Controlling Abnormal Grain Growth In Friction Stir Welded Al-Li 2195 Spun Formed Domes

NASA desires to create a net-shaped dome for cryogenic fuel tanks from Al-Li 2195 alloy using spin forming. This application requires large plates with diameters greater than 18 ft. Commercial plate is not available in sizes large enough to fulfill this requirement, but large blanks can be fabricated by joining smaller plates with friction stir welding (FSW).

Certain technical challenges must be overcome when spin forming Al-Li alloy 2195 with FSWs. FSWs that have undergone solution heat treatment (SHT) can develop abnormal grain growth (AGG) in the nugget zones, which significantly reduces their mechanical properties. AGG can also arise during post-weld annealing and is sensitive to plastic deformation of the weld.

NASA–Marshall Space Flight Center used two approaches to mitigate AGG. First, the weld parameters were changed to increase grain size and reduce the thermodynamic forces driving AGG. Second, the onset of AGG was delayed by increasing the heating rate, simultaneously activating many recrystallization nucleation sites, which resulted in the formation of a finer grain structure during SHT.  This study also investigated how AGG might be affected by forming degree.

Different heat inputs were achieved by conducting conventional friction stir welding (C-FSW) at three tool rotation speeds.  Low heat input produced a finer grain structure, but AGG occurred throughout the weld nugget zone during a post-weld anneal at 800 °F. The weld nugget zone formed coarser grains when FSW was performed at higher rpm's, achieving a more stable microstructure during post-weld annealing.

Increasing the heating rate to the SHT temperature resulted in the formation of a finer and more uniform grain structure during SHT.  AGG behavior was also dependent upon both grain size and plastic strains. A small-grained microstructure developed in FSWs that had been deformed to 39%, with grain size refined even further when SHT was conducted in a salt bath that accelerated the heating rate. A significantly coarser grain size developed in FSWs that had been deformed to 15%, for which fast heating in a salt bath was less effective in refining grain size.