Synthetic Solution for Cores & Molds?
An investigation looks at how sintered bauxite holds up to the demanding thermal conditions of the casting process.
Scott Giese, Univ. of Northern Iowa, Cedar Falls, Iowa, and Rafael Curimbaba Ferreira, Groupo Curimbaba, Poços de Caldas, Brazil
(Click here to see the story as it appears in the July issue of Modern Casting.)
As the demand of molding aggregates to meet special casting requirements increases, sands such as chromite and zircon have been introduced to the industry with the goal of producing defect-free, high quality castings while minimizing metalcasters’ post-processing costs. The challenge of synthetic sand producers is supplying molding aggregates that are comparable to naturally occurring sands at a substantially lower cost. Changes in resin demand levels, possible thermo-chemical reactions during mixing and higher permeability issues have become concerns for metalcasters who adopt these aggregates.
Casting trials were performed to evaluate the physical properties of synthetic aggregate materials directly produced from bauxite and compared to properties and casting performance of silica and chromite sand. This case study investigated the abilities of synthetic sand (sintered alumina produced directly from the mining source) to meet the harsh thermal conditions of the casting process while developing molding properties to address metal penetration and veining defects associated with ferrous casting applications.
The trials evaluated physical properties for each sand sample for AFS-GFN, tensile profiles, specific surface area, pH, acid demand value (ADV), base permeability, bulk and tapped density, specific heat capacity and linear expansion. Step-cone castings and Gertzman castings were poured for evaluating casting quality performance in low carbon, low alloy steel, class 35 gray iron and 60-40-18 ductile iron. Tensile cores and cores for casting analysis were prepared using the furan nobake, phenolic ester-cured nobake and phenolic urethane cold box binder systems. Physical properties were determined to be comparable to the base line sands investigated.
At the onset of the research work, three major molding requirements—resin demand properties development associated with the synthetic sand, surface quality and refractoriness—were addressed to assess the feasibility of advancing the technology of a synthetic sand. To properly assess the performance of the synthetic sand system compared to natural alternatives, researchers performed tests for physical properties with three binder systems and ferrous casting experiments using two standard tests.
The sintered bauxite product has demonstrated satisfactory casting performance in a laboratory and metalcasting environment. Subsequent investigation using an improved sintered bauxite product showed improvement in metal penetration resistance. Based on the laboratory results and industrial trial, the sintered bauxite aggregate meets casting requirements for ferrous applications without veining defects.
Specific Surface Area, pH, Acid Demand Value and Base Permeability
According to the test results, the Bauxite 40/50 aggregate exhibited a surface area half of silica, indicating sintered bauxite could require less binder, though not in the same percentage reduction, based on the baseline surface area of silica sand. The binder demand for sintered bauxite would be comparable to chromite sand. However, chromite exhibited lower surface area, which is unexpected because of the sand distribution and acicular nature of chromite. Based on this observation, bauxite might have surface porosity that increases the surface area and could influence binder demand. The most notable feature of the bauxite product was the exceptional permeability. Though the advantage of lower permeability would be related to a decrease in gas-related defects, the increase in permeability would lead to a higher propensity of metal penetration defects.
Bulk & Tapped Density
The sintered bauxite has a density between chromite and silica sand, indicating molds and cores using sintered bauxite would be lighter than chromite sand and heavier than silica sand. Additionally, sintered bauxite would require less binder than chromite based solely on the density.
As seen in Fig. 1, silica, as expected, goes through the alpha-beta phase transition at 1,045F (580C) and starts undergoing a cristobalite transition at 2,410F (1,300C). Bauxite 40/50, bauxite 40/100, and chromite display lower linear expansion than round grain silica. The bauxite 40/100 exhibited a lower linear expansion rate up to 1,475F (800C) than the bauxite 40/50 product but then the expansion rate increases up to the identified sintering point. Chromite starts sintering near 2450F (1350C) while bauxite 40/50 and bauxite 40/100 start sintering at 2,100F (1,150C).
Specific Heat Capacity
The bauxite 40/100 showed a rise in thermal heat capacity to approximately 800C, starting at 0.75 J/g C and increasing to roughly 1.4 J/g C (Fig, 2). Chromite 40/100 showed a thermal capacity range of 0.65 to 1.4 J/g C for the temperature range investigated. In the case of silica 50/140, the specific heat capacity increased from 0.8 to 1.1 J/g C to approximately 575C followed by a decrease to 1.0 J/g C at 650C with a subsequent increase to 1.1 J/g C at 800C. This change in thermal heat capacity between the range of 575C and 650C can be attributed to the phase change of silica. Based on the results, sintered bauxite 40/100 has a thermal heat capacity between silica 50/140 and chromite 40/100.
Tensile Strength Results
The significant contributor to higher tensile properties of silica sand for the furan binder system can be attributed to the higher surface area and broad screen distribution, providing numerous binder bridge contact points. The bauxite 40/50 was comparable in strength, though slightly lower, than silica sand. When tested using the phenolic ester-cured binder system, no observable differences were detected between all three aggregates. For the phenolic urethane binder system, the bauxite 40/50 was observed to have the lowest tensile strength. Because bauxite 40/50 is a sintered aggregate, it has a tendency to absorb the phenolic urethane resin. However, the bauxite 40/50 tensile strength profile would be acceptable for metalcasting applications.
Step Cone Casting Analysis
The casting analysis results for coated and uncoated cores produced using the phenolic ester-cured binder system poured with a class 35 iron are shown in Table 1. The best performing aggregate was the bauxite 40/50 with a graphite coating. The significant contributing factor to the overall index was penetration resistance. When comparing the index values for uncoated cores, penetration resistance was comparable to chromite sand, which can be attributed to the grain fineness value for bauxite and chromite. Silica sand performed the worst for coated and uncoated aggregate cores based on the overall index value, mainly due to a propensity to form veining defects.
Table 2 shows the casting analysis results for coated and uncoated cores produced using the furan binder system. Bauxite 40/50 performed the best in overall defect resistance with the coated and uncoated bauxite ranking in the top three best overall performing aggregate material. The major attribute was its veining resistance because of the low thermal expansion properties of bauxite 40/50. However, the penetration resistance was markedly reduced, attributable to low grain fineness and high permeability of bauxite.
Table 3 shows the casting analysis results for coated and uncoated cores produced using the phenolic urethane coldbox binder system. The overall defect index value of the bauxite aggregate performed noticeably better than silica sand but was slightly worse that chromite sand. Similar to the furan binder system, lower penetration resistance was the contributing factor to the overall defect ranking.
Additional step cone castings were poured to evaluate three bauxite products that were engineered to combat metal penetration. The aggregates investigated were bauxite 40/70, bauxite 40/100 and bauxite 70/140. Screen distribution and GFN values for these aggregates are presented in Tables 4 and 5. The purpose of the testing was to broaden the screen distribution and increase the GFN to decrease the high susceptibility of metal penetration observed with the bauxite 40/50 product. Since no veining defects were observed with the bauxite product, only metal penetration was evaluated upon sectioning the castings. Additionally, only class 30 gray iron and low carbon, low alloy steel were poured into phenolic urethane cold box cores. Noticeable improvement in metal penetration resistance was observed for both ferrous alloys, demonstrating synthetic sands are viable alternative aggregate materials if engineered for specific applications.
Gertzman Casting Analysis
Analysis of Gertzman cores was performed to assess metal penetration, sintering and veining defects of bauxite 40/50, silica sand and chromite sand when subjected to a high metallostatic height (approximately 18 in.) and severe heat flow.
The bauxite 40/50 product performed better than silica sand and comparable to chromite sand for gray and ductile iron castings. The uncoated cores exhibited less metal penetration than silica sand but were observed to have a very rough surface. For coated cores, the degree of metal penetration was reduced for the bauxite 40/50 aggregate but, again, a rough surface was observed. Both of these observations for coated and uncoated bauxite 40/50 for gray iron is a strong indication that the high permeability of the product is detrimental in surface quality. This was also confirmed with the ductile iron castings. In this case, silica sand did not exhibit any metal penetration, though substantial veining, for graphite coated and uncoated cores. For ductile iron, metal penetration is not commonly seen on coated or uncoated cores because of the higher surface energy between ductile iron and the molding aggregate. Bauxite 40/50 did have appreciable metal penetration but was observed to have significant rough surface features on the entire surface of the cores, confirming the screen distribution of Bauxite 40/50 is not appropriate for casting applications.
Under extreme conditions such as steel casting temperatures, bauxite 40/50 did not perform as well as silica and chromite sands. The major observation was the fused mass surrounded by steel penetrating the synthetic aggregate, making it extremely difficult to remove and rapidly degrading the cutting saw blade. Chromite and silica sand was observed to have extensive metal penetration, though not as severe with chromite. However, depending on the depth of metal penetration, shallow penetration could be removed.
Another series of Gertzman tests were performed to investigate the synthetic aggregate bauxite 40/100 product. The experimental binder system used for the test cores was 1.25% phenolic urethane nobake poured with a class 35 gray iron at 2,600F.
As expected, the silica sand exhibited massive metal penetration in the uncoated condition and extensive veining defects for the zircon coated cores. The bauxite 40/100 product did not exhibit any veining defects for the coated and uncoated condition. Slight metal penetration was observed for both the uncoated and zircon coated cores. However, when comparing to bauxite 40/50, significant reduction in metal penetration was observed, confirming aggregate distribution contributes significantly to penetration resistance for the bauxite product. The bauxite 40/100 product performed equally well when compared to chromite and zircon sand.
Industrial Casting Trial
A casting trial at a steel casting facility was performed to advance the product from the laboratory environment to actual part production, measuring the performance of the bauxite product to silica sand. A complex stainless steel valve body with two internal cores was selected as the test casting, because the solidification pattern around the two cores produces heat saturation, giving a higher propensity to cause metal penetration, aggregate sintering and veining. Because of this casting geometry, the metalcaster coated the two cores with a zircon water-based solution. To increase the casting difficulty of the bauxite 40/100 product, the two cores were not coated.
The stainless steel casting was poured at 2,850F and allowed to cool for approximately four hours before shakeout and blast cleaning. The internal features were observed for surface finish, comparing the zircon-coated silica sand cores to the uncoated bauxite 40/100 cores. For both castings, no metal penetration or veining defects was observed. The bauxite 40/100 cored area showed a slightly rougher surface than the zircon coated silica sand cored area. However, the manager indicated the surface finish of the bauxite 40/100 product was accepted and would be included into the finish casting lot.
The bauxite aggregate’s performance is comparable to other metalcasting aggregates. No significant physical properties issues were observed to affect the performance for three different sand binder systems. The heat capacity property of bauxite is similar to silica sand, expansion characteristics comparable to zircon and chromite sand, and chemical properties analogous to the aggregate materials investigated. Through some modification with the screen distribution, the bauxite aggregate performed as well as silica sand, chromite sand and zircon sand. The series of evaluation tests clearly illustrated bauxite can be used as synthetic sand for the metalcasting industry.