Friction Stir Processing (FSP) is derived from Friction Stir Welding (FSW), which was developed in 1991 at Cambridge, UK as an effective, inexpensive, solid state welding technique for aluminum alloys that are generally difficult to fusion weld. Researchers found that FSW greatly refined the microstructure of the material in the weld, which contributed to the increase in mechanical properties compared to the base material. Investment cast titanium is becoming more popular in the aerospace industry because large, cost affective, one-piece shapes can be formed to replace parts that were previously made by mechanically fastening several components together. However, areas of these cast shapes experience greater amounts of stress at the surface, which cause them to fail sooner than expected. Our research is interested in FSP of titanium in order to increase the mechanical properties in order to keep parts in service longer.

In order to understand FSP on the microstructure of Ti-6Al-4V, samples were sectioned with an abrasive water jet from the cast material and FSP region for metallographic analysis using characterization techniques such as OIM, SEM, and EDS. Analysis suggests that there was significant re-crystallization in the stir zone and to a lesser extent, in the transition zone. The heat affected zone showed a wide range of complex microstructures due to the thermal gradient across the processed zone. The stir zone, adjacent to the surface, showed extensive grain refinement forming 1 to 2 µm diameter equiaxed primary α grains from coarse β grains that were on the order of one millimeter. It is expected that the fine grain structure will be better for crack nucleation resistance. The transition zone, between stir zone and heat affected zone, showed a combination of deformed lamellae from the base material and finer grained α . The heat affected zone showed signs of recovery in the coarse lamellar microstructure. The coarse lamellar structure is beneficial to crack propagation resistance.