alloys, foundries may increase their addition of silicon or in other words run silicon higher on the chemical range for silicon to increase fluidity in the alloy. Shell composition and makeup


will have an effect on fluidity in a few different ways. The permeability of the shell is greatly important in the efforts to fill out thin sections, which will be discussed in detail later in the paper. Another factor is the surface that makes up the internal cavity of the shell. The coarseness of the prime coat or any ash that may remain in the shell cavity are sources of friction, which by definition, resist flow. Imagine pouring water onto glass vs sandpaper and ignore any wetting effects. You intuitively know that water will flow with ease on glass but slow and laborious on sand paper. In a similar way, albeit less drastic, the effect can be seen when pouring shells of different surface roughness. Viscosity is the measure of a fluids ability to resist deformation at a given shear rate. Put another way, it is a measure of how “thick” the material is; the classic example being water vs honey. Generally speaking, many alloys have similar viscosities when they are fully liquid, making the difference negligible in many cases. However, things change once the metal begins to solidify or if the material forms solid stable oxides. Solidification rates and the solidification morphology changes the viscosity of the material and its ability for flow through thin sections as well as oxide films on film forming alloys. Oxides and oxide films also have an


implications to fluidity. Aluminum for example forms a very stable oxide film on its surface which, combined with its surface tension, can have a profound influence on the flow characteristics of the material. As discussed earlier, sources of friction can restrict the flow and even “thicken up” the material. Some materials have a sludgy appearance during filling due to their interaction with the atmosphere and the formation of oxides and oxide films. Pourers will tell you that these alloys are difficult to pour and the result can be defects in the casting.


®


An effects not often considered is capillary repulsion, the opposite effect of capillary action, which becomes important due to the non-wetting effect of the shell (1). The adhesive forces, molecular attraction between unlike molecules, is less than the cohesive forces, intermolecular bonding, in casting which pulls the material into itself rather than pulling the material through a section by “griping” the walls. Measuring fluidity is not done frequently in the foundry, but there are tests that are used and have been performed to measure fluidity. The fluidity spiral is probably the most well- known and is a mold that has a constant thickness that forms a spiral, mainly to limit the size of the mold you would need to produce, and after the metal is poured you measure the length that the metal traveled; that’s your fluidity. Another test is pouring linear sections but of decreasing thicknesses. The idea that you will get to a thickness where the metal will not be able to travel very far and the information you can obtain is both your minimum thickness that you can pour and the length at which you can fill that thickness.


Temperature Temperature is by far the first thing that is considered when addressing misrun defects or even figuring out if you would be able to pour a given section size or not. There are however many different temperatures one must know and the first is pouring temperature. Pouring temperature is the temperature at which the mold begins to fill. One thing to point out here is that pouring temperature is different from tap temperature and can be different for each shell if multiple shells are poured out of a given heat. Using a lip pour or teapot ladle is an efficient way of delivering liquid metal to the shell but during the pouring process metal is rapidly losing temperature as the pourer fills the shells. Depending on how many shells are being poured at once and the pour weight, there would be losses as high as 100°F in temperature out of the ladle. Knowing what the actual initial temperature is, is important when


determining the root cause of the misrun defect. Furnace pouring somewhat eliminates this issue, but presents challenges in shell handling, setup on the pouring floor, and control of the metal delivery. Pouring practices will be different depending on how the foundry is setup, but knowing your process will be a key factor in determining what your temperatures should be. To know better what the pouring temperature should be for a given alloy and process, one must first consider other important temperatures of the material. The


liquidus and solid temperature fraction is the


temperature at which the metal begins to solidify. As the metal cools below the liquidus temperature, solidification starts


increases.


Just under the liquidus temperature however a stable dendritic network hasn’t been formed yet and so although there is solid phase starting to grow, the material maintains its ability to flow; albeit with increasing difficulty as solid fraction increases. A pouring temperature is usually determined by how many degrees above the liquidus temperature a particular alloy needs to be poured at to fill out the cavity. This temperature difference between the pouring temperature and the liquidus temperature


is called the superheat.


The superheat is sometimes determined with trial and error and is based on the experience of the foundry as well as industry guidelines. However,


this


doesn’t mean that the accepted rules cannot be challenged. As we will see later, there may be methods to explore to lower the pouring temperature and maintain a full casting tree at the same time. As solid fraction continues to increase during the solidification process however, there will come a point where the metal no longer has the ability to flow. This point at which the flow of material essentially stops in dendritic materials is called its coherency point. The Coherency point is where dendrites in a material form a coherent structure that can inhibit or eliminate the ability for the material to flow. Even


Continued on pg 20 November 2022 ❘ 21


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