Pore formation in cast metals and alloys

Abstract

Porosity occurs in cast solidifying metals and alloys due to negative pressures generated during solidification contraction, and pressure developed by gases dissolved in the motten metal. Both the above processes may act either together or separately to produce such shrinkage or gas defects (collectively termed pores). They are generally unwanted and constitute a major industrial problem. This paper is an attempt to review up-to-date knowledge of the conditions of pore formation in cast metals and alloys. Various mechanisms responsible for pore nucleation and growth are summarized, and experimentally evaluated using an unfed type of mould with aluminium alloy castings. The observations are in support of a non-nucleation mechanism of pore formation playing a major role in the occurrence of such defects in cast metals. Further, in gas-containing alloy melts the critical amounts of gas required for single and multiple pore nucleation have been determined quantitatively and are listed in the text. The gas contents of the melts were measured using an apparatus based on the “first bubble technique”. It is also experimentally observed that under poor feeding conditions more than one of the non-classical nucleation mechanisms may be functional at the same time for the formation of such defects.