Meeting Out-Of-Production And Low Volume Aircraft Casting Demand Through The Use Of Additive Technologies

Cast magnesium parts are used extensively in aerospace applications owing to their high strength-to-weight ratios, intrinsic load damping capabilities and superior bending stiffness / isotropic structural characteristics. Although a multi-billion dollar industry worldwide, that segment of the industry dedicated to adherence to the quality standards of aerospace is quite small in comparison. The technology utilized to create patterns, cores, and tooling attendant to the fabrication of these castings has remained essentially unchanged for centuries, relying often upon artisanship in both tool fabrication and pouring execution. These pattern fabrication techniques are a critical and integral step of the casting process, and are typically the process stage with the longest associated lead times. Traditional hand fabrication techniques will necessarily introduce considerable expense into process total cost. Patterns are produced by a diminishing pool of skilled trade resources, and thus impose a lack of agility to new product development programs. Innovative additive technologies may be utilized to streamline the casting process, to facilitate the shortening of tool development times, and enable the accurate simulation of tool development. This in turn can have the affect of reducing delivery time to product end users engaged in both prototype development, reinforce aftermarket support of legacy aircraft, and to "peak shave" current production demand. Attendant to this study, a cast magnesium gearbox subassembly was topographically laser-scanned and the results of this scan converted to a CAD model. Internal passages, or cores, were then modeled in, machined details de-featured, and the resultant model dimensionally verified. From this CAD model, a rigging model was produced. Digital simulations were conducted that examined molten material velocities, Reynolds numbers, liquid phase evolution profiles, and internal temperature gradients. These factors, if not given due consideration, would compromise final product quality. Through stereolithographic technique, a three-dimensional disposable pattern was printed in segments using a specialized sand and foundry-grade resin and assembled. A first article pour was conducted, resulting in a cast product.