Metal Powder Bed Fusion Research Articles

Experimental and analytical investigations of the removal of spatters by various process gases during the powder bed fusion of metals using a laser beam

The powder bed fusion of metals using a laser beam (PBF-LB/M) is increasingly being utilized in industrial applications. This is due to several advantages over conventional manufacturing processes when it comes to the fabrication of complex part designs. However, the process still poses various challenges that have to be overcome. One of these challenges is the formation of a significant amount of spatters and fumes. These could attenuate the laser beam or decrease the powder reusability. To lower their negative impact on the process and the mechanical properties of the parts, a process gas flow is used in PBF-LB/M to remove these by-products from the processing zone. This study was, therefore, dedicated to investigating the potential of various gases on the removal of spatters. The focus was placed on argon, helium, and their mixtures. After theoretical considerations determining the range of applicable gas flow velocities, the experimental results unveiled the real spread of spatters over the powder bed and their characteristics. Whilst the removal of spatters was found to be worse for an argon–helium gas mixture at comparable gas flow velocities, increasing the velocity turned out to be a proper measure to enhance the removal for low-density gases. At this flow condition, the use of the argon–helium gas mixture led to a similar removal of spatters and the creation of a lower spatter mass in total (reduced to 40%) compared to argon.

A Brief History of the Progress of Laser Powder Bed Fusion of Metals in Europe

Abstract The progress of additive manufacturing (AM) within the last few decades has been phenomenal, progressing from a polymeric technique to a method for manufacturing metallic aerospace components. We take a look at various technological advances which have helped paved the way for this growth, focussing on European input, as currently, 54% of AM machines are sold by European manufacturers (Wohlers, Campbell, Diegel, Kowen, Mostow, and Fidan, 2022, “Wohlers Report 2022: 3D Printing and Additive Manufacturing Global State of the Industry,” Wohlers Associates, ASTM International, Fort Collins, Colo., Washington, DC). We take deep dives into several critical topics including sensing and monitoring, preheating, and multi-laser technology and illustrate how these develop from research ideas into industrial products. Finally, an outlook is provided, highlighting the topics currently gaining research traction, and which are expected to be the next key breakthroughs.

Experiments with Near-Field Microwave Imaging for Powder Bed Fusion Metal Additive Manufacturing

Experiments to evaluate the use of Near-Field Microwave (NFMW) imaging for the Powder Bed Fusion quality control are here reported. A NFMW sensor was employed together with general purpose instrumentation to perform measurements over a stainless steel 316 sample produced by laser PBF. Although with limited sensitivity, results showed that NFMW sensors can be used to evaluate the proper layer consolidation while relying on the electrical conductivity difference between powder and consolidated metal. Results also highlight the sensor high spatial resolution, well appreciated for PBF layer-wise imaging.

Evaluating Capacitive Imaging for Powder Bed Fusion Metal Additive Manufacturing

This work evaluates the use of co-planar capacitive sensors to provide layer-wise imaging results for the Powder Bed Fusion quality control. A new sensor was designed together with a digital readout electronics which allowed for high-resolution localized measurements. The system was evaluated in different surface features produced by laser PBF with stainless steel 316 powder. Features were tested with and without the presence of the original powder with results showing that the powder presence masked the output for most of the features. This suggests that the ability of carrying charges within an alternating electrical field is similar between powder and the consolidated metal, and that, the concerning signal contribution is much higher than that caused by the conductivity difference between powder and consolidated metal.

A Comparative Investigation of Properties of Metallic Parts Additively Manufactured through MEX and PBF-LB/M Technologies.

In this study, the research on 316L steel manufactured additively using two commercially available techniques, Material Extrusion (MEX) and Laser Powder Bed Fusion of Metals (PBF-LB/M), were compared. The additive manufacturing (AM) process based on powder bed synthesis is of great interest in the production of metal parts. One of the most interesting alternatives to PBF-LB/M, are techniques based on material extrusion due to the significant initial cost reduction. Therefore, the paper compares these two different methods of AM technologies for metals. The investigations involved determining the density of the printed samples, assessing their surface roughness in two printing planes, examining their microstructures including determining their porosity and density, and measuring their hardness. The tests carried out make it possible to determine the durability, and quality of the obtained sample parts, as well as to assess their strength. The conducted research revealed that samples fabricated using the PBF-LB/M technology exhibited approximately 3% lower porosity compared to those produced using the MEX technology. Additionally, it was observed that the hardness of PBF-LB/M samples was more than twice as high as that of the samples manufactured using the MEX technology.

Advantages of Injection Mold with Hybrid Process of Metal Powder Bed Fusion and Subtractive Process

This paper focuses on the hybrid process combining metal additive manufacturing (AM) and subtractive processing developed for application to injection molds. The basic concept is a combination of laser powder bed fusion of metal powder and subtractive processing. This process is characterized by alternating buildup and milling processes. Even the inner surface of deep grooves, which conventionally required electrical discharge machining, can be machined with small-diameter tools with a short flute length. Therefore, molds with complex shapes that previously required electrical discharge machining can be manufactured in a single process. Moreover, a dimensional accuracy and surface roughness of levels equal to those achieved by machining with the machining center can be ensured. In the hybrid process, it is necessary to minimize the surplus solidified area (which is the area milled by the small-diameter tool). Therefore, the formation mechanism of the surplus solidified region is verified. It is shown that the power distribution of the laser beam significantly affects the size (width and depth) and density distribution of the excessively solidified region. In addition, the effective value of metal AM mold is introduced. The 3D cooling circuit improves the efficiency of the injection molding process. If the temperature balance between the cavity side and core side is achieved, the distortion of the molded product would be suppressed. If the cooling effect is promoted, the molding cycle would be shortened substantially. Second, the effect of the gas vent function by a permeable structure is explained through actual examples. The effect of the gas vent function by the permeable structure is explained. It is indicated that stable molding can be achieved. In addition, the appearance defects of molded products can be reduced when the air inside the cavity is exhausted sufficiently from the mold through the permeable structure.

In-situ porosity prediction in metal powder bed fusion additive manufacturing using spectral emissions: a prior-guided machine learning approach

Numerous efforts in the additive manufacturing literature have been made toward in-situ defect prediction for process control and optimization. However, the current work in the literature is limited by the need for multi-sensory data in appropriate resolution and scale to capture defects reliably and the need for systematic experimental and data-driven modeling validation to prove utility. For the first time in literature, we propose a data-driven neural network framework capable of in-situ micro-porosity localization for laser powder bed fusion via exclusively within hatch strip of sensory data, as opposed to a three-dimensional neighborhood of sensory data. We further propose using prior-guided neural networks to utilize the often-abundant nominal data in the form of a prior loss, enabling the machine learning structure to comply more with process physics. The proposed methods are validated via rigorous experimental data sets of high-strength aluminum A205 parts, repeated k-fold cross-validation, and prior-guided validation. Using exclusively within hatch stripe data, we detect and localize porosity with a spherical equivalent diameter (SED) smaller than 50.00μ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$50.00\\,\\upmu $$\\end{document}m with a classification accuracy of 73.13±1.57%\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$73.13\\pm 1.57\\%$$\\end{document} This is the first work in the literature demonstrating in-situ localization of porosities as small as 38.12μm\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$38.12\\,\\upmu m$$\\end{document} SED and is more than a five-fold improvement on the smallest SED porosity localization via spectral emissions sensory data in the literature. In-situ localizing micro-porosity using exclusively within hatch-stripe data is a significant step towards within-layer defect mitigation, advanced process feedback control, and compliance with the reliability certification requirements of industries such as the aerospace industry.

Numerical investigation of balling defects in laser-based powder bed fusion of metals with Inconel 718

Balling is one of the main single melt track defects limiting stable production of dense parts with laser-based powder bed fusion of metals (PBF-LB/M) additive manufacturing. A fundamental understanding of balling therefore is critical for process control and optimization. Violent interface dynamics present a severe challenge for mesh-based numerical schemes, and can be dealt with by particle methods. In this study, we focus on investigating single melt tracks by smoothed particle hydrodynamics (SPH). The considered regime is characterized by long liquid melt pools and remelting of the previous layer. The balling mechanism is driven by Marangoni and capillary forces. The resulting convective melt flux leads to the accumulation of melt in the tail region and leaves an exposed area of marginal layer build-up. In addition to the competing mechanisms of solidification and spreading, a competition between spreading and melt contraction shortly after the melt pinch-off is observed. We find a similar balling behavior yet different ball size at different laser power settings. We stress that SPH with accurate interface modeling has the capability to quantify balling processes and to improve the understanding of balling mechanisms.

Influence of a vibrating build platform on the density and hardness of parts made by laser powder bed fusion of metals

Influence of a vibrating build platform on the density and hardness of parts made by laser powder bed fusion of metals

Process development for the hybrid additive manufacturing of metallic structures on polymer substrates

3D hybrid additive manufacturing describes the combined application of different additive manufacturing processes in order to manufacture a component with almost unlimited design freedom and free choice of materials. One challenge in the hybrid additive manufacturing lies in the different conditions of individual additive manufacturing processes. For example, in the combined application of the laser powder bed fusion (PBF-LB) of metals with the fused filament fabrication (FFF) of polymers, damage can occur in the plastic due to the higher processing temperatures of PBF-LB. Therefore, in this work a process strategy for the buildup of metallic structures in the PBF-LB process on polymer base structures was developed. By reducing the laser energy density in combination with increasing the height of the first layer, a processing window could be identified. The built hybrid components were investigated with micro-computed tomography and the effects of different processing parameters on the interface between the polymer and metal part are discussed. Finally, a strategy to increase the strength of the interface is presented.

Ultrasonic Powder Atomization for Additive Manufacturing

The following article presents a special case of metal powder production, ultrasonic metal atomization. In this case, ultrasound technology is based on the capillary wave phenomenon. We verify the suitability of the produced powders for 3D metal printing with various tests. In the case of prints with a metal powder bed fusion (PBF), the properties of the raw material of the powder are extremely important. The main results of the tests carried out in the article (SEM images, EDS composition analysis, sieve analysis) were described.

Experimental Assessment and Micromechanical Modeling of Additively Manufactured Austenitic Steels under Cyclic Loading

The present work deals with the cyclic deformation behavior of additively manufactured austenitic stainless steel 316L. Since fatigue experiments are complex and time‐consuming, it is important to develop accurate numerical models to predict cyclic plastic deformation and extrapolate the limited experimental results into a wider range of conditions, considering also the microstructures obtained by additive manufacturing. Herein, specimens of 316L steel are produced by powder bed fusion of metals with laser beams (PBF‐LB/M) with different parameters, and cyclic strain tests are performed to assess their deformation behavior under cyclic loads at room temperature. Additionally, a micromechanical model is set up, based on representative volume elements (RVE) mimicking the microstructure of the experimentally tested material that is characterized by electron backscatter diffraction (EBSD) analysis. With the help of these RVEs, the deformation‐dependent internal stresses within the microstructure can be simulated in a realistic manner. The additively manufactured specimens are produced with their loading axis either parallel or perpendicular to the building direction, and the resulting anisotropic behavior under cyclic straining is investigated. Results highlight significant effects of specimen orientation and crystallographic texture and only a minor influence of grain shape on cyclic behavior.

Laser beam shape optimization in powder bed fusion of metals

Laser beam shape optimization in powder bed fusion of metals

Regeneration of the Damaged Parts with the Use of Metal Additive Manufacturing-Case Study.

The paper shows the results related to regeneration possibilities analysis of a damaged slider removed from a hydraulic splitter that was repaired using additive manufacturing (AM), laser-based powder bed fusion of metals (PBF-LB/M) technology. The results demonstrate the high quality of the connection zone between the original part and the regenerated zone. The hardness measurement conducted at the interface between the two materials indicated a significant increase equal to 35% by using the M300 maraging steel, as a material for regeneration. Additionally, the use of digital image correlation (DIC) technology enabled the identification of the area where the largest deformation occurred during the tensile test, which was out of the connection zone between the two materials.

Optimisation of Imaging Confocal Microscopy for Topography Measurements of Metal Additive Surfaces

Additive manufactured surfaces, especially metal powder bed fusion surfaces, present unique challenges for measurement because of their complex topographies. To address these measurement challenges, optimisation of the measurement process is required. Using a statistical approach, sensitivity analyses were performed on measurement settings found on a commercial programmable array scanning confocal microscope. The instrument measurement process parameters were compared by their effects on three quality indicators: the areal surface texture parameter Sa, measurement noise, and number of non-measured points. An analysis was performed using a full factorial design of experiments for both the top and side surfaces of test surfaces made from Inconel 718 and Ti-6Al-4V using powder bed fusion. The results indicated that measurements of metal additive surfaces are robust to changes in the measurement control parameters for Sa, with variations within 5% of the mean parameter value for the same objective, surface, and measured area. The number of non-measured points and the measurement noise were more varied and were affected by the choice of measurement control parameters, but such changes could be predicted by the statistical models. The contribution offered by this work is an increased understanding of imaging confocal microscopy measurement of metal additive surfaces, along with the establishment of good practice guidance for measurements.

Influence of Novel Beam Shapes on Laser-Based Processing of High-Strength Aluminium Alloys on the Basis of EN AW-5083 Single Weld Tracks

The commonly used Gaussian intensity distribution during the laser-based processing of metals can negatively affect melt pool stability, which might lead to defects such as porosity, hot cracking, or poor surface quality. Hot cracking is a major factor in limiting production rates of high-strength aluminium alloys in laser-based processes such as welding or the powder bed fusion of metals (PBF-LB/M). Going away from a Gaussian intensity distribution to ring-shaped profiles allows for a more even heat distribution during processing, resulting in more stable melt pools and reduced defect formations. Therefore, the aim of this study is to investigate the influence of different laser beam profiles on the processing of high-strength aluminium alloys by using a multicore fiber laser, allowing for in-house beam shaping. Single weld tracks on the aluminium alloy EN AW-5083 are produced with varying laser powers and weld speeds, as well as different beam profiles, ranging from Gaussian intensity distribution to point/ring profiles. The molten cross sections are analyzed regarding their geometry and defects, and the surface roughness of the weld tracks is measured. By using point/ring beam profiles, the processing window can be significantly increased. Hot cracking is considerably reduced for weld speeds of up to 1000 mm/s compared to the Gaussian beam profile. Furthermore, the melt pool width and depth are more stable, with varying parameters for the point/ring profiles, while the Gaussian beam tends to keyhole formation at higher beam powers. Finally, a strong decrease in surface roughness for the point/ring profiles, accompanied by a significantly reduced humping effect, starting even at lower beam powers of 200 W, can be observed. Therefore, these results show the potential of beam shaping for further applications in laser-based processing of high-strength aluminium alloys.

Alternative Approach to Modeling of Nucleation and Remelting in Powder Bed Fusion Additive Manufacturing

In powder bed fusion (PBF) of metals, energy of a laser or electron beam is utilized to form near‐net‐shaped parts from a powder bed. A very promising application of PBF lies in the direct control of the resulting microstructure by adjusting process parameters. Especially the intentional tilting of grains in one certain direction offers a complete new field of activity for additively produced parts specially designed for a given load case. Nucleation is essential for utilization of this effect. Herein, an alternative approach for modeling nucleation in PBF of metals is introduced. The model is not intended to cover every physical effect of this extreme complex phenomenon. Based on observations reported from different researchers in the field of PBF, a heuristic approach is applied to consider new grains within a cellular automata (CA)‐based crystal growth model for PBF. Utilizing the implicit capability of the CA to model competitive growth of grains, this approach does not require any additional computational capacities. The numerical calculations are carried out using a message–passing interface (MPI) parallelization on a high‐performance computing (HPC) cluster located at the Regional Research Center Erlangen. The numerical results are validated by means of experiments and electron backscatter diffractometric measurements.

Effect of Process Variables on Microstructure, Flow, and Tensile Properties of Nickel‐Based Superalloys with Designed Porous Structures Manufactured by Laser‐Based Powder Bed Fusion

The nickel‐based superalloys Haynes 282 and Inconel 625 are processed using laser‐based powder bed fusion of metals (PBF‐LB/M) to manufacture designed materials (DMs) with configurable open‐porous morphology. Such structures can be utilized in large gas turbines, for example, as high‐frequency dampers, heat exchangers, and for transpiration cooling features. This research investigates the effects of the applied volume energy input and laser deflection angle (as calculated by position) on the macro‐ and microstructure and tensile and flow properties of DMs made from Haynes 282. Furthermore, a novel combination of DMs and cubic lattice structures made from Inconel 625 has been investigated regarding potential improvements. The substantial interface between the two features, DM and lattice structure, is visually analyzed using microcomputed tomography. The analysis of the hierarchical DMs reveals that the combination of lattice structures and DMs is manufacturable and has benefits in terms of mechanical strength while having less significant effects on flow properties.

Linking process parameters with lack-of-fusion porosity for laser powder bed fusion metal additive manufacturing

Linking process parameters with lack-of-fusion porosity for laser powder bed fusion metal additive manufacturing

Support-free laser-based powder bed fusion of metals using pulsed exposure strategies

Several studies demonstrate the potential of pulsed exposure strategies for improving spatial accuracy, surface quality, and manufacturability of low-angle overhangs in laser-based powder bed fusion of metals. In this paper, those fundamental potentials are transferred to the support-free manufacturing of heat exchanger structures with partial horizontal overhangs made of Ti6Al4V. The pulsed exposure with pulse repetition rates of 20 kHz and pulse duration of 25 µs enabled the support-free manufacturing of these complex structures with densities of more than 99%. A comparison of the Archimedean density determination with optical density determination using micrographs indicate permeability of the specimens below an applied volume energy density of 30 J/mm3 due to open porosity. Furthermore, the pulsed manufactured structures show an improved flow behavior within the heat exchanger compared to specimens manufactured with continuous exposure strategies.