1. Introduction

    Casting quality is primarily influenced by the casting process and subsequent treatments such as heat treatment, hot isostatic pressing (HIPing) etc. Casting techniques developed to improve the quality of cast aluminium alloys can be categorized as (i) control of the liquid metal quality prior to casting; (ii) control of the pouring of liquid metal into a mould; and (iii) control of casting microstructures and defects during solidification. Because of the commercial and technological importance of cast aluminium alloys, these processes have been the subjects of extensive research for several decades aimed at improving the quality of cast alloys.

    Unfortunately, liquid aluminium is usually laden with entrained oxide films. The entrainment process ensures that the films are doubled-over and frozen into castings as cracks. If care is not taken to remove oxide films suspended in the melts, they can lead to a variety of problems [1, 2]: (i) loss of fluidity and feeding properties such that the amount of microporosity may increase; (ii) increase in gas porosity with gascoated films acting as sites for heterogeneous initiation of gas or shrinkage cavities; (iii) increase in the tendency for hot tearing because oxide films may act as sites for the hetero-geneous nucleation of hot tears; (iv) leakage defects resulting from folded oxide films providing leakage paths by connecting wall to wall of castings, with bubble trails and non-wetted confluences of melt fronts being also particularly troublesome with respect to leak-tightness; (v) poor machinability giving increasing tool wear and poor surface finish becauseof the extreme hardness of oxides; and (vi) reduction in the me-chanical properties and reliability of castings because oxide films as cracks in castings introduce structural weakness, causing reduction and scatter of strength, ductility, and fatigue resistance, etc. In summary, therefore, the entrained oxide cracks are a source of concern because they are often not detectable by normal non-destructive testing techniques but are highly damaging to casting quality.

``````