Influence of Processing Parameters on Microstructure, Crystallographic Texture, and Tensile Behavior in Dissimilar Friction Stir-Welded Ti–6242 SG and Ti–54M

Dissimilar friction stir welding (FSW) of titanium alloys offers a route to spatially optimized aerospace structures, but the asymmetric thermomechanical environment produces heterogeneous microstructures and textures whose interaction with mechanical performance is not well characterized. A near-α/α+β combination of titanium alloys, Ti–6242 SG (advancing side, ADV) and Ti–54M (retreating side, RET), was welded across a matrix of rotation speeds (225–325 rpm) and traverse speeds (100–150 mm·min−1), spanning rotation-to-traverse-speed ratios N/v of 1.80–2.75, which was used throughout as an empirical processing index that orders the conditions of this matrix rather than as a measure of specific heat input. Microstructure, phase identification, relative diffracted-intensity trends, and crystallographic textures were characterized by 2D-XRD at three cross-section locations (ADV, weld nugget center [CEN], RET) and correlated with transverse tensile properties and fracture locations. Partial pole figures were plotted in the simple-shear reference frame with ideal-orientation overlays, intensities in multiples of a random distribution (m.r.d.). The CEN develops the strongest textures, dominated by a basal {002}α component (20–31 m.r.d.) with poles near the normal direction; this concentration lies away from the ideal shear fiber loci and is more readily explained by orientation inheritance during the β→α transformation on cooling than by direct shear, although unambiguous identification of variant selection would require orientation-resolved measurements. The RET develops {101}α and {100}α pole concentrations clustering near the ideal P-fiber loci, consistent with deformation-related texture development, intensifying with both rotation and traverse speed. The ADV shows mixed textures varying non-monotonically with parameters. Two conditions of nearly identical N/v obtained from different parameter combinations (225 rpm/125 mm·min−1 and 275 rpm/150 mm·min−1) nevertheless develop measurably different streak morphologies, microstructures, textures, and tensile responses, showing directly that N/v orders but does not determine the thermomechanical state. Yield strength is uniform (≈900–940 MPa) across the full matrix, consistent with a Schmid-factor estimate in which the basal-near-ND CEN texture gives a very low resolved shear stress on basal systems under transverse loading; joint efficiencies reach ≈90–96%. Ductility, in contrast, tracks consolidation quality rather than texture severity: fracture strain rises almost monotonically with N/v, from ≈0.6–1.4% at N/v ≈ 1.8 (defect-driven, erratic failure) to ≈5.4–6.0% at N/v = 2.60, despite the latter condition carrying the strongest RET pyramidal texture. Full-field strain measurement shows the weld nugget to carry the lowest strain and the highest apparent stiffness of any zone in every condition for which the load record is reliable, with strain accumulating on the advancing side. Consolidated conditions fracture on the advancing side where deformation concentrates, whereas the lowest N/v and longest-exposure conditions fracture in the nugget center; all fracture surfaces are ductile, with the crack path following continuous α layers at prior-β grain boundaries. A favorable processing range within the investigated parameter matrix is N/v ≈ 2.2–2.6, with the best overall combination at 325 rpm and 125 mm·min−1 (N/v = 2.60: UTS ≈ 1010 MPa, ≈5.4–6.0% elongation). Within the parameter range examined here, consolidation quality is the first-order design variable for this dissimilar system, with the zonal texture architecture setting the yield strength level.