Microstructural factors governing hardness in friction-stir welds of solid-solution-hardened Al alloys
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INTRODUCTION
ALUMINUM (Al) alloys are roughly classified into precipitation-hardenable alloys and solid-solution-hardened alloys. Friction-stir welding creates a softened region around the weld center in the precipitation-hardened Al alloys.[1–8] The present authors[7,8,9] have examined the hardness profile associated with the microstructure in a friction-stir weld of precipitation-hardened Al alloy 6063-T5. The hardness profile was found to be strongly affected by precipitate distribution rather than grain size in the weld. A similar effect of precipitates on the mechanical properties has been found in friction-stir welds of some other precipitation-hardened Al alloys, such as 7075-T651,[10] 6061-T6,[2,3] and 7050T7451.[11] Such softening is caused by dissolution and growth of strengthening precipitates during the thermal cycle of the welding. Generally, friction-stir welding does not result in softening in the solid-solution-hardened Al alloys.[1,12,13] Some studies[14,15,16] have suggested that microstructural factors govern hardness in the welds of the solution-hardened Al alloys. Svensson et al.[14] observed the microstructures of the stir zone and the base material in the weld of Al alloy 5083-O by optical microscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM) and then correlated the mechanical properties with the microstructure. The hardness profile was roughly uniform in the weld. The stir zone had fine equiaxed grains with a lower density of large particles (1 to 10 m) and a higher density of small particles (0.1 to 1 m). They concluded that the hardness profile mainly depends on dislocation density, because the main YUTAKA S. SATO, Research Associate, SEUNG HWAN C. PARK, Graduate Student, and HIROYUKI KOKAWA, Professor, are with the Department of Materials Processing, Graduate School of Engineering, Tohoku University, Sendai 980-8579, Japan. Manuscript submitted June 19, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
hardening mechanism for this alloy is strain hardening. On the other hand, Murr et al.[15] performed Vickers hardness (Hv) tests and TEM observations of a friction-stir weld of Al alloy 1100 with a microdendritic structure and a dislocation cell structure. The hardness profile across the stir zone was roughly homogeneous. The TEM micrographs showed that the grain size of the stir zone was close to the dislocation cell size of the base material. They explained that the small difference between the recrystallized grain size and the dislocation cell size resulted in the homogeneous hardness profile in the weld. Recently, the present authors[16] applied frictionstir welding to an equal-channel angular (ECA) pressed Al alloy 1050. Friction-stir welding reproduced fine grains with a grain size of about 0.61 m in the stir zone, which achieved suppression of the hardness reduction. These studies have suggested that the hardness is mainly affected by the grain size in friction-stir welds of solid-solution-hardened Al alloys. Consequently, the relationship betw
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