Life Estimation of SR-FSW Pin Tools for NASA Application

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Self-reacting friction stir welding (SR-FSW) is one of the processes used in the fabrication of the liquid oxygen and hydrogen tanks that comprise NASA’s Space Launch System’s (SLS) rocket. The SR-FSW tool assembly both clamps and stirs the material by means of rotation and translation through the work piece being welded. One weld tool component of interest is manufactured from MP159, a cobalt-based alloy with excellent mechanical properties at elevated temperatures. Failure of the welding tool during production would result in downtime and require structural mitigation of the rocket body. The repaired weld produced may then exhibit less than desirable mechanical properties. Given the Certified Weld Procedure (CWP) used during production, the lineal inches of weld in a single production “pass”, and the requirement for frequent tool replacement, a weld pin-tool failure during production would result from low cycle fatigue (LCF) damage accumulation. Unfortunately, there is limited data available in the literature for this material’s fatigue behavior at elevated temperatures. This work has generated a statically significant strain life curve of MP159 at 800°F with fully reversed loading at a frequency of f = 1 Hz. Subsequent tests have produced data to support an exploratory strain life curve at a frequency of f = 2.4 Hz. Additionally, a combined kinematic and isotropic hardening constitutive model has been calibrated to the materials elevated temperature cyclic response. The constitutive model is used to determine the history-dependent deformation response of the tool at critical geometric locations as a function of loading conditions, boundary conditions and weld pin tool material evolution. Finally, a weld pin tool failure analysis engine has been implemented in MATLAB. This engine estimates the life of the weld pin tool as a function of accumulated damage resulting from any number of loading scenarios, or combined loading scenarios. Failure of the pin tool is predicted by use of Miner’s rule. The failure algorithm has the ability to combines damage from weld start up, steady state operation, and tool “pull out” as well as any combination of these individual operational components. The analysis engine may be used to determine safe operational conditions for the weld tool given weld procedures, or it may be used to redesign welding pin tool geometry to mitigate failures during production, thereby reducing production cost and schedule as well as ensuring the most structurally sound weld possible.

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