Powder Bed-based Additive Manufacturing Research Articles

Manufacturing and aging behavior of CuCr1Zr powder: Analysis and modelling of close-coupled gas atomization of CuCr1Zr and its impact on the melt beam disintegration and analysis powder oxidation kinetics

Powder bed-based additive manufacturing methods represent an increasingly vital role in industrial applications. Specifically, there has been considerable focus on highly reflective metals, such as CuCr1Zr, in recent years. However, current research primarily revolves around developing appropriate manufacturing parameters e.g. laser powder bed fusion (PBF/LB-M) and assessing the corresponding mechanical and electrical properties of CuCr1Zr. The impact of manufacturing routines and oxidation behavior of CuCr1Zr powder are seldom discussed in additive manufacturing. Therefore, the present study addresses the analysis of powder production and the oxidation behavior of CuCr1Zr powder via close-coupled gas atomization to analyze the disintegration of a CuCr1Zr melting jet during gas atomization of CuCr1Zr by using a high-speed camera. Critical conditions for secondary disintegration were calculated as a function of the initial gas pressure. Additionally, calorimetric measurements and isothermal oxidation experiments were conducted to quantify oxide formation and growth. According to the results, initial oxide formation occurs after 42 days with CuCr1Zr powder at room temperature in a nitrogen atmosphere. Subsequently, oxide growth follows a logarithmic law, associated with the formation and conversion of Cu2O to CuO.

Investigation of Effects of Spreading Parameters on Powder Bed Quality in Additive Manufacturing

Powder bed-based additive manufacturing processes such as laser powder bed fusion, binder jetting, and electron beam melting are commonly utilized in various critical areas such as medical, aviation, and energy. Common to all these operations, the powders are first spread onto the built platform in a layer-by-layer fashion and selectively fused or bound with a suitable method. The quality of the process depends on several process parameters, including how the powders are spread onto the built platform. Powder spreading is an important step in these operations and can affect various process outputs. In this study, the powder spreading is numerically investigated using the discrete element method to determine the effects of the layer thickness, rotation, and transition velocities selected as parameters with a powder spreader roller. The relationship between powder spreading parameters and powder volume packing fraction was investigated using the 'Hertz-Mindlin and Johnson-Kendall-Robert’s (JKR) contact model considering the interactions between particle-particle and particle-built plate, has been added to the numerical model. A Design of Experiment combined with analysis of variance (ANOVA) was utilized to gain a broader understanding on the relationship between process parameters and green density and dynamic angle of repose.

Binder jetting in gears

Due to near netshape production, powder metallurgically manufactured (PM) gears have a high potential to increase cost and resource efficiency. Compared to conventionally machined gears, the advantages of PM gears are the different local density distribution in the component and the potential integration of secondary design elements. These advantages are accompanied by the reduction of weight and the possibility of optimizing the NVH-behavior of a gear. PM gears may be manufactured by both, die pressing and additive manufacturing (AM). The powder bed-based additive manufacturing processes for metals can be classified into binder-based (e.g. Binder Jetting, BJT) and beam-based thermal processes (e.g. Laser Powder Bed Fusion, LPBF). Due to the specific process technology, the manufacturing of PM gears by die pressing is only economical for large batch sizes in series production. For small batches, AM offers an approach to manufacture gears that meet the requirements in terms of quality, strength, acoustics and economic efficiency of the manufacturing process.This report describes the potential of BJT gears made of stainless steel 17-4PH (X5CrN-CuNb16-4) with a relative density of ρ0,rel ≈ 99% regarding the tooth bending strength. The reliability of the process and thus the tooth bending strength can be increased further by a specific adjustment of process parameters. Subsequently, the gears are tested with regard to the tooth bending strength on the pulsator test rig. The results are summarized in a SN-curve.

Mechanical properties and microstructural characteristics of 316L stainless steel fabricated by laser powder bed fusion and binder jetting

In this work, the tensile test coupons and lattice structure of 316L stainless steel (SS) samples were manufactured by two powder bed-based additive manufacturing (AM) techniques-laser powder bed fusion (LPBF) and binder jetting (BJ) processes. The microstructure and mechanical properties of these components were investigated through a scanning electron microscope, hardness measurement, tensile tests, and compression tests. The results showed that the grain size of the LPBF sample was smaller than that of the BJ sample. This difference led to the microhardness of LPBF and BJ samples being 239.16 ± 4.90 HV and 208.71 ± 4.15 HV, respectively. The relative density of LPBF samples was 99.20 ± 0.30% and the BJ samples was 96.54 ± 0.54%. The LPBF samples had better mechanical properties, with tensile strength and compressive yield strength of 669.92 ± 18.68 MPa and 75.65 ± 5.77 MPa, respectively, while the tensile strength and compressive yield strength of the BJ samples were 437.55 ± 10.11 MPa and 78.65 ± 3.57 MPa, respectively. The experimental data showed good consistency with the simulation results (error within 10%) in both the compression modulus and yield strength for the lattice structures. These findings look promising for the future design of lattice structures with high mechanical properties using LPBF and BJ techniques.

Additive manufacturing for the automotive industry: on the life-cycle environmental implications of material substitution and lightweighting through re-design

The automotive sector has recently been taking measures to reduce fuel consumption and greenhouse gas emissions for the mobility of ground vehicles. Light-weighting, via material substitution, and the re-designing of components or even a combination of the two, have been identified as a crucial solution. Additive manufacturing (AM) can be used to technologically complement or even replace conventional manufacturing in several industrial fields. The enabling of complexity-for-free (re) designs is inherent in additive manufacturing. It is expected that certain benefits can be achieved from the adoption of re-design techniques, via AM, that rely on topological optimisation, e.g., a reduced use of resources in both the material production and use phases. However, the consequent higher specific energy consumption and the higher embodied impact of feedstock materials could result in unsustainable environmental costs. This paper investigates the case of the light-weighting of an automobile component to quantify the outcomes of the systematic integration of re-designing and material substitution. A bracket, originally cast in iron, has been manufactured by means of a powder bed-based AM technique in AlSi10Mg through an optimized topology. Both manufacturing routes have been evaluated through a comparative Life Cycle Assessment (LCA) within cradle-to-grave boundaries. A 69%-lightweighting has been achieved, and the carbon dioxide emissions and energy demands of both scenarios have been compared. Besides the use-phase-related savings in terms of both energy and carbon footprint due to the lightweighting, the results highlight the environmental trade-offs and prompt the consideration of such a manufacturing process as an integral part of sustainable product development.

Influence of particle size distribution on surface roughness in powder bed fusion - A contribution to increase resource efficiency

High roughness, especially on downskin surfaces, limits the high potential of powder bed-based additive manufacturing. Additionally, resource efficiency is low due to necessary powder preparation by sieving and sifting. In this examination, the potential of high output powder (maraging tool steel 1.2709), which was only marginally prepared after atomizing, was analysed in terms of roughness at different surface orientations on both the upskin and the downskin surface. The high output powder was compared to both, a powder with increased fineness and a commercially available powder, showing great potential regarding resource efficiency and roughness, especially for internal channels.

Directed energy deposition of pure copper using blue laser

Additive manufacturing (AM) technology has progressed dramatically over the last decade, allowing for rapid manufacturing of components with complex geometries and diverse feedstocks. AM of previously challenging materials such as pure copper has thus been made easier with extended processing routes. However, only powder bed-based AM processes (typically equipped with near IR lasers or an electron beam) have previously been demonstrated for producing bulk pure Cu parts with well-defined geometries, where certain disadvantages and limitations abound. This study demonstrates the first bulk Cu parts with well-defined geometries built by the blown powder, directed energy deposition (DED) process with a blue laser. Near full density (up to 99.6 %) parts with a volume of 1000 mm3 were produced, which is the densest pure Cu part reported so far in laser AM, but at a significantly lower energy density compared to similar volume parts built using near IR lasers. A larger part with a volume of 8000 mm3, the largest volume of pure Cu reported to date in laser AM, was also fabricated using the same build parameters with relative density up to 94.1 %. Microstructure analyses were performed and the influence of different building conditions were examined.

Electron Beam-Melting and Laser Powder Bed Fusion of Ti6Al4V: Transferability of Process Parameters

Metal powder bed-based Additive Manufacturing (AM) technologies, such as Electron Beam-Melting (EBM) and Laser Powder Bed Fusion (LPBF), are established in several industries due to the large design freedom and mechanical properties. While EBM and LPBF have similar operating steps, process-specific characteristics influence the component design. The differences in the energy coupling lead to differing solidification conditions, microstructures, and, thus, mechanical properties. The surface finish and geometrical accuracy are also affected. As opposed to LPBF, EBM powder layers are preheated prior to selective melting. In this study, similar volume energy densities in LPBF and EBM were used to manufacture Ti6Al4V test geometries to assess the process transferability. Since the energy coupling of LPBF and EBM differ, heat source absorption was considered when calculating the volume energy density. Even when a similar volume energy density was used, significant differences in the component quality were found in this study due to specific respective process constraints. The extent of these constrains was investigated on voluminous samples and support-free overhanging structures. Overhang angles up to 90° were manufactured with LPBF and EBM, and characterized with regard to the relative density, surface roughness, and geometric compliance.

A novel approach for powder bed-based additive manufacturing of compositionally graded composites

In this feasibility study, a novel selective powder raking approach is used for E lectron B eam P owder B ed F usion (EB-PBF) to produce fully dense and compositionally graded composites. The adaption of the chemical composition along the thickness direction of the printed parts is based on the application of a powder blend composed of powders with different particle sizes combined with varying layer thicknesses during EB-PBF. The microstructure of EB-PBF-processed samples with a composition gradient is investigated. The distribution of different phases in the as-built samples is discussed. Based on the results derived in this work, this approach can be easily extended to other powder bed-based AM technologies as well as to other material systems to produce functionally graded materials.

Influencing the Size and Shape of High-Energy Ball Milled Particle Reinforced Aluminum Alloy Powder.

High-energy ball milling represents an efficient process for producing composite powders consisting of ceramic particles dispersed in a metallic matrix. However, collision events, plastic deformations, and cold welding during the milling lead to a flake or block-like shape of the resulting composite powders. Further consolidation of such irregularly shaped powders by powder bed-based additive manufacturing technologies can be challenging because of their low flowability and low bulk density. In this work, different approaches, including milling process parameters (speed, process control agent atmosphere) and post-treatments (mechanical and thermal), are investigated on their suitability to influence the particle shape, especially concerning the roundness of the composite powders consisting of the aluminum alloy AlSi10Mg with 5 vol% SiC and Al2O3 reinforcement. It is found that milling with menthol as a process control agent leads to the finest composite powder compared to other milling parameters, with the lowest particle roundness of 0.39 (initial powders 0.84). No success in rounding the milled composite powder could be achieved through mechanical post-treatment in a planetary ball mill. On the other side, the thermal spraying of, e.g., SiC reinforced AlSi10Mg powder resulted in a 77–82% relative roundness. A remarkable change in the microstructure and the shape of the composite powders could also be observed after heat treatment in tube furnaces at a temperature above the melting point of AlSi10Mg. The best result in terms of improved roundness (relative to around 85%) was obtained for Al2O3 reinforced at 600 °C. A further increase of the temperature to 700 °C resulted in a moderate coarsening of powders with Al2O3 and extensive sintering of powders with SiC, presumably due to a different distribution inside the matrix.

Effects of Particle Size Distribution with Efficient Packing on Powder Flowability and Selective Laser Melting Process.

The powder bed-based additive manufacturing (AM) process contains uncertainties in the powder spreading process and powder bed quality, leading to problems in repeatability and quality of the additively manufactured parts. This work focuses on identifying the uncertainty induced by particle size distribution (PSD) on powder flowability and the laser melting process, using Ti6Al4V as a model material. The flowability test results show that the effect of PSDs on flowability is not linear, rather the PSDs near dense packing ratios cause significant reductions in flowability (indicated by the increase in the avalanche angle and break energy of the powders measured by a revolution powder analyzer). The effects of PSDs on the selective laser melting (SLM) process are identified by using in-situ high-speed X-ray imaging to observe the melt pool dynamics during the melting process. The results show that the powder beds made of powders with dense packing ratios exhibit larger build height during laser melting. The effects of PSD with efficient packing on powder flowability and selective laser melting process revealed in this work are important for understanding process uncertainties induced by feedstock powders and for designing mitigation approaches.

Inverse Determination of Johnson-Cook Parameters of Additively Produced Anisotropic Maraging Steel.

In powder bed-based additive manufacturing (AM), complex geometries can be produced in a layer-wise approach. Results of material science experiments regarding material property identification, e.g., tensile strength, show interdependencies between the test load direction and the layer orientation. This goes hand-in-hand with the measured cutting force, changing with the relative angle between cutting direction and layer orientation in orthogonal cutting tests. However, due to the specific process characteristics, the layer orientation results in anisotropic material properties. Therefore, during machining, the material behaves depending on the buildup direction, which influences the cutting process. To predict this behavior, a simplified inverse approach is developed to determine the buildup direction-dependent parameters of a modified Johnson–Cook model for cutting simulation. To qualify these cutting models, mainly the cutting force and additionally the chip formation examined during orthogonal cuts are used. In the present paper, the influence of the laser-powder-bed-fusion (LPBF) process parameters on subtractive post-processing are shown. A good agreement between verification experiments and simulations is achieved.

Electrostatic powder attraction for the development of a novel recoating system for metal powder bed-based additive manufacturing

The field of electrostatics and electrophotography offers great potential for applying powder layers for powder bed-based additive manufacturing processes, such as powder bed fusion of metals using a laser beam (PBF-LB/M). The authors introduce a highly flexible powder layer forming process using the electrophotographic principle. The focus is on full-area powder attraction by electrostatic mass transport of a typical gas atomized PBF-LB/M steel alloy (20MnCr5). A simulation model was created to predict the formation of the electric field. This model was used to determine parameter preselection for experimental validation. After successful full-area powder attraction, a selective powder attraction approach was investigated.

Numerical simulation and experimental measurement of pressureless sintering of stainless steel part printed by Binder Jetting Additive Manufacturing

Binder Jetting-Metal Additive Manufacturing (BJ-MAM) is a powder bed-based additive manufacturing technology which deposits liquid binder droplets to join powder particles to form complex shaped structures (i.e., green parts). As the printing process is done around room temperature, BJ-MAM is largely immune to the distortion and cracking issues that can be prevalent in melting based powder bed processes. However, a main issue for BJ-MAM is the part shrinkage and distortion during high-temperature sintering. The densification and deformation behaviors during pressureless sintering of green parts printed by BJ-MAM were studied in this paper. Experimentally, cantilever- and bridge-shaped coupons with varying beam lengths were printed using 316L stainless steel powder; these coupons produced different extents of deformation after sintering. Based on the existing modeling approaches in the powder metallurgy literature, a finite element model was developed incorporating an elastic-viscoplastic constitutive equation for computing both uniaxial equivalent creep strain and volumetric swelling strain. Two methods, a viscosity based and a power-law creep based, were further evaluated for calculating the uniaxial equivalent creep strain. Material property data used in the constitutive equation such as viscosity and creep parameters were collected from the literature, critically reviewed, and then inputted into the model. Other salient features of the model included thermal-mechanical property data that were dependent on both relative density and temperature as well as frictional contact between the part surface and the furnace wall under gravitational load. The calculated quantities such as shrinkage, final relative density, and deformed shapes were compared with the respective experimental data across different part geometries.

Numerical simulation of the flow behavior and powder spreading mechanism in powder bed-based additive manufacturing

The flow behavior of the powder layer significantly affects subsequent dynamic behavior and final quality of the parts fabricated via powder bed-based additive manufacturing. High powder layer quality is the key scientific problem and technical difficulty, however, the dynamic flow behavior of powder particles and its relationship between powder layer quality were not well understood. In this paper, the influences of typical factors including process parameters (relative layer height and blade velocity) and powder properties (particle size and surface energy coefficient) on powder layer quality of polyamide 6 were systematically investigated by discrete element method to understand the mechanism of the powder layer packing behavior at the particle scale. The packing density, structure uniformity, normalized layer thickness, and surface uniformity were introduced to quantitatively characterize the powder layer quality. The mechanisms at the particle scale, including the blocking of particle, force arch, particle velocity distribution, and cohesion effect were discussed in detail. The results showed that a decreasing value particle size/layer height can effectively weaken the blocking effect of particles, improving the continuity and stability of powder flow and inducing a dense and uniform powder layer. The particles with high average velocity in the gap between the blade and baseplate will move forward after the blade leaves, significantly reducing the layer thickness and packing density. Meanwhile, the cohesion force was determined by the particle size and surface energy coefficient, and the cohesion effect dominated the flow behavior of the particles with a small particle size d = 10μm or a large surface energy coefficient γ = 0.2mJ/m2. These findings can provide useful guidance in understanding the flow behavior at the particle scale and improving the powder layer quality.

Slurry development for lithography-based additive manufacturing of cemented carbide components

This article describes slurry development for lithography-based additive manufacturing (AM) of cemented carbide components. Due to their superior material properties compared to steels, tungsten carbide‑cobalt hardmetals are an exciting material for manufacturing tools. AM technologies' geometric freedom makes it possible to design more efficient tools compared to other manufacturing techniques. These technologies are known as resource-efficient and sustainable processes. The material that is not required for manufacturing is re-used for the production of additional components. The high energy input in laser powder bed-based AM processes leads to strong grain growth and brittle phases, so the achievable mechanical properties are significantly limited. Lithography-based additive manufacturing shows high potential due to the manufacturing of a polymer-bound green body and a subsequent heat treatment. Tungsten carbide‑cobalt (88–12 wt%) slurries were developed with a solid content of 40 vol%, based on the resin's reactive components. Dispersants made it possible to increase the maximum processable volume fraction of solid particles. A thixotropic agent helped stabilize the slurry. Rheological investigations characterized these effects. The influence of the photoinitiator concentration and the volume fraction of tungsten carbide and cobalt particles on the curing depth was analyzed. Prepared slurries and manufactured multi-layered green bodies were evaluated for homogeneity using quantitative microstructure analysis.

Mechanical Properties of Selective Laser-Melted Components of Alsi10mg for Prototype Vehicles

Fabrication of aluminum alloy components by traditional high-pressure die casting (HPDC) requires cost- and time-consuming tooling of steel dies, which makes HPDC uneconomic for producing low-volume components or prototypes. In comparison, powder bed-based additive manufacturing, e.g. selective laser melting (SLM), enables rapid prototyping and production of even complex-shaped components directly from computer-aided design models without needing expensive tools. However, SLM prototype components must have almost identical mechanical properties to HPDC serial components in order to emulate their functionality under different load conditions. In this work uniaxial tensile properties of cast alloy AlSi10MnMg (EN AC-43500) in condition T7, i.e. with 120-170 MPa yield stress, 200-240 MPa tensile strength and 9-12 % strain at fracture, shall be attained using selective laser melting of powder alloy AlSi10Mg (EN AC-43000). These properties were achieved by tailored heat treatment. Furthermore, the effect of hot isostatic pressing (HIP) was investigated. The results of the tensile tests confirmed the basic feasibility of substituting HPDC components with SLM components for prototyping. In particular, similar tensile strength and uniform strain were achieved for SLM samples in condition O, i.e. for SLM samples which were only annealed.

A novel experimental method for in situ strain measurement during selective laser melting

ABSTRACT Selective laser Melting (SLM), a powder bed-based additive manufacturing technology, has been developed and applied in multiple industrial fields in the last decade. However, the distortion and swelling in the SLM process resulting from thermal stress cannot be predicted subject to measurement. In this work, an in situ distortion measurement system applied to the SLM process is presented. The distortion behaviour of component under laser scanning can be precisely recorded in real-time by this system. The detailed evolution and driving force of specimen distortion in the SLM process are discussed based on the experimental results. The distortion in single laser scanning presents a strong instantaneous upward motion of the central section during laser heating and a relatively slow downward recovery motion of the central section during cooling. The distortion behaviour of the sample with and without a layer of metal powder are compared, and laser scanning on the bare sample surface leads to a significantly higher residual distortion. The influence of SLM parameter variables (such as scanning speed, laser power, scanning width, layer thickness and scanning times) on SLM distortion is also analysed. At last, the stress distribution of laser melting is verified by the high-resolution EBSD analysis.

Measuring procedures for surface evaluation of additively manufactured powder bed-based polymer and metal parts

Powder bed-based additive manufacturing has become increasingly important for industrial applications. In the light of this, qualitative considerations such as the geometrical accuracy, the resulting mechanical properties, and the surface quality of additively manufactured parts must be taken into account. Optical measuring techniques such as confocal laser scanning microscopy, fringe projection and focus variation as well as profilometers are evaluated here, to determine the surface quality of powder bed-based manufactured parts. Even though these surface evaluation methods are established commercially, no standardized measuring procedure has yet been established. Within an experimental study the validity and accuracy of surface measurement methods are evaluated below, taking the limitations of each measurement system and the comparability of areal surface textures into account. The examinations are carried out with the powder materials EN-AW2024, Ti-6V-4Al and PA12, which are processed by electron beam melting, and laser beam melting of metals and polymers. Guidance for a consistent and comparable surface evaluation is thereby provided.

Fabrication of Single Crystals through a µ-Helix Grain Selection Process during Electron Beam Metal Additive Manufacturing

Selective Electron Beam Melting (SEBM) is a powder bed-based additive manufacturing process for metals. As the electron beam can be moved inertia-free by electromagnetic lenses, the solidification conditions can be deliberately adjusted within the process. This enables control over the local solidification conditions. SEBM typically leads to columnar grain structures. Based on numerical simulation, we demonstrated how technical single crystals develop in IN718 by forcing the temperature gradient along a µ-Helix. The slope of the µ-Helix, i.e., the deviation of the thermal gradient from the build direction, determined the effectiveness of grain selection right up to single crystals.