Powder Bed Fusion System Research Articles

Applying systems engineering principles to develop an open source laser based metal powder bed fusion system

Purpose Conventional powder bed fusion systems, with their high costs, proprietary nature and restrictive fees, limit research opportunities. This study aims to unveil an affordable, open-source hardware, open-source software laser-based metal powder bed fusion system. Recognizing the distinction between DIY and open-source hardware is crucial for widespread acceptance. Design/methodology/approach The authors present a comprehensive system architecture using object process methodology for functions and architecture, a design structure matrix to model system dependencies and classical technical drawing exploded views for select subsystems. Modularization enables high adaptability, fostering potential adoption. Findings The fully open system enables unrestricted research, mirroring common industrial metal laser-based powder bed fusion (L-PBF) systems. While “open” systems are available for purchase, they remain closed-source, lacking source code and technical drawings sharing, hindering contribution and co-development. The authors’ is the pioneering and sole open-source metal L-PBF system, boasting 1,500+ print hours. A series of industrial and academic adopters are currently implementing the system. Originality/value The open system, slicer software and controller offer unique process control, supporting multimaterial printing. The authors shared the design on the OpenAM GitHub page under the CERN-OHL-P v2 Open Source Hardware license. While it is functional for additive manufacturing (e.g. aluminum, tool steel, titanium and stainless steel), the entire process chain is actively evolving, ideal for co-development with the additive manufacturing community.

Additive manufacturing of flexible thermoplastic polyurethane (TPU): enhancing the material elongation through process optimisation

AbstractThermoplastic polyurethane (TPU) is used to produce elastomeric parts with superior wear/abrasion resistance, toughness, shock absorption properties, and flexibility, even at low temperatures. The production of this material through additive manufacturing (AM) techniques has been increasing because of the possibility of tuning the mechanical properties using structural design and build process parameters. Despite the data being limited, TPU produced by AM, mainly based on material extrusion, is much stiffer than the corresponding produced by conventional manufacturing, and, therefore, it shows a limited elongation. This study presents the mechanical characterization of TPU produced by the infrared light powder bed fusion (PBF-IrL) system (HP multi-jet fusion), which has recently been introduced. The properties are compared with TPU produced by open (3ntrA4) and closed (Markforged) material extrusion (MEX) systems. For the open FDM, the effects of the processing conditions are investigated to improve the material elongation and UTS with respect to the data reported in the literature for AM and conventional manufacturing. For this reason, an extensive and comprehensive review has been carried out. Compared to material extrusion, PBF-IrL TPU specimens showed higher Young’s modulus, but poorer A%. Considering the samples produced by MEX and compared to previous results in the literature, the properties obtained in this study are superior both in terms of UTS and A%.

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.

Bi-metallic lattice structures manufactured via an intralayer multi-material powder bed fusion method

The laser powder bed fusion technology is widely utilized for manufacturing intricate metallic components, overcoming design limitations inherent in traditional subtractive methods. Recent advancements in powder bed fusion have extended to the creation of metal multi-material structures, allowing for functionally graded and tailored properties within a single component. While functionally graded lattice structures are commonly achieved by spatially varying the relative density and/or topology of the lattice, multi-material lattices are rarely studied particularly via metal additive manufacturing where multi-material methods are a relatively recent development. This study applies an accessible enhancement to a standard powder bed fusion system for multi-material additive manufacturing, and investigates printed and heat-treated bi-metallic lattices containing regions of both 316L and 17–4PH stainless steel. The bi-material interface is examined in lattice and bulk samples, and the compression behaviour of bi-metallic lattices is investigated experimentally and computationally. The material interface is shown to be robust as built and under loading, and the bi-metallic lattices exhibit greater energy absorption than single-material samples. The inclusion of the ductile 316L in bi-metallic lattices also delays the propagation of local failure and shear bands initiated in the more brittle 17–4PH. Finite element analysis simulates the lattice compression which includes strut fracture and as-built diameters informed by computed tomography, enabling further investigation and analysis of multi-material lattice designs. This work demonstrates the benefit of multi-material additive manufacturing by providing improved performance over single-material designs.

Compositionally graded titanium to aluminum processed by laser powder bed fusion process: Microstructure evolution and mechanical properties

A compositionally graded Ti–Al alloy with a designed composition range from pure Ti to pure Al was successfully produced by a laser powder bed fusion (L-PBF) system equipped with two powder feeders. The compositionally graded Ti–Al profile is confirmed by scanning electron microscopy, X-ray diffraction analysis, electron backscatter diffraction microscopy, and transmission microscopy. The graded material is free of macro-horizontal cracks, but micro-cracks appear at the center of the sample. Varying the Ti and Al ratio along the build direction induces microstructural changes, transforming α+β-Ti into solid solution, creating intermetallics (Ti and Al), and yielding an Al phase with diverse morphologies. Additionally, the Ti–Al ratio's influence on microstructure formation and phase evolution results in varying mechanical properties among the graded material's layers. The results demonstrate that the L-PBF process can achieve continuous compositional grading of Ti–Al materials in a single run. Moreover, this study hold the potential to facilitate the design and production of numerous compositional variants for high entropy alloys and multi-materials, enabling future investigation into microstructural evolution, phase transformation, and physical and mechanical properties.

Eliminating cracks in Ti–47Al–2Cr–2Nb/Ti–6Al–4V micro-laminated composites fabricated by dual-material laser powder bed fusion

The low ductility of titanium aluminide at room temperature has been one of the factors that severely limit its applications. In this study, Ti–6Al–4V (Ti64) alloys were combined with Ti–47Al–2Cr–2Nb (TiAl) alloys to make micro-laminated composites with optimal ply configurations to eliminate crack formation in TiAl and improve the ductility. A dual-material laser powder bed fusion (LPBF) system was used to fabricate the TiAl/Ti64 micro-laminate samples. The presence of cracks, elemental composition and mechanical properties were experimentally characterised and analysed to understand the effect of ply configuration and the mechanism of eliminating cracks. It was found that smaller TiAl ply thickness and lower TiAl volume fraction result in fewer cracks formed in the micro-laminates. Elemental composition analysis reveals that the Al content in the first two powder layers (i.e., 200 μm thick in total) of a TiAl ply is reduced to 10%–35% due to the partial mixture of the TiAl ply and the proceeding Ti64 ply in the LPBF. The reduced Al content leads to combined α-Ti, β-Ti and α2-Ti3Al phases instead of γ-TiAl phases, thus eliminating the formation of cracks within the first 200 μm thick region of the TiAl ply. This study demonstrates that crack-free and dense TiAl/Ti6 micro-laminated composites can be produced with the TiAl ply thickness reduced to 200 μm and the TiAl volume fraction reduced to 25% (Ti64 ply thickness 400 μm). Ductility of the crack-free micro-laminates at room temperature is comparable to or even exceeds the properties of TiAl/Ti64-type laminates made by other manufacturing processes, which may imply good machinability. Note that further research is required to investigate the properties and applications of the micro-laminates at elevated temperatures.

Benchmarking L-PBF Systems for Die Production: Powder, Dimensional, Surface, Microstructural and Mechanical Characterisation

While conventional die manufacturing techniques often lead to limitations in production speed and design intricacy due to labour-intensive procedures like machining and casting, Additive Manufacturing (AM) emerges as a key player offering substantial potential for cost reduction and process improvement in mass production. This study benchmarks four leading Laser Powder Bed Fusion (L-PBF) systems for producing maraging steel (EN 1.2709) dies. Despite the shared material and technology, variations in dimensional accuracy, surface finish, and microstructure were observed among the maraging steel parts. SEM/EDS, EBSD, hardness testing, and dimensional analysis revealed system-specific performance differences. Additionally, select parts underwent heat treatment and tensile testing, demonstrating the impact of post-processing on mechanical properties. These results offer valuable guidance for industrial stakeholders considering AM, highlighting the importance of supplier selection and process optimisation for achieving consistent part quality and unlocking the full potential of AM technologies.

Additive manufacturing inert gas flow path strategies for multi-laser powder bed fusion systems and their impact on lattice structure mechanical responses

Multi-laser Additive Manufacturing systems hold great potential to increase productivity. However, adding multiple energy sources to a powder bed fusion system requires careful selection of a laser scan and inert gas flow strategy to optimize component performance. In this work, we explore four different laser scan and argon flow strategies on the quasi-static compressive mechanical response of Body Centered Cubic lattices. Three strategies employ a swim lane method where laser pathing tends to progress parallel to argon flow. Method one only uses a single laser while method two uses four, both with the laser path working against the argon flow. The third method uses four lasers, each operating in their own lane like the second method, but the laser pathing progresses with the argon flow. The fourth method has all four lasers operating in quadrants and the laser pathing trends against the argon flow.The single-laser strategy generally had the lowest mechanical responses compared to the other three strategies. A quadrant strategy generally had the highest quasi-static mechanical responses and was at least 25% greater in stiffness, yield force, ultimate force, and energy absorption when compared to the single laser strategy. However, the four-laser swim strategy where the laser pathing tends against the argon flow was found to be statistically similar to the quadrant strategy. It is hypothesized that spatter introduced onto the powder layer from the melt pool and particle entrainment may be worse for laser pathing which trends with the argon flow direction. Additionally, the additional energy added to the build volume helps to mitigate inter-layer cool time which reduces temperature gradients. This shows that multi-laser AM systems have an impact on part performance and potentially shows lattices built with multi-laser AM systems may have certain advantages over single-laser AM systems.

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.

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.

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.

Experimental Analysis and Spatial Component Impact of the Inert Cross Flow in Open-Architecture Laser Powder Bed Fusion

Laser-based powder bed fusion is an additive manufacturing process in which a high-power laser melts a thin layer of metal powder layer by layer to yield a three-dimensional object. An inert gas must remove process byproducts formed during laser processing to ensure a stable and consistent process. The process byproducts include a plasma plume and spatter particles. An NC sensor gantry is installed inside a bespoke open-architecture laser-based powder bed fusion system to experimentally characterize the gas velocity throughout the processing area. The flow maps are compared to manufactured samples, where the relative density and melt pools are analyzed, seeking a potential correlation between local gas flow conditions and the components. The results show a correlation between low gas flow velocities and increased porosity, leading to lower part quality. Local flow conditions across the build plate also directly impact components, highlighting the importance of optimizing the gas flow subsystem. The experimental flow analysis method enables optimization of the gas flow inlet geometry, and the data may be used to calibrate the computational modeling of the process.

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.

Assessing the feasibility of using an irregularly shaped Ta and spherical Ti bimodal powder blend on the powder spreadability process in powder bed fusion systems

The cost of additive manufacturing can be reduced if economical non-spherical powders are used. However, current flowability tests are inadequate to determine whether these powders can flow and spread well in powder bed systems. This paper presents a novel tool called the Universal Powder Bed designed to analyse the layer-spreading process of an electron beam melting system. The spreadability and feasibility of using a novel bimodal powder comprising of irregularly shaped tantalum and spherical titanium is investigated against three measures: area fraction of build surface covered with powder, surface roughness and composition of the powder bed. The characterisation techniques revealed that increasing layer thicknesses and slower rake speeds produced more efficiently packed powder layers and decreased the effects of segregation between opposite sides of the powder bed. This study shows that despite the segregation observed with bimodal powders, acceptable levels of spreadability can be produced by refining the spreading parameters.

CHANGE IN MICROSTRUCTURE AND HARDNESS OF ADDITIVELY MANUFACTURED AISI H13 STEEL BY HEAT TREATMENT AND NITRIDING PROCESSES

AISI H13 steel samples were additively manufactured using a laser powder bed fusion (LPBF) system. The effect of annealing tem-perature, quenching &amp; tempering, and nitriding were determined. The microstructure and properties of the samples were investigated using optical microscopy, scanning electron microscopy, electron backscattered diffraction, electron probe micro-analysis, X-ray diffraction, roughness measurement, and a hardness tester. The results show that the as-built AISI H13 steel sample had a roughness on the surface and pores inside. The microstructure consisted of martensite and retained austenite. The average hardness was 460 HV, and the porosity was 0.086 %. The annealing process helped homogenize the microstructure, increase the density, and reduce the porosity and hardness of the LPBF-manufactured sample. The quenching process helped increase the hardness of the steel to the maximum of 787 HV, then the tempering process reduced the hardness to 572 HV. Heat treatment and nitriding processes tended to increase the martensite block size, reduce the retained austenitic content, and precipitate the V-Mo-rich carbide in the LPBF-manufactured AISI H13 steel. After nitriding was conducted, the nitriding case depth was 87 um, and the surface hardness increased up to higher than 1020 HV due to the formation of CrN and Fe3-4N.

Prototype Design for Grading Structures in Powder Bed Fusion Processes.

While targeted alignment in certain additive manufacturing (AM) methods such as material extrusion (MEX) and stereolithography (SLA) has been well documented in the research community, a method for targeted alignment of added fillers or fibrous materials in powder bed fusion (PBF) AM devices has yet to be successfully achieved. Similarly, incorporation of multimaterials does not work easily with any of the AM technologies. This study creates a prototype design that could be integrated into a PBF system to allow for multimaterial layer deposition and alignment of powders and powder blends. The rheological properties of polyamide powder and a range of polyamide composite blends (incorporating milled carbon fibre, graphite flakes, polytetrafluoroethylene, and glass microspheres) in different concentrations were studied, and together with the particle size distribution and particle morphology analysis were applied for the design of a prototype hopper for incorporation in the PBF system to create targeted multimaterial deposition. Different concept designs, multichambered and multi-hopper with hopper angles calculated specifically for the composite blend powders selected, were proposed. Initial deposition trials outside a PBF process were tested, and the deposited layers were measured.

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 [...]

A comprehensive and comparative study of microstructure and mechanical properties for post-process heat treatment of AlSi7Mg alloy components fabricated in different laser powder bed fusion systems

AlSi7Mg (F357) alloy specimens were fabricated in two different laser powder bed fusion (LPBF) systems: EOS M290 and SLM 280HL. Vertical (Z) and horizontal (XY) orientations were fabricated, and five different thermal post processes were applied to samples, individually. According to ASTM F3318-18, the considered thermal conditions were as-built, stress relieved (SR1), HIP, T6 and HIP+T6. Subsequently, the individual samples were aged at 140 °C and 177 °C for 100 h and for up to 1000 h. Tensile specimens were machined down from the aged samples and tested as per ASTM E8/E8M-21. While the yield stress (YS), elongation (%), and Vickers microindentation hardness (HV) were somewhat different for the as-built components, the general trends for the different heat treatments were essentially the same. As-built and SR1 treated microstructures were dominated by microdendritic cells, while the HIP, T6 and T6 + HIP component microstructures consisted of recrystallized grains containing eutectic Si particles of various sizes and shapes within the grain interiors and the grain boundaries; which gave rise to wide-ranging mechanical properties. As an example of these widely-ranging mechanical properties, it was observed that components fabricated in the Z or build direction in the EOS system exhibited a YS, elongation, and HV of 225 MPa, 13%, and HV120, while when HIPed and unaged exhibited values of 87 MPa, 25%, and HV51, respectively. These same HIPed components when aged at 177 °C for 1000 h exhibited values of 81 MPa, 41% and HV44. The mechanical properties of the unaged, HIPed and aged fabricated in Z direction in SLM system were 85 MPa, 31%, HV51, and 80 MPa, 42%, and HV47, respectively, providing support for LPBF system fabrication compatibility. These measured mechanical property values represent a small fraction of the more than 1600 mechanical property measurements (YS, UTS, elongation, and hardness (HV)) in this study.