Propulsion system development requires new, more affordable manufacturing techniques and technologies in a constrained budget environment, while future in-space applications will require in-space manufacturing and assembly of parts and systems. Marshall is advancing cuttingedge commercial capabilities in additive and digital manufacturing and applying them to aerospace challenges. The Center is developing the standards by which new manufacturing processes and parts will be tested and qualified. Rapidly evolving digital tools, such as additive manufacturing, are the leading edge of a revolution in the design and manufacture of space systems that enables rapid prototyping and reduces production times. Marshall has unique expertise in leveraging new digital tools, 3D printing, and other advanced manufacturing technologies and applying them to propulsion systems design and other aerospace materials to meet NASA mission and industry needs. Marshall is helping establish the standards and qualifications “from art to part” for the use of these advanced techniques and the parts produced using them in aerospace or elsewhere in the U.S. industrial base.

Additive manufacturing

BACKGROUNDMetallic components are the cornerstone of modern industries such as aviation, aerospace, automobile manufacturing, and energy production. The stringent requirements for high-performance metallic components impede the optimization of materials selection and manufacturing. Laser-based additive manufacturing (AM) is a key strategic technology for technological innovation and industrial sustainability. As the number of applications increases, so do the scientific and technological challenges. Because laser AM has domain-by-domain (e.g., point-by-point, line-by-line, and layer-by-layer) localized forming characteristics, the requisite for printing process and performance control encompasses more than six orders of magnitude, from the microstructure (nanometer- to micrometer-scale) to macroscale structure and performance of components (millimeter- to meter-scale). The traditional route of laser-metal AM follows a typical “series mode” from design to build, resulting in a cumbersome trial-and-error methodology that creates challenges for obtaining high-performance goals.ADVANCESWe propose a holistic concept of material-structure-performance integrated additive manufacturing (MSPI-AM) to cope with the extensive challenges of AM. We define MSPI-AM as a one-step AM production of an integral metallic component by integrating multimaterial layout and innovative structures, with an aim to proactively achieve the designed high performance and multifunctionality. Driven by the performance or function to be realized, the MSPI-AM methodology enables the design of multiple materials, new structures, and corresponding printing processes in parallel and emphasizes their mutual compatibility, providing a systematic solution to the existing challenges for laser-metal AM. MSPI-AM is defined by two methodological ideas: “the right materials printed in the right positions” and “unique structures printed for unique functions.” The increasingly creative methods for engineering both micro- and macrostructures within single printed components have led to the use of AM to produce more complicated structures with multimaterials. It is now feasible to design and print multimaterial components with spatially varying microstructures and properties (e.g., nanocomposites, in situ composites, and gradient materials), further enabling the integration of functional structures with electronics within the volume of a laser-printed monolithic part. These complicated structures (e.g., integral topology optimization structures, biomimetic structures learned from nature, and multiscale hierarchical lattice or cellular structures) have led to breakthroughs in both mechanical performance and physical/chemical functionality. Proactive realization of high performance and multifunctionality requires cross-scale coordination mechanisms (i.e., from the nano/microscale to the macroscale).OUTLOOKOur MSPI-AM continues to develop into a practical methodology that contributes to the high performance and multifunctionality goals of AM. Many opportunities exist to enhance MSPI-AM. MSPI-AM relies on a more digitized material and structure development and printing, which could be accomplished by considering different paradigms for AM materials discovery with the Materials Genome Initiative, standardization of formats for digitizing materials and structures to accelerate data aggregation, and a systematic printability database to enhance autonomous decision-making of printers. MSPI-oriented AM becomes more intelligent in processes and production, with the integration of intelligent detection, sensing and monitoring, big-data statistics and analytics, machine learning, and digital twins. MSPI-AM further calls for more hybrid approaches to yield the final high-performance/multifunctional achievements, with more versatile materials selection and more comprehensive integration of virtual manufacturing and real production to navigate more complex printing. We hope that MSPI-AM can become a key strategy for the sustainable development of AM technologies. Download high-res image Open in new tab Download PowerpointMaterial-structure-performance integrated additive manufacturing (MSPI-AM).Versatile designed materials and innovative structures are simultaneously printed within an integral metallic component to yield high performance and multifunctionality, integrating in parallel the core elements of material, structure, process, and performance and a large number of related coupling elements and future potential elements to enhance the multifunctionality of printed components and the maturity and sustainability of laser AM technologies.

Material-structure-performance integrated laser-metal additive manufacturing.

Functionally graded materials (FGMs) represent a class of novel materials in which compositions/constituents and/or microstructures gradually change along single or multiple spatial directions, resulting in a gradual change in properties and functions which can be tailored for enhanced performance. FGMs can be fabricated using a variety of well-established processing methods; however, it is also known that there are inherent drawbacks to existing synthesis methods. As an emerging technology that provides a high degree of control over spatial resolution, additive manufacturing (AM) provides an intriguing pathway to circumvent the drawbacks of currently available methods. AM involves the selective deposition of individual layers of single or multiple materials, and as such it offers the potential of local control of composition and microstructure in multiple dimensions; such process conditions, in principle, can be tailored to construct complex FGMs with multi-dimensional and directional gradient structures. In this review paper, our current understanding of important issues, such as modeling, processing, microstructures and mechanical properties, as related to FGMs produced via AM, are described and discussed in an effort to assess the state of the art in this field as well as to provide insight into future research directions.

Additive manufacturing of functionally graded materials: A review

Recent advances in multi-material 3D and 4D printing (time as the fourth dimension) show that the technology has the potential to extend the design space beyond complex geometries. The potential of these additive manufacturing (AM) technologies allows for functional inclusion in a low-cost single-step manufacturing process. Different composite materials and various AM technologies can be used and combined to create customized multi-functional objects to suit many needs. In this work, several types of 3D and 4D printing technologies are compared and the advantages and disadvantages of each technology are discussed. The various features and applications of 3D and 4D printing technologies used in the fabrication of multi-material objects are reviewed. Finally, new avenues for the development of multi-material 3D and 4D printed objects are proposed, which reflect the current deficiencies and future opportunities for inclusion by AM.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/advs.201902307

Multi-Material 3D and 4D Printing: A Survey

In the circular economy, products, components, and materials are aimed to be kept at the utility and value all the lifetime. For this purpose, repair and remanufacturing are highly considered as proper techniques to return the value of the product during its life. Directed Energy Deposition (DED) is a very flexible type of additive manufacturing (AM), and among the AM techniques, it is most suitable for repairing and remanufacturing automotive and aerospace components. Its application allows damaged component to be repaired, and material lost in service to be replaced to restore the part to its original shape. In the past, tungsten inert gas welding was used as the main repair method. However, its heat affected zone is larger, and the quality is inferior. In comparison with the conventional welding processes, repair via DED has more advantages, including lower heat input, warpage and distortion, higher cooling rate, lower dilution rate, excellent metallurgical bonding between the deposited layers, high precision, and suitability for full automation. Hence, the proposed repairing method based on DED appears to be a capable method of repairing. Therefore, the focus of this study was to present an overview of the DED process and its role in the repairing of metallic components. The outcomes of this study confirm the significant capability of DED process as a repair and remanufacturing technology.

Application of Directed Energy Deposition-Based Additive Manufacturing in Repair

To understand processing ability and measure resultant interfacial and thermal properties of Inconel 718 and copper alloy GRCop-84, bimetallic structures were fabricated using laser engineering net shaping (LENS™), a commercially available additive manufacturing technique. It was hypothesized that additively combining the two aerospace alloys would form a unique bimetallic structure with improved thermophysical properties compared to the Inconel 718 alloy. Two approaches were used: the direct deposition of GRCop-84 on Inconel 718 and the compositional gradation of the two alloys. Scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray Diffraction (XRD), Vickers microhardness and flash thermal diffusivity were used to characterize these bimetallic structures to validate our hypothesis. The compositional gradation approach showed a gradual transition of Inconel 718 and GRCop-84 elements at the interface, which was also reflected in the cross-sectional hardness profile across the bimetallic interface. SEM images showed columnar grain structures at the interfaces with Cr2Nb precipitate accumulation along grain boundaries and the substrate-deposit interface. The average thermal diffusivity of the bimetallic structure was measured at 11.33 mm2/s for the temperature range of 50 °C–300 °C; a 250% increase in diffusivity when compared to the pure Inconel 718 alloy at 3.20 mm2/s. Conductivity of the bimetallic structures increased by almost 300% compared to Inconel 718 as well. Such structures with designed compositional gradation and tailored thermal properties opens up the possibilities of multi-material metal additive manufacturing for next generation of aerospace structures.

Additive manufacturing of Inconel 718—Copper alloy bimetallic structure using laser engineered net shaping (LENS™)

Bimetallic structures belong to a class of multi-material structures, and they potentially offer unique solutions to many engineering problems. In this work, bimetallic structures of Inconel 718 and Ti6Al4V (Ti64) alloys were processed using laser engineered net shaping (LENS™). During LENS™ processing, three build strategies were attempted: direct deposition, compositional gradation and use of an intermediate bond layer. Inconel 718 and Ti64 alloys exhibit thermal properties mismatch along with brittle intermetallic phase formation at the interface resulting in delamination. For a successful build, the use of a compositional bond layer (CBL) was employed, which was a mixture of a third material - Vanadium Carbide - with the parent alloys to form an intermediate layer used in bonding the two immiscible alloys. A crack-free bimetallic structure of Inconel 718 and Ti64 was demonstrated. Scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction and Vickers hardness were used to characterize these bimetallic structures. XRD analysis indicated presence of Cr3C2 phases. CBL improved the bonding strength by avoiding formation of brittle intermetallic phases such as TiNi3 and Ti2Ni as well as reducing thermal stresses at the interface. Our results successfully demonstrate the formation of Inconel 718 and Ti64 bimetallic structures using a laser-based commercially available additive manufacturing approach.

Additive manufacturing of Inconel 718 – Ti6Al4V bimetallic structures

Additive manufacturing in repair: Influence of processing parameters on properties of Inconel 718

Significant efforts have been made to treat bone disorders through the development of composite scaffolds utilizing calcium phosphate (CaP) through additive manufacturing techniques. However, the incorporation of natural polymers with CaP during 3D printing is difficult and remains a formidable challenge in bone and tissue engineering applications. The objective of this study is to understand the use of a natural polymer binder system in ceramic composite scaffolds using a ceramic slurry-based solid freeform fabricator (SFF). This was achieved through the utilization of naturally sourced gelatinized starch with hydroxyapatite (HA) ceramic in order to obtain high mechanical strength and enhanced biological properties of the green part without the need for cross-linking or post processing. The parametric effects of solids loading, polycaprolactone (PCL) polymer addition, and designed porosity on starch-HA composite scaffolds were assessed through mechanical strength, microstructure, and in vitro biocompatibility utilizing human osteoblast cells. It was hypothesized that starch incorporation would improve the mechanical strength of the scaffolds and increase proliferation of osteoblast cells in vitro. Starch loading was shown to improve mechanical strength from 4.07 ± 0.66 MPa to 10.35 ± 1.10 MPa, more closely resembling the mechanical strength of cancellous bone. Based on these results, a reinforcing mechanism of gelatinized starch based on interparticle and apatite crystal interlocking is proposed. Morphological characterization utilizing FESEM and MTT cell viability assay showed enhanced osteoblast cell proliferation in the presence of starch and PCL. Overall, the utilization of starch as a natural binder system in SFF scaffolds was found to improve both green strength and in vitro biocompatibility.

Starch-Hydroxyapatite Composite Bone Scaffold Fabrication Utilizing a Slurry Extrusion-Based Solid Freeform Fabricator.

Bimetallic structures provide a unique solution to achieve site-specific functionalities and enhanced-property capabilities in engineering systems but suffer from bonding compatibility issues. Materials such as titanium alloy (Ti6Al4 V) and stainless steel (SS410) have distinct attractive properties but are impossible to reliably weld together using traditional processes. To this end, a laser-based directed energy deposition (DED) system was used to fabricate bimetallic joint of Ti6Al4 V and SS410 keeping niobium (Nb) as a diffusion barrier layer. Both shear and compression tests were used to characterize the joint’s strength, and compared with the base materials. The bimetallic-joint shear and compressive yield strengths were 419 ± 3 MPa (∼114% of SS410) and 560 ± 4 MPa (∼169% of SS410), respectively. The increase in interfacial shear and compressive yield strengths over the base material indicates strong metallurgical bonding between the base materials and the interlayer, Nb. Proof-of-concept part for direct application of the bimetallic joint was demonstrated by welding base metals, end-to-end, to the joint. The interfacial microstructures, elemental diffusion and phases, including failure modes were examined using secondary and backscatter electron imaging, X-ray diffraction (XRD) and energy dispersive spectroscopy (EDS). The bimetallic-joint interfaces were free from brittle intermetallic compounds such as FeTi and Fe2Ti that are generally responsible for weak bond strength.

Bonny Onuike

Papers

Additive manufacturing of Inconel 718—Copper alloy bimetallic structure using laser engineered net shaping (LENS™)

Additive manufacturing of Inconel 718 – Ti6Al4V bimetallic structures

Additive manufacturing in repair: Influence of processing parameters on properties of Inconel 718

Starch-Hydroxyapatite Composite Bone Scaffold Fabrication Utilizing a Slurry Extrusion-Based Solid Freeform Fabricator.

Functional Bimetallic Joints of Ti6Al4V to SS410.