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.

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.

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.

AFRL researchers are expanding the knowledge of microstructure-property relationships, corrosion and failure modes, and life-cycle benefits in friction-stir-welded materials. They are also developing databases and process specifications so that manufacturers employing these FSW tools can consistently achieve desirable and predictable properties, enabling FSW to be qualified for use in the manufacture of major structural assemblies such as reusable cryotank applications for space.

Besides the AFRL activity, automotive, aerospace, and shipbuilding companies are also vigorously pursuing FSW technology to join not only aluminum alloys but also steels and, more recently, titanium alloys. Research is rapidly progressing in such areas as novel tool design, process parameter optimization, and FSW process modeling. As a result of these advances, FSW could soon produce joints with mechanical properties better than fusion-welded or mechanically fastened joints and provide cost-effective methods for repairing defects in metal surfaces.

Dr. Kumar V. Jata, Dr. Lee Semiatin, Dr. Reji John, and Dr. Peter S. Meltzer (General Dynamics), of the Air Force Research Laboratory's Materials and Manufacturing Directorate, wrote this article. For more information visit http://www.afrl.af.mil/techconn_index.asp. Reference document ML-H-06-01.


Air Force Research Laboratory Technology Horizons Magazine

This article first appeared in the December, 2006 issue of Air Force Research Laboratory Technology Horizons Magazine.

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