An improved friction stir shear localization model and applications in understanding weld formation process in alloy Ti-6-4

Abstract

This paper presents a coupled shear localization model incorporating both pin/workpiece contact and three-dimensional (3D) heat flow environment generated by friction heating between tool shoulder and workpiece. It represents a significant improvement over an early shear localization model presented by the same authors (2014, 2015) which was focused on shear localization process in front of pin such that shear band development can be treated as a one-dimensional (1D) thermal viscoplastic problem, without considering shoulder/workpiece heating effects and contact interactions between pin and workpiece. The current model includes a novel analytical-based 3D transient heat flow solution incorporating detailed spatial resolution regarding heat generation at shoulder/workpiece interface, which provides a transient temperature environment within which shear localization process can now be examined in a greater detail. Also, contact interactions between pin and workpiece are now treated explicitly by introducing a classical contact mechanics solution for describing contact behavior between a cylinder (pin) and cylindrical cavity (surrounding base metal). This refined shear localization model has been shown capable of providing more realistic and reasonable estimations for both temperature and welding torque when compared with available experimental data. A robust numerical procedure has also been developed and implemented for determining optimum welding speed for a given combination of stir tool rotational speed and welding travel speed, which shows a good agreement with experimental results for a number of applications.