Beam-based Additive Manufacturing Research Articles

Experimental investigation of material extrusion additive manufacturing of copper with high powder loading rate

Beam-based additive manufacturing has challenges in fabricating pure copper due to high reflectivity. Material extrusion additive manufacturing (MEAM) avoids these issues with low costs, making it suitable to fabricate complex structures in pure copper. In this work, the effects of several printing parameters on the geometry of the samples are studied from single line, thin wall to cube using homemade copper feedstocks with high powder loading rate (65 vol%). The results show that the single line is most affected by layer thickness, while the thin wall has stricter requirements for printing temperature and printing speed. The geometric accuracy of the cube demonstrates large parametric tolerance due to the overlap and support between the lines. Central composite face-centered (CCF) design and central composite design (CCD) are used to analyze and optimize process parameters for densification. The measured porosity at the optimized printing (printing temperature = 177 °C, air gap = −0.05 mm, layer thickness = 0.3 mm) and sintering parameters (sintering temperature = 1045 °C, holding time = 4 h) is in good agreement with the predicted values. Air gap and sintering temperature have the greatest influence on the porosity, and other parameters within a certain range of on-demand adjustments do not have a significant effect. The ultimate tensile strength and elongation of the sintered samples under the optimal parameters are 153.9 ± 6.1 MPa and 31.36 ± 3.24%, respectively. No printing defects are found at the fracture and the elongation is good, showing the effectiveness of the parameter optimization.

Influence of Hot Isostatic Pressing on Microstructure and Tensile Properties of Nickel-Free Stainless Steel for Metal Binder Jetting

Additive manufacturing processes continue to grow in popularity. Hereby, metal binder jetting (MBJ) has a high potential for series production of highly complex parts with added value compared to other production technologies. Applications already exist in thermal management, filtering, or fluid distribution. As with beam-based additive manufacturing processes, the materials available for MBJ are still limited. Especially in the area of nickel-free stainless steels; for example, for medical applications, there are hardly any options available. Therefore, this study investigates a newly developed nickel-free stainless steel specifically designed for the MBJ process. Particular attention is paid to the microstructure and the mechanical properties such as hardness and tensile strength. In addition, the investigation focuses on the potential of hot isostatic pressing (HIP) after sintering to favorably influence the microstructure and the mechanical properties. The results show that in the as-sintered state, a maximum porosity of 2.8% is present, which can be completely removed by hot isostatic post-densification. HIP post-densification improves both the yield strength and the tensile strength by about 10%. The increase in elongation at break is around 50%.

Filament extrusion-based additive manufacturing of 316L stainless steel: Effects of sintering conditions on the microstructure and mechanical properties

Filament extrusion-based additive manufacturing of metals offers an alternative to the widespread beam-based counterparts. The microstructure obtained from extrusion-based techniques differs greatly from the ones obtained by beam-based additive manufacturing, as a sintering process is used, in contrast to the rapid solidification of a melt pool. In this study, the microstructure of 316L stainless steel fabricated by filament extrusion is investigated as a function of debinding and sintering conditions. High-speed nanoindentation correlated with energy-dispersive X-ray mapping is employed for microstructural characterization. High sintering temperatures of 1350 °C, an atmosphere of pure H2, and a cooling rate of 60 K/m are found to result in the optimal microstructure. High densities are obtained due to accelerated densification, enabled by the introduction of diffusion paths due to δ-ferrite formation. At the same time, hard phases like oxides or σ-precipitates with detrimental effects on the mechanical properties can be avoided. It is shown that the porosity can be quantified by analysis of hardness and modulus data from nanoindentation mapping. The values obtained are in good agreement with optical and Archimedes immersion method measurements. Tensile tests of 3D-printed and sintered specimens show excellent ductility and strength in comparison to literature. We demonstrate that 3D printing of 316L filaments and sintering with the optimized conditions results in material properties comparable to bulk values.

Electron beam-based additive manufacturing of Fe93.5Si6.5 (wt.%) soft magnetic material with controllable magnetic performance

Crack-free Fe-Si soft magnetic material with various microstructural features is additively manufactured by means of Electron beam Powder Bed Fusion (E-PBF). Owing to different microstructural features caused by using different processing parameters, E-PBF-processed Fe93.5Si6.5 (wt.%) shows adjustable magnetic performance (e.g., power losses and saturation magnetic flux density) and various electrical properties. The correlation between the processing parameters used during E-PBF and the corresponding microstructural features as well as the resulting magnetic and electrical properties is discussed.

Microstructural and Mechanical Properties of a Solid-State Additive Manufactured Magnesium Alloy

Abstract In recent years, additive manufacturing (AM) has gained prominence in rapid prototyping and production of structural components with complex geometries. Magnesium alloys, which have a strength-to-weight ratio that is superior compared with steel and aluminum alloys, have shown potential in lightweighting applications. However, commercial beam-based AM technologies have limited success with magnesium alloys due to vaporization and hot cracking. Therefore, as an alternative approach, we propose the use of a near net-shape solid-state additive manufacturing process, additive friction stir deposition (AFSD), to fabricate magnesium alloys in bulk. In this study, a parametric investigation was performed to quantify the effect of process parameters on AFSD build quality including volumetric defects and surface quality in magnesium alloy AZ31B. In order to understand the effect of the AFSD process on structural integrity in the magnesium alloy AZ31B, in-depth microstructure and mechanical property characterization was conducted on a bulk AFSD build fabricated with a set of acceptable process parameters. Results of the microstructure analysis of the as-deposited AFSD build revealed bulk microstructure similar to wrought magnesium alloy AZ31 plate. Additionally, similar hardness measurements were found in AFSD build compared with control wrought specimens. While tensile test results of the as-deposited AFSD build exhibited a 20% drop in yield strength (YS), nearly identical ultimate strength was observed compared with the wrought control. The experimental results of this study illustrate the potential of using the AFSD process to additively manufacture Mg alloys for load bearing structural components with achieving wrought-like microstructure and mechanical properties.

Microstructure evolution during reduction and sintering of 3D-extrusion-printed Bi2O3+TeO2 inks to form Bi2Te3

As an alternative to beam-based additive manufacturing, 3D ink-extrusion additive manufacturing is studied here for thermoelectric Bi2Te3, starting from Bi2O3+TeO2 oxide precursor powders. In situ synchrotron XRD in flowing H2 at elevated temperatures reveals the complex phase evolution upon co-reduction leading to the formation of Bi2Te3, Bi2TeO5 and Bi2TeO2. Sintering trials performed using optimal temperatures identified by in situ XRD show that low heating rates and extensive holding times are required to achieve full co-reduction to pure Bi2Te3. The formation of liquid Bi at the temperatures required for oxide reduction leads to local transient-liquid-phase sintering, creating a coarse-grained porous structure. To limit the amount of free Bi, compositional homogenization prior to reduction is achieved by oxide pre-sintering in air, allowing to access the high temperatures required for interdiffusion. After co-reduction of the complex Bi2Te4O11+Bi2Te2O7+TeO2 pre-sintered ceramic, fine-grained, Te-rich n-type Bi2Te3 is obtained, achieving a zT of ∼0.4 between 373 and 423 K. The present study demonstrates the feasibility of synthesizing thermoelectrics from oxide precursors, with the potential of simplifying the processing chain and reducing cost to obtain 3D-extruded thermoelectric parts with complex shapes and architectures.

Cold Spray Additive Manufacturing: Microstructure Evolution and Bonding Features

ConspectusMost metal additive manufacturing (AM) methods involve the melting or sintering of feedstock powder or wire using an energy source (laser, electron beam, or electric arc). Solid-state AM, sometimes also known as non-beam-based AM, is a process in which the deposited material does not melt and is built up layer-by-layer, typically through severe plastic deformation. Initially considered to be a coating technique, by virtue of its high deposition rate, cold spray additive manufacturing (CSAM) is attractive as a solid-state AM technology. In the CSAM process, metal powder particles are impacted onto a substrate at a supersonic velocity and relatively low temperature. The CSAM process reduces or eliminates many problems associated with melting or beam-based AM methods, making the CSAM technically attractive for a wide range of applications, such as in the aerospace, automobile, marine, biomedical, machinery, and energy sectors.In this Account, the author briefly reviews the setup of a cold spray system and discusses the strengths and drawbacks of CSAM compared with thermal sprays and beam-based AM processes, as well as applications in relevant industries. The author summarizes the bonding mechanisms proposed for the cold spray process. The focus of this Account is to review the microstructure evolution of several typical model metals (copper, nickel, aluminum, and titanium) during the cold spray process. The author shows a large variety of microstructure characteristics (recrystallized grains, annealing twins, shear bands, submicron-sized grains, deformation twins, and nanometer-sized grains) in cold-sprayed copper, dynamic recrystallization of cold-sprayed nickel, and the formation of refined grains even below 10 nm in size in cold-sprayed aluminum. The magnitude of the stacking fault energy of as-sprayed materials considerably influences the microstructure after cold spray and postprocessing. Due to the relatively low thermal conductivity and ductility of titanium, cold spraying of titanium shows both a high deposition efficiency and relatively high porosity of as-sprayed parts, as well as a heterogeneous microstructure. Moreover, the Account introduces the postprocessing heat treatment and nanoindentation characterization of cold-sprayed materials. A fundamental understanding of microstructure evolution during and after the cold spray process is essential for optimal mechanical properties. Lastly, the author provides a perspective on applying old lessons and taking advantage of new techniques and materials, including powder characterization, hybrid additive manufacturing, machine learning for searching process windows, nanomechanical testing, and emerging alloys (high-entropy alloys, nanocrystalline alloys, and quasicrystals), to advance the research and applications of CSAM.

Revealing the influence of electron beam melted Ti-6Al-4V scaffolds on osteogenesis of human bone marrow-derived mesenchymal stromal cells

Porous Titanium-6Aluminum-4Vanadium scaffolds made by electron beam-based additive manufacturing (AM) have emerged as state-of-the-art implant devices. However, there is still limited knowledge on how they influence the osteogenic differentiation of bone marrow-derived mesenchymal stromal cells (BMSCs). In this study, BMSCs are cultured on such porous scaffolds to determine how the scaffolds influence the osteogenic differentiation of the cells. The scaffolds are biocompatible, as revealed by the increasing cell viability. Cells are evenly distributed on the scaffolds after 3 days of culturing followed by an increase in bone matrix development after 21 days of culturing. qPCR analysis provides insight into the cells’ osteogenic differentiation, where RUNX2 expression indicate the onset of differentiation towards osteoblasts. The COL1A1 expression suggests that the differentiated osteoblasts can produce the osteoid. Alkaline phosphatase staining indicates an onset of mineralization at day 7 in OM. The even deposits of calcium at day 21 further supports a successful bone mineralization. This work shines light on the interplay between AM Ti64 scaffolds and bone growth, which may ultimately lead to a new way of creating long lasting bone implants with fast recovery times.

Comparison of wire-arc and powder-laser additive manufacturing for IN718 superalloy: unified consideration for selecting process parameters based on volumetric energy density

Wire-arc additive manufacturing (WAAM) as well as powder-laser additive manufacturing (PLAM) are both promising cladding additive manufacturing (AM) techniques for fabricating large IN718 superalloy parts. Correct selection of initial process parameters is an important prerequisite to ensure the success of subsequent AM stage and its dynamic adjustment. For comparing the relationship between the process parameters and their influence on the quality of WAAM and PLAM cladding beads, the relationship between the energy density and the most controllable parameters was comparatively studied from the perspective of unified energy dissipation and the weldability. The equal effective volumetric energy density can be obtained under different combinations of WAAM and PLAM parameters. The defects such as porosity formation, centerline grain boundary, and liquation cracking are mainly affected by the cladding speed, rather than the effective volumetric energy density. During WAAM and PLAM, the corresponding effective volumetric energy density range which can avoid internal and external defects is in the theoretical weldable zone of IN718 superalloy. The key to obtain a defect-free cladding bead is to properly control the energy input and its distribution. The high material utilization under equal effective power and cladding speed is the main reason why WAAM is more efficient than PLAM. According to the energy dissipation hypothesis and related formulas, different energy beam-based AM processes can be further compared under the equal energy input, which provides a basis for the selection of initial process parameters and the dynamic adjustment of main parameters.

Development of a Novel High-Temperature Al Alloy for Laser Powder Bed Fusion

The number of available materials for Laser Powder Bed Fusion is still limited due to the poor processability of many standard alloys. In particular, the lack of high-strength aluminium alloys, widely used in aerospace and automotive industries, remains a big issue for the spread of beam-based additive manufacturing technologies. In this study, a novel high-strength aluminium alloy for high temperature applications having good processability was developed. The design of the alloy was done based on the chemical composition of the widely used EN AW 2618. This Al-Cu-Mg-Ni-Fe alloy was modified with Ti and B in order to promote the formation of TiB2 nuclei in the liquid phase able to stimulate heterogeneous nucleation of grains and to decrease the hot cracking susceptibility of the material. The new Al alloy was manufactured by gas atomisation and processed by Laser Powder Bed Fusion. Samples produced with optimised parameters featured relative density of 99.91%, with no solidification cracks within their microstructure. After aging, the material revealed upper yield strength and ultimate tensile strength of 495 MPa and 460 MPa, respectively. In addition, the alloy showed tensile strength higher than wrought EN AW 2618 at elevated temperatures.

Deformation-based additive manufacturing of 7075 aluminum with wrought-like mechanical properties

Owing to the occurrence of hot cracking during solidification and vaporization of solute elements during melting, beam-based additive manufacturing has encountered serious problems in printing high-strength, nonweldable Al alloys. After aging, the mechanical properties of these alloys are often substantially inferior to their wrought alloy counterparts. Here, a deformation-based additive process is used for printing high-strength 7075 Al alloy (i.e., AA7075), wherein frictional heating is leveraged to enable rapid plastic deformation. The as-printed material consists of a refined, equiaxed microstructure and shows no surface or interface porosity. After proper solution treatment and aging, the yield strength, ultimate tensile strength, and elongation of the printed AA7075 are measured as 477 MPa, 541 MPa, and 8.2%, respectively. These values are a notable improvement from those published for beam-based additive manufacturing and are comparable to the typical properties of wrought alloy AA7075. With a comparison to the feed material, these values account for 95% of the yield strength, 94% of the ultimate tensile strength, and 78% of the elongation of the wrought AA7075-T6 with the exact same composition. Additional printing and testing show that wrought-like mechanical properties can be consistently achieved using this approach without adding new elements or nanoparticles.

3D-printed tungsten sheet-gyroids via reduction and sintering of extruded WO3-nanopowder inks

Additive manufacturing of objects with complex geometries from refractory metals remains very challenging. Here, we demonstrate the fabrication of tungsten sheet-gyroids via 3D ink-extrusion of WO3 nano-powder followed by hydrogen reduction and activated sintering with NiO additions, as an alternative route to beam-based additive manufacturing of tungsten and other high melting metals and alloys. The microstructure and mechanical properties of the tungsten sheet-gyroids are measured for various wall architectures and processing conditions. The original gyroid architecture, separating two equally-sized volumes, is modified to achieve double-wall gyroids (with three separate volumes) with higher relative densities. The compressive properties of these single- and double-walled gyroids are compared to cross-ply lattice structures at 20 and 400 °C, below and above the ductile-to-brittle transition temperature of tungsten. Gyroids are similarly stiff but have lower peak stresses and absorption energy as compared to cross-plies, due to a more severe multiaxial stress state. Based on architecture changes (number, spacing and width of walls), the mechanical properties of the printed gyroids can be tailored to their application requirements.

Modeling and Simulation of Microstructure Evolution for Additive Manufacturing of Metals: A Critical Review

Beam-based additive manufacturing (AM) of metallic components is characterized by extreme process conditions. The component forms in a line-by-line and layer-by-layer process over many hours. Locally, the microstructure evolves by rapid and directional solidification. Modeling and simulation is important to generate a better understanding of the resultant microstructure. Based on this knowledge, the AM process strategy can be adapted to adjust specific microstructures and in this way different mechanical properties. In this review, we explain the basic concepts behind different modeling approaches applied to simulate AM microstructure evolution of metals. After a critical discussion on the range of applicability and the predictive power of each model, we finally identify future tasks.

Solid-State Metal Additive Manufacturing: A Review

Although a significant amount of technical, commercial, and academic resources have been invested in laser and electron beam-based additive manufacturing (AM) of metals and alloys over several decades, challenges and limitations associated with repeated local melting and processing complexity along with the cost of equipment and operation have sparked interest in research on solid-state AM methods as an alternative. This paper reviews the capabilities and challenges of major solid-state metal AM techniques by dividing it into two broad categories (plastic deformation based and sinter based) depending on the metallurgical bonding mechanisms, range of processible alloys, and resulting microstructures. The limited and recent data available in literature show that, while deformation-based AM techniques are primarily limited to relatively ductile alloys, a larger variety of materials are suitable for manufacturing through sinter-based AM. Deformation-based methods generally refine the microstructure by recrystallization, while in most cases sinter-based AM methods lead to grain growth due to high-temperature processing and a more isotropic microstructure. Among the solid-state AM methods summarized here, the binder jetting and additive friction stir AM methods stand out with isotropic microstructures and mechanical properties close to the wrought properties.

Kinetics of alloy formation and densification in Fe-Ni-Mo microfilaments extruded from oxide- or metal-powder inks

3D ink-extrusion of powders followed by sintering is an emerging alternative to beam-based additive manufacturing, capable of creating 3D metallic objects from 1D-extruded microfilaments. Here, in situ synchrotron X-ray diffraction and tomography are combined to study the phase evolution, alloy formation and sinter-densification of Fe-20Ni-5Mo (at.%) microfilaments. The filaments are <200 µm in diameter and are extruded from inks containing a blend of Fe2O3+NiO+MoO3 µm-sized oxide particles. Blended oxide inks show rapid reduction and homogenization during heating to 1373 K in H2 accompanied by fast densification and interdiffusion. The resulting homogenous Fe-20Ni and Fe-20Ni-5Mo alloys reach near-full density within minutes at 1373 K. When using Ar-5%H2, co-reduction and interdiffusion are slower and the sequence of reduction is changed. During H2 co-reduction, Fe2O3 and NiO show synergistic effects. The onset temperature of reduction is mutually reduced and the conversion rates to Fe3O4 and Ni are increased. Fe-20Ni-5Mo microfilaments printed with coarser, elemental powder (~3 µm) show slower sintering and compositional homogenization as compared to inks of blended oxide, as the coarser metal particles provide lower surface/volume ratio and higher diffusion distances. Using micron-size oxide powders (rather than coarser metallic particles) accelerates kinetics of reduction, sintering and interdiffusion which reduces costs and energy in 3D ink-printing, improves filament surface quality, but doubles the extent of shrinkage.

Beamless Metal Additive Manufacturing.

The propensity to manufacture functional and geometrically sophisticated parts from a wide range of metals provides the metal additive manufacturing (AM) processes superior advantages over traditional methods. The field of metal AM is currently dominated by beam-based technologies such as selective laser sintering (SLM) or electron beam melting (EBM) which have some limitations such as high production cost, residual stress and anisotropic mechanical properties induced by melting of metal powders followed by rapid solidification. So, there exist a significant gap between industrial production requirements and the qualities offered by well-established beam-based AM technologies. Therefore, beamless metal AM techniques (known as non-beam metal AM) have gained increasing attention in recent years as they have been found to be able to fill the gap and bring new possibilities. There exist a number of beamless processes with distinctively various characteristics that are either under development or already available on the market. Since this is a very promising field and there is currently no high-quality review on this topic yet, this paper aims to review the key beamless processes and their latest developments.

Review on electron beam based additive manufacturing

PurposeElectron beam-based additive manufacturing (EBAM) is an emerging technology to produce metal parts layer-by-layer. The purpose of this paper is to systematically address the research and development carried out for this technology, up till now.Design/methodology/approachThis paper identifies several aspects of research and development in EBAM.FindingsElectron beam has several unique advantages such as high scanning speed, energy efficiency, versatility for several materials and better part integrity because of a vacuum working environment.Originality/valueThis paper provides information on different aspects of EBAM with the current status and future scope.

Developing New Materials for Electron Beam Melting: Experiences and Challenges

Lack of industrially available materials for additive manufacturing (AM) of metallic materials along with the promises of materials with improved or unique properties provides a strong drive for developing new process/material combinations. As powder bed technologies for metallic materials are relatively new to the market, and to some extent are only maturing, developers of new process/material combinations have certain challenges to overcome. Firstly, basic knowledge on the behavior of materials (even those well established for other applications) under extreme conditions of melting/solidification with beam-based AM methods is far from being adequate. Secondly, manufacturing of the equipment is up to date driven by industrial application, thus optimization of the AM machines for small test batches of powders is still belongs to research and development projects. Also, majority of the powder manufacturers are primarily driven by the market development, and even they are well aware of the demands imposed by the powder bed AM machines, availability of small test batches of adequate powders may be problematic or at least quite costly for the R&amp;D oriented users. Present paper describes the experiences in developing new materials for EBM A2 machine by Arcam EBM, modified for operating with powder batches of 100-200 ml and less. In particular it discusses achievements and challenges of working with powders from different materials with specifications far beyond the range suggested by machine manufacturer. Also it discusses the possibility of using blended rather than pre-alloyed powders for achieving both composite-like and alloyed materials in the same part by steering electron beam energy deposition strategy.

Numerical simulation of residual stress in laser based additive manufacturing process

Minimizing the residual stress build-up in metal-based additive manufacturing plays a pivotal role in selecting a particular material and technique for making an industrial part. In beam-based additive manufacturing, although a great deal of effort has been made to minimize the residual stresses, it is still elusive how to do so by simply optimizing the processing parameters, such as beam size, beam power, and scan speed. Amid different types of additive manufacturing processes, Direct Metal Laser Sintering (DMLS) process uses a high-power laser to melt and sinter layers of metal powder. The rapid solidification and heat transfer on powder bed endows a high cooling rate which leads to the build-up of residual stresses, that will affect the mechanical properties of the build parts. In the present work, the authors develop a numerical thermo-mechanical model for the measurement of residual stress in the AlSi10Mg build samples by using finite element method. Transient temperature distribution in the powder bed was assessed using the coupled thermal to structural model. Subsequently, the residual stresses were estimated with varying laser power. From the simulation result, it found that the melt pool dimensions increase with increasing the laser power and the magnitude of residual stresses in the built part increases.

Erratum to: Modeling the Microstructure Evolution During Additive Manufacturing of Ti6Al4V: A Comparison Between Electron Beam Melting and Selective Laser Melting

Beam-based additive manufacturing (AM) is an innovative technique in which parts are built layerwise, starting from the material in powder form. As a developing manufacturing technique, achievement of excellent mechanical properties in the final part is of paramount importance for the mainstream adoption of this technique in industrial manufacturing lines. At the same time, AM offers an unprecedented opportunity to precisely control the manufacturing conditions locally within the part during build, enabling local influence on the formation of the texture and microstructure. In order to achieve the control of microstructure by tailoring the AM machine parameters, a full understanding and modeling of the heat transfer and microstructure evolution processes is needed. Here, we show the implementation of the non-equilibrium equations for phase formation and dissolution in an AM modeling framework. The model is developed for the Ti6Al4V alloy and allows us to show microstructure evolution as given by the AM process. The developed capability is applied to the cases of electron beam melting and selective laser melting AM techniques to explain the significantly different microstructures observed in the two processes.