Pavel Sherstnev

A state parameter-based model for static recrystallization interacting with precipitation

In the present work, we develop a state parameter-based model for the treatment of simultaneous precipitation and recrystallization based on a single-parameter representation of the total dislocation density and a multi-particle multi-component framework for precipitation kinetics. In contrast to conventional approaches, the interaction of particles with recrystallization is described with a non-zero grain boundary mobility even for the case where the Zener pressure exceeds the driving pressure for recrystallization. The model successfully reproduces the experimentally observed particle-induced recrystallization stasis and subsequent continuation in micro-alloyed steel with a single consistent set of input parameters. In addition, as a state parameter-based approach, our model naturally supports introspection into the physical mechanisms governing the competing recrystallization and recovery processes.

Integrated Modeling of Strength Evolution in Al-Mg-Si Alloys during Hot Deformation

In the present work we develop a physically based model of strength evolution during hot deformation of Al-Mg-Si alloys. The goal is to predict a change of material strength taking into account the impact of microchemistry, i.e. the influence of solutes and precipitates on strengthening and softening mechanisms. The material strengthening is considered in the present work in terms of solid solution strengthening (the Labusch-Naborro model), work hardening (the advanced one-parameter Kocks model), as well as precipitation strengthening due to the stress contribution of non-deformable particles, i.e. dispersoids (the Orowan by-pass). The material softening is described by dynamic recovery through thermal activation of dislocation climb. For the precipitation kinetics the computational thermodynamics code MatCalc (Materials Calculator) was used. The model was validated by comparison with experimental data of compression tests of the 6xxx series aluminium alloys and a reasonable agreement of the simulated and measured flow stress curves was found.

Analysis of Plastic Flow during Friction Stir Spot Welding Using Finite Element Modelling

Friction stir spot welding (FSSW) as a variant of the linear friction stir welding is implemented in automotive industry as a partial replacement of resistance spot welding for aluminium. FSSW as a solid state joining technology, primarily takes advantage of severe thermoplastic deformation, to achieve the joining between two parts, which can be from the same material or even dissimilar. In this paper, the coupled thermo-mechanical viscoplastic finite element formulation is presented based on the character of FSSW. The model was calibrated by comparing temperature history obtained from the simulation with experimental data and subsequently used to investigate the effective strain distribution in the weld zone as well as the material flow and the shape of the stir zone.

Simultaneous Precipitation and Recrystallization during Hot Deformation of Ti, Nb and V Microalloyed Steel

Recrystallization is a major means for controlling the grain size of steel during hot deformation. Usually, small grain sizes deliver superior mechanical properties. To aid the grain size controlling effect of recrystallization, small precipitates of carbo-nitride particles can be utilized to hinder the movement of grain boundaries. Interestingly, these particles are not only effective during grain growth, but also during recrystallization. In the present work, a recently developed state-parameter based model is introduced that is capable of describing both, the individual processes of static recrystallization, dynamic and static recovery and precipitation as well as the mutual interaction of these mechanisms in the course of elevated temperature processing. The evolution of state parameters within the model is discussed and the simulation results are compared to experimental information. Within our approach, a vast amount of experimental data for microalloyed steel is reproduced on basis of a single set of input parameters

Modeling of Static and Geometric Dynamic Recrystallization during Hot Extrusion of Al-Mg-Si Alloy

Under certain conditions of extrusion temperature and strain rate Al-Mg-Si alloys produce coarse recrystallized grains at and near the surface. Current FEM models are able to analyze grain size evolution for extruded profiles, but cannot predict the coarse recrystallized grains near the surface. A new model using DEFORM 2D and local state variables such as strain, strain rate and temperature is compared with Al-Mg-Si rods extruded at 440°C and 500°C for two extremes of strain rate. The model is found to be sensitive to the processing conditions and to accurately predict the recrystallized grain size and fraction.

Prediction of the Yield Strength during a Heat Treatment of Deformed Al-Mg-Si Alloys

The present work focuses on a physical description of phenomena occurring during the heat treatment of a cold deformed aluminium sheet. The major practical interest lies in the prediction of microstructural changes and their impact on the yield strength. The material softening part was described due to thermally activated glide of dislocations. The precipitation kinetics are calculated by using the computational thermodynamics program MatCalc (MATerials CALCulator). The model was validated by comparison with experimental data from tensile tests of cold deformed and heat treated sheets. Finally, it was shown that the model can be used to predict the yield strength during heat treatment of Al-Mg-Si alloys.