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Metal Casting Technologies : December 2006
METAL Casting Technologies December 2006 45 wider this temperature interval for a fixed rate of heat transfer, the longer the time available for crystals to grow and hence the more difficult feeding becomes. THE THERMAL CHARACTERISTICS OF THE MOULD The thermal conductivity of the mould influences the rate of heat transfer from the casting and hence affects the thermal gradients within the casting. The higher the thermal conductivity and heat capacity of the mould material, the greater the rate of heat transfer from the casting and the shorter the time interval between liquidus and solidus. Hence thermal gradients will be steeper and crystal growth will be shorter, thus creating more favourable feeding conditions. Sand moulds are of relatively low thermal conductivity and tend to give rise to low thermal gradients within the casting, particularly with heavier casting sections. Mould materials such chromite or zircon sand have higher thermal conductivities and heat capacities than silica sand and can help to increase thermal gradients and improve casting soundness, particularly in light casting sections. THERMAL CONDUCTIVITY OF THE SOLIDIFYING ALLOY The effect of an alloy of high thermal conductivity, such as copper-base, is to reduce thermal gradients within the solidifying casting due to the rapid homogenization of temperature across the casting section. Crystal growth is hence longer and feeding becomes more difficult. SOLIDIFICATION TEMPERATURE The higher the solidification temperature of an alloy, the greater the rate of heat transfer and the steeper the thermal gradients across the casting section will become. With high solidification temperatures, therefore, crystal growth is inhibited and feeding becomes more favourable. SOLIDIFICATION MODULUS As solidification modulus, or solidification time, increases, thermal gradients across the casting section are reduced. Crystal growth and solidification bandwidth is increased leading to shallower thermal gradients and an increase in internal porosity. EFFECT OF SOLIDIFICATION MECHANISM ON SHRINKAGE DISTRIBUTION The widely variable freezing modes that are displayed by copper-base alloys result in very different forms of porosity within the casting and feeder. Generally speaking, short freezing range alloys show deep pipes in the feeders as metal is supplied to the casting throughout the majority of the solidification interval. Internal porosity within the casting can take the form of small open cavities which occur near the end of solidification when feed metal is cut off by the merging of parallel solidification fronts: this is commonly called center- line shrinkage. Another form of shrinkage exhibited by short freezing range alloys is open cavities at inadequately fed thermal centers and isolated "heavy sections". With alloys of long freezing range, feeders often show minimal pipe, as the 'mushy' solidification mode will only allow liquid flow for a part of the total solidification time. Finely dispersed porosity can exist throughout the entire casting section, with coarser concentrations at parts of slower cooling such as junctions and under feeder heads. Under normal foundry conditions, it is virtually impossible to achieve absolute soundness in extremely long freezing range alloys such as tin or phosphor bronze and it is not unusual for up to 60% of the total liquid and solidification shrinkage of this type of alloy to be dispersed throughout the entire casting section. Figure 2. Short freezing range aluminium bronze casting showing deep piping in the feeders Figure 1. Typical porosity forms in short freezing range alloys