In situ characterisation of entrainment defects in liquid Al-Si-Mg alloy

Entrainment defects occur frequently in aluminium alloy castings during mould filling, and are very detrimental to both mechanical properties and reliability. The formation mechanisms and influences of these defects have been discussed previously, but the behaviour of the defects in the liquid metal and their evolution during solidification has not been studied in detail. In this research, samples of Al-Si-Mg alloy A356 that contained entrainment defects were scanned using ultra-fast synchrotron-based X-ray tomographic microscopy at the TOMCAT beamline of the Paul Scherrer Institut. The samples were directly viewed at both room temperature and in a fully liquid state. The reconstructed images showed three different entrainment defect morphologies, namely, entrained pores, tangled double oxide films and closed cracks. The evolution of the morphology of the entrainment defects was studied to better understand morphological changes of the defects, and the relationship between entrainment defects and microporosity.