Castability

Castability is the suitability of a metal or an alloy to be processed by casting . Castability is made up of a number of so-called casting properties that have an influence on the casting process or the quality of the casting. This includes the flow and mold filling capacity , the power supply capacity , the shrinkage and shrinkage , the heat crack tendency , the gas absorption , the penetration and segregation .

Castability is therefore a collective term for various material properties that play a role in casting. However, they can depend on the casting process used. Sand casting alloys are suitable for example only slightly for the die casting . Analogous to castability, machinability , formability and suitability for welding are understood to mean the suitability of a material for machining , forming or welding .

Fluidity

The flowability indicates how far the melt can flow into the casting mold before it solidifies. To measure it, one mostly uses horizontal gutters that are rolled up in a spiral. Good flowability is particularly important for thin-walled components in order to ensure complete filling of the mold and avoid rejects . The melt in the casting spiral or the casting mold slowly cools down. If there is a smooth-walled, rough-walled or shell-forming solidification , this reduces the cross-section that is available for the melt to flow and the flow rate decreases until it dies when the solidus temperature is reached . In the case of a slurry or sponge-like solidification, on the other hand, the viscosity of the melt increases very sharply after the liquidus temperature has been reached, so that the flow of material can also stall before the solidus temperature is reached. Typical casting defects with insufficient fluidity are cold welds and unfilled areas.

The heat balance has an important influence on fluidity. It is usually better, the higher the temperature of the melt. Alternatively, it is also possible to use molds or molds made of a material that does not conduct temperature well. For this reason, the flowability in a sand mold is better than in a graphite mold that is a good conductor of heat. The hypereutectic aluminum-silicon alloys with a Si content of around 20% generate heat when they solidify, which improves the flowability. Increasing the casting speed also improves the flowability. A low surface tension or viscosity has a positive effect on the flowability. Both can be influenced via the temperature and the composition of the melt. Oxide layers on the melt surface impair the flowability.

The flowability of pure metals is usually very good because they solidify with smooth walls. As the proportion of alloying elements increases, it decreases, as rough-walled or spongy solidification occurs. In the eutectic, however, it is again very good because of the shell-forming solidification.

Mold filling capacity

Qualitative progression of the mold filling capacity of an aluminum alloy as a function of the pressure level at different temperatures compared to the calculated mold filling capacity

Bolt test to determine the mold filling capacity of a melt during casting. View from above.

The mold filling capacity describes how well the geometry of the mold is transferred to the casting. Edges and corners are of particular interest here. In the case of the exclusively liquid melt, it depends on the surface tension , the metallostatic pressure prevailing in the melt and the density of the melt. A partially solidified melt has a poorer mold filling capacity. If the mold filling capacity is too low, sharp contours of the mold in the casting are only shown rounded. The mold filling capacity can be increased by increasing the casting temperature, heated molds, thermal insulation of the molds in order to keep the melt liquid until the end of the mold filling and an increase in pressure either through a higher pressure head during gravity casting or a higher piston pressure during die casting . The surface tension usually decreases with increasing temperature, which has a positive effect on the mold filling capacity - the exceptions are copper and cast iron . Further possibilities are the addition of elements that lower the surface tension or influence the solidification morphology . Oxidations have a negative influence on the mold filling capacity.

If the melt is exclusively liquid, its mold filling capacity can be calculated from the density of the melt , the metallostatic pressure head , the surface tension and the acceleration due to gravity . ${\ displaystyle F}$ ${\ displaystyle \ rho}$ ${\ displaystyle h}$ ${\ displaystyle \ sigma}$ ${\ displaystyle g}$

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In practice, the real mold filling capacity differs from the calculated theoretical one. The higher the melt temperature and the lower the pressure head, the better the results agree. At higher melt temperatures, the probability is higher that it is actually only in liquid form, which can also explain the deviations. The temperature therefore only has an influence below a certain transition temperature. From this transition temperature onwards, the melt is present as a liquid throughout the entire mold filling process. However, the formula for the calculation shows the basic possibilities for influencing the mold: The higher the density and the pressure height, the better the mold filling capacity and the lower the surface tension. The latter can be reduced via the composition of the melt and usually by increasing the temperature. However, an increase in temperature also leads to a greater tendency to oxidize and absorb gas. A better mold filling capacity, due to the higher surface wetting, results in a higher contact area between the melt and the mold, which leads to greater heat loss, which reduces the flowability.

For the measurement of mold filling capacity which has been in practice studs sample proved. It consists of two cylindrical, upright bolts that touch each other. They are poured over with the melt. The further it has penetrated into the gap between the bolts, the better the mold filling capacity. The melt forms a curve in this gap, which points in the direction of the gap. A radius and thus also a diameter can be assigned to this rounding. The mold filling capacity then corresponds to the so-called reciprocal diameter (1 / D). It therefore has the unit 1 / mm.

Feeding capacity, shrinkage and shrinkage

At the latest after the mold has been filled with the liquid melt, it will cool down. With almost all cast materials, the density increases and the workpiece volume decreases. This process is known as liquid shrinkage . Therefore, liquid melt must be able to flow into the mold. For this purpose, so-called feeders are attached to the workpiece in the mold , from which melt can flow in until it has solidified. Solidification shrinkage occurs during solidification. This is followed by shrinkage of the solid, which, like the solidification shrinkage, can no longer be compensated for by peasants. The total volume deficit for most materials is around 11–13%, for cast iron it is around 3 to 6%. In the case of special cast iron alloys with silicon, expansion can even occur.

Hot crack tendency

Hot crack behavior of aluminum: Qualitative course of the crack length depending on the silicon content at different temperatures above the respective liquidus temperature (also changes with the silicon content)

Star mold for measuring the tendency to hot cracks.

The shrinkage and shrinkage of the workpiece can lead to cracks if it is hindered, for example by the mold or the casting itself. If the melt has not yet completely solidified, it can flow into the cracks that have arisen and close them again . Hot cracks can be recognized by the fact that their fracture surface is scaled and tarnished. Some dendrites can be seen and the fracture surface is intergranular, i.e. it runs along the grain boundaries. In the case of cold cracks, on the other hand, a bare, fine-grain fracture surface can be seen that runs through the grains themselves, i.e. is transcrystalline. The tendency towards hot cracks is highly dependent on the material. In the case of materials that tend to form dendrites in the structure , these can trap the residual melt inside the casting and thus prevent it from flowing into the cracks. The tendency to hot cracks can be examined with the ring test. Here a ring-shaped casting is produced that is prevented from shrinking by a ceramic core in the center of the mold. Another possibility is the use of star molds . Several rods of different lengths, which are arranged in a star shape, extend from the sprue. At their ends they have thickenings so that they cannot contract. The mechanical stresses occurring in the rods are then proportional to the length of the rods. The longest non-cracked bar can then be used to infer the tendency towards hot cracks. The tendency to hot cracking of steel and malleable cast iron is relatively poor, that of cast iron and eutectic aluminum-silicon alloys is very good.

Gas intake

Cast piece made of AlMg 3 with pores

In principle, gases can dissolve in the liquid melt . The relationship between pressure and solubility at constant temperature is described by Sievert's law . When the melt cools down, the solubility for gases that migrate from the melt to the surface also decreases. If the casting has already solidified at the edges, the gas that escapes from the melt that is still liquid inside cannot leave the casting and forms cavities that can be recognized as pores or blowholes and reduce the strength. Hydrogen, in particular, is problematic because it dissolves well because of its small atomic size. In the melt it is present as a monatomic solution. When it escapes, it reassembles into diatomic molecules that require more volume so that it can stretch. In principle, however, these can also compensate for shrinkage and shrinkage.

Oxidation tendency

Oxidation tendency describes the tendency of the hot melt to form oxides with the oxygen in the air . Aluminum forms an oxide layer on its surface that prevents further oxidation (so-called passivation ). Magnesium, on the other hand, has to be cast in a protective gas atmosphere , as otherwise it would pull the oxides into the interior of the casting. There the hard oxides would have a notch effect that would reduce strength. In addition, liquid oxides impair the flowability and the mold filling capacity and reduce the ductility , which increases the tendency towards hot tearing. Oxidation can be counteracted by using protective gases such as sulfur dioxide , which are, however, not environmentally friendly. Other options are filters in the pouring runner or cleaning of the melt by gas flushing , degassing tablets or covering salts .

Tendency to penetrate

In sand casting, penetration is the penetration (penetration) of the melt into the molding sand. It is noticeable on the casting through adhering grains of sand and poor surface quality (roughness). Penetration is used if the grains of the molding material have not reacted chemically with the melt, otherwise it is mineralization or burning. A measure of the tendency to penetrate is the contact angle , which indicates the angle between a liquid surface and a solid. Ideally, it is 180 °. For sand, quartz , zirconium sand, loam , clay and graphite it is close to 180 °.

literature

Stephan Hasse (Ed.): Giesserei-Lexikon 2008 . 19th edition, Fachverlag Schiele & Schön, Berlin 2007, ISBN 978-3-7949-0753-3 .
Alfred Herbert Fritz (Ed.): Manufacturing technology . 11th edition, Springer Fachmedien, Berlin Heidelberg 2015, ISBN 978-3-662-46554-7 .

Web links

Castability on Ingenieurkurse.de - with an illustration of a casting spiral, accessed on April 1, 2016
Druckguss In: gta.htw-aalen.de , accessed on April 1, 2016

Individual evidence

↑ Stephan Hasse (Ed.): Foundry Lexicon , Schiele & Schön, Berlin, 18th edition, 2001, p. 504 (keyword “castability”).
↑ Matthias Bünck: Casting properties in: Andreas Bührig-Polaczek, Walter Michaeli, Günter Spur (ed.): Handbuch Spanen , Hanser, Munich, 2014, p. 28.
^ ^A ^b Alfred Herbert Fritz (ed.): Manufacturing technology . 11th edition, Springer Fachmedien, Berlin, Heidelberg, 2015, p. 46.
↑ ^a ^b Matthias Bünck: Casting properties in: Andreas Bührig-Polaczek, Walter Michaeli, Günter Spur (ed.): Handbuch Spanen , Hanser, Munich, 2014, p. 28f.
^ Matthias Bünck: Casting properties in: Andreas Bührig-Polaczek, Walter Michaeli, Günter Spur (eds.): Handbuch Spanen , Hanser, Munich, 2014, p. 30.
^ ^A ^b Matthias Bünck: Casting properties in: Andreas Bührig-Polaczek, Walter Michaeli, Günter Spur (eds.): Handbuch Spanen , Hanser, Munich, 2014, pp. 32–34.
^ Alfred Herbert Fritz (Ed.): Manufacturing technology. 11th edition, Springer Fachmedien, Berlin, Heidelberg, 2015, p. 51.
↑ ^a ^b Matthias Bünck: Casting properties in: Andreas Bührig-Polaczek, Walter Michaeli, Günter Spur (eds.): Handbuch Spanen , Hanser, Munich, 2014, p. 36.
↑ Stephan Hasse (ed.): Foundry Lexicon , Schiele & Schön, Berlin, 18th edition, 2001, p. 939 (keyword "penetration")
^ Alfred Herbert Fritz (Ed.): Manufacturing technology. 11th edition, Springer Fachmedien, Berlin, Heidelberg, 2015, p. 54.

[1] Stephan Hasse (Ed.): Foundry Lexicon , Schiele & Schön, Berlin, 18th edition, 2001, p. 504 (keyword “castability”).

[2] Matthias Bünck: Casting properties in: Andreas Bührig-Polaczek, Walter Michaeli, Günter Spur (ed.): Handbuch Spanen , Hanser, Munich, 2014, p. 28.

[fritz46-3] A ^b Alfred Herbert Fritz (ed.): Manufacturing technology . 11th edition, Springer Fachmedien, Berlin, Heidelberg, 2015, p. 46.

[buenck28f-4] Matthias Bünck: Casting properties in: Andreas Bührig-Polaczek, Walter Michaeli, Günter Spur (ed.): Handbuch Spanen , Hanser, Munich, 2014, p. 28f.

[5] Matthias Bünck: Casting properties in: Andreas Bührig-Polaczek, Walter Michaeli, Günter Spur (eds.): Handbuch Spanen , Hanser, Munich, 2014, p. 30.

[buenck34-34-6] A ^b Matthias Bünck: Casting properties in: Andreas Bührig-Polaczek, Walter Michaeli, Günter Spur (eds.): Handbuch Spanen , Hanser, Munich, 2014, pp. 32–34.

[7] Alfred Herbert Fritz (Ed.): Manufacturing technology. 11th edition, Springer Fachmedien, Berlin, Heidelberg, 2015, p. 51.

[buenck36-8] Matthias Bünck: Casting properties in: Andreas Bührig-Polaczek, Walter Michaeli, Günter Spur (eds.): Handbuch Spanen , Hanser, Munich, 2014, p. 36.

[9] Stephan Hasse (ed.): Foundry Lexicon , Schiele & Schön, Berlin, 18th edition, 2001, p. 939 (keyword "penetration")

[10] Alfred Herbert Fritz (Ed.): Manufacturing technology. 11th edition, Springer Fachmedien, Berlin, Heidelberg, 2015, p. 54.