Pierre Archambault

Inverse Resolution of the Heat- Transfer Equation: Application to Steel and Aluminum Alloy Quenching

Two applications of the numerical method for the inverse heat conduction problem are presented. This numerical method calculates the surface temperature and heat flux using an internal experimental temperature evolution. In the case of aluminum alloys, the question of stability and sensitivity to error measurements is investigated and applied to actual quench cooling. For steel application (high heating and cooling rates), a new calculation procedure is developed to solve the problem of solution stability due to the nonmonotonous initial temperature profile generated by the superficial heating. This new calculation procedure allows the martensite tempered zones to be explained and localized.

Modeling of Precipitation Coupled with Thermodynamic Calculations

A precipitation model is developed based on three physical mechanisms, ie nucleation, growth and coarsening of the precipitates. The model follows the particle size distribution by using a finite volume method. Inputs of thermodynamic data are required, such as the driving force for nucleation and equilibrium concentrations. An ideal dilute solution, an ideal solution and a model fully coupled with the thermodynamic calculation software Thermo-Calc®[l] are used to describe the Gibbs energy of the different phases. The influence of using these different thermodynamic approximation on the prediction of the precipitation model is studied.

Phase transformations and generation of heat treatment residual stresses in metallic alloys

Today, a better understanding of the development of microstructures and internal stresses during heat treatment is achieved through modelling and numerical simulations.The key point is to take into account the phase transformations and the induced couplings with the thermomechanical behaviour of the material.These phenomena and their present modelling are reviewed for different metallic alloys, mainly steels, aluminium and titanium alloys.

SECTION 9.12 – Models for Stress-Phase Transformation Couplings in Metallic Alloys

This chapter discusses the models for stress-phase transformation couplings in metallic models.v The models described in the chapter, which concern phase transformation kinetics and thermomechanical behavior, is used for many steels and high-strength aluminium alloys submitted to various thermal histories (heating, cooling, isothermal holding) and to stress states that lead to small plastic strains (a few percent). Typical applications are the modeling of heat treatment processes and also welding processes. A global approach based on the Johnson Mehl Avrami formalism is developed that assumes an additivity principle, or an approach that explicitly includes nucleation and growth laws. The thermomechanical behavior of the material during the phase change is described by phenomenological behavior laws that include the thermoelastic and plastic-viscoplastic behavior of the stable multiphase material and the effect of the phase transformations. For precipitation at very small volume fractions, the approach through mixture laws is no longer valid, and a model describing both mechanical softening due to heterogeneous precipitation and hardening due to homogeneous precipitation is proposed.