In general, the most serious problem facing foundrymen is porosity. For this reason, it is worth while considering in de- tail. The amount of porosity in a casting depends on several factors. Listed roughly in order of importance they are:


• solidification rate • gas content • metal cleanliness • pressure in the casting • modification, and • grain refinement.


Four of the above factors were studied by Fang and Granger.7 Their castings had a water-cooled chill at one end and were solidified directionally, so feeding was more than adequate to prevent shrinkage. The amount of porosity and the aver- age size of the pores were measured by quantitative metal- lography at various distances from the chill. The solidifica- tion rate at these locations was determined by thermocouples in the mold. The amount of porosity formed in their castings is plotted in Figure 14.


Oxide films are also an import source of casting defects. A thin oxide film forms on the surface as soon as liquid aluminum comes in contact with air. If the liquid is quiescent, this oxide stays on the surface and does not affect metal quality. But if there is any turbulence or splashing, the oxide film is mixed (or folded) into the melt; and the quality of the casting suffers. The generation of oxide films has been studied in great detail by Campbell and his associates. An excellent review26


of their


research has been published and is highly recommended. This work clearly establishes the importance of ‘folded in’ oxides, which have aptly been called ‘bi-films’ by Campbell.


One important practical implication on metal quality is that oxide films assist in porosity formation.


Aluminum readily picks up hydrogen gas from humidity in the atmosphere. Hydrogen has low solubility in solid met- al—only 6% of the gas soluble in the liquid—so it tends to form dispersed micro-pores during solidification. Because of the high surface tension of liquid metal, it is not possible for pores to nucleate homogeneously, so gas pores form on oxide films in the melt. In fact, if the metal is filtered careful- ly, no porosity will form, even if significant amounts of gas are dissolved in the metal. This phenomenon was first docu- mented by Brondyke and Hess27


and Rooy and Fischer.28


If one looks carefully at pores found in aluminum castings under a microscope, one usually sees remnants of an oxide film. An example of an oxide in contact with a pore is shown in Figure 15. In this case, it is clear that an oxide on the surface of the metal was ‘folded into’ the bath, by excessive metal turbulence and/or splashing.


To sum up the preceding discussion, not only do oxide films act as defects in their own right, but they also are a necessary


International Journal of Metalcasting/Winter 11 Figure 15. Pore nucleating on a folded oxide film in A356 alloy. 17


precursor for the formation of porosity. In both cases, they have a significant deleterious effect on quality. With these concepts firmly in mind, it will be instructive to consider how practices in the foundry determine casting quality.


Effect of Metal Treatment and Transfer on Quality


In many foundries it is possible to see this sequence of op- erations. Metal is pumped into a crucible. The crucible is then transferred by fork lift or crane, whereupon the metal is poured into a holding furnace. Then a ladle is used to dip out of the holding furnace, and to pour metal into the mold. Each transfer of liquid metal produces an aluminum ‘water- fall’. The resulting splashing generates oxide films, which are folded into the liquid and carried into the casting.


In contrast to the above, a level-pour transfer has been em- ployed for many years by primary aluminum suppliers. A launder (or trough) carries liquid metal from the melting furnace to the casting pit. Once the launder is filled, the metal flows in the lower part of the channel, and the sur- face is quiescent. Because the transfer is level, there are no ‘waterfalls’, and oxide films are not ‘folded into’ the melt. Normally the metal also passes through an in-line de- gasser and a filter box. As a result of these metal transfer and treatment practices, it is possible to produce castings which are very low in gas, and nearly free from oxide films or other inclusions.


A good example of the quality produced in this way is can stock. Can producers take large 3xxx alloy slabs and roll them into thin sheet, which is drawn into shapes needed for the can body. If you crush an empty aluminum can, you will quickly appreciate how thin the body of the can has become. If there are inclusions in the cast metal, they cause pin-holes in the wall of the can. Each can is tested for holes before filling with a beverage. When I first came into the alumi- num industry, the ‘standard’ reject rate for can stock was one in ten thousand. Now it is closer to 20 ppm. The level-


300x


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