Design of a Helicopter Structural Member Using Friction Stir Welding

Friction Stir Welding (FSW )is evaluated in this paper to determine if it can be used to produce high strength, low weight, and low cost structural airframe components for helicopters. Various FSW I-Beam Structural members were designed and an optimized design was chosen based on the above criteria, and ultimately form, fit, and function in a rotorcraft environment; all designs assumed fabrication from standard aluminum 7075-T6 sheet stock and 7075-T6511 extruded shapes. Two I-Beams measuring 24” X 5.5” X 2” were FSW utilizing 7075-T6 aluminum sheet for the web sections, while the caps and vertical stiffeners were made from 7075-T6511 “T” and “C” extrusions, respectively. The test beams were subsequently processed and finished using standard chromic acid anodize and primed using MIL grade epoxy. The two fabricated beams were subjected to static and fatigue evaluations and the results were compared to previous 4-PT Bending fatigue testing conducted on I-Beams fabricated from Riveted Sheet Metal (RSM) and Thin Walled Machined (TWM) aluminum plate. While, fatigue testing of the FSW I-Beams revealed similar results to the Riveted I-Beam, a serious reduction in fatigue life was exhibited when compared to the TWM I-Beam structure. Because the failure origin of the first test beam was at a FSW keyhole located at a vertical stiffener, keyholes in the second test beam were drilled, reamed, and filled with a fastener, changing the failure origin to a row of Hi-Lok® fasteners in the bottom cap; the TWM I-Beam displayed similar failure modes. Although the FSW beams exhibited lower fatigue strengths than the TWM beams, failures did not occur at the FSW. Future improvements may include repositioning keyholes to the neutral axis or filling keyholes to reduce stress concentration, making upper and lower T-caps thicker and making C-channel vertical stiffeners integral to caps (via FSW). Preliminary developments and results on FSW structural members are very promising. In lieu of MIL-HDBK type data for Friction Stir Welded aluminum alloys and joints, further design and development is warranted to optimize the FSW process for manufacturing structural members for rotorcraft.