Precipitation sequence in friction stir weld of 6063 aluminum during aging
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I. INTRODUCTION
MOST of commercial aluminum (Al) alloys are strengthened by precipitation or solution hardening. Fusion welding of precipitation-hardenable Al alloys produces a fusion zone consisting of an as-cast coarse microstructure with solute gradients near the dendrite boundaries.[1–6] Solute segregation to the dendrite boundaries during solidification is unavoidable. In addition, strengthening precipitates are dissolved during fusion welding. These microstructural changes often lead to a significant deterioration of strength in the weld. Postweld aging to restore mechanical properties is not effective because of solute loss in the matrix introduced by the solute segregation.[3–6] Therefore, solution heat treatment and aging are needed to effectively improve the weld properties, but postweld solution heat treatment (SHT) is not practical for many applications of Al alloy fusion welds.[5,6] Friction stir welding is a new solid-state welding process invented at TWI (Cambridge, United Kingdom) in 1991.[7] It has enabled us to butt weld Al alloys, which are often difficult to fusion weld, without voids, cracking, or distortion. Recently, this process for Al alloys has captured the attention of the fabrication industry.[8] Friction stir welding does not produce as-cast coarse microstructure and solute segregation in the welds because it is a solid-state process.[9–15] However, softening occurs in the weld zone to some extent, since the density of strengthening precipitates is remarkably low in fine recrystallized grains around the weld center.[7–13] The density reduction deteriorates the weld properties around the weld center, but
postweld aging may improve the properties effectively. Mahoney et al. have shown an effect of postweld aging on tensile properties of friction stir welded 7075 Al.[10] They have reported that postweld aging produces an increase in the volume fraction of fine strengthening precipitates, which leads to the improvement of strength and the loss of ductility. The authors have reported the microstructural evolution, especially the precipitation sequence, of 6063 Al during friction stir welding in a previous article.[13] The friction stir weld of 6063 Al is softened around the weld center. The softening is characterized by dissolution and growth of the precipitates. No fusion zone is detected in the overall weld. Since the softened zone around the weld center is virtually solutionized by the thermal cycle of the welding, postweld aging should improve the weld properties just as in frictionstir-welded 7075 Al. Aging responses and the precipitation sequence in friction stir weld of 6xxx (Al-Mg-Si) series alloys have not been characterized well yet. The purpose of the present study was to systematically investigate aging responses in a friction stir weld of 6063 Al. In the as-welded and postweld-aged 6063 Al, hardness profiles were measured, and then precipitate size and distribution were observed by transmission electron microscopy (TEM). Aging responses at various locations in the weld were di
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