Laser-powder bed fusion (L-PBF) is a metal additive manufacturing (AM) process involving layer-by-layer metal parts fabrication. It has been successfully used in the literature to manufacture nickel alloy 718 parts. However, the process parameter selection has generally been based on either intensive experimental methods or expensive computational simulations. As such, these methods test a limited region of the parameter space. Thus, hindering the evaluation of possible parameter combinations that can prove superior to those in the tested space. Moreover, most of the previous studies combine the process parameters into single metrics, e.g., linear energy density (LED) or volumetric energy density (VED). Both metrics have been shown in the literature to provide an incomplete understanding of the produced parts and inconsistent mechanical properties and microstructures. This paper uses a systematic framework developed by the authors that covers the entire scanning speed laser power (v-P) parameter space, estimates the maximum hatch spacing at each v-P combination, and detects the good printability region. This framework defines the good printability region as the region that is free of the main possible porosity defects, namely, lack of fusion, keyholing, and balling. Furthermore, to cover the effect of the fourth process parameter (layer thickness), the framework is applied to 60 and 90 & mu;m layer thicknesses. This extension of the framework is performed to test the possibility of increasing the volumetric build rate (VBR) by increasing the layer thickness. Finally, the printability is validated using density and porosity measurements, while performance is evaluated using tensile tests. Using the framework, parts with 99.99% densification were fabricated successfully for both layer thicknesses. Moreover, tensile strength values of 1059.1 and 1063.5 MPa and volumetric build rates of 7.10 and 6.92 mm3/s were achieved for the as-printed parts for the 60 and 90 & mu;m layer thicknesses, respectively. However, these VBR values are constrained by the conservative hatch spacing criterion used in the framework. Further optimization of the hatch spacing can provide higher VBR values for the 90 & mu;m layer thickness.

Laser-powder bed fusion (L-PBF) is a commonly employed additive manufacturing (AM) technique in manu-facturing 3D metallic parts with complex geometries from powder materials. It is a complex process with more than 100 process parameters affecting the properties of fabricated parts. Studies on AM of nickel alloy 718 are currently focusing on investigating the effect of the process parameters and heat treatment on final part properties. Nevertheless, the effect of different powder characteristics such as particle size distribution (PSD), the presence of fine particles, powder reuse, and moisture content has not been studied comprehensively in the literature. In this study, two types of powder: virgin and recycled, as well as four parameters: PSD, which is represented by the median particle size and the particle size range, moisture content, and volumetric energy density (VED), were studied. To reduce experimental cost and burden, statistical design of experiment (DOE) techniques were utilized. Then, the measured results were analyzed using analysis of variance (ANOVA) to ascertain the effect of the different powder characteristics on multiple quantities of interest (QoIs). Different densification and mechanical tests were performed, and their outputs were studied as the QoIs. This study revealed that VED, powder type, PSD range, and median are the most common significant factors for most QoIs. Meanwhile, moisture content had a significant effect only on the Charpy impact energy. Besides, the study showed that the presence of fine particles < 20 mu m improved the densification of the fabricated parts but reduced the mechanical properties.

The mechanical properties of 3D printed polymers parts are process parameter dependent. Defects such as inadvertent voids between deposited rasters and layers lead to weakness in produced parts, which results in inferior mechanical properties as compared to injection molding. An alternative method to change energy absorption and stiffness of a polymer is hybrid additive manufacturing (AM). Hybrid-AM is the use of additive manufacturing with one or more secondary processes that are fully coupled and synergistically affect part quality, functionality, and/or process performance. In this study, fused filament fabrication (FFF) was coupled with layer-by-layer shot peening to study the dynamic mechanical properties of ABS 430 polymer using dynamic mechanical analysis (DMA). FFF is a heated extrusion process. Shot peening is a mechanical surface treatment that impinges a target with a stochastically dispersed, high velocity stream of beads. Compressive residual stress was imparted to preferential layer intervals during printing to modify the elasticity (stiffness), viscosity, toughness, and glass transition temperature. Viscoelastic and dynamic mechanical properties are important to the performance of polymers in automotive, aerospace, electronics, and medical components. Coupling printing and peening increased the storage and loss moduli as well as the tangent delta. DMA results suggest that preferential layer sequences exist that possess higher elasticity and better absorb energy upon sinusoidal dynamic loading.

This article highlights work at Oak Ridge National Laboratory’s Manufacturing Demonstration Facility to develop closed-loop, feedback control for laser-wire based Directed Energy Deposition, a form of metal Big Area Additive Manufacturing (m-BAAM), a process being developed in partnership with GKN Aerospace specifically for the production of Ti-6Al-4V pre-forms for aerospace components. A large-scale structural demonstrator component is presented as a case-study in which not just control, but the entire 3D printing workflow for m-BAAM is discussed in detail, including design principles for large-format metal AM, toolpath generation, parameter development, process control, and system operation, as well as post-print net-shape geometric analysis and finish machining. In terms of control, a multi-sensor approach has been utilized to measure both layer height and melt pool size, and multiple modes of closed-loop control have been developed to manipulate process parameters (laser power, print speed, deposition rate) to control these variables. Layer height control and melt pool size control have yielded excellent local (intralayer) and global (component-level) geometry control, and the impact of melt pool size control in particular on thermal gradients and material properties is the subject of continuing research. Further, these modes of control have allowed the process to advance to higher deposition rates (exceeding 7.5 lb/hr), larger parts (1-meter scale), shorter build times, and higher overall efficiency. The control modes are examined individually, highlighting their development, demonstration, and lessons learned, and it is shown how they operate concurrently to enable the printing of a large-scale, near net shape Ti-6Al-4V component.

Sensing and closed-loop control are critical attributes of a robust 3D printing process, such as Directed Energy Deposition (DED), in which it is necessary to manage geometry, material properties, and residual stress and distortion. The present research demonstrates multiple modes of closed-loop melt pool size control in laser-wire based DED, a form of large-scale metal additive manufacturing. First, real-time closed-loop melt pool size control through laser power modulation was demonstrated for intralayer control of bead geometry. Aspects such as controller tuning, response time, interaction with primary process variables, and disturbance rejection are presented. Next, an interlayer trend in laser power during the printing of layered components was documented, which inspired the development of novel modes of control. A controller that modulates print speed and deposition rate on a per-layer basis was developed and demonstrated, enabling the control of either average melt pool size alone or average laser power in coordination with real-time melt pool size control. This work demonstrates that accumulated heat in components under construction can be exploited to maintain process stability as print speed and deposition rate are automatically increased under closed-loop control. This has major implications for overall production efficiency. Control modes are characterized in terms of their effect on local bead geometry, global part geometry, and interlayer effect on energy density, among other factors.

A variety of techniques have been utilized in metal additive manufacturing (AM) for melt pool size management, including modeling and feed-forward approaches. In a few cases, closed-loop control has been demonstrated. In this research, closed-loop melt pool size control for large-scale, laser wire-based directed energy deposition is demonstrated with a novel modification, i.e., site-specific changes to the controller setpoint were commanded at trigger points, the locations of which were generated by the projection of a secondary geometry onto the primary three-dimensional (3D) printed component geometry. The present work shows that, through this technique, it is possible to print a specific geometry that occurs beyond the actual toolpath of the print head. This is denoted as extra-toolpath geometry and is fundamentally different from other methods of generating component features in metal AM. A proof-of-principle experiment is presented in which a complex oak leaf geometry was embossed on an otherwise ordinary double-bead wall made from Ti-6Al-4V. The process is introduced and characterized primarily from a controls perspective with reports on the performance of the control system, the melt pool size response, and the resulting geometry. The implications of this capability, which extend beyond localized control of bead geometry to the potential mitigations of defects and functional grading of component properties, are discussed.

The current study investigates the influence of layer-by-layer shot peening (SP) on the low velocity impact properties of P430 acrylonitrile butadiene styrene (ABS) parts processed by fused filament fabrication (FFF). The experiment examines the relationships between layer peening frequency and build orientation with the impact performance. Preferential layers were shot peened to form cumulative mechanical properties that influenced low strain rate impact behavior. Samples peened every three, four, and five layers were compared to surface-only peened and non-peened samples. Two different printing orientations were examined. Low strain rate impact behavior after hybrid additive manufacturing was assessed by drop tower impact and Charpy impact. Results revealed that layer peening frequency and printing orientation influence ABS’ ability to absorb energy and resist fracture. SP between printed layers enhanced the impact strength of ABS. Less frequent peening and side aligned printed parts resulted in a higher toughness, impact strength, and breakage resistance.