Bed Fusion Additive Manufacturing Research Articles

The commissioning of a hybrid multi-material 3D printer

Additive Manufacturing (AM) has rapidly become an important technology in both research and industry. This development has allowed the evolution of 3D printers which are able to print complex geometries at low costs and faster than traditional methods. Despite this, most of these printers are either only for using one material or one technology. This limits a lot its use in different sectors such as aeronautics, automotive or health, because multi-material prototypes are needed. For example, surgeons need surgical planning prototypes for preoperative planning. These 3D printed prototypes have mainly been manufactured using just one technology. As a result, the prototypes have some main limitations: (1) do not actually mimic the anatomical structures of the human body, (2) high costs specially for Material Jetting and Powder Bed Fusion AM technologies. Therefore, the aim of present manuscript is the design, development, and commissioning of a hybrid multi-material 3D printer.

Method for Rapid Modeling of Distortion in Laser Powder Bed Fusion Metal Additive Manufacturing Parts

The simulation and modeling of part-level distortion and residual stress in diverse metal additive manufacturing (AM) geometries has great potential to enable the rapid adoption of this technology in engineering design. Moreover, the use of additive manufacturing component libraries (CLs) offer a computationally efficient means of quantifying these part-level defects resultant from AM processing. We report on how the individual simulations of simple shapes, potential entries in a CL, can be superimposed to provide an indication of distortion and residual stresses in complex geometries. Laser powder bed fusion AM was used to construct test geometries of varied shapes and their combinations in the form of CLs in an effort to characterize location-dependent and feature-dependent distortion distributions. Blue light scanning was used to experimentally measure 3D distortions in order to investigate the interaction between the component shapes and local boundary conditions. Overall, part-level distortions were highly dependent on test component geometry, local boundary conditions, and shape combination. Commercial finite element software was used to verify experimental trends and to make predictions of distortion. The use of CLs resulted in over 20 times savings in computational cost while reproducing overall trends in distortion for test geometry assemblies. Therefore, it is anticipated that the use of CLs for L-PBF AM geometries has demonstrated potential to facilitate efficient simulations of full component AM assemblies, thereby reducing the need for costly trial-and-error-type experimental analysis.

Measuring the Residual Stress and Stress Corrosion Cracking Susceptibility of Additively Manufactured 316L by ASTM G36-94

Residual stress is a contributor to stress corrosion cracking (SCC) and a common byproduct of additive manufacturing (AM). Here the relationship between residual stress and SCC susceptibility in laser powder bed fusion AM 316L stainless steel was studied through immersion in saturated boiling magnesium chloride per ASTM G36-94. The residual stress was varied by changing the sample height for the as-built condition and additionally by heat treatments at 600°C, 800°C, and 1,200°C to control, and in some cases reduce, residual stress. In general, all samples in the as-built condition showed susceptibility to SCC with the thinner, lower residual stress samples showing shallower cracks and crack propagation occurring perpendicular to melt tracks due to local residual stress fields. The heat-treated samples showed a reduction in residual stress for the 800°C and 1,200°C samples. Both were free of cracks after >300 h of immersion in MgCl2, while the 600°C sample showed similar cracking to their as-built counterpart. Geometrically necessary dislocation (GND) density analysis indicates that the dislocation density may play a major role in the SCC susceptibility.

Phase and structural changes during heat treatment of additive manufactured CrFeCoNi high-entropy alloy

Additive manufacturing (AM) of high-entropy alloys (HEAs) is a new challenge in the Material Science and Advanced Manufacturing fields. In the AM processing procedure, heat treatments after fabrication are often beneficial to stabilize microstructure and properties, while limited reports are available for AM HEAs. In the current study, the effect of a post-printing heat treatment at 400–1000 ℃ for 24 h and for 21 days on the changes in structures and phase compositions of an AM CrFeCoNi alloy prepared by the laser powder bed fusion AM technique is presented to better understand a heat treatment-microstructure-property relationship of the AM HEA. Heating up to 600 ℃ demonstrated the polygonization process in the alloy. Grain growth was observed in the alloy upon heating over 700 ℃, while a preferred texture is observed along the build direction after annealing at 900 ℃ for 24 h. The formation of the secondary phase was revealed, and it is associated with the impurities of the initial CrFeCoNi powder. The AM CrFeCoNi system demonstrates excellent phase stability inthe solid solution for all annealing temperatures.

Temperature Distribution Design Based on Variable Lattice Density Optimization and Metal Additive Manufacturing

Additive manufacturing (AM) is employed for fabricating industrial products with complex geometries. As topological optimization is suitable for designing complex geometries, studies have combined AM and topological optimization, evaluating the density optimization of lattice structures as a variant of topological optimization. The lattice structures of components fabricated via AM comprise voids. Models designed using topological optimization should be modified to ensure structures suitable for AM. As the lattice unit can be easily fabricated using AM with fewer design modifications, this study uses lattice density optimization for an industrial AM product. We propose a method of optimizing the lattice distribution for controlling the surface temperature uniformity of industrial products, such as molds. The effective thermal conductivity of the lattice is calculated using the homogenization and finite element methods. The effective thermal conductivity changes depending on the internal pore sizes. The proposed methodology is validated using a 3D example; the minimization problem of surface temperature variations in the target domain is considered. The variable density of the embedded lattice in the target domain is optimized, and we experimentally validated the performance of the lattice unit cell and optimal 3D structure using metal powder bed fusion AM.

Prediction of Extreme Value Areal Parameters in Laser Powder Bed Fusion of Nickel Superalloy 625

Important to the success of additive manufacturing (AM) is the ability to inspect and qualify parts. The research community is pushing to identify correlations between part function and surface topography, yet little guidance specific to AM surface measurement exists. Thus, development of inspection methods for surface finish are Required. In laser powder bed fusion (LPBF) AM, parts are built through a complex process with many variables, and the length scales of interest on the surface cover a wide range. Full characterization of the surface is time consuming and costly as high resolution in surface measurements decreases field-of-view (FoV), requiring stitching multiple FoVs to cover large areas. Statistical methods exist to estimate the maximum value based on a sample of FoVs, but are not yet commonplace in AM surface measurement. The goal of this work is to understand the use of these statistical methods in the estimation of maximum area valley depth (Sv) of a surface, an extreme value parameter, for which researchers have already found relationship to fatigue life. This work also investigates the effect of microscope objective, measurement region size, and nesting index of areal filters on Sv. A large (i.e., greater than 40 mm × 40 mm) planar LPBF surface is fabricated in nickel superalloy 625 and measured using a focus variation microscope with a 10x objective and again with a 20x objective. Results show that there is little difference in the maximum value of Sv between the two objectives, but the nesting index does have some effect. Results also show that a Type 1 Generalized Extreme Value, or Gumbel, distribution can be used to accurately estimate the maximum value of Sv for a surface from a small set of measurements, providing a framework for users to develop inspection routines that balance measurement time and accuracy of estimation.

Modeling the effects of coordinated multi-beam additive manufacturing

In additive manufacturing (AM), it is necessary to know the influence of processing parameters in order to have better control over the microstructure and mechanical performance of the part. Laser powder bed fusion (LPBF) AM is beneficial for many reasons; however, it is limited by the thermal solidification conditions achievable in the available processing parameter ranges for single-beam processing methods. Therefore, this work investigates the effect of multiple, coordinated heat sources, which are used to strategically modify the melting and solidifying in the AM process. To model this, existing thermal models of the LPBF process have been modified to include the effects of multiple, coordinated laser beams. These computational models are used to calculate melt pool dimensions and thermal conditions (thermal gradient and cooling rate). Furthermore, the results of the simulations are used to determine the influence of the distance between the coordinated laser beams in different configurations. The multi-beam scanning strategies modeled in the present study are shown to alter both melt pool shape and size, and the thermal conditions at the onset of solidification. However, these variations are not shown to result in alterations in the grain morphology of Ti-6Al-4V components. This predictive method used in this research provides insight into the effects of using multiple coordinated beams in LPBF, which is a necessary step in increasing the capabilities of the AM process.

Elastic and failure characteristics of additive manufactured thin wall lattice structures with defects

The design and realization of lightweight thin wall lattice structures assisted by additive manufacturing (AM) has the potential for a broad range of engineering applications. Due to the more pronounced material and feature defects that commonly occur with AM lattice structures, the application of these structures are currently hindered by the lack of adequate understanding of the effects of material defects on various structural performance, including the fracture characteristics. In this work, an analytical modeling-based analysis was carried out with two types of lattice designs, including the re-entrant auxetic and the diamond structures, in order to understand the effect of material defects on the elastic and fracture characteristics of these structures. The material defect characteristics were represented by introducing material property variability (strength and elastic modulus) functions using stochastic normal distribution functions, which were experimentally established in this study. The models provided good predictions of various fracture characteristics of the lattice structures fabricated by laser powder bed fusion AM (L-PBF-AM) process. Furthermore, the models were employed to investigate the effect of varying material property variabilities on the fracture characteristics of the two types of lattice structures. The results showed that the existence of material property variability not only result in reduced initial fracture strength, but also lead to some unexpected characteristics such as the increase of retaining strength and the divergence of fracture patterns. In addition, it was found that the auxetic and diamond structures exhibit significantly different sensitivities to material property variabilities, which translates into various implications for their use in different potential applications.

Electron Beam Melting: From Shape Freedom to Material Properties Control at Macro- and Microscale

Electron beam melting (EBM) is one of the constantly developing powder bed fusion (PBF) additive manufacturing technologies (AM) offering advanced control over the manufacturing process. Development of the additive manufacturing today is targeting both widening of the available materials classes, and introducing new manufacturing modalities. Present research is related to the new possibilities in tailoring different properties within additively manufactured components effectively adding “fourth dimension to the 3D-printing”. Through manipulating beam energy deposition (scanning strategy) it is possible to tailor quite different material properties selectively within each manufactured component, including crystalline and amorphous state, effective material density, as well as mechanical, thermal, electrical and acoustic properties. With the blends of precursor powder, it is also possible to acquire by choice both metal-metal composite and completely alloyed material. Specific examples are given in relation to the EBM, but majority of the conclusions are valid for the other PBF techniques as well.

Layer-wise engineering of grain orientation (LEGO) in laser powder bed fusion of stainless steel 316L

Additive manufacturing (AM) has opened new possibilities for site-specific microstructure control of metal alloys. In laser powder bed fusion (LPBF) AM, the microstructure can be locally tailored by manipulating the solidification conditions point to point. In this work, we demonstrate this capability by varying the laser scanning angle during LPBF to produce stainless steel samples with controlled crystallographic textures. The resulting microstructures range from strongly textured blocks of arbitrary thickness and shape to gradient crystallographic textures. This strategy paves the way to a new generation of additively manufactured metals with optimized, site-specific properties to suit a wide range of applications.

Mechanical Behavior of Electron Beam Powder Bed Fusion Additively Manufactured Ti6Al4V Parts at Elevated Temperatures

Abstract Ti6Al4V is one of the vital metal alloys used in various industries including aerospace, especially at high-temperature applications, because of having high strength-to-weight ratio, and high melting temperature. Manufacturing these metal parts by the conventional subtractive methods have been challenging due to the difficulty involved with the cutting and machining it. However, additive manufacturing (AM) offers a convenient way for shaping this metal into the desired complex parts. Although different powder bed fusion (PBF) AM processes are time and cost effective, degradation of mechanical properties during high-temperature applications could be a concern for parts produced by them. Therefore, this study focuses on the anisotropic and high-temperature elastic and plastic behaviors of Ti6Al4V parts made using electron beam powder bed fusion (EB-PBF) process. Mechanical properties, like modulus of elasticity, 0.2% yield strength, ultimate tensile strength (UTS), and percent elongation, have been determined at 200 °C, 400 °C, and 600 °C temperatures from the samples produced in different build orientations. Considerable anisotropic behavior and temperature dependency were observed for all the analyzed properties. At 600 °C, various softening mechanisms such dislocation glide, grain boundary slip, and grain growth were anticipated to be activated reducing the flow stress and increasing the elasticity. Fractography analysis on fractured surfaces of the samples reveals various defects, including partially melted or unmelted powder particles near the surface and subsurface areas. Those internal and external defects are analyzed further using X-ray computed tomography (CT) and surface profilometer to show their effect on the anisotropic behaviors.

Additively manufactured novel Al-Cu-Sc-Zr alloy: Microstructure and mechanical properties

An in-depth understanding of microstructure and resultant properties is paramount in the design of a novel alloy system, especially for additive manufacturing (AM). The present investigation aims to characterize a prototypical AM Al alloy with great potential for structural applications. An Al-1.5Cu-0.8Sc-0.4Zr alloy designed using integrated computational material engineering was printed using the laser powder bed fusion AM process. This novel alloy shows promising combination of strength and ductility in as-built and peak-aged conditions. This improvement in the tensile properties is attributed to the presence of both coherent L12 Al3Sc/Al3(Sc,Zr) precipitates and Cu-rich regions. The microstructures were studied via extensive microscopy at different length scales using X-ray microscopy, scanning electron microscopy, and transmission electron microscopy. Fractography revealed that the columnar grain boundaries in as-built condition allow easy slip transfer as compared to the equiaxed grains, with the apex of the melt pool acting as the crack nucleation site. However, the peak aged condition resulted in improved strength while marginally sacrificing ductility due to precipitates decorating dislocations, grain boundaries and melt pool boundaries thus acting as obstacles to slip transfer.

Towards process consistency and in-situ evaluation of porosity during laser powder bed additive manufacturing

There is a synergy between welding and additive manufacturing with reference to spatial and temporal variations of heat transfer. In this research, in-situ measurements of heat transfer conditions are considered as a viable qualification methodology for additive manufacturing (AM). Infrared imaging (IR) was performed within a laser powder bed fusion (L-PBF) AM machine equipped with an IR camera. Infrared thermal signatures as a function of space and time, while processing Ti6Al4V and 316L stainless steel powders, were extracted and analysed. The analyses correlated the defect evolution at low- and high-heat input conditions to thermal decay and integrated intensities. The IR based results were validated by processing a 316L cylinder with engineered porosities and detecting the same with ground truth data from computed tomography.

An Overview of Additive Manufacturing Technologies-A Review to Technical Synthesis in Numerical Study of Selective Laser Melting.

Additive Manufacturing (AM) processes enable their deployment in broad applications from aerospace to art, design, and architecture. Part quality and performance are the main concerns during AM processes execution that the achievement of adequate characteristics can be guaranteed, considering a wide range of influencing factors, such as process parameters, material, environment, measurement, and operators training. Investigating the effects of not only the influential AM processes variables but also their interactions and coupled impacts are essential to process optimization which requires huge efforts to be made. Therefore, numerical simulation can be an effective tool that facilities the evaluation of the AM processes principles. Selective Laser Melting (SLM) is a widespread Powder Bed Fusion (PBF) AM process that due to its superior advantages, such as capability to print complex and highly customized components, which leads to an increasing attention paid by industries and academia. Temperature distribution and melt pool dynamics have paramount importance to be well simulated and correlated by part quality in terms of surface finish, induced residual stress and microstructure evolution during SLM. Summarizing numerical simulations of SLM in this survey is pointed out as one important research perspective as well as exploring the contribution of adopted approaches and practices. This review survey has been organized to give an overview of AM processes such as extrusion, photopolymerization, material jetting, laminated object manufacturing, and powder bed fusion. And in particular is targeted to discuss the conducted numerical simulation of SLM to illustrate a uniform picture of existing nonproprietary approaches to predict the heat transfer, melt pool behavior, microstructure and residual stresses analysis.

Assessment of grain structure evolution with resonant ultrasound spectroscopy in additively manufactured nickel alloys

Despite the advantages of metal additive manufacturing (AM), ensuring integrity and reproducibility for built components is a barrier to the implementation of AM components in critical applications. Component qualification necessitates Non-Destructive Evaluation (NDE), but existing NDE frameworks are insufficient for the rapid and cost effective screening of variable AM components. In this study, alterations in laser powder bed fusion (LPBF) AM process parameters were characterized using resonant ultrasound spectroscopy (RUS). Samples that were subjected to a Hot Isostatic Press (HIP) and Heat Treatment (HT) post-processing step exhibited changes in resonance frequencies that varied in magnitude and direction with the type of resonance mode. The initial build direction prior to HIP and HT had a negligible effect on resonant frequency changes after recrystallization. The change in resonant frequencies at each process condition was predicted using Finite Element Modeling (FEM) informed with Electron Backscatter Diffraction (EBSD) data. FEM identified that the experimentally measured change in resonant response between the initially textured state and the recrystallized state was dominated by grain orientation dependent changes in elasticity. The EBSD-estimated elastic constants and FEM results were validated using experimental laser vibrometry and RUS inversion of elastic constants. RUS Inversion by Bayesian inference and Hamiltonian Monte Carlo has not been used to characterize an AM nickel-base alloy prior to this work. These results demonstrate that RUS is capable of detecting part to part microstructure variability between built AM components.

A Fractographic Analysis of Additively Manufactured Ti6Al4V by Electron Beam Melting: Effects of Powder Reuse

Metal additive manufacturing (AM) is being rapidly adopted in the aerospace and biomedical industries. Powder bed fusion AM processes are leading this trend. To maximize process economy, excess “unmelted” powder retrieved from the build chamber is used in subsequent build cycles. The metal properties and component reliability could undergo degradation with powder reuse. This study investigates the effects of powder reuse on fracture surface characteristics of Ti6Al4V specimens fabricated by electron beam melting AM over 30 sequential build cycles. Optical microscopy and scanning electron microscopy were used to evaluate the changes in fracture surface features of tensile failures with powder reuse. Macroscopically, slant fractures were most common in early builds, which transitioned to orthogonal fracture surfaces with poorly defined shear lips with increasing reuse. Regardless of the build number, the fracture origins were consistently from the as-built surfaces. Microscopically, ductile features such as micro-void coalescence were evident throughout the 30 build cycles. However, increasing flute content with reuse suggests that rising oxygen levels causes solution strengthening and limits the participation of active slip systems. These results highlight the importance of surface roughness and powder oxidation to metal performance in AM, and the evolution of fractographic features with powder reuse.

Preliminary Characterization of Glass/Alumina Composite Using Laser Powder Bed Fusion (L-PBF) Additive Manufacturing.

Powder bed fusion (PBF) additive manufacturing (AM) is currently used to produce high-efficiency, high-density, and high-performance products for a variety of applications. However, existing AM methods are applicable only to metal materials and not to high-melting-point ceramics. Here, we develop a composite material for PBF AM by adding Al2O3 to a glass material using laser melting. Al2O3 and a black pigment are added to a synthesized glass frit for improving the composite strength and increased laser-light absorption, respectively. Our sample analysis shows that the glass melts to form a composite when the mixture is laser-irradiated. To improve the sintering density, we heat-treat the sample at 750 °C to synthesize a high-density glass frit composite. As per our X-ray diffraction (XRD) analysis to confirm the reactivity of the glass frit and Al2O3, we find that no reactions occur between glass and crystalline Al2O3. Moreover, we obtain a high sample density of ≥95% of the theoretical density. We also evaluate the composite’s mechanical properties as a function of the Al2O3 content. Our approach facilitates the manufacturing of ceramic 3D structures using glass materials through PBF AM and affords the benefits of reduced process cost, improved performance, newer functionalities, and increased value addition.

Design for metal powder bed fusion: The geometry for additive part selection (GAPS) worksheet

In industry, Design for Additive Manufacturing (DfAM) is currently synonymous with expert knowledge and external consultants for many companies. Particularly in higher cost technologies, such as metal powder bed fusion, component design requires extensive additive manufacturing (AM) knowledge. If a part is improperly designed, then it can cause thousands of dollars of lost time and material through a failed print. To avoid this situation, specialists must be consulted throughout the printing process; however, the shortage of trained personnel familiar with AM can create a bottleneck during design. In order to help businesses identify candidate parts for Powder Bed Fusion (PBF) AM, this paper presents a DfAM worksheet to help engineers, drafters, and designers select good part candidates with little prior knowledge of the specific technology. This worksheet uses data from the literature to support the values used for design guidance. Example components are shown to demonstrate the worksheet process. Ratings of these components are then compared with expert raters’ assessments of their suitability for fabrication with PBF from a geometric standpoint. In addition to introducing the worksheet, preliminary user feedback about the worksheet is presented, and future work is discussed.

Multi-physics continuum modelling approaches for metal powder additive manufacturing: a review

PurposeThis paper aims to present a systematic approach in the literature survey related to metal additive manufacturing (AM) processes and its multi-physics continuum modelling approach for its better understanding.Design/methodology/approachA systematic review of the literature available in the area of continuum modelling practices adopted for the powder bed fusion (PBF) AM processes for the deposition of powder layer over the substrate along with quantification of residual stress and distortion. Discrete element method (DEM) and finite element method (FEM) approaches have been reviewed for the deposition of powder layer and thermo-mechanical modelling, respectively. Further, thermo-mechanical modelling adopted for the PBF AM process have been discussed in detail with its constituents. Finally, on the basis of prediction through thermo-mechanical models and experimental validation, distortion mitigation/minimisation techniques applied in PBF AM processes have been reviewed to provide a future direction in the field.FindingsThe findings of this paper are the future directions for the implementation and modification of the continuum modelling approaches applied to PBF AM processes. On the basis of the extensive review in the domain, gaps are recommended for future work for the betterment of modelling approach.Research limitations/implicationsThis paper is limited to review only the modelling approach adopted by the PBF AM processes, i.e. modelling techniques (DEM approach) used for the deposition of powder layer and macro-models at process scale for the prediction of residual stress and distortion in the component. Modelling of microstructure and grain growth has not been included in this paper.Originality/valueThis paper presents an extensive review of the FEM approach adopted for the prediction of residual stress and distortion in the PBF AM processes which sets the platform for the development of distortion mitigation techniques. An extensive review of distortion mitigation techniques has been presented in the last section of the paper, which has not been reviewed yet.

Electron beam additive manufacturing of Ti6Al4V: Evolution of powder morphology and part microstructure with powder reuse

Additive Manufacturing (AM) processes for metals are advancing at a rapid pace. Among many attractive qualities, AM relaxes design constraints and can significantly reduce material waste in comparison to subtractive manufacturing processes. However, there are some fundamental issues that must be addressed for metal AM to become prevalent in aerospace. In powder bed fusion AM, powder reuse from previous build cycles is desired to improve process economics. However, there is limited understanding of the contributions from powder reuse to particle and part quality. The present study investigates this topic in electron beam melting (EBM) powder bed fusion AM of a titanium alloy (Ti6Al4V) over 30 build cycles (∼480 h of build time) and powder reuse. Results show that nearly all aspects of the process are influenced by powder reuse. Specifically, the particle size distribution tightens, largely due to fewer particles with small diameter. Particle damage increases with reuse, which includes surface deformation (reduction in sphericity), partial melting and/or particle fusion and fracture. In regard to the built metal, the microstructure exhibits increasingly finer basket weave strucutre with greater surface area to volume ratio of the part. While there are no apparent trends in α volume fraction with powder reuse, an increase in α-lath thickness is observed . In the analysis of composition, no substantial changes in the Al and V alloy content are apparent, nor in Fe, H and N. However, the O concentration of the powder increased significantly with reuse, in fact, it exceeded the concentration limit (0.2%) in just 11 build cycles. Overall, powder reuse should be considered carefully in the development of titanium parts for performance critical applications by EBM AM.