Design and manufacture of a functional 3D-printed Stirling engine: A case study involving bound metal deposition of 17-4 PH

B.D. Ellis, Q. Campell, M. Filiault, A. Scopel, K. Rooney
University of Maine,
United States

Keywords: additive manufacturing, bound metal deposition, design, manufacturing


Growing at approximately 21% per year, additive manufacturing (AM) has primarily been utilized in applications requiring short lead times, geometric complexity, reduced part weights, reduced part counts, enhanced functionality, or a combination of the aforementioned criteria. For example, biomedical applications benefit from AM’s ability to create complex geometry, whereas economically viable AM jet engine fuel nozzles are 25% lighter, contain 95% few parts, and have five times the useful life of similar subtractive manufactured jet engine fuel nozzles. Recent advances in AM machinery have resulted in fused filament fabrication (FFF) and bound metal deposition (BMD) processes, in which a metal-binder composite material is extruded to print a green-state part. The green-state part is then placed within a debinding fluid to remove the vast majority of the binder before the remaining metal part is densified via sintering. Although FFF/BMD machinery and raw material costs are expected to be significantly less than the costs for metal powder bed fusion machinery and raw materials, FFF/BMD processes are relatively new, meaning that relatively few engineers and designers understand how to design parts and functional assemblies utilizing FFF/BMD. This research addresses this problem by detailing the design and manufacture of a functional 3D-printed Stirling engine utilizing a Desktop Metals Studio system and 17-4 PH materials. The beta-type Stirling engine shown in Figure 1 is powered by the flow of heat from a heat source (not shown) to the working fluid via the hot-side heat exchanger. Expansion of the working fluid causes the piston to translate, which causes the flywheel to rotate. The working fluid is then cooled to the initial temperature by rejecting heat to the atmosphere via the cold-side heat exchanger. The Stirling engine consists of 11 AM parts, all of which contain design, engineering, and manufacturing challenges. These challenges include: (1) directionally-dependent and differential shrinkages (cf. Figure 2); (2) post-AM processing to obtain sufficient surface finishes and geometric tolerances to permit sliding at piston-cylinder and piston arm-flywheel knuckle interfaces; (3) fatigue resistance; (4) wear resistance; (5) reducing weight and costs; and (6) defining parts and printing processes to reduce the need for support material during printing. Presented results will include empirical data from process and material characterization studies; lessons learned during the design, engineering, and manufacturing of the Stirling engine; and an analysis of the costs and time required to manufacture the Stirling engine. This work is significant in that recently-commercialized FFF/BMD AM machinery will create new economically feasible design and performance domains. These domains need to be explored through work such as this to help companies determine how metal AM can be utilized within current or future applications.