Laser Directed Energy Deposition Research Articles

Control of microstructure, defects and mechanical properties in direct energy deposited SS316L/Inconel 718 functionally graded material via mechanical vibration

In this study, a mechanical vibration assisted laser direct energy deposition was used to fabricate a functionally graded material composed of stainless steel 316L and Inconel 718. The effect of mechanical vibration on the microstructure, defects and mechanical properties of the prepared functionally graded material was studied. The results indicate that in the sample without the assistance of mechanical vibration, plenty of brittle Laves phase and NbC carbides form and induce the cracking in the specific compositional gradient range from 60 % stainless steel 316L/40 % Inconel 718 to 40 % stainless steel 316L/60 % Inconel 718. The Vickers hardness obtains a minimal value near the compositional region of 50 % stainless steel 316L/50 % Inconel 718. The tensile properties are terrible with an ultimate tensile strength of 309.7 MPa and an elongation of 7.8 %. In the sample with the assistance of mechanical vibration, the synchronous mechanical vibration could effectively restrain the cracks formation in the compositional gradient region by suppressing the brittle phases precipitation, weakening the dislocation aggregation and refining the grain size, and resultantly achieve decent tensile properties with an ultimate tensile strength of 642.6 MPa and an elongation of 22.4 % and significantly improve the deterioration of hardness in the compositional gradient region.

Microstructure, microhardness, tensile and fatigue investigation on laser shock peened Ti6Al4V manufactured by high layer thickness directed energy deposition additive manufacturing

Thin layer thickness in additive manufacturing (AM) has always been a hindering factor leading to long completion times. Increasing layer thickness is accompanied by a higher tendency of surface cracks and inferior mechanical properties. These can be improved with laser shock peening (LSP). Emphasizing on the high layer thickness, this paper elucidates the high-layer thickness laser directed energy deposition (LDED) AM and the effects of LSP on the fabricated parts. Primary focus was given to the investigation of surface microcracks, microstructures, tensile and fatigue properties. Ti6Al4V bulk sample was prepared using the LDED process. Tensile, fatigue, and other samples were extracted to investigate the above-mentioned properties using different characterization tools such as optical microscope, scanning electron microscope, Vickers microhardness tester, and universal testing machine. Results showed proper depositions without major defects despite the high-layer thickness. Micro-cracks were limited to the surface only, and typical Widmanstätten patterns with parallel α lath and mixed α+β microstructures are observed. Yield strength (YS) varied from 730 MPa to 940 MPa, while ultimate tensile strength (UTS) ranged from 799.5 MPa to 953.75 MPa. Even though laser peening significantly improved micro-hardness by 49.81 % on average, not all fatigue samples observed enhanced fatigue life. A maximum improvement of 40.34 % in fatigue performance was observed in the LDED+LSPed samples. The strain hardening, plastic deformation, and grain refinement during the LSP process contributed to the enhanced mechanical properties. Some samples’ reduced fatigue life may be due to the anisotropic nature of additively manufactured parts, internal defects, and surface cracks on the specific samples. Considering high layer thickness, vertical orientation of samples, and as-prepared samples, our research shows a promising high layer thickness additive manufacturing process. The current research can be used as a base for further study on other properties and characteristics.

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.

On the control of epitaxial growth and stray grains during laser-directed energy deposited Ni-based single crystal superalloy

Laser-directed energy deposition (L-DED) is considered as an effective repair method for damaged single-crystal (SX) superalloy turbine blades. However, controlling stray grains (SGs) and epitaxial growth is still the major problem. This study investigated the influences of pre-solution treatment and intrinsic heat treatment (IHT) on the formation of SGs and epitaxial growth of L-DED René N5 SX superalloy. The γ/γ’ eutectics and coarse γ’ precipitates at the inter-dendrite region can be eliminated after pre-solution treatment procedures before the L-DED experiment. A lower temperature in pre-solution treatment resulted in the retention of residual γ/γ’ eutectics and coarse γ’ precipitates. Thus, the aggravated element segregation within the local melt pool and interface collapse during the L-DED process can result in stray grains (SGs) formation. Improved pre-solution treatment procedures can help reduce SGs caused by the residual carbides at the bottom of the molten pool. Besides, the SG formation in the deposited layers can be attributed to the recrystallization as a result of high thermal stress during the IHT in thermal cycles. The optimized pre-solution treatment procedure reduced the elemental segregation of the substrates, resulting in a larger primary dendrite arm spacing (PDAS) from the bottom of the deposited layers. Due to IHT during thermal cycles in the deposited layers, the partial aging treatment promoted the re-precipitation and growth of γ’ phase. Meanwhile, the optimized pre-solution treatment procedure decreased the size of the heat-affected zone (HAZ) caused by thermal cycles. In summary, by optimizing the substrates' pre-solution treatment procedures to eliminate the γ/γ’ eutectics, the formation of SGs resulting from substrate segregation can be controlled. This work provides the potential guidance of substrate pre-treatment for L-DED repair of SX superalloys.

Microstructural evolution and wear resistance of composite WC10%-Ni60AA cermet built by laser directed energy deposition

Microstructural evolution and wear resistance of composite WC10%-Ni60AA cermet built by laser directed energy deposition

Laser-directed energy deposition of bioactive glass on Ti-6Al-7Nb titanium alloy substrate with highly refined grain structure

The bioactive glass S520 was applied to an ultrafine-grained Ti-6Al-7Nb titanium alloy by a laser-directed energy deposition (LDED) process, in order to create a biocompatible material without potentially toxic vanadium for use in load bearing biomedical implants. The laser cladding process influenced the substrate's structure by melting the metallic material at the surface and infusing it with bioactive glass. Subsequently, the melted titanium alloy crystallised, resulting in the formation of relatively large grains. Deeper into the material, laser-induced heat triggered martensitic transformation, leading to the formation of α′ acicular martensite. In the lower regions of the HAZ, a distinctive band with refined grains was identified. Detection of certain amounts of Al directly in the bioactive glass raises concerns about potential toxicity as the glass dissolves in the human body. The nearly complete reduction in P concentration after 14 days highlights the high bioactivity of the material produced.

Towards a Digital Twin Framework in Additive Manufacturing: Machine Learning and Bayesian Optimization for Time Series Process Optimization

Laser directed-energy deposition (DED) offers notable advantages in additive manufacturing (AM) for producing intricate geometries and facilitating material functional grading. However, inherent challenges such as material property inconsistencies and part variability persist, predominantly due to its layer-wise fabrication approach. Critical to these challenges is heat accumulation during DED, influencing the resultant material microstructure and properties. Although closed-loop control methods for managing heat accumulation and temperature regulation are prevalent in DED literature, few approaches integrate real-time monitoring, physics-based modeling, and control simultaneously in a cohesive framework. To address this, we present a digital twin (DT) framework for real-time model predictive control of process parameters of the DED for achieving a specific process design objective. To enable its implementation, we detail the development of a surrogate model utilizing Long Short-Term Memory (LSTM)-based machine learning which uses Bayesian Inference to predict temperatures across various spatial locations of the DED-built part. This model offers real-time predictions of future temperature states. In addition, we introduce a Bayesian Optimization (BO) method for Time Series Process Optimization (BOTSPO). Its foundational principles align with traditional BO, and its novelty lies in our unique time series process profile generator with a reduced dimensional representation. BOTSPO is used for dynamic process optimization in which we deploy BOTSPO to determine the optimal laser power profile, aiming to achieve desired mechanical properties in a DED build. The identified profile establishes a process trajectory that online process optimizations aim to match or exceed in performance. This paper elucidates components of the digital twin framework, advocating its prospective consolidation into a comprehensive digital twin system for AM.

Effect of TiB2 Addition on the Microstructure and Mechanical Properties of Laser-Directed Energy Deposition TiAl Alloy

The microstructure characteristics of TiAl alloy prepared by laser-directed energy deposition (L-DED) are coarse columnar grains parallel to the building direction, which results in serious mechanical properties and anisotropy and limits its application. In the present study, TiB2 can be used as an effective grain refiner due to the extremely high Q value (growth inhibition factor; the larger the Q value of an alloying element, the stronger its grain refinement effect.) of B. With TiB2 addition, TiAl alloys prepared by laser-directed energy deposition with the microstructure of full equiaxed grains were obtained, and the grain size was significantly reduced by about 30% with 0.45 wt.% TiB2. This value has been further increased to 45% when adding 0.9 wt.% TiB2. Moreover, the γm phase was nearly eliminated and the width of (α2 + γ) lamellar was significantly decreased, which has positive effects on mechanical properties. Meanwhile, TiB2 precipitates uniformly distribute in the matrix, as a reinforced particle to increase the hardness and compressive strength of the alloys. The microhardness of the TiAl alloy increased with the increasing content of TiB2. The addition of TiB2 improved the room and high-temperature compressive properties of TiAl alloy while slightly increasing its ductility. These findings have important guiding significance for expanding the application of TiAl alloys.

Hybrid Analytical-Numerical Modeling of Surface Geometry Evolution and Deposition Integrity in a Multi-Track Laser-Directed Energy Deposition Process

Abstract Modeling multitrack laser-directed energy deposition (LDED) is different from single-track deposition. There is a temporal variation in the deposition geometry and integrity in a multitrack deposition, which is not well understood. This article employs an analytical model for power attenuation and powder catchment in the melt pool in conjunction with a robust fully coupled metallurgical-thermomechanical finite element (FE) model iteratively to simulate the multitrack deposition. The novel hybrid analytical–numerical approach incorporates the effect of preexisting tracks on melt pool formation, powder catchment, geometry evolution, dilution, residual stress, and defect generation. CPM 9V steel powder was deposited on the H13 tool steel substrate for validating the model. The deposition height is found to be a function of the track sequence but reaches a steady-state height after a finite number of tracks. The height variation determines the waviness of the deposited surface and, therefore, the effective layer height. The inter-track spacing (I) plays a vital role in steady-state height evolution. A larger value of I facilitates faster convergence to the steady-state height but increases the surface waviness. The FE model incorporates the effects of differential thermal contraction, volume dilation, and transformation-induced plasticity. It predicts the deposition geometry and integrity as a function of inter-track spacing and powder feed rate. The insufficient remelting of the substrate or the preceding track can induce defects. A method to predict and mitigate these defects has also been presented in this article.

In-situ process monitoring and adaptive quality enhancement in laser additive manufacturing: A critical review

Laser Additive Manufacturing (LAM) presents unparalleled opportunities for fabricating complex, high-performance structures and components with unique material properties. Despite these advancements, achieving consistent part quality and process repeatability remains challenging. This paper provides a comprehensive review of various state-of-the-art in-situ process monitoring techniques, including optical-based monitoring, acoustic-based sensing, laser line scanning, and operando X-ray monitoring. These techniques are evaluated for their capabilities and limitations in detecting defects within Laser Powder Bed Fusion (LPBF) and Laser Directed Energy Deposition (LDED) processes. Furthermore, the review discusses emerging multisensor monitoring and machine learning (ML)-assisted defect detection methods, benchmarking ML models tailored for in-situ defect detection. The paper also discusses in-situ adaptive defect remediation strategies that advance LAM towards zero-defect autonomous operations, focusing on real-time closed-loop feedback control and defect correction methods. Research gaps such as the need for standardization, improved reliability and sensitivity, and decision-making strategies beyond early stopping are highlighted. Future directions are proposed, with an emphasis on multimodal sensor fusion for multiscale defect prediction and fault diagnosis, ultimately enabling self-adaptation in LAM processes. This paper aims to equip researchers and industry professionals with a holistic understanding of the current capabilities, limitations, and future directions in in-situ process monitoring and adaptive quality enhancement in LAM.

Effect of laser power and deposition sequence on microstructure of GRCop42 - Inconel 625 joints fabricated using laser directed energy deposition

The joining of Inconel 625 and GRCop42 using additive manufacturing is required for thermal management of high operating temperature components such as rocket combustion chambers. Though prior efforts have been made to use laser directed energy deposition to join these materials, the impact of laser power and deposition sequence on microstructure of these joints is not well understood. In this study, Inconel 625 onto GRCop42 and GRCop42 onto Inconel 625 joints are fabricated by powder directed-energy-deposition at various laser powers and subsequently subjected to characterization in terms of present defects, grain morphology, and phases. Results show lack-of-fusion free Inconel 625 onto GRCop42 joints can be fabricated by increasing the laser power in the first layer. Substrate remelting in GRCop42 onto Inconel 625 joints is found to result in a melt pool composition which induces liquid-state immiscibility, resulting in a Cu-deprived liquid solidifying to form crack prone islands. Increasing laser power decreases the embrittlement of these islands due to the precipitation of a lower volume fraction of intermetallic phases.

Microstructure, tribological property and high temperature tensile property of Co-Cr-Ni-W alloy parts fabricated by laser directed energy deposition

In this study, Co-Cr-Ni-W alloy parts were fabricated on the surface of 304 stainless steel (SS) substrate by laser directed energy deposition (LDED). Microstructure, microhardness and tribological behavior of as-deposited samples and 750 °C–950 °C aging-treated samples were tested. Carbides precipitated from Co-Cr-Ni-W alloy, and martensite transformation from γ-Co (FCC) to ε-Co (HCP) occurred in high temperature. The changes of microstructure caused by aging treatment could increase the microhardness and change wear mechanism. Tensile property of Co-Cr-Ni-W alloy at 750 °C–950 °C was also investigated. Co-Cr-Ni-W alloy formed a hard-sliding structure under high temperature, leading to high tensile strength. The main fracture mode of Co-Cr-Ni-W alloy was brittle cleavage fracture.

Anomalous twin boundary formation in magnesium alloys by rapid solidification

The aim of this work is to investigate the formation of an anomalously high fraction of twin boundaries (TBs) during laser directed energy deposition (DED) of a Mg-3Al-1 Zn (wt.%) alloy. With that goal, single tracks are deposited using different combinations of scan speed and powder feed rate. The melt pool dimensions are related to the DED parameters. Irrespective of the processing conditions, the resulting microstructures are always formed by fine grains with a very weak texture. However, some DED parameter combinations give rise to a fraction of TBs that is significantly higher than that corresponding to a random texture. These TBs are characterized by an irregular morphology and by the presence of segregated atoms, solute clusters, and nanoparticles. Additionally, the lattice in the vicinity of the TBs is highly distorted and contains a high population of stacking faults. All these characteristics set them apart from conventional tensile TBs, which exhibit a large degree of coherency and limited segregation. Rapid solidification is thus presented here as a novel avenue to design TBs with unique properties in Mg alloys. This work paves the way for future studies on grain boundary design via additive manufacturing methods, a field that is still in its infancy.

The microstructural evolution and mechanical response of laser direct energy deposition Inconel 718 alloy based on simulation and experimental methods

During laser directed energy deposition (LDED), the complex microstructure created by multiple rapid heating and cooling cycles results in the uneven performance of the parts. This work investigates the microstructure evolution and mechanical properties of LDED Inconel 718 samples with different deposit shapes of single-track and thin-wall. The crystal plasticity constitutive and statistical microstructure modeling combined with experiments were performed to explore the influence of microstructure on tensile property. Compared to the thin-wall samples, the single-track sample has smaller sub-grains because of the higher cooling rate, resulting in higher hardness. Furthermore, the anisotropy of the mechanical properties is not changed by the scanning strategy, although the grain size and the yield strength with the Hall-Petch relation are affected by the scanning strategy. The slip system with maximum and secondary Schmid factors is more easily activated during deformation. The rotational behavior of the grains suggests that the anisotropy of mechanical properties is closely linked to the dominant texture of the 〈001〉 orientation crystals, including the dominant direction of the grain long axis and the ratio of the long and short axis. The results can provide guidance for understanding the deformation mechanism of LDED parts and optimizing the strength design.

Mechanical Behavior and Shape‐Memory Effects of NixTi1−x Alloys with Tunable Mixing of Ni/Ti Powders by Laser‐Directed Energy Deposition

As it is known, controlling NiTi alloy's microstructure is an urgent problem in additive manufacturing, which directly affects the mechanical properties of the alloy. In laser‐directed energy deposition (LDED) technology, the simultaneous powder feeding equipment with double powder barrels can effectively control the material ratio and realize the generation of NixTi1−x alloys with different atomic ratios. First, the performance differences of NiTi alloy under different thermal treatments are explored. Based on this, the optimal thermal treatment process is used to adjust the microstructure of NiTi alloy fabricated by LDED by changing the Ni/Ti atomic ratio to improve mechanical properties. The results indicate that Ni4Ti3 precipitates can induce the R phase, change the phase transition, and improve the mechanical properties in Ni‐rich NiTi alloys. Besides, the mechanical properties of NixTi1−x alloy can be enhanced by adjusting the Ni/Ti atomic ratio to change the content of Ni4Ti3 precipitates. Ni52Ti48 alloy has a tensile strength of 771 MPa, an elongation of 11.6%, and a recovery rate of 95.2%.

Fabrication of thin silicon flakes through dealloying from laser directed energy deposition Ca-Si alloys

Abstract In this study, thin silicon flakes were fabricated through dealloying from laser directed energy deposition (LDED) Ca-Si alloys, and the solidification mechanism of the lamellar eutectic microstructure was studied. With 1200 W laser power and 450 mm/min scanning speed, the LDED Ca-Si alloys have a fine lamellar eutectic microstructure and are easily separated from copper substrates. After dealloying in the diluted HCl solution for 6 h, the thin silicon flakes ranging in thickness from 100 nm to 0.5 μm were obtained. The formation of the thin flakes was attributed to fine lamellar Si phases in precursor alloys. The rapid solidification of laser processing could refine the microstructure, especially Si phases, and improve the chemical homogeneity of the material. Combining laser processing and dealloying, a cost-effective and scalable strategy was devised to fabricate fine powder from a laser processing optimized microstructure.

Alloying on the high-temperature oxidation behavior of Nb-Ti-Si-based alloy additively manufactured by laser directed energy deposition

Nb-Si-based alloys have important application prospects in the new generation of high-thrust-to-weight ratio aero-engines hot-end components. Understanding how alloying affects the microstructure and oxidation resistance of Nb-Si-based alloys is crucial for their applications. In this work, the effects of Zr, Cr and Mo on the microstructure and high-temperature oxidation resistance of Nb-23Ti-14Si-based alloys prepared by laser directed energy deposition (LDED) were investigated. Results showed that the oxidation resistance at 1250 °C of as-deposited Nb-23Ti-14Si-based alloys enhanced by Zr, Cr and Mo alloying, of these, Cr is the most significant, followed by Zr and Mo. The oxide film of Nb-23Ti-14Si-based alloys have three distinct oxide layer: the inner, middle, and outer oxide layer. The formation of SiO2 layer in the Nb-23Ti-14Si-based alloys' outer oxide layer when Zr and Cr alloying, respectively. Furthermore, following oxidation at 1250 °C for 20 h, and the weight gain per unit area is around 32.52 mg/cm2, and the creation of CrNbO4 layer when Cr content is up to 10 at.%.

Enhanced wear resistance of TiC/Ti6Al4V composites through changing TiC morphologies in laser direct energy deposition

The wear performance of particle-reinforced metal matrix composites is significantly influenced by the morphologies of ceramic particles. In this study, the friction and wear characterization of titanium matrix composites (TMCs) with different TiC morphologies was studied. This TMCs, consisting of 20 wt% of TiC, was prepared by laser direct energy deposition (LDED). Different LDED parameters are employed to changing the morphologies of TiC. Two distinct TiC morphologies, equiaxed and dendritic, are achieved by varying the laser scanning speed and laser power during the LDEDed TMCs. The friction and wear tests of TMCs with different TiC morphologies are carried out by using a ball-on flat reciprocating tribometer. The results reveal that TMCs with different TiC morphologies manifest divergent wear mechanisms. TMCs with equiaxed TiC dominates three-body wear due to the fracture debonding and pulling out of TiC, resulting in a higher wear rate. TMCs with dendritic TiC mainly dominates two-body wear, facilitated by the protection of hard dendrites and the tribolayer, which leads to a 59% improvement in wear resistance compared to TMCs with equiaxed TiC. This work provides a method of TiC morphologies changing to achieve an enhanced wear performance for TMCs.

Design of dissimilar material joint for defect-free multi-material additive manufacturing via laser-directed energy deposition

Additive manufacturing technology has advanced beyond creating optimized features, from strengthening materials to make them lightweight to fabricating multi-material combinations that offer functionalities beyond the capabilities of individual materials. In this study, a lamination method for laser-directed energy deposition (LDED) is developed to achieve dense multi-material features, and a design that combines different and dissimilar materials is developed. To evaluate these novel developments, two materials—AISI 316L stainless steel and Inconel 625—are introduced. Tensile specimens, fabricated via multi-material additive manufacturing using LDED, are subjected to tensile tests that are recorded on video for digital image correlation. After the tests, fracture surface analyses of the fractured specimens are performed via scanning electron microscopy, and optical monitoring analyses are performed on the specimens that are not subjected to the tensile tests. The results indicate that the specimens demonstrate varied mechanical properties due to the influence of lamination direction and order, which affect the formation of critical cracks and pores.

Effects of hatch spacing on pore segregation and mechanical properties during blue laser directed energy deposition of AlSi10Mg

Hatch spacing is a crucial parameter for achieving superior mechanical properties during the process of laser directed energy deposition (L-DED) process. However, the optimum hatch spacing is based on trial and error approaches using pre-existing experience. In this paper, we have systematically compared the porosity characteristics, microstructure evolution, and thermal gradients in double tracks of AlSi10Mg under various hatch spacings during blue laser directed energy deposition (BL-DED). A peculiar phenomenon of pore segregation is observed at the boundary of the overlapping zone of adjacent deposited tracks, where the porosity is almost 8 times that of other areas in the molten zone. Pore segregation consistently exists under different hatch spacings, with 90% of the pores being below 10 μm in diameter. As the hatch spacing increases, the occurrence of pore segregation reduces significantly within the double tracks. Pore segregation tends to occur more frequently at the boundaries of Si eutectic networks due to the cellular growth pattern of α-Al in the solidification process, which pushes the pores towards the Si-based eutectics. The electron backscatter diffraction results reveal that the grain orientations of the double tracks are closely aligned with the <101> direction, and the overlapping area exhibits a mixing characteristic of fine equiaxed and columnar grain structures along the building direction. The BL-DED AlSi10Mg parts exhibited an ultimate tensile strength of 400±4MPa with minimum pore segregation under the hatch spacing of 1.6mm (80% of the width of the single track). This study provides a novel insight into the hatch spacing during BL-DED and the relationship among pore defects, developed Si eutectic networks, and solidification structures. It also challenges the conventional selection strategies for hatch spacing and suggests potential enhancements in the quality and mechanical characteristics of BL-DED fabricated parts.