Electrochemical Machining of Refractory Metals for Aerospace Applications

Hypersonic systems travelling above the speed of Mach 5 impart intense thermal stress on the airframe, requiring the use of refractory alloys to withstand the heat. The leading edges of hypersonic airframes and scramjet engine intakes are required to be sharp, smooth, and highly accurate to reduce drag and prevent unwanted shock waves due to manufacturing errors. Additionally, rocket engines, both for in-space maneuvering and primary propulsion, often utilize refractory materials to counter the high chamber temperatures while requiring thin-walled designs to reduce mass. Refractory materials are difficult to employ in high speed applications for two key reasons. First, the materials have challenging machining properties. Tungsten, for example, is naturally abrasive and prone to vibrations that damage cutting tools while niobium has a tendency to gall and tear, requiring secondary operations to improve surface finish and fatigue life. Second, refractory materials are often used in thin-wall applications where the contact forces in conventional machining can easily deform the part. Therefore, conventional machining of refractory materials, especially of thin-walled structures, is expensive, time-consuming, and prohibits the implementation of higher performance hypersonic and scramjet features. In response, Voxel proposes adapting Pulsed Electrochemical Machining (PECM) to process refractory alloys with thinner walls, sharper edges, and better surface finish than any existing manufacturing processes and thereby enable the manufacturing of higher performance hypersonic and scramjet engine components which are critical to national security. Through a partnership with Duke University, Voxel is developing an ECM electrode coating that allows machining of refractories without tool damage, thereby preserving all of the traditional ECM advantages for refractory metal. During this Phase II effort, we will 1) establish the expected lifetime of the coating under a range of process conditions, 2) optimize the ECM process for tungsten and niobium, and 3) demonstrate the process through fabrication of a scramjet engine leading edge.