Research Status and Development Trend of Foam Ceramic Filters for Liquid Casting Alloys

1.2 It has suitable refractoriness and low coefficient of thermal expansion, so that it does not soften deformation and crack under the action of high temperature molten metal for a long time.

1.3 It has excellent high temperature chemical stability, so it is not affected by high temperature metal liquid and avoids contamination of molten metal.

2   Development status of foreign foam ceramic filters

The earliest ceramic foam filter was a foam ceramic filter for aluminum alloy developed by RR Mollard and N. Davison of Consolidated Aluminum in the United States in 1978 under the trade name Selee/Al. In 1984, a foam ceramic filter Selee/Fe for filtering black metal was developed. Selee/Al is a bonded corundum filter with aluminum phosphate (ALPO ₄ ) as a binder. Its maximum temperature is 1400 ° C, and it can also be used for other non-ferrous alloys. Selee/Fe is mainly sintered type. According to different application objects, the following refractory aggregates can be used: mullite, corundum, MgO partially stabilized ZrO ₂ and its composite materials; in addition, bonded SiC Selee/Fe. They have a pore size of 15, 25 ppi (number of holes per inch length) and a thickness of 19 to 25 mm. Their chemical composition and main physical properties and their application range are shown in Table 1. If the ceramic foam filter is preheated before use, the maximum use temperature can be increased. The corundum sintered ceramic foam filter with a size of 100 × 100 × 25 mm costs about US$3, and the same size zirconia toughened corundum (ZTA) ceramic filter has a price of US$4.6. A comparative test conducted by RR Mollard et al. on Al-Mg alloys showed that the ceramic foam filter can increase the tensile strength of the alloy by about 4% and the elongation by about 14%. Filtration of as-cast ductile iron with Selee/Fe 15ppi and 25ppi filters increases the fatigue strength by 21.4% and 30.6%, and the filtration efficiency is as high as 97%. The reject rate of castings is greatly reduced after filtration (see Table 2).

Table 2 US foam ceramic filter Selee application examples

In the early 1980s, Hi-Tech Ceramics Co., Ltd. also developed four kinds of foam ceramic filters for filtering high-temperature alloys. The refractory aggregates are mullite, corundum, zirconia toughened corundum (ZTA) and Magnesium oxide partially stabilizes zirconia (PSZ). The refractoriness of these filters is 1650~1800°C, the average pore diameter is Φ0.5~1.4mm, the skeleton thickness is 0.23~0.69mm, and the high temperature creep (the hourly deformation of the sample after 3h at 0.034MPa and 1500°C) The amount is 0.074~0.749%/h, and the filtration efficiency is as high as 92~99%.

Astro Met also produces foam ceramic filters for alloy filtration of aluminum, magnesium, copper, cast iron, cast steel and stainless steel. The material is mainly corundum, zirconia mixed with titanium oxide and silica. The company's research shows that foamed ceramic filters bonded with phosphate or chromate will crack in the range of 1093 ~ 1260 ° C; mullite sintered filters have good thermal shock resistance, but Softening around 1593 ° C; these two ceramic foam filters are generally not suitable or less useful for cast steel and cast iron alloy filtration.

The United States has also successfully developed a ceramic foam filter for SiC-Al ₂ O ₃ composite sintered ductile iron filtration, which has a composition of 50% SiC, 40% Al ₂ O ₃ , and the rest is a sintering aid, a binder, and the like.

Japan has also developed rapidly in the development and application of foam ceramic filters for foundry, which are made of cordierite (2MgO·Al ₂ O ₃ ·5SiO ₂ ), mullite (3Al ₂ O ₃ ·2SiO ₂ ), Al ₂ O ₃ , SiC, Si ₃ N ₄ and the like. Bridge Stone Co., Ltd. has developed three types of ceramic foam filters, namely 6, 13, 22 ppi, with a porosity of 80 to 90%, a bulk density of 0.35 to 0.60 g/cm ³ , and a thickness of 10~25mm.

Japan's Shinagawa White Co., Ltd. also produces foam ceramic filters made of Al ₂ O ₃ , SiC and Si ₃ N ₄ . According to the public

Division reported: When filtering non-ferrous alloys, the life of Si ₃ N ₄ is the longest; when filtering steel liquids above 1600 ° C, it is preferable to use composite oxides such as ZrO ₂ , MgO and Cr ₂ O ₃ .

Generally, the ceramic foam filter is subjected to "end treatment", that is, the same ceramic material as the body is used to block the peripheral pores so as to block the flow of the fluid to the surroundings and increase the strength. Some of the foam ceramic filter skeleton surfaces in Japan are coated with a resin to increase the strength, but this tends to cause pores in the casting.

Japan Shiqiao Zhengyong has successfully developed a sintered foam ceramic filter with a directionality (the elliptical cross section is elliptical) with a large number of perforated layers in the thickness direction, thereby increasing the number of collisions between the filtered fluid and the filter skeleton. To improve the filtering effect. Shiqiao Zhengyong's filter for cast iron filtration is made of SiC and has a thickness of 22mm. It has been successfully cast in the production by using dozens of filters (separately placed in each gate) to successfully cast up to 20 tons. / box of ordinary gray castings.

The patent for the ceramic foam filter applied by Toshiba to Toshiba in Germany in 1987 is based on Si ₃ N ₄ , plus 1% or more of Al ₂ O ₃ , SiO ₂ , TiO ₂ , MgO, ZrO ₂ and As a sintering aid, Cr ₂ O ₃ has a sintering temperature of 1500 to 1800 ° C, a bulk density of 0.7 g/cm ³ , and a compressive strength of 11 MPa. When there is no sintering aid, the compressive strength is only 1 MPa.

The development and development of foam ceramic filters for foundry in the UK also started earlier. In the early 1980s, the famous British Foseco company successfully developed a foam ceramic filter (model Sivex) for filtering non-ferrous alloys (especially aluminum-based and copper-based alloys), and filtering most of the cast iron and partially high melting point. Copper alloy foam ceramic filter (model Sedex), pore size specifications of 10, 20, 30ppi three, open porosity up to 90%, it can remove slag much smaller than 10um; filter aluminum alloy is generally used 10 and 20 ppi filters. The tool wear of the filtered aluminum alloy die-casting parts is reduced by 50% compared with the unfiltered castings; the filtration can increase the fatigue strength of the ferrite ductile iron by about 10%, and the tool wear is reduced by 0.04~0.1mm; After the steel box body adopts the filtering technology, the casting process yield rate is increased from 39% to 63% due to the simplification of the casting system. In the late 1980s, Foseco introduced a ceramic foam filter (model Stelex) for filtering steel castings with an open porosity of approximately 70%.

The patent for the first foam ceramic filter in the UK was Rolls-Royce from Britain's Fairey Ceramics. The company's ceramic foam filters are available for aluminum, copper, and magnesium alloys, but not for molten steel filtration. British aerospace industry castings used foam ceramic filters in the late 1980s to reach 25,000 pieces per month.

In addition to the United States, Japan and the United Kingdom, Germany and Switzerland have also successfully developed foam ceramic filters for casting alloy filtration, and have been widely used in castings of various materials such as sand casting, die casting and investment casting. The material composition of a ceramic foam filter obtained by Swiss Aluminum Co., Ltd. in 1988 is 50% or more of SiC, 30% or more of SiO ₂ , and the balance is Al ₂ O ₃ . Among them, SiO _{2 is} used as a gel binder. The ceramic foam filter is mainly used for the filtration of cast iron alloys.

Comprehensive analysis of the above results and Table 1 data, you can see:

2.1 The strength and service temperature of the sintered foam ceramic filter are higher than those of the bonded foam ceramic filter of the same material.

And its scope of application is wider, but the cost is higher.

2.2 In the same type of filter, the higher the open porosity, the lower the bulk density and strength.

2.3 The highest order temperature from high to low foam ceramic filter material order: partially stabilized zirconia - corundum

- Silicon carbide - mullite (high alumina bauxite). Some of the stabilized zirconia is particularly suitable for various high-temperature alloys. Corundum is more suitable for cast steel, silicon carbide is more suitable for cast iron, and mullite (high alumina bauxite) is suitable for cast iron and non-ferrous alloy.

2.4 When the corundum foam ceramic filter is toughened by adding zirconia, the strength is reduced, but the thermal shock resistance is significantly improved.

This will expand the range of applications.

3   Development status of domestic foam ceramic filters

The development of foam ceramic filters for foundry alloys began in the early 1980s. The foam ceramic filter that Harbin University of Science and Technology first developed in 1982 can only be used for the filtration of aluminum alloy. Since then, the school has developed foam ceramic filters that can be used for ferrous metal filtration. During this period, Shenyang Foundry Research Institute, Shanghai Machinery Manufacturing Technology Research Institute, Hubei Electromechanical Research and Design Institute, Nanchang Aviation Industry College, Dongfeng Motor Corporation and other units have also carried out research work on foam ceramic filters, and have achieved fruitful results. .

The filter for ferrous metals (CFF-I type) developed by Harbin University of Science and Technology in the mid-eighth year uses high alumina bauxite as the refractory aggregate with a hole size of 15 ppi and a thickness of 22 mm. After that, a CFF-II filter was developed by adding a small amount of Y ₂ O ₃ and CeO ₂ as a sintering aid. The sintering holding time was shortened from several hours to about 10 minutes, and the sintering temperature was also reduced by about 200 °C. However, the performance of the product does not change much. In order to improve the ability of the foam ceramic filter to resist brittle fracture when filtering high-temperature alloy, the school added ZrO ₂ toughening agent and TiO ₂ , refractory clay and other sintering aids in the corundum powder base, and used aluminum dihydrogen phosphate. For the binder, a foam ceramic filter for cast steel was developed at a sintering temperature of 1560 ° C, and its main physical properties were significantly improved (see Table 1). The calibration test of various metal liquids shows that the foam ceramic filter can effectively remove slag and some gases such as non-metallic inclusions, improve the metallographic structure of the casting, refine the grains, and improve the tensile strength of the aluminum alloy. 4~5%, the elongation rate is increased by 7~11%; the tensile strength of copper alloy can be increased by 22%, the elongation rate is nearly doubled, the hardness is increased by 8~20HB; the flexural strength of gray iron can be increased by 5%. The tensile strength is increased by about 16%; the tensile strength of ductile iron is increased by about 17%, the elongation is increased by about 40%, the hardness is increased by about 10%, and the hardness distribution is more uniform; the elongation of stainless steel can be increased by 67%, impact Increased toughness by 18%.

The Shenyang Foundry Research Institute has also developed a foam ceramic filter for cast iron with an open porosity of 80%, a density of 0.4 g/cm ³ , a maximum casting temperature of 1400 ° C and a flow rate of 15 kg/cm ² · min. The 1400 ° C molten iron can last for 10 minutes without breaking, and can be used for iron castings weighing up to 1.7 tons.

The foam ceramic filter developed by Dongfeng Motor Co., Ltd. has three materials, namely aluminum bauxite (Al ₂ O ₃ 50%, SiO ₂ 35%), Al ₂ O ₃ (80% Al ₂ O ₃ , 20% SiO _{2 ).} And SiC (45% SiC, 45% Al ₂ O ₃ ), the hole density is 10, 20 ppi and other specifications, the thickness is 22mm. The application of the company to the SiC foam ceramic filter in the ductile iron production line shows that the strength and elongation of ductile iron are increased by 15.3% and 12.5%, respectively, and the dispersion is reduced, and the casting process yield ratio is higher than that of the non-filtering. The use of honeycomb straight-hole ceramic filter increases by 2.8%, the casting rejection rate is reduced by about 50%, the tool life during machining is more than doubled, and good economic and technical benefits have been achieved.

The foam ceramic filter material developed by Shanghai Machinery Manufacturing Technology Research Institute is mainly made of high alumina bauxite with a porosity of 80%, a thickness of 15mm, a compressive strength of 1.5MPa, a flexural strength of 0.3MPa and a sintering temperature of 1500~. At 1600 ° C, the sintering time is about 10 h.

Nanchang Institute of Aeronautics and Astronautics focuses on pure magnesium oxide foam ceramic filters for magnesium alloy filtration. Since the standard formation of magnesium oxide is very low, the molten magnesium at high temperature is very easy to form MgO with some standard free oxides such as SiO ₂ and Al ₂ O ₃ , which will cause Al ₂ O ₃ and SiO _{2 .} The ceramic skeleton of the filter which is a ceramic base material is rapidly eroded and disappears, and the reaction product such as MgO enters the magnesium liquid to seriously contaminate the magnesium liquid. The school has developed a high-temperature solid-phase sintered pure magnesium oxide foam ceramic filter for the characteristics of magnesium alloy. The ceramic skeleton of this filter itself is not dense, and the apparent porosity is 5 to 10%. This microscopically rough porous ceramic skeleton is not only easy to adsorb fine inclusions, but also adsorbs flux inclusions in the molten state in the magnesium liquid (MgO and the flux are completely wetted). After purifying ZM-5 magnesium alloy liquid by pure MgO foam ceramic filter, there is almost no inclusion in the casting, the tensile strength can be increased by 25.2%, the elongation is increased by 82%, and the magnesium alloy of the casting system can be completely recovered. It can float out when the alloy is remelted. Nanchang Institute of Aeronautics and Astronautics also uses silicon carbide as the ceramic base material and a small amount of boride as a sintering aid. The foam ceramic filter for cast iron is fired at a lower sintering temperature, which has higher strength and lower cost. High temperature hot metal impact for more than 3min. The comparison between the filter and the high temperature resistant fiber filter on the ductile iron crankshaft shows that the average diameter of the graphite sphere is significantly reduced and the distribution is more uniform than when using the fiber filter.

Hubei Mechanical and Electrical Research and Design Institute began research on foam ceramic filters in 1988. According to different working conditions, mullite (MF type), corundum (AF type), zirconia-mullite was developed. (ZMF type) and zirconia-corundum (ZAF type) four kinds of foam ceramic materials, the main technical properties and application range are also shown in Table 1, the open porosity is 75~85%, normal temperature compression The strength and high temperature compressive strength are 2.5~3.5Mpa and 1.3~2.0MPa respectively, the refractoriness is 1640~1800°C, and the maximum operating temperature is up to 1650°C, which is close to the international advanced level. The production process of this series of ceramic foam filter is characterized by: selecting a new type of auxiliary agent with small thermal expansion coefficient, high refractoriness, excellent sintering capacity and ceramic slurry suspension and dispersion ability; ceramic slurry selected The binder is an inorganic binder having high strength, low gas generation and low cost. Through experimental and regression analysis, the institute found that with the decrease of open porosity, the compressive strength increases along the power function of open porosity, and the filtration efficiency also increases, but the flow resistance to liquid alloy also increases; At a rate of about 80%, the filter has the best combination of filtration efficiency, strength and flow resistance. The application practice of this series of ceramic foam filters in the production sites of more than ten foundry enterprises shows that through its efficient filtration of various inclusions and refinement of crystal phase structure, casting defects can be significantly reduced or eliminated; The tensile strength, impact toughness and hardness of gray iron are increased by 6.9%, 14.3% and 19.4%, respectively, which is equivalent to an increase of grade grade; the flexural strength, impact toughness and hardness of high chromium alloy wear-resistant cast iron can be improved respectively. 14%, 15% and 30%, and the above mechanical properties can be reduced by 33~67%, that is, the performance is more stable; the tensile strength, yield point and elongation of carbon steel can be increased by 4%, 13% and 6% or so. Further research in the hospital in recent years has enabled foam ceramic materials to be successfully applied in the fields of filtration and purification of high-temperature exhaust gas, high-temperature heat insulation, high-temperature and high-humidity noise reduction and noise reduction.

4 Trends in foam ceramic filters

As mentioned above, in the past two decades, through the efforts of foundry workers, foam ceramic filters and their application in the field of liquid casting alloy filtration have been greatly developed. According to incomplete statistics, nearly ten foam ceramic filter production lines have been put into production in China, and the annual production has reached several million pieces. However, according to estimates, the potential annual demand for China's foundry industry is about 100 million pieces. Therefore, in the new century, the research, production and application promotion of foam ceramic filters still need to be strengthened, and the focus should be placed on the following aspects:

4.1 Development and production of new low-cost foam ceramic filters with low sintering temperature and short sintering time.

4.2 Develop and produce new foam ceramic filters suitable for various active alloys with high temperature and physical properties.

4.3 Develop and produce shaped foam ceramic filters suitable for special casting processes such as investment casting and metal casting.

4.4 In-depth study of the filtration and purification mechanism of foam ceramic filters and the mechanism of influence on the metal solidification process.

4.5 The system studies the application of foam ceramic filters, including the choice of aperture and thickness, the placement and construction of the gating system, the pouring temperature and speed, and the control of the metal hydraulic head.

4.6 Carry out serialization and standardization of foam ceramic filters.