Closed-Loop Control of the Thermal Stir Welding Process to Enable Rapid Process / Part Qualification

Thermal Stir Welding (TSW) provides advancement over the more conventional Friction Stir Welding (C-FSW) process because it separates the primary processes variables thereby allowing independent control of metal stirring and forging from the stir zone temperature. However, the feedback for precise control of the stir zone temperature, and hence the process parameters to sustain that temperature within a narrow range, does not currently exist on the TSW machine at the NASA Marshall Space Flight Center (MSFC). At present, the current state of the art for the selection of process parameters for both TSWing and C-FSWing parameters is highly empirical and by nature is based on phenomenological knowledge. In response to this need, Keystone is proposing this Phase II STTR project to demonstrate the feasibility of closed-loop control of the TSW process and to enable the establishment of a analytically derived processing map to accelerate process understanding and selection of parameters for a given material and pin tool design. The close-loop control system will enable sustainment of a steady-state temperature at the stir rod as a function of spindle RPM and the travel velocity for a given z-axis loading and stir rod design. Use of this theoretically derived processing map will provide guidance in the optimization of the process parameter domain for solid-state welding of a given material. This capability will in turn enable rapid process qualification of the TSW process and components produced by the process.

The advancement of stir welding technologies has application well beyond NASA applications. It is envisioned this technology will become important to the Navy for ship building applications, as well as to the larger commercial ship building industry for construction of ship hulls and superstructure. The process will also be useful to fabrication of large tankage and piping for the chemical processing and transportation industry. Potential NASA applications for the TSW technologies we propose developing and implementing would be solid and liquid rocket motor casing, liquid rocket nozzle extensions, and other high temperature components; more specifically, a nozzle skirt extension for the J2X engine and 4130 steel external casing for the NASA sounding rocket.

The proposed technology will enable the creation of processing maps to characterize the solid-state joining of metals by thermal stir welding. The closed-loop temperature control system (applicable to other types of stir welding) will allow higher fidelity, stir welding of alloys of interest to NASA for manufacturing space craft and heavy lift vehicles. The analytical simulation of the stir welding process will enable more rapid and less costly formation of processing maps of a given material. The control system and analytical simulation combined will enable formation of a processing map for Haynes 230 (the alloy of choice by NASA for the J2X nozzle extension) and establishment of a rapid process qualification methodology for thermal stir welded components. Keystone also recently demonstrated over 60-feet of solid-state titanium welds for evaluations and for build of a partial section of a littoral ship structure built by Litton Industries. With the success of this demonstration ONR is considering extension of the project to include further mechanical/structural testing of the hull and superstructure section. Is it this type of foundational work, enabled by the NASA TSW process, Keystone anticipates emergence into commercial ship building applications for the TSW process.

This is early/mid-stage (TRL 5) — the most realistic path in is NASA SBIR/STTR, which funds small businesses and research institutions to develop technology aligned with NASA's needs (equity-free, phased funding). Check whether a current SBIR/STTR solicitation topic overlaps with this project's technology area, or contact the project directly (above) to ask.

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