FSW may also produce significant economic advantages. The process joins aluminum alloys fairly rapidly— about 4 mm/sec—with low heat input and without the costly shielding gases and filler materials required in fusion welding processes. The aerospace industry also uses substantial quantities of fasteners to join metallic structures— literally millions of fasteners in fabricating a large cargo or passenger aircraft. Thus, eliminating fasteners in aerospace structures by incorporating FSW joints would provide manufacturers considerable cost and weight savings.

Researchers from AFRL’s Metals, Ceramics, and Nondestructive Evaluation Division friction-stir-welded a number of aerospace aluminum alloys, including 7050-T7451, an alloy widely used in military and commercial aircraft manufacturing, to assess the effects of the process on microstructure and mechanical properties. Optical microscopy and transmission electron microscopy (TEM) examination of the welded joint’s weld-nugget region showed that FSW transforms the initial millimeter-sized, pancake-shaped grains to fine, 1-5 μ dynamically recrystallized grains. The TEM examination also demonstrated that the FSW process redissolves the strengthening precipitates in the weld-nugget region. Furthermore, in the HAZ, the FSW process preserved the initial grain size and increased both the size of the strengthening precipitates and that of the precipitatefree zone (PFZ) by a factor of five (see figure). The AFRL team was the first to explain the continuous dynamic recrystallization process in friction stir welds.

The team also performed a series of mechanical tests on friction-stir-welded aluminum alloys. To stabilize the welded material, the test samples first underwent a postweld heat treatment (120°C for 24 hrs to create an as-welded [as-FSW] +T6 temper). As expected of any weldment, tensile specimens loaded transverse to the weld direction exhibited a slight reduction in strength level and an elongation in the as-FSW condition and also revealed that the fracture occurred in the HAZ.