The effect of friction stir processing on 5083-H321/5356 Al arc welds: Microstructural and mechanical analysis

Friction stir processing (FSP) is used locally to modify the microstructure and thus mechanical properties of 5083-H321/5356 aluminum gas metal arc welds (GMAWs). Four specimen approaches were examined: as-arc welded, weld toe FSP (with arc weld on either the advancing or the retreating side of tool), and weld crown FSP. Microstructures within the fine-grained FSP region contained smaller constituent particles. Mg₂Si and Al₆(Fe,Mn), than those particles found in the arc weld nugget, heat-affected zone (HAZ), and base-metal (BM) locations. The FSP improved the monotonic tensile strength, yield strength, and elongation of 5083-H321/5356 Al arc welds by 6 to 9 pct, 7 to 13 pct, and 46 to 80 pct, respectively. The addition of FSP produced a 30 pct increase in the load necessary to reach 10⁷ cycles during four-point bending fatigue. An analysis of strengthening mechanisms determined that solid-solution, grain-size, and precipitation strengthening made contributions to the calculated yield strength of the BM, are weld nugget, and FSP regions. In addition, the strength mechanism analysis demonstrated that FSP increased the amount of grain-size strengthening and precipitate strengthening by nearly 110 MPa, when compared to the arc weld nugget.