AFS Transactions 01-048 (Page 1 of 10) Mechanisms of Bubble Trail Formation in Castings M. Divandari 1 , J. Campbell 2 1 The School of Metallurgy and Materials, Iran University of Science and Technology, Narmak, 16765, Tehran, Iran 2 The School of Metallurgy and Materials, IRC in Materials, University of Birmingham, Birmingham, UK Copyright 2001 American Foundry Society ABSTRACT Bubble trails appear to be a common defect in castings, but have not been investigated so far. The phenomenon had been predicted only quite recently. This study has been carried out in aluminum alloys by artificially introducing air bubbles into the mould cavities and subsequent examination of fracture of specimens taken from the casting. Bubble trails have also been found on the fracture surfaces of Zn-Al die casting components. The defect is the result of the splitting and reforming of the oxide film on a rising bubble, and the coming together of the film at its wake. Three different forms of bubble trail were found and the mechanism of their formation is described here. INTRODUCTION One of the defects created during the pouring process is the bubble trail. The bubble trail is the name coined by one of the authors [1] to describe the defect remaining in film-forming alloys after the passage of a bubble through the melt; " Since air, water vapour and other core gases are normally all highly oxidising to the liquid metal, a bubble of any of these gases will react aggressively, oxidising the metal as it progresses, leaving in its wake the bubble trail; the collapsed tube of oxide as a crack resembling an old sack. The trail is a serious threat to the mechanical strength and integrity of the casting". It seems that bubble trails could be an important source of other problems, especially leak tightness, in castings. This is in addition to their effect on the fatigue and other mechanical properties. The main sources of bubbles in castings are: I) Poor running system design causing surface turbulence in the metal flow and leading to entrained air bubbles. II) Thermal interaction between the molten metal and the mould or core leading to large core blow defects. Serious oxide films, which can cause leaks and mechanical cracking, can be caused by the outgassing of cores through the melt, causing bubbles to rise through the metal, and leaving an oxidised bubble trail in its wake. A succession of such bubbles from a core is very damaging to the upper parts of the casting. The bubble from the core contains a variety of gases, including water vapour, which are highly oxidising to metals such as aluminum and higher melting point metals. Bubble trails from core blows are usually particularly noteworthy for their characteristically thick and leathery double oxide skin (Figure 1(a)), which is probably why core blows result in such efficient leak defects through the upper sections of castings [2]. III) Chemical interaction between the molten metal and the mould or core materials. Bubbles arising as a result of gases diffusing into castings rarely cause the bubbles to grow sufficiently large to become detached, and thus mobile. These bubbles are generally small and are trapped among the growing dendrites. Their lack of mobility ensures that they do not create bubble trail. Thus they are not considered further in this report. OXIDE FILMS The main feature of the bubble trail is the oxide film, and the irregular pore, like a collapsed-in pipe, along the length of its centre. The outer surface of the oxide film is wetted by (i.e. in atomic contact with) the melt, but is unwetted by its inner surface which is microscopically rough and mostly folded as the result of motion of liquid metal. The inner, unwetted surfaces of the oxide film represent an unbonded interface in the liquid and therefore effectively constitute a crack even if the surfaces appear to be in good contact. The character of the oxide film could have a pronounced effect on the defects which may form in casting. Although much research has been done on the oxide film on aluminum alloys, the fact is that almost all of these researches have been done on tranquil liquid metal [3-7]. There is relatively little data on the characterization of oxide films, in flowing liquids. More importantly these earlier studies have mostly concentrated on the oxide film formation at long time (minutes and hours) while in the situation of interest to casting, the liquid metal is oxidising in only a very short time, like 0.1 second for the case of the creation of bubble trails, and probably 0.01 second for some types of severe surface turbulence.