Microporosity characteristics in Al-Si foundry alloys

Although the importance of aluminium as a material for a broad variety of technical applications is already very high, the general trend of weight reduction in the mobility sector has the potential to boost the application of aluminium in structural components as well as in housings, covers and the powertrain even further. The aluminium cast process is and remains a very competitive process to produce highly complex and integrated, near-net-shape components up to very large production volumes. This is true, although the capability to predict and thus control the amount and size of microporosity is still worthy of improvement. To achieve better control of microporosity, a better understanding of the causes, the nucleation, the growth, and the final characteristics of microporosity in aluminium cast alloys are necessary. This work aims to expand and deepen the understanding and knowledge of microporosity by looking at the fundamental principles of solidification, and the evaluation of almost 100 micro XCT scans taken from industrial production and laboratory casting experiments. A pore volume-weighted approach is applied to enable the comparison of XCT data generated at three different spatial resolutions. The weighing procedure leads to linear correlations between average porosity and the maximum and mean pore volume and size for up to four characteristic specimen populations. The four different specimen populations can be related to the hydrogen content as well as the local cooling rate. At low hydrogen levels, the differences in solidification morphology due to chemical composition determine the amount of average porosity. The maximum pore size increases strongly with average porosity, and only very low local cooling rates change the observed low sphericity pore morphology. At high hydrogen levels, the sensitivity of the mean and maximum pore volume and size is lower in respect to average porosity, and the local cooling rate strongly affects the observable high sphericity pores. In the transition zone between low and high hydrogen levels, a mixture of high and low sphericity pores can be observed. For low hydrogen levels, the analysis of the mean pore sphericity shows a constant drop with increasing average porosity. At medium to high hydrogen levels samples show a constant and high mean sphericity value. A single linear relation between the mean and maximum pore volume respectively the mean and maximum pore size can be established. These correlations are independent of all varied experimental parameters and therefore constitute a new and fundamental characteristic of microporosity in aluminium cast alloys. The presented findings improve the understanding of the characteristics of microporosity and allow the prediction of important pore distribution measures. The developed correlations will find application in the Integrated Computational Materials Engineering (ICME) approach and are the first step in a through process modelling framework