Modelling of Phase Transformation During Homogenization of Ternary Aluminum Alloys
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Modelling of Phase Transformation During Homogenization of Ternary Aluminum Alloys Thorsten Hofmeister, Klaus Greven, Andreas Ludwig, Peter R. Sahm, Foundry Institute, RWTH-Aachen, Germany ABSTRACT The microstructure determines for the most part mechanical properties of castings. This is the case especially with age-hardening aluminum alloys. To predict the phase transformation in cast parts during and after solidification a sophisticated approach to a coupled modeling of various simulation phenomena is presented. This approach couples a diffusion equation solver which considers the online estimation of thermodynamic equilibria with a macroscopic model for temperature calculation. The coupled micro-model predicts the dendrite solidification of ternary alloys. To simulate the phase transformation in temperature zones below solidification a homogenization model is added. Permanent mould casting experiments with ternary aluminum alloys are used to validate these models.
INTRODUCTION The crystallographic basis for age-hardening is the creation of a precipitation structure that severely strains the solvent matrix and thereby causes hardening. Some numerical techniques on the micro level have been developed to predict the local evolution of microstructure and the formation of microsegregation as a function of the global cooling conditions. Reviews of analytical, semi-empirical and numerical methods can be found in [1,2]. As demonstrated by Boettinger et al. [3,4], the on-line use of thermodynamic programs is advisable to enhance the accuracy of thermodynamic data for multi-component alloys. Micro and macro models should ideally be closely coupled to take into account mutual influences between microstructural evolution and temperature changes during solidification and subsequent cooling. This work also presents an approach to simulate the homogenization as the first step in the age-hardening process.
CONSTITUTION OF THE AL-CU-MG-SYSTEM Al-Cu-Mg is a typical age-hardening alloy. The solidification of a AlCu4Mg-alloy starts in the Al-rich corner with the solidification of α–dendrites. The eutectic groove is reached at an amount of 34wt%Cu in the liquid. At this point α– and θ-phase (Al2Cu) solidify. Finally at 502°C the ternary point is reached. The remaining liquid changes into α-, θ- and S-phase (Al2CuMg). Precipitation hardening consists of three steps: solution treatment, quenching and aging, figure 1. These steps are performed at defined temperatures for a given time interval.
Y8.11.1
Solution treatment
Quenching
Aging
annealing temperature 520-530°C annealing time 8-16 h
water temperature 20-80°C
aging temperature 170-180°C aging time 7-10 h
Texture before solution treatment:
Texture after quenching:
θ
Texture after aging:
precipitate Grain boundary
S α
supersaturated
α
Grain boundary
Figure 1: Precipitation hardening of Aluminum alloys.
NUMERICAL PROCEDURE The described simulations on the macro level are performed by means of the in-house package CASTS, a 3D finite element code. Detailed descripti
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