Powder Bed System Research Articles

On the Effect of Volumetric Energy Density on the Characteristics of 3D-Printed Metals and Alloys

Selective Laser Melting (SLM) is a specific 3D printing technique under Additive Manufacturing (AM) metal technologies. SLM is considered to be a precise rapid AM process combined with a powder bed system for producing customized metal products with a tailored microstructure and shape. Differences in the printing parameters can lead to differences in the surface as well as macroscopic mechanical characteristics of the manufactured parts and components. This work aims at quantifying the effect of the Volumetric Energy Density (VED) used in the SLM processing of various metals and alloys. Metallic specimens printed with different VED values were subjected to surface characterization as well as tensile deformation. Their surface roughness, yield stress and toughness were subsequently used to verify a linear relationship between roughness and VED, and a linear behavior between yield stress/toughness and VED was proposed. Predictive models were formulated for estimating the roughness/yield stress/toughness of the produced specimens with respect to the VED used in their production. The models’ predictions will provide insight into the 3D printing parameters, thus minimizing the cost and effort of the 3D printing procedure, in applications where surface quality and strength are important.

Assessing the feasibility of using an irregularly shaped Ta and spherical Ti bimodal powder blend on the powder spreadability process in powder bed fusion systems

The cost of additive manufacturing can be reduced if economical non-spherical powders are used. However, current flowability tests are inadequate to determine whether these powders can flow and spread well in powder bed systems. This paper presents a novel tool called the Universal Powder Bed designed to analyse the layer-spreading process of an electron beam melting system. The spreadability and feasibility of using a novel bimodal powder comprising of irregularly shaped tantalum and spherical titanium is investigated against three measures: area fraction of build surface covered with powder, surface roughness and composition of the powder bed. The characterisation techniques revealed that increasing layer thicknesses and slower rake speeds produced more efficiently packed powder layers and decreased the effects of segregation between opposite sides of the powder bed. This study shows that despite the segregation observed with bimodal powders, acceptable levels of spreadability can be produced by refining the spreading parameters.

A novel additive manufacturing compression overmolding process for hybrid metal polymer composite structures

Metal polymer composites combining low density, high strength composites with highly ductile and tough metals have gained traction over the last few decades as lightweight and high-performance materials for industrial applications. However, the mechanical properties are limited by the interfacial bonding strength between metals and polymers achieved through adhesives, welding, and surface treatment processes. In this paper, a novel manufacturing process combining additive manufacturing and compression molding to obtain hybrid metal polymer composites with enhanced mechanical properties is presented. Additive manufacturing enabled deposition of polymeric material with fibers in a predetermined pattern to form tailored charge or preform for compression molding. A grade 300 maraging steel triangular lattice is first fabricated using AddUp FormUp350 laser powder bed system and compression overmolded with additively manufactured long carbon fiber-reinforced polyamide-6,6 (40% wt. CF/PA66) preform. The fabricated hybrid metal polymer composites showed high stiffness and tensile strength. The stiffness and failure characteristics determined from the uniaxial tensile tests were correlated to a finite element model within 20% deviation. Fractographic analyses was performed using microscopy to investigate failure mechanisms of the hybrid structures.

The influence of a large build area on the microstructure and mechanical properties of PBF-LB Ti-6Al-4 V alloy

This study investigated the print homogeneity of Ti-6Al-4 V alloy parts, when printed over a large build area of 250 times 250 times 170 mm3, using a production scale laser powder bed additive manufacturing system. The effect of part location across this large build area was investigated based on printed part porosity, microstructure, hardness, and tensile properties. In addition, a Hot Isostatic Pressing (HIP) treatment was carried out on the as-built parts, to evaluate its impact on the material properties. A small increase in part porosity from 0.01 to 0.09%, was observed with increasing distance from the argon gas flow inlet, which was located on one side of the build plate, during printing. This effect, which was found to be independent of height from the build plate, is likely to be associated with enhanced levels of condensate or spatter residue, being deposited at distances, further from the gas flow. Despite small differences in porosity, no significant differences were obtained for microstructural features such as prior β grain, alpha lath thickness, and phase fraction, over the entire build area. Due to this, mechanical performances such as hardness and tensile strengths were also found to be homogenous across the build area. Additionally, it was also observed based on the lattice constants that partial in-situ decomposition of {alpha }^{^{prime}}to alpha +beta phases occurred during printing. Post HIP treatment result showed a decrease of 7 and 6%, in the yield strength (YS) and ultimate tensile strength (UTS), respectively, which was associated with a coarsening of alpha lath widths. The potential of the laser powder bed system for large area printing was successfully demonstrated based on the homogenous microstructure and mechanical properties of the Ti-6Al-4 V alloy parts.

Study on powder particle behavior in powder spreading with discrete element method and its critical implications for binder jetting additive manufacturing processes

ABSTRACT Understanding powder bed system behaviour in powder spreading is a fundamental issue in binder jetting additive manufacturing (BJAM). This work established a discrete element model incorporating a parallel bond model to compatibly depict local cross-links between powder particles. BJAM parameters including layer thickness, gap compensation, recoat speed, rotation speed, and layer number were studied quantitatively for their effects on recoated powder's packing density and microscopic pore size and bonded layer's breakage and layer shift. Evolutions and influence mechanisms on both layer shift and bond breakage were further elucidated. Some practical implications include: gap compensation corresponding to an ideal recoated powder structure is ∼75 μm; rotation speed should be controlled at 40–120 rad/s to avoid low-rotation-speed layer shift surge and high-rotation-speed breakage; layer shift occurring at a certain stage is irreversible and must deserve well-maintained. This research can provide theoretical guidance for developing BJAM and even support-free powder bed – based additive manufacturing.

The joining of CP-vanadium and Ti–6Al–4V using the Electron Beam Melting Additive Manufacturing method

The use of electron beam welding for dissimilar welding (DW) of commercially pure (CP) vanadium to Ti–6Al–4V has been investigated via ARCAM S12, an additive manufacturing powder-bed system. Investigations of bead-on-plate welds for Ti–6Al–4V were first conducted to identify the process parameters for full penetration welds with a minimum energy input of 37 mA at a traverse speed of 7 mm/s. Vanadium bead on plate welds produced a penetration of approximately 75%, which was enough to proceed onto DW experiments. Defect-free full penetration welds were produced. The DW weld zone microstructure revealed an elongated dendritic structure comprised of bulky βTi grains and a fine substructure of α' laths. Thermal imaging (TI) showed an increment in radiance temperature ahead of the melt pool, indicating that there is a minimum energy required before keyhole welding is present, confirming mathematical calculations. Mechanical characterisation finds a fair range of hardness across both base metals (BM), heat affected zones (HAZ) and fusion zones (FZ). With no yield plateau in tensile test curves, the material is confirmed to fail on the side with lower mechanical properties, i.e., vanadium, which draws a fair process window for dissimilar welding between Ti6Al4V and vanadium alloys.

Volumetric heat source model for laser-based powder bed fusion process in additive manufacturing

Laser-based powder bed fusion (L-PBF) is one of the most widely used additive manufacturing methods. Its most striking feature is the melt pool in the powder bed, which is formed by a laser heat source. Many physical phenomena occur as a result of the interaction of the heat source with metal powders. Spattering, melting and sintering to each other of the metal powders (phase and shape change), liquid metal movement induced by buoyancy force and surface tension gradient, high evaporation and resulting recoil pressure, fluctuations and deteriorations on the liquid metal surface, plume type structures can be given as examples. The most important point in terms of L-PBF and similar methods is to predict the melt pool dimensions and temperature distribution in the powder bed. However, the phenomena mentioned above complicate the process considerably and require high calculation costs. Volumetric heat sources are a frequently used prescription in terms of simplifying the modeling of the process and reducing the calculation cost. However, such heat source models may contain many non-standard parameters and do not always produce reasonable results. In this study, a dynamic heat source model is presented for use in powder bed systems. Some discussions related to the problems such as uncertainties in absorptivity ratios used in L-PBF are also presented.

Two-Scale Optimization and Generation of Anisotropic Cellular Designs in the Context of Additive Manufacturing

Within the context of gradient-based optimization of cellular structures, we aim to optimize designs in a two-scale setting and generate respective geometric descriptions that are additively manufacturable in powder-bed systems. We analyze representative unit cells for their suitability and present a workflow to generate cellular designs by means of a parameterized unit cell, for which an implicit surface description is described. After performing homogenization-based optimization, we compare the predicted performance of the homogenized model to a full-resolution finite element (FE) analysis. Throughout the whole process, special attention is paid to the consistent integration of manufacturing limitations in terms of horizontal overhang and feature size.We discuss the performance of cellular components with maximized stiffness. Our numerical experiment for a single-load scenario demonstrates that using isotropic micro-structures is not beneficial, even when graded. We further observe that using anisotropic micro-structures leads to improvements only in very specific cases, e.g. with a uniformly distributed load. However, the conclusion may be different for anisotropic cells with non-uniform parameterization and other loading scenarios. Instead of conducting further tedious investigations for stiffness maximization, we finally demonstrate the benefit of cellular designs for a thermo-mechanical example.

Characterization of Tantalum–Titanium Powders with Universal Powder Bed (UPB) System for Electron Beam Melting Process

Electron beam melting (EBM) is a powder bed fusion additive manufacturing (AM) technique that produces three-dimensional (3D) parts by fusing metallic powders with a high-energy electron beam. Powder has been major issue for the properties of the final parts. Hence, establishing the effect of powder properties on the final product properties is important for producing parts of quality. In this study, Ta–60Ti, Ta–50Ti, Ta–40Ti was a blend of tantalum and titanium powders. The blend of powder was characterized in terms of the particle size distribution (PSD), bulk and tap density, flowability in Hall Funnel flowmeter and particle morphology. In addition, a universal powder bed (UPB) system was used to characterize the powder behavior raking because the UPB system allows a quick assessment of the powder spreading behavior in AM processes. Surface roughness’s of the powder after raking with universal powder bed (UPB) system, the microstructural heterogeneity, defect distributions were investigated.

LENS manufactured γ-TNB turbine blade using Laser “in situ” alloying approach

Abstract:A hollow γ-TNB turbine blade was 3D printed in this studying using the –R Optomec LENS machine from the elemental powders of aluminium, niobium and titanium making use of the laser “in situ” alloying approaching. The printed blade was characterised of a nearly lamellar β microstructure in the As-built state. The microstructure of the blade post heat treated was characterised of grain growth and coarsening and the formation of the γ phase which was of the result of the transformation of β. This transformation was also observed in the As-built state and is reported here for the first time. A massive crack that was observed half-way through in the built was attributed to the thermal shocks that are experienced by the almost immediately after manufacturing. The EDS and Map taken on the As-built and heat treated samples conclude that there was no segregation in the alloying element during manufacturing and that the blade was of the dual phase. Hardness results indicated the heat treated sample was 91 HV0.5 lower in hardness when compared to the As-built component. The successful print of this hollow blade indicate that γ-TNB and other Ti-Al alloys can be printed with the LENS but if a crack free sample was to be achieved the set-up had to be manipulated or addition resources must be added to adapt the set-up. Meanwhile the successes of this study show that LENS is going to be considered as a cost-effective manufacturing tool in the future for 3D printing Ti-Al and other metallic structure that would have improved properties when compared to traditional manufacturing technique such as casting and the powder bed systems.

Prediction of initial transient behavior with stationary heating during laser powder bed fusion processes

Laser powder bed fusion (L-PBF) has become a popular additive manufacturing process. To minimize the need to perform experimental trial-and-error based searching, physical models can be used to determine process parameters to achieve desirable process conditions. In this paper, a physics-based predictive model is presented to describe the underlying physics of L-PBF during the initial highly transient process of molten pool and keyhole formation during spot heating, i.e., zero-velocity, finite-duration laser energy addition to a Ti-6Al-4V/Ti-6Al-4V powder bed system. The simulation presented herein uses a computationally-efficient volumetric energy addition method to describe energy transfer to the particles during melting and ray-tracing to account for attenuated beam energy reaching the substrate through the powder bed. Validation of the model was performed against imagery captured from synchrotron measurements of a static L-PBF process which showed a keyhole welding mode for each of two power settings. The model presented herein was able to describe the laser energy absorption, molten bead formation, keyhole welding mode, and showed excellent correlation of keyhole and molten bead dimensions for each of the cases presented.

Laser Operating Windows Prediction in Selective Laser-Melting Processing of Metallic Powders: Development and Validation of a Computational Fluid Dynamics-Based Model.

The rapidly ascending trend of additive manufacturing techniques requires a tailoring of existing solidification models and the development of new numerical tools. User-friendly numerical models can be a valid aid in order to optimize operating parameter ranges with the scope to extend the modelling tools to already existing or innovative alloys. In this paper a modelling approach is described simulating the generation of single tracks on a powder bed system in a selective laser melting process. The approach we report attains track geometry as a function of: alloy thermo-physical properties, laser speed and power, powder bed thickness. Aim of the research is to generate a numerical tool able to predict laser power and speed ranges in manufacturing porosity-free printed parts without lack of fusion and keyhole pores. The approach is based on a simplified description of the physical aspects. Main simplifications concern: the laser energy input, the formation of the pool cavity, and the powder bed thermo-physical properties. The model has been adjusted based on literature data providing the track’s geometry (width and depth) and relative density. Such data refer to different alloys. In particular, Ti6Al4V, Inconel625, Al7050, 316L and pure copper are considered. We show that the printing process presents features common to all alloys. This allows the model to predict the printing behavior of an alloy from its physical properties, avoiding the need to perform specific experimental activities.

Development of an innovative, high speed, large-scaled, and affordable metal additive manufacturing process

This paper introduces a new additive manufacturing (AM) process which significantly improves the productivity of the current metal AM technologies by combining binder jet printing and stereolithography principles. Three dimensional objects can be printed on a powder bed system where photopolymerization takes place by selectively curing the suspensions containing metallic powder and ultraviolet curable resin in a layer-by-layer fashion. Integration of a digital light projection module allows a high-speed production with dimensional accuracy within 100 µm. Printed parts are sintered at appropriate temperatures to attain metal parts with the final density above 97% and homogenous microstructure without residual stress.

An Initial Investigation of Tungsten Inert Gas (TIG) Torch as Heat Source for Additive Manufacturing (AM) Process

Additive Manufacturing (AM) processes are gaining more steam in the last years, due to a series of advantages, such as capability of producing complex parts never possible before, low stock to be removed, complex parts in exotic materials, etc. However, energy efficiency, high costs and longer fabrication time, could be some of the drawbacks in the future. Welding processes can be used to produce pieces by AM techniques and their application has been intensified lately. Commonly, AM processes use MIG/MG and TIG and primarily with wire as stock material. Welding offers the advantage of producing large parts in lower time and material consistency is always good, since those processes have been greatly improved. In contrast, welding workpieces, generally, result with rough surfaces and poor geometric quality, which makes them not readily acceptable for most of the common machinery applications. The present work investigates the use TIG process with metal powder as stock material to produce workpieces, which are to be post processed by machining. The AM process is based on the Powder Bed Fusion (PBF) system. A simple one-axis moving system was produced to test the whole concept. Powder was preferred since it is readily available in variety of metallic alloys. Two powder grain sizes (Gs), three travel speed values (f) and three current levels (C) were tested producing weld beads of AISI H13 on a substrate of AISI 1020 plate. Beads were assessed by measuring external dimensions and looking at their microstructures. Additional tests were performed building straight wall stacking 10 layers with the best welding parameters found with the first trials. Results indicate that materials with good internal quality could be produced when making single beads and also building straight walls. These first trials show that the proposed process can be used as a promising hybrid process, using AM (PBF-TIG) and conventional machining at the same equipment. Future work will concentrate on adapting the welding and powder bed system in a machining center to further study the process.

Three-dimensional semi-elliptical modeling of melt pool geometry considering hatch spacing and time spacing in metal additive manufacturing

A semi-elliptical moving heat source approach is used to predict the in-process temperature profile inside the build part during laser-based metal additive manufacturing (AM) processes. The laser has a constant heat strength which releases its energy continuously on the semi-infinite medium. It is assumed that the medium is initially at room temperature. In the proposed analytical model, some details are considered to predict the melt pool geometry more accurately and realistically. The thermal material properties are considered to be temperature dependent since the existence of the steep temperature gradient affects the magnitude of the thermal conductivity and specific heat, and as a result, it changes the heat transfer mechanisms. Moreover, the melting/solidification phase change is considered using the modified heat capacity. The multi-layer aspect of the metal AM part is considered in the modeling of the temperature profile, since the thermal interaction of the successive layers has an influence on heat transfer mechanisms. The prediction of the temperature profile of an AM part is the building block for the prediction of the thermal stress, residual stress, part distortion, and microstructure evolution. Adding more details of the AM processes to the analytical models will help to increase the accuracy of the results. In this paper, the effect of time spacing (time delay between two irradiations) and hatch spacing on thermal material properties and melt pool geometry are studied. The effect of the number of scans on melt pool geometry is also investigated. The proposed model can be used to predict the temperature profile and melt pool geometry in laser-based metal additive manufacturing configurations of either direct metal deposition (DMD) or selective laser melting (SLM). In order to validate that the proposed model can capture the physical aspects of both powder bed systems, such as SLM, and powder feed systems, such as DMD, two sets of parts are chosen which are built using SLM and DMD processes and the predicted melt pool size is compared to experimental values.

Thermal and molten pool model in selective laser melting process of Inconel 625

Nowadays, additive manufacturing via topology optimization creates new opportunities for weight reductions in aerospace industry where high fly-to-buy ratio is desired. Selective laser sintering of advanced engineering materials like nickel super alloys are also expanding to reduce the cost and time of the manufacturing in aerospace industry. Elevated temperature and temperature gradients are critical factors in selective laser sintering of metals and they significantly affect the quality and integrity factors of produced parts such as microstructures, porosity, residual stresses, and distortions. Therefore, the aerospace industry needs advanced simulation tools to predict the temperatures, temperature gradients, and molten pool geometries to better understand the physics of the selective laser melting process as well as for the process optimizations. This article introduces an adjustable finite element-based multi-physics and multi-software platform thermal model, for laser additive manufacturing in powder bed systems to predict the transient temperature and the molten pool geometry. The developed model is able to simulate 3D transient temperature and molten pool shape in the laser additive manufacturing process by including the features of melting and solidification, porous media, and temperature-dependent thermal material properties for different materials. A set of experiments of Inconel 625 is carried out in order to measure the size of the molten pool and to validate the developed thermal model. An experimental study on temperature distribution carried out with titanium and an experimental study on molten pool sizes carried out with Inconel 625 in the literature are also compared with the developed thermal model. The estimation errors of the developed model are in the range of 11–18%.

Change in microstructure of selectively laser melted AlSi10Mg alloy with heat treatments

In the present study, we examined changes in the microstructure and mechanical properties of AlSi10Mg alloy, initially fabricated using selective laser melting (SLM) combined with a powder-bed system, by applying heat treatments at temperatures of either 300 or 530°C. The as-fabricated samples exhibited a characteristic microstructural morphology and {001} texture. Melt pools corresponding to the locally melted and rapidly solidified regions were found to be composed of several columnar α-Al grains surrounded by fine eutectic Si particles. A fine dislocation substructure consisting of low-angle boundaries is present within the columnar α-Al grains. At elevated temperatures, fine Si phase precipitates within the columnar α-Al phase and coarsening of the eutectic Si particles occurs. These fine Si particles inhibit grain growth in the α-Al matrix, resulting in the microstructural morphology and [001] texture observed in the heat-treated samples. The dislocation substructure disappears in the columnar α-Al grains. Furthermore, the formation of a stable intermetallic phase occurs, reaching microstructural equilibrium after long-term exposure. The as-fabricated specimen exhibits a high tensile strength of approximately 480MPa. The strength is independent of the tensile direction, that is, normal and parallel to the building direction. In contrast, the tensile ductility is found to be direction-dependent, and is therefore responsible for a fracture preferentially occurring at a melt pool boundary. The direction-dependence of the tensile ductility was not found in the specimen that had been heat-treated at 530°C. The present results provide new insights into the control of the direction-dependence of the tensile properties of AlSi10Mg alloys fabricated by SLM.

Comparison of porcelain bond strength of different metal frameworks prepared by using conventional and recently introduced fabrication methods

Statement of problemMost studies evaluating dental laser sintering systems have focused on the marginal accuracy of the restorations. However, the bond strength at the metal-ceramic interface is another important factor that affects the survival of restorations, and currently, few studies focus on this aspect. PurposeThe purpose of this in vitro study was to compare the porcelain bond strength of cobalt-chromium (Co-Cr) metal frameworks prepared by using the conventional lost-wax technique, milling, direct metal laser sintering (DMLS), and laser cusing, a direct process powder-bed system. Material and methodsA total of 96 metal frameworks (n=24 in each group) were prepared by using conventional lost-wax (group C), milling (group M), DMLS (group LS), and direct process powder-bed (group LC) methods according to International Organization for Standardization standard ISO 9693-1. After porcelain application, a 3-point bend test was applied to each specimen by using a universal testing machine. Data were statistically analyzed using 1-way ANOVA and Tukey honest significant difference tests (α=.05). Failure types at the metal-ceramic interfaces were examined using stereomicroscopy. Additionally, 1 specimen from each group was prepared for scanning electron microscopy analysis to evaluate the surface topography of metal frameworks. ResultsThe mean bond strength was 38.08 ±3.82 MPa for group C, 39.29 ±3.51 MPa for group M, 40.73 ±3.58 MPa for group LS, and 41.24 ±3.75 MPa for group LC. Statistically significant differences were observed among the 4 groups (P=.016). All groups, except for LS, exhibited adhesive and mixed type bond failure. ConclusionsBoth of the laser sintering methods were found to be successful in terms of metal-ceramic bond strength.

Effect of process parameters on the surface characteristics of AlSi12 samples made via Selective Laser Melting

In the past decade there has been a rapid growth in additive manufacturing and its applications in a wide range of industries. Selective laser melting (SLM) is one such technology, which uses a powder-bed system to selectively sinter/melt the powdered material resulting in a dense structure. However, an optimized set of process parameters have to be established for each material system to maximizethe performance of the components built in the SLM machine. In this research work, several experimental trials were carried out to investigate the influence of the laser power and defocus distance of the laser beam on the surface quality of the parts fabricated using AlSi12 alloy powders. The as-built samples were characterised using 3D profilometry, scanning electron microscopy, and microhardness testing. From the results obtained, it was concluded that a defocus distance of -3 mm at a laser power of 95% were the best combination of laser parameters to achieve good surface finish on the as-built AlSi12 parts on SLM.

Influence of porcelain firing and cementation on the marginal adaptation of metal-ceramic restorations prepared by different methods

Statement of problemMarginal adaptation plays an important role in the survival of metal-ceramic restorations. Porcelain firings and cementation may affect the adaptation of restorations. Moreover, conventional casting procedures and casting imperfections may cause deteriorations in the marginal adaptation of metal-ceramic restorations. PurposeThe purpose of this in vitro study was to compare the marginal adaptation after fabrication of the framework, porcelain application, and cementation of metal-ceramic restorations prepared by using the conventional lost-wax technique, milling, direct metal laser sintering (DMLS), and LaserCUSING, a direct process powder-bed system. Alterations in the marginal adaptation of the metal frameworks during the fabrication stages and the precision of fabrication methods were evaluated. Material and methodsForty-eight metal dies simulating prepared premolar and molar abutment teeth were fabricated to investigate marginal adaptation. They were divided into 4 groups (n=12) according to the fabrication method used (group C serving as the control group: lost-wax method; group M: milling method; group LS: DMLS method; group DP: direct process powder-bed method). Sixty marginal discrepancy measurements were recorded separately on each abutment tooth after fabrication of the framework, porcelain application, and cementation by using a stereomicroscope. Thereafter, each group was divided into 3 subgroups according to the measurements recorded in each fabrication stage: subgroup F (framework), subgroup P (porcelain application), and subgroup C (cementation). Data were statistically analyzed with univariate analysis of variance (followed by 1-way ANOVA and Tamhane T2 test (α=.05). ResultsThe lowest marginal discrepancy values were observed in restorations prepared by using the direct process powder-bed method, and this was significantly different (P<.001) from the other methods. The highest marginal discrepancy values were recorded after the cementation procedure in all groups. ConclusionsThe results showed that the direct process powder-bed method is quite successful in terms of marginal adaptation. The marginal discrepancy increased after porcelain application and cementation.