Friction Stir Welding of titanium-aluminum-vanadium alloy sheet and plate for aerospace structures.

摘要

Friction Stir Welding of aluminum alloys has been studied extensively and applied successfully in a variety of applications and industries. Welding of high strength-high temperature materials, such as titanium, is relatively new. Friction Stir Welding of titanium alloys is of particular interest in the aerospace industry for the fabrication of near net shape structures. The primary purpose of this research was to develop the process for Friction Stir Welding titanium Ti-6AI-4V alloy sheet and plate material. Process conditions capable of producing defect free joints in thicknesses ranging from 3 mm sheet to 12 mm plate were successfully identified. Most of the weld nuggets possessed a microstructure indicative of process temperatures which had exceeded the beta transus of the material. The weld grain size increased with joint thickness, due to the higher processing temperatures and slower cooling rates. The effect of process parameters on microstructure, temperature and material flow were also evaluated. It was found that the process parameters directly influenced the weld temperature, process loads, microstructure and even superplastic performance of the joint. Spindle speed controlled the peak weld temperatures, while the feed rate dictated exposure times. High spindle speeds and/or low feed rates result in higher peak weld temperatures, lower process loads, larger grained microstructures, less superplastic performance and potentially the formation of voids. Conversely, low spindle speeds and/or high feed rates led to lower temperatures, higher process loads, finer grained microstructures, tool wear and lack of penetration. A theoretical process model was also proposed in this dissertation. This model was validated by successfully predicting experimentally observed trends. Tensile properties and crack growth behavior of the joints were comparable to the as-received material properties while the fatigue and fracture toughness performance was within 20% of the baseline. Post weld heat treatment temperatures above 870°C led to improved fatigue performance and elongations, with only a small reduction in strength. The capability for industrial application of this process was also demonstrated by producing complex joint configurations, which are needed for parts in most structural aerospace applications.