Cellular Ceramics / 5
.1.pdf
401
Part 5
Applications
Cellular Ceramics: Structure, Manufacturing, Properties and Applications. Michael Scheffler, Paolo Colombo (Eds.)
Copyright 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 3-527-31320-6
403
5.1
Liquid Metal Filtration
Rudolph A. Olson III and Luiz C. B. Martins
5.1.1
Introduction
In most molten-metal processing operations, the acts of melting, transporting, and alloying the metal in preparation for casting into desired shapes introduces undesirable nonmetallic inclusions into the melt. Molten metals are highly reactive and tend to interact with gases and refractories during processing steps; undesirable phases are absorbed either as liquid or solid. Examples of the introduction of such inclusions are molten aluminum reacting with atmospheric oxygen to form solid aluminum oxide, or molten cast iron reacting with atmospheric oxygen to form liquid slag phases.
Once metal is cast, these inclusions can result in defects that render the product unusable. In some cases, the inclusions are too small or too few to be detected in the as-cast part, and the defects are not detected until much further along in the process in machining and forming steps. Many modern advanced manufacturing systems rely on complete elimination of defects in the production path to facilitate and enhance productivity. Thus, to efficiently cast defect-free metal products, it is necessary to remove these nonmetallic inclusions from the melt, and one of the most effective methods of performing this task is the use of ceramic foam filters.
Ceramic foam filters (CFFs) were introduced in 1974 for filtration of molten aluminum used in the production of wrought aluminum alloys [1]. Commercial application in the aluminum industry started in 1976. Filtering of single-part mold castings also started with aluminum in 1977, followed by cast iron in 1983. Today, filtration through cellular ceramics plays a major role in processing several metals. CFFs are used for more than 50 % of the wrought aluminum cast in the world today, representing a total of about 650 000 filters per year. Filter sizes in this application range from 18 to 66 cm square, and the standard thickness is 5 cm. The production of cast iron parts is the second largest use. More than 50 % of cast iron parts produced today are filtered with CFFs. This usage represents a total of 400 000 000 filters per year with sizes ranging from 35 mm square to 150 300 mm; thickness typically ranges from 13 to 32 mm. Other metal products, such as aluminum castings, steel castings, copper alloys, and high-temperature superalloys, are filtered routinely with CFFs.
Cellular Ceramics: Structure, Manufacturing, Properties and Applications.
Michael Scheffler, Paolo Colombo (Eds.)
Copyright 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
ISBN: 3-527-31320-6
404 Part 5 Applications
Most CFFs used in molten-metal filtration are manufactured by the foam replication technique. In the process, polyurethane foam is coated with ceramic slurry and the resulting part is dried and fired. During firing, the polyurethane foam within the ceramic coating vaporizes and exits the structure, leaving behind the porous ceramic foam. The cycle time and temperature necessary to generate a bonded filter (for example, silica-bonded silicon carbide or phosphate-bonded alumina) is generally 1–2 h and about 1200 C, whereas those for sintered ceramic filters (for example, zirconia-toughened alumina or partially stabilized zirconia) is generally 1–2 d and in excess of 1500 C. The foam replication manufacturing process is capable of generating large volumes of filters with acceptable properties at relatively low cost.
5.1.2
Theory of Molten-Metal Filtration
Three filtration mechanisms operate alone or in combination in a filtration application. These are sieving, cake formation, and deep-bed filtration. Fig 1 shows a schematic of these three filtration modes. In most applications, deep-bed filtration is the dominant mechanism due to the large surface energies present in these systems and the small size of the inclusions that are removed. When an inclusion interacts with the filter wall, the strength of adhesion must be great enough to resist the force of continuous flowing metal and prevent its being swept away and reintroduced to the metal stream. The adhesion strength is correlated with the sum of the interfacial energies between inclusion/filter, metal/filter, and metal/inclusion. The change in free energy for separation of an inclusion from the melt to the filter wall is given as:
DG = cif – cmf – cmi |
(1) |
where DG is the Gibbs free energy, and c the interfacial energy [2–3]. For the inclusion to remain fixed at the filter wall, the free energy must be sufficiently less than zero. This can be enhanced by reaction between the inclusion and the filter material, or by the metal remaining nonwetting to the filter and inclusion materials. An example of this scenario is provided in Fig. 2, where the metal has withdrawn at the interface because it does not wet the filter or the inclusion.
Filter 
Filter
Filter
Sieving Cake Deep Bed
Fig. 1 Three possible modes of filtration.
5.1 Liquid Metal Filtration 405
Filter wall
Solid |
Molten |
Metal |
inclusion |
metal |
Withdrawal |
Fig. 2 Schematic of a solid inclusion particle at the interface between the filter and the molten metal.
Smith, Aubrey, and Miller [4] presented a model for filtration of liquid aluminum for wrought alloy application by CFFs. This model assumes that CFFs function as deep-bed filters, whereby the majority of the retained inclusion particles are smaller than the filter pore size and therefore are retained through the depth of the filter structure. Due to the high surface area and tortuous path inherent to CFFs, inclusions typically have a short transport distance to the filter wall. Figure 3 better illustrates deep-bed capture of inclusion particles through the depth of a CFF, with a concentration that is highest at the filter inlet surface and decreases towards the outlet surface. Figure 4 is a scanning electron micrograph of inclusions captured near the entrance of a CFF.
Filter performance models published in the literature are based on trajectory flow modeling, interception-collector theory, and probabilistic modeling. These models predict removal efficiency based on the characteristics of the filter media (cell size, density, thickness), metal flow rate (melt velocity), and the characteristics of the inclusion system (size, density, wetting characteristics) [5–8]. Smith, Aubrey, and Miller [4] constructed an interception-collision filtration model based on a model developed by Grandfield et al. [8]. The analysis assumed that gravity collection and
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Fig. 3 Schematic of deep-bed filtration [4].
406 Part 5 Applications
Metal flow direction
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Filter
Fig. 4 Scanning electron micrograph showing typical inclusion retention at the entrance to a CFF used to filter aluminum. Note the large number of alumina skins.
direct particle collision with the CFF are the two most important parameters controlling removal efficiency. The total filter collection efficiency gt is defined as:
gt = gc + gg |
(2) |
where gc is the direct particle collision collection efficiency, and gg the gravity collection efficiency.
The collection efficiency for direct particle collision was defined as:
gc = 3 Rp/Rc |
(3) |
where Rp is the radius of the inclusion particle, and Rc the radius of the filter particle.
The gravity collection efficiency was defined as:
gg = Up/(Up+U1) |
(4) |
where the Stokes settling velocity of the inclusion particle Up was defined as:
Up = 2 g( p– m)(Rp)2/9 g |
(5) |
where g is the acceleration due to gravity, g the melt viscosity, p the particle density, and m the melt density, and the metal velocity through the filter U1 as:
U1 = Me/ mA |
(6) |
where M is the mass metal flow per unit time, A the filter area, and e the filter-bed porosity.
5.1 Liquid Metal Filtration 407
For deep bed filtration, the particle concentration Cz through depth z is calculated according to:
Cz = Co exp(–3 gt(1–e)z/4 eRc) |
(7) |
where Co is the incoming concentration of inclusions.
Manipulation of the above equation allows the removal efficiency to be calculated as a function of filter pore size, melt velocity, filter thickness, and inclusion-particle size. Figures 5 and 6 are examples of how the model can be used to predict filtration
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Inclusion Particle Size - Microns
Fig. 5 Effect of pore size on filtration efficiency. Removal efficiency decreases as ppi decreases (used with permission from TMS, Warrendale, PA, USA, www.tms.org).
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Fig. 6 Gain in removal efficiency from a second-stage fine-pore filter.
408 Part 5 Applications
efficiency as a function of real-life variables [4, 9]. Figure 6 represents the efficiency of a staged filtration system, discussed in Section 5.1.3. Smith, Aubrey, and Miller [4] also numerically validated the model. Thus, removal efficiency tends to increase as filter pore size decreases, melt velocity decreases, filter thickness increases, and inclusionparticle size increases. The flipside to this scenario is that as filter pore size is reduced and thickness increased, pressure drop across the filter also increases, and as the melt velocity decreases, production rate also decreases; lower production rate and excessive pressure drop across the filter are both undesirable, so a balance must be struck.
5.1.3
Commercial Applications
5.1.3.1
Aluminum
Molten aluminum must be properly treated before filtration is performed in order to obtain acceptable properties in the final product. Waite [10] provides an excellent description of these processing steps. Once filtered, the aluminum is cast and wrought into final shape. “Wrought” aluminum signifies that products are rolled or extruded, which typically requires very clean metal. For these products, the required extent of filtration is dictated by the quality requirements of the final product. Common inclusions in molten aluminum that are removed by filtration are solids such as oxide skins, magnesium aluminate spinels, and borides, and liquids such as magnesium chloride salts and salt–oxide agglomerates.
Aluminum foils are produced commercially down to 5 mm in thickness; extremely clean metal is required. Metal used for aluminum beverage cans has one of the highest quality requirements; inclusion content is typically in parts per billion. Requirements for quality of aluminum extrusions result from processing needs such as increased extrusion speed or process yield, but can also depend on the sensitivity of final product requirements, such as copier drums and tubing.
Phosphate and/or silica are commonly used to bond the alumina aggregate to form the filter body. The filter composition is typically 80–90 % alumina with the balance phosphate and/or silica binder. The binder allows the filter to be manufactured with sufficient properties at high volume, relatively low temperature, nearzero part shrinkage, and low cost.
Selection of the filter pore size for a process is critical. The unit of measure for pore size is pores per inch (ppi). The pore size of CFFs for wrought aluminum varies from 20 to 70 ppi. Pore size selection is a function of incoming metal quality and final product requirements. Filters with 20–40 ppi are used for common applications, and with 50–70 ppi for high-end products. Pore size has a tremendous influence on both the flow rate of aluminum through the filter and the head required to prime the filter. Filtration of wrought aluminum alloys is performed at flow rates from 5 to 1200 kg min–1. Filter use is semicontinuous; a furnace load of between 10 000 and 100 000 kg is cast through a disposable filter. The specific flow rate in
5.1 Liquid Metal Filtration 409
mass flow per filtration area varies from about 0.1 to 0.2 kg min–1 cm–2. When aluminum is first introduced to the filter, a thin skin of alumina forms at the interface between the metal and the filter. A head pressure is required to break through this skin and allow metal to prime the filter; the finer the pore size, the greater the head required for priming. In this application, it is imperative that the pore size across the filter is relatively uniform. If not, when the filter is primed and metal begins to flow, it will take the path of least resistance and only part of the filter will be primed.
Filtration requires the use of custom equipment designed to support the filter in the path of the molten metal and also provide means to preheat the filter before the molten metal is allowed to contact it. Molten aluminum is typically filtered at about 750–800 C, and bonded alumina filters, although cost-effective, are not designed to withstand considerable thermal shock. Thus, carefully controlled preheating techniques are required to ensure the filter does not fail by thermal shock when molten metal is introduced. Preheating is especially critical for large filters of 60 cm square or more.
The patented staged filtration system is designed to use two filters in series [11]. These systems are commonly used in the production of wrought aluminum. Pictures and schematics of such systems are displayed in Figs. 7–9. The 60 cm filter has coarser pore size and removes much of the larger-sized inclusions from the melt, whereas the 50 cm filter is of finer pore size and removes the smaller inclusions that escape capture in the 60 cm filter. This system enables a plant to run relatively unclean metal yet generate a high-quality end product.
Fig. 7 Duplex 50 cm/60 cm staged filtration system with gas preheat lid. The filter is preheated from the top. The system uses 60 cm filters in series with underlying 50 cm filters.
The 60 cm filters are coarser (used with permission from TMS, Warrendale, PA, USA, www.tms.org).
410 Part 5 Applications
Preheat |
burner |
60 cm filter |
50 cm filter |
Fig. 8 Schematic illustration of a staged filtration bowl. Preheating of the filter is performed from the bottom by using a high-velocity burner (used with permission from TMS, Warrendale, PA, USA, www.tms.org).
outlet
filter inlet
Fig. 9 Top view of the inside of a filter bowl with reactionbonded alumina filter inside. The two dots on the corners of the filter disappear when the filter is preheated to the appropriate temperature (used with permission from TMS, Warrendale, PA, USA, www.tms.org).
5.1.3.2
Iron Foundry
Cast iron parts are used in a large number of applications, including automotive and mechanical assemblies. Iron can be easily cast into intricate shapes, producing a part highly similar to the final desired geometry, greatly reducing additional conversion.