Ceramic foam filter technology for aluminum foundries.
Although only implemented by aluminum foundries since the late 1970s, ceramic foam filters are increasingly being used to prevent nonmetallic materials from entering the mold cavity, resulting in premium-quality castings demanded by today's market.
The ceramic foam filter was originally developed for producing wrought aluminum alloys where extreme microcleanliness was required for stringent product applications, such as aluminum can body and lid stock, foil, aircraft extrusion stock, memory disk, forging stock and wire. The first application in the foundry industry was for premium quality aluminum castings for aerospace applications. Since that time, it has spread into the commercial casting and automotive applications.
Placed in the running systems of sand molds, filters can produce castings with little or no metallic inclusions. X-ray examination, fluid penetrant inspection, mechanical properties testing and metallographic examinations reinforce this fact.
The mechanism of nonmetallic inclusion removal may occur through screening, deep bed or cake filtration. Screens made of metal or fiberglass operate primarily by the mechanism of screening filtration. Screening filtration occurs when inclusions larger than the openings in the filter media are physically unable to pass through.
The ceramic foam filter operates primarily by deep bed filtration. Deep bed filtration efficiency is governed by the total 3-D surface area of the filter material. Inclusion retention is possible throughout the entire filter thickness. The reticulated structure of the ceramic foam filter allows for a high internal surface area and enhances deep bed filtration. Filtration efficiency is easily improved by reducing the filter pore size, thereby increasing the internal surface area.
Cake filtration mainly occurs when previously captured inclusions prohibit remaining inclusions from entering the filter structure. The presence of inclusions effectively blocking the entrance of the filter media is required for cake filtration. Screens and ceramic foams may function as cake filters in the presence of many inclusions.
Filtration with ceramic foam filters offers several key benefits that improve the quality of premium aluminum castings. These include:
* improved mechanical properties;
* reduced dye-penetrant indications;
* reduced x-ray evidence of nonmetallics;
* reduced rework/scrap;
* improved machining properties.
The presence of nonmetallic inclusions is detrimental to the production of premium quality castings. High attainable mechanical properties are a key aspect that is readily lost in the presence of inclusions and oxide skins. The mechanical properties of high-strength aluminum alloys are very sensitive to notches or imperfections in the casting. Filtration effectively reduces inclusion levels and subsequent notch effects caused by inclusions. Conventional techniques for attempting to remove inclusions are ineffective, and the use of metal or fiber screens isn't sufficient to keep the metal truly free of harmful nonmetallics.
Premium quality castings generally restrict the acceptability of surface defects that are revealed by dye penetrant inspection. Research has shown the level of such defects is reduced when ceramic foam filtration is involved. A marked decrease, or total elimination, of indications has been noted.
Another benefit that is gained from the use of ceramic foam filters is the reduction in the x-ray evidence of nonmetallics. The presence of x-ray defects under these circumstances is often the basis for the rejection of expensive castings. The most significant benefit is the reduction of defects visible in those areas that are designated as "structurally critical."
Nonmetallics act as nucleating sites for the formation of hydrogen gas porosity. Therefore, a reduction in the content of inclusions will also produce a concomitant reduction in porosity due to gas. The filter doesn't keep out the hydrogen; it merely reduces the propensity for the formation of gas porosity.
A major benefit that may be gained from the use of filters is the great reduction in rework. The elimination of defects, either of a surface or subsurface nature, adds considerable cost to the price of the casting--provided these defects do not occur in critical areas. Furthermore, if blending or other finishing techniques results in the loss of dimensional integrity, the loss of the casting may occur. Repair is not an efficient means for competitively producing premium quality castings.
Another major benefit of foam filtration--one not necessarily observed by foundries--is reduced machining problems. The presence of the spinel form of aluminum oxide can be extremely detrimental to tool life. In addition, nonmetallics will affect the finishing characteristics of the machined casting. The sheen and luster that are characteristic of machined aluminum are lost in the presence of nonmetallic inclusions.
The effect of filtration on the mechanical properties of cast test bars is shown in Fig. 2-5. The test bars were produced from alloys Almag (AA535) and 356-T6, respectively. Almag is a high magnesium-containing alloy that develops reasonable strength without the need for heat treatment. The high magnesium content, however, results in the development of oxide skins during the casting process. As a consequence, reduced property levels and the need for extensive repair are often encountered.
Although high-strength levels are not the usual requirement for Almag 35, pressure tightness is almost always a required property. Filtration effectively reduces inclusions and skins, and improves the pressure tightness of Almag 35.
Figure 2 compares the tensile strengths of filtered and unfiltered Almag 35 alloy. It may be concluded that the use of ceramic foam filtration has resulted in about a 20% increase in attainable tensile strength. In addition, the effect on percent elongation shown in Fig. 3, is markedly improved by filter use. The improved mechanical properties of Almag 35 may allow for increased applications for the alloy.
In a similar manner, the tensile strengths and elongations of filtered and unfiltered 356 alloy in the T6 temper are plotted in Fig. 4 and 5, respectively. Although the improvement in tensile strength is not marked, the use of a filter shows about a 25% increase in percent elongation. Another observable benefit of filtration is the reduced spread in the data. Consistency is a driving force for quality programs, and it enhances the utility of the casting process.
It becomes reasonable to conclude, therefore, that the ceramic foam filter is very effective in preventing the introduction of nonmetallics in the mold. Therefore, optimum mechanical properties, either in their level or their reproducibility, are achievable with ceramic foam filters.
To obtain an optimum mold fill rate, it is important to calculate the proper filter size for each specific casting application. The filter must be large enough so that it doesn't retard or choke the flow of molten metal or become blocked with retained inclusions before the casting is completed. To achieve as close to the original unfiltered flow as possible, the filter must be several times larger than the choke area of the gating system. The following casting parameters must require careful consideration.
Flow rate factors--filter pore size number (10, 15, 25) and choke area of the gating system.
Blockage factors--filter pore size number (10, 15, 25) and melt quality including: melting stock, alloy chemistry, melt treatment and quantity of metal to be filtered.
To achieve an adequate flow rate, the filter must be sized so its frontal area is several times greater than the total choke area in the gating system. As the filter area/choke area increases, the mold fill rate increases exponentially. At filter area/choke area ratios less than four, the filter significantly increases the pour time in comparison with unfiltered pouring.
At filter area/choke area rations above eight, the pour time is virtually unaffected by the filter. If the ratio is in the four-to-eight range, a slight increase in pouring time may be evident.
It is generally recommended to use a filter area/choke area ratio of at least four, but for thin-sectioned castings the ratio should be higher. Finer pore sizes (#15 and 25) slightly increase the filter flow restriction and may require a slight increase in the filter area. Multiplying the choke area by the desired filter/choke area ratio yields the required filter size.
It is important that the effective filter is used for the calculation. The effective filter area is defined as the inlet area of the filter that isn't covered by the print in the mold. The blockage factors are to be used as guidelines. Many foundries with sound metal processing practices are able to pass much more aluminum per square inch than mentioned in the table.
Table 1. Blockage Factors of #10, 15 and 25 Filters. Filter Pore Size Blockage Factor (lb/sq in) #10 (coarse) 15 #15 (medium) 10 #25 (fine) 5
The largest of the required filter area between the flow rate calculation and the blockage factor calculation should be used.
Proper installation is essential for optimum casting results. An expansion in the gating system is required to accommodate the larger filter surface area and to provide a seating surface to mechanically retain the filter in the gating system.
Figures 6 and 7 show horizontal and vertical filter installation drawings, respectively. Both figures illustrate the expansion of the runner adjacent to the filter and the subsequent contraction after the filter. A rectangular sprue is recommended to avoid vortexing during the pour.
Filters 3 x 3 in. or smaller may be used in the horizontal or vertical position. Larger filters should be used in the horizontal position only. If larger filters are to be used vertically, then some type of mechanical support is recommended along the top, unsupported edge.
It has been shown that the integration of ceramic foam filters into the running system of the casting setup is useful in removing nonmetallics from the casting. This results in a reduction in radiographic indications of nonmetallics and gas porosity, and a reduction/elimination of dye penetrant indications.
Additional benefits include rework reduction and improved machining properties. It has also been shown that reduction of the amount of nonmetallics in the casting by filtration contributes to the casting quality by improving the mechanical properties or enhancing the reproducibility of the properties.
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|Author:||Davidson, Nathan J.|
|Date:||Jul 1, 1993|
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