Characterization and modeling of the influence of the ageing treatment on the precipitation process and the mechanical behavior of the AlSi10Mg(Cu) aluminum alloy

A major objective of the current engine development projects is the downsizing of engines, which leads to more demanding specifications for the design of aluminum cylinder head castings such as higher operating temperatures or increased combustion pressures. As a result, these internal combustion engine components are subjected to more severe thermal loads related to the start-operate-stop cycles of the engine which produces low-cycle fatigue loading conditions in the material. Therefore, the initial ageing condition of the alloy as well as the subsequent thermal loading during the component service have a great impact on the cylinder head mechanical response and lifetime predictions. For this reason, mechanical models which can consider the ageing condition in the alloy are highly appreciated in this field. A comprehensive analysis of the effect of the artificial ageing on the precipitation process of the age-hardenable AlSi10Mg(Cu) aluminum alloy from T6 to T7 condition and its influence on the mechanical behavior of the alloy at elevated temperatures is presented in this work, considering the influence of temperature and time ageing conditions of interest. The influence of the artificial ageing on the material microstructure is carried out considering the most important microstructure compounds usually present in Al-Si-Mg alloy systems. A complete quantitative characterization of the Mg2Si precipitation distributions covering a broad range of ageing conditions is obtained using the small angle neutron scattering (SANS) technique, complemented with high-resolution transmission electron microscopy (HTEM). This information is used to fit Robson's precipitation model for the prediction of the precipitation distribution as function of time and temperature. Based on the measured precipitation behavior, a sigmoidal interface energy function is added to Robson's model. As a result a unique set of modeling parameters is obtained for the whole precipitation process. Therefore, Robson's model is shown to be a powerful tool for predicting the evolution of these nanometer-scale particles in industrial and complex ageing processes. The influence of the ageing condition on the mechanical response of the alloy to different loading conditions is also examined. Hardness measurements and tensile tests are performed at room temperature. At higher temperatures, creep and low-cycle fatigue tests are carried out to analyze the relationship existing between the precipitation distributions and the mechanical response of the alloy. The predominant strengthening mechanism at this high temperature regime is determined from these results. The information obtained from this analysis is of great importance for the mechanical modeling using Steck's viscoplastic material model considering both isotropic and kinematic hardening. This constitutive mechanical model is further developed in this work to consider combined cyclic and relaxation loading conditions by using a unique set of modeling parameters independent of each other and of temperature. According to the results presented, Steck's model is especially useful at elevated temperatures, of interest in this work. Finally and based on the physic principles of both precipitation and material models, Steck's material model is extended to consider the alloy ageing condition in the kinematic and isotropic hardening components. The results obtained for an intermediate overaged condition of the alloy using the extended Steck's constitutive model make this model combined with Robson's precipitation model a good approach to predict the mechanical behavior of the AlSi10Mg(Cu) aluminum cast alloy at high temperatures considering the potential effect of further thermal loading, which is of great interest for its industrial application in future cylinder head design projects