Powder Layer Research Articles

In situ Alloyed Medium Entropy Alloy Fabricated by Laser Powder Bed Fusion: Microstructure and Cryogenic Performance

An investigation is conducted into the feasibility to produce in situ alloyed medium entropy alloy containing four principal elements by laser powder bed fusion of the blended elemental powders. The effect of scan strategy, layer thickness, and laser power on in situ alloying is examined. It is revealed that the melting of Cr particles could be enhanced by increasing the remelting times of each building layer through the re‐scan strategy or reducing the powder layer thickness and by increasing the melting pool depth via large laser power. Besides, the combination of elemental uniformity and the low porosity in the printed alloy can be achieved. The accuracy of the nominal composition in the as‐printed sample could be ensured by the large remelting times combined with middle laser power, while an excessively large laser power could cause the loss of Cr element. Based on the high repetitive remelting process, the mechanism of pores elimination during in situ alloying is discussed. After the optimization, the as‐printed Fe60Co15Ni15Cr10 alloy fabricates with the re‐scan strategy or high laser power shows a tensile strength of ≈1140 MPa and an elongation of ≈0.39 under cryogenic loading. The as‐printed alloy overcomes the strength–ductility trade‐off at cryogenic temperature through the transformation‐induced plasticity.

A Simulation Study on the Effect of Layer Thickness Variation in Selective Laser Melting

Abstract Selective laser melting (SLM) has gained prominence in the manufacturing industry for its ability to produce lightweight components. As the raw material used is in powder form, the stochastic nature of the powder distribution influences the powder layer thickness and affects the final build quality. In this paper, a multi-layer multi-track simulation study is conducted to investigate the effect of stochastic powder distribution on the layer thickness and plastic strain in a printed geometry. A faster simulation approach is employed to simulate multiple layers. First, the powder distribution and the melt layer thickness of the first layer are obtained from discrete element method (DEM) and computational fluid dynamics (CFD) simulations respectively. Next, the melt layer thickness of the first layer is used as an input to the finite element (FE) based structural mechanics solver to predict the deformation and layer thickness of subsequent layers. Two nominal layer thicknesses 67.4 μm and 20 μm were considered. Two particle size distribution (PSD) configurations and two scanning strategies were tested. The results showed that variation in PSD and scanning strategy leads to variation in layer thickness which in turn leads to variation in the plastic strain that is known to drive the deformation. However, the nominal layer thickness of 20 μm was found to be less influenced by the PSD configuration. The proposed simulation approach and the insights achieved can be used as inputs in the part-scale simulations for geometric robustness evaluation in the early design stages of SLM products.

CFD–DPM Simulation Study of the Effect of Powder Layer Thickness on the SLM Spatter Behavior

Selective Laser Melting (SLM) has significant advantages in manufacturing complex structural components and refining the alloy microstructure; however, spatter, as a phenomenon that accompanies the entire SLM forming process, is prone to problems such as inclusions, porosity, and low powder recovery quality. In this paper, a Computational Fluid Dynamics–Discrete Particle Method (CFD–DPM) simulation flow for predicting the SLM spatter behavior is established based on the open-source code OpenFOAM. Among them, the single-phase flow Navier–Stokes equation is used in the Eulerian framework to equivalently describe the effect of metal vapor and protective gas on the flow field of the forming cavity, and the DPM method is used in the Lagrangian framework to describe the metal particle motion, and the factors affecting the particle motion include particle–particle collision, particle–wall collision, fluid drag force, gravity, buoyancy force, and additional mass force. In addition, the equivalent volume force and fluid drag force are used to characterize the fluid–particle coupling interaction. For the spatter behavior and powder bed denudation phenomenon, the calculation results show that the spatter height and the drop location show a clear correlation, and the powder bed denudation phenomenon is caused by the high-speed gas flow, causing the surrounding gas to gather in the forming area, which in turn drives the motion of the powder bed particles. For the effect of powder layer thickness on spatter and powder bed denudation, the calculation results show that the effect of powder layer thickness on the number of spatters is large (when the thickness was increased from 50 μm to 100 μm, the number of spatters increased by 157%), but the effect on spatter height and drop location distribution is small. When the powder layer thickness is small, the width of the denudation zone is significantly larger, but when the powder layer reaches a certain thickness, the width of the denudation zone does not show significant changes. It should be noted that the presented model has not been directly validated by experiments so far due to the difficulty of tracking the large-scale motion of SLM spatter in real time by current experimental means.

In-situ monitoring of powder bed fusion of polymers using laser profilometry

Process monitoring allows for unique opportunities to capture information at intermediate layers during fabrication for part qualification, but also to improve process robustness by identifying issues before yield or quality is reduced. An industrial-grade laser profilometer has been integrated for the first time in a commercial polymer laser sintering machine and used to monitor powder bed quality and potentially qualify parts during production. Powder layer quality was measured in different conditions and changed drastically as a function of recoating speed, preheating temperature but also particle shape. The effective layer thickness was obtained for the first time in PBF of polymers, and gradually increased with the layer count until reaching a plateau at the nominal layer thickness. Powder layer density could be calculated as well and lies between 34 and 65 %, depending on the powder type and layer count. Finally, curling increased with decreasing preheating temperature, ranging from 70 µm to 350 µm i.e. until collision with the recoater. Based on the measured degree of curling, mitigation strategies could be integrated using a closed-loop feedback control. Effective process monitoring such as the one provided with laser profilometry may enable the development of new materials featuring more complex processing conditions, but also larger machines where thermal uniformity might be an issue.

Neutron Bragg edge imaging for strain characterization in powder bed additive manufacturing environments

Spatially resolved studies of crystalline structures, e.g. lattice spacings, are enabled by recording the transmitted spectra in neutron Bragg edge imaging. The recorded signals are, however, a result of through-thickness averaging of the probed specimen in the beam direction. Therefore, it is challenging to extract the strain distribution when the strain varies across the thickness, which applies for studies on different materials or material states along the beam. Here we introduce the approach to disentangle contributions to the recorded signals, i.e. separating the transmission spectra of two different material states. This is particularly applicable to powder bed additive manufacturing environments where operando strain characterization of the printed specimen using neutrons is intended. In this work, Laser Powder Bed Fusion (PBF-LB/M)-built 316L and IN718 samples embedded in their corresponding powders are used, extracting the desired spectra of the printed specimen. The disentanglement is proven to be satisfactory by obtaining coinciding strain maps of identical specimens embedded in powder layers of different thicknesses. Furthermore, the obtained residual strain distributions of 316L samples were verified by conventional neutron diffraction with lower spatial resolution due to the gauge volume averaging.

Difference between powder bed density and green density for a free-flowing powder in binder jetting additive manufacturing

In binder jetting additive manufacturing, the final part density is closely related to powder bed density and green density. Though powder bed density is considered one of the most critical factors to control and predict green density, their difference has not been fully understood. In this work, experiments and simulations were employed to investigate their difference for a free-flowing powder in a counter-rotating roller powder spreading system. Using an alumina powder with excellent flowability, both experiments and simulations show that powder bed density is higher than green density. Furthermore, simulations provide an in-depth understanding of the density difference. Specifically, in the powder bed density scenario where particles are able to move across different powder layers, a slower powder flow with a smooth velocity transition is observed. In the green density scenario where powder is spread on a printed powder layer, the confined layer space generates a powder flow with higher velocity and more turbulence. As a result, the powder packing structure in the green density scenario has more void space near layer interface than that in the powder bed density scenario. This paper reports a reason for the difference between powder bed density and green density for the first time.

On the effect of building platform material on laser-powder bed fusion of a Ni-base superalloy HAYNES® 282®

Additive manufacturing (AM) by laser powder bed fusion (LPBF) involves melting of layers of powder onto a substrate, called a building platform. Due to cost or convenience considerations, building platform materials rarely match the LPBF material, especially for high temperature materials. To ensure tolerances in component geometries, AM components are often stress-relieved/heat-treated while still attached to the building platform. It is therefore important to understand the effect of dissimilar building platform materials on the properties of the built-up material. These effects may be particularly important for high performance materials such as Ni-base superalloys used for critical applications in the aerospace and energy industries. To investigate this effect, samples of a Ni-base superalloy HAYNES® 282® were built onto a carbon steel building platform in several configurations. The samples were removed from the building platform after heat treatment and subjected to detailed composition analysis and microstructural characterization to investigate the effect of the building platform material on the properties of the additively manufactured part. Room temperature and high temperature tensile testing were used to characterize the material. Results showed no risk of large-scale chemical composition change, or mechanical property degradation of built-up material from on-platform heat treatment.

Cylindrical interfaces repair technique using electric resistance welding of metal powder materials

The study aims to increase the manufacturability of cylindrical interfaces repair by restoring the outside surface geometry using electric resistance powdered welding. The paper presents the technique of coating a worn outside cylinder surface using metal powders and mesh by giving the results of the quality indicators study. The research allowed identifying optimal parameters for the metal powder supply depending on the metal mesh size and the powder's technological properties. Theoretical studies have been undertaken to substantiate the technological process parameters. The nomographic chart developed to determine the angle of powder supply to the electric resistance welding zone allows predicting the coating thickness and varying within 0.3 … 1.5 mm. The powder brand, the diameter of the electrode roller, part and mesh are considered when developing the chart. Due to the triaxial compression under the electrode rollers' force and the mesh deformation, the powder layer experiences additional compaction and is practically sintered to a compact state. The coating obtained under optimal conditions has a porosity of 2 … 6%. Laboratory and operational tests allowed justifying the applicability of coatings in interfaces operating using lubrication and experiencing moderate loads. The electric resistance welding provides high anti-friction properties of resulting metal coatings (1.9 times higher than hardened steel 45, and HD80 (high duty) cast iron by 1.6 times). The developed technology makes it possible to restore cylinder parts worn to 1 mm with wear resistance higher than that of new parts such as the support surfaces of drive shafts, screw rotors, mould guides, etc.

Structure, phase composition and mechanical characteristics of layered composite materials based on TiB/xTi-Al/α-Ti (x = 1, 1.5, 3) obtained by combustion and high-temperature shear deformation

The effect of the ratio of titanium and aluminum in the Ti–Al intermetallic layer on the microstructure, phase composition, and mechanical characteristics of layered composite materials based on TiB/xTi-Al/α-Ti (x = 1, 1.5, 3) is studied systematically. The layered composite materials are obtained by combustion and high-temperature shear deformation. The green sample is a compressed sandwich blank consisting of two Ti–B and Ti–Al powder layers and an α-Ti titanium sheet. The mechanisms of phase and structure formation that occur during synthesis and subsequent high-temperature shear deformation in ceramic and intermetallic layers are described. It is shown that the synthesis between the initial components proceeds according to the reaction-diffusion mechanism. It has been found that the structure, grain size (from 200 nm to 15 μm), phase composition, and mechanical characteristics of the composite can be tuned by changing the initial composition and the delay time before applying external pressure. It is shown that the mechanical characteristics of the resulting composites depend on the location of macrolayers relative to the applied load during high-temperature shear deformation and have the following values: composite surface hardness up to 1800 HV, elastic modulus up to 300 GPa, elastic recovery up to 43, stress intensity factor up to 25.3 MPa m1/2, bending strength up to 650 MPa. The mechanical characteristics of the obtained composites are compared with known analogs, and the advantages are revealed.

Acoustics for in-process melt pool monitoring during metal additive manufacturing

Intrinsic to most metal additive manufacturing (AM) processes are melt pools generated from directed energy sources like lasers. Melt pools are critical as they function to join powder layers to previous layers during the process. Their criticality extends deeper as most porosity in AM parts stems from melt pool behavior while melt pool solidification dictates the parts’ microstructure. Significant breakthroughs in metal AM are severely hindered by the lack of access to experimental tools to study melt pools and modeling that do not fully capture the complex physics. Thus, melt pool related defects are often difficult to predict in occurrence and location while determining optimal process parameters to eliminate defects is extremely challenging and costly. Furthermore, the many exciting opportunities such as realizing new AM alloys, developing gradient materials/structure, and tailoring microstructure to intended applications are possible only with further understanding of melt pool behavior. With these clear needs, in-process acoustics have been proposed as plausible experimental tools for studying melt pool behavior. This presentation will provide an overview of the current activity in this area in addition to the specific needs the acoustics community can potentially address.

Effect of processing parameters on texture and variant selection of as-built 300 maraging steel processed by laser powder bed fusion

Among the materials that might be manufactured with laser powder bed fusion (LPBF), one can highlight maraging steels, with excellent weldability, strength and fracture toughness. However, the effects of the processing parameters and the mechanisms governing the as-built texture are not clear yet. A recent publication showed a low texture index in the prior austenite, in contrast to other alloys subjected to LPBF with the same strategy. Authors suggested several hypotheses, although no conclusions were drawn. This work aims to investigate these findings by using a 300 maraging steel processed under different conditions, i.e. different printer, powder layer thickness and laser emission mode. To do so, X-Ray Diffraction, Electron Backscattered Diffraction and Scanning Electron Microscopy have been used. Results show that the heat treatment intrinsic to the LPBF process does not affect the prior austenite grains, whose texture and morphology remain unchanged throughout the process. Also, for the studied ranges, the microstructure texture is not related to the powder layer thickness or to the laser emission mode, although it could be affected by the laser power or the scan strategy. Finally, a low degree of variant selection has been observed, where the selected variants are those that contribute to a martensite cubic rotated texture.

Laser powder bed fusion of titanium aluminides using sequential thermal scanning strategy

Titanium Aluminides (TiAls) are good candidates for many applications in the automobile, aerospace, and energy industry because of their high strength, toughness, corrosion, and oxidation resistance at high temperatures. Nevertheless, the low ductility and fracture toughness makes processing of TiAls extremely difficult. Laser powder bed fusion (L-PBF) can additively manufacture high-quality components of various materials. The micro layer-based nature of this process allows for design freedom and near-net shape products. The rapid cooling rates during the L-PBF process may cause defects during the manufacturing of hard materials such as TiAls. These rapid cooling rates lead to high induced residual stresses and crack formation. Several heating strategies and post-heat treatments methods are presented in the open literature for reducing the cracks formed during the L-PBF of TiAl. This study presents a new, novel scanning strategy for mitigating the cracks and reducing residual stresses in the L-PBF of Ti-48Al-2Cr-2Nb. This scanning strategy, referred to as sequential thermal scanning (STS) strategy, applies three thermal cycles in each layer: pre-heating selective area of the powder, melting a layer of powder, and post-heating the solidified layer. The laser process parameters used in each step of the STS strategy were determined and explained in this study. The thermal cycles applied in this scanning strategy were verified using an infra-red thermal camera. Sets of process parameters were studied in the melting step of the STS strategy using a factorial design of experiments. These sets of process parameters cover a large range of volumetric energy density (17–2917 J/mm3). The powder morphology, composition, and behavior were studied. The TiAl fabricated parts were investigated for bulk density, surface and internal cracks, balling defects, internal pores, and surface quality. The spatter formation was studied using a high speed camera. Process maps for the relationship between the laser process parameters and part defects were formulated. Residual stresses were measured to understand the influence of the STS strategy and its effects on the surface features and cracks. The results showed that the STS strategy could be used to produce parts of approximately 99 % density without internal pores. In addition, the STS strategy minimized the amount of internal micro-cracks in TiAl parts. This study contributes to the mitigation of internal defects in the L-PBF of TiAl using a sequential thermal scanning (STS) strategy.

A practicable and reliable test for metal powder spreadability: development of test and analysis technique

A crucial step in the powder bed metal additive manufacturing process is the formation of a thin layer of powder on top of the existing material. The propensity of the powder to form thin layers under the conditions used in additive manufacturing is critically important, but no test method has yet been established to measure this characteristic, which is sometimes referred to as spreadability. The current work spreads a single layer of powder using commercial equipment from the paint and food industries and derives the density of a layer of powder, which is of a similar thickness to that in additive manufacturing. Twenty-four powders from eight suppliers have been tested and the density of the layers has been measured as a function of various parameters. Twenty-two of the powders successfully form thin layers, with a density of at least 40% of each powder’s apparent density. Hall flow time did not correlate with the spread layer density, although the two powders that did not spread did not pass through the Hall funnel. The roughness of the plate onto which the powder was spread, the recoater speed, the layer thickness, particle size and aspect ratio all affect the measured layer density. Results of the new test are repeatable and reproducible. These findings can be used to develop a test for spreadability for metal powders that can be used for additive manufacturing, which will help to improve the quality of printed components.

A review on process parameters, microstructure and mechanical properties of additively manufactured AlSi10Mg alloy

Additive Manufacturing is a layer over layer material depositing process to fabricate 3D shape objects. In this process laser completely melts one powder layer and fuses it with another. Metal additive manufacturing process like selective laser melting can produce intricate shapes which are not possible to produce by conventional methods. This capability has enabled designers to come up with innovative part designs which add value to either overall manufacturing process or product performance, also fulfilling the demands for reducing the cost and manufacturing time. The material selection, process parameters and post process are the significant stages in additive manufacturing to achieve the desired properties of output product. The present paper provides comprehensive review in a systematic manner on selective laser melting printing parameters, microstructure, heat treatment and mechanical properties of additively manufactured AlSi10Mg aluminium alloy and based on the review; few arenas for future research are highlighted.

Dry powder coating in additive manufacturing

Dry powder coating is used in many industries to tailor the bulk solid characteristics of cohesive powders. Within this paper, the state of the art of dry coating of feedstock materials for powder based additive manufacturing (AM) processes will be reviewed. The focus is on feedstock materials for powder bed fusion AM processes, such as powder bed fusion of polymers with a laser beam and powder bed fusion of metals with lasers or an electron beam. Powders of several microns to several ten microns in size are used and the feedstock’s bulk solid properties, especially the flowability and packing density are of immanent importance in different process steps in particular for powder dosing and spreading of powder layers onto the building area. All these properties can be tuned by dry particle coating. Moreover, possibilities to improve AM processability and to manipulate the resulting microstructure (c.f. grain refinement, dispersion strengthening) by adhering nanoparticles on the powders will be discussed. The effect of dry coating on the obtained powder properties along the whole AM process chain and the resulting part properties is assessed. Moreover, appropriate characterization methods for bulk solid properties of dry-coated AM powders are critically discussed.

Binder Jet 3D Printing of 316L Stainless Steel: Orthogonal Printing and Sintering Process Optimization

Binder jet 3D printing (BJ3DP) is an additive manufacturing technique that uses a liquid binder to selectively deposit powder layer by layer to fabricate 3D parts followed by sintering. Herein, the printing parameters, sintering temperature, and holding time of 316L stainless steel (SS) materials produced by BJ3DP technique are systematically investigated. Based on the L9 orthogonal experimental design, the green parts can achieve the highest relative density of ≈55.9%, when the layer thickness, binder saturation, and roller traverse speed are 150 μm, 35%, and 100 mm s−1, respectively. Five green parts printed using the optimal parameters are subsequently sintered under different temperatures and holding time. All the as‐sintered samples consist of a primary γ‐Fe phase and a small amount of δ‐ferrite phase. After sintering at 1435 °C for 7.5 h, the as‐sintered samples obtain the highest relative density of ≈92.1% along with the most excellent mechanical properties, displaying ultimate tensile strength of ≈558 MPa and an elongation of ≈41%. The mechanical properties of these samples sintered at 1435 °C for 7.5 h are also superior to those of the annealed 316L SS in ASTM A276/A276M‐17.

Cold-Sprayed Multilayer Thermal Barrier–Catalytic Coatings for Engine Pistons: Coatings Design and Properties

Yttrium-stabilized zirconia thermal barrier coatings (TBCs) of combustion chambers and piston crowns are used most frequently to increase the chamber temperature and the internal combustion engine efficiency. The development of multilayer metal matrix composite coating is of great importance to diminish the ceramic thermal barrier coating’s brittleness and susceptibility to degradation providing the similar thermal insulation. Our group is developing multilayer TBCs based on intermetallic (Fe-Al) compounds combined with alternating zirconia-based layers made by low-pressure cold spraying (LPCS) and sintering. The Fe-Al intermetallic phase was synthesized during reaction sintering of stainless steel and Al particles in the powder layer previously obtained by cold spraying. A double-nozzle low-pressure cold-spraying gun was used to deposit two layers (stainless steel and Al-YSZ) per one track. The effect of the breaking of the brittle ZrO2 particles due to impingement with the substrate results in the formation of a relatively homogeneous structure with ZrO2 particle size of 3–10 μm. Cold-spray deposition of additional Cu-Ni-Graphene catalytic layers on the TBCs is developed to improve performance and emissions of engines. The microstructure, thermal conductivity, thermal shock behavior and microhardness of TBCs were examined and discussed.

Controllable tensile performance of additively manufactured Al–Fe alloy

Achieving both high strength and ductility is challenging in additive manufacturing of Al alloys using the laser powder bed fusion (L-PBF) process by altering the laser conditions to control the microstructural characteristics. The current work focuses on the influence of varying laser energy density (ED), which is adjusted by different sets of laser power (P) and scan speed (v), on the microstructure-tensile property relationships of the L-PBF-fabricated Al–2.5%Fe alloy under varying laser conditions (P = 204–230 W, v = 450–600 mm/s) with a fixed powder layer thickness (t = 30 μm) and hatch distance (h = 30 μm). With increasing ED (= P/v·t·h), a slight increase in the average size of the columnar α-Al grains was found in the experimental samples concurrently with a higher fraction of <001>-oriented grains. Nanosized particles of the Al6Fe phase with an increased tendency to interconnect were observed in the matrix inside the melt pool, and the θ phase formed within the submicron-sized cellular structure along the melt pool boundaries (MPBs). A low tensile ductility, induced by the local microstructural inhomogeneities across MPBs, was found in all specimens tensile-deformed along the building direction, which was characterized by a preference fracture along the MPBs. At higher applied ED, the tensile ductility of the experimental Al–2.5Fe alloy was significantly improved with only a slight decrease in tensile strength. The difference in concentration of Fe across the MPBs in the matrix was also moderately diminished, which is consistent with the reduced local difference in microhardness across the MPBs under higher ED conditions. Therefore, the present results demonstrate that local microstructural inhomogeneities across MPBs can be controlled by the laser conditions, thereby producing both high strength and good ductility in the L-PBF processed Al–Fe alloys.

Effect of elevated pressure on gas-solid flow properties in a powder feeding system

Abstract In view of the powder feeding system, a multi-physical coupling model of the gas-powder-piston was established based on the Euler-Euler two-fluid model. The numerical simulation method was applied to explore the effects of dense gas-solid flow characteristics under different operating pressures. The results show that gas-solid pulsations at different operating pressures are mainly concentrated in the upper part of the powder tank. An elevated operating pressure efficiently decreases the powder layer area (εp = 0.1) fluctuation. As the operating pressure increases from 0.5 MPa to 3.0 MPa, the rising time and fluctuation rate of pressure are reduced by 71.4% and 62.3%, respectively, and the pressure in the tank has a long stabilization period. Meanwhile, the variation of the instantaneous powder flow rate is more stable and its average value is closer to the theoretical. A high-pressure environment is more conducive to the stable transportation of powder.

A framework for graph-base neural network using numerical simulation of metal powder bed fusion for correlating process parameters and defect generation

Powder bed fusion (PBF) is the most common technique used for metal additive manufacturing. This process involves consolidation of metal powder using a heat source such as laser or electron beam. During the formation of three-dimensional(3D) objects by sintering metal powders layer by layer, many different thermal phenomena occur that can create defects or anomalies on the final printed part. Similar to other additive manufacturing techniques, PBF has been in practice for decades, yet it is still going through research and development endeavors which is required to understand the physics behind this process. Defects and deformations highly impact the product quality and reliability of the overall manufacturing process; hence, it is essential that we understand the reason and mechanism of defect generation in PBF process and take appropriate measures to rectify them. In this paper, we have attempted to study the effect of processing parameters (scanning speed, laser power) on the generation of defects in PBF process using a graph-based artificial neural network that uses numerical simulation results as input or training data. Use of graph-based machine learning is novel in the area of manufacturing let alone additive manufacturing or powder bed fusion. The outcome of this study provides an opportunity to design a feedback controlled in-situ online monitoring system in powder bed fusion to reduce printing defects and optimize the manufacturing process.