Modeling of banded structure in friction stir weld in strain rate–hardening materials of Zener–Hollomon type

This article presents a shear localization model for simulating the shear band formation process uniquely associated with friction stir welding. By introducing a thermal and plastic deformation boundary layer definition, the shear band formation process can be modeled as shear localization phenomena using one-dimensional coupled elastic visco-plastic model. Material constitutive behavior is assumed to follow Zener–Hollomon constitutive equation. With this model, shear band width, formation time and propagation speed can be theoretically estimated as a measure of friction stir weldability. As such, the shear band propagation speed serves as a theoretical estimate of the maximum possible welding speed under given base material and welding conditions, such as stir pin rotational speed and torque level. The model is shown to provide reasonable estimates of shear localization parameters when compared with recent experimental data on titanium alloy Ti-6Al-4V. With this model, some fundamental questions such as why titanium alloys are more difficult to weld than aluminum alloys or steels can be more quantitatively addressed. And consequently, potential means for mitigating some of the difficulties in friction stir welding of some of the alloys can be theoretically examined to provide guidance for cost-effective development of welding process parameters for new applications.