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

Abstract

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.

An alternative pressure-dependent velocity boundary condition for modeling self-reacting friction stir welding

“…In the constant velocity boundary, the material velocity at interface is directly assigned as a fraction of tool velocity as,   rr b tool vv (10) where  represents the degree of sticking and an averaging effect of the shear layer [12]. As  increases, the predicted width of deformation zone increases accordingly since higher boundary velocity intensifies the material movement in the pin vicinity.…”

“…Pei and Dong [9] incorporated this 1D shear localization model into FSW process modeling, with different boundary and initial conditions. In their following study [10], they used Zener-Hollomon material constitutive model instead of Johnson-Cook model, which was previously used in [6,9]. They calculated the width, formation time and propagation speed of shear layer.…”

Ultralow Temperature Densification of a Titanium Alloy by Spark Plasma Sintering

“…At this stage, samples have attained a packing density of 62.5% (high-temperature regimes) and 77% (low-temperature regimes), equivalent to the reduced surface area in contact with the die. Therefore, at high-temperature regimes, particles experience contact stresses of merely 92 MPa, whereas that sintered at low-temperature regimes undergo up to 693 MPa in stresses exceeding the elastic limit of the alloy at %300 C (YS 300 C ¼ 651 MPa) [17,18] suggesting that the induced particle deformation is instantaneous. Figure 2b shows the mass transport mechanism under Hertzian contact schematically which creates varying conditions for dislocation movement and densification for stresses exceeding yield stress.…”