Laser-based Directed Energy Deposition Research Articles

A physical simulation-machine learning model for optimal process schemes in laser-based directed energy deposition process

The major challenge faced is the definition of optimal process variables for rich quality of fabricated parts in laser-based directed energy deposition (DED-LB) processes. However, predicting the optimal process scheme using machine learning models is still challenging owing to the need for a large amount of training experimental data with high costs in DED-LB. In view of this, a physical simulation (PS)-machine learning (ML) model using a relatively small accurate data set of 31 simulation data for optimal process schemes is proposed in DED-LB process in this study. Firstly, a powder-scale high precision phenomenon model incorporating the mass transfer, phase transformations and heat transfer and using Lagrangian particle model to add mass is developed to depict the DED-LB process. Then, a Gaussian process regression (GPR) agent model is established to rapidly and accurately predict the geometry and dilution rate of deposition tracks under different manufacturing parameters based on the high precision simulation results. Finally, a PS-ML model for process parameter optimization is developed using the particle swarm algorithm (PSO), and the optimized parameters are experimentally validated. The results show that the developed powder-scale model and GPR model results are consistent with the Inconel 718 alloy experimental results. The proposed PS-ML model can increase the accuracy of the ML models even with a small simulation data set. None of PS-ML model optimization results has a relative error of more than 3% to the experimental results, and the dilution rate is reduced by up to 61.66% compared to the experimental design parameters without optimization. The proposed physical simulation-machine learning model in this study enables inexpensive and accurate optimization of DED-LB process parameters.

Effect of Pre-Heating on Residual Stresses and Deformation in Laser-Based Directed Energy Deposition Repair: A Comparative Analysis.

Laser-directed energy deposition (DED), a metal additive manufacturing method, is renowned for its role in repairing parts, particularly when replacement costs are prohibitive. Ensuring that repaired parts avoid residual stresses and deformation is crucial for maintaining functional integrity. This study conducts experimental and numerical analyses on trapezoidal shape repairs, validating both the thermal and mechanical models with experimental results. Additionally, the study presents a methodology for creating a toolpath applicable to both the DED process and Abaqus CAE software. The findings indicate that employing a pre-heating strategy can reduce residual stresses by over 70% compared to no pre-heating. However, pre-heating may not substantially reduce final distortion. Notably, final distortion can be significantly mitigated by pre-heating and subsequently cooling to higher temperatures, thereby reducing the cooling rate. These insights contribute to optimizing DED repair processes for enhanced part functionality and longevity.

The effect of the laser beam intensity profile in laser-based directed energy deposition: A high-fidelity thermal-fluid modeling approach

Modeling the thermal and fluid flow fields in laser-based directed energy deposition (DED-LB) is crucial for understanding process behavior and ensuring part quality. However, existing models often fail to accurately predict these fields due to simplifying assumptions, particularly regarding powder particle-induced attenuation in laser power and energy density distribution, and the variable material properties and process parameters. The present work introduces a high-fidelity multi-phase thermal-fluid model driven by a combination of the discrete element method (DEM) and the finite volume method (FVM). Incorporating an enhanced attenuation model for laser energy enables a more precise approximation of powder particle-induced attenuation effects in the laser power and energy density distribution. The study focuses on the influence of laser beam intensity profiles during DED-LB of austenitic stainless steel (AISI 316 L), with model validation conducted through experimental measurements of deposited track dimensions for different beam shapes. The results of numerical simulations demonstrate the critical impact of powder-induced attenuation on the laser power and intensity profiles. Neglecting laser energy attenuation, a common assumption in numerical simulations of DED-LB, leads to overestimations of the absorbed energy of the laser beam, affecting thermal and fluid flow fields, and melt pool dimensions. The present study unravels the complex relationship between the attenuation coefficient (due to the powder stream) and powder stream characteristics, describing the variations of the attenuation coefficient with changes in the powder mass flow rate and powder stream incidence angle. The findings show the critical effects of laser beam shaping on melt pool behavior in DED-LB, with square beams inducing larger melt pool volumes and circular beams creating smaller but deeper melt pools. The proposed enhanced thermal-fluid modeling framework offers a robust approach for optimizing laser-based additive manufacturing across diverse materials and laser systems.

Crack healing via electropulsing treatment applied to additive-manufactured TiC/316L stainless steel composites

Three coupons of TiC/316L stainless steel composites were fabricated by laser-based directed energy deposition (DED-LB). One coupon is crack-free, and the other two coupons have cracks inside. Electropulsing treatment (EPT) was applied to heal the cracks, with various current densities and 75 pulses on each sample. The cracks with widths of 0–200 μm can be healed by EPT. The microstructure of the healed region consists of refined fractured TiC particles and resolidified fine austenite grains. The EPT is able to potentially be applied to heal the defects or cracks in additive-manufactured metal-matrix composites.

Microstructure and fracture toughness of a Nb–Ti–Si-based in-situ composite fabricated by laser-based directed energy deposition

Laser-based directed energy deposition (LDED), a significant integrated preparation technique combining material design and production, has been progressively used to manufacture Nb–Ti–Si-based in-situ composites. However, it is commonly known that Nb–Si-based in-situ composites have low room-temperature fracture toughness. Herein, we have designed a novel Nb–40Ti–10Si–5Al–2V (at.%) composite, the microstructure of LDED-built composite is made up of Nbss and γ-Nb5Si3, the formation of γ-Nb5Si3 precipitate in the Nbss phase with an orientation relationships (ORs) of [1110]γ//[011]Nbss, (10 1‾ 0)γ//(011)Nbss and (1 2‾ 11)γ//(200)Nbss. The composite has Nbss, Nb3Si and γ-Nb5Si3 phases after heat treatment at 1400 °C for 30 h, and δ-Nb11Si4 precipitate has formed in the Nbss phase with an ORs of [100]Nbss//[010]δ, (010)Nbss//(010)δ. The composite processing Nb–40Ti–10Si–5Al–2V (at.%) has outstanding toughness (21.62 MPa·m1/2). It has ascertained that the room-temperature fracture toughness of Nb–Si-based in-situ composite is positively impacted by the high concentration of Ti.

Fabrication of architecturally designed steel for improving isotropy of mechanical properties using directed energy deposition

As an excellent illustration of architectural alloy designs, multilayered structures, which incorporates high-strength and high-ductility alloys in their layered composition, can accomplish unusual combinations of strength and ductility. This is possible because the continuous microstructure consisting of a single phase without a phase boundary in each layer results in a uniform distribution of strain. However, the multilayered structure shows anisotropic mechanical properties due to discontinuity from the phase boundary in the perpendicular direction (z-direction). In this work, an alloy system with a 3D continuous microstructure with two distinct steels was manufactured using laser-based directed energy deposition. The alternately deposited lines of two distinct alloys, along with the rotated consecutive layers, contribute to creating a woven-like structure. This architectural structure features continuous microstructures in multiple directions. Successfully manufactured both multilayered and woven-like systems demonstrated advanced yield strength and uniform elongation compared to each constituting alloy in the xy plane. In the z-direction, however, only the woven-like system exhibited improved properties as in the xy plane; the multilayered system did not. This isotropy in the mechanical properties of a woven-like system results from a 3D continuous microstructure in the z-direction. The strain analyses confirmed the deformation of the brittle body-centered cubic phase and partitioning to the ductile phase which are key mechanisms of mechanical property enhancement with continuous microstructure.

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.

Multi-objective optimisation of process parameters for laser-based directed energy deposition of a mixture of H13 and M2 steel powders on 4Cr5Mo2SiV1 steel

ABSTRACT In present paper, a universal method for multi-objective parameter optimisation of additive manufacturing processes was proposed and successfully applied to laser-based directed energy deposition (DED) experiments with mixtures of H13 and M2 steel powders that were deposited on 4Cr5Mo2SiV1 hot work die steel. The DED experiments were designed and completed based on the response surface method with 13 groups of laser parameters. The microstructure of the deposited alloy steel was observed and its mechanical properties were tested. The deposited steel alloy achieved an ultimate tensile strength (UTS) of 1821 ± 30 MPa with a reasonable elongation of approximately 4.5%, and the bond strength specimens achieved a bond toughness of ∼10.66% with a moderate UTS (1329 ± 28 MPa). A multi-objective optimisation method was proposed based on response surfaces which were established according to microstructural characteristics and mechanical properties data. It provided a basis for achieving high strength or high toughness DED-fabricated steel alloys.

Investigation of thermal behavior of powder stream and molten pool during laser-based directed energy deposition

Laser-powder interaction and molten pool evolution are two main physical processes in laser-based directed energy deposition (L-DED), which greatly affect the morphology and quality of the deposition layer. In this research, a comprehensive three-dimensional analytical-numerical model which integrates a laser-powder interaction model and material deposition model is developed to study the multi-physics coupling characteristics in L-DED process. The concentration of the powder stream is modeled based on conservation of mass and is in good agreement with the experimental result which is captured by high-speed camera. The effects of main parameters, including laser power, powder feeding rate, powder feeding angle and defocus length on the laser attenuation and the temperature distribution of the in-flight powder particles are analyzed. The results show that the laser attenuation rate increases from 0.72% to 1.36% with the powder feeding angle increasing from 45 deg. to 65 deg-. However, the laser attenuation rate is almost the same with the defocus length of 25 mm, 35 mm and 45 mm. Moreover, the peak powder temperature on the deposition surface increases with the increase of powder feeding angle and decrease of defocus length. Accordingly, the heat and mass input conditions at the molten pool surface for the material deposition model, including powder mass flux, effective laser intensity and powder temperature are obtained from the laser-powder interaction model. The heat transport and fluid dynamics of the molten pool are discussed based on the calculated results from material deposition model. Finally, the calculated molten pool geometry shows good agreement with the experimental results with the relative error <8.5%. This work is helpful in process optimization from the point of view of adjusting the parameters related to the powder stream and deeper understanding of laser-powder interaction and molten pool evolution during the L-DED process.

Computational Modeling of Directed Energy Deposition of Titanium Aluminide Plate Geometries

In this work, transient thermomechanical analysis of the laser-based Directed Energy Deposition (DED) process was carried out to predict melt pool dimensions, thermal cycles and residual stress during Titanium Aluminide (TiAl) plate deposition. The predicted melt pool length, width, and depth increased as the laser power was increased from 250 W to 350 W at a constant travel speed of 15 mm/s. The melt pool length increased from 0.425 mm to 0.55 mm, while the depth increased from 0.1 to 0.29 mm, and the width increased from 0.412 to 0.5 mm. The plate's edges (tracks 1 and 6) were found to experience a slightly higher temperature than the middle tracks, which may contribute to higher thermal gradients at the edges. The highest Principal stress of 140 MPa was found in track 1. The magnitude of Principal stresses diminished as track deposition progressed from track 2 to 6 due to the variation of the thermal gradient along the tracks.

Fatigue crack growth behavior of an additively manufactured titanium alloy: Effects of spatial and crystallographic orientations of α lamellae

Laser-based directed energy deposition (LDED) enables the rapid near-net-shape fabrication of large-scale titanium components for aerospace applications. However, the fatigue failure of the LDED-produced titanium alloys hinders their widespread use in critical load-bearing structures subjected to cyclic loading. Here, we investigate the fatigue crack growth behavior of LDED-produced Ti-6Al-2Zr-Mo-V alloy in both as-deposited and heat-treated states. The two states of samples consist of α and β laths with the majority of α laths showing distinct crystallographic and spatial orientations. They show distinct cracking behaviors: (i) Fatigue cracks with lengths lower than ∼400 µm grow along α/β boundaries and are surrounded by limited plastic deformations in the as-deposited sample, while in the heat-treated sample, they grow along basal or prismatic planes of the α lamellae associated with substantial plastic deformations; (ii) Fatigue cracks with lengths over ∼400 µm are primarily deflected by α/β boundaries in the as-deposited sample, but are retarded by severe plastic deformations in the heat-treated sample; (iii) Secondary cracks appear along the α/β boundaries in the as-deposited sample, whereas in the heat-treated sample they initiate in the interior and preferentially along the prismatic plane of the α lamellae due to localized shearing and plastic deformation. The fatigue crack growth behaviors are strongly correlated with orientations (both crystallographic and spatial ones) and the size of the α lamellae in the titanium samples. These findings highlight the significance of simultaneously tuning the spatial orientations of α lamellae and their crystallographic orientations to enhance the fatigue crack growth resistance of LDED-produced titanium alloys.

Laser-Based Directed Energy Deposition of Ceramic Nanoadditivated AA7075 Powder Alloys

AA7075 is one of the most resistant aluminium alloys, so it is frequently used in very demanding industries as aeronautics or defence. However, the 7075 alloy falls into the non-weldable category thus hardly processable through additive manufacturing processes, and specially on laser-based DED (Directed Energy Deposition). The low absorption together with cracking behaviour remain a challenge for the industrialisation of these processes. Alloying with minor elements or addition of nano-reinforcement have been proven as a successful approach to increase its manufacturability. In this work, the feasibility of printing 7075 with nano-TiC as additive was evaluated. Two compositions with 0.5 and 2% in weight were developed by dry mixing. The powders were characterized by scanning electron microscopy (SEM) and flowability was compared with the unreinforced alloy. With the optimal laser process parameters, 3D coupons were printed to be characterized microstructurally, thermally, and mechanically. Process monitoring using thermal and high-speed cameras was carried out to gain insight into the thermal behaviour of the melt-pool and resulting process stability. After printing, aspect ratio of single tracks was measured, and dilution was also evaluated. Although addition of 0.5% of n-TiC promotes a slight improvement on the alloy, allowing it to be mechanically tested, it still presents some defects as porosity. By increasing the content up to 2%, both the quality and the mechanical performance were enhanced significantly.

Development of a laser preheating concept for directed energy deposition

In today’s manufacturing, additive manufacturing processes enable the production of complicated three-dimensional structures that are hard to be manufactured with traditional manufacturing processes. Due to its high build rate, the laser-based directed energy deposition (DED-LB) process is an attractive and versatile process to manufacture these kinds of components. In addition to the production of components, DED-LB is used for repair or coating applications. The DED-LB process consists of a multitude of complex thermal mechanisms with high heating and cooling rates of 5 × 102 up to 5 × 105 K/s. For materials with high hardness or low thermal conductivity like tool steels, cast iron, or tungsten carbide, these high cooling rates can lead to defects in the microstructure like cracks, pores, or delamination between the substrate and the deposited structures. By preheating the substrate, the cooling rates can be reduced and defects can be eliminated. In this paper, a preheating cycle was developed, which uses the laser of a DMG MORI LT 65 DED hybrid machine as a moving heat source for the substrate preheating. For this cycle, process parameters, a tool path strategy, and a temperature control system were developed. The impact of the elaborated concept was shown by depositing tungsten carbide in a nickel matrix on an S235 steel substrate.

Melt pool monitoring and process optimisation of directed energy deposition via coaxial thermal imaging

In Laser-based Directed Energy Deposition of metal powder, the use of optimised parameters allows the deposition of defect-free material, while diverging from these optimised parameters can typically result in high porosity, high dilution or different track geometry. One of the main challenges when building complex geometries is that the geometrical and thermal conditions of the deposition are constantly changing, which requires to adjust the process parameters during the production. In order to facilitate this process, sensors such as thermal cameras can be used to extract data from the process and adapt the parameters to keep the process stable despite external disturbances. In this research, different signals extracted from a coaxial thermal camera are investigated and compared for process optimisation. To investigate such possibilities, five overlapped tracks are deposited at constant laser powers in order to extract average pixel values as well as the melt pool area, length, width and orientation. The behaviour of each track deposition is modelled as a function of the laser power, and these models are used to calculate and test laser power reduction strategies based on different signals. The results show that the melt pool area is the most relevant signal to use for an efficient process control, resulting in a stable process with only ±1.6 % of signal variation from track to track.

Ultrasonic in situ measurements for hybrid metal additive manufacturing

Hybrid additive manufacturing (AM) of metals includes synergistic secondary processes such as milling or peening that impart changes to the component for improved performance. Although a valuable concept, implementation is difficult to verify due to a current lack of in situ inspection methods. In this presentation, two different strategies are described with respect to ultrasonic inspections for hybrid metal AM. Both are used for inspections during laser-based directed energy deposition with milling as the hybrid process. The first uses a transducer mounted below the build plate. This configuration allows waves to interrogate the entire sample during all additive and subtractive manufacturing steps. Experiments with Ti6Al4V show the changes in ultrasonic wave speed and attenuation that track the manufacturing temporally. The second inspection approach uses ultrasonic surface waves at the top surface of the sample. These measurements are used on samples of 316L stainless steel at the end of additive steps as well as before and after milling to quantify localized information regarding the most recent layers. Wave speed and diffuse scattering information show the variability with respect to sample geometry, build height, and process parameters. Finally, advantages and limitations of both approaches and prospects for validated parts are discussed.

The effects and utility of homogenization and thermodynamic modeling on microstructure and mechanical properties of SS316/IN718 functionally graded materials fabricated by laser-based directed energy deposition

The effects and utility of homogenization and thermodynamic modeling on microstructure and mechanical properties of SS316/IN718 functionally graded materials fabricated by laser-based directed energy deposition

Study on the in situ strengthening and toughening mechanism of H13 tool steel/WC-12Co composite using laser-based directed energy deposition

To overcome the unworkability of hot work molds in conventional manufacturing, laser additive manufacturing (AM) is widely adopted to print molds/dies with complex internal structures. However, in the AM process, some hot work steels easily crack (e.g., H11 and H13 tool steel) or exhibit dissatisfactory wear resistance (e.g., MS1 maraging steel). To tackle this issue, toughened functionally graded metal matrix/ceramic composite materials can be fabricated on existing molds by laser directed energy deposition (L-DED). Contrary to previous studies that clad Co on WC to avoid the decomposition of ceramic, in this work, we innovatively utilized a type of WC-12Co powder with a substructure to accelerate its decomposition and improve the toughness of composites. It was found that the unmelted tens-micrometer-magnitude WC-12Co powder and in situ synthesized nano WC coexist in the laser-deposited H13 steel/WC-12Co composites to act as the reinforcement phase. Particularly, all the brittle phases (WC and FexWxC) are wrapped by soft γ phases, alleviating the coefficient of thermal expansion (CET) mismatch between materials and the residual stress generated by laser AM. Consequently, defect-free deposits with varied WC contents are manufactured by L-DED and exhibit high hardness and superior wear resistance at both room and elevated temperatures due to the second phase and grain refinement strengthening mechanisms. These findings provide a disparate metal matrix composite design route for laser additive manufacturing to improve the toughness of metal matrix/ceramic composite materials and obtain exceptional performance.

Effects of laser rescanning on microstructure and mechanical properties of Al-10Si-1Mg alloy produced by laser-directed energy deposition

Laser-based additive manufacturing (AM) is a powerful tool for the production of complex metal parts. In this study, the Al-10Si-1Mg (wt%) alloy was fabricated using the laser-based directed energy deposition (L-DED) process. Additionally, the laser rescanning (LRS) process was used to fabricate samples under the same L-DED processing parameters. The microstructure and mechanical properties of the samples were investigated. Microstructural analysis showed the presence of a unique microstructure containing relatively coarse dendrites near the laser melt-pool boundary and fine equiaxed grains inside the melt pool. LRS was shown to effectively alter the grain size and texture of the alloy. Tensile tests in directions vertical and transverse to the L-DED BD showed that only the L-DED-processed alloys exhibited strong anisotropy in their mechanical properties. The LRS-processed samples exhibited significant improvements in strength and elongation with significantly reduced anisotropy compared to the L-DED-only processed samples. Under the same laser volumetric energy density conditions, increasing the laser scanning speed could effectively reduce the porosity, which resulted in a significant improvement in the mechanical properties with and without the LRS.

Partially melted powder in laser based directed energy deposition: Formation mechanism and its influence on microstructure

The powder–melt pool interaction behavior is crucial in laser-based directed energy deposition (LDED). Partially melted particles, which are formed as a result of this interaction, significantly influence on the microstructure and mechanical performance of multi-material and metal-matrix composites fabricated via LDED. However, the presence of partially melted particles is a contentious issue that has been overlooked in single-material LDED studies. Furthermore, the investigation of partially melted particles is hindered by the difficulties in direct observation. To overcome this obstacle, this study was conducted using a single-bead Ti–6Al–4V printing experiment with a relatively high oxygen content to distinguish partially melted particles directly. The formation mechanism of the partially melted particles was revealed through experimental studies combined with numerical analysis using a self-established model. Additionally, the influence of partially melted particles on the grain structure of LDED–fabricated parts was investigated in a low–oxygen environment. The partially melted particles tend to survive close to the surface of the deposited layer. As the penetration depth increased, the particle size decreased and the aspect ratio increased. The formation of partially melted particles collectively depends on the laser power, scanning velocity, powder size and powder feed speed, differing from the common conclusion that an insufficient input energy results in poor powder melting behavior. Furthermore, a Ti–6Al–4V sample with high–fraction equiaxed grains was fabricated using optimized processing conditions. The partially melted particles significantly affected the solidification behavior. In addition to the heterogeneous nucleation mechanism caused by the partially melted particles, a novel seed crystal mechanism was proposed to support the abnormal formation of equiaxed grains. This study highlights the importance of partially melted particles in LDED, and provides useful insights into in-situ microstructural control in LDED.

Semi-Continuous Functionally Graded Material Austenitic to Super Duplex Stainless Steel Obtained by Laser-Based Directed Energy Deposition

In this work, a semi-continuous functionally graded material (FGM) between an austenitic and a super duplex stainless steel was obtained. These materials are of great interest for the chemical, offshore, and oil and gas sectors since the austenitic stainless steel type 316L is common (and not so expensive) and super duplex stainless steels have better mechanical and corrosion resistance but are more expensive and complex in their microstructural phases formation and the obtention of the balance between their main phases. Using directed energy deposition, it was possible to efficiently combine two powders of different chemical compositions by automated mixing prior to their delivery into the nozzle, coaxially to the laser beam for melting. A dense material via additive manufacturing was obtained, with minimum defectology and with a semi-continuous and controlled chemical compositional gradient in the manufactured part. The evolution of ferrite formation has been verified and the phase fraction measured. The resulting microstructure, austenite/ferrite ratio, and hardness variations were evaluated, starting from 100% austenitic stainless-steel composition and with variants of 5% in wt.% until achieving 100% of super duplex steel at the end of the part. Finally, the correlation between the increase in hardness of the FGM with the increase in the ferrite phase area fraction was verified.