Cellular Ceramics / 5

5.1 Liquid Metal Filtration 411

Filtration is needed to meet the quality requirements established for complexshaped parts manufactured in large-scale assembly lines such as engine blocks, crankshafts, and disk-brake rotors. These parts are individually cast in separate sand molds. Total cast weight per mold typically varies from 2 to 200 kg, and a mold may contain one or more parts. Filters are placed in the feeding system within the mold. Schematics of two feed systems are shown in Fig. 10. Silica or clay-bonded silicon carbide filters are used, ranging in size from 35 to 185 mm square, typically with pore size from 3 to 20 ppi.

The filter size and pore size selected for a process depend mainly on the type of iron being cast. Gray iron generally contains a lesser amount of inclusions; therefore a relatively high filter loading of 0.35–0.5 MPa and relatively fine pore size from 10 to 20 ppi can be used; filter loading is simply the mass of metal that can be poured through unit filter area. Ductile iron contains more inclusions, so filter loading must be lower and pore size coarser. Filter loads typically vary from 0.21 to 0.28 MPa and pore sizes from 3 and 10 ppi. Filter size is dictated by the need to keep the filter to choke area ratio preferably higher than 6. The choke is the smallest passage in the liquid-metal feeding system and controls metal flow velocity. The rule above prevents the filter from becoming a flow restriction and modifying mold feeding, which can greatly affect product yield and quality.

Filter

Fig. 10 Typical liquid-metal feeding canals with filter placed either horizontally or vertically.

Liquid inclusions are quite common in cast iron. Silicon, iron, and manganese from the melt can be oxidized to form SiO2–FeO–MnO slag. A slag composed of these oxides can melt at temperatures as low as 1170 C [12]. Capture of liquid slag by a silica-bonded silicon carbide ceramic foam filter is illustrated in Fig. 11 [13].

Filtering molten iron is a difficult task; the filter encounters a rapid temperature swing followed by very high mechanical load at high temperature. The casting time is only as much as 45 s, but the temperature of the molten iron is in excess of 1400 C. The filter is not preheated when molten iron is introduced, so the temperature climbs very quickly from 20 to over 1400 C. The filter is then quickly loaded with inclusions, and the head pressure above the filter continues to rise, generating significant loads.

Silica-bonded silicon carbide is an excellent material for use in iron filtration. The filter cannot be preheated, so it must have outstanding thermal shock resistance, which is afforded by the high thermal conductivity of silicon carbide. The silica

412 Part 5 Applications

MnSiO3 Slag

Filterilter

Fig. 11 Scanning electron micrograph in backscatter mode showing liquid manganese silicate slag wetted on a silicabonded silicon carbide filter (used with permission of the American Foundry Society, Schaumburg, IL, USA, www.afsinc.org).

matrix and silicon carbide aggregate create a strong high-temperature bond when the composition is well designed. Filters formulated according to Hoffman and Olson [14] have modulus of rupture values greater than 0.7 MPa when ramped quickly to 1428 C and tested at 45 s, which is a relatively long casting time. The composition, as well as the rheological characteristics of the slurry, is crucial to obtaining sufficient high-temperature strength.

Another important feature of ceramic foam filters in foundry applications is flow modification of the molten iron as it enters the casting. Turbulence in the molten metal stream as it enters the casting must be avoided at all costs, as it tends to promote formation of entrained gas bubbles and oxide inclusions. Water modeling has shown that ceramic foam greatly reduces the turbulence in a liquid stream as compared to screening-type filters [3]. Real-time X-ray images of molten metal flowing into a mold have shown that ceramic foam filters considerably reduce the turbulence of the melt and inhibit formation of these defects [15].

5.1.3.3

Steel

Steel is commonly used for complex mechanical parts such as pump and valve bodies. Steel is cast in sand molds in the nearly the same process as discussed above for iron. Steel sand castings are one of the most demanding filtration applications, even more so than iron, due to casting temperatures in excess of 1560 C and low metal fluidity, as well as the practical limitation that the filter cannot be preheated. For these reasons, the choice of filter compositions is limited to partially stabilized zirconia. Magnesium is commonly used as the stabilizing agent.

The extent of zirconia stabilization in the filter is critical to its performance. If the extent of stabilization is too great, the filter can fail catastrophically due to thermal- shock-related phenomena. If the extent of stabilization is insufficient, the filter can

5.1 Liquid Metal Filtration 413

Filter

Metal +

Alumina deoxidation products

Filter

Fig. 12 Photomicrograph showing Al2O3 deoxidation inclusions bridging across the filter pore opening. Alloy type: CN-7M.

Filter type: partially stabilized zirconia (used with permission of the American Foundry Society, Schaumburg, IL, USA, www.afsinc.org)

fail due to poor high-temperature strength. The composition and its manufacture are optimally designed to prevent failure by these two mechanisms.

Filtration of steel for castings is crucial because the presence of surface or subsurface oxide macro-inclusions can significantly impair their machinability, mechanical properties, and pressure-tightness. Oxide macro-inclusions are primarily the result of reoxidation during pouring. These inclusions must be removed in the gating system prior to entering the mold cavity.

Filter

Filter

414 Part 5 Applications

Total cast weight per mold varies from 2 to 2000 kg, and a mold may contain one or more parts. Filter sizes ranges from 5 to 20 cm square with thickness of 2 or 3.8 cm. Common filter pore sizes are 3–5 and 6–10 ppi. Filter dimensioning is a strong function of the type of steel being cast. Aluminum-deoxidized steels are filtered at 0.17 MPa; silicon-deoxidized steels and stainless steels can be poured at up to 0.7 MPa.

Both solid and liquid inclusions are present in molten steel. Alumina particles form when aluminum is use to deoxidize the steel. These fine particles can easily agglomerate and be retained by the filter, as shown in Fig. 12, and liquid oxide slags are common, such as the calcium aluminate slag shown in Fig. 13 [16]. The large surface energies existing at these elevated temperatures allow the filter to collect both types of inclusions, as shown in the figures.

5.1.4

Summary

Ceramic foam filters provide an efficient and effective means of removing undesirable inclusions from molten metal. They must be refractory, corrosion-resistant, demonstrate sufficient thermal-shock resistance and high-temperature strength for the application, and be cost-competitive. In all molten-metal processing applications, the porosity must be sufficiently continuous and of consistent pore size from filter to filter to ensure the repeatability of filter performance in molten-metal processing operations, and the pore size must be appropriately selected to achieve optimum processing conditions and acceptable product. The deep-bed filtration mechanism is the most important, and capture of inclusions is based on interfacial energies between the molten metal, filter body, and inclusion type. Research continues in this area, and new CFF designs continue to be introduced to the marketplace in all areas of molten-metal filtration.