The effect of friction stir processing on 5083-H321/5356 Al arc welds: Microstructural and mechanical analysis
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PAST research on structural aluminum alloys demonstrated lower fatigue resistance in gas metal arc welds (GMAWs) when compared to base-metal (BM) properties (e.g., References 1 and 2). Fatigue behavior of GMAWs can be accommodated by increasing the reinforcement at the arc weld location, thereby increasing component weight. However, there is a continual emphasis on decreasing the cost or weight of a given structure. Friction stir processing (FSP) is a technique that produces local microstructural modification and, when applied to GMAWs, improves the microstructure and corresponding mechanical properties at the weld toe and crown locations. Reasons commonly cited for lower fatigue resistance of full penetration GMAWs include the following: a weaker filler metal than the base metal (an undermatched weld), defects within the weld nugget such as solidification porosity, and stress concentrations at the weld bead.[3,4] Stress concentrations at the weld toe are the most important factor influencing the fatigue behavior of Al GMAWs; thus, removal of the weld bead increases fatigue resistance.[5] In addition, fatigue behavior of weldments can be improved further with either dressing of the arc weld bead or the introduction of compressive residual stresses at the weld location[4] (e.g., shot peening, surface rolling, or low plasticity burnishing[6]). However, with the weld bead removed, surface and near-surface defects within the filler metal become the primary fatigue initiation sites,[7] and production of a porosity-free arc weld can be problematic. The basic concepts of FSP are the same as those of friction stir welding (FSW).[8] In both cases, a rotating tool, with shoulder and probe, is plunged into a workpiece and translated along the desired path. Rotation of the tool produces frictional heating and corresponding plastic deforma-
tion within the workpiece. The extensive plastic deformation produces a narrow zone of softened material. This material is transferred around the tool as the tool traverses forward along the selected tool path. Consolidation of the material directly behind the tool produces a fully recrystallized, void-free, fine-grained microstructure. In both FSW and FSP, all material deformation occurs in the solid state; thus, no melting occurs. The fundamental difference between FSW and FSP is that no joining occurs in FSP; thus, no interface (contact line) is present. The objective of FSP is to modify microstructures to improve local properties within a structure, e.g., a GMAW within a large structure where FSW cannot be applied. The FSW is effective in producing butt joints with better mechanical properties than GMAWs.[9] However, the limitations of FSW include relatively high applied loads; slow travel speeds, especially at deep penetration depths, e.g., .12 mm; and the need for a robust system to react the high applied loads. An alternative to FSW is to friction stir process the surface of gas metal arc welds. Processing the surface reduces the loads (relative to FSW), such that a small portable system can
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