Effect of graphite interlayer quantity on weld formation and defect evolution in AA7075–AZ31 dissimilar friction stir welding

In the present work, AA7075 aluminium alloy and AZ31 magnesium alloy were friction stir welded under three graphite interlayer conditions: 0, 1 and 2 g. All welding trials were conducted using a square-profile HCHCr tool at a rotational speed of 1000 rpm, traverse speed of 15 mm/min and axial force of 2 kN. This study is intended as a preliminary exploratory investigation of the influence of graphite quantity under one fixed processing condition rather than as a process-optimization study. The graphite-free and 2 g graphite conditions did not produce stable bonded regions. In contrast, the 1 g graphite condition generated a comparatively more continuous weld region, although substantial defects remained. FESEM examination of the 1 g condition revealed interfacial discontinuities, void-like regions, crack-like features, fragmented material zones and non-uniform flow structures, demonstrating incomplete consolidation. EDS analysis confirmed the simultaneous presence of Al, Mg and localized carbon-rich regions within the analysed weld zone, indicating partial incorporation of graphite but heterogeneous distribution. Accordingly, the results demonstrate an observed non-monotonic trend in weld formation under the investigated conditions, rather than establishing a universal quantitative relationship between graphite quantity and weld quality. A limited graphite addition was associated with improved local continuity relative to the graphite-free condition, whereas excessive graphite was associated with severe disruption of interfacial consolidation.