Laser Powder-bed Fusion Additive Manufacturing Research Articles

An automated computational framework to construct printability maps for additively manufactured metal alloys

In metal additive manufacturing (AM), processing parameters can affect the probability of macroscopic defect formation (lack-of-fusion, keyholing, balling), which can, in turn, jeopardize the final product’s integrity. A printability map classifies regions in the processing space where an alloy can be printed with or without porosity defects. However, the creation of these printability maps is resource-intensive. Previous efforts to generate printability maps have required single-track experiments on pre-alloyed powder, limiting the utilization of these printability maps for the high-throughput design of printable alloys. We address these challenges in the case of Laser Powder Bed Fusion AM (L-PBF-AM) by introducing a fully computational, predictive approach to create printability maps for arbitrary alloys. Our framework uses physics-based thermal models and a variety of defect formation criteria. We benchmark the predictive ability of the proposed framework against literature data for the following commonly printed alloys: 316 Stainless Steel, Inconel 718, Ti-6Al-4V, AF96, and Ni-5Nb. Furthermore, we deploy the framework on NiTi-based Shape Memory Alloys (SMAs) as a case study. We scrutinize the accuracy of various sets of defect criteria and use these accuracy measurements to create an uncertainty-aware probabilistic framework capable of predicting the printability maps of arbitrary alloys. This framework has the potential to guide alloy designers to potentially easy-to-print alloys, enabling the co-design of high-performing printable alloys.

Laser powder bed fusion of high-strength crack-free Al7075 alloy with the in-situ formation of TiB2/Al3Ti-reinforced phases and nucleation agents

The existence of solidification cracks caused by columnar grains in precipitation-hardened aluminium alloys limit the applicability of Al7075 components manufactured via laser powder bed fusion (LPBF) additive manufacturing. A novel approach was developed to co-incorporate submicron-sized B and micron-grade Ti6Al4V to eliminate hot cracks and to effectively transform coarse columnar grains into fine equiaxed grains, thus improving the mechanical performance of LPBF-fabricated modified Al7075 material. The grain refinement was mainly attributable to the heterogeneous nucleation promoted by the combination of in-situ-formed L12-Al3Ti and TiB2 nano-sized phases. After an optimised T6 heat treatment, excellent comprehensive mechanical properties were achieved, with a tensile strength of 460 MPa and an elongation of 13 %. This research provides an efficient and cost-effective path for addressing crack-sensitive metallic materials used for LPBF additive manufacturing processes.

On morphology and roughness of upskin surfaces in laser powder-bed fusion additive manufacturing – Contouring strategy effects

The surface quality of inclined parts made by Laser powder-bed fusion (L-PBF) additive manufacturing is inferior to those made by conventional formative and subtractive manufacturing techniques. The inherited step edges formed during such layer-by-layer fabrications and partially melted or non-melted particles attached to the newly formed surface are the major causes of poor surface finish of L-PBF parts. This study is a part of a research framework, in which the effects of different contouring strategies and scan parameters on the surface morphology of inclined parts are investigated by both experimental and numerical methods. In particular, the effect of two different contour scan strategies, namely pre-contouring and post-contouring, on the upskin surface roughness of 30⁰ inclined parts is analyzed. The inclined specimens were fabricated with Ti6Al4V powder using an EOS M270 system and the upskin surfaces were measured by a white light interferometer. In addition, a multi-track multi-layer numerical approach using coupled discrete element method and thermo-fluid simulation was also performed to understand the inclined surface formation in L-PBF.The findings show that the pre-contouring strategy with high linear energy density (LED) (0.4 J/mm) results in average surface roughness (Sa) as low as 7.43 µm, whereas the lowest LED (0.05 J/mm) resulted in the highest Sa of 33.74 µm. Larger melt pool dimensions were obtained in the high LED pre-contouring simulations, which indicates a better material fusion resulting in a good surface finish. The numerical simulation showed that the upskin surface roughness is also affected by the raster scan, if the raster scanning LED is higher than that of pre-contour scans. For the post-contouring strategy studied, higher and lower LEDs were used in inner and outer contour scanning, respectively. It is indicated that the inner contour scanning with a high LED reduces the surface roughness because of a larger melt pool size, irrespective of outer contour scanning. The lowest Sa obtained was 8.24 µm for the highest LED inner contour (0.05 J/mm) case and the highest Sa was 19.95 µm, for the parts made with the lowest LED inner contour scan (0.075 J/mm) strategy. The coupled discrete-element and thermo-fluid-based simulations offer insights into the sloped surface formation and the results supported the experimental findings in qualitative manner.

Machine learning predictions of spatter behavior in LPBF additive manufacturing

The adoption of additive manufacturing (AM) technologies, particularly Laser Powder Bed Fusion (LPBF), has been rapidly increasing in industries requiring high precision and complex geometries. Despite its advantages, LPBF faces challenges related to defects that affect material quality, with spatter formation being a significant concern. Spatters—tiny particles ejected during the printing process—can adversely affect the final product’s integrity by altering surface roughness and contributing to defects. This study introduces a comprehensive approach to predict the ejection velocity and direction of spatter particles using a suite of machine learning (ML) algorithms, including Random Forest, Gaussian Process Regression, Support Vector Machine, Regularized Linear Regressions, Gradient Boosting Trees, and Neural Networks. Our analysis reveals that the Neural Network model outperforms others, achieving prediction accuracies of 97.58% for spatter velocity and 88.22% for ejection direction, thus offering a substantial improvement in understanding and controlling spatter-related defects in LPBF processes. The practical implications of these predictions are profound, enabling manufacturers to adjust AM parameters in real time to minimize defects and enhance product quality. This study not only fills a gap in the current literature by providing a detailed comparative analysis of multiple ML algorithms for spatter ejection prediction but also paves the way for future research into real-time monitoring and control systems in AM.

Vibration absorption of as-built and post-heat-treated Ti-6Al-4V alloy fabricated by laser powder bed fusion additive manufacturing

Vibration absorption of the Ti-6Al-4V alloy fabricated by laser powder bed fusion additive manufacturing (LPBF-AM) was evaluated following as-built and post-heat treatment (i.e., solution treatment followed by water quenching). The base plate was heated at 50°C or 200°C during building. Results showed that the vibration absorption of the as-built Ti-6Al-4V alloy was higher when the base plate was heated at 50°C than at 200°C. Further, the vibration absorption indicated strong anisotropy, with the highest vibration absorption in the direction perpendicular to the building direction and transverse to the laser scanning direction. However, after solution treatment followed by water quenching, the anisotropy in the vibration absorption of the LPBF-AMed Ti-6Al-4V alloy practically disappeared, and relatively high values were obtained parallel to each direction.

Sawtooth scanning strategy for additive manufacturing

PurposeThis study aims to improve the acceleration in the additive manufacturing (AM) process. AM tools, such as extrusion heads, jets, electric arcs, lasers and electron beams (EB), experience negligible forces. However, their speeds are limited by the positioning systems. In addition, a thin tool must travel several kilometers in tiny motions with several turns while realizing the AM part. Hence, acceleration is a more significant limiting factor than the velocity or precision for all except EB.Design/methodology/approachThe sawtooth (ST) scanning strategy presented in this paper minimizes the time by combining three motion features: zigzag scan, 45º or 135º rotation for successive layers in G00 to avoid the CNC interpolation, and modifying these movements along 45º or 135º into sawtooth to halve the turns.FindingsSawtooth effectiveness is tested using an in-house developed Sand AM (SaAM) apparatus based on the laser–powder bed fusion AM technique. For a simple rectangle layer, the sawtooth achieved a path length reduction of 0.19%–1.49% and reduced the overall time by 3.508–4.889 times, proving that sawtooth uses increased acceleration more effectively than the other three scans. The complex layer study reduced calculated time by 69.80%–139.96% and manufacturing time by 47.35%–86.85%. Sawtooth samples also exhibited less dimensional variation (0.88%) than zigzag 45° (12.94%) along the build direction.Research limitations/implicationsSawtooth is limited to flying optics AM process.Originality/valueDevelopment of scanning strategy for flying optics AM process to reduce the warpage by improving the acceleration.

Achieving strength-ductility synergy of laser powder bed fusion Ti–6.5Al–2Zr–1Mo–1V alloy by regulating laser power

In general, the Ti–6.5Al–2Zr–1Mo–1V (TA15) components fabricated via laser powder bed fusion (L-PBF) exhibit high strength and low ductility. Herein, we report a novel approach to enhance the comprehensive mechanical properties of L-PBF TA15 alloy by adjusting the laser power. Samples processed using the optimal laser power exhibit a grid structure of alternating wide and narrow prior-β grains (PBGs), the inside of which is composed of a fully martensitic microstructure. The paper discusses in detail the alterations in the microstructure of samples processed at both low and high laser powers, clarifying the relationship between microstructure and mechanical properties. Thinner martensite contributes to higher strength, while a homogenous microstructure improves ductility. These findings provide valuable insights for controlling microstructure and achieving strength-ductility synergy in L-PBF additive manufacturing of titanium alloys.

Spatter transport in a laser powder-bed fusion build chamber

The adverse effects of spatter particles are well known in laser powder-bed fusion (LPBF) additive manufacturing. To prevent the deposition of spatter particles, an inert gas flow is commonly used to transport these spatters away from the build plate. However, the inert gas flow does not remove all spatters due to varying spatter sizes and ejection angles. Therefore, it is essential to understand and predict spatter trajectories to achieve superior LPBF parts. The present study focuses on numerical modelling of spatter trajectories in Renishaw AM250 using an Eulerian-Lagrangian discrete phase model. The argon velocity profile and spatter trajectories with and against the flow are computed for various materials, sizes and ejection angles. The simulation results are validated with experimental results and show a presence of uneven flow due to inlet geometry along with varying flow profiles across the build height due to inlet location. Spatter analysis shows three methods which result in spatter deposition. Spatter particles either fall directly on the build plate, are transported by the airflow or are re-directed in the recirculation zone. The findings presented in this work indicate the importance of build chamber design along with material-based parameter optimization that results in maximum spatter removal.

A Gaussian Process-Based extended Goldak heat source model for finite element simulation of laser powder bed fusion additive manufacturing process

Laser powder bed fusion (L-PBF) additive manufacturing (AM) is a key enabling technology to manufacture highly complex and integrated metallic structures. In L-PBF AM process, the melting of the metal powders and the layers underneath can be governed by either “conduction mode” or “keyhole mode”, with the keyhole mode reportedly leading to porosity and decreased strength and ductility by many studies. In part scale simulations, finite element (FE) model is often used to study the temperature distribution during printing and to predict the residual stress, where a volumetric heat flux with a Gaussian or a double ellipsoidal (Goldak) distribution is often applied as the laser heat source. However, the above heat source models can only capture the melt pool shape in the conduction mode, and fail to capture the transition to keyhole melting mode when the process parameters change. To overcome this inaccuracy, an extended Goldak heat source model is proposed by introducing a laser penetration term as a function of laser parameters obtained from a Gaussian-Process (GP) model. The model is validated by “2D pad” AlSi10Mg L-PBF experiments under a wide range of laser power, scan speed, and laser focus offset, and the results show the model successfully captures the measured melt pool shape in all conditions.

Mechanical properties of high-entropy alloys CoCrFeNiMnWx manufactured by laser powder-bed fusion additive manufacturing using mechanically alloyed plasma spheroidized powders

Mechanical properties of high-entropy alloys CoCrFeNiMnWx manufactured by laser powder-bed fusion additive manufacturing using mechanically alloyed plasma spheroidized powders

Spectral Behavior of Fiber Bragg Gratings during Embedding in 3D-Printed Metal Tensile Coupons and Cyclic Loading.

Additive manufacturing (AM) enables the spatially configurable 3D integration of sensors in metal components to realize smart materials and structures. Outstanding sensing capabilities and size compatibility have made fiber optic sensors excellent candidates for integration in AM components. In this study, fiber Bragg grating (FBG) sensors were embedded in Inconel 718 tensile coupons printed using laser powder bed fusion AM. On-axis (fiber runs through the coupon's center of axis) and off-axis (fiber is at 5° and 10° to the coupon's center of axis) sensors were buried in epoxy resin inside narrow channels that run through the coupons. FBGs' spectral evolutions during embedment in the coupons were examined and cyclic loading experiments were conducted to analyze and evaluate the sensor integration process, complex strain loading, process flaws, and sensing performance. This study also demonstrates that the AM process-born deficiencies such as poor surface finish and staircase effects can be detrimental to the embedded sensors and their sensing performance.

Influence of Pre- and Post-Contouring Strategies to Downskin Sloped Surfaces in Laser Powder-Bed Fusion (L-PBF) Additive Manufacturing.

Among various metal additive manufacturing (AM) technologies, L-PBF is known for fabricating intricate components. However, due to step edges and powder particle attachments, attaining a good surface finish is challenging, especially on downskin surfaces. Contour scanning has potential to improve surface quality because such scanning may dominate the surface formation of sloped features. This study evaluates the effects of pre- and post-contouring strategies on the sloped downskin surfaces fabricated using a commercial L-PBF system with Ti6Al4V powder. L-PBF parts printed at inclination angles 30°, 45° and 60° were investigated. A double-contouring approach with varying processing conditions was employed and surface characteristics were analyzed using data acquired by white light interferometry. The average surface roughness, Sa, surface skewness, Ssk, and percentage area of powder particles attached onto surfaces were statistically evaluated. The lowest Sa obtained for pre- and post-contoured samples is 14.08 µm and 18.88 µm, respectively. For both strategies, the combination of a low laser power and a high scan speed on the interface of downskin surface and underneath powder results in smoother surfaces. However, while comparing both strategies, pre-contouring gives better surface finish for samples built at similar processing conditions, with a difference of nearly 5 µm in Sa.

Qualitative and quantitative characterization of powder bed quality in laser powder-bed fusion additive manufacturing by multi-task learning

Qualitative and quantitative characterization of powder bed quality in laser powder-bed fusion additive manufacturing by multi-task learning

Investigation of melt pool dynamics and solidification microstructures of laser melted Ti-6Al-4V powder using X-ray synchrotron imaging

Titanium alloy (Ti-6Al-4V) is widely used in additive manufacturing (AM) industry. However, as laser powder-bed fusion (PBF-L) additive manufacturing (AM) advances towards reliable production of titanium parts, a thorough understanding of the process-structure-properties (PSP) relationships remain to be fully understood. A study of the laser melting was paired with high-speed X-ray synchrotron imaging at the 32-ID beamline of the Advanced Photon Source at Argonne National Laboratory. Simultaneous melting and imaging was carried out on a Ti-6Al-4V powder layer held in a custom device designed to mimic single-track scans of the PBF-L process at different laser power levels, powder size distributions, and cover gas environments (Ar and He) on top of AM Ti-6Al-4V base metal. It was found that the thickness of the powder layer significantly affected the melt behavior: too much powder led to the formation of molten droplets that wetted the surface of the titanium, yet did not contribute to a uniform melting profile. Residual gas pores in the atomized powder were also observed to contribute to the pores observed in the melt pool, with the porosity of the powder (defined as volume of pores divided by total material volume) constant with powder size distribution (i.e., larger particles contained more entrapped gas, which increased final part porosity). When varying Ar or He through the same gas flow meter settings and nozzle, the difference in flow rates likely contributed more to the resultant porosity of the solidified material than did the thermal conductivity of the gasses, with He being the greater contributor to porosity. The microstructure of the heat affected zone contained α′, α, and an increased β phase fraction relative to the base material. The crystallographic texture of the melt pool region adopted that of the base metal.

Laser Powder Bed Fusion Printing of CoCrFeMnNi High Entropy Alloy: Processing, Microstructure, and Mechanical Properties

Equiatomic CoCrFeMnNi high entropy alloy (HEA) powder was processed by laser powder bed fusion (LPBF) additive manufacturing (AM). The properties of the spherical pre-alloyed CoCrFeMnNi powder were characterized and its processability using LPBF AM was systematically investigated through the volumetric energy density (VED) based on the surface roughness, defects (micro-cracks and porosity) and densification. After optimization, LPBF processing at a VED of 104 J/mm3 achieved highly dense and crack-free vertical and horizontal test specimens with a porosity fraction lower than 0.01% and micro-pores having a mean size of, respectively, 25.9 μm and 13.4 μm, as determined from X-ray micro-computed tomography (μCT) inspection. Scanning electron microscope (SEM) analysis of the as-built (AB) CoCrFeMnNi processed at a VED of 104 J/mm3 showed a heterogeneous solidification microstructure, consisting of columnar grains with a cellular subgrain structure, and electron backscattered diffraction (EBSD) revealed a crystallographic texture mainly along the < 100 > direction. Post treatment with hot isostatic pressing (HIP) was effective in closing the remnant micro-pores in the bulk volume of the AB CoCrFeMnNi. Also, the cellular sub-grain structure in the AB CoCrFeMnNi completely disappeared after HIP and the resulting microstructure consisted of recrystallized equiaxed grains with annealing twins. The room temperature tensile response was anisotropic for AB CoCrFeMnNi with horizontally built specimens exhibiting higher strength and fracture strains (global and local) compared to vertically built ones; HIP reduced the anisotropy in the tensile properties and led to similar tensile strength with elongation values that were ~ 50% higher than in the AB condition. The HIPed CoCrFeMnNi also displayed higher Charpy impact toughness and absorbed energy at both room and liquid nitrogen temperatures compared to the AB material. Examination of the fracture surfaces after tensile and Charpy impact testing revealed ductile features with characteristic dimpled appearance and pointed to the important role of the remnant micro-pores on failure in the AB CoCrFeMnNi. Tribological assessments pointed to the superior low-stress abrasion resistance of AB and HIPed CoCrFeMnNi compared to 316L stainless steel (SS), which was included in this study to reinforce the analysis. SEM observations revealed that scratching and micro-fracture are the dominant wear mechanisms for the CoCrFeMnNi HEA, whereas ploughing and cutting parallel to the abrasive flow direction are the dominant mechanisms for 316L SS. To the authors’ knowledge, this study is the first to evaluate and report the low-stress abrasion resistance of any high entropy alloy. To understand the corrosion behavior, polarization curves of AB and HIPed CoCrFeMnNi were measured in 3.5 wt% NaCl and 1N H2SO4 solutions, and the results were compared to those of 316L SS. The findings indicate that AB and HIPed CoCrFeMnNi outperform 316L SS in a chloride-containing environment, but not in an acid-containing environment. Additionally, observations of hydrogen permeability revealed that AB CoCrFeMnNi permeates a lower volume of hydrogen atoms (by ~ 5 times) compared to 316L SS, despite its higher (by nearly 3 times) diffusion coefficient. Electrochemical hydrogen permeation data showed that the concentration of atomic hydrogen in the sub-surface of AB and HIPed CoCrFeMnNi was, respectively, about 32 and 26 times lower than in 316L SS. This study provides important material–structure–property data and indicates a promising outlook for LPBF of the CoCrFeMnNi HEA with high-performance.

Apparent properties of porous support structure with imperfections in metal additive manufacturing

In the laser powder bed fusion (L-PBF) additive manufacturing (AM) of metals, support structure is necessary for overhang shaped components. This support is porous with periodic 2D lattice microstructure. The purpose of this paper is to predict numerically the apparent properties of porous support made by titanium alloy, Ti-6Al-4V. One of the problems involved in L-PBF AM is the geometrical imperfections, which are also seen in the microstructure of the support. Micro-CT imaging revealed the variation in strut width and the disconnections. They were parameterized and statistically measured for two cases with different design parameter that determines the density of the 2D lattice microstructure. Based on the statistically measured data, three probabilistic microstructure models were generated per one design parameter. The asymptotic homogenization method was applied to calculate the apparent Young’s moduli, shear moduli and thermal conductivities assuming orthotropic material model. The calculated results were compared with those based on the CAD data without imperfections. Finally, the apparent properties were correlated with the design parameter so that they can be used in the process simulation in the design phase before manufacturing.

Solute segregation in a rapidly solidified Hastelloy-X Ni-based superalloy during laser powder bed fusion investigated by phase-field and computational thermal-fluid dynamics simulations

Solute segregation significantly affects material properties and is a critical issue in the laser powder-bed fusion (LPBF) additive manufacturing (AM) of Ni-based superalloys. To the best of our knowledge, this is the first study to demonstrate a computational thermal-fluid dynamics (CtFD) simulation coupled multi-phase-field (MPF) simulation with a multicomponent-composition model of Ni-based superalloy to predict solute segregation under solidification conditions in LPBF. The MPF simulation of the Hastelloy-X superalloy reproduced the experimentally observed submicron-sized cell structure. Significant solute segregations were formed within interdendritic regions during solidification at high cooling rates of up to 1.6 × 106 K s−1, a characteristic feature of LPBF. Solute segregation caused a decrease in the solidus temperature (TS), with a reduction of up to 38.4 K, which increases the risk of liquation cracks during LPBF. In addition, the segregation triggers the formation of carbide phases, which increases the susceptibility to ductility dip cracking. Conversely, we found that the decrease in TS is suppressed at the melt-pool boundary regions, where re-remelting occurs during the stacking of the layer above. Controlling the re-remelting behavior is deemed to be crucial for designing crack-free alloys. Thus, we demonstrated that solute segregation at the various interfacial regions of Ni-based multicomponent alloys can be predicted by the conventional MPF simulation. The design of crack-free Ni-based superalloys can be expedited by MPF simulations of a broad range of element combinations and their concentrations in multicomponent Ni-based superalloys.

Additive manufacturing of defect-free TiZrNbTa refractory high-entropy alloy with enhanced elastic isotropy via in-situ alloying of elemental powders

Laser powder-bed fusion (L-PBF) additive manufacturing presents ample opportunities to produce net-shape parts. The complex laser-powder interactions result in high cooling rates that often lead to unique microstructures and excellent mechanical properties. Refractory high-entropy alloys show great potential for high-temperature applications but are notoriously difficult to process by additive processes due to their sensitivity to cracking and defects, such as un-melted powders and keyholes. Here, we present a method based on a normalized model-based processing diagram to achieve a nearly defect-free TiZrNbTa alloy via in-situ alloying of elemental powders during L-PBF. Compared to its as-cast counterpart, the as-printed TiZrNbTa exhibits comparable mechanical properties but with enhanced elastic isotropy. This method has good potential for other refractory alloy systems based on in-situ alloying of elemental powders, thereby creating new opportunities to rapidly expand the collection of processable refractory materials via L-PBF.

Performance Improvement for the CuCrZr Alloy Produced by Laser Powder Bed Fusion Using the Remelting Process.

Owing to the high optical reflectivity of copper powder, the high-performance fabrication of copper alloys in the laser additive manufacturing (AM) field is problematic. To tackle this issue, this study employs the remelting process during laser powder bed fusion AM to fabricate defect-free and high-performance CuCrZr alloy. Compared to the non-remelting process, the remelting process yields finer grains, smaller precipitates, denser dislocations, and smaller dislocation cells. It realizes not only the dense molding of high laser reflectivity powders but also excellent mechanical properties and electrical conductivity (with an ultimate tensile strength of 329 MPa and conductivity of 96% IACS) without post-heat treatment. Furthermore, this study elucidates the influence of complex thermal gradients and multiple thermal cycles on the manufacturing process under the remelting process, as well as the internal mechanisms of microstructure evolution and performance improvement.

Perspectives on the unusual electrochemical corrosion of Nickel Aluminum Bronze (NAB) alloy fabricated through laser-powder bed fusion additive manufacturing

This research delves into the passivation and electrochemical corrosion response of as-printed and annealed L-PBF NAB, comparing their performance with wrought NAB counterparts. The results unveil that the interplay between microstructural uniformity, higher aluminum content, residual stress, and the presence of martensitic nanotwins substantially enhances the integrity of the corrosion film and improves the uniform corrosion resistance of as-fabricated NAB. Nonetheless, this combination adversely impacts the localized corrosion resistance. Remarkably, annealing, despite its detrimental effect on passivation and uniform corrosion behavior, ameliorates resistance to localized attacks. In contrast, the wrought equivalents exhibit inferior electrochemical corrosion performance across all experimental conditions.