David Piot

Integrated modelling of precipitation during friction stir welding of 2024-T3 aluminium alloy

Abstract The microstructure and hardness profiles across a friction stir welded joint of a 2024-T3 aluminium alloy have been simulated by thermal and microstructure models. Thermal modelling is based on a semi-analytical model, which analyses the complex material flow by standard velocity fields. The precipitation model is based on classical nucleation, growth and coarsening mechanisms and can be combined with precipitation hardening laws. Applying these models in succession gives simulated hardness profiles, which have been compared to experimental hardness values after friction stir welding under equivalent conditions. The positions of maximum and minimum hardness are well reproduced, but the simulated hardness variation across the weld is less pronounced than the experimental one. The simulated microstructures in well characterised zones of the joint have been compared to previous TEM observations and shown to be generally in agreement. The results demonstrate the feasibility of making realistic hardness predictions from physically based models on a standard personal computer.

Combined Effects of Grain Boundary Convection and Migration in Dynamic Phase Transformations

During large strain deformation of polycrystals, grain or interphase boundaries are driven by the material flow, which is a convection movement. By contrast, upon static recrystallization or grain growth, their motion takes place with respect to matter, which is referred to as grain boundary or interphase migration. During hot working, where dynamic phase transformations commonly occur, convection and migration operate simultaneously. According to local geometrical (e.g., prescribed velocity field, grain boundary curvature) and physical (e.g., grain boundary mobility, dislocation densities) conditions, they can reinforce or oppose each other, but generally combine in more complex ways. The aim of this work is to analyze such effects on the basis of simple analytical approaches. The results suggest that second phase particles or grains dynamically generated (i.e., during straining) exhibit approximately equiaxed shapes.