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Metal Casting Technologies : December 2006
BacktoBASICS Figure 3. Typical porosity distribution in long freezing range alloys Figure 4. Long freezing range leaded bronze casting showing minimal piping in the feeders METAL Casting Technologies December 2006 FEEDING OF COPPER-BASE ALLOYS Feeding of alloys exhibiting a short freezing range It has long been recognized that a necessary prerequisite for the production of sound castings in alloys of short freezing range is that the metal in the mould shall begin to solidify at points remote from the feeder heads and that solidification shall proceed progressively towards the feeders which are the last parts to solidify, all liquid and solidification shrinkage thus being contained in them while the casting itself is sound. This type of continuous solidification is sometimes called 'directional solidification' which is defined as the establishment of conditions that develop a solidification front that is substantially V-shaped in a longitudinal cross-section, with the large end of the V directed towards the feeders. This idealistic scenario is not often achieved in practice due to the complexity of casting design and the difficulties in establishing adequate thermal gradients across the entire casting section. Generally speaking, for satisfactory feeding of short freezing range alloys, feeders must be placed over thermal centers of the casting. They must solidify after that part of the casting to which they are connected, and they must have sufficient volume to compensate for the liquid and solidification shrinkage of the alloy. Due consideration should also be given to the feeding range of the particular alloy. The feeding range may be defined, as the distance a feeder will modify thermal gradients in a uniform casting section so as to promote directional solidification. Feeding of alloys exhibiting a long freezing range The concept of directional solidification has little relevance with alloys of long freezing range and attempts to achieve directionality of solidification, particularly in heavy casting sections, will usually have an adverse effect on soundness by merely concentrating porosity into localized areas. This is particularly so in many long freezing range copper-base alloys in which the difficulty in feeding porosity is aggravated by the high thermal conductivity of these alloys. High thermal conductivity in the liquid will help maintain nearly uniform thermal gradients throughout the solidifying casting. The high specific heat and latent heat of these alloys will also aggravate this condition. Generally, the goal in feeding such alloys is not to totally eliminate porosity but to ensure it is dispersed as evenly as possible throughout the casting section. An example of this practice is with leaded bronze bushes which are often cast without feeders so that thermal gradients are kept as uniform as possible. It is often desirable for feeders to compensate only for superheat and a portion of solidification shrinkage so as not to excessively extend solidification time. These alloys have virtually no feeding range and under normal foundry conditions achieving high degrees of soundness is virtually impossible. ● Reference: R. W. Ruddle: The Solidification of Castings. Institute of Metals J. Campbell: Castings. Butterworth- Heinemann Ltd., 1991 R. W. Ruddle: The Solidification of Castings. Institute of Metals. 1957 46