Abstract

Friction stir welding (FSW) is an advantageous solid state joining method for many different engineering designs. It was invented in 1991 by TWI and since that time a tremendous amount of research has been executed to understand and implement FSW in a myriad of materials in numerous industries such as aerospace, ship building, train, automotive, energy, agriculture and others. Most industries have strict weld inspection processes to validate a weld before it is put into service. One of the main weld characteristics that is measured is the size of any subsurface volumetric voids. The current standard to test friction stir welds for voids are ultrasound and/or radiography. Both of which elongate the value stream of a FSW part. To simplify the manufacturing process some researchers have focused their efforts to develop a new non-destructive evaluation (NDE) method for FSW that uses properties of the weld that can be measured during welding. One of the most useful properties that can be measured during the FSW process are forging forces. Since FSW is a thermo-mechanical process measured forces provide unique feedback that other measurable properties cannot. Since 2004 many different techniques have been developed to use welding forces to determine FSW quality. Most approaches use a training set of welds to calibrate the proposed technique and then have tested it on another set of welds. Very few have been applied their proposed technique across many different machines and welding tools. Thus, the application of such specialized NDE methods have been limited in industry. In addition, industry process can significantly vary. This variation would require a huge training set that could be too arduous to justify its application. These two points show that for industrial implementation of a novel NDE method would require generalization where it could be employed in various applications. Lastly, the notion that NDE process that use measured properties during welding is cheaper than traditional processes has not been verified. Within this dissertation a generalized NDE method was evaluated in three facets. First the savings of the NDE method for a high-volume application was verified via simulation. Second, the performance of the NDE method was quantified when applied outside of calibration. Lastly, the method was applied across multiple welding machines, aluminum alloys, FSW tools, thicknesses to validate that it can be applied to various welding environments. Within this work a key finding was that time domain force data is sufficient to determine weld quality and spectral calculations are not required.

Degree

PhD

College and Department

Ira A. Fulton College of Engineering; Mechanical Engineering

Rights

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