Static spheroidization and its effect on superplasticity of fine lamellae in nugget of a friction stir welded Ti-6Al-4V joint

Abstract

• A new spheroidization mechanism- termination coalescence in Ti-6Al-4 V alloy was discovered. • The special α/β interface with Burgers orientation relationship in the FSW Ti alloys was directly observed for the first time. • The kientic difference of the new spheroidization mechanism and the classical one was quantified compared. • This study provided a new simple and low-cost method without deformation to prepare the spheroidized structure. • Similar superplastic abilities of the base material and the nugget zone were achieved. The spheroidization of the lamellar structure can greatly contribute to the superplasticity of the nugget zone (NZ) of Ti alloy welds, which is the key to achieve the integral superplastic forming of welds for the fabrication of large-scale complex components. However, the spheroidization process is complex and costly since it cannot be obtained generally, unless the lamellae suffers from a large deformation. In this study, the static spheroidization was achieved for the fine lamellae structure in the nugget of a friction stir welded (FSW) Ti-6Al-4V joint, particularly by the annealing without any deformation. The special α/β interface obeying a Burgers orientation relationship (BOR) after FSW was first time directly observed, whose effect on the spheroidization was discussed. A new static spheroidization mechanism with the gradual coalescence of the adjacent lamellae was discovered, which we named as “termination coalescence”. There was a slower coarsening rate in the lamellar structure than in the classical equiaxed one, due to the BOR in the lamellae, although both of them exhibited a volume diffusion character during annealing. Consequently, the similar superplasticity can be achieved for the base material and NZ after annealing. This study can provide a new way to the spheroidization and a theoretical basis for the integral superplastic forming of welds during production.