Laser Powder Bed Fusion System Research Articles

Challenges in characterizing additively manufactured AlSi10Mg using X-ray Laue micro-beam diffraction

Abstract Additive manufacturing of metals using for example laser powder bed fusion systems generally results in grains of complex shapes with cellular structure of submicron sizes, accompanied by a high dislocation density. This paper presents preliminary results from characterizing an AlSi10Mg alloy manufactured by L-PBF using non-destructive three-dimensional X-ray Laue micro-beam diffraction. Both synchrotron and laboratory X-ray methods are used. The aim is to identify challenges in characterizing these microstructural features and to propose future research directions to address them.

Laser Powder Bed Fusion and Heat Treatment of the Martensitic Age‐Hardenable Steel (1.2709)

The primary objective of this study is to clarify the fundamental question of whether, in principle, it is possible to dispense with a prior solution annealing process in favor of a direct aging heat treatment for specimens of maraging stainless steel grade X3NiCoMoTi18‐9‐5 (1.2709) produced by laser powder bed fusion (LPBF). The waiver of a solution annealing process would significantly increase the process efficiency and thus support a sustainable and resource‐friendly production of such components. Therefore, the hardness, microstructure, and the present phases of specimens in as‐built + aged condition (AB + A) and solution‐annealed + aged (SOL + A) are examined during this study. Initially, an extended parameter study is performed using a Renishaw AM 250 LPBF system equipped with a pulsed mode laser system to achieve the highest possible apparent density. As test specimens, small cubes are produced for parameter study and are analyzed for porosity by means of optical microscopy. To investigate the relationship between microstructure and hardness in different material states, one series of specimens is aged directly after LPBF processing in the as‐built state (AB + A). For comparison, the other series was solution annealed at 820 °C for 60 min, quenched in water and then aged (SOL + A). A maximum hardness value of 614 HV1.0 is achieved for specimen aged at 490 °C for 120 min in as built condition (AB + A), while 624 HV1.0 was measured for specimen aged at 490 °C for 180 min in conventionally solution annealed + aged (SOL + A) condition. Significant austenite reversion is not observed at aging temperature of 490 °C in both cases. Aging of specimens at temperatures of 540 and 600 °C resulted in reduction of specimen hardness due to higher percentage of austenite reversion. No significant difference between the hardness values of AB + A and SOL + A specimens is observed. It can therefore be concluded that, in principle, conventional solution annealing and ageing can be dispensed with in favor of direct aging. However, as the results are based on small sized specimens, further investigations into the scalability are needed.

Review of In Situ Detection and Ex Situ Characterization of Porosity in Laser Powder Bed Fusion Metal Additive Manufacturing

Over the past decade, significant research has focused on detecting abnormalities in metal laser powder bed fusion (L-PBF) additive manufacturing. Effective online monitoring systems are crucial for enhancing process stability, repeatability, and the quality of final components. Therefore, the development of in situ detection mechanisms has become essential for metal L-PBF systems, making efficient closed-loop control strategies to adjust process parameters in real time vital. This paper presents an overview of current in situ monitoring systems used in metal L-PBF, complemented by ex situ characterizations. It discusses in situ techniques employed in L-PBF and evaluates the applicability of commercial systems. The review covers optical, thermal, acoustic, and X-ray in situ methods, along with destructive and non-destructive ex situ methods like optical, Archimedes, and X-ray characterization techniques. Each technique is analyzed based on the sensor used for defect detection and the type or size of defects. Optical in situ monitoring primarily identifies large defects from powder bed abnormalities, while thermal methods detect defects as small as 100 µm and keyholes. Thermal in situ detection techniques are notable for their applicability to commercial devices and efficacy in detecting subsurface defects. Computed tomography scanning excels in locating porosity in 3D space with high accuracy. This study also explores the advantages of multi-sensor in situ techniques, such as combining optical and thermal sensors, and concludes by addressing current research needs and potential applications of multi-sensor systems.

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.

Energy efficiency of Gaussian and ring profiles for LPBF of nickel alloy 718

Additive manufacturing (AM) has the potential for improving the sustainability of metal processing through decreased energy and materials usage compared to casting and forging. Laser powder bed fusion (LPBF) of high-temperature alloys such as nickel alloy 718 is one of the key modalities supporting this effort. One of the major drawbacks to LPBF is its slow build speed on the order of 5–10 cubic centimeters per hour print speed. This experimental study investigates how to increase the productivity of the LPBF process by switching from a traditional Gaussian laser shape to a ring laser shape using a nLight multi-modal laser. The objective is to increase productivity, reducing energy consumption and time, without sacrificing mechanical properties by switching to the ring laser thereby improving the sustainability of LPBF. Results include measuring the energy consumption of an Open Additive LPBF system during 718 printing and comparing the microstructure and mechanical properties of the two different lasers.

Additive manufacturing of multi-materials with interfacial component gradient by in-situ powder mixing and laser powder bed fusion

Defects (e.g., pores and cracks) and high stress concentration derived from the significant mutation in materials are the critical challenge for multi-material structures fabricated by laser powder bed fusion (LPBF). Introducing a transition zone between dissimilar materials with a composition gradient at the interface may be a promising approach to resolve or minimize these problems. In this work, three distinct CuSn10 copper alloy (CuA)/316 L stainless steel (SS) multi-material gradient structures (i.e., CS, SC-1, and SC-2) were fabricated by a LPBF system with in-situ powder mixing and feeding capability. A buffer layer material based on the mixture of the two base materials was introduced at the interface between CuA and SS. The effects of the building strategies and interfacial composition gradient transition paths on the interfacial bonding characteristics were investigated. The microstructure evolution and the underlying interfacial bonding mechanisms were discussed. It was found that the interfacial bonding behaviors were critically dependent on the occurrence of liquid metal embrittlement induced by Cu penetration into Fe grains, as well as the width and microstructure gradient of the diffusion zone and high stress concentration at the interface. A non-uniform melting behavior of CuA and SS powders could be induced by the distinct thermal-physical properties between the two materials, which could lead to lack-of-fusion pores at the interface. In the CS sample, printing SS on the CuA with a buffer layer material could inhibit the formation of Cu-penetration cracks and increase the width of the diffusion zone, which may enhance the interfacial metallurgical bonding. This work may enhance the essential comprehension of improving interfacial bonding strength of LPBF-processed multi-material structures.

A One-Step Novel Method to Fabricate Multigrade Ti6Al4V/TiN Composites Using Laser Powder Bed Fusion

Ti6Al4V is the most used alloy for implants because of its excellent biocompatibility; however, its low wear resistance limits its use in the biomedical industry. The additive manufacturing (AM) of Ti6Al4V is a well-established technique that is being used in many fields. However, the AM of Ti6Al4V composites is currently under investigation, and its manufacture using laser powder bed fusion (L-PBF) would result in a great benefit for many industries. The one-step novel concept proposed uses a gas-controlled L-PBF system that enables the AM of layers with different compositions. Six millimeter-edged cubes of Ti6Al4V were manufactured in an Ar atmosphere and coated with in situ Ti6Al4V/TiN layers by using an Ar–N2 mixture given the direct reaction between titanium and nitrogen. Unreinforced Ti6Al4V presented a martensitic microstructure, and TiN reinforcement dendrites and a minor Ti2N phase were gradually introduced into an α + β basketweave titanium matrix. The composites’ microhardness, nanohardness, and elastic modulus were 2, 3, and 1.5 times higher, respectively, than those of the Ti6Al4V. Porosity levels (caused by a lack of fusion, trapping gases, and interdendritic porosity), ranged from 7 to 12% (most measured 20–40 µm) and increased with the reinforcement content (15 to 25%). A scaled-up, proof-of-concept design of a hip implant stem was 3D printed using this nitriding method. Since the neck of the stem (top part) is more susceptible to the fracture and fretting corrosion process, the resulting graded material part consisted of unreinforced Ti6Al4V at the bottom and Ti6Al4V/TiN at the top. This change was controlled by gradually adding nitrogen to the atmosphere; moreover, it was found that the more nitrogen in the chamber, the more TiN reinforcement formed in the part. A microhardness of ~450 HV0.1 was measured at the bottom and gradually increased to ~900 HV0.1, with the increment corresponding to the in situ TiN reinforcement amount.

Parameter prediction and laser alignment for synergistic Multi-Laser Powder Bed Fusion

Industry has adopted the use of multiple lasers within one laser powder bed fusion system to increase productivity. The scan field of each laser either partially or fully overlaps with that of other lasers. However, each laser is operated to be independent of the other lasers, with measures in place to make sure they do not interfere with each other. There are potential benefits to operating multiple lasers in close proximity of each other in synchronized scan strategies, but there are significant challenges to overcome, including parameter optimization, laser synchronization, scan field alignment, and the drift thereof during a long print job. This work highlights different approaches to mitigate and overcome these challenges.

Implications of topology optimization on the ecological efficiency of the LPBF process

Laser Powder Bed Fusion (LPBF) offers possibilities towards a sustainable manufacturing due to high powder material recycling rates or manufacturing of lightweight and topology optimized (TO) components. In order to evaluate sustainability respectively resource consumption, Life Cycle Assessment (LCA) is employed, and a virtual model of the physical LPBF process is developed. For conclusive modeling, however, primary data of resource flows and energy consumption must be known. Especially the influence of topology optimization on the resource consumption in LPBF and on the resulting footprint of manufactured geometries is barely investigated. In the present work, TO approaches on an aerospace component are used to reduce the components volume. Two iterations of the component are built on a commercial LPBF system and primary energy data is measured. A LCA model has been developed to compare the ecological impacts of these iterations. Results indicate that by applying TO, the footprint can be reduced.

Towards data driven quality monitoring: alignment and correlation of photodiode-based co-axial melt pool monitoring signals to part quality in laser powder bed fusion

Online quality control of advanced manufacturing processes often utilize process monitoring as a qualification method to reduce or eliminate time consuming and costly destructive and non-destructive tests. The need for process monitoring based qualification becomes more apparent with the advent of laser based additive manufacturing for complex geometries and lot-size-one production. However, the usability of the process monitoring solution heavily relies on the accuracy of the established alignment and assignment of process signals to specific part and defects herein. This research proposes a data fusion strategy for data pre-processing to structure and align multi-source sensor data for improving correlation accuracy in process monitoring. A laser powder bed fusion system equipped with a photodiode-based co-axial melt pool monitoring is used for the study. First, the monitoring system collects photon emission data from the melt pool using photodiodes with high frequency (>100 kHz) which are aligned spatially and temporally using position data (xy-coordinates) of the galvanometer scanner and the laser ON/OFF signals. Next, a clustering approach is used to assign each photodiode signal with the individual parts on the build platform. Finally, correlation between melt pool monitoring signals and part level porosity is studied using micro-computed X-ray tomography (μCT) from a build job containing parts produced under varying process conditions.

Cracking mechanisms and effect of extensive preheating in CM247LC and IN713LC Ni-base superalloy processed by Laser Powder Bed Fusion

Laser powder bed fusion processing of non-weldable γ’-strengthened Ni-based superalloys is challenging due to various cracking mechanisms that occur during the solidification and cooling stages of the materials. The current study offers an insight into the failure mechanisms of the CM247LC and IN713LC alloys, identifying the pre-heating temperatures which can mitigate cracking phenomena during processing. The adopted methodology involved the alloy processing by a standard Laser powder bed fusion system followed by microstructural characterization. It was confirmed that the development of cracks with a length in the range of 100–500 µm became evident in both alloys. High-temperature preheating strategies were then investigated using a simplified approach based on single laser track remelting. At pre-heating levels up to about 900 °C, corresponding to measured cooling rates of the order of 104 °C/s, hot cracks were still observed in the alloys due to γ’ precipitation and segregation of carbide former elements at the cell boundaries according to both microstructure observations and thermodynamic predictions. Preheating conditions at higher temperatures targeting levels above the γ’-solvus temperature, exceeding 1200 °C could promote slower cooling rates, leading to full suppression of cracks in the investigated γ’-strengthened alloys.

Numerical Investigation of the Thermal Distortion in Multi-Laser Powder Bed Fusion (ML-PBF) Additive Manufacturing of Inconel 625

Metal additive manufacturing, especially laser powder bed fusion (L-PBF), is increasingly being used to fabricate complex parts with fine features. Emerging L-PBF systems have large build volumes and several lasers that operate simultaneously. Hence, they can produce large and complex parts at reduced costs and short build times. However, the thermal distortion remains a critical challenge. Hence, a thorough understanding of the impact of multiple lasers on part distortion in multi-laser PBF (ML-PBF) is imperative. Although experimental investigation is possible, a more conducive approach is to design and create suitable predictive models to understand the impact of multiple lasers consolidating a part into layers. To fulfill this goal, in this study, a commercially available and widely used thermo-mechanical model, Netfabb, was used to investigate the effects of multiple lasers for complex scan patterns such as raster, spiral, and Hilbert on the temperature distribution and thermal distortion. The results show that the thermal distortion is minimal for the spiral scan pattern. Additionally, multiple lasers were found to decrease the build time (as expected) while maintaining or reducing the thermal distortion compared with their single-laser counterparts for all scan patterns (except Hilbert). Therefore, the newly developed ML-PBF predictive model is capable of providing critical insights into the effects of using multiple lasers, thereby opening new possibilities for the faster production of complex parts. In the future, small-scale computational models will be expanded to include large-scale parts, and probabilistic models will be developed to establish correlations.

Fringe projector with submillimeter fringe spacing at a meter-scale field of view.

State-of-the-art fringe projection systems generate fringe patterns using digital light projectors (DLP). The axial uncertainty is limited by the smallest fringe period and is directly related to the pixel count. This results in limited accuracy of current DLP systems that affect applications such as in situ measurements for laser powder bed fusion systems, where a submillimeter fringe period is needed for field-of views larger than 500m m×500m m. This work presents a scalable fringe projection technique that enables the generation of stable fringe patterns over a large field of view spanning several meters while maintaining submillimeter fringe periods. This system uses geometric phase gratings to enable variable fringe spacing and fringe orientation capabilities. The system shears a coherent beam in the Fourier plane using a pair of geometric polarization gratings. The separation between the gratings directly affects the fringe spacing, and the orientation of the gratings affects the fringe orientation. The depth of focus is only limited by the coherence of the light source, enabling high fringe periods even on tilted planes. The system is designed with a single path configuration, making the system more robust to environmental noise. With a rotating linear polarizer, we demonstrate that phase-shifting methods could be employed to acquire phase information about the object. This paper employs a single-shot Fourier transform phase estimation technique to process the intensity maps acquired using projected fringe patterns. Further, we demonstrate the capabilities of the system to produce submillimeter fringe spacing and the ability to project fringes on larger scales for measurements.

High cycle fatigue resistance of laser powder bed fused Ti–6Al–4V alloys with processing-induced porosity: Towards damage-tolerant design of printed components

Three groups of Ti–6Al–4V specimens were printed on an industrial laser powder bed fusion system using three volumetric energy densities (47, 38 and 30 J/mm3), called High, Optimal and Low (HED, OED and LED), and subjected to stress relief and annealing heat treatments. The reference OED group had the smallest amount of porosity <0.005%, while the two other groups (HED and LED) had similar levels of porosity (0.03–0.05%), but formed by flaws of different nature: gas pores (HED) and lack-of-fusion defects (LED). First, chemical composition, microstructure and porosity features of each group of specimens were analyzed. Then, uniaxial tensile testing, axial fatigue testing, fractography analysis and fatigue crack growth testing were carried out. Similar chemical compositions and bi-modal microstructures were observed in all the specimens. Micro-computed tomography (μCT) confirmed the targeted porosities and types of pores for each group of specimens, with only the LED specimens having a significant number of pores larger than 0.07 mm. Tensile properties were similar for all the specimens with the exception of elongation to failure, which was significantly lower for the LED specimens as compared to the HED and OED specimens (∼11 vs. ∼14%). Fatigue testing of the HED, OED and LED specimens showed runout (107) stress ranges (Δσ) of, respectively, 450, 495 and 360 MPa, while fatigue crack growth testing resulted in a threshold stress intensity value of ΔK = 3.9 MPa m½ for all the specimens. A Kitagawa-Takahashi diagram was built as a function of the μCT-measured defect size and showed to be an appropriate while conservative tool for the defect tolerant design of Ti–6Al–4V LPBF components. A strong influence of the defect size and proximity to the surface on the fatigue behavior was observed, while the defect shape was shown to have little incidence on this property. Overall, this study showed that the influence of pore size on the fatigue resistance becomes particularly significant once a certain pore size threshold (√area ≈0.2 mm) is exceeded, and that this eventuality coincides with a ∼20% decrease in the volumetric laser fusion energy density as compared to its optimum value.

Patterned keyhole porosity formation in laser powder bed fusion caused by local disturbances in the shielding gas flow

Porosity is a major challenge in laser powder bed fusion systems (PBF) and a crucial contributor to fatigue life. The current approach to remedy this challenge is thermal management, which involves optimizing process parameters to minimize porosity formation from repeated stacking of layers to produce a fully dense part. However, auxiliary mechanisms other than laser power, scan speed, and hatch spacing simultaneously affect the repeatability of the process. For example, the irregular flow of shielding gas over a powder bed disrupts the melt’s pool convective heat transfer resulting in keyhole porosity. This study aims to explain the defect formation associated with the print process caused by local disturbances of the shielding gas flow pattern and the interaction of adjacent rasters. Additively manufactured 316 stainless steel cuboids were investigated by x-ray computed tomography. Results indicated that pores were spatially aligned in a pattern and uniformly distributed. A comparison of the CT scans and the print file showed that the pore pattern aligned with the layout of print cells during the melting of odd layers. During odd layers, the position of the recoater disrupted the shielding gas flow. The disrupted flow created regions with low gas velocity, which led to heat retention at the boundary of the print cells. The heat retained generated a concentrated occurrence of keyhole defects since the melt pools were deeper.

Thermophysical properties of additively manufactured (AM) GRCop-42 and GRCop-84

Additive manufacturing (AM) has enabled new opportunities of high-performance alloys for use in complex components across the aerospace, industrial, automotive, and energy industries. These industries have growing interest in copper alloys and AM for high-performance heat exchangers such as rocket engine combustion chambers. GRCop copper alloys, such as GRCop-84 (Cu-8Cr-4 Nb at%) and GRCop-42 (Cu-4Cr-2 Nb at%), have high thermal conductivity and high mechanical properties at elevated operating temperatures. In this paper, thermophysical properties for GRCop-84 and GRCop-42 produced using laser powder bed fusion (L-PBF) are examined. While L-PBF parameters and resulting microstructures can be achieved in a laboratory environment, those parameters do not always translate well into a production environment, and the variations across multiple machines and powder chemistries must be understood. The repeatability and reproducibility of samples across the commercial supply chain is critical for designers to ensure parts meet operational requirements. Thermophysical properties, such as thermal conductivity and thermal expansion are key design attributes. Thermal conductivities for eight GRCop-42 blocks and two GRCop-84 blocks were measured from room temperature up to 700 ℃. Thermal expansion was also measured from 20 °C to 1000 °C. These GRCop alloy parts were made with gas atomized powders from different vendors and with different L-PBF systems and parameters. Equations for the thermal conductivity and expansion are given. The variation of thermal conductivities for L-PBF GRCop parts is estimated to be less than + /− 4%. Based on phase analysis, compositional analysis, and thermal conductivity measurements, the small variations in thermal conductivities for the L-PBF GRCop parts are elucidated. The thermophysical property variations can be used to establish process capabilities and design guidelines for ongoing commercial use and further research.

Improved Productivity with Multilaser Rotary Powder Bed Fusion Additive Manufacturing.

Laser powder bed fusion (LPBF) enables the fabrication of intricate, geometrically complex structures with a sufficiently fine surface finish for many engineering applications with a diversity of available feedstock metals. However, the production rate of LPBF systems is not well suited for mass production in comparison to traditional manufacturing methods. LPBF systems measure their deposition rates in 100's of grams per hour, while other processes measure in kilograms per hour or even in the case of processes such as forming, stamping, and casting, 100's of kilograms per hour. To be widely adopted in industry for mass production, LPBF requires a new scalable architecture that enables many orders of magnitude improvement in deposition rate, while maintaining the geometry freedom of additive manufacturing. This article explores concepts that could achieve as much as four orders of magnitude increase in the production rate through the application of (1) rotary table kinematic arrangements; (2) a dramatic number of simultaneously operating lasers; (3) reductions of laser optic size; (4) improved scanning techniques; and (5) an optimization of toroidal build plate size. To theoretically demonstrate the possibilities of production improvements, a productivity analysis is proposed for synchronous reluctance motors with relevance to the electric vehicle industry, given the recent increase in the diversity of printable soft magnetic alloys. The analysis provides insights into the impact of the architecture and process parameters necessary to optimize rotary powder bed fusion for mass production.

Effect of high preheating on the microstructure and mechanical properties of high gamma prime Ni-based superalloy manufactured by laser powder bed fusion

Laser powder bed fusion (L-PBF) is an additive manufacturing method with potential applications in the aerospace industry. Currently, IN738LC has been a challenging engineering material to work with as it is inherently brittle due to its high γ′ volume fraction. In this study, crack-free IN738LC was manufactured on a customized L-PBF system capable of preheating to 700 °C. A wide processing parameter range (58–83 J/mm3) could be selected to achieve solid bulk fabrication with a density of 99.95%. The microstructure and mechanical behavior difference between preheating at 200 °C and 700 °C were assessed. In addition to achieving cracking suppression, the high preheating temperature resulted in a strong textured columnar microstructure with finely dispersed carbide particles along the grain boundaries of as-built parts, and superior room temperature ultimate tensile strength of 1612 MPa associated with good ductility of 18% was achieved. Work hardening rate evolution that differed from 200 °C preheating was thought to cause abnormal anisotropic tensile behavior. The influence of preheating on the melt pool morphology, the development of fluid dynamics, and the thermal behavior were discussed according to simulations with the discrete element model (DEM) and volume of fluid (VOF) method.

Measuring the Cooling Behavior of Melt Pools in L-PBF by Pyrometry.

This study aims to measure the cooling rates or, more precisely, the cooling durations of single laser tracks by pyrometry within the laser powder bed fusion (L-PBF) process. Two-color, as well as one-color pyrometers are tested within this work. Regarding the second, the emissivity of the investigated 30CrMoNb5-2 alloy is determined in-situ within the L-PBF system in order to measure temperature instead of arbitrary units. This is done by heating up printed samples and verifying the measured pyrometer signal by comparing it to values obtained by thermocouples attached to the samples. In addition, the precision of two-color pyrometry is verified for the given setup. Following the verification experiments, single laser track experiments are conducted. The obtained signals prove to be partially distorted mainly due to by-products such as smoke and weld beads arising from the melt pool. To encounter this problem, a new fitting method is presented and experimentally validated. Melt pools resulting from different cooling durations are analyzed by EBSD. These measurements show areas of extreme deformation or potential amorphization correlating with the cooling durations. The obtained cooling duration can be used for the validation of simulations as well as for the correlation of corresponding microstructure and process parameters.

Correction: Dimopoulos et al. Multi-Response Optimization of Ti6Al4V Support Structures for Laser Powder Bed Fusion Systems. J. Manuf. Mater. Process. 2023, 7, 22

In the original publication [...]