Powder Bed Fusion Method Research Articles

Mapping of residual stresses in as-built Inconel 718 fabricated by laser powder bed fusion: A neutron diffraction study of build orientation influence on residual stresses

Manufacturing of functional (ready to use) parts with the powder bed fusion method has seen an increase in recent times in the field of aerospace and in the medical sector. Residual stresses (RS) induced due to the process itself can lead to defects like cracks and delamination in the part leading to the inferior quality of the part. These RS are one of the main reasons preventing the process from being adopted widely. The powder bed methods have several processing parameters that can be optimized for improving the quality of the component, among which, build orientation is one. In this current study, influence of the build orientation on the residual stress distribution for the Ni-based super-alloy Inconel 718 fabricated by laser-based powder bed fusion method is studied by non- destructive technique of neutron diffraction at selected cross-sections. Further, RS generated in the entire part was predicted using a simplified layer by layer approach using a finite element (FE) based thermo-mechanical numerical model. From the experiment, the part printed in horizontal orientation has shown the least amount of stress in all three directions and a general tendency of compressive RS at the center of the part and tensile RS near the surface was observed in all the samples. The build with vertical orientation has shown the highest amount of RS in both compression and tension. Simplified simulations results are in good agreement with the experimental value of the stresses.

Characterization and Qualification of LPBF Additively Manufactured AISI-316L Stainless Steel Brackets for Aerospace Application

Additive manufacturing or 3D printing is recognized as a revolutionary type of processing to replace the traditionally fabricated components, including castings, wrought products and multiple piece assemblies thereof. 3D printing helps to realize complicated parts within a short time, with minimum material wastage and allowing higher level of design optimization. Two types of brackets in stainless steel AISI-316L grade for aerospace applications were realized through laser powder bed fusion method. Detailed characterization, analysis and structural testing were performed on the powder, test coupons and the component itself to qualify these brackets for the intended application. Significant amount of material removal, excessive machining time and associated problems such as residual stresses and warpage envisaged in the conventional manufacturing route have been avoided. The mechanical properties meet the requirements specified in ASTM F 3184-16 standard and were similar to or better than vis-a-vis that can be achieved through forged route manufacturing. Minor distortion was noticed on thin wall regions which have been avoided in the next hardware by distortion compensation and adding extra stock at thin sections which can be removed by minimum machining. Non-destructive testing was performed by macro CT and brackets were found to be free from defects > 100 µm. Further, there is scope for topology optimization through design for additive manufacturing (DfAM) and achieving weight savings for future requirements.

Fundamental Investigations on Additive Manufacturing Methodby Selective Solidification Using Electrical Discharge

We propose an additive manufacturing (AM) method using electrical discharge with twin electrodes. Today, inexpensive three-dimensional (3D) modeling machines for fused deposition modeling (FDM) with resin fibers are used in various fields. On the other hand, laser sources and electron beam guns as heat sources for the powder bed fusion (PBF) method with metal powder are generally expensive. This impedes the wide use of modeling machines. In the proposed AM method, part of the powder layer is selectively solidified by electrical discharge from the twin electrodes through the surface of a metal powder layer. It is expected that the use of a discharge circuit as an inexpensive heat source will reduce the total machine cost and the overall machine size. A three-dimensional shape with overhangs was successfully fabricated by laminating thin layers.

Effect of Heat Treatments on Fatigue Properties of Ti-6Al-4V Alloy Fabricated by EBM Additive Manufacturing

The powder bed fusion method using a heat source for 3D printing can be applied to fabricate the geometrical shapes of some parts, and enables rapid melting and cooling. 3D EBM additive manufacturing can supply a near-net-shape or net-shape and precise parts better than micro scale casting and precision casting. Casting parts have residual stress in the matrix and their microstructure, which can affect the dynamic properties, but if they are heat-treated under the optimal conditions, the residual stress can be reduced to enhance the dynamic-mechanical properties of 3D printed parts. In this study, Ti-6Al-4V bulk bars were fabricated by EBM additive manufacturing, and the microstructures, residual stress, and various interior defects of the specimens were analyzed. The residual stress and surface tension were measured and calculated in different specimens heat-treated at 850~950 ℃ . The relationship between the residual stress and surface tension was explained using the related equations and the influence on the high cycle fatigue properties was evaluated.

Resource analysis model and validation for selective laser melting, constituting the potential of lightweight design for material efficiency

Selective Laser Melting (SLM) offers significant potential for a sustainable way of production. Raw material in form of metallic powder can directly be reused and the selective nature of the process offers new potential for resource economization. We introduce a mathematical model, which allows conclusions about the influence of parameters like part volume (influenced by lightweight design) and exposure parameters onto the resource consumption in an SLM process. For this purpose, time and energy consumption are classified in process shares as a function of volume and process parameters. The introduced approach is validated by experimental methods under the consideration of part volume, exposure parameters and batch size. While the approach shall be independent of the manufactured material, the experiments are executed for the aluminum alloy AlSi10Mg. The measurements quantify the impact of the part volume and process parameters on the resource consumption and provide recommendations for improvements regarding an increased material efficiency. Additionally, the established model can be used to analyze manufacturing costs for single parts or series productions. The results illustrate the importance of lightweight design methods for an efficient and sustainable production by powder bed fusion methods like SLM.

Crush performance of additively manufactured maraging steel microlattice reinforced plates

Additive metal manufacturing techniques, in particular laser-based powder bed fusion methods, are revolutionary in their capabilities to fabricating new classes of lightweight and complex materials called metallic microlattices. In this paper, a microlattice structure was designed for energy absorption purposes and further additively manufactured using maraging steel (Maraging300) powder through Laser-powder bed fusion (L-PBF) technique. In addition, several cylindrical bars and cubes in horizontal and vertical directions were manufactured to perform uniaxial tensile and compression tests on bulk L-PBF Maraging300. The manufactured microlattices were characterized using different electron microscopy techniques including scanning electron microscopy and micro-CT analysis to ensure that the desired structural integrity was achieved. In addition, the microlattice was then crushed using a universal mechanical testing machine to evaluate its performance experimentally under quasi-static uniaxial compressive loading conditions. Along with the crush, a nonlinear finite element model with predictive capabilities was then developed using commercial package (LS-DYNA) for axial crush of the microlattice to compare its expected performance. Two finite element models are developed using the as-built geometry from the printed lattice, and the as-designed geometry. The effect of model parameters is discussed and very good agreement between the experimental results and the finite element prediction was observed. Finally statistical analysis of the model and failure analysis of the crushed lattice is presented.

Compositionally-tailored steel-based materials manufactured by electron beam melting using blended pre-alloyed powders

The paper presents the prospects of additive manufacturing (AM) in metal, using the powder bed fusion (PBF) method Electron Beam Melting (EBM) in fabrication specific steel-based alloys for different applications. The proposed approach includes manufacturing of metals from blended pre-alloyed powders for achieving in situ alloying and the material microstructure tailoring by controlling electron beam energy deposition rate EBM tests were conducted with the blends of 316L stainless steel and Colferoloys 103 and 139, corrosion- and abrasion-resistant iron based materials commonly used for plasma spray coating. Thorough microstructure analysis of the manufactured sample was carried out using electron microscopy and measurements of microhardness and elastic modulus was carried out using nanoindentation. It is concluded that implementation of blended powder pathway in PBF AM allows to widen the scope of available materials through diminishing the dependence on the availability of pre-alloyed powders. Together with beam energy steering this pathway also allows for an effective sample microstructure control at different dimensional scales, resulting in components with unique properties. Therefore, the implementation of ‘blended powder pathway’ in PBF AM provides a possibility of manufacturing components with the composite-like and homogeneous zones allowing for the microstructure control and effectively adding a “4th dimension” to “3D printing".

A study on elimination of failures resulting from layering and internal stresses in Powder Bed Fusion (PBF) additive manufacturing

ABSTRACTThe study is focused on the manufacturing of fully dense and porous Ti6Al4V parts by Powder Bed Fusion (PBF) method. In the scope of the study, the reasons of the impact of the powder recoating system and internal stress-induced failures in the manufacturing of tensile specimens were investigated and solution proposals were presented. Results that were determined by implementing the most suitable solution were investigated. In this study, failures that occurred during manufacturing were prevented by analyzing the powder recoating system. As a result of the study, the negative effects, which were caused by the impact of the recoater blade on the parts, were prevented. It was observed that the samples were bent due to the forces that were generated by the recoating system movement during manufacturing. The bending conditions were eliminated when the samples were manufactured in a barrel. Thus, no deformation occurred in the part. The negative effects of internal stresses in samples were removed by using the cooling rate concept and FEA analysis. Finally, assumptions of simulations were verified by manufacturing samples with the same machine parameters.

Quantification of location-dependence in a large-scale additively manufactured build through experiments and micromechanical modeling

Wire-based directed energy deposition additive manufacturing techniques (AM) permit the rapid production of large-scale structural components which are not currently possible using the more common powder bed fusion (PBF) AM methods. However, due to larger melt pool widths and higher energy inputs than PBF methods, local thermal history effects produce significant location-dependent microstructure, porosity, and mechanical behavior that necessitates thorough quantification of this emergent technology. Wire + Arc Additive Manufacturing (WAAM) was used to produce austenitic stainless-steel single bead walls in order to statistically quantify the variation of critical material properties within the build. Individual grain geometric properties evaluated using electron back scatter diffraction at different points in the build were well fit by a three-parameter Weibull cumulative distribution function, yet sufficiently different from averaged values. X-ray diffraction for each location disclosed a strong wire texture in the build direction, leading to anisotropic elastic moduli values that were well described by directionally-dependent modulus predictions obtained from diffraction peak analysis. Location-dependent mechanical behavior was examined and accurately captured by an elasto-viscoplastic model based on the Fast-Fourier Transforms (EvpFFT) using the local microstructure orientation data as input. Overall, a high-quality build was realized, with minimal porosity of less than 0.32%, and median yield and tensile strength values of approximately 320.4 ± 8.0 MPa and 531.6 ± 8.2 MPa, respectively. In conclusion, mean values for mechanical behavior within the wall build were found to closely resemble single pass weldments, with statistical variation between individual locations mainly occurring in the weld direction due to local thermal history effects.

The effect of texture on the anisotropy of thermophysical properties of additively manufactured AlSi10Mg

The process of additive manufacturing (AM) has rapidly developed over the past two decades and is now addressing the needs of industry for fast production of samples with tailored properties and complex geometries. One of the most common alloys fabricated from powder using the Laser Powder Bed Fusion (L-PBF) method is AlSi10Mg. The effects of the inherent anisotropy and existing porosity in AM AlSi10Mg were investigated in terms of thermophysical properties, namely thermal conductivity, diffusivity, heat capacity and thermal expansion. These properties were measured in the two principal directions, namely parallel and perpendicular to the printing direction (i.e., in the Z- and X-directions, respectively). In both cases, the sample showed abnormal thermal expansion and conductivity, as compared to a conventionally fabricated sample. After heat treatment, macro- and microstructure analysis confirmed that thermally induced porosity (TIP) had occurred. The anisotropic behaviors of thermal conductivity, diffusivity and thermal expansion were found to be related to the texture, preferred orientation and pore distribution of the aluminum grains in the L-PBF-treated samples.

Surface roughness effect on multiaxial fatigue behavior of additive manufactured metals and its modeling

Additive manufacturing (AM) has become a very popular topic recently due to its many advantages including short build cycles, convenience of customization, and most importantly the ability to build components with complex geometry. However, the surface condition of additive manufactured components is not always satisfactory, particularly with respect to fatigue performance. This is because the as-built surface tends to be rough and post surface treatments or processes such as machining and polishing may not be applicable to all AM parts. On the other hand, since many components are under cyclic loading consisting of normal and shear stresses, multiaxial fatigue behavior is one of the most important aspects to evaluate. This paper evaluates the surface roughness effect on fatigue behaviors of Ti-6Al-4V alloy samples additively manufactured by laser-based powder bed fusion method (L-PBF). Fully reversed axial, torsional, and combined axial-torsion fatigue tests were conducted on specimens with different surface conditions and with different post heat treatments (annealed and HIP). Fatigue life predictions were made using linear elastic fracture mechanic with satisfactory results, as compared to experimental results.

Printability and microstructural evolution of Ti-5553 alloy fabricated by modulated laser powder bed fusion

In this research, the printability of Ti-5553 alloy is assessed using a modulated laser powder bed fusion method. Cylindrical samples were printed with a wide range of volumetric energy density (VED). Density evaluation was practiced by the Archimedes method and X-ray computed tomography (XCT). Surface roughness analysis and hardness mapping were further used to characterize the as-built samples. In addition, the microstructure was studied using optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD) techniques. It was observed that low and high VED values resulted in an increase in the level of porosity. The highest relative density of 99.92% and surface roughness of < 12 μm were achieved while using the VED of 112 J/mm3, resulting in a uniform hardness distribution equal to 295 ± 10 HV. In addition, the characterization by electron microscopy revealed evidence for the presence of ω phase in the sample with the highest density. It was also observed that the use of rather high VEDs gave rise to the in situ precipitation hardening due to nucleation of α-Ti needles in the β-Ti phase matrix. However, due to the inhomogeneous size distribution and volume fraction of the α-Ti needles along the building direction, a non-uniform hardness was obtained when high VEDs were applied.

Additive manufacturing of glass with laser powder bed fusion

AbstractIts transparency, esthetic appeal, chemical inertness, and electrical resistivity make glass an excellent candidate for small‐ and large‐scale applications in the chemical, electronics, automotive, aerospace, and architectural industries. Additive manufacturing of glass has the potential to open new possibilities in design and reduce costs associated with manufacturing complex customized glass structures that are difficult to shape with traditional casting or subtractive methods. However, despite the significant progress in the additive manufacturing of metals, polymers, and ceramics, limited research has been undertaken on additive manufacturing of glass. In this study, a laser powder bed fusion method was developed for soda lime silica glass powder feedstock. Optimization of laser processing parameters was undertaken to define the processing window for creating three‐dimensional multilayer structures. These findings enable the formation of complex glass structures with micro‐ or macroscale resolution. Our study supports laser powder bed fusion as a promising method for the additive manufacturing of glass and may guide the formation of a new generation of glass structures for a wide range of applications.

Correlation between surface tension and fatigue properties of Ti-6Al-4V alloy fabricated by EBM additive manufacturing

The powder bed fusion method using a heat source for 3D printing can be applied to fabricate the geometrical shapes of some parts, and enables rapid melting and cooling. 3D EBM additive manufacturing can supply a near-net-shape or net-shape and precise parts better than micro scale casting and precision casting. Casting parts have residual stress in the matrix and their microstructure, which can affect the dynamic properties, but if they are heat-treated under the optimal conditions, the residual stress can be reduced to enhance the dynamic-mechanical properties of 3D printed parts. In this study, Ti-6Al-4V bulk bars were fabricated by EBM additive manufacturing, and the microstructures, residual stress, and various interior defects of the specimens were analyzed. The residual stress and surface tension were measured and calculated in different specimens heat-treated at 850–950 °C. The relationship between the residual stress and surface tension was explained using the related equations and the influence on the high cycle fatigue properties was evaluated.

Additive Manufacturing: Assessing Metal Powder Quality Through Characterizing Feedstock and Contaminants

The quality of powder feedstock for additive manufacturing (AM) metal powder bed fusion methods can significantly affect the quality of parts built from it. Particle size distribution (PSD) and shape factors influence flowability as well as the thickness and uniformity of each powder layer in the build box. For high-performance applications such as aerospace, medical, power generation and military, it becomes important to identify critical additional factors: the types, numbers and sizes of particulate contaminants that may be present in the powder. This is true for virgin, used and blended powders. Contaminants may be introduced during powder manufacture (e.g., ceramic insulation fragments from gas atomization equipment), handling (building insulation, talc) or possibly during the build process itself. Contaminants contained within a batch of powder can be physically built into an additive part when they are incorporated into the melt pool, and they can remain as discrete particulates or non-fused interfaces that act as stress concentrators. Their presence may decrease fatigue life by increasing the likelihood of fatigue crack initiation. This article describes three methods to rapidly and quantifiably characterize powder feedstock. (1) Computer-controlled scanning electron microscopy (CCSEM) provides quantitative size and shape parameters, as well as fine surface details from individual images on a particle-by-particle basis in large populations of powder. (2) Energy-dispersive spectroscopy (EDS) can be included, providing insights into variations within a batch of powder, as well as contaminant compositions. (3) For critical applications, the heavy liquid separation (HLS) method physically extracts low-density contaminants from a sample of powder metal down to part-per-billion detection limits to allow direct examination of contaminants and enhance identification and prevention of their sources. Altogether, these methods permit direct comparisons among powder metal samples. Better quantification of powder characteristics aids determination of suitability for end uses.

Developing processes for manufacturing metal aviation technology components using powder bed fusion methods

Favorable parameters for selective melting methods using electron and laser radiation have been established to obtain the required geometric, physical and mechanical characteristics of thin-walled parts for aviation purposes from H18N9T (analogue AISI 321) and Ti6Al4V alloys. Parts were manufactured and field tests were carried out on the stand. It has been shown that the technological processes developed using the SLM and SEBM methods can be recommended for the manufacture of thin-walled parts working in conditions of rapidly changing deformations.

Study of powder bed fusion method with metal

Study of powder bed fusion method with metal

Depth-sensing time-dependent response of additively manufactured Ti-6Al-4V alloy

Depth-sensing (instrumented) indentation testing technique is a robust, reliable, convenient and non-destructive characterization method to study small-scale mechanical properties and rate-dependent plastic deformation in metals and alloys at ambient and elevated temperatures. In the present paper, depth-sensing indentation creep behavior of an additively manufactured, via laser powder bed fusion (L-PBF) method, Ti-6Al-4V alloy is studied at ambient temperature. Indentation creep tests were performed through a dual-stage scheme (loading followed by a constant load-holding and unloading) at different peak loads of 250 mN, 350 mN, and 450 mN with holding time of 400 s. Creep parameters including creep rate, creep stress exponent, and indentation size effect were analyzed, according to the Oliver and Pharr method, at different additive manufacturing scan directions and scan sizes. To assess processing parameter/ microstructure/ creep property correlations in the additively manufacture Ti-6Al-4V alloy, microstructural quantitative analyses (i.e. optical microscopy and scanning electron microscopy) were performed as well. The findings of this study, according to stress exponent values, showed that the controlling mechanism of the creep at ambient temperature for the examined L-PBF Ti-6Al-4V is mainly glide-controlled dislocation creep. These findings were compared against traditionally processed Ti-6Al-4V as well.

Influence of post treatment on microstructure, porosity and mechanical properties of additive manufactured H13 tool steel

Additive manufacturing (AM) is an attractive manufacturing technology in tooling applications. It provides unique opportunities to manufacture tools with complex shapes, containing inner channels for conformal cooling. In this investigation, H13, a widely used tool steel, was manufactured using a laser powder bed fusion method. Microstructure, tensile mechanical properties, hardness, and porosity of the AM H13 after stress relieve (SR), standard hardening and tempering (SR + HT), and hot isostatic pressing (SR + HIP + HT) were investigated. It was found that the microstructure of directly solidified colonies of prior austenite, which is typical for AM, disappeared after austenitizing at the hardening heat treatment. In specimens SR + HT and SR + HIP + HT, a microstructure similar to the conventional but finer was observed. Electron microscopy showed that SR and SR + HT specimens contained lack of fusion, and spherical gas porosity, which resulted in remarkable scatter in the observed elongation to break values. Application of HIP resulted in the highest strength values, higher than those observed for conventional H13 heat treated in the same way. The conclusion is that HIP promotes reduction of porosity and lack of fusion defects and can be efficiently used to improve the mechanical properties of AM H13 tool steel.

Microstructure, Solidification Texture, and Thermal Stability of 316 L Stainless Steel Manufactured by Laser Powder Bed Fusion

This article overviews the scientific results of the microstructural features observed in 316 L stainless steel manufactured by the laser powder bed fusion (LPBF) method obtained by the authors, and discusses the results with respect to the recently published literature. Microscopic features of the LPBF microstructure, i.e., epitaxial nucleation, cellular structure, microsegregation, porosity, competitive colony growth, and solidification texture, were experimentally studied by scanning and transmission electron microscopy, diffraction methods, and atom probe tomography. The influence of laser power and laser scanning speed on the microstructure was discussed in the perspective of governing the microstructure by controlling the process parameters. It was shown that the three-dimensional (3D) zig-zag solidification texture observed in the LPBF 316 L was related to the laser scanning strategy. The thermal stability of the microstructure was investigated under isothermal annealing conditions. It was shown that the cells formed at solidification started to disappear at about 800 °C, and that this process leads to a substantial decrease in hardness. Colony boundaries, nevertheless, were quite stable, and no significant grain growth was observed after heat treatment at 1050 °C. The observed experimental results are discussed with respect to the fundamental knowledge of the solidification processes, and compared with the existing literature data.