Laser-based Powder Bed Fusion Research Articles

Additive manufacturing of metallic glass from powder in space

Additive manufacturing of metals – and in particular building with laser-based powder bed fusion – is highly flexible and allows high-resolution features and feedstock savings. Meanwhile, though space stations in low Earth orbit are established, a set of visits to the Moon have been performed, and humankind can send out rovers to explore Venus and Mars, none of these milestone missions is equipped with technology to manufacture functional metallic parts or tools in space. In order to advance space exploration to long-term missions beyond low Earth orbit, it will be crucial to develop and employ technology for in-space manufacturing (ISM) and in-situ resource utilisation (ISRU). To use the advantages of laser-based powder bed fusion in these endeavours, the challenge of powder handling in microgravity must be met. Here we present a device capable of building parts using metallic powders in microgravity. This was proven on several sounding rocket flights, on which occasions Zr-based metallic glass parts produced by additive manufacturing in space were built. The findings of this work demonstrate that building parts using powder feedstock, which is more compact to transport into space than wire, is possible in microgravity environments. This thus significantly advances ISRU and ISM and paves the way for future tests in prolonged microgravity settings.

The competing role of defects and surface roughness on the fatigue behavior of additively manufactured AlSi10Mg alloy

The fatigue behavior of AlSi10Mg alloy processed by laser-based powder bed fusion of metals (PBF-LB/M) in as-built (AB) and surface treated conditions, is assessed. A new chemo-mechanical polishing (CMP) surface treatment is applied to AB specimens to achieve an average surface roughness of Sa≈ 0.3 μm which is more than two orders of magnitude lower than that of AB specimens (Sa≈ 48.6 μm). The CMP surface treatment led to a significant improvement in fatigue strength of the alloy as compared to the AB material. The fatigue life of AB + CMP specimens is discovered to be governed by surface or near surface defects, whereas the deepest surface roughness valleys play a dominant role in controlling the fatigue behavior of the AB material. A simplified fracture mechanics-based fatigue life modeling approach was developed by combining real-time defects distribution data from 3D computed tomography (CT) and surface profilometry measurements. The model provided a quick and effective medium for obtaining reasonable first-order fatigue life approximations for AlSi10Mg alloy fabricated using PBF-LB/M.

Static load-carrying behavior and material properties of additively manufactured gears (PBF-LB/M, 16MnCr5)

PurposeNumerous metals can be processed using the additive manufacturing process laser-based powder bed fusion of metals (PBF-LB/M, ISO/ASTM 52900). The main advantages of additive manufacturing technologies are the high degree of design freedom and the cost-effective implementation of lightweight structures. This could be profitable for gears with increased power density, combining reduced mass with considerable material strength. Current research on additively manufactured gears is focused on developing lightweight structures but is seldom accompanied by simulations and even less by mechanical testing. There has been very little research into the mechanical and material properties of additively manufactured gears. The purpose of this study is to investigate the behavior of lightweight structures in additively manufactured gears under static loads.Design/methodology/approachThis research identifies the static load-carrying capacity of helical gears with different lightweight structures produced by PBF-LB/M with the case hardening steel 16MnCr5. A static gear loading test rig with a maximum torque at the pinion of T1 = 1200 Nm is used. Further focus is set on analyzing material properties such as the relative density, microstructure, hardness depth profile and chemical composition.FindingsAll additively manufactured gear variants show no failure or plastic deformation at the maximum test load. The shaft hub connection, the lightweight hub designs and the gearing itself are stable and intact regarding their form and function. The identified material characteristics are comparable to conventionally manufactured gears (wrought and machined), but also some particularities were observed.Originality/valueThis research demonstrates the mechanical strength of lightweight structures in gears. Future research needs to consider the dynamic load-carrying capacity of additively manufactured gears.

Investigation of Wear and Friction Behavior of Cobalt-Chromium-Molybdenum Alloy Produced by Laser Powder Bed Fusion

This study investigates the sliding wear resistance of the Co-28Cr-6Mo alloy manufactured by laser-based powder bed fusion through linear reciprocating sliding wear tests. Varied loads and sliding speeds, in both dry and lubricated contact scenarios, were examined using a hardened steel ball as a counter material. Microhardness profiles were obtained to understand the alloy’s mechanical behavior, revealing consistent average microhardness values of 409.67 HV through the thickness and 404.05 HV along the wear track. The coefficient of friction (COF) indicated a decrease in COF with an increase in the applied normal load in both dry and lubricated contacts. On the other hand, the COF increased with higher sliding speeds, and remained stable under lubricated contact but fluctuated in dry conditions over time. Increased applied load and sliding speed correlated with escalated wear rates. Investigation of the impact of different lubricants on tribological properties highlighted synthetic gear oil as the most effective in reducing wear rate among tested lubricants. Wear mechanisms were elucidated through scanning electron microscopy and energy-dispersive X-ray spectroscopy. The presented results can serve as a guide for selecting suitable operating conditions and lubricants to reduce wear in Co-Cr-Mo alloy applications.

Operando tomographic microscopy during laser-based powder bed fusion of alumina

Laser-based Powder Bed Fusion (LPBF) of oxide ceramics enables fabrication of objects with complex three-dimensional shapes. However, mechanical properties of dense LPBF-manufactured ceramics are poor due to large amount of structural defects. Here, we perform the operando tomographic microscopy during LPBF of a magnetite-modified alumina to gain a deeper understanding of the underlying mechanisms. The effect of the laser energy density on the surface roughness, powder denudation zone and porosity formation mechanisms are investigated. Increasing laser power results in significant increase of the melt pool width, but not its depth and no melt pool depression is observed. Forces due to the recoil pressure are not seen to significantly influence the melt pool dynamics. Increasing power allows to avoid fusion porosity but enhances formation of spherical porosity that is formed by either reaching boiling point of liquid alumina, or by introducing gas bubbles by injection of hollow powder particles into the liquid.

Influence of ring-shaped beam profiles on spatter characteristics in laser-based powder bed fusion of metals

Despite the maturity of laser-based powder bed fusion of metals (PBF-LB/M), some barriers prevent the manufacturing process from fully being established in the industry. One drawback is spatter formation, which is disadvantageous to PBF-LB/M for three main reasons. First, adhering spatter can initiate coater collision, resulting in process failure. Second, large adhering spatter may cause lack-of-fusion defects as they require more energy to remelt sufficiently compared to unprocessed powder. Furthermore, big nonadhering spatter cannot be recycled as powder. The recycling of small spatter particles potentially results in degraded material properties. Ring-shaped beam profiles have been established for deep penetration welding to reduce spatter formation. Investigations on ring-shaped beam profiles in PBF-LB/M focus on improving productivity and process robustness. Qualitative spatter reduction in PBF-LB/M using ring-shaped beam profiles has also been reported. This publication quantitatively examines the influence of ring-shaped beam profiles on spatter formation in PBF-LB/M. Image processing algorithms of on-axis high-speed images are utilized for spatter detection and tracking. A self-developed spatter segmentation is used to determine the spatter size. A Laplacian of Gaussian filter is combined with a Kalman tracker to count and track the spatter. The results show that spatter formation is highly influenced by the beam profile and the chosen process parameters. Considering the melt track width, ring-shaped beam profiles could reduce the number of spatter per fused area. High numbers of spatter are generated when parameter sets result in balling. Moreover, spatter velocity is primarily dependent on the introduced dimensionless enthalpy.

Multi-sensor monitoring of powder melting states via melt pool optical emission signals during laser-based powder bed fusion

The instability of the powder melting states is a common problem in laser-based powder bed fusion of metals (PBF-LB/M). Optical signal monitoring based on melt pool emissions has great potential to solve this problem, but there is a lack of research on the inherent correlations between the optical emissions and powder melting states. In this paper, a synchronous monitoring method of multi-sensor signals based on automatic trigger by numerical control programming was proposed. Then the relationships between the characteristic changes in melt pool signals and the powder melting state variations were carefully examined based on the statistical method. The results showed that the data collection method achieved synchronous monitoring of multi-sensor signals in the target process. The combination of the melt pool light intensity and melt pool area can effectively capture powder melting state variations owing to the changes of powder thickness. Among them, the melt pool light intensity can accurately monitor powder melting state variations caused by the powder thickness of 30 μm to 210 μm. While the melt pool area is susceptible to powder melting state variations caused by the powder thickness of 210 μm to 300 μm. Meanwhile, two emission characteristics (mean intensity and standard deviation) of the optical emissions were closely related to the melting states and stability of the powder.

Fracture toughness and wear resistance of heat-treated H13 tool steel processed by laser powder bed fusion

In today's industry, complex parts made of steel are a reality thanks to the introduction of additive manufacturing, which offers technological improvements in conventional and large manufacturing industries that rely on molds and dies for mass production. However, the rapid solidification, remelting and reheating during the process and the uneven properties of the parts might present some difficulties in expanding its application in some sectors. For example, H13 tool steel offers exceptional properties, but finding the correct post-heating treatment for additively manufactured parts with reliable and repeatable properties is still a challenge. In this study, we focused on investigating different heat treatments to better understand the microstructure evolution and consequently the mechanical behavior under quasi-static loads, fracture toughness and wear resistance. Conclusions could be drawn on the structure-property relationships involving the strengthening and toughening mechanisms, as well as the evolution of the martensitic microstructure, retained austenite and carbides in parts processed by laser-based powder bed fusion. We found that low temperature tempering (550 °C) provides the lowest fracture toughness (KQ = 36 MPa.m0.5, J = 6.5 kJ.m−2, δ = 2.0 µm), the highest mechanical strength (YS up to 2270 MPa) and the lowest wear rate (circa 2.0 ×10−5 mm3/N.m). On the other hand, high temperature tempering (650 °C) with prior austenitizing and quenching provides the best fracture toughness (KQ = 87 MPa.m0.5, J = 70 kJ.m−2, δ = 40 µm), average mechanical properties (YS = 1104 MPa) and wear rate (3.4 ×10−5 mm3/N.m). Thus, different heat treatment processes can be applied depending on the requirements of the project.

Benchmark models for conduction and keyhole modes in laser-based powder bed fusion of Inconel 718

Benchmark models for conduction and keyhole modes in laser-based powder bed fusion of Inconel 718

Simulation-based process optimization of laser-based powder bed fusion by means of beam shaping

Simulation-based process optimization of laser-based powder bed fusion by means of beam shaping

Laser based additive manufacturing of tungsten: Multi-scale thermo-kinetic and thermo-mechanical computational model and experiments

Coupling a multi-scale thermo-kinetic and thermo-mechanical model with critical experiments, the present study investigates the role of process parameters on the resulting densification, cracking behavior, and microstructural evolution in laser-based powder bed fusion (LPBF) of pure tungsten. Thermal management strategies comprising shorter scan length (1 mm) and/or substrate preheating (873 K) were explored to achieve higher densities and mitigate cracks. A finite element (FE) based thermo-kinetic and thermo-mechanical computational model was developed to simulate the process spanning from the melt pool scale to the component level. FE simulations indicated pronounced heat accumulation with reduced scanning lengths. Such increased heat accumulation resulted in melt pools with larger dimensions (width and depth) and more favorable rheological characteristics resulting in printed tungsten parts with higher densities. Using an input laser fluence (ILF) of 28.6 J/mm2 and a shorter scan length of 1 mm, a relative density of maximum 99.2% was achieved, the highest reported value using LPBF in case of pure tungsten. FE simulations indicated nearly 5-fold reduction in thermal gradients and 2-fold reduction in cooling rates associated with the processing conditions corresponding to low degree of cracking (ILFs:28.6 J/mm2 and ILF 15.9 J/mm2 augmented with substrate preheating at 873 K) compared to the processing conditions of high degree of cracking (ILF: 15.9 J/mm2). Further, high ILF samples were characterized by significant extent of in-grain misorientations and low angle boundaries indicating that process induced in-situ restoration mechanisms are operative, thereby potentially aiding in reduction of cracking.

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.

Machinability study on laser-based powder bed fusion processed hypo-eutectic aluminium alloy composite (AlSi10Mg + NbC)

Understanding the machinability of additively manufactured composite materials is crucial for efficient manufacturing. The study aimed to evaluate the machinability of the composite material (AlSi10Mg + Niobium carbide (NbC)) using the Electro-discharge machining (EDM) process and to investigate the influence of different printing orientations on the EDM process. The Plackett–Burman design was initially employed to determine the significant EDM parameters. Subsequently, the Box–Behnken Design was followed to obtain the optimal settings on the EDM setup. The validation experiment based on the composite desirability approach confirmed the maximum material removal rate (MRRmax) and minimum tool wear rate (TWRmin). The identified configuration of 3 wt. % NbC reinforcement in AlSi10Mg, printed in the 0° orientation, exhibited the best performance in MRRmax: 146.43 mg/min, TWRmin: 0.00072 mg/min, and surface finish of 2.813 µm. These findings enhance the understanding of composite machinability and can assist in optimizing the EDM process for advanced materials.

Innovative Process Strategies in Powder-Based Multi-Material Additive Manufacturing

Multi-material additive manufacturing (AM) attempts to utilize the full benefits of complex part production with a comprehensive and complementary material spectrum. In this context, this research article presents new processing strategies in the field of polymer- and metal-based multi-material AM. The investigation highlights the current progress in powder-based multi-material AM based on three successfully utilized technological approaches: additive and formative manufacturing of hybrid metal parts with locally adapted and tailored properties, material-efficient AM of multi-material polymer parts through electrophotography, and the implementation of UV-curable thermosets within the laser-based powder bed fusion of plastics. Owing to the complex requirements for the mechanical testing of multi-material parts with an emphasis on the transition area, this research targets an experimental shear testing set-up as a universal method for both metal- and polymer-based processes. The method was selected based on the common need of all technologies for the sufficient characterization of the bonding behavior between the individual materials.

PLA-Mg composites by laser-based powder bed fusion – A preliminary study

PLA-Mg composites by laser-based powder bed fusion – A preliminary study

Effect of HfO2 dispersoids on the microstructure of a Ni-Cr-Al-Ti superalloy processed by laser-based powder-bed fusion

Effect of HfO2 dispersoids on the microstructure of a Ni-Cr-Al-Ti superalloy processed by laser-based powder-bed fusion

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.

Comprehensive review on residual stress control strategies in laser-based powder bed fusion process– Challenges and opportunities

Comprehensive review on residual stress control strategies in laser-based powder bed fusion process– Challenges and opportunities

Influence of post-process heat treatments on the fatigue crack propagation behaviour of a PBF-LB/M AlSi10Mg alloy

The microstructure has a great influence on short fatigue crack growth in metallic materials. Laser-based Powder Bed Fusion AlSi10Mg alloys exhibit in the as-built condition a fine fibrous Si structure and a supersaturated solid solution of Si in the α-Al matrix, which is significantly modified by heat treatments starting already at temperatures under 260 °C. This study focuses on the influence of post-process heat treatments on the microstructural evolution and the resulting fatigue crack growth resistance. As compared to the as-built condition, two heat treatments at 265 °C/1 h and at 300 °C/2 h are found to be beneficial to the fatigue crack growth resistance of the investigated material.

Microstructure and mechanical characterization of additively manufactured Fe11Cr8Ni5Co3Mo martensitic stainless steel

This study investigates the impact of various printing processes on the microstructure, grain orientation, dislocation density, residual stresses, and mechanical properties of Fe11Cr8Ni5Co3Mo0.2Si0.16Al0.12 V martensitic aged stainless steels produced through laser-based powder bed fusion (LPBF) using different processing parameters. The LPBF samples displayed varying densities, dislocation densities, and grain orientations with different combinations of process parameters. Sample 3 showed the highest densification effect, dislocation density ranged from 6.23 × 1013 m−2-1.164 × 1014 m−2. The horizontally crystallographic texture is primarily 〈111〉 orientation, it was observed that sample 3 also had a 〈001〉 orientation. The building direction has strong Gaussian texture {110} 〈001〉. The residual stress is compressive residual stress, and the maximum residual stress is depicted in sample 6, which is −222.8 ± 16.8 MPa. The optimal combination for the ultimate tensile strength and elongation was achieved using a laser power of 200 W, scanning speed of 1000 mm/s, hatch spacing of 100 μm, layer thickness of 30 μm, and laser energy density of 66.7 J/mm3. The total yield strength increase in the LPBF MSS samples, 60% can be attributed to the intensification of dislocations, with the contributions to intensity ranked in the order of σd>σt>σgb>σp.