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Controlling Force and Depth in Friction Stir Welding

The proportionality between penetration force and penetration depth is exploited.

Feedback control of the penetration force applied to a pin tool in friction stir welding has been found to be a robust and reliable means for controlling the depth of penetration of the tool. This discovery has made it possible to simplify depth control and to weld with greater repeatability, even on workpieces with long weld joints.

Prior to this discovery, depths of penetration in friction stir welding were controlled by hard-tooled roller assemblies or by depth actuators controlled by feedback from such external sensors as linear variable-differential transformers or laser-based devices. These means of control are limited:

  • A hard-tooled roller assembly confines a pin tool to a preset depth that cannot be changed easily during the welding process.
  • A measurement by an external sensor is only an indirect indicative of the depth of penetration, and computations to correlate such a measurement with a depth of penetration are vulnerable to error.

ImageThe present force-feedback approach exploits the proportionality between the depth and the force of penetration. Unlike a depth measurement taken by an external sensor, a force measurement can be direct because it can be taken by a sensor coupled directly to the pin tool. The reading can be processed through a modern electronic servo control system to control an actuator to keep the applied penetration force at the desired level. In comparison with the older depth-control methods described above, this method offers greater sensitivity to plasticizing of the workpiece metal and is less sensitive to process noise, resulting in a more consistent process.

In an experiment, a tapered panel was friction stir welded while controlling the force of penetration according to this method. The figure is a plot of measurements taken during the experiment, showing that force was controlled with a variation of 200 lb (890 N), resulting in control of the depth of penetration with a variation of 0.004 in. (0.1 mm).

This work was done by Glynn Adams, Zachary Loftus, Nathan McCormac, and Richard Venable of Lockheed Martin Corp. for Marshall Space Flight Center.

Title to this invention has been waived under the provisions of the National Aeronautics and Space Act {42 U.S.C. 2457(f)}, to Lockheed Martin Corp. Inquiries concerning licenses for its commercial development should be addressed to:

Lockheed Martin Corp.
Attn. General Counsel
P.O. Box 29304
New Orleans, LA 70189
Telephone No. (504) 257-4786

Refer to MFS-31310, volume and number of this NASA Tech Briefs issue, and the page number.