Microstructure Evolution during Friction Stir Welding of Mill-Annealed Ti-6Al-4V

Abstract

In this study, mill-annealed Ti-6Al-4V plates were successfully friction stir welded over a wide range of processing parameters using a tungsten-1 pct La 2 O 3 tool. Two K-type thermocouples embedded in the tool indicated that approximately 25 pct of the heat generated during welding was transferred out of the workpiece and into the tool. The thermocouple data, combined with observations of the microstructure, indicated that the stir zone of all welds exceeded the b transus. The microstructure and texture of two representative welds made just above and high above the b transus were investigated with scanning electron microscopy and electron backscatter diffraction (EBSD). The b phase orientations were reconstructed with a fully automated technique from the as-collected a phase data through knowledge of the Burgers orientation relationship. The results suggest that the fine b grains in the stir zone are formed from the base material ahead of the advancing tool by dissolution of secondary and primary a phase, and there is no further recrystallization. These grains subsequently deform by slip and rotate toward the orientations that are most stable with respect to the shear deformation induced by the tool. In the highest temperature weld, diffusion tool wear in the form of periodically spaced bands provided an internal marker of the tool/workpiece interface during welding. The flow patterns evident within the tungsten-enriched bands suggest that flow is considerably more chaotic on the advancing side than in the central stir zone.

Microstructure and Texture Evolution during Friction Stir Processing of Fully Lamellar Ti-6Al-4V

“…The strains during the rest of the process are more elusive and modeling efforts are limited by the uncertainty of what is physically happening at the interface between the tool and the material being actively deformed, e.g., if the tool is rigidly coupled to the workpiece or if there is some sliding. Experimental observations on aluminum [58] and titanium [62] alloys suggest that there is a stochastic swapping between interfacial sticking and slipping during FSW. [58] While the total strain in the stir zone is unknown, the lack of microtexture and the development of a nearly random distribution of grain orientations is a practically important result that can be understood on the basis of crystal plasticity theory.…”

Achieving superior low-temperature superplasticity for lamellar microstructure in nugget of a friction stir welded Ti-6Al-4V joint

“…In other words, it is harsh and complex to control the nugget temperature to be just near the β transus by adjusting the FSW process. Actually, a fully lamellar microstructure was usually obtained in the nugget under most of FSW parameters [18][19][20]. Therefore, it is of great significance to study whether it is possible to obtain similar superplastic deformation ability for the lamella-structured nugget and the BM.…”

Microstructural Evolution in Ti-5111 Friction Stir Welds

“…The equiaxed prior b grains appear to be a product of continuous dynamic recrystallization [41] above the b transus temperature. The lamellar a microstructure within them is characteristic of deformation above the b transus, as observed in prior friction stir welding/processing studies on pure titanium, [42] Ti-5111, [43][44][45] and Ti-6Al-4V [14,[46][47][48][49][50][51][52] alloys. The relatively small prior b grain size and the narrow width of the grain boundary a indicates that the cooling rate from the b phase was relatively fast.…”

“…This abrupt transition from BP to SZ is characteristic of titanium FSWs [16,42,46,48,49,52] and contrasts with aluminum FSWs, which exhibit more extensive TMAZs. [2][3][4][5]8,55,56] While tool geometry (e.g., the use of threads to enhance material flow in aluminum friction stir welds) may explain some of these differences, the disparity in thermal conductivity and flow behavior of the two materials is probably more significant.…”