Laser Additive Manufacturing Process Research Articles

Investigating the effects of powder feeding rate on microstructure transformation in linear laser beam additive manufactured thick-walled structures

The coarse columnar crystals that grow through multiple layers are commonly observed in laser additive manufactured structures, with fractures tending to occur in regions where there are abrupt changes in grain morphology. This can lead to a degradation of the mechanical properties of the samples. In this study on laser additive manufacturing, thick-walled structures made of 304 stainless steel were created using a linear beam spot with a rectangular powder feeding nozzle. By adjusting the powder feeding rate to influence the thermal cycling characteristics during the laser additive manufacturing process, a hierarchical grain structure that combines coarse and fine equiaxed grains was achieved, enhancing the forming efficiency and overall mechanical performance of the structure. The results from the thermal cycling characteristics and microstructure analysis indicate that increasing the powder feeding rate during the additive manufacturing process can decrease the hierarchical cooling rate, temperature gradient, and heat accumulation effect. This reduction in turn decreases the grain size and facilitates the transformation of columnar crystals into equiaxed crystals. Furthermore, the transformation of low-angle grain boundaries into high-angle grain boundaries in the interlayer region helps to reduce stress concentration, weaken anisotropic tendencies, and mitigate intergranular fracture tendencies, ultimately improving the mechanical properties of the overall structure. In actual engineering applications, the powder flow can be adjusted to control the temperature gradient and cooling rate of the deposition layer, thereby altering the grain morphology and optimizing the mechanical properties of additive manufacturing parts.

Laser powder bed fusion in-situ alloying of W-Y alloy: Microstructure, mechanical properties and cracking suppression

The microcracks caused by impurity oxygen are recognized as one of the main challenges in additive manufacturing of tungsten. In this study, yttrium was incorporated into the tungsten matrix to suppress crack formation, and W-Y alloys with minimal cracks were successfully obtained by laser powder bed fusion (LPBF). It is found that during the LPBF process, yttrium combines with impurity oxygen to generate intergranular and intragranular precipitated particles in situ, which effectively suppress cracking behavior due to impurity oxygen. Furthermore, these intracrystalline nanoprecipitate are cubic Y2O3 particles that have formed a semi-coherent interface with the tungsten matrix. These dispersed particles significantly improved the mechanical properties of W-Y alloy. The results offer a corresponding strategy for crack suppression of tungsten alloys in laser additive manufacturing processes, and would provide a benificial additive manufacturing strategy to other refractory alloys.

Microstructure and properties of underwater in-situ wire-based laser additive manufactured duplex stainless steel

ABSTRACT A novel underwater in-situ wire-based laser additive manufacturing (ULAM) technology is proposed for the in-service repair of underwater components in nuclear power plant. Duplex stainless steel (DSS) obtained in air and underwater environments were analysed using material characterisation and testing methods. The effects of underwater additive environments on the microstructure evolution, mechanical properties and corrosion resistance of the specimens were investigated. The results show that the laser heat input is consumed to balance the heat loss of the water-cooled base material during the underwater laser additive manufacturing process, leading to a reduction in the heat input to the molten pool. Underwater specimen exhibit a two-phase balance, with small ferrite grain boundary angles, resulting in better tensile strength and corrosion resistance. Laser reheat treatment leads to a phase change in microstructure, which can enhance the microhardness and the tensile strength. The ULAM system can meet the requirements of actual engineering for cladding layer.

Research about the microcrack mechanisms in molybdenum and corresponding suppression strategies during laser additive manufacturing process

In this study, the mechanism of microcracks in the laser powder bed fusion of molybdenum was analyzed, and the corresponding solutions were proposed. Through specially designed single-track experiment and finite element simulation analyses, we found that thermal stress was the main cause of cracking during the laser powder bed fusion process. The cracks were generated almost synchronously with the laser scanning process, and thus optimizing the process parameters alone could not inhibit the occurrence of cracks. Herein, alloying strategy was proposed and proved to be an effective way to solve the cracking problems. Specifically, by adding Cr and Ti elements, the high-temperature mechanical properties of Mo were improved through solid solution strengthening mechanism, hence the thermal stress induced cracking was suppressed effectively. In addition, according to the thermodynamic calculations, it was found that Cr and Ti had an inhibitory effect on the segregation of impurity at the grain boundaries and this conduce to improve the strength as well.

Influences of the composition design on the microstructure and properties of laser additive manufacturing TiAl alloy coatings

The utilization of titanium aluminide (TiAl) alloys is widespread in the high-temperature sectors of the aerospace industry, despite their challenging manufacturing process and inherent low toughness. Laser additive manufacturing (LAM), as a solution to these challenges, enhances the capability to form complex components and mechanical properties. The influence of process parameters, heat treatment methods, and reinforcing components on the microstructure of TiAl alloys was studied during the LAM process. The principles of designing TiAl alloys were emphasized from the action mechanism of reinforcing components. A classification standard for the component design was proposed based on the different roles of β-stabilizing elements, α-stabilizing elements, rare earth elements, and composites (a combination of the aforementioned components). Furthermore, the work discussed the evolution mechanism of the microstructure and the strengthening mechanism of mechanical properties in TiAl alloys prepared using LAM. These included solid solution strengthening, precipitation strengthening, grain boundary strengthening, grain refinement, and lattice distortion toughening. Finally, future research directions in LAM TiAl alloys were proposed—the innovative design of TiAl alloys through the structural or interfacial control, the development of multiscale characterization techniques for microstructures, and the advancement of high-performance TiAl alloys using data-driven machine learning approaches.

Laser additive manufacturing of a CoCr-based medium-entropy alloy: Impressive stacking fault-induced nanotwinning strengthening through rapid solidification

The directed energy deposition (DED) technique was used to deposit a CoCrNiWC medium entropy alloy (MEA) through a robotic-assisted laser additive manufacturing (LAM) machine. The microstructural features, including solidification grains, structural defects, and phase transformations, along with uniaxial tensile behavior in different orientations, were comprehensively studied. Furthermore, a quantitative assessment was carried out to evaluate the contribution of strengthening mechanisms through apprising stacking fault constant of 11362 MPa nm for the developed MEA, enrolling the impact of FCC to HCP crystal structure transition for the cobalt matrix after rapid cooling LAM solidification in the solid-state board. In addition, the presence of numerous ordinary screw dislocations interacting with each other, stacking faults (SFs), nanotwins, and refined grains led to their hindering as the critical factor for strengthening. The SF intersections resulted in the formation of effective Lomer-Cottrell (LC) locks and nanotwins, which were barriers to slip and attributed to dislocation interactions. Accordingly, the rapid cooling solidification of the LAM process contributed to impressive mechanical properties of deposited CoCr-based MEA with remarkable yield and tensile strengths of ∼895 and 1410 MPa, confirming the direct contributions of statistically stored dislocations upon impact SF and nanotwining strengthening.

Numerical simulation of multi-field coupling behavior and heat and mass transfer mechanism in laser additive manufacturing process

Numerical simulation of multi-field coupling behavior and heat and mass transfer mechanism in laser additive manufacturing process

Research on Thermocouple Integration Method of Hollow Thin-Walled Turbine Rotor Blade Surface

The hollow thin-walled structure is increasingly used in advanced aero-engine turbine rotor blades, and the traditional surface temperature measurement method based on groove buried couple is no longer applicable. In this paper, a new thermocouple integration method is proposed. The laser additive manufacturing process is used to generate thermocouple embedding channels on the surface of turbine blades. After the thermocouple is embedded, it is pre-fixed, and the thermocouple protection is realized by supersonic flame spraying technology. The strength of the thermocouple integrated structure of the turbine blade is verified by finite element analysis. Based on CFD analysis, the influence of thermocouple integration on the efficiency, pressure ratio and temperature distribution of the turbine blade is studied. The high-speed rotation test shows that the thermocouple integrated structure has high strength and reliability and meets the measurement requirements. This method can provide technical support for the surface temperature measurement of hollow thin-walled turbine rotor blades.

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 on the structure and wear performance of laser additively manufactured Cu45Zr45Al6Ti4 metallic glass coating

Laser powder bed fusion (LPBF) technology presents significant potential for the production of metallic glass (MG) coatings. This work investigated the influence of laser power on the structure and wear performance of a CuZr-based metallic glass coating with composition of Cu45Zr45Al6Ti4 on Ti substrate. It was observed that the melt pool, which serves as the fundamental building block for constructing MG coatings in LPBF, undergoes a transition from conduction mode to keyhole mode, and subsequently to extended mode with increasing laser power. The MG coatings prepared using low power laser (140 W) and medium power laser (240 W) both exhibited a nearly fully amorphous state. However, at a laser power of 360 W, noticeable crystallization occurred in the coating, as evidenced by SEM observation. The wear rate of Cu45Zr45Al6Ti4 MG coating was measured to be 4.67 × 10−6 mm3·N−1·m−1, which is significantly lower than that of the Ti substrate (6.38 × 10−4 mm3·N−1·m−1). This study reveals the relationship between the laser power and morphology of the melt pool, as well as the structure and wear performance of the LPBF-fabricated MG coatings, offering guidance for optimizing the laser additive manufacturing process of wear-resistant MG coatings.

Correlating Laser Additive Manufacturing Process with Heat Source Coefficients by Integrated Experimental and Numerical Models

Correlating Laser Additive Manufacturing Process with Heat Source Coefficients by Integrated Experimental and Numerical Models

Fabrication of crack-free aluminum alloy 6061 parts using laser foil printing process

PurposeThis paper aims to present the fabrication of 6061 aluminum alloy (AA6061) using a promising laser additive manufacturing process, called the laser-foil-printing (LFP) process. The process window of AA6061 in LFP was established to optimize process parameters for the fabrication of high strength, dense and crack-free parts even though AA6061 is challenging for laser additive manufacturing processes due to hot-cracking issues.Design/methodology/approachThe multilayers AA6061 parts were fabricated by LFP to characterize for cracks and porosity. Mechanical properties of the LFP-fabricated AA6061 parts were tested using Vicker’s microhardness and tensile testes. The electron backscattered diffraction (EBSD) technique was used to reveal the grain structure and preferred orientation of AA6061 parts.FindingsThe crack-free AA6061 parts with a high relative density of 99.8% were successfully fabricated using the optimal process parameters in LFP. The LFP-fabricated parts exhibited exceptional tensile strength and comparable ductility compared to AA6061 samples fabricated by conventional laser powder bed fusion (LPBF) processes. The EBSD result shows the formation of cracks was correlated with the cooling rate of the melt pool as cracks tended to develop within finer grain structures, which were formed in a shorter solidification time and higher cooling rate.Originality/valueThis study presents the pioneering achievement of fabricating crack-free AA6061 parts using LFP without the necessity of preheating the substrate or mixing nanoparticles into the melt pool during the laser melting. The study includes a comprehensive examination of both the mechanical properties and grain structures, with comparisons made to parts produced through the traditional LPBF method.

Physics guided heat source for quantitative prediction of IN718 laser additive manufacturing processes

Challenge 3 of the 2022 NIST additive manufacturing benchmark (AM Bench) experiments asked modelers to submit predictions for solid cooling rate, liquid cooling rate, time above melt, and melt pool geometry for single and multiple track laser powder bed fusion process using moving lasers. An in-house developed Additive Manufacturing Computational Fluid Dynamics code (AM-CFD) combined with a cylindrical heat source is implemented to accurately predict these experiments. Heuristic heat source calibration is proposed relating volumetric energy density (ψ) based on experiments available in the literature. The parameters of the heat source of the computational model are initially calibrated based on a Higher Order Proper Generalized Decomposition- (HOPGD) based surrogate model. The prediction using the calibrated heat source agrees quantitatively with NIST measurements for different process conditions (laser spot diameter, laser power, and scan speed). A scaling law based on keyhole formation is also utilized in calibrating the parameters of the cylindrical heat source and predicting the challenge experiments. In addition, an improvement on the heat source model is proposed to relate the Volumetric Energy Density (VEDσ) to the melt pool aspect ratio. The model shows further improvement in the prediction of the experimental measurements for the melt pool, including cases at higher VEDσ. Overall, it is concluded that the appropriate selection of laser heat source parameterization scheme along with the heat source model is crucial in the accurate prediction of melt pool geometry and thermal measurements while bypassing the expensive computational simulations that consider increased physics equations.

Recent progress in carbide‐based composite materials fabricated by laser additive manufacturing processes

AbstractFabricating and introducing new materials with new features is one of the most important goals for many scientific groups. Today, additive manufacturing (AM) is known as one of the new production methods, which can create an excellent opportunity for achieving this goal. As one of the ongoing production processes, the laser AM (LAM) methods have the proper capacity for producing new advanced materials like carbide, nitride, and boride‐based materials. Hence, this work aims to review carbide‐based materials produced by LAM techniques with the composite viewpoint. These materials can act as a matrix, reinforcement, or tertiary component in advanced composites. With this viewpoint, these composite materials were introduced in eight separate sections, which are tungsten carbide, titanium carbide, silicon carbide, niobium carbide, chrome carbide, vanadium carbide, and boron carbide, as well as high entropy alloy composites.

Microstructure Evolution and Frictional Wear Behavior of Al–18Si–4Mg–2Mn–0.5Fe–xCu Alloys Fabricated by Laser Additive Manufacturing

The microstructure evolution, corrosion resistance, and wear resistance of Al–18Si–4 Mg–2Mn–0.5Fe–xCu (x = 0,1,2,3,4,5 wt%) alloys prepared by laser additive manufacturing (LAM) process are studied. It is found that the addition of Cu not only produces Cu‐rich strengthened phase, but also promotes the precipitation of the second phase. With the addition of Cu, the corrosion resistance of the alloy decreases, which is due to the increase of potential difference caused by the increase of the strengthening phase. The addition of Cu can effectively inhibit corrosion cracking. The hardness and wear resistance of the alloy increase with the increase of Cu content under the synergistic strengthening of the Q‐Al5Cu2Mg8Si6 phase and Al2Cu phase. When the Cu content is 3 wt%, the hardness and wear resistance of the alloy are the best. However, with the increase of Cu content, part of the Q phase is transformed into the Al2Cu phase; this synergistic strengthening effect is weakened, and the hardness and wear resistance of the alloy are reduced, but still higher than that of the alloy without Cu addition, which indicates that the addition of Cu can effectively improve the hardness and wear resistance of the alloy.

A single-sensor multi-scale quality monitoring methodology for laser-directed energy deposition: Example with height instability and porosity monitoring in additive manufacturing of ceramic thin-walled parts

Laser Additive Manufacturing (LAM) faces various technical challenges, encompassing issues with dimensional accuracy, mechanical properties, and processing-related defects, such as surface roughness, cracking, and residual porosity. Currently, the quality of safety-critical parts produced by LAM processes, which is still improved through trial-and-error adjustments of multiple process variables, have limited real-time monitoring capabilities. This study introduces an innovative method for real-time quality assessment of laser-directed energy deposition (LDED) procedures across multiple scales (macroscale, mesoscale, and microscale) utilizing a single sensor. This approach is founded on two fundamental components: the single-sensor system for data acquisition and the algorithm for simultaneous multi-scale quality monitoring. For the data acquisition system aspect, a single-sensor, high dynamic range (HDR), multi-scale information acquisition system has been developed to mitigate intense radiation, eliminate plume disturbances, and rectifies backlight shadows, facilitating clearer images of deposition contours and the molten pool region. In the realm of data monitoring algorithms, a physics model-driven supervisory algorithm is employed to enable monitoring of contour height instability, while a convolutional neural network (CNN), driven by image data, is utilized for porosity prediction. Then, this algorithm adeptly tackles the challenge of simultaneously monitoring macroscopic contour and mesoscale porosity. Regarding the validation of the method, the outcomes of single-sensor multi-scale quality monitoring in additive manufacturing of ceramic thin-wall parts indicate a 100% recognition rate for deposition contour and molten pools, contour feature recognition with a relative error of under 0.05%, porosity prediction accuracy surpassing 99%, and a monitoring time of 80.4 ms per frame. Single-sensor multi-scale real-time quality monitoring can serve as a methodological support for future real-time quality control in LDED.

Listening for defects in laser additive manufacturing in real time

Researchers from Switzerland's Ecole Polytechnique Fédérale de Lausanne (EPFL) have developed a pioneering approach to defect detection in laser additive manufacturing processes.

Using artificial neural networks to model single bead geometries processed by laser-wire direct energy deposition

Wire-feed laser additive manufacturing processes have gained researchers’ attention because of their potential to reduce material waste, guarantee accuracy, increase material quality and density, and produce a wide dimensional range of final products. Nevertheless, printing materials with desired geometrical properties of the beads is still challenging in such processes. This might be attributed to the need for more sufficient experimental data and precise modeling approaches. In this study, an architecture based on Artificial Neural Networks (ANNs) is developed to model the bead geometries (width, height, and area), considering the wire feed rate, laser power, and travel speed as process parameters. A design-of-experiment based on full factorial design is considered for processing single beads with a Fraunhofer coaxial wire-feed laser system. Inconel 625 wire with a diameter of 1.14 mm and stainless steel substrate are utilized as the experimental materials. Geometrical data is obtained using a laser scanner model RA-7525 SE with 0.026mm volumetric accuracy. The beads’ geometrical details are provided as the feeding data for the proposed ANN. For each bead, a length of 10 mm is considered to calculate the average geometrical parameters, which increases the accuracy of the dataset in comparison to the values acquired via a macroscopic picture of the cross-section of each weld bead. A variety of hyperparameters are chosen and compared regarding precision criteria, including Mean Square Error (MSE), to increase the model‘s accuracy. A train-test separation strategy is considered to evaluate the model‘s accuracy on independent data points. The outcome of this research is an ANN-based geometry prediction model that can be utilized to enhance the development of offline path planners and optimize process parameter selection for a precise geometry toward process control.

Insights into the applicability of laser energy density in directly preparing Al2O3-based eutectic ceramics by laser additive manufacturing

Laser additive manufacturing (LAM) has become a promising technique to directly fabricate high-performance Al2O3-based eutectic ceramics, but the applicability of laser energy density (LED), a comprehensive index commonly used to regulate the LAM process of metallic parts, in preparing brittle ceramic samples with narrower laser processing windows is not clear yet. In this paper, Al2O3/GAP/ZrO2 eutectic ceramics are prepared by LAM to investigate the effects of LED on forming quality and melting dimensions of the as-deposited eutectic samples. The results indicate that the reliability of using LED as a design parameter depends on the way how it is changed. There is a response relationship between the LED and experimental results when individually adjusting laser power P or scanning speed v, but abnormal phenomena are detected when simultaneously changing P and v. It is found that the melting dimensions obtained under higher LED can be unexpectedly smaller. Moreover, there is a significant difference in the melting dimensions despite using the same LED. In addition, 50 layers thin-wall structured Al2O3/GAP/ZrO2 eutectic specimens prepared under a constant LED present significant differences in the forming quality. It is revealed that the contribution of P to LED is much greater than that of v, which is not consistent with the same influence degree reflected in the calculation formula. Thus, there are significant limitations to utilizing LED to regulate the LAM process of brittle ceramics in consideration of the sensitivity of their forming quality to changes in laser parameters.

Effect of Combined Direct Current Electric Field and Pulsed Magnetic Field on the Transient Melt Pool in Laser Additive Manufacturing Process

Effect of Combined Direct Current Electric Field and Pulsed Magnetic Field on the Transient Melt Pool in Laser Additive Manufacturing Process