**3. Physical-chemical reactions on the liquid metal-ceramic coating sand mould boundary**

In order to properly choose the ceramic coating required, it is necessary to be familiar with the phenomena related to physical-chemical and thermo-dynamic changes carried out on the liquid metal-mould boundary during the phase of inflow, cool down and solidification of castings. Basic physical and physical-chemical processes carried out on the metal-mould contact surface are the following:


During sand casting, oxidation atmosphere prevails within the mould cavity, influencing alloy components to oxidize first; afterwards, reactions among the metal oxides and mould blend are carried out. In order to obtain a quality casting surface, it is necessary to examine the interaction between metal oxides and mould blend, as well as pore formation and mould blend maceration with metal. When liquid metal penetrates the mould pores formed under the influence of capillary forces, complex reactions of chemical nature between the mould material and liquid metal are carried out. Depending on the mould blend composition, oxidation atmosphere has different oxidation ability which depends on CO, CO2, H2 and O2 contents. Character of the oxides formed on the metal surface is determined by the ratio of individual components from the gas stage composition, as well as by specific properties of the alloy being casted. It should be stressed that chemical action of metal oxide and mould blend is also determined by oxidation of some elements out of the blend composition. Pore formation is prevented by an appropriate choice of a mould blend either not reacting with metal oxides or forming solid compounds on the liquid metal-mould contact surface. Therefore, for example, manganese steel with 13% Mn gives unsuitable casting surface during sand casting, due to active activity of MnO and SiO2. Being alloyed with aluminium, the same kind of steel does not have the faults mentioned above. It is explained by the fact that Al2O3 oxide skim with melting temperature of 2050°C together with an easily meltable eutecticum Al2O3· SiO2 with melting temperature of 1545 °C forms a layer protecting the metal from further oxidation. When carbon steels are casted in the moulds made out of mould blend, iron oxide (FeO) prevails in the metal oxide composition. At the steel casting temperature, these oxides are highly overheated (1370°C) causing their hastened activity and fluidity. Figure 1 shows a schematic of interaction between the metal oxides and mould material. Metal oxides being formed on the melt surface penetrate the pores and, reacting with quartz sand grains, form silicates of volatile compositions of (FeO)m(SiO2)n type, influencing casting surface

Ceramic Coating for Cast House Application 265

b. interaction between metal oxides and mould sand

Mould blend maceration with metal has a decisive influence on the casting surface quality. It is known that metal and alloy melt, if not "contaminated" with oxides, do not macerate the mould walls. If a melt is oxidized, i.e. if there is a thin oxide skim on the melt surface, mould wall maceration will depend on the oxide properties. For instance, if a steel cast contains certain amount of iron oxides (if a melt disoxidation process has not been fully completed or if the melt oxidizes due to a casting process) maceration will happen, i.e. send will be burnt on the casting surface, Figure 3.a. Steel cast is most frequently disoxidized by aluminium; then, an aluminium oxide skim is present on the cast surface, preventing the melt from further oxidation, thereby preventing a sand burn fault on the casting surface. If these oxides are not easily dissolved in the basic metal, the metal will then penetrate the mould wall pores only if the pores are sufficiently sized, Figure 3.b (Ballman 1988, Svarika

Fig. 2. Schematic view of iron silicate formation on the metal – mould contact boundary

a. sintered sand on the casting surface b. mould punching on thicker cross

Fig. 3. Faults on casting obtaining in quartz sand – based moulds

sections of castings

SiO2

Fe+FeO (I+II) (FeO) (SiO ) <sup>m</sup> <sup>2</sup> <sup>n</sup>

c. silicate layer with volatile composition

FeO (I+II)

O2

through pores

(Svarika 1977)

1977, Tomović 1990).

a. metal surface oxidation

Fe+FeO (I)

FeO (II)

quality, Figure 1.a. Silicates formed will penetrate the mould wall if they have a low viscosity, while metal might penetrate the surface pores formed. It has been shown that depending on the metal temperature (1100°, 1300°, 1500°C) and mould material quality, mould is macerated with oxides ( in the case of sand-clay, iron chromite, chromemagnesite), and silicates are intensively formed (with sand-clay blend). Examinations concerning liquid metal casting into the sand mould showed that iron oxide (FeO) reacted with the SiO2 sand grains and formed an easily meltable silicate. Thus created silicate fills the space between the sand grains and gets suppressed by liquid metal into the mould interior, Figure 1 b. Further, by interaction between iron oxide and silica dioxide SiO2 , a thin membrane may be formed, disabling metal penetration into the mould pores, (Figure 1.c). Pore formation in the mould blend is influenced by the character of the oxides formed, as well as their actions with the mould surface (Cibrik 1977, Svarika 1977, Tomović 1990).

Fig. 1. Schematic view of non-metallic layer formation on the metal-mould boundary (Svarika 1977)

Iron silicate formation on the metal-mould contact boundary is carried out in three stages, Figure2. At the first stage, metal oxidation is carried out through the mould pores (Figure 2.a), second stage is featured by absorption of iron oxide dissolved in metal on the sand grain surface and by its reaction with SiO2 (Figure 2.b), while silicate with volatile composition (FeO)m(SiO2)n , is formed at the third stage (Figure 2.c). As this process is carried out in an excessive SiO2 , there is no possibility that easily meltable iron silicates will be formed. Absence of these mobile silicates (with a low viscosity), as well as duration of a forming process, make metal penetration either difficult or disabled. In case that the silicates are hard to melt, i.e. if they have a high viscosity, they may represent a barrier for metal penetration into the mould pores, like with sintered material(Cibrik 1977, Svarika 1977, Tomović 1990).

quality, Figure 1.a. Silicates formed will penetrate the mould wall if they have a low viscosity, while metal might penetrate the surface pores formed. It has been shown that depending on the metal temperature (1100°, 1300°, 1500°C) and mould material quality, mould is macerated with oxides ( in the case of sand-clay, iron chromite, chromemagnesite), and silicates are intensively formed (with sand-clay blend). Examinations concerning liquid metal casting into the sand mould showed that iron oxide (FeO) reacted with the SiO2 sand grains and formed an easily meltable silicate. Thus created silicate fills the space between the sand grains and gets suppressed by liquid metal into the mould interior, Figure 1 b. Further, by interaction between iron oxide and silica dioxide SiO2 , a thin membrane may be formed, disabling metal penetration into the mould pores, (Figure 1.c). Pore formation in the mould blend is influenced by the character of the oxides formed, as well as their actions with the mould surface (Cibrik 1977, Svarika 1977,

metal

metal layer of gaseouse

silicates sand

a. b. c.

grains

Fig. 1. Schematic view of non-metallic layer formation on the metal-mould boundary

the mould pores, like with sintered material(Cibrik 1977, Svarika 1977, Tomović 1990).

Iron silicate formation on the metal-mould contact boundary is carried out in three stages, Figure2. At the first stage, metal oxidation is carried out through the mould pores (Figure 2.a), second stage is featured by absorption of iron oxide dissolved in metal on the sand grain surface and by its reaction with SiO2 (Figure 2.b), while silicate with volatile composition (FeO)m(SiO2)n , is formed at the third stage (Figure 2.c). As this process is carried out in an excessive SiO2 , there is no possibility that easily meltable iron silicates will be formed. Absence of these mobile silicates (with a low viscosity), as well as duration of a forming process, make metal penetration either difficult or disabled. In case that the silicates are hard to melt, i.e. if they have a high viscosity, they may represent a barrier for metal penetration into

Tomović 1990).

clay

(Svarika 1977)

sand oxides

silicates

metal

a. metal surface oxidation through pores

b. interaction between metal oxides and mould sand

c. silicate layer with volatile composition

Fig. 2. Schematic view of iron silicate formation on the metal – mould contact boundary (Svarika 1977)

Mould blend maceration with metal has a decisive influence on the casting surface quality. It is known that metal and alloy melt, if not "contaminated" with oxides, do not macerate the mould walls. If a melt is oxidized, i.e. if there is a thin oxide skim on the melt surface, mould wall maceration will depend on the oxide properties. For instance, if a steel cast contains certain amount of iron oxides (if a melt disoxidation process has not been fully completed or if the melt oxidizes due to a casting process) maceration will happen, i.e. send will be burnt on the casting surface, Figure 3.a. Steel cast is most frequently disoxidized by aluminium; then, an aluminium oxide skim is present on the cast surface, preventing the melt from further oxidation, thereby preventing a sand burn fault on the casting surface. If these oxides are not easily dissolved in the basic metal, the metal will then penetrate the mould wall pores only if the pores are sufficiently sized, Figure 3.b (Ballman 1988, Svarika 1977, Tomović 1990).

a. sintered sand on the casting surface b. mould punching on thicker cross sections of castings

Fig. 3. Faults on casting obtaining in quartz sand – based moulds

Ceramic Coating for Cast House Application 267

reducing friction and preventing liquid metal penetration. They thus provide for smooth surfaces of castings. Utilization of ceramic coatings minimizes the risk of mould cavity

Different to sand mould or metal mould casting, where liquid metal flows into the mould cavity, with expendable polymer pattern casting (EPC casting process), patterns and inflow systems made of polymers are retained in the mould until liquid metal flows in ("full mould" casting). In contact with liquid metal, polymer pattern degradation and expansion process is carried out violently, during a relatively short time, and is followed by moulding crystallization. In order to attain a high quality and cost effective production of castings by EPC casting process, it is necessary to achieve the balance of the expandable polymer patternliquid metal-ceramic coating-sand mould system at the stage of metal inflow, polymer pattern degradation and expansion and casting formation and solidification, Figure 4.a. It requires a systematic research of both complex notions and processes carried out in the pattern and the notions and processes carried out in the metal-pattern contact zone, as well as in the metalceramic coating-sand contact zone (Aćimović-Pavlović et al. 2007,2010,2011, Brome 1988).

coat Liquid polystyren

accumulation

Liquid metal flow

casting's surface

Coat layer

b) liquid products accumulation on

Polymer pattern

d) major permeability: less coat layers' thickness and bigger sand grains

 ceramic coat

Sand

Polystyrene patern which is been decomposing

**4. Physical-chemical properties on the liquid metal-ceramic** 

**coating-expandable pattern-sand mould boundary** 

Polymer

Gassy products of decomposition

coat-pattern-sand

Liquid metal

 ceramic coat

Sand

a) System balance: liquid metal-ceramic

Polymer pattern

c) minor permeability: higher coat layers' thickness and smaller sand grains

Fig. 4. EPC process: The role of ceramic coating

Sand

ceramic

erosion and liquid metal contamination.

From the aspect of liquid metal action to the mould or core made of typical mould and core blends (quartz sand or some other sand and a binding agent, various additions or impurities), sand blend features, such as refractoriness and permeability, have a crucial influence at the casting stage.

Examinations of surface faults on the iron alloy castings obtaining into mould and core blends with quartz sand point to the fact that a higher melting temperature, as well as oxidation atmosphere, i.e. an immediate contact between SiO2 and FeO, cause appearance of a sintered layer on the casting surface. Defects of mould and core blends with quartz sand may be eliminated by replacing this kind of sand with highly refractory sands based on zircon, olivine, sinter magnesite, chromite, corundum and other, or by application of protective ceramic coatings for moulds and cores (Aćimović et al. 1994, Burdit 1988 Clegg 1978 , Shivukumar et al. 1987).

Melted metal often contains large amounts of dissolved gases; they are likely to disappear from the mould cavity mostly through the mould blend. When choosing a blend permeability, one should bear in mind that blends with different sand grain sizes may cause surface defects on castings. In case of high blend permeability with large sand grains, flowing metal, due to its viscosity, can easily get into intermediate areas among the sand grains, forming a very rough surface of castings after solidification. Large sand grains and spacious intermediate areas make melted metal flow more difficult due to high friction, thereby abrupting individual grains, causing a change of mould cavity shape and contaminating liquid metal itself. Surface defects on castings being present as impurities of mould or core blend are mostly originated from a mould not being firm enough but also from the inflow system being improperly sized or due to carelessness during mould and core manufacture. Apart from impurities, certain swollen parts are frequently noticed on castings on the spots where mould or core material is taken off by the melt flow. It does not happen when a very tiny sand grain is concerned, but the metal flowing in would become ruffled due to low friction. Besides, permeability of such sand blend would have an extremely low value (Cibrik 1977, Svarika 1977)

While interpreting the role of ceramic coatings, their effect on sand moulds or cores and the processes being carried out on the sand mould-ceramic coating-liquid metal contact surface, it can be concluded that the coatings increase relative refractoriness of sand blend, since:


Ceramic coatings influence a better metal fluidity, because its firm particles get into intermediate areas filling the empty spots among the sand grains on the moulding surface,

From the aspect of liquid metal action to the mould or core made of typical mould and core blends (quartz sand or some other sand and a binding agent, various additions or impurities), sand blend features, such as refractoriness and permeability, have a crucial

Examinations of surface faults on the iron alloy castings obtaining into mould and core blends with quartz sand point to the fact that a higher melting temperature, as well as oxidation atmosphere, i.e. an immediate contact between SiO2 and FeO, cause appearance of a sintered layer on the casting surface. Defects of mould and core blends with quartz sand may be eliminated by replacing this kind of sand with highly refractory sands based on zircon, olivine, sinter magnesite, chromite, corundum and other, or by application of protective ceramic coatings for moulds and cores (Aćimović et al. 1994, Burdit 1988 Clegg

Melted metal often contains large amounts of dissolved gases; they are likely to disappear from the mould cavity mostly through the mould blend. When choosing a blend permeability, one should bear in mind that blends with different sand grain sizes may cause surface defects on castings. In case of high blend permeability with large sand grains, flowing metal, due to its viscosity, can easily get into intermediate areas among the sand grains, forming a very rough surface of castings after solidification. Large sand grains and spacious intermediate areas make melted metal flow more difficult due to high friction, thereby abrupting individual grains, causing a change of mould cavity shape and contaminating liquid metal itself. Surface defects on castings being present as impurities of mould or core blend are mostly originated from a mould not being firm enough but also from the inflow system being improperly sized or due to carelessness during mould and core manufacture. Apart from impurities, certain swollen parts are frequently noticed on castings on the spots where mould or core material is taken off by the melt flow. It does not happen when a very tiny sand grain is concerned, but the metal flowing in would become ruffled due to low friction. Besides, permeability of such sand blend would have an

While interpreting the role of ceramic coatings, their effect on sand moulds or cores and the processes being carried out on the sand mould-ceramic coating-liquid metal contact surface, it can be concluded that the coatings increase relative refractoriness of sand blend, since:

 They prevent chemical reactions from happening on the metal-mould contact surface, i.e. they disable formation of iron oxides (FeO) due to presence of reduction atmosphere

 There is no direct contact between sand mould and liquid metal due to reduction atmosphere formation, thus avoiding a heat shock in the moment of contact with liquid

 Temperature gradient is created on the metal-mould contact surface due to presence of the gas film mentioned above, thus avoiding a sintering process, as a consequence of lighter allotropic modifications of quartz (SiO2) in case of sand mould being

The coatings have a more suitable grain size and a lower quantity of adverse matters

Ceramic coatings influence a better metal fluidity, because its firm particles get into intermediate areas filling the empty spots among the sand grains on the moulding surface,

influence at the casting stage.

1978 , Shivukumar et al. 1987).

extremely low value (Cibrik 1977, Svarika 1977)

metal flowing into the mould cavity,

than refractory blend.

created by burndown of the coating layer applied,

immediately heated up to the liquid metal temperature,

reducing friction and preventing liquid metal penetration. They thus provide for smooth surfaces of castings. Utilization of ceramic coatings minimizes the risk of mould cavity erosion and liquid metal contamination.
