Modelling precipitation hardening in an A356+0.5 wt%Cu cast aluminum alloy

Abstract

The behavior of a A356+0.5 wt%Cu alloy used to manufacture cylinder heads was studied. Samples were solutionized, quenched and aged at 200 °C for 0.1, 1, 10 and 100 h. TEM characterization showed that for the short aging durations (up to 10 h), the dominating hardening precipitates were β″ rods, while for the long aging duration (100 h), the dominance shifted to the Q-phase (Q′, Q″ precipitates). The length and diameter of the β″ rods were measured to produce size distributions which were later used to calibrate and validate the precipitation model. The physics-based precipitation kinetics model relies on classical nucleation/growth/coarsening equations adapted for the precipitation of Mg–Si precipitates in the aluminum matrix. Indirect coupling to Thermo-Calc software was used in order to determine the essential thermodynamic variables such as the driving force for precipitation and the solubility product for the model. Recent developments regarding the correction of the growth rate equations and the curvature effect were used to take into account the elongated morphology of precipitates. A Kampmann-Wagner Numerical (KWN) based model was used to track the evolution of the size distributions during nucleation, growth and coarsening of the β precipitates. The yield strength of the alloy was modelled using the Pythagorean sum of the contributions of intrinsic strength, solid solution strengthening and precipitation hardening. Both models showed good accuracy when compared to experimental results.