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Metal Casting Technologies : December 2006
BacktoBASICS T Solidification and feeding of copper-base castings INTRODUCTION he solidification modes of copper-base alloys vary widely. Freezing ranges, or the temperature interval between liquidus and solidus, can vary from a few degrees for an aluminium bronze to over 200 degrees for a tin bronze. It is important for foundrymen to have a sound knowledge of solidification behavior and its effect on feeding practice, as the correct approach to methoding castings in one alloy could have disastrous results when applied to another. SOLIDIFICATION MECHANISMS For proper consideration of feeding practice, it is first necessary to understand how the alloy under consideration solidifies. It is not sufficient to merely classify copper-base alloys according to similar generic names such as brasses or bronzes and assume a similar approach to feeding, as these alloys cover the entire spectrum of solidification mechanisms. From a practical point of view, however, it is generally sufficient to classify alloys according to their freezing ranges, even a rather crude division into two broad categories of 'long' and 'short' freezing range is usually sufficient for practical purposes. SOLIDIFICATION OF SHORT FREEZING RANGE ALLOYS Common commercial copper-base alloys, that fall into this category include: ■ Deoxidized Copper ■ Aluminium Bronze ■ Manganese Bronze ■ Sand-casting Brass When an alloy of short freezing range cools in a sand mould, that portion of the liquid which first reaches the liquidus temperature begins to solidify. This usually occurs at the casting to mould interface where heat extraction is greatest. The chilling action of the mould wall results in the formation of a skin of solid metal surrounding the liquid. With further extraction of heat through this skin of solid metal, the liquid begins to freeze onto it and the shell of solid increases in thickness. The solid and liquid portions are separated by a relatively sharp line of demarcation -- the solidification front -- which advances steadily towards the center of the casting with further heat extraction. The crystal growth on the solidification front is relatively short and corresponds to the start of freeze at their apex and the end of freeze at their bases. Short freezing range alloys encourage directional solidification even at relatively low thermal gradients. SOLIDIFICATION OF LONG FREEZING RANGE ALLOYS Common commercial copper-base alloys that fall into this category include: ■ Phosphor Bronze ■ Leaded Bronze ■ Gun Metal ■ Leaded Gun Metal With long freezing range alloys, the development of directional solidification can be difficult. Although a thin skin of solid may initially form on the mould walls, solidification does not readily proceed progressively inwards to the thermal center of the casting. Instead, solidification begins by the advance from the mould walls towards the interior of a 'nucleation wave' corresponding with the liquidus isotherm. At some later time, a second 'end-of-freeze wave' corresponding with the solidus isotherm moves away from the mould wall and pursues the 'nucleation wave' towards the center of the casting. Freezing thus begins at each location in the casting when the nucleation wave passes it, and ends there when the end- of-freeze wave reaches it. In general, there are three distinct zones during the solidification of a long freezing range alloy: a completely liquid zone at the thermal center of the casting: a zone of solid metal at the mould walls: and a region of partial solidification between the liquid and solid zones. In extreme examples of long freezing range alloys, such as heavy section tin bronze, where the long freezing range and slow cooling rate combine to produce low thermal gradients, there can be liquid and solid phases co-existing throughout the entire casting section. FACTORS THAT INFLUENCE SOLIDIFICATION MECHANISMS There are many factors that influence the solidification mode of a particular alloy. The solidification range of an alloy, as measured in temperature degrees, is not the only indicator of solidification mode. Rather, it is the time interval between the start and end of freeze and the thermal properties of the alloy and mould material that determines how an alloy will solidify. Some of the major factors which effect solidification mechanisms include: THE SOLIDIFICATION RANGE OF THE ALLOY This is a fundamental characteristic of a particular alloy as indicated by its phase diagram and is the temperature difference between the start and end of solidification. The By J. F. Meredith, Casting Solutions Pty Ltd METAL Casting Technologies December 2006 44