This invention establishes a process to define a viable self-reacting friction stir weld (SR-FSW) schedule (parameter settings) for a given material combination. The focus of this process results in a SR-FSW schedule that is insensitive to intentional changes or normal process variation in pin force at a given rotation and travel speed. Viable is defined as a weld schedule that is usable in a production environment and is able to accommodate normal production variations.

Traditionally, a design of experiment process was utilized to optimize the weld schedule settings (rotation speed, travel speed, and pin force, plus interactions) versus the weld tensile strength. In some cases, pin force was held constant during experimentation, and only the rotation speed and travel speed settings were varied. Additionally, previous SR-FSW development did not focus on determining at what point a weld schedule (rotation speed, travel speed) was insensitive to large changes in pin force.

Pin force insensitivity is a characteristic that makes the weld schedule more robust in a production environment (joint fit-up, chillbar contact, product form, etc.). Additionally, this new process is quicker and more effective (smaller panel count and development time) than historical weld development methodologies (design of experiments) utilized.

The methodology was to:

  1. Select a series of parameters that bounds the “welding range” (combination of travel and rotation). Each combination of travel and rotation should be run at a high and low pin force. Welding range is defined as set of parameters where the panels are physically joined in a complete weld regardless of internal defects.
  2. Run an experiment with selected parameters (weld, non-destructive evaluation, mechanical testing, and macro evaluation).
  3. For each travel speed, plot the minimum tensile values as a function of RPI (rotation/travel speed).
  4. For each RPI, point-distinguish between the high and low pin force.
  5. Evaluate data against the expected behavior of the effect of pin force, switching from high force providing the higher strength to low force providing the higher strength as RPI increases.
  6. Perform iterative steps to locate the RPI for a given travel where the minimum strength is pin-force-independent.

This work was done by Joseph Murphy, Jerry Majors, Jason Calmes, and Richard Kinmonth of Lockheed Martin for Johnson Space Center. For further information, contact the JSC Technology Transfer Office at (281) 483-3809. MSC-25480-1


NASA Tech Briefs Magazine

This article first appeared in the October, 2014 issue of NASA Tech Briefs Magazine.

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