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•Metal additive manufacturing in aerospace comprehensively reviewed.•Discussion of advantages and benefits of metal additive manufacturing in aerospace.•Limitations and challenges ...described in context of current technology.•Successful examples of metal additive manufacturing in aerospace demonstrated.•Future growth potential and promising areas discussed.
Metal additive manufacturing involves manufacturing techniques that add material to produce metallic components, typically layer by layer. The substantial growth in this technology is partly driven by its opportunity for commercial and performance benefits in the aerospace industry. The fundamental opportunities for metal additive manufacturing in aerospace applications include: significant cost and lead-time reductions, novel materials and unique design solutions, mass reduction of components through highly efficient and lightweight designs, and consolidation of multiple components for performance enhancement or risk management, e.g. through internal cooling features in thermally loaded components or by eliminating traditional joining processes. These opportunities are being commercially applied in a range of high-profile aerospace applications including liquid-fuel rocket engines, propellant tanks, satellite components, heat exchangers, turbomachinery, valves, and sustainment of legacy systems. This paper provides a comprehensive review of metal additive manufacturing in the aerospace industry (from industrial/popular as well as technical literature). This provides a current state of the art, while also summarizing the primary application scenarios and the associated commercial and technical benefits of additive manufacturing in these applications. Based on these observations, challenges and potential opportunities are highlighted for metal additive manufacturing for each application scenario.
The high‐speed synchrotron X‐ray imaging technique was synchronized with a custom‐built laser‐melting setup to capture the dynamics of laser powder‐bed fusion processes in situ. Various significant ...phenomena, including vapor‐depression and melt‐pool dynamics and powder‐spatter ejection, were captured with high spatial and temporal resolution. Imaging frame rates of up to 10 MHz were used to capture the rapid changes in these highly dynamic phenomena. At the same time, relatively slow frame rates were employed to capture large‐scale changes during the process. This experimental platform will be vital in the further understanding of laser additive manufacturing processes and will be particularly helpful in guiding efforts to reduce or eliminate microstructural defects in additively manufactured parts.
The high‐speed synchrotron X‐ray imaging technique was synchronized with a custom‐built laser‐melting setup to capture the dynamics of laser powder‐bed fusion processes in situ. Various significant phenomena, including vapor‐depression and melt‐pool dynamics and powder‐spatter ejection, were captured with high spatial and temporal resolution.
The purpose of this study was to perform a comparative analysis of powder-bed-based additive manufacturing (AM) technologies during the production of metallic components using Inconel 625 powder ...material. The AM technologies explored in this study include electron beam powder bed fusion (EPBF), laser powder bed fusion (LPBF), and binder jetting technology. Samples were fabricated in two build directions (X and Z build orientations) for this evaluation process, where all specimens underwent a hot isostatic pressing (HIP) post-process. The comparison was made in terms of microstructure and mechanical properties including ultimate tensile strength (UTS), yield strength (YS), percent elongation, and modulus of elasticity (E). Microstructural characterization showed evidence of equiaxed grain formation for binder jetting and LPBF parts, whereas EPBF parts displayed a more columnar grain formation parallel to the build direction. Six specimens were tested per technology, three built in the X orientation and three built in the Z orientation. All six specimens were built in a single run of each AM machine. Results indicated that all three technologies are capable of meeting the minimum requirements of the ASTM F3056-14 standard for parts produced in the X orientation, with properties that are similar to wrought Inconel 625. In the Z orientation, however, only LPBF was able to meet the minimum standard requirements. Through the comparative analysis of the mechanical properties, this work showed that LPBF outperformed the other technologies in a majority of the evaluated properties, followed by EPBF and binder jetting. An analysis of the fracture surfaces of tensile specimens was also performed, and it indicated ductile fracture (dimple rupture) for the specimens produced with all three of the AM technologies studied. Nevertheless, the characterization also showed certain differences in the fractured surfaces, such as the presence of un-sintered powder particles for the binder jetting processed Inconel 625, or the development of the so called woody structure for the EPBF processed material. This study can be used to determine distinct characteristics between the three powder-bed-based technologies for the fabrication of Inconel 625 that can further include other technologies and materials using similar approaches.
We review the progress of additive manufacturing effort on refractory metal tungsten and tungsten alloys. These materials are excellent candidates for a variety of high temperature applications but ...extremely challenging to fabricate via additive manufacturing due to a series of existing issues during the manufacturing. We outline these issues and discuss the current understanding and progress to tackle them. Laser powder-bed-fusion, laser directed-energy-deposition, and electron beam powder-bed-fusion are three common techniques that have been applied to additively manufacture pure tungsten. This overview discusses current observations and understanding on the issues associated with each of these techniques. We identify future research opportunities in additive manufacturing of refractory metals.
Powder Bed Fusion (PBF) techniques constitute a family of Additive Manufacturing (AM) processes, which are characterised by high design flexibility and no tooling requirement. This makes PBF ...techniques attractive to many modern manufacturing sectors (e.g. aerospace, defence, energy and automotive) where some materials, such as Nickel-based superalloys, cannot be easily processed using conventional subtractive techniques. Nickel-based superalloys are crucial materials in modern engineering and underpin the performance of many advanced mechanical systems. Their physical properties (high mechanical integrity at high temperature) make them difficult to process via traditional techniques. Consequently, manufacture of nickel-based superalloys using PBF platforms has attracted significant attention. To permit a wider application, a deep understanding of their mechanical behaviour and relation to process needs to be achieved. The motivation for this paper is to provide a comprehensive review of the mechanical properties of PBF nickel-based superalloys and how process parameters affect these, and to aid practitioners in identifying the shortcomings and the opportunities in this field. Therefore, this paper aims to review research contributions regarding the microstructure and mechanical properties of nickel-based superalloys, manufactured using the two principle PBF techniques: Laser Powder Bed Fusion (LPBF) and Electron Beam Melting (EBM). The ‘target’ microstructures are introduced alongside the characteristics of those produced by PBF process, followed by an overview of the most used building processes, as well as build quality inspection techniques. A comprehensive evaluation of the mechanical properties, including tensile strength, hardness, shear strength, fatigue resistance, creep resistance and fracture toughness of PBF nickel-based superalloys are analysed. This work concludes with summary tables for data published on these properties serving as a quick reference to scholars. Characteristic process factors influencing functional performance are also discussed and compared throughout for the purpose of identifying research opportunities and directing the research community toward the end goal of achieving part integrity that extends beyond static components only.
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•Review of academic and industrial contributions over the last 25 years.•Practice and evaluation methodologies are explored.•The capabilities of PBF processes and the resulting material properties are linked.•Opportunities for further research are stated to help direct the research community.•A benchmark of mechanical properties obtained to date for PBF are provided.
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•The heterogeneities in the microstructure and mechanical properties of AM Ni-based superalloys are reviewed.•The origins of heterogeneities are linked to the variations in thermal ...conditions throughout the build.•A short case study is presented.•Strategies to minimize microstructure heterogeneity are discussed.
The adaptation of additive manufacturing (AM) for Ni-based superalloys has gained significance in aerospace and power-generation industries due to the ability to fabricate complex, near-net-shape components on-demand and with minimal material waste. Besides its advantages, challenges remain in metal AM, especially for printing complex alloys such as superalloys. These challenges are often linked to heterogeneity in the as-fabricated parts and continue to limit the practical applications of AM products. A thorough understanding of the relationship between the complex AM process and the resulting microstructure heterogeneity needs to be established before mitigation strategies can be developed. The ability to fabricate more homogeneous Ni-based superalloy parts is expected to unlock not only better mechanical properties but also additional fields of applications.
This review aims to summarize the current understanding of heterogeneities in the microstructure and mechanical properties of AM Ni-based superalloys. Microstructure heterogeneities discussed include heterogeneity in the chemical composition, phase constitution, porosity, grain and dendrite morphology, and solid-state precipitates. Related heterogeneities in hardness, tensile, creep, fatigue, and residual stress are discussed to represent mechanical properties, and mitigation strategies are summarized. The origins of heterogeneity in the as-fabricated parts are linked to the variations in AM thermal conditions caused by the complex thermal histories.
The combination of refined microstructures (induced by rapid cooling) and melt pool-induced mesostructures in AlSi10Mg fabricated using laser powder bed fusion (LPBF) – a widely used additive ...manufacturing technique – impart high strength and fracture toughness. Further exploitation of such property combinations requires a detailed understanding of how the processing conditions control the micro- and mesostructures and, in turn, the mechanical performance, especially regarding fracture resistance. Towards this end, the crack resistance curve (R-curve) behavior in different orientations of LPBF-fabricated AlSi10Mg alloys processed with different layer thickness, hatch spacing, and scan strategies was evaluated and correlated with micro- and mesostructural features such as grain size and grain orientation, texture, cell morphology, and melt pool arrangement. Results show a strong anisotropy in both tensile stress-strain behavior and fracture toughness with layer thickness and hatch spacing controlling strength and scan strategy dictating fracture resistance. In terms of tensile stress-strain behavior, the arrangement of melt pool boundaries with respect to loading direction results in anisotropy in ductility whereas strength is controlled by grain size and cellular structure. In case of fracture toughness, measurements show that failure is dominated primarily by melt pool morphology and hence the mesostructure that is controlled by scan strategy. They furthermore reveal, that, despite the pronounced anisotropy in the R-curve behavior the presence of such mesostructure enables a level of damage-tolerance in AlSi10Mg that cannot be achieved in a cast alloy.
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Powder bed fusion (PBF) uses an energy beam to scan a powder bed surface, heat it locally, and consolidate the material to form a part. The choice of energy beam paths is typically based on user ...experience. Simple beam path strategies (e.g., raster or spiral) are used and the thermal field analysed post hoc. We refer to this approach as solving the direct problem, which involves many modelling and experimental iterations until a close to satisfactory, but not optimal, thermal field is achieved. In contrast, we propose a rational approach, solving an inverse problem to control heat placement, which we illustrate using laser beam PBF. Starting from the desired thermal field to be induced in the powder bed, we show how to formulate and solve the Inverse Heat Placement Problem (IHPP) so that a set of time-varying beam parameters (path, scan speed, energy delivered) is obtained in a single iteration. Solving the IHPP takes the craftsmanship out of beam path planning and puts it on a sound scientific and mathematical basis. This will enable a step change in the quality and complexity of customised macro-, meso-, and microscale properties of PBF built parts.
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•Inverse heat placement problem to enable more constant temperature distribution on the target surface has been formulated.•Optimised (non-linear) laser beam path of time varying scan speed and power has been simulated.•Experimental validateions of the optimised laser beam path was demonstrated on an open controller AM system.
•Torsion fatigue of powder bed fusion additive manufactured (AM) Ti-6Al-4V.•Surface finish and heat treatment effects on torsion fatigue of AM Ti-6Al-4V.•Damage mechanism and effect of defects in ...torsion fatigue of AM Ti-6Al-4V.•Torsion fatigue behavior and failure mechanism of wrought Ti-6Al-4V.
Additive manufacturing (AM) is a state of the art technology enabling fabrication of complex geometries, in addition to providing other advantages as compared to the traditional subtractive manufacturing methods. Powder bed fusion (PBF) is one of the most commonly used metal AM processes that uses laser to melt particles on the bed of metallic powder. Ti-6Al-4V is a common alloy made by this process and has applications in many industries, in particular aerospace and medical industries. Understanding mechanical performance of the additively manufactured materials and components for critical load bearing applications is still in early stages. As such components are typically subjected to cyclic loading, fatigue failure is a major consideration in their design. In addition, due to the multiaxial nature of the loading and/or complex geometries manufactured by AM, the state of stress often includes both normal and shear stresses. However, all the studies so far on fatigue behavior of additive manufactured metals have only considered axial loading, resulting in normal stresses. This study is on torsional fatigue behavior producing shear stresses and, therefore, addresses a major gap in understanding the mechanical behavior of additive manufactured metals in general and for PBF Ti-6Al-4V alloy in particular. In this study, thin-walled tubular specimens made of both wrought and AM Ti-6Al-4V were subjected to monotonic as well as cyclic torsional loads to study and compare their shear deformation and fatigue behaviors. Failure mechanisms in different life regimes and the effects of heat treatment and surface finish were also evaluated.
An adaptive laser power control strategy for Selective Laser Melting (SLM) has been developed using data from a co-axial photodiode monitoring system with 200 KHz temporal resolution. A supervised ...machine learning based algorithm outputs variable laser power along the scanning path based on mechanistic features. The approach was implemented on a commercial machine and demonstrated an average 12 % reduction in porosity size and 65 % reduction in the standard deviation of porosity size measured by X-Ray Computed Tomography (CT) compared to parts built with constant laser power. This approach is scalable and its precalculated nature is compatible with regulatory concerns.
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