Effect of heat treatment and iron content on the microstructure and mechanical properties of a secondary Al-Si-Cu alloy

In this work the effect of heat treatments on the microstructure and mechanical properties of a secondary Al-Si-Cu alloy is studied. A Design of Experiments (DoE) was developed in order to optimize the heat treatment parameters and analyze the dissolution and fragmentation of the iron-rich inclusions as well as the improvement of the mechanical properties by non-equilibrium heat treatments. The iron-rich intermetallic inclusions were characterized as-cast and after heat treatment both by two-dimensional metallography and three-dimensional computed-tomography. The results of the microstructural characterization were statistically analyzed and comparison between the two-dimensional and the three-dimensional analysis established with the aim of determining the accuracy of the classic two-dimensional microstructural analysis. The three-dimensional characterization of defects served as base for the analysis of the interaction between defects, especially between iron-rich inclusions and pores. Fatigue tests were performed with the objective of studying the effect of iron-rich inclusions on the fatigue behavior of the secondary alloy. The fracture surfaces of the post-mortem fatigue specimens were analyzed by micro computed-tomography and scanning electron microscopy. The effect of the iron content on the microstructural and mechanical properties was studied with as-cast alloy and under the effect of non-equilibrium heat treatment in order to study the effect of these inclusions on the fracture behavior of the material. The three-dimensional reconstructions enabled to determine the interaction between defects and the cross sectional fracture surfaces were analyzed by scanning electron microscopy with the aim of analyzing the influence of these inclusions on the crack propagation mechanism in secondary cast Al-Si-Cu alloy.