Improving Casting Quality Through Sand, Metallurgy
A sampling of white paper technical presentations from the 119th Metalcasting Congress focus on sand and metal for improved cast components.
A Modern Casting Staff Report
(Click here to see the story as it appears in the May issue of Modern Casting.)
Every year, Metalcasting Congress is anchored by a peer-reviewed technical program featuring panel discussions as well as white paper technical presentations that reveal the results of recent and ongoing research designed to improve the quality of metalcasting operations.
We have selected three of the white papers that were presented in April to summarize below. These well-reviewed technical papers from leading experts in the field represent lab studies that could have a practical impact on cast component quality.
Comparing Sand Additives for Steel Castings (15-006)
Ralph Showman and Eric Scheller, ASK Chemicals, Dublin, Ohio
Sand additives commonly are used in sand molds and cores for steel castings. The additives can impact casting quality both physically and chemically by reducing casting defects like veining, metal penetration and surface finish. Chemical interactions between the steel and mold/core materials like carbon pickup or dissolved gas also can be minimized.
Historically, iron oxides have been the additives of choice for steel castings. Both red iron oxide (hematite, Fe2O3) and black iron oxide (magnetite, Fe3O4) have been used successfully. However, iron oxides also can have negative impacts both in cost and casting quality. A number of engineered sand additives (ESAs) have been developed that are advertised to provide lower cost, improved veining resistance, or fewer gas defects, etc., but little comparative data has been available.
A design of experiments was developed to compare the performance of several ESAs to red and black iron oxides. Two different additive levels, high and low for each type, were selected. Two sands were selected, both round grain high purity silica with AFS GFN of 51 and 34. The additives tested included:
- Sand additive SX, a synthetic ESA consisting primarily of iron oxide but with a grain size similar to sand and a rounded particle shape.
- RIO, red iron oxide or hematite (Fe2O3).
- BIO, black iron oxide or magnetite (Fe3O4).
- Additive V2, a mineral ESA with some red iron oxide.
- Additive I900, a mineral ESA with some red iron oxide.
- V400, a mineral ESA based on red iron oxide.
- V2003, a mineral ESA that contains spodumene, a lithium ore.
- IL, a naturally occurring mineral called ilmenite.
The impact on the mold/core properties and on casting quality was measured over a range of process parameters, including work time/strip time, tensile strength, humidity resistance, permeability and smoke opacity. This may provide some additional insight on which sand additive should be used to achieve a particular set of desired results.
The results of the design of experiments highlight the strengths and weaknesses of various sand additives and provide insight as to which sand additive should be used for specific casting results in steel castings and what, if any, trade-offs can be expected on mold/core properties. However, the effects of the sand additives on mold/core properties outlined by the results of this design of experiments are limited to the PUNB systems, as other binder systems have different mechanisms of reaction.
The V400 ESA was notable in that it had a significant effect on slowing the work time/strip time but it also reduced the smoke opacity to about 50% of that seen with the other additives. Additional work is underway to further evaluate and quantify these effects.
Use of Sintered Bauxite in Molding Applications (15-075)
Scott Giese, Univ. of Northern Iowa, Cedar Falls, Iowa
Since the 1990s, synthetic sands have been introduced to the metalcasting industry to address the limited availability of specialty sands and cost associated with transporting these sands over long distances to the foundry. Derived and processed from lower cost ceramic materials, synthetic sands have been developed to meet molding and casting requirements attained from natural occurring specialty sands. The challenge of synthetic sand producers is producing a comparable molding aggregate at a substantially lower cost. Additionally, changes in resin demand levels, possible thermo-chemical reactions when molding aggregate stream are mixed, and higher permeability issues leading to casting surface changes have been a concern in adopting these types of molding aggregates.
This case study presents the preliminary investigation of engineering a synthetic sand 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.
At the onset of the research work, three major molding requirements were addressed to assess the feasibility of advancing the technology of a synthetic sand:
- resin demand properties development associated for the synthetic sand.
- surface quality.
The synthetic sand investigated was sintered alumina produced directly from the mining source. To achieve these three research goals, physical properties with three binder systems and ferrous casting experiments using two standard tests were performed.
A casting trial at a steel alloy casting facility was performed to advance the product from the laboratory environment to actual part production, measuring the performance of the sintered bauxite product to silica sand used by the foundry. To observe the performance of the bauxite product, a complex stainless steel valve body with two internal cores was selected as the test casting. After casting and shakeout, the internal features of the stainless steel valve 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 were observed. The bauxite 40/100 cored area showed a slightly rougher surface than the zircon coated silica sand cored area. However, the foundry manager indicated the surface finish of the bauxite 40/100 product was accepted and would be included into the finish casting lot.
Assessment of the bauxite aggregate indicates performance is comparable to other aggregates available in the metalcasting industry. No significant physical properties issues were observed to affect the mold-making performance of the bauxite product for three different sand binder systems. Physical property evaluation showed 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 product subjected to laboratory and actual casting conditions performed equally 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.
Alternatives to the Al-Si Eutectic System in Aluminum Casting Alloys (15-089)
Theodoros Koutsoukis and Makhlouf Makhlouf, Worcester Polytechnic Institute, Advanced Casting Research Center, Worcester, Mass.
The majority of the traditional aluminum casting alloys are based on the aluminum-silicon eutectic system because of its excellent casting characteristics, including good fluidity and resistance to hot tearing. Unfortunately, the solidus in this system does not exceed 1,071F (577C) and the major alloying elements in traditional aluminum casting alloys, e.g., zinc, magnesium, and copper, further depress the alloy’s liquidus temperature. Moreover, these alloying elements have high diffusivity in the aluminum solid solution and their diffusivity increases with increased temperature. Therefore, while casting alloys based on the aluminum-silicon eutectic and traditional strengthening elements have good room temperature strength, their strength is rapidly degraded when they are used at temperatures higher than 482F (250C). Furthermore, silicon interacts with aluminum and iron—which is invariably present in commercial aluminum casting alloys—to form harmful intermetallic compounds, such as the β-(Al5SiFe) phase, which adversely impacts the alloys’ ductility, or with precipitation strengthening elements.
In many applications, stability of mechanical properties at high temperature is the primary need, not high strength at room temperature. In this study, the binary aluminum-nickel and aluminum-iron eutectics, as well as the ternary aluminum-iron-nickel eutectic are considered as alternatives to the aluminum-silicon eutectic for constituting aluminum casting alloys. The purpose of this investigation is to measure the casting ability (i.e., the tendency to hot tear and the ability to fill the mold cavity), the room and elevated temperature (662F or 350C) tensile properties (i.e., ultimate tensile strength, yield strength and elongation) of these eutectic compositions, and compare them to their counterparts for the aluminum-silicon eutectic and, when necessary, also to those of commercial grade A390, A206 and 413.0 alloys.
The three alternative eutectic systems investigated, namely the Al-Ni eutectic, the Al-Fe eutectic and the Al-Ni-Fe eutectic, exhibit very good casting ability and tensile properties at both room temperature and elevated temperature; and in this regard, they compare favorably with the Al-Si eutectic. Among the four eutectics, the Al-Ni eutectic has the highest strength at room temperature, but Ni is significantly more expensive than Si and Fe. The Al-Fe eutectic has excellent casting ability and is inexpensive compared to the Al-Ni eutectic, but it has the lowest room temperature strength among the four eutectics. The elevated temperature (662F or 350C) tensile properties of the Al-Ni, Al-Fe and Al-Fe-Ni eutectics are superior to those of the Al-Si eutectic with the 662F (350C) tensile strength of the Al-Ni-Fe eutectic being the highest among all four eutectics. Among the four eutectic systems investigated, the Al-Fe-Ni eutectic is the most thermally stable.
The Al-Fe-Ni eutectic combines characteristics of both the Al-Ni and Al-Fe systems, i.e., good casting ability, good elevated temperature tensile properties, and low cost. Therefore it is the most promising of the eutectic systems for high temperature applications.
The three papers summarized in this article were presented at the 119th Metalcasting Congress in Columbus, Ohio.