Directed Energy Deposition Process Research Articles

Analysis of self-supporting conformal cooling channels additively manufactured by hybrid directed energy deposition for IM tooling

Additive manufacturing (AM) of injection moulding (IM) tools has attracted significant interest in the polymer manufacturing industry for quite some time. However, hybrid manufacturing (HM) using directed energy deposition (DED), which involves concurrent additive and subtractive manufacture, has not been a commonly used process for IM tooling manufacture. This is apparent despite several advantages over the prevalent laser-powder bed fusion (L-PBF) alternative, including higher build rate, lower cost and integrated machining to directly achieve higher tolerances and surface finish. A key reason for this low utilisation is the limited ability of DED processes to produce circular channel profiles typically used in IM tooling, due to stricter constraints on the manufacturability of overhanging geometry. To address this, a range of self-supporting IM cooling channel profiles suited for hybrid laser and powder-based DED manufacture are proposed in this work. Numerical and experimental evaluations are conducted of the cooling performance of several non-circular conformal cooling channel (NCCC) profiles to identify a profile which achieves the maximum heat transfer for a constant cross-sectional area and coolant flow rate. Experimental studies included AM builds to evaluate the DED manufacturability of the selected NCCC profile on a conformally cooled HM benchmark model, followed by cooling performance characterisation, including a comparison against a reference L-PBF manufactured benchmark model. In conclusion, a shape correcting factor is obtained using response surfaces. This factor is used to convert thermal performance calculations for non-circular profiles to a conventional circular channel profile to simplify the DED manufacturing process for non-circular IM cooling channels.

Influence of the laser-beam intensity distribution on the performance of directed energy deposition of an axially fed metal powder

This paper investigates the influence of laser-beam intensity distribution (LBID) on the performance of the annular laser-beam directed energy deposition (DED) process with axial powder delivery. Three different LBIDs: Gaussian-like (G-LBID), top-hat-like (TH-LBID) and ring (R-LBID) at two LBID diameters were used. The process performance was characterised qualitatively in terms of the melt-pool shape and the process stability and quantitatively by powder-catchment efficiency, selected geometrical and metallurgical properties of the clad. The observed influence of LBID on process performance, as determined by the relationship between LBID and powder stream density distribution (PSDD), decreased with increasing mean surface-energy density and was more significant at larger LBID diameter. The highest powder-catchment efficiencies (90% and 87%) were achieved with the G-LBID and TH-LBID, whose high-intensity centre is aligned with the peak of the Gaussian-like PSDD. A lower powder-catchment efficiency of 77% was achieved with the R-LBID, whose high-intensity region is located at the edge of the melt pool with minimum powder density. However, this also results in the highest and most uniform dilution, the highest metallurgical bond ratio and the lowest lack of fusion porosity at the clad-substrate interface. In addition, the process was stable at the lower values of mean surface-energy density with R-LBID, while balling instability was observed with G-LBID and TH-LBID. It can be concluded that the use of R-LBID at lower values of mean surface-energy density improves the performance of the DED process with axial powder feed in terms of process stability and metallurgical properties of the clad.

Phase-Field Simulation of Solidification Evolution in Directed Energy Deposition Process of GH3536 Superalloys

Phase-Field Simulation of Solidification Evolution in Directed Energy Deposition Process of GH3536 Superalloys

Enhanced localized corrosion resistance of Ni-based alloy 625 processed by directed energy deposition additive manufacturing

We report a significant improvement in localized corrosion resistance of Ni-based alloy 625 processed by directed energy deposition (DED) in 7.6 mol/L LiCl at 50 °C. Disparities observed in polarization behavior between DED and wrought alloy 625 are analyzed through compositional and microstructural differences. The exceptional localized corrosion resistance of the DED alloy likely stems from the refinement of secondary phases resultant from the DED process. The prototypical precipitates larger than ≈1 μm in the wrought material, primarily Nb,Ti-enriched face centered cubic nitrides with the stoichiometry of MN where M = Nb, Ti, Cr, served as pit initiation sites and likely degraded the corrosion resistance.

Effect of oblique angle on the formability, crystal growth behavior and microstructure in laser direct energy deposited thin-wall structure of single-crystal superalloy

Laser directed energy deposition process shows a tremendous potential to additively repair and manufacture single-crystal components in a near-net shape. For the specific non-vertical structure such as the tilted and torsional sidewalls of single-crystal components, achieving complete monocrystalline nature in the manufactured zone is still a great challenge and hazy. In this study, oblique thin-wall structures of single-crystal superalloy were fabricated by coaxial laser directed energy deposition process. The effect of oblique angle on the formability, crystal growth behavior and microstructure formation in thin-wall structures of single-crystal superalloy was discussed. The results showed that the critical oblique angle for good formability of thin-wall structure is 30°. A poor-quality and shapeless deposited bead forms once the oblique angle researches 40°. The thin-wall structure inherits the [001]/<100> crystalline orientation from parent single-crystal substrate through the continuous epitaxial growth of ultrafine columnar dendrites. The competition and alternate development of primary trunks-secondary arms-tertiary arms achieve the multiplication and reproduction of ideal columnar dendrites along the [001]/<100> crystalline orientation in the overhanging region of the thin-wall structure. Periodic segregation forms among the columnar dendrites. Epitaxial-growth continuity of the columnar dendrites with consistent crystalline orientation of substrate can be successfully achieved in large-scale tilted and torsional thin-wall structures of single-crystal superalloy with proper processing parameters. These findings are of great importance in the microstructure control for additively manufacturing complex single-crystal components by laser directed energy deposition process.

Microstructure and mechanical properties of a dilute Mg-Gd-Y-Zr alloy fabricated using wire-arc directed energy deposition additive manufacturing

Mg-Gd-Y-Zr is an important lightweight material in the aerospace field. However, the engineering applications of large-scale Mg-Gd-Y-Zr alloy components are significantly limited by macrosegregation and microstructure coarsening during the casting process. In recent years, wire-arc directed energy deposition (DED), which has high design and manufacturing freedom, provides a new route to manufacture large-scale metal parts. In this work, a dilute Mg-3.2Gd-0.6Y-0.5Zr (wt.%) alloy, which had a degree of alloying comparable to AZ31 commercial alloys, was fabricated using a wire-arc DED process based on tungsten inert gas (TIG) welding, the microstructure evolution, tensile properties, and impact toughness were investigated. Due to the multiple grain refinement effects, the DED sample displayed a fine-grain structure (12.3 ± 7.4 μm), which was comparable to that of the wrought counterpart. Combined with the ductilizing effects of the fine grain and the Gd/Y solutes, the DED sample achieved a high ductility, which was better than most reported additive-manufactured Mg alloys as well as wrought Mg alloys. The good plastic deformation capacity also endowed the DED sample with good crack propagation resistance under dynamic impact load. We hope this work can help guide the further development of high-performance Mg alloys that are specially designed for the wire-arc DED additive manufacturing process.

Wire Based Directed Energy Deposition of JBK-75

Applications and adoption of metal additive manufacturing (AM) are increasing for fabrication of low volume, complex components with novel materials, as well as replacement parts. While the use of powder bed fusion-based processes have been widely used to build complex components with fine feature resolution, there is a volume limitation. Expanding the application of metal AM will rely on other processes that remove this build size constraint. These processes are referred to as Directed Energy Deposition (DED) and can use either powder or wire feedstock. Wire based DED provides the highest deposition rates which shortens the fabrication time making it attractive for fabrication of large parts replacing traditional wrought billets or castings. In this study, an iron-based austenitic superalloy (JBK-75) was deposited using an arc-based, wire-fed (AW)-DED process. The material was metallographically characterized and quasi-static mechanical properties were obtained. The resulting microstructure and mechanical properties are compared with conventional wrought and cast forms of JBK-75 subjected to the same heat treatments. As compared to wrought material, the AW-DED grain size was larger after the heat treatment, although the strengths were similar. Improved homogenization was observed after heat treatment in the AW-DED specimens as compared to the cast specimens.

Microstructural evolution in laser-based directed energy deposition of 316 L stainless steel with interlayer deformation

There has been a significant industrial interest in additive manufacturing (AM) technologies such as directed energy deposition (DED) due to their ability to produce complex geometries with controlled microstructures. More recently, AM processes have been hybridized with plastic deformation technologies to achieve further benefits. In this experimental work, we systematically investigate the microstructural evolution in a DED process, selectively coupled with interlayer deformation using 316 L stainless steel. Our results revealed that the region below the interlayer deformed surface comprised of a recrystallized zone and a retained deformation zone with increased hardness in both zones. The region above the interlayer deformed surface experienced a refined solidification at both the grain and the sub-grain levels, which was attributed to the change in nucleation conditions due to the interlayer deformation. Moreover, microstructural evolution was found to vary significantly under different deformation levels and DED parameters. The extent of the recrystallized zone increased with increasing interlayer deformation level and decreased with faster scan speed. The findings provide comprehensive insights into the microstructural evolution in AM processes coupled with interlayer deformation and could pave the way for quick and cost-effective methods to engineer microstructures for different applications.

Energy and mass assessment of the laser Directed Energy Deposition process (DED-LB) for reduced environmental impact

ABSTRACT Additive Manufacturing (AM) processes have the capability to promote a more rational use of resources, reduce generated waste, as well as repair damaged components to extend their lifespan. With the aim of contributing to a more efficient and environmentally friendlier manufacturing process, this research work investigates the energy consumption of the Laser Directed Energy Deposition process (DED-LB) through the introduction of new parameters in its characterisation, which are energy and mass efficiency. First, the energy efficiency of the laser generator itself has been characterised. Based on the results obtained, it was possible to conclude that pulsed laser operation strategies allow a reduction in consumption of up to 9% compared to a continuous laser operation mode. Second, a design of experiments was carried out to relate the energy and mass efficiency of the deposition process to the input parameters: laser power, feed rate, and mass rate. The parameter that most influences the energy efficiency of the process is the mass rate, where high rates are advisable to maximise its value. On the other hand, it is concluded that an increase in the mass rate or a reduction in the feed rate results in an increase in the mass efficiency

Effects of Oxidized Metal Powders on Pore Defects in Powder-Fed Direct Energy Deposition.

Laser-based additive manufacturing processes, particularly direct energy deposition (DED), have gained prominence for fabricating complex, functionally graded, or customized parts. DED employs a high-powered heat source to melt metallic powder or wire, enabling precise control of grain structures and the production of high-strength objects. However, common defects, such as a lack of fusion and pores between layers or beads, can compromise the mechanical properties of the printed components. This study focuses on investigating the recurrent causes of pore defects in the powder-fed DED process, with a specific emphasis on the influence of oxidized metal powders. This research explores the impact of intentionally oxidizing metal powders of hot work tool steel H13 by exposing them to regulated humidity and temperature conditions. Scanning electron microscopy images and energy-dispersive X-ray spectroscopy results demonstrate the clumping of powders and the deposition of iron oxides in the oxidized powders at elevated temperatures (70 °C for 72 h). Multi-layered depositions of the oxidized H13 powders on STD61 substrate do not show significant differences in cross sections among specimens, suggesting that oxidation does not visibly form large pores. However, fine pores, detected through CT scanning, are observed in depositions of oxidized powders at higher temperatures. These fine pores, typically less than 250 µm in diameter, are irregularly distributed throughout the deposition, indicating a potential degradation in mechanical properties. The findings highlight the need for careful consideration of oxidation effects in optimizing process parameters for enhanced additive manufacturing quality.

Enhanced Microstructure and Wear Resistance of Ti-6Al-4V Alloy with Vanadium Carbide Coating via Directed Energy Deposition.

Ti-6Al-4V alloys are known for their suboptimal tribological properties and are often challenged by durability issues under severe wear conditions. This study was conducted to enhance the alloy's wear resistance by forming a hardened surface layer. Utilizing directed energy deposition (DED) additive manufacturing with a diode laser, vanadium carbide particles were successfully integrated onto a Ti-6Al-4V substrate. This approach deviates from traditional surface enhancement techniques like surface hardening and cladding, as it employs DED additive manufacturing under parameters akin to those used in standard Ti-6Al-4V production. The formed vanadium carbide layer achieved a remarkable thickness of over 400 µm and a Vickers hardness surpassing 1500 HV. Pin-on-disk test results further corroborated the enhanced surface wear properties of the Ti-6Al-4V alloy following the additive-manufacturing process. These findings suggest that employing vanadium carbide additive manufacturing, under conditions similar to the conventional DED process with a diode laser, significantly improves the surface wear properties of Ti-6Al-4V in metal 3D-printing applications.

A Rate- and Temperature-Dependent Thermomechanical Internal State Variable Model of the Directed Energy Deposition Process

A Rate- and Temperature-Dependent Thermomechanical Internal State Variable Model of the Directed Energy Deposition Process

Microstructural Analysis of Grain Morphology and Precipitations on Molybdenum Cladding Layers Fabricated by the Direct Energy Deposition Process

Microstructural Analysis of Grain Morphology and Precipitations on Molybdenum Cladding Layers Fabricated by the Direct Energy Deposition Process

Impact of Boundary Parameters Accuracy on Modeling of Directed Energy Deposition Thermal Field

Within the large Additive Manufacturing (AM) process family, Directed Energy Deposition (DED) can be used to create low-cost prototypes and coatings, or to repair cracks. In the case of M4 HSS (High Speed Steel), a reliable computed temperature field during DED process allows the optimization of the substrate preheating temperature value and other process parameters. Such optimization is required to avoid failure during the process, as well as high residual stresses. If 3D DED simulations provide accurate thermal fields, they also induce huge computation time, which motivates simplifications. This article uses a 2D Finite Element (FE) model that decreases the computation cost through dividing the CPU time by around 100 in our studied case, but it needs some calibrations. As described, the identification of a correct data set solely based on local temperature measurements can lead to various sets of parameters with variations of up to 100%. In this study, the melt pool depth was used as an additional experimental measurement to identify the input data set, and a sensitivity analysis was conducted to estimate the impact of each identified parameter on the cooling rate and the melt pool dimension.

Piezoelectric-laser ultrasonic inspection and monitoring of thin-walled structure fabricated by directed energy deposition process based on guided waves

Thin-walled metallic structures produced by the Directed Energy Deposition (DED) Additive Manufacturing (AM) process are prone to various fabrication defects, which hinder the wider applications of the technique in practice. In-situ inspection and monitoring methodologies are in high demand for improved quality control of printed parts. This paper presents an ultrasonic guided-wave-based method and a prototype that can potentially be used for in-situ inspection of thin-walled structures produced by DED. Lamb waves are excited by a Lead zirconate titanate (PZT) piezoelectric transducer bonded on the DED substrate remotely from the thin wall. The substrate works as a waveguide to transmit the waves which then propagate along the thin wall. A non-contact laser vibrometer is applied to measure the guide wave signals by scanning the surface of the thin wall. The mechanisms of guided wave generation and propagation along the substrate and printed part are theoretically studied. It allows for choosing proper inspection parameters to enhance the measurement sensitivity of guided waves and help interpret the signals for defect detection. Experiments were conducted with DED-produced stainless steel (316L) thin-walled structure. The new method is demonstrated in one example to detect and localize a small defect caused by inconsistent powder delivery of a fabricated thin wall sample, via analysing the B-scan ultrasonic guided wave signals. The new technique provides strong potential for in-situ online monitoring of the DED process.

Advances in Ultrasonic-Assisted Directed Energy Deposition (DED) for Metal Additive Manufacturing

Directed Energy Deposition (DED), a branch of AM processes, has emerged as a significant technique for fabricating large metal components in sectors such as aerospace, automotive, and healthcare. DED is characterized by its high deposition rate and scalability, which stand out among other AM processes. However, it encounters critical issues such as residual stresses, distortion, porosity, and rough surfaces resulting from rapid melting and solidification. As a novel advancement, Ultrasonic-Assisted Directed Energy Deposition (UA-DED) integrates ultrasonic oscillations into DED aimed at addressing these challenges. Herein, the latest research related to the UA-DED process and the current major challenges of the DED process, residual stresses, porosity, and crack defects are critically reviewed. Subsequently, the paper also details the working principle and system components of UA-DED technology and reviews the material improvement by introducing UA into the DED process, grain, porosity, tensile properties, and deposition defects. The most critical optimization methods of process parameter variables for UA and the different material interaction mechanisms between UA and DED processes are identified and discussed in detail. Finally, the perspectives on the research gap and potential future developments in UA-DED are also discussed.

Thermal analysis and microstructure evolution of TiC/Ti6Al4V functionally graded material by direct energy deposition

Metal/ceramic functionally graded materials (FGMs) have attracted significant attention due to their ability to combine the unique mechanical properties of metals with the high-temperature resistance of ceramics while mitigating the thermal stresses arising from the differences in their thermal expansion coefficients. In this study, the directed energy deposition (DED) technology is applied to manufacturing TiC/Ti6Al4V FGMs with TiC contenting up to 50 wt%. However, the variations in composition gradients and the complex thermal cycling processes lead to difficulties in the microstructure evolution and control. Therefore, a combined approach involving finite element (FE) simulations and experiments was employed to investigate the mechanisms underlying microstructural transformations under various gradient thermal cycles. The results showed that the morphology of TiC in the FGM from top to bottom is as follows: coarse dendritic TiC + unmelted TiC, granular primary TiC + underdeveloped dendritic TiC, chain-like eutectic TiC + granular eutectic TiC. Meanwhile, both α-Ti and β-Ti exhibited equiaxed transformation, with their sizes decreasing continuously with the gradient. This is due to the strong orientation relationship between TiC particles and the titanium matrix, allowing α-Ti and β-Ti to form non-uniform nucleation within the TiC phase. In addition, the chain-like eutectic TiC and dendritic primary TiC showed a certain degree of fragmentation and granulation after undergoing thermal cycling. The microstructural evolution mechanism in different gradient regions during the DED process under the coupling effect of thermal cycling and ceramic composition was clarified by combining the FE simulation results. The specimen at the bottom of FGMs exhibits strong and ductile (tensile strength: 1296 MPa, elongation: 7.17 %), which are mainly attributed to the Hall Petch strength and solid solution strengthening. The brittle phase of dendritic TiC and unmelted TiC under high gradients leads to decreased tensile properties.

Comprehensive study of the cold-temperature directed energy deposition: Multi-physics modelling and experimental validation

Currently almost all the directed energy deposition (DED) processes are performed under ideal lab conditions with the room temperature close to 20 °C. By contrast, the cold-temperature DED has potentially onsite use in cold environments, such as the winter in the Northern Hemisphere, where the ambient temperatures are often near or below freezing point temperature (0 °C). However, there is a lack of study on the cold-temperature DED process due to its unique temperature conditions. To fill this gap, the authors employed both multi-physics modelling and experimental approaches to study the DED process at cold temperature (CT) −3 °C, and compare to the room temperature (RT) 20 °C DED process. Stainless Steel 316L (SS316L) was selected as powder material. The multiple physics in the DED process involving laser heat source, local melting, rapid cooling, solidification, phase change, evaporation, and fluid-gas interactions were modeled using the computational fluid dynamics (CFD) – volume of fluid (VOF) approach. To validate the model, the DED experiments were conducted using a cryogenic DED platform in which the DED process was chilled to customized low temperatures. This high-fidelity model predicted accurate thermal results and deposit geometries for both the CT and RT DED cases. The results show that under sub-freezing conditions, the deposit size of the CT-DED is ∼62.1 % bigger than the RT-DED in the mean area throughout the length of the single-track thin wall. Furthermore, the CT-DED results show a steeper thermal gradient and at the same time a lower heat flux near the substrate. This high-fidelity modelling framework can be effectively expanded to study more cold-temperature additive manufacturing cases, like the Arctic exploration and in-Space manufacturing.

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.

Powder degradation as a consequence of laser interaction: A study of SS 316L powder reuse on the laser directed energy deposition process

The feedstock capture efficiency on powder laser directed energy deposition (L-DED) is becoming a big challenge in the industrial use of the L-DED process for the manufacturing of large-scale AM parts. The powder capture efficiency is dependent on process optimization and the toolpath. The current literature presents a vast range of usual powder efficiency, between 3% and 32% and in some specific cases exceeding 90%. In L-DED, the powder-gas jet stream interacts with the laser beam adding material locally onto the substrate. Part of this material is captured by the melt pool. The not captured material that is affected by the laser beam suffers degradation. In the literature, there is a lack of studies related to powder reuse in the L-DED process. This paper presents a comprehensive study on the consequence of laser interaction with SS 316L metal powder particles during the L-DED process using a range of different powder characterization techniques to assess the powder morphology, size distribution, chemical composition, followability, and density. The study was conducted within eight powder reuse cycles, without adding virgin material to the powder batch. Reduction of particle size distribution range, increase in circularity, and improvement in the powder flowability were identified as consequences of powder reuse. The result of laser interaction with particles was further explored by scanning electron microscopy, presenting the continuous modification of the particles across the eight reuse cycles. The oxygen content on the particles was also measured to access the O2 pick-up as a consequence of particle heating.