High Speed Friction Stir Welding for Assembly and Repair of Space Structures

Abstract

The Space Transportation System has been planned as an evolving system, implementing new technologies that increase safety and reliability of Reusable Launch Vehicles, and innovative materials and joining technologies have been developed at NASA Marshall Space Flight Center (MSFC).In the framework of the Friction Stir Welding upgrade of the Space Shuttle External Tank, it has been spent an effort in defining a simple and reliable process, that take advantage of the high rotational speed of the pin tool. The author conducted at NASA MSFC a feasibility study for the High Speed Friction Stir Welding (HSFSW) process and the research consisted of three main phases: 1. theoretical modeling of the HSFSW process. 2. design and manufacturing of the HSFSW machine. 3. DOE and lab analyses of the experimental joints. The High Speed FSW process has been modeled implementing the “Rotating plug model” developed at NASA, and based on the Plastic-Analytic approach, which consist in superposing two flows : a primary flow comprising a plug of metal rotating with the pin tool and shearing over a cylindrical surface, and a secondary flow driven by threads on the pin and resembling a ring vortex around the pin. The model has been tested during phase 3 with some modifies to validate it in the high speed rotation condition: Parametric evaluation of conditions for surface slip. Flow stress at high temperatures and influence of the material characteristics (Al2219-Al2195). Once determined the main process parameters it has been designed the FSW system. It consisted in three main sub phases : 1. motor characteristics and machine design 2. process control 3. pin tools design. The factors that influenced this phase has been requirements imposed for Space application and for a technology transfer. Essential characteristics of the High Speed FSW machine are : light weight, low power consumption, high rotational spindle speed, forces and torque equilibrium of the whole system, easy to handle. The third phase consisted in planning the tests, according with the DOE method, considering the theoretical model values and the machine parameters. The main parameters are : Material (composition, thickness, surface) Pin tool (length, diameter, taper angle) Shoulder (length, diameter, taper angle) Machine (travel speed, RPM, angle, penetration) Finally lab tests has been conducted to evaluate the quality of the joints (Macro pictures, Xray, Ultrasonics). The standard NASA testing procedure for welded Space vehicles components has been adopted and the results showed excellent microstructural and mechanical properties of the FSW welded joints.The feasibility study was successfully completed, proving a new technology, with possible application in Space. Long duration space flights requires a repair technique in Space and compared to all the other joining techniques available, the High Speed Friction Stir Welding appears to be the best one, for its simplicity , reliability , quality and safety. The material is new and original, as part of a research conducted at MSFC, NASA, and its content has not been presented to previous meetings. The author take responsibility to deliver the paper in Bremen.