Microstructural Evolution in Ti-5111 Friction Stir Welds

• Published in: Metallurgical and materials transactions. A, Physical metallurgy and materials science Physical metallurgy and materials science, 2011-08, Vol.42 (8), p.2312-2322
• Main Authors: Knipling, K. E., Fonda, R. W.
• Format: Article
• Language: English
• Subjects: Characterization and Evaluation of Materials; Chemistry and Materials Science; Diffraction; Materials Science; Metallic Materials; Metallurgy; Microstructure; Nanotechnology; Structural Materials; Surfaces and Interfaces; Thin Films; Titanium alloys; Welding
• ISSN: 1073-5623; 1543-1940
• DOI: 10.1007/s11661-011-0630-2

The microstructural evolution occurring during friction stir welding of a near-α titanium alloy, Ti-5111, has been examined by backscattered electron imaging and electron backscatter diffraction. The unaffected baseplate (BP) microstructure consists of millimeter-scale prior β grains containing ~100 μm large colonies of aligned α laths, related to each other by a strain-accommodating Burgers orientation relationship. The α laths are separated by fine, 100 to 150-nm-thick, interlath β ribs. A heat-affected zone (HAZ) is observed ~1.5 to 2.5 mm from the tool surface, characterized by a thickening of the β ribs and the formation of secondary α platelets within them closer to the tool. There is a narrow thermomechanically affected zone (TMAZ), comprised of outer and inner regions, observed ~1.0 to 1.5 mm from the tool surface. Deformation is first observed in a ~200-μm-wide outer TMAZ, where the microstructure is refined through an increase in fine secondary (α laths) α laths and the lattice orientations rotate to align the close-packed directions with the shear direction (SD). Continued deformation closer to the tool produces periodic shear bands within a ~300-μm-wide inner TMAZ, resulting in alternating regions of material that are deformed below and above the β transus. Material in the stir zone (SZ) within ~1 mm of the tool surface consists of fine (~10 to 20-μm diameter) equiaxed prior β grains that are delineated by ~500-nm-thick α and contain 150-500-nm thick α laths. The texture exhibits both bcc and hcp shear texture components, indicating that this material exceeded the β transus during welding.