Master of Science in Mechanical Engineering (MSME)
Friction Stir Welding (FSW) has been adopted as one of the major welding processes for joining Aluminum. Many aerospace and marine structures are welded through this novel FSW processes currently. The Lockheed Martin Space Systems (LMSS), Michoud Operations, in New Orleans is continuously pursuing Friction Stir Welding technologies in its efforts to advance fabrication of the external tanks of the space shuttle. Recently, a reduction in mechanical strength (embrittlement) has been observed especially in self-reacting (SR) friction stir welds. This strength reduction was attributed to Residual Oxide Defects (ROD) but the exact reasons for this type of behavior needed to be investigated. NASA-Lockheed Martin provided the FSW samples of Aluminum 2195 and 2219 and is interested to find out the existence and consequences of ROD from these samples. The existence of ROD could compromise the structural integrity of the external tanks and could result catastrophic brittle failures. It is also found that certain FSW processing parameters would yield these reduced mechanical properties. The strength of FSW Aluminum panels generally decreases with increasing tool travel-rate, decreasing rotation speed, and offset of the weld seam to the retreating side of the FSW tool. The microstructure of welds exhibiting these strength reductions as well as welds that behaved as expected are examined to determine microstructural effects of processing parameters. Both SEM and TEM works have been conducted on these self-reacting FSW specimens provided by NASA. Scanning Electron Microscopy (SEM) shows that these weld conditions are accompanied by large è precipitates along the grain boundary for both Al- 2219 and Al -2195. Transmission Electron Microscopy (TEM) also shows the precipitates to be è - particles (Al2Cu), and intermetallics (Al7Cu2Fe) in the Al-2219, and T1 (Al2CuLi) and TB (Al7Cu4Li) particles in the Al-2195. The large size and heavy non-linear distribution of these precipitates, especially on the advancing side of the weld-seam may influence these properties. There appears to be no signs of Residual Oxide Defects in the micrograph samples analyzed in this study. A more complete understanding of these phenomena is necessary to ensure consistent and predictable weld properties.
Document Availability at the Time of Submission
Release the entire work immediately for access worldwide.
Taylor, David Eric, "Effects of processing parameters on the embrittlement of self reacting friction stir welds" (2009). LSU Master's Theses. 57.