Mechanism of heterogeneous
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I.
INTRODUCTION
THE production of castings free from porosity is one of the major challenges in the foundry industry. This inevitable casting defect deteriorates the soundness of castings resulting in decreased mechanical properties. It is generally accepted that porosity in castings is caused by the cooperative effects of shrinkage and dissolved gases. The existence of a bubble in the interdendritic liquid of a solidifying alloy necessitates the mechanical equilibrium between the gas pressure in the bubble and the sum of the local pressures and the surface tension force. Therefore, the gas content should be large enough to balance the high surface tension force of liquid metal. The negative pressure developed because of volumetric shrinkage during solidification helps the bubble to overcome the pressure requirement. Over the last few decades, several investigations tl-s,7-~TJ have been made dealing with various aspects of porosity formation. However, the mechanism by which such pores are nucleated in the liquid metal or alloy is still far from being completely understood. Pore nucleation is regarded as formally similar to the nucleation of a solid from the liquid? Jj A bubble cannot begin at zero size, as the surface tension pressure becomes infinite at this radius. Hence, there is a critical radius of the bubble, r*, below which it is not capable of surviving and above which it will tend to grow. Numerous efforts have been carried out in the past to predict the activation barrier (fracture pressure) in order to P.S. MOHANTY, Graduate Student, Department of Mining and Metallurgical Engineering, McGill University, Montreal, is also with the Department des Sciences Appliqu6es, Universit6 du QuEbec Chicoutimi, Chicoutimi. F.H. SAMUEL, Alcan-NSERC Industrial Research Chair Professor, is with the Department des Sciences Appliqu6es, Universit6 du QuEbec ~ Chicoutimi, Chicoutimi, PQ, Canada G7H 2B1. I.E. GRUZLESKI, Chair and Professor, is with the Department of Mining and Metallurgical Engineering, McGill University, Montreal, PQ, Canada H3A 2A7. Manuscript submitted October 6, 1992. METALLURGICAL TRANSACTIONS A
create a pore in a liquid metal and to assess the possibility of achieving this pressure in normal castings. It has been recognized that nucleation of pores in liquid aluminum would require extremely high gas contents and shrinkage pressures, of the order of 30,000 atm. t2'3] Several studies t2-sl have shown that the real expected shrinkage pressure drop and gas pressure are not very significant compared to the fracture pressure. Thus, homogeneous nucleation of pores is extremely difficult and is unlikely to occur in practice. In castings, nucleation of pores can be expected to occur primarily at heterogeneous sites. It has been experimentally shown in transparent systems that for rapid depressurization, nearly all initial bubbles are nucleated on existing surfaces, t61 Heterogeneous sites are inherent in almost all normal castings, with inclusions forming the most important category of such sites in the melt.
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