The National Aeronautics and Space Administration’s (NASA) George C. Marshall Space Flight Center (MSFC) is partnering with The University of Tennessee in development of large aerospace structures via robotically controlled wire arc additive manufacturing (WAAM) of aluminum alloys, primarily aluminum 2319. Objective benchmarks include fabrication of thin-wall and bulk geometries with both gravity and non-gravity aligned features. Demonstration of these structures will encompass welding process parameter development via pulsed gas metal arc welding process (GMAW) for the base layer and cold metal transfer (CMT) deposition for all subsequent layers. Work is also on-going to advance path planning algorithms for printing bulk, non-gravity aligned (NGA) features and structures. Finally, in-situ melt pool monitoring capabilities will be conducted for future implementation into NGA process controls.
BULK NGA PROCESS CONDITIONS: Baseline CMT deposition parameters were explored using a Fronius VR 7000 power supply and single-wire torch with gravity aligned single wall structures defining robot velocity, arc waveform settings, wire feed speed, contact-tip-to-workpiece distance (CTWD) and interpass temperature. Gravity aligned sample parts were created using these parameters to ensure material quality prior to advancing into NGA thin wall structures. Experiments will be conducted to characterize melt pool behavior with torch orientation angle to develop robot configurations necessary for NGA printing. Previous work in gravity aligned bulk structures is being used to develop the process required to create NGA bulk structures. This background consists of elements such as multi-pass depositions, bead overlaps, start-stop interfaces, and material filling. Process performance is determined based on weld bead consistency, part dimensional accuracy, and minimizing build defects. Final material characterization including microstructures and mechanical properties will be conducted on the printed geometry and compared to characterization of samples from the development process.
BULK NGA PATH PLANNING PROCESSES: Advancement in creating bulk walled geometry capabilities is a necessary integration into UT’s previously developed path planning program. NGA printing is an existing capability of the software but has only been applied to thin wall, single pass applications in low carbon steel. Further exploration of NGA robot trajectories to accommodate bulk features with exterior contour paths, subsequent infill patterns, overlapping beads, and side walls is required for modification to the original software capability. These aspects of the software must be optimized with respect to melt pool motion and shape to minimize form errors and build defects. Path plans designed for thin-walled NGA structures provide a solid baseline when tool pathing for thick-walled NGA structures. Bead behavior is expected to be similar yet modified in NGA paths when transitioning from thin to thick walls with the addition of multiple passes and/or weave pattern width. Another consideration is ensuring proper heat input to the base plate. Pulsed GMAW is used on the base layer of thin wall structures to ensure proper heating of the base plate but may be rendered irrelevant since multiple passes or weave patterns provide increased heating.
IMPLEMENTATION OF IN-SITU MELT POOL MONITORING: Work has also been started to monitor NGA melt pools in-situ with an eye toward future process control. Multi-mode sensing is conducted via optical imaging, infrared thermography, and acoustic monitoring to monitor the single-wire CMT welding torch. Additional performance elements of the welding torch are also monitored using a LabView integrated data acquisition program and includes torch tip position, orientation, arc current, arc voltage and wire feed speed. A Xiris XIR-1800 short wave IR camera is the primary source for melt pool monitoring. Thermal imaging will be of used to capture welding interpass temperature and deposited bead geometry via a FLIR A35 thermal camera. Weld pool stability will also be analyzed and recorded via acoustic monitoring. The intent is to relate process signatures with their corresponding weld bead characteristics to inform an operator of potential issues during material deposition. This knowledge will be applied to non-gravity aligned experiments, working towards real-time feedback control of contours, overlaps, and start-stop positions.
RESULTS: Weld test beads were deposited and adjustments to printing process parameters were made to determine proper voltage, current, wire feed speed, CTWD and tool path speed. After base layer process conditions were determined, beads were stacked to determine layer printing process parameters. These process conditions were established as job numbers to be called during printing by the robot program. A diamond geometry was designed to show weld bead behavior in both acute and obtuse angled corners. The path plan was successfully printed in aluminum 2319 using the aforementioned process and sent for metallurgy to examine printed material microstructures and to quantify the presence of porosity.
CONCLUSIONS: Process parameters and tool paths have been defined and successfully applied for thin wall gravity aligned structures in aluminum 2319. It has been found that while thin wall geometries require pulsed GMAW for base layers this may not be required for thick-walled structures. On-going work is focused on printing a NASA defined thin-wall structure. Data acquisition from this print will be used to inform further thick wall geometry path plan development and subsequent NGA structures.