Effect of Tool Pin Diameter on the Tensile Strength of Friction Stir Welded AA6061 Joints: Implications for Railway Carbodies

Friction stir welding (FSW) is a solid-state joining process that is increasingly used to fabricate aluminum railway carbodies. This study examined the effect of tool pin diameter on the tensile properties of FSW joints in 6 mm thick AA6061-T6 aluminum plates. High-speed steel tools with a 13 mm flat shoulder and straight cylindrical pins of 3, 4, and 5 mm diameter were fabricated by turning, and butt joints were welded on a conventional milling machine at a rotational speed of 1130 rpm, a traverse speed of 20 mm/min, and a tilt angle of 3°. Three joints were produced for each pin diameter (n = 3), and one transverse tensile specimen was tested from each joint. Results are reported as mean ± standard deviation and were analyzed using one-way ANOVA, the Kruskal–Wallis test, and Spearman’s rank correlation. The mean tensile strength decreased from 210.06 ± 5.87 MPa with the 3 mm pin to 190.96 ± 26.57 MPa and 146.36 ± 62.26 MPa with the 4 and 5 mm pins, respectively, while the scatter increased with pin diameter. Owing to the small sample size and large scatter, the differences among group means were not statistically significant (p = 0.206); however, pin diameter was negatively correlated with tensile strength (ρ = −0.74, p = 0.023). Visual inspection revealed less weld flash with the 3 mm pin, consistent with the smaller volume of material displaced by the pin. Within the tested range, the 3 mm pin provided the highest and most consistent tensile strength.