NDE of Friction Stir Welds in Aerospace Applications

Friction Stir Welding (FSW) is a solid state joining process, which utilizes a cylindrical, shouldered pin tool with a radiused tip that is rotated and plunged into the weld joint. Frictional heating beneath the shoulder, and surrounding the pin tip causes the material to plasticize, intermix and consolidate into a weldment without melting the parent material. FSW in aluminum alloys has many advantages such as low distortion and shrinkage, excellent mechanical properties, and no porosity. However, the propensity of the FSW process to create detrimental defects does exist, and is dependent on FSW parameter limits and controls. Inspection processes for FSW must also be selected and implemented concurrent with the new weld process. This paper describes the efforts by Lockheed Martin and NASA to find proper NDE techniques for detecting and characterizing the anomalies that may be caused by operating outside the envelope of optimized FSW parameters. Potential defects are identified and the results of the exploration of numerous NDE techniques including visual, liquid penetrant, multiple ultrasonic methods, eddy current and conductivity are discussed. Friction Stir Welding Developing and implementing new processes to enhance the performance, reliability and safety of aerospace hardware is a primary ongoing objective for both government and industry programs. TWI in Cambridge, UK, invented friction Stir Welding [1] in the early 90's and Lockheed Martin began its development activities in 1995. FSW development continued at Marshall Space Flight Center (MSFC) through 2001 for various NASA applications including man-rated flight hardware. Friction Stir Welding is accomplished with both monolithic and multiple piece pin tools rotating at several hundred RPM and traversing a square butt weld joint of the same design configuration used for fusion welding. A plunge load is imparted through a spindle, driven by a FSW machine and reacted against a backside anvil. Frictional heating under the pin tool and around the pin tip generate sufficient heat to locally plasticize the aluminum alloys to be welded. Tool rotation during the FSW process imparts a material flow in three dimensions to the plasticized weldment, causing complete mixing of the alloys. Consolidation of the weldment occurs via an extruding/forging action under the pin tool shoulder as the pin tool is traversed down the length of the weld. See Figure 1 for a schematic representation of the FSW process. FSW enjoys a number of advantages over fusion welding processes including the elimination of welding consumables such as gas, filler wire and electrodes. As a joining process based on frictional heating due to mechanical work, FSW has only three primary weld variables to control. These are plunge force, rotation speed and weld travel speed.