Microstructural Evolution in Ti-5111 Friction Stir Welds

Abstract

The microstructural evolution occurring during friction stir welding of a near-a 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 b grains containing~100 lm large colonies of aligned a laths, related to each other by a strain-accommodating Burgers orientation relationship. The a laths are separated by fine, 100 to 150-nmthick, interlath b ribs. A heat-affected zone (HAZ) is observed~1.5 to 2.5 mm from the tool surface, characterized by a thickening of the b ribs and the formation of secondary a 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-lm-wide outer TMAZ, where the microstructure is refined through an increase in fine secondary (a laths) a laths and the lattice orientations rotate to align the close-packed h11 " 20i directions with the shear direction (SD). Continued deformation closer to the tool produces periodic shear bands within a~300-lm-wide inner TMAZ, resulting in alternating regions of material that are deformed below and above the b transus. Material in the stir zone (SZ) within~1 mm of the tool surface consists of fine (~10 to 20-lm diameter) equiaxed prior b grains that are delineated by~500-nm-thick a and contain 150-500-nm thick a laths. The texture exhibits both D 1 ð " 1 " 12Þ½111 bcc and P 1 ð1 " 100Þ½11 " 20 hcp shear texture components, indicating that this material exceeded the b transus during welding.

Thermal Stir Welds in Titanium

“…By comparison, the width of the thermomechanically affected zone in a near-alpha Ti-5111 alloy conventional friction stir weld was only about 500 lm. [15] In this transition region, an intermediate structure and orientation is observed between the small, equiaxed red/ orange grains of the base plate and the larger red/blue acicular grains of the weld nugget. These transitional grains are about twice as large as the base plate grains and still retain a convex grain shape.…”

“…The tool shoulder produces a majority of the heating in thin sheet welds, while the pin produces a majority of the heating in thick workpieces. [4] While there have been many studies on FSW in titanium alloys, few of those [5][6][7][8][9][10][11][12][13][14][15] have focused on alpha or near-alpha titanium alloys. Most of those studies have focused on thin section (from 2-to 6.35-mm-thick sheet) welding, where the heat generated by the shoulder can more readily be transferred to the base of the weld during welding.…”

“…Most of those studies have focused on thin section (from 2-to 6.35-mm-thick sheet) welding, where the heat generated by the shoulder can more readily be transferred to the base of the weld during welding. While FSW of thicker (12.7 mm) plate has been performed on alpha [7] and near-alpha [6,10,15] titanium alloys, no stable welding conditions (tool design, tool rotation, weld speed, and welding forces) have been found to consistently achieve defect-free welds in such thicker sections of these alloys.…”

Location and Orientation Specific Material Property Evaluation of Friction Stir Welded Ti‐5111: A Microsample Approach

“…[20] Figure 12 shows a plan view photograph of the weld showing this rotation, and compares the UTSs of transverse SZ samples with longitudinal samples located on the outer part of the SZ. [20] Figure 12 shows a plan view photograph of the weld showing this rotation, and compares the UTSs of transverse SZ samples with longitudinal samples located on the outer part of the SZ.…”

“…[20] Figure 12 shows a plan view photograph of the weld showing this rotation, and compares the UTSs of transverse SZ samples with longitudinal samples located on the outer part of the SZ. [20] The top of the weld shows an average SZ value of 978 MPa while the longitudinal direction shows outer SZ values of 934 and 997 MPa on the advancing and retreating sides, respectively. Figure 12 shows that the microsample measured transverse SZ UTS is comparable to the outer SZ properties in the longitudinal direction.…”