Directed Energy Deposition Process Research Articles

Numerical investigation of thermal behavior during the laser-assisted directed energy deposition process for additive manufacturing

Numerical investigation of thermal behavior during the laser-assisted directed energy deposition process for additive manufacturing

Transient and steady models for blue laser directed energy deposition

Laser Directed Energy Deposition (DED) is an important process for additive manufacturing because of the advantage of high manufacturing speed. 450 nm blue laser has a higher absorption rate than traditional infrared laser for highly reflective materials such as Al and Cu. However, it is still unclear how the blue laser affects the particles and the molten pool flow in the DED process. In this work, we first designed two models to reveal the mechanism of the molten pool behaviors in the blue laser DED process: the transient model and the steady deposition model. The models are validated by in-situ and ex-situ experiments respectively. It is found that the three stages can be concluded for a particle impacting the molten pool: particle impacts, surface oscillates and the molten pool recovers. The particles whose diameter is less than 100 µm are necessary for steady deposition. The influence of process parameters on DED is investigated, and a suitable process parameters window is found. we also verified the superiority of the blue laser for DED of highly reflective materials through the simulation models. The current study provides new insights into the mechanism from the interaction in the molten pool scale to single-track forming in the blue laser DED, and can provide theoretical guidance on the application of blue laser DED of highly reflective materials.

Enhancing Metal Forging Tools and Moulds: Advanced Repairs and Optimisation Using Directed Energy Deposition Hybrid Manufacturing

This article investigates the efficacy of directed energy deposition (DED) processes in repairing forging tools and moulds, comparing mechanical properties between specimens fabricated from conventional sheet metal and those manufactured by DED techniques. A comparative analysis reveals significant mechanical differences between subtractive and DED-manufactured specimens, emphasising the nuanced balance between tensile strength and ductility in DED-produced components influenced by layering. Notable insights from scatter plot analyses highlight distinct material behaviours, particularly layer-dependent tendencies in DED-manufactured specimens. Regression-based predictive models aid in understanding material behaviours, aiding in informed material selection for manufacturing processes. Additionally, this article underlines the advantages of DED-based repair processes, highlighting precision, material efficiency, reduced lead times, and cost-effectiveness. The article studies die and mould repair, tool restoration, and critical considerations like material compatibility and quality assurance. The study concludes by emphasising the role of hybrid manufacturing in extending product lifecycles, in conformity with specific mechanical requirements, and fabricating complex geometries, despite potential higher costs in materials and technologies. Overall, this research demonstrates the efficacy of DED processes in enhancing component reliability and lifespan in metalworking industries.

Potential of thermography for the monitoring of DED-Arc processes

Thermography is a technique that uses, e.g., an infrared camera to visualize and measure the temperature of an object. It is often used in industrial and scientific applications to identify areas of heat loss, overheating, and other thermal anomalies. In the context of the DED-Arc (Direct Energy Deposition) process, thermography can be used to monitor the process and evaluate the temperature profile of the produced part. This can help to ensure the quality and reliability of the product, as well as to predict the resulting mechanical properties of the produced part. However, for other AM processes like LPBF (laser powder bed fusion), thermography is already used in industrial applications while for DED-Arc it is still a challenge to reliably determine the dynamically changing emission coefficient, as the emissivity strongly depends on the surface conditions. This means the emission coefficient differs for changes in surface conditions like impurities from soot and annealing colors. This work focuses on the potential of thermography for monitoring the DED-Arc process. A workflow for generating a calibration function for the emission coefficient ε is presented. In the context to the focus of this work, the resulting ε(T) function differentiates between the first three deposited layers and shows the change of emissivity for higher temperatures. This function is then used to correct the measured temperature profile with regard to different surface conditions and thus emission coefficients of a DED-Arc part.Highlights1. Temperature-dependent function for correcting the emissivity for DED-Arc with steel was defined.2. Emissivity is also influenced by the purity of the surface, leading to higher emissivity coefficients in layers that are previously deposited.3. t8/5 times are also affected by a wrongly set emission coefficient; deviations from the corrected t8/5 time depend on the cooling speed.

Development of 1D-convolutional Neural Network-based Height Profile Prediction Model in Directed Energy Deposition Process Using Melt-pool Image Data

Directed energy deposition (DED) process is a representative metal additive manufacturing technology that uses a flexible deposition head mainly used for repairs in space and marine industries. The DED process saves time and money as it repairs only damaged parts and components. Therefore, a geometric control is important to fill the volume of the target damaged area economically and accurately. However, efficiency depends on process parameters such as laser power, scanning speed. This study proposes a one-dimensional convolutional neural network (1D-CNN) model to predict the height profile of the DED parts utilizing melt-pool image data. First, DED experiments were performed for a total of nine cases considering laser power and scanning speed as parameters. The collected melt-pool image data was pre-processed and only those related to the regions of interest were extracted. Initially, a total of 15 features were extracted from size, shape, location, and brightness from the melt-pool images. Then, 10 critical ones, selected through a permutation feature importance evaluation method, were input to the 1D-CNN algorithm to predict height profiles of the deposited layers. In testing phase, a mean absolute percentage error (MAPE) of 9.55% was achieved, and thus, applicability of the proposed model was verified.

Over-deposition assessment of Direct Energy Deposition (DED) using melt pool geometric features and Machine Learning

Over-deposition’s influence on the DED process is significant in terms of process stability and final part quality. Most recent studies on quality assessment are based on online monitoring of the melt pool and comparing it with pre-specified thresholds. In this study an online machine learning model is developed, capable of identifying two over-deposition defect types, satellite and wire-oscillation, by using frames captured by an optical camera targeting the melt pool. The features extracted from the images included five geometrical melt pool features and the operating laser power and were successfully used to achieve a 94.4% classification accuracy regardless of the final part geometry.

Deposit characteristics, morphology and microstructure regulation of single-track nickel-based alloy using quasi-continuous-wave laser direct energy deposition

A quasi-continuous-wave laser direct energy deposition (QCW-DED) process was employed to regulate the deposit characteristics, molten pool morphology and microstructure of single-track nickel-based alloy. The variation of the deposit characteristics with laser pulse frequency was investigated through experiment and numerical simulation. The molten pool flow pattern and the dendrite growth mode were described based on the observations of the deposition morphology and microstructure in the cross- and longitudinal-sections. It was concluded that the QCW-generated single-track deposits decreased in width and depth, yet increased solely in height as the pulse frequency increased. A “U” shaped structure composed of substrate materials was observed in the molten pool at the low pulse frequency. The flow in the molten pool of QCW specimens was alternately dominated by the Marangoni force and impact force, whereas the fluid flow in the molten pool formed by continuous wave (CW) was entirely dominated by the Marangoni force. The disappearance of the “U” shaped structure and the rapid remelting effect of the molten pool underlie the epitaxial growth of dendrites at the high pulse frequency. In the CW specimen, disorganized dendrites were produced in the heat flow direction along the molten pool boundary.

Hybrid directed energy deposition process coupled with plastic deformation

Laser directed energy deposition (LDED) process has unique advantage in rapid forming of large-sized metal components, gradient material/structural components, or repairing/remanufacturing worn parts. However, the high residual stress and strong anisotropy in mechanical properties of the as-deposit components limit the application of LDED technology in the manufacturing of key structural components. To overcome these problems, various hybrid additive manufacturing (HAM) technologies have been developed, such as plastic deformation, ultrasonic or magnetic field assisted LDED processes to improve the quality and the mechanical properties, where these coupled processes are carried out either simultaneously or cyclically with the LDED process. The hybrid additive manufacturing, while retaining the advantages of individual forming process, avoids the mutual interference between each process and reducing the adverse effects generated if used separately. Hybrid additive manufacturing processes fundamentally change the underlying physical mechanisms of molten pool dynamics, microstructural evolution, temperature and thermal stress gradient in additive manufacturing, thereby optimizing the microstructure and performance of the manufactured components. In this paper, the key technical features of the hybrid additive manufacturing process coupled with plastic deformation were described in details, and the resulting differences in microstructure, residual stress, and mechanical properties of the prepared samples were systematically analyzed. The developing trend of hybrid additive manufacturing processes in coupling mechanisms, parameter optimization, and equipment have been discussed.

Investigating the microstructure and mechanical behavior of optimized eutectic Al[sbnd]Si alloy developed by direct energy deposition

Wire arc additive manufacturing (WAAM) is a direct energy deposition process majorly used to fabricate large and medium-sized components. However, the process encounters defects, such as anisotropic microstructure and mechanical properties, residual stresses, excessive oxidation, and geometrical irregularities. Hence, efforts have been made to address the issues by optimizing the process parameters and incorporating in situ and post-process treatment to improve mechanical properties. In line with this, the present work deals with statistical-based process parameter optimization based on a multicriteria approach. The responses were quantified, statistically analyzed, and correlated with the process parameters. Travel speed exhibited a minor effect on all responses, while the current and wire feed speed significantly affected the responses. The percentage porosity was more sensitive to welding current than the other parameters. The maximum ultimate tensile strength (UTS) of 193.73 ± 3.93 MPa and yield strength (YS) of 108.09 ± 12.28 were displayed by Wall 2. YS and UTS showed marginal variation in vertical and horizontal directions, while % elongation showed significant variation. The desirability Wall (DW) exhibits better-forming quality due to the minimization of the contact angle, and fibrous silicon was observed between the layers. In contrast, it appears to be fragmented silicon at the interfacial region due to remelting and breakdown of Si. However, there were a few limitations to the DW wall's mechanical properties due to coarser dendrites and bulk eutectic at the interface. The average grain size of the bottom and top region for the optimized parameter is 111.156 μm and 173.66 μm, respectively. The observed texture components in the top region include C{001} 〈110〉, A2* (111) [112¯], A (11¯1) [110], and B (11¯2) [110], exhibiting a maximum texture intensity of 10.414. In the bottom region, the identified texture components are B¯ (1¯12¯) [1¯1¯0], B (11¯2) [110], A (11¯1) [110], C{001} <110> with a maximum texture intensity of 10.110.

Wire-arc directed energy deposition of Ti–6Al–4V alloy using CO2 active interpass cooling: phase composition, grain orientation, texture strength, dislocation density and mechanical property

In this study, active interpass cooling using various compressed CO2 gas strategies was applied to wire-arc additively manufactured Ti–6Al–4V alloy, and as-manufactured microstructure characteristics, including phase composition, grain growth and microhardness, were fully explored by using Electron Backscattered Diffraction and hardness tester. Results show that rapid interpass cooling brings obvious martensite phase transformation, primarily from lamellar α grains to acicular α grains, to the deposited component. The microstructure tends to be fine-grained, disordered and displays reduced texture strength and increased dislocation density through visible coloration. This feature also exhibits high hardness values that brings an improvement in mechanical strength to deposited metal. Moreover, it is found cooling time variation could obtain better grain refinement than altering cooling gas flow rate during deposit, which is probably related to effective heat dissipation. The research findings can provide significant insight into microstructural evolution mechanism in wire-arc directed energy deposition process, and benefits to properties control.

Microstructure and properties of (diamond + TiC) reinforced Ti6Al4V titanium matrix composites manufactured by directed energy deposition

In this study, (diamond + TiC) reinforced Ti6Al4V titanium matrix composites (DT-TMCs) were fabricated using the directed energy deposition (DED) technique to enhance multiple properties of the Ti6Al4V alloy. The phase composition, microstructure, microhardness, wear resistance, and thermal conductivity of the composites were investigated. The findings demonstrate that the DT-TMCs consist of diamond, in-situ formed TiC (including eutectic TiC and primary TiC), α-Ti and β-Ti. The formation of in-situ TiC is attributed to the partial dissolution of the diamond into the Ti6Al4V matrix during the DED process. The superior hardness of in-situ TiC, compared to that of Ti6Al4V alloy, plays a pivotal role in enhancing the overall hardness of the DT-TMCs. Owing to the high hardness and excellent thermal conductivity of diamond, the wear resistance and thermal conductivity of the DT-TMCs are superior to those of the unmodified Ti6Al4V alloy. With the diamond volume fraction increasing from 0% to 15%, the microhardness of the DT-TMCs raises from 333.99 HV0.2 to 438.15 HV0.2, the wear rate decreases from 207.514 × 10−5 mm³·N−1·m−1 to 2.256 × 10−5 mm³·N−1·m−1, and the thermal conductivity increases from 6.686 W·m−1·K−1 to 13.613 W·m−1·K−1. Additionally, the in-situ TiC exhibits superior thermal conductivity compared to the Ti6Al4V alloy, which also contributes to the improved thermal conductivity of the DT-TMCs.

Material flow and heat transfer characteristics in flat-top laser composite arc-based directed energy deposition

In this study, a novel flat-top laser composite arc-based directed energy deposition process is proposed, and the material flow and heat transfer behaviors are investigated for both arc-only and hybrid flat-top laser modes. Transient CFD models are developed to simulate the energy of the laser and arc using the surface heat source and double-ellipsoid body heat source models, respectively, and validated through experimentation. The results show that the high-temperature metal fluid flows outward from the center of the molten pool and then backward in the front part. The flat-top laser does not alter the flow field pattern but allows for the spread of more hot liquid metal towards the lateral edges. Furthermore, it is discovered that the flat-top laser reduces the contact angle and promotes a stable molten pool temperature.

Multi-regional microstructure control using ultrasonic-assisted directed energy deposition for Al-Cu alloy

The uneven microstructure and element distribution between the inner-layer zone (INZ) and the inter-layer zone (ITZ) resulting from the thermal cycling effect during the directed energy deposition process with the electric arc energy source (DED-Arc) can lead to inferior mechanical properties. In order to address this problem, the ultrasonic treatment of the 2319 Al-Cu alloy during the DED-Arc process is investigated to control the multi-regional microstructure quantitatively. Therefore, the effect of ultrasonic vibration during the DED-Arc process on grain morphology, distribution of precipitates and mechanical properties in different regions are explored in detail. The formation mechanism of differential microstructure is elucidated by using an ultrasonic-heat-flow coupling simulation model for the DED-Arc process. The results indicate that ultrasonic vibration refines the average grain size, leading to 16% and 40% reductions at the molten pool center (MPC) and boundary (MPB), respectively. However, the grains in the semi-melting zone (SMZ) experience coarsening due to the reduced temperature gradient after ultrasonic treatment. The enhanced melt flow rate is considered responsible for both the homogenization of Cu and the fragmentation of the θ-Al2Cu phase. The higher subgrain boundary density in the ITZ leads to an average microhardness of approximately 60 HV, which is lower than that found in the INZ (72.3 HV). Meanwhile, ultrasonic treatment increases the average strength and elongation by 38.1% and 17.5%, respectively, mainly through precipitation strengthening, which results in a strength increment of 91.6 MPa. The slip directions tend to be consistent after ultrasonic vibration, which is conducive to crystal sliding and displays excellent ductility.

Benchmarking study between Lagamine and Comsol Solvers for Finite Element Thermal Analyses of Directed Energy Deposition Process

This work presents a benchmarking study between Lagamine, an in-house developed finite element (FE) code, and COMSOL Multiphysics® (Comsol) commercial software in thermal analyses to investigate their capability in modeling complex manufacturing processes. For this purpose, two case studies, including a NAFEMS benchmark for heat transfer with convection and a Directed Energy Deposition (DED) of a bulk sample, were used as test cases. The simulation models using Lagamine and Comsol solvers for each case were described. The underlying algorithms and theories, as well as the software development, are investigated. The computational results indicate slight differences between Lagamine and Comsol solutions in both case studies. For the NAFEMS test case, the results obtained with the Comsol solver appear to be less dependent on the mesh size than those obtained with Lagamine. For the DED test case, within the chosen configurations of Lagamine and Comsol codes, the maximum difference in the highest peak temperatures obtained from the two codes is about 20%. From an engineering point of view, it is suggested to determine the parameters of the FE model consistently with the selected FE code to provide the best match with experimental observations.

Prediction of Ni-based alloy microstructure in wire arc additive manufacturing from cellular automata model

Wire Arc Additive manufacturing (WAAM) process is a class of directed energy deposition processes with wire feedstock. The process enables a higher build rate than the powder-based additive process and integrates multiple custom wire feedstock to induce spatial variation of microstructure and composition. The lower speed employed in WAAM reduces the cooling rate and resultant residual stress compared to the powder bed fusion process. The systematic epitaxy across the layer creates a directional/columnar microstructure for Ni-superalloy. Here, we report cellular automata model development to predict the microstructure evolution with surface and bulk nuclei for single track and multiple layers. The cellular automata model was developed to account for secondary element addition, predict segregation, local melting, and latent heat release, as well as predict crystallographic texture/Euler angles from orientation information. We experimentally validated the layer-level microstructure and texture from SEM and EBSD, respectively, and found agreement with the CA model developed.

Corrosion and stress corrosion cracking characteristics of 4043 aluminium alloy fabricated through directed energy deposition process

Additive manufacturing (AM) has emerged as a crucial element of Industry 4.0. In the pursuit of fabricating lightweight components, AM of aluminium alloys has become the centre of focus in recent years. This study investigates aluminium alloy (ER4043) components fabricated by arc-based directed energy deposition (DED-arc) for their corrosion and stress corrosion cracking (SCC) behaviour in 0.6 M NaCl solution at room temperature. Electrochemical characteristics in 0.6 M NaCl solution were investigated using open circuit potential (OCP), potentiodynamic polarization (PDP) and electrochemical impedance spectroscopy (EIS) tests. Electrochemical results revealed that the DED samples fabricated using a tungsten inert gas (TIG) welder as the energy source demonstrated better resistance against corrosion when compared to those developed through cold metal transfer (CMT) based DED. The prolonged exposure of samples in 0.6 M NaCl facilitated the growth of a stable passive oxide film leading to enhanced corrosion resistance. Slow strain rate tests (SSRT) conducted in air and 0.6 M NaCl solution showed that DED samples are susceptible to SCC. The fractography of the fractured SSRT samples indicated the mode of failure to be intergranular.

Development of Fe–Cr–Si deposited layer manufactured by laser directed energy deposition process

In this study, the mechanical and corrosion characteristics of a corrosion-resistant layer made of stainless steel (STS) 316 L and Fe–Cr–Si alloy powder were investigated using laser-directed energy deposition (DED). In the STS 316 L deposited specimen, both the substrate and deposited layer were face-centred cubic (FCC). The deposited Fe–Cr–Si layer was clearly separated from the substrate because it was composed of body-centred cubic (BCC). Despite the phase differences, the surface of the Fe–Cr–Si-deposited layer showed a lower corrosion rate than that of the STS 316 L. All the deposited specimens exhibited typical high-temperature tensile behavior. However, the Fe–Cr–Si deposited layer at 600 °C showed a notable reduction in strength and increased elongation compared to the room temperature (RT) and 300 °C test results owing to the carbide concentration and phase transformation in the deposited layer. Because nuclear facilities mainly operate at temperatures below 600 °C, Fe–Cr–Si materials can also be used as nuclear piping coating materials. This study provides a mechanism for the high-temperature properties and corrosion resistance of the Fe–Cr–Si deposited layer and makes it competitive for application in fourth generation nuclear power systems.

Thermo-fluid dynamics and morphology evolution of nickel-based superalloy in a multilayer laser directed energy deposition process

The complex thermo-fluid dynamics of the molten pool determine the dimension accuracy and mechanical properties of laser directed energy deposition (L-DED) components. In this work, a three-dimensional (3D) multiphase computational fluid dynamics (CFD) model based on the Volume of Fluid (VOF) method was proposed to establish the relationship between the thermo-fluid dynamics and the molten pool morphology evolution of René N5 nickel-based superalloy in L-DED. The single-track and ten-layer deposition processes under unidirectional and bidirectional scanning strategies were studied. Results show that the predicted deposition morphology and dimensions are in good agreement with the experimental samples. The heat accumulation in the multilayer deposition process increases the size of the molten pool, thereby increasing the width of the deposit. Different from the single-layer deposition driven by the Marangoni force, gravity is the key factor in the multilayer deposition process without the side support. The hump structure at the starting position under unidirectional scanning is attributed to the backward flow of the melt of the molten pool, while the decline structure at the end position is enhanced by the forward flow of the melt. At the starting position of each layer under bidirectional scanning, the expansion of the molten pool resulted from the heat accumulation and mass addition increases the width of the two ends and compensates for the deposition height. This work can provide a theoretical basis for understanding the uneven morphology characteristics and the dimensional accuracy control in L-DED.

Industrial IoT system for laser-wire direct energy deposition: data collection and visualization of manufacturing process signals

Industry 4.0, also known as the Fourth Industrial Revolution, is a term used to describe the current trend of automation and data exchange in manufacturing and other industries. The Internet of Things (IoT) plays a crucial role in Industry 4.0 by connecting devices, machines, and products to the Internet and enabling real-time data exchange. Moreover, additive manufacturing is a key developing manufacturing technology in Industry 4.0. New technologies such as data analysis with Artificial Intelligence and machine vision are widely used in optimization. However, in a lab environment, these technologies depend on the data collected from the process. For such work, the researchers should be able to focus on their core research rather than on the development of infrastructure to collect and analyse the data. This research presents an open software and hardware IoT solution to monitor a laser wire direct energy deposition system installed in a cartesian type 3-axis machine tool. The IoT solution adopts three open-source tools for core issues, such as 1) interoperability, flexibility, and availability; 2) data storage; and 3) data visualization of sensor data and manufacturing process signals. The system architecture is based on one or more edge devices connected to sensors and forwarding their data toward a local API endpoint. The endpoint is created with Node-RED, an open-source visual flow-based development tool for IoT data. Node-RED forwards the data to an open-source InfluxDB database. Finally, the data is visualized with an open-source Grafana application. The system is prototyped, designed, implemented, and tested in a lab environment to monitor a laser-wire direct energy deposition process. The significance of such a flexible IoT data collection system for research and development projects can be integral. Thus, providing savings in time and money can substantially speed up the development of new technologies where the value arises from the sensor data and its analysis.

Significant dislocation strengthening of stainless steel 316L via co-directed energy deposition of silica

This study aims at fully utilizing dislocation strengthening for higher hardness of stainless steel 316 L through two methods: 1) Multi-layer directed energy deposition (DED) is utilized to intensify dislocation generation induced by thermal cycling compared to single-layer DED. 2) Ultra-low thermal expansion coefficient silica particles are incorporated into a 316 L substrate, utilizing thermal misfit stresses between particles and the matrix. Silica particles, dispersed within the substrate, enable both multi-layer DED and surface DED. Nine samples (labeled as 1 L–9 L, respectively) with varying deposition layers from 1 to 9 are prepared. As deposition layers increase, silica particles agglomerate and elongate, promoting micro-crack formation in the matrix. Increasing deposition layers is found to alter gradually the solidification structure from planar to cellular and columnar dendrites, and transform the primary austenite to martensite (approximately 100 % at 9 L). This phase transformation is driven by oxygen-affine Cr and Mn elements diffusing into silica particles. Surprisingly, the hardness is significantly increased from about 170 HV at the base metal, 180–220 HV at 1 L–5 L, and 270 HV at 6 L, to maximum 300–330 HV at 8 L. This improvement is theoretically explained by significant dislocation strengthening mechanism (Δρ ∼ 6.70 × 1014 m−2) amplified by the multi-layer silica DED process. Furthermore, distinct dislocation structures including dislocation entanglement around silica particles, patterned dislocation walls, and nano-lamellae pile-ups of dislocations are found at 8 L from transmission electron microscope analysis. The strengthening origins and the potential of SiO2/316 L bulk composites are briefly explored.