Investigation of Metallurgical and Mechanical Behaviors of AA6061-AA2017 Joints Produced by Single- and Double-Pass Friction Stir Welding: Influence of Base Material Position and Post-Weld Shot Peening Technique

<div>The growing demand for lightweight, high-strength materials in marine and aerospace structures has promoted the use of friction stir welding (FSW) for welding dissimilar aluminum alloys. However, tensile residual stresses and microstructural heterogeneities often degrade weld integrity. This study investigates the combined impact of base material positioning, single- and double-pass FSW, and post-weld shot peening (SP) on the metallurgical and mechanical properties of AA6061–AA2017 joints. Five welding configurations were examined to evaluate how varying base material positions on the advancing and retreating sides affect material flow and mechanical behavior. Post-weld SP effectively presented compressive residual stresses, reduced surface defects, and refined surface grains. The average grain size in the stir zone was reduced from 5.2 μm (single-pass) to 2.0 μm (double-pass U-turn) after SP, confirming significant grain refinement through dynamic recrystallization. Mechanical testing revealed that double-pass FSW with opposite weld direction (U-turn) followed by SP achieved the highest performance, with ultimate tensile strength (UTS) improving from 246 MPa to 289 MPa (≈17% increase) and tensile elongation rising from 8.78% to 9.41%. Microhardness in the heat-affected zone improved up to 127 VHN, countering thermal softening effects. The synergistic effect of double-pass welding and SP enhanced homogeneity, fatigue resistance, and surface integrity. The results establish SP as an efficient post-FSW treatment for dissimilar aluminum joints, offering quantifiable improvements in strength and ductility, making the process highly suitable for demanding marine and aerospace structural applications.</div>