Defense Tech Briefs - Friction Stir Welding of Aerospace Materials

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Friction Stir Welding of Aerospace Materials

Dec 01 2006

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Friction stir welding promises advantages over conventional joining processes.

AFRL scientists are studying a unique metal joining process— friction stir welding (FSW)—for building major structural assemblies. FSW is a solid-state welding process that forces a spinning tool along the joint line, heating the abutting components by friction and producing a weld joint formed by strong plastic mixing (stirring) of the two components’ constituent materials. FSW promises to be a highly efficient and cost- effective alternative to the conventional fusion welding routinely used for joining structural alloys on military and civilian aircraft.

Bright-field TEMs of the 7050-T7451 subgrain structure (left) and coarsened precipitates in the grain interior and along the grain boundaries and PFZ of a friction-stir-welded joint (right)

Some of the important advantages FSW offers over fusion welding include the ability to weld structural aluminum alloys (particularly alloys in the 7xxx series), better retention of baseline material properties, fewer welding defects, lower residual stresses, and improved dimensional stability of the welded structure. The material that flows around the tool undergoes extreme levels of plastic deformation, and a very recrystallized grain structure forms in the center of the weld. This region of the weld, commonly referred to as the nugget zone, is part of the weld’s heat-affected zone (HAZ). The surrounding material, which constrains the nugget metal and undergoes deformation via the spinning tool’s passage, comprises the remainder of the HAZ and experiences much lower plastic strains. Because the FSW process does not melt or recast the welded material, microstructural material transformations occur during the weld’s cooldown— essentially taking place in the material’s solid state.

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