Friction-Stir Welding Effects on Microstructure and Fatigue of Aluminum Alloy 7050-T7451 K.V. JATA, K.K. SANKARAN, and J.J. RUSCHAU Aluminum alloy 7050 was friction-stir welded (FSW) in a T7451 temper to investigate the effects on the microstructure and mechanical properties. Results are discussed for the as-welded condition (as-FSW) and for a postweld heat-treated condition consisting of 121 °C for 24 hours (as-FSW + T6). Optical microscopy and transmission electron microscopy (TEM) examination of the weld-nugget region show that the FS welding process transforms the initial millimeter-sized pancake-shaped grains in the parent material to fine 1 to 5 m dynamically recrystallized grains; also, the FS welding process redissolves the strengthening precipitates in the weld-nugget region. In the heat-affected zone (HAZ), the initial grain size is retained, while the size of the strengthening precipitates and of the precipitate- free zone (PFZ) is coarsened by a factor of 5. Tensile specimens tested transverse to the weld show that there is a 25 to 30 pct reduction in the strength level, a 60 pct reduction in the elongation in the as-FSW condition, and that the fracture path is in the HAZ. The postweld heat treatment of 121 °C for 24 hours did not result in an improvement either in the strength or the ductility of the welded material. Comparison of fatigue-crack growth rates (FCGRs) between the parent T7451 material and the as-FSW + T6 condition, at a stress ratio of R = 0.33, shows that the FCG resistance of the weld- nugget region is decreased, while the FCG resistance of the HAZ is increased. Differences in FCGRs, however, are substantially reduced at a stress ratio of R = 0.70. Analysis of residual stresses, fatigue- crack closure, and fatigue fracture surfaces suggests that decrease in fatigue crack growth resistance in the weld-nugget region is due to an intergranular failure mechanism; in the HAZ region, residual stresses are more dominant than the microstructure improving the fatigue crack growth resistance. I. INTRODUCTION line is desired. The height of the pin is slightly smaller than the thickness of the alloy plates that are being joined, so the COMPARED to many of the fusion-welding processes penetration of the pin into the work pieces stops as soon as that are routinely used for joining structural alloys, friction- the shoulder of the cylinder makes contact with the surface stir (FS) welding is an emerging solid-state joining process [1–12] of the work piece. The rotating pin (extending from the in which the material that is being welded does not melt cylindrical shoulder) produces the stirring action in the mate- and recast. Therefore, when alloys are friction-stir welded rial along the bond line and produces the required thermome- (FSW), phase transformations that occur during the cool chanical deformation. Frictional heating is produced from down of the weld are of a solid-state type. Due to the absence the interaction of the cylinder shoulder with the work piece of parent-metal melting, the new FS welding process is and the downward applied forging pressure. To produce a observed to offer several advantages over fusion welding. longitudinal weld, the work piece assembly is translated The benefits that stand out most are welding of difficult-to- relative to the shoulder and pin assembly. To produce an weld aluminum alloys such as the 7xxx series, better retention ideal defect-free weld, the revolutions per minute of the of baseline material properties, fewer weld defects, low cylinder shoulder-pin assembly, travel speed, downward residual stresses, and better dimensional stability of the forging force, and pin tool design have to be optimized. welded structure. Also, FS welding is an environmentally Although the development of FS welding technology to cleaner process, due to the absence of a need for the various make complex welds is proceeding at an extremely rapid gases that normally accompany fusion welding. pace, primarily due to the efforts of the industry, understand- A schematic of the FS welding assembly is shown in ing of the microstructural transformations that occur during Figure 1. The FS welding process uses a nonconsumable the welding process and of the postweld mechanical proper- pin made from a high-strength material that extends from a ties [5–13] has been slow. There has also been significant activ- cylindrical shoulder. The shoulder and the pin rotate at sev- ity in addressing the much needed modeling research, in eral hundred revolutions per minute. The work pieces that order to understand the weld- and parent-metal constitutive are to be joined are firmly clamped to the work table, and behavior, [14,15] heat transfer, and thermomechanical analysis the pin is plunged into the work pieces where the weld bond of FS welding. [9,16] The present research is aimed at understanding the micro- structural and mechanical-property changes brought about K.V. JATA, Senior Research Materials Scientist, is with the Materials and Manufacturing Directorate, Air Force Research Laboratory, AFRL/ by FS welding of a high-strength Al-Zn-Mg-Cu alloy, 7050- MLLM, Wright-Patterson Air Force Base, OH 45433. K.K. SANKARAN, T7451. T7451 is an overaged temper, specially developed Technical Fellow, is with the Boeing Company, St. Louis, MO 63166. J.J. by the aluminum industry to optimize corrosion resistance RUSCHAU, Research Engineer, is with the Materials Engineering Division, and mechanical properties, and is widely used by aircraft University of Dayton Research Institute, Dayton, OH 45469-0136. Manuscript submitted September 22, 1999. manufacturers. METALLURGICAL AND MATERIALS TRANSACTIONS A U.S. GOVERNMENT WORK VOLUME 31A, SEPTEMBER 2000—2181 NOT PROTECTED BY U.S. COPYRIGHT