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

The turbine blade test parts were manufactured by the selective laser melting (SLM) process using a nickel-based pre-alloyed Inconel (IN) 718 powder. Various mechanical post-processing techniques, such as barrel finishing (BF), shot peening (SP), ultrasonic shot peening (USP), and ultrasonic impact treatment (UIT), were applied to improve the surface layer properties of the SLM-built specimens. Effects of mechanical surface treatments on surface topography, porosity, hardness, and residual stress were studied. In comparison with the SLM-built state the surface roughness (Sa = 5.27 μm) of the post-processed specimens were respectively decreased by 20.6%, 26.2%, and 57.4% after the BF, USP, and UIT processes except for the SP-treated ones. The Sz parameter was reduced in all treated SLM-built specimens except for the SP-treated ones. The surface microhardness of the SLM-built specimen (~390 HV0.025) was increased after the BF (by 14.2%), USP (by 23.8%), UIT (by 50%), and SP (by 66.5%) processes. The deepest hardened layers were formed after the UIT and SP processes. Residual porosity of the SLM-built specimen was decreased by 23.1%, 40.6%, 55%, and 84% after the BF, SP, USP and UIT processes, respectively. The UIT process formed a densified subsurface layer of significantly reduced porosity (0.118%). All mechanical surface treatments successfully transformed the tensile residual stresses generated in SLM-built specimen into the compressive residual stresses (−201.4...510.7 MPa). The thickness of hardened, densified and compressed near-surface layers ranges from ~80 μm after BF to ~140 μm after USP, and ~180 μm after SP and UIT processes, which correlates to the accumulated energy and deformation extent of the treated surface. The effect of the accumulated energy on the outcomes of the applied surface treatments is also addressed.

Post-processing of the Inconel 718 alloy parts fabricated by selective laser melting: Effects of mechanical surface treatments on surface topography, porosity, hardness and residual stress

• Common defects and anomalies in powder bed fusion metal additive manufacturing. • Formation mechanism and practical mitigation strategies are discussed. • Defects/anomalies are classified as powder-related, processing-related, and post-processing related issues. • Properties such as mechanical behavior and corrosion resistance of defective parts are discussed. • Current challenges, gaps, and future trends are discussed. Metal additive manufacturing is a disruptive technology that is revolutionizing the manufacturing industry. Despite its unrivaled capability for directly fabricating metal parts with complex geometries, the wide realization of the technology is currently limited by microstructural defects and anomalies, which could significantly degrade the structural integrity and service performance of the product. Accurate detection, characterization, and prediction of these defects and anomalies have an important and immediate impact in manufacturing fully-dense and defect-free builds. This review seeks to elucidate common defects/anomalies and their formation mechanisms in powder bed fusion additive manufacturing processes. They could arise from raw materials, processing conditions, and post-processing. While defects/anomalies in laser welding have been studied extensively, their formation and evolution remain unclear. Additionally, the existence of powder in powder bed fusion techniques may generate new types of defects, e.g., porosity transferring from powder to builds. Practical strategies to mitigate defects are also addressed through fundamental understanding of their formation. Such explorations enable the validation and calibration of models and ease the process qualification without costly trial-and-error experimentation. 

Defects and anomalies in powder bed fusion metal additive manufacturing

This is an accepted manuscript of an article published by Elsevier in Additive Manufacturing on 02/10/2021, available online: https://doi.org/10.1016/j.addma.2021.102378 
The accepted version of the publication may differ from the final published version.

Fused Deposition Modelling: Current Status, Methodology, Applications and Future Prospects

Architectured structures, particularly functionally graded lattices, are receiving much attention in both industry and academia as they facilitate the customization of the structural response and harness the potential for multi-functional applications. This work experimentally investigates how the severity of density and unit cell size grading as well as the building direction affects the stiffness, energy absorption and structural response of additively manufactured (AM) short fibre-reinforced lattices with same relative density. Specimens composed of tessellated body-centred cubic (BCC), Schwarz-P (SP) and Gyroid (GY) unit cells were tested under compression. Compared to the uniform lattices of equal density, it was found, that modest density grading has a positive and no effect on the total compressive stiffness of SP and BCC lattices, respectively. More severe grading gradually reduces the total stiffness, with the modulus of the SP lattices never dropping below that of the uniform counterparts. Unit cell size grading had no significant influence on the stiffness and revealed an elastomer-like performance as opposed to the density graded lattices of the same relative density, suggesting a foam-like behaviour. Density grading of bending-dominated unit cell lattices showcased better energy absorption capability for small displacements, whereas grading of the stretching-dominated counterparts is advantageous for large displacements when compared to the ungraded lattice. The severity of unit cell size graded lattices does not affect the energy absorption capability. Finally, a power-law approach was used to semi-empirically derive a formula that predicts the cumulative energy absorption as a function of the density gradient and relative density. Overall, these findings will provide engineers with valuable knowledge that will ease the design choices for lightweight multi-functional AM-parts.

Effect of density and unit cell size grading on the stiffness and energy absorption of short fibre-reinforced functionally graded lattice structures

Ocular prosthesis is either a readymade stock shell or custom made prosthesis (CMP). Presently, there is no other technology available, which is either superior or even comparable to the conventional CMP. The present study was designed to fabricate ocular prosthesis using computer aided design (CAD) and rapid manufacturing (RM) technology and to compare it with custom made prosthesis (CMP). The ocular prosthesis prepared by CAD was compared with conventional CMP in terms of time taken for fabrication, weight, cosmesis, comfort, and motility. Two eyes of two patients were included. Computerized tomography scan of wax model of socket was converted into three dimensional format using Materialize Interactive Medical Image Control System (MIMICS)software and further refined. This was given as an input to rapid manufacturing machine (Polyjet 3-D printer). The final painting on prototype was done by an ocularist. The average effective time required for fabrication of CAD prosthesis was 2.5 hours; and weight 2.9 grams. The same for CMP were 10 hours; and 4.4 grams. CAD prosthesis was more comfortable for both the patients. The study demonstrates the first ever attempt of fabricating a complete ocular prosthesis using CAD and rapid manufacturing and comparing it with conventional CMP. This prosthesis takes lesser time for fabrication, and is more comfortable. Studies with larger sample size will be required to further validate this technique.

An innovative method of ocular prosthesis fabrication by bio-CAD and rapid 3-D printing technology: A pilot study.

Enhancement of surface characteristics of direct metal laser sintered stainless steel 316L by shot peening

Influence of γ-alumina coating on surface properties of direct metal laser sintered 316L stainless steel

Influence of Vapour Smoothing Process Parameters on Fused Deposition Modelling Parts Surface Roughness at Different Build Orientation

Fused deposition modeling is used in industry, mainly for prototyping the designs modeled through CAD software. The most common build material used in this technology is ABS (Acrylonitrile Butadiene Styrene) plastic and it is one of the common methods used to produce ABS parts of mid quantity series of high complexity objects. But due to stair step effect, the products produced by this method have high roughness compared to ABS parts produced by conventional method like injection molding process. So FDM parts are generally electroplated to meet the industry requirements. However, electroplating has a significant effect on the roughness and the dimension of the parts and this effect varies significantly at different part orientation. Therefore, in this work, experimentation is done to study the electroplating effect on FDM parts surface roughness and dimension at various surface angles. From the experimented data base of electroplated test parts, advanced interpolation method is used to predict the surface roughness and change in dimension of electroplated FDM parts at other orientation. Interpolation helps to find out the best build orientation for any significant features of a part to get good/ required surface finish and gives dimension allowance required before electroplating. The proposed methodology is validated with a case study of prediction model using NACA 0012 airfoil.

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M. Sugavaneswaran

Papers

An innovative method of ocular prosthesis fabrication by bio-CAD and rapid 3-D printing technology: A pilot study.

Enhancement of surface characteristics of direct metal laser sintered stainless steel 316L by shot peening

Influence of γ-alumina coating on surface properties of direct metal laser sintered 316L stainless steel

Influence of Vapour Smoothing Process Parameters on Fused Deposition Modelling Parts Surface Roughness at Different Build Orientation

Effect of electroplating on surface roughness and dimension of FDM parts at various build orientations