| Abstract Scope |
INTRODUCTION: Nickel aluminum bronze (NAB) is a complex alloy commonly selected for marine applications due to its cavitation strength and bio-fouling resistance. Historically a cast alloy, recent work has explored wire-arc additive manufacturing (WAAM) of NAB for large scale structures using cold metal transfer (CMT) welding. By utilizing this process, long lead times for initial castings can be avoided as a near-net-shape fabricated component can be machined into the final product. This can reduce waste and logistical challenges associated with material acquisition, as well as allowing for geometric complexity not possible with cast components.
APPROACH: Our process combines nine degrees of robotic freedom with five-axis machining capability contained within a hybrid manufacturing cell that also includes robotic structured light metrology. A near-net-shape component is fabricated layer-wise in nickel aluminum bronze on a mild steel substrate using a Fronius CMT process, which is then evaluated using a GOM ATOS-Q structured light scanner. This 3-dimensional digital representation is then evaluated for part distortion. It also serves to inform toolpath generation for 5-axis machining utilizing a HAAS UMC750 machining center. This process can be repeated iteratively until the final part geometry is realized.
To accomplish this in NAB, process development started with the exploration and verification of parameters informed by literature descriptions of welding. Initial development was performed using gas metal arc welding (GMAW) deposition using a Miller Pipeworx 400 manual welding power supply. During this development, it was discovered that bronze, stainless steel, and mild steel base substrates could be used, with the latter being chosen for its favorable cost and availability. Due to the reciprocating nature of the wire feeding profile of CMT and its ability to produce higher material deposition rates at reduced heat input, initial GMAW parameters found in literature proved inadequate in the WAAM development space. These parameters were intended for traditional joining processes and/or the repair of materials, and it was found that heat input/management, shielding gas coverage, and contact-tip-to-workpiece-distance (CTWD) were critical to a successful WAAM part in NAB.
Due to the layerwise nature of WAAM, reduced heat input during deposition is required, with the exception of the initial layer. This is due to the reduction of the cross-sectional area of the substrate as successive layers are produced when compared to traditional joining processes. In addition to heat input, interpass temperature was also found to be an integral part of the NAB process to ensure proper material cooling and not overheat the printed structure as additional layers are deposited. While literature has indicated 150-200⁰C as a favorable interpass region, our work showed ~100⁰C as an ideal target, and this value was used to inform the dwell time between print layers.
Due to the omnidirectional nature of the movement of a robot-mounted GMAW torch, the torch does not maintain a constant orientation with the direction of travel. Because of this, a symmetrical shielding gas profile is ideal to maintain an inert atmosphere around the deposition site in any direction. Initial test deposits were done with a dual-wire torch being operated in a single-wire configuration which produced an asymmetrical gas profile and inconsistent material deposits. This system was later replaced with a traditional single-wire torch and it associated shielding gas nozzle to improve deposited material quality.
After initial parameter development was completed, geometries were developed to define an average layer height for the chosen parameters, as well as to explore geometry-based material deposition challenges, such as overhangs, parallel walls, and acute corners. This knowledge culminated in the final fabrication of a large, thin-walled part with 350 layers and a total build time of ~30 hours printed intermittently over a three-day period. Preliminary mechanical properties have proven consistent with prior WAAM NAB efforts.
FUTURE DEVELOPMENT: Future development is necessary to focus on automating and refining printing process parameters, such as interpass temperature control and contact to workpiece distance. Work is also necessary to explore non-gravity aligned deposition techniques.
CONCLUSIONS: Process conditions necessary for the successful deposition of stable and high-quality nickel aluminum bronze structures have been developed. With this manufacturing technique, exemplar geometries have been demonstrated and culminated in the ability to fabricate a large, NAB thin-walled part consisting of ~350 layers over a 30hour print cycle using intermittent print cycles. |