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Solid-State Large Aluminum Additive Manufacturing Replacements


OUSD (R&E) CRITICAL TECHNOLOGY AREA(S): Trusted AI and Autonomy OBJECTIVE: Additively manufacture (AM) aluminum alloy 7XXX (Al-Zn-Mg-Cu) or equivalent material via solid-state for replacement of forged legacy components with long lead times and logistics tail. DESCRIPTION: As the need for sustainment of aging US armed forces aircraft continue to rise and will continue to rise with the introduction of Future Vertical Lift (FVL) [1], there is a growing necessity for supplementing the supply chain for long logistic components to maintain fleet readiness. As a disruptor of traditional manufacturing, AM has come into focus as a leading technology to fabricate components, supplementing hard to procure aerospace components [2]. This is possible due to AM systems offering all-in-one turnkey manufacturing solutions, providing benefits in reducing production costs associated with build time and waste material of traditional manufacturing methods [3]. However, for fusion-based AM processes (e.g. selective laser melting and electron-beam melting), certain alloys suffer from poor weldability impeding fabrication via AM [4], and are typically limited to smaller parts that must fit within 1 sqft. sealed environments for processing. One such alloy system is Al-Zn-Mg-Cu (AA7XXX) aluminum alloys, which comprise the majority of the structural materials used in aerospace across the DoD and industry including FVL offers. It is well established that the AA7XXX family is traditionally considered unweldable, and when subjected to high thermal gradients, hot cracking occurs in the microstructure. Therefore, fusion-based AM, in which high thermal gradients are introduced into the microstructure similar to welding, typically results in hot cracking and material anisotropy when fabricating or repairing AA7XXX. These deleterious defects within the microstructure reduce the mechanical performance of the material, beyond allowable limits for aviation applications. To alleviate the detrimental-effects to the microstructure, AA7XXX powders for fusion AM have been enhanced with additional alloying elements (e.g. Scandium). However, the introduction of these new additives raises concerns on material response when compared to traditional AA7XXX, and how it will respond during typical aerospace service conditions. Thus, there is need for a 1-to-1 replacement of traditionally high strength, low weight forged aerospace materials to preclude the inherent uncertainties with AM aluminum materials. Nascent solid-state AM techniques have been proven to be capable of depositing traditional materials, like AA7XXX, due to the low thermal requirements to deposit the material. As a result, the microstructure is not thermally stressed to the same degree as fusion-based AM and is not subject to the same negative effects observed when processing with conventional alloys as the input feedstock. Additionally, solid-state techniques are more modular and are not limited to the geometric constraints governed by inert build chambers or laser interactions, permitting significantly larger build areas. However, the low resolution and characterization of the alloys for aerospace components has left technological gaps to permit adoption for aviation applications. The goal of this topic is to identify a solid-state AM processes that can 3D print traditionally unweldable aerospace materials without adding additional alloying elements to the bulk material for a true 1-to-1 replacement of components. The solid-state AM process will demonstrate the feasibility of printing a large aviation component free from contamination and additional inoculants. Then after successful printing, an optimized process will produce a final aerospace component as a demonstration. PHASE I: Demonstrate the feasibility of printing a large, full-sized aviation component (build area/volume larger than 1sqft/1ft3) via a friction-based solid-state additive manufacturing method utilizing a high-strength alloy (e.g. 7XXX). This component will serve as both a technology demonstrator and a first article cut up. Initial microstructural and mechanical characterization will be performed by extracting material samples from the first article component to demonstrate a lack of process related defects, porosity, and contaminants, with an initial evaluation of mechanical performance. Phase I deliverables include a report detailing first article production and evaluation of the sectioned component for process defects and optimization plan for the material and process. PHASE II: Following the initial successful demonstration using solid-state AM to produce a print with a 7XXX aluminum alloy, process optimization will be conducted to further refine parameters. The optimized parameters will then be used to establish repeatability through analysis of process structure property (PSP) relationships and mechanical testing. Material samples shall be evaluated in the final post-processed condition. Extensive microstructural evaluation utilizing a combination of optical and electron microscopy and X-ray spectroscopy and tomography provides an in-depth analysis of the microstructural evolution to elucidate production and post-processing effects on the final prototypes. This includes inspections on density, phase identification and dispersion, and granular characterization. Additionally, mechanical performance of the optimized component shall be evaluated with tensile and fatigue, with detailed observations on damage mechanisms and failure modes using microscopy. Test and evaluation techniques shall follow ASTM standard procedures to be documented and contrasted against legacy aviation material requirements. Complete data and manufacturing instructions from process preparation to post-processing shall be delivered in a phase II report along with a second finished component fabricated with the optimized and substantiated material developed under this effort. PHASE III DUAL USE APPLICATIONS: The civilian and defense sectors would benefit from this developed technology as an alternative means to rapidly produce large scale, long lead wrought aluminum forgings with that match original requirements of the legacy component that would be otherwise difficult to match through current additive manufacturing methods. DoD may pursue this technology for transition into the larger organic industrial base, as a close out report with all data and documentation necessary to fully replicate large parts within the defense industrial base. Successful delivery of manufacturing instructions will be transferable to the Jointless Hull activities in relation to the Next Generation Combat Vehicle (NGCV) in direct collaboration with DEVCOM Ground Vehicle Systems Center. Thus, successful demonstration of solid-state AM producing an aluminum alloy component with 1-to-1 equivalent material will increase Army readiness and reduce logistical timeframe for component procurement across ground and aviation systems. REFERENCES: 1. Dixon, M., 2006, The Maintenance Costs of Aging Aircraft: Insights From Commercial Aviation, RAND Corporation, Santa Monica, CA. 2. Liu, R., Wang, Z., Sparks, T., Liou, F., and Newkirk, J., 2017, Aerospace Applications of Laser Additive Manufacturing, Woodhead Publishing, Sawston, UK. 3. Berman, B., 2012, “3-D Printing: The New Industrial Revolution,” Bus. Horiz., 55(2), pp. 155–162. 4. Cevik, B., 2018, “Gas Tungsten Arc Welding of 7075 Aluminum Alloy: Microstructure Properties, Impact Strength, and Weld Defects,” Mater. Res. Express, 5(6), p. 066540. 5. Reschetnik, W., Brüggemann, J. P., Aydinöz, M. E., Grydin, O., Hoyer, K. P., Kullmer, G., and Richard, H. A., 2016, “Fatigue Crack Growth Behavior and Mechanical Properties of Additively Processed En AW-7075 Aluminium Alloy,” Procedia Struct. Integr., 2, pp. 3040–3048. KEYWORDS: Additive Manufacturing, Solid-State, Forging, Aluminum, Replacements, Process-Structure-Property
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