machined (M) with no PRR or SER. Optical and digital surface roughness characterization and stress-controlled fatigue testing were performed. Results revealed that while AB-samples showed significantly rough surface compared to HP-samples, their fatigue performance did not reduce. In contrast, M-samples had a slight improvement in roughness but exhibited a fatigue life increase by a factor greater than 17. This was attributed to the PRR, specifically the solidification valleys between the neighboring melt pools. Factographic analysis showed that the PRR-driven fatigue crack initiation is a critical determinant of fatigue performance.

Microstructure and friction properties of NiTi2-based composite layer prepared by ultrasonic assisted underwater wet laser deposition

An in situ TiC-enhanced NiTi2-based composite deposition layer was prepared by ultrasonic assisted underwater wet laser deposition. The evolution of the microstructure and interface state, and the friction properties in the air and brine solution environments of the deposition layer were analyzed and researched. The results demonstrated that the thermal and steady-state cavitation from ultrasound effects increase the total energy of the melt pool and promote mixing. The dilution rate of the ultrasonic assisted underwater laser deposition layer increases to 86.72 % compared with that of the underwater laser deposition layer (78.75 %). Ultrasonic reduced underwater laser deposition defects such as lack of fusion and weld slag, and enhanced the wettability between the deposition layer and substrate. The contact angle of the UWNT layer (24.40°, in average) is smaller than the WNT layer (29.80°, in average). Ultrasonic shortened the contact time between the deposition layer surface and the solution, which reduced the reaction of the Ni element with the brine solution. It also transformed the deposition layer from hypereutectoid microstructure to hypoeutectic microstructure and from planar interface to cellular interface. With the effect of the ultrasonic field, the elements in the deposition layer become more uniformly distributed and the secondary phases undergo spheroidization. In the ultrasonic assisted deposition layer, stress-induced FCC-Ti was discovered. Ultrasonic field also reduced the dispersion of the microhardness and increased the average microhardness to 863.3 ± 45.1 HV0.3. Tribology properties show that ultrasonic field effectively improved the wear resistance and friction stability of the ultrasonic assisted deposition layer. The COF of the ultrasonic assisted deposition layer was less affected by the environment, with values of 0.25 and 0.24 observed in air and brine solution, respectively. Overall, the research results in this paper can provide a new insight to improve the surface properties of NiTi2-based alloy underwater laser deposition.

Synchronized Multi-Laser Powder Bed Fusion (M-LPBF) Additive Manufacturing: A Technique for Controlling the Microstructure of Ti–6Al–4V

One of the technological hurdles in the widespread application of additive manufacturing is the formation of undesired microstructure and defects, e.g., the formation of columnar grains in Ti-6Al-4V—the columnar microstructure results in anisotropic mechanical properties, a reduction in ductility, and a decrease in the endurance limit. Here, we present the potential implementation of a hexagonal array of synchronized lasers to alter the microstructure of Ti–6Al–4V toward the formation of preferable equiaxed grains. An anisotropic heat transfer model is employed to obtain the temporal/spatial temperature distributions and construct the solidification map for various process parameters, i.e., laser power, scanning speed, and the internal distance among lasers in the array. Approximately 55% of the volume fraction of equiaxed grains is obtained using a laser power of P = 500 W and a scanning speed of v = 100 mm/s. The volume fraction of the equiaxed grains decreases with increasing scanning velocity; it drops to 38% for v = 1000 mm/s. This reduction is attributed to the decrease in absorbed heat and thermal crosstalk among lasers, i.e., the absorbed heat is higher at low scanning speeds, promoting thermal crosstalk between melt pools and subsequently forming a large volume fraction of equiaxed grains. Additionally, a degree of overlap between lasers in the array is required for high scanning speeds (v = 1000 mm/s) to form a coherent melt pool, although this is unnecessary for low scanning speeds (v = 100 mm/s).