Investigation of the weld characteristics of AA6016-T4 friction stitch welds in overlap configuration and the influence of stitch length on static and fatigue strength

High-strength, age-hardenable aluminum alloys in car body construction challenge conventional joining methods — especially in mixed-material body structures, as fusion welding is prone to hot cracking and to hydrogen porosity. As a solid-state process, friction stir welding circumvents these challenges and typically produces joints with a higher strength than fusion welding, particularly in high-strength aluminum alloys. While friction stir spot welding guns are commercially available, the joints produced with them exhibit significantly lower strength compared to linear welds. To address this issue, a friction stir welding gun capable of producing short stitch welds was developed for a possible application in car body manufacturing. This work investigates friction stitch welds in AA6016-T4 sheet overlap joints and quantifies how the stitch length influences static strength, fatigue performance, hardness, and microstructure, compared to a continuous friction stir welded (FSW) joint. Short stitch welds obtained the highest lap-shear strength, achieving up to 83% joint efficiency, while longer welds reached between 65% and 68%. Metallography confirmed overlap-specific features, such as cold-lap imperfections in the weld, and showed that tool reentry can locally fragment the oxide line and diminish cold-lap severity, improving static strength. The fatigue performance of the stitch welds was lower than that of the strongest static condition, with short stitches particularly susceptible to notch effects due to overlap-specific features and reentry-related porosity. In general, intersecting stitch welds can surpass continuous FSW in static strength, but fatigue optimization will require mitigating the severity of the cold lap and reentry imperfections, for example, through adapted tool and pin designs. • It is possible to produce interlocking stitch welds in an overlap configuration. • Interlocking stitch welds provide better static strength values than conventional FSW. • Fatigue strength is decreased due to imperfections at the interlock point. • Hook geometry and cold lap imperfections are decisive for strength properties.