Historically, foundries worldwide have been forced to rely on organic-based core and mold binders, and the use of refractory coatings, along with sand additives, when the hot strengths of the binders are insufficient to produce salable castings.
In 1995, Cadic Corp., and its subsidiary CTI Inc., introduced the Cadic Convert Process, which uses a liquid mixture of silicon oxide and sodium in alcohol to treat resin-bonded molds and cores. In practice, cores and molds produced using organic heat- and amine-cured resins were dipped or immersed in the ethanol solution, dried, and subsequently fired in a kiln to convert the organic-bonded article to an inorganicbonded mold or core. 1, 2, 3, 4
This post-production treatment of organic-bonded aggregates resulted in the formation of organo-silicates that were converted to ceramics when fired at elevated temperatures, and poured hot to produce thin-wall (2 mm or less) stainless steel castings.
Laboratory evidence, supported by foundry trials, confirmed that the silica compound formed a crystal lacuna surrounding the sodium (Na) at room temperatures and allowed the Na ion to retain its high degree of reactivity.
The subsequent high-temperature treatment resulted in the conversion of quartz to tridymite, thereby increasing the high-temperature properties of the core or mold.
Because of the high degree of reactivity, the Na ion is utilized in this process as the alkali oxide Na2O. In this new process, the nano silica composition allows the Na ion to maintain its reactivity from room temperature to the boiling point of the compound.
With the elimination of a post-production treatment for the core or mold using ethanol as a carrier, the resulting new development makes the Cadic Convert Process considerably simpler, because the nano silica compounds can be added to the sand mix prior to the production of the core or mold in any organic resin-bonded processes, such as phenolic urethanes and shell sand mixtures.
Formation of silica compounds
Preparation of nano silica compounds is accomplished by blending alcoholic solutions of silicon oxide and sodium under controlled conditions. The resulting product is α-trydimite when reacted at 117°C.
The Na ion is captured within the lacuna or shell of the crystal formed without losing its reactivity of metallic sodium. When exposed to temperatures of approximately 800°C, the alkali oxide is formed and as the crystal begins to melt the Na ion reaches its boiling point, and begins to evaporate.
As the temperature is increased to 867°C, the silica crystal is converted into β-tridymite by the alkali oxide. As the temperature increases above 867°C, the Na ion evaporates and the black silica composition becomes a white tridymite crystal.
Thermal analysis of the SiO2-Na compound confirms that SiO2 is deoxidized at 778.91°C, and that the Na ion oxidizes at 882.91°C, generating Na2O. Further investigation confirms that the Na ion evaporates at 1,044.96°C.
Crystalline silica formation — Both TEM at room temperature and X-ray diffraction at 900°C confirm the formation of the trydymite crystal.
The quartz crystal converts into α-tridymite at lower temperatures but the conversion of quartz to β-trydimite at higher temperatures passes through cristobalite in the intermediate phase. The conversion of the crystal of the silica composition is similar to the conversion of quartz to tridymite.
The crystal lacuna, or shell, of α-tridymite has a radius of approximately 1.2 Å. The Na ion of the crystalline shell acts as a catalyst in the formation of the larger crystalline shell, as it expands during formation to 1.75 Å in the 800°C temperature range.
The Na ion, with a 1.1 Å radius expands and deoxidizes the SiO2, with the result that Na2O is liberated and forced out of the shell as a black by-product. Further temperature increases up to the boiling point of Na results in the distillation of Na, and the remaining silica crystal is converted to β-Tridymite at 867°C by the generated oxidizing alkali.
Improved heat resistance of organic-based polymeric materials
Because of the strong reducing properties of the Na ion at 200°C, the nano silica compound reacts with the organic polymers and forms an organic/silica compound that exhibits significantly improved hot strength properties.
Thermal deflection test — Using the thermal deflection test, the organic mold with the silica-coated sand is compared with the conventional organic mold.
On this evidence, we can observe that the silica composition formed organic silica compound at 200°C; that the heat resistance of the polymeric materials was improved by equal to or more than 300°C; and that the shape of tridymite remains at 1,000°C.
Nano-Silica Coated Sand Composition — Cera-Beads (a mullite product containing artificial spherical sand) of 80-mesh particle size were coated with an organic phenolicurethane no-bake binder (Ashland Casting Solutions’ Pep Set product) and the nano-silica compound. Using scanning electron microscopy (SEM) to observe the effects, it was concluded that the Na contents evaporate, whereas the tridymite silica assumes a “binding state” and the alumina silicate maintains a granular structure.
Basic core and mold technology — Essentially all organic binders are compatible with the nano-silica compound. In addition, a wide variety of grain size and distribution are compatible.
The Na ion provides a strong reducing atmosphere and evaporates at high temperatures.
Reduced organic resin levels may be employed, because of the increase in the final strength of the converted binder. Veining, erosion, and burn-in can be reduced due to the higher thermal properties of ceramics.
Artificial sands are easily adapted to the new technology.
Post-treatment of molds and cores, for example baking, or exposure to kiln temperatures, as required in previous ceramic technologies, are not required.
Ceramic Cores — In addition to the conventional sands employed in metalcasting processes, artificial sands, such as Cera- Beads, offer very little expansion and are very compatible with the new process because their low-expansion characteristics contribute to their performance in high-temperature environments.
High-temperature molding
In 1995, Cadic Corp. introduced the Cadic Convert Mold Process to the international foundry industry, for the production of thin-wall stainless steel castings. While this process enjoys some success, acceptance has been limited due to the requirement for post-treatment of an organic-bonded mold with an ethanol solution of Na and Si oxides, followed by drying, and subsequently firing in a 1,000°C kiln.
The new Convert Mold Process eliminates the need for posttreatment with alcohol solutions, and offers several advantages.
Hot Tear — The mmE (metal mold equilibrium) curve, invented in the 1970s, is useful in investment casting when hightemperature molds are used. The temperature of high-temperature molds increases immediately after casting. Therefore, the mmE temperature at which the solidification point of the metal becomes equal to the mold temperature is of great importance. Thin-wall cast steel can be produced under excellent conditions, as it solidifies before reaching the mmE temperature. On the other hand, the restraint provided by the mold due to its thermal expansion at ZDT (Zero Ductility Temperature), at which the strength of metal becomes zero, must be taken into account. Otherwise, “hot tear” will occur. Particularly in the casting of 1-mm-thick cast steel products in the effort to attain smoother movement of the molten steel, it is more effective to increase the mold temperature than to increase the casting temperature.
When a 1,000°C mold is used, the thermal temperature must be minimized to prevent the restraint of molten metal by the mold.
Grain size — In 1946, Dr. Nicholas J. Grant reported that grain size does not affect the mechanical properties of cast steel. In stress rupture tests, larger grain sizes will indicate a longer rupture life. Those experimental results are reflected in today’s “Directional Solidification” and “Single Crystallization” technologies for investment casting of turbine blades for aircraft manufacturing.
Gas defect — The products are free of gas defects, as the mold emits no gas during casting.
Gravity pouring — As the fluidity of molten steel is high, casting can be conducted easily under normal gravity in high-temperature molds, with no need for counter gravity or suction during the pouring process.
Large casting yield — Casting yield is generally improved and a large feeder head normally is not used, reducing the total amount of metal required for casting.
Over the course of history, the process of converting molten metal into usable shapes has improved continuously thanks to innovative refinements to the casting process. While both productivity and quality have been significantly improved, basic complications associated with the casting process continue to require additional refinements to production methods.
Historically, the industry’s need for thinner or lighter castings has been limited by molding and coremaking capabilities, and by the metal being cast.
As a result of the development and use of the nano silica cell compositions, another advance in casting technology can be achieved with conventional casting processes and materials. Notable improvements include the fact that steel casting can be used to produce aircraft products, and for lighter and more heat-resistant components to improve the fuel efficiency of automotive products. In addition, the nano silica cell composition can be expected to enhance production of other advanced materials and products, such as alkaline ion batteries and the inorganicity of the polymeric materials.
| Nobuyoshi Sasaki is the president and chief executive officer of Cadic Corp., Yokohama, Japan. Contact him at [email protected], or visit www.cadic-global.com |
References
1. Noboyushi Sasaki, “The Development of a Process for Manufacturing Mould for Thin Wall Castings,” BCIRA World Conference Best Paper Awards in York England (1996)
2. Nobuyoshi Sasaki, William L. Tordoff, “Thin-Wall Steel Casting Process for Exhaust Components,” Society of Automotive Engineers, Inc., 012BECB-43 Paris France (2002)
3. Nobuyoshi Sasaki, “Hollow Castings for Commercial and Automotive Industries an Alternate Cost Effective Solution,” The 9th World Conference on Investment Casting in San Francisco USA (1996)
4. Nobuyoshi Sasaki, USP5569320 USP5611848, EP066 12 45 B1 1996, Oct. 29, “Process for Preparing Refractory Molded Articles and Binder Therefore”
5. S.B Holmsquist, “Conversion of Quartz to Tridymite,” J. Am. Ceram. Soc. 44[2] (1961)