A physically-based constitutive model with internal state variables (ISVs) is established, it is used to describe the flow stress and microstructure evolution of Ti-6Al-4V alloy in the superplastic forming (SPF). The ISVs in the constitutive model includes the dislocation density, grain size, the volume fraction of dynamic recrystallization. The flow stress consists of σ_fd, σ_ta and σ_GB, which are related to forest dislocation, thermal activation and grain boundary sliding (GBS), respectively. The material constants of the constitutive model are determined by the genetic algorithm (GA) optimization. A modeling method path to optimize the flow stress model is established, which is on the basis of the errors between the predicted and experimental flow stresses. In the modified flow stress constitutive model, the grain rotation (GR) is applied as a hardening mechanism, and the void is treated as a softening mechanism. A new GR model is proposed to describe the flow stress which is related to the GR. The modified constitutive model can accurately predict the evolution of yield stress, grain size and flow stress in SPF. With the calculation results of multi-scales constitutive model, the mechanism of Ti-6Al-4V in SPF is discussed, and a new deformation map with dominant mechanisms for Ti-6Al-4V is obtained.

This work provides a comprehensive investigation of multi-walled carbon nanotubes (MWCNTs) inducement in weldment and their apparent effect on the microstructure, %elongation and ultimate fracture behavior of Al-Mg-Si alloy referring modified tungsten inert gas (TIG) welding joints. Serious experimental work is carried out at 1 wt%, 1.5 wt%, and 2 wt% of MWCNTs to provide a gradually increasing heterogeneous nucleation. The behavior of grain morphology showed the pure field of epitaxial growth without MWCNTs, and the forestry type morphology for 1 wt% MWCNTs at low welding currents (160 A), though there was a noticeable conversion into equiaxed (EQZ) grains filled with inter-dendritic particles at high welding currents (180 A and 200 A) for 1.5 wt% and 2 wt% of MWCNTs. Moreover, the formation of a cellular type network above the fusion line predominated initially at all parameters. Conversely, fine EQZ grains were formed as they moved upward into the welded zone (WZ) explicitly at a high heat input. A conceptual pictorial model is presented in the study which summarized the behavior of morphological changes at the utilized parameters. The welded joints have demonstrated an increasing trend of strength and %elongation in contrast to joints without added MWCNTs. Comparative results have shown an exceptional increment of 71 to 76% and 67 to 75% of elongation up to ultimate tensile strength (UTS), and a fracture point that was clinched for 1 wt% and 1.5 wt% MWCNTs at 180A. From macro to micro-examination of the fracture surfaces, pure ductile modes constituting elliptical cup and cone type isotropic flow was evident in all specimens. Detailed confirmation of the pull-out fracture mode of MWCNTs has highlighted in the scanning electron microscope (SEM) images that intimated a methodical contribution in load-transfer from matrix to the fiber under axial load. Overall, a concise en-route for MWCNTs inducement is well-appointed through tube fillers along with an activating facilitator (TiO₂) in contrast to stereotype fillers for improved behavior termed as modified TIG welding joint process in study.