In the past two decades, there have been significant advancements in the field of additive manufacturing (AM) of titanium alloy (Ti-6Al-4V) and other metallic components for aerospace applications.
Generally, metallic brackets used in spacecraft applications are machined from the bulk-rolled or extruded products of the specific alloy. Using an electron beam melting-based (EBM) additive manufacturing process, several Ti- 6Al-4V alloy brackets were produced, finish-machined, and tested to determine their mechanical properties compared to the bulk alloy.
Initially, the 3D CAD model of each of the bracket drawings was developed to include slightly excess surface layers for machining to a smooth surface finish. Multiple parts were built simultaneously in the available chamber size. Preliminary cost and benefit analysis was also performed to assess the merit of using AM for future spacecraft components. Under the same EBM processing conditions, multiple Ti-6Al-4V rod specimens were built to prepare the subscale test specimens for mechanical property measurements.
Results of these tests indicated that the average material properties were quite comparable to the wrought alloy. Each of the manufactured parts was inspected using a nondestructive test method to ensure the quality of the finished product. A few componentlevel destructive tests validated that the component had adequate margin of safety to survive operational load conditions.
Ti-6Al-4V is an attractive, lightweight material for spacecraft structures, as it provides an excellent combination of high strength, low density, high modulus, low coefficient of thermal expansion, and higher operational temperature than aluminum alloys. While spacecraft structures are mostly constructed from carbon/polymer matrix composites, titanium alloys are used for several brackets, fittings, propulsion tubing lines, and support tubes.
Metal AM, or direct digital manufacturing (DDM), is a layer-by-layer technique of producing 3D parts directly from its 3D CAD models. At the outset, it offers designers a unique tool to envision innovative and integrated designs, eliminating the iterative cycle of generating several versions of the drawings.
Metal AM processes include two key inputs: type of raw material, and the energy source to form the part. For example, the raw material could be in powder or wire form, and energy sources are generally either a laser or electron beam. Metal AM systems can be grouped into three broad categories: 1) powder bed, 2) laser powder injection, and 3) freeform fabrication. In a typical powder bed system, the laser or electron beam energy is directed on the powder bed (held in a vacuum or inert environment) to melt the powder to form the desired shape consistent with the 3D CAD model. In a laser powder injection system, the powder is fed through a nozzle and the laser beam melts the powder. The freeform fabrication AM processes include e-beam deposition of metal wire, ultrasonic consolidation of metal layers, and arc deposition of powder and wire. In this work, several Ti-6Al-4V parts were manufactured using the EBM process.
Typical composite structure of the spacecraft bus includes several aluminum and titanium alloy-based brackets used for primary and secondary loadbearing applications. Overall, quantities of the Ti-6Al-4V components with similar geometry are significantly lower than other aerospace applications. However, a few representative components were selected to assess the feasibility of fabricating thin-walled brackets and fittings.
Each of the as-deposited brackets had about a 2-mm buildup on all surfaces to allow for finish machining, consistent with the tolerances indicated in the drawings of the bulk-machined units. Driven by the severe mass constraints, most of the brackets are designed to minimum thickness to satisfy the worst-case load conditions. Figure 1 shows two of the brackets in the as-deposited and fully machined conditions. As-deposited components exhibited surface finish (Ra) of ~0.05 mm. A few experiments were conducted to evaluate the use of electropolishing to obtain an acceptable smooth surface finish with very limited success.
Results of tensile tests indicated that Z-orientation-processed tensile specimens do exhibit slightly higher tensile strength than the XY-orientation specimens; measured properties of EBMprocessed Ti-6Al-4V at 121 °C indicate ~10% reduction in strength, compared to RT; stress-strain plots show that EBMprocessed specimens exhibited about ≥14% elongation; and each set of tensile property data exhibited quite low coefficient of variation, suggesting uniform quality of as-deposited specimens.
As-fractured surfaces seem to indicate a localized region of incomplete melt or impurity in two of the tested specimens (XY-1 and Z-1). Presence of localized incomplete melt can be attributed to insufficient EB energy to create a consistent melt pool in the thick powder-bed layer (>100-150 microns). Prior to processing the parts, a few initial trials were done to optimize the layer thickness (the thinner the better), flatness, and number of passes so as to minimize any incidence of incomplete melt.
Each of the as-processed components was fully machined to the final dimensions to generate confidence in the viability of using metal AM for the spacecraft structure. Results of material property tests and mechanical testing of EBM Ti-6Al-4V components gave confidence in the quality and robustness of parts produced by additive manufacturing.
Results of material property tests, mechanical testing, and the quality control documentation of each EBM processing run gave the designers the confidence to insert the technology for the secondary support structure applications. Consequently, four sets of waveguide brackets were selected for use on the Juno spacecraft structure (see Figure 2). These brackets successfully endured the system-level tests, including vibration and thermal cycling.