Z. S. Loftus, Lockheed Martin Space Systems, New Orleans, LA

Summary:

The feasibility of friction stir welding (FSW) Beta 21S titanium in the annealed condition has been investigated. Determinations of appropriate pin tool material, optimum processing parameters, tooling challenges, and environmental requirements were also made. Welding was performed at the University of South Carolina and Lockheed Martin Space Systems – Michoud Operations using MTS friction stir welding equipment.

Because of the relatively high melting points and reactivity of titanium alloys, FSW of titanium presents a number of challenges that are not issues when welding lower melting point materials such as aluminum and magnesium. Due to the temperature at which the process must operate, the most critical issue is the durability of the pin tool material in the welding environment. Additionally, titanium is a reactive alloy that must be shielded from interstitial contamination at the welding temperature. Finally, an appropriate anvil material (low thermal conductivity and good hot strength) must be chosen. A description of the approach taken to successfully overcome these challenges is presented.

It was determined that Beta 21S could be successfully friction stir welded within a relatively large process window. The annealed material in the as-welded condition can also be post weld heat treated to obtain higher strength at the expense of ductility. Experimental results include transverse tensile properties, microhardness, and cross-sectional metallography. Recommendations are given for future process developments in FSW of titanium which include the near-alpha and alpha-beta alloys in addition to the beta alloy presented.