Research aims for improved castings to attract customers.

The foundry industry must make special efforts to woo dubious customers into seeing the added value of castings as a manufacturing option.

The industry has made great strides internally through research, recognizing the need for improvements in casting processes and innovations that take advantage of all available or emerging technology. But when it comes to turning that research into new casting jobs and customers, the industry has fallen short.

In addition to focusing on casting improvements, foundries must convince potential customers that metalcasting is a viable and attractive option. Recognizing this need, the industry has responded through various research projects aimed at achieving both casting improvements and documenting industry capabilities as the means to improve products.

AFS and its coalition partners have more than 50 active research projects (many with that goal) at a total investment of close to $15 million. In an attempt to begin the transfer of some of those research findings to the industry, this article examines some recently completed projects as well as some ongoing ones.

The research discussed is funded by AFS, the Defense Logistics Agency (DLA) through the American Metalcasting Consortium (AMC), the U.S. Dept. of Energy (DOE) Office of Industrial Technology through the Cast Metals Coalition (CMC), the United States Council for Automotive Research (USCAR), and the NASA Marshall Space Flight Center through the Solidification Design and Control Consortium (SDCC).

Mold Media Thermo Physical Properties

Background: Precise thermal data for all characteristics of a casting process is required to provide accurate predictions from any physics-based casting simulation program. Sensitivity analyses indicate that the accuracy of the predictions is severely affected by inaccuracies in the thermal input properties (specific heat, density, thermal diffusivity and thermal conductivity) for the molding materials.

The generic thermal data for molding materials used by most simulation engineers is more than 20 yr old. Significant changes in mold and core binder technology have occurred during this period, along with the advent of more sophisticated measurement techniques. Any inaccuracies in molding media thermal properties are expected to magnify as the industry moves toward thin-walled casting capability to improve productivity and meet customer needs.

Goal: The AFS Engineering Div.'s Process Design and Modeling Committee (1-F) undertook a research project to quantify the thermal characteristics of several mold materials (green sand, urethane chemically bonded molding sand, chemically bonded shell molding sand and furan chemically bonded molding sand). The Molding Methods and Materials Div. (Div. 4) provided guidance on seven types of molding materials to test and helped collect samples for testing at K+P Agile, Inc., Naperville, Illinois.

Process: Computer simulations and validating foundry trials were conducted to compare the older generic data with the new data.

Results: Generic mold material thermal data produces a slightly conservative analysis with a negative safety margin (the distance from the bottom of the pipe to the riser/casting interface divided by the poured height of the riser) for a tightly rigged cube (Fig. la). The new mold material data improved the prediction of feeding, riser piping and thermal profiles and solidification time (Fig. 1b).

Foundries can use this data to increase the accuracy of simulations, using them as a tool to meet customer specifications on the first try. As customers demand thin walled castings, foundries will have better insight into integrating design quality into the process.

Note: Since the new mold material thermophysical data has not been extensively validated, users are advised to conduct some validation efforts within their foundry on castings with known results (riser piping, cooling curve data, etc.) prior to extensively applying the data.

Porosity Formation in White Iron

Background: For most iron foundries, porosity occurs sporadically. However, even occasional outbreaks can be costly since a small amount of porosity can reduce the mechanical properties of castings significantly. When porosity is detected, castings are scrapped and re-melted. If porosity is undetected, poor quality parts are shipped to the consumer. Neither case is desirable.

Goal: The Cast Iron Div.'s Special Irons Committee (5-D) began evaluating factors affecting the erratic occurrence of porosity in white cast irons at the DOE Albany Research Center.

Process: A series of vacuum induction melts produced ingots with a low content of gaseous elements. Induction melts were performed under gas pressures higher than ambient pressure to in-vestigate the relationship between gas pressure, melt gas content and porosity level.

Results: It was found that there is a critical level of nitrogen content for each composition-- above that threshold, nitrogen contributes to the formation of porosity. White iron can pick up a significant amount of nitrogen from the atmosphere during induction melting. The principal causes of gas pick-up from air are excessive superheat and extended hold times. Using a lower melt temperature limits the amount of nitrogen absorbed by the molten iron (Fig. 2). In less than 30 mm, nitrogen saturates the melt.

Keeping dissolved gases out of the melt is as important as techniques to remove gas from the melt. Limiting porosity formation through shorter holding times and lower melt temperatures means fewer scrapped castings for the foundry and less opportunity for undetected porosity reaching the customer.

Reducing superheat and hold times not only increases productivity--it also improves quality.

Note: DOE's Office of Industrial Technology has funded a new project through the CMC to quantify the critical gas threshold in various cast irons below which porosity formation is innocuous. With this knowledge, it will develop a practical model to predict porosity formation.

Cooling Rate Effect on Ductile Iron Properties

Background: Little information exists on the effect of section size on ductile iron mechanical properties. In addition, no design factors exist for the mechanical properties of machined samples versus samples with an intact as-cast surface.

Goal: Because of this lack of information, the Quality Control Committee (5-J) of the AFS Cast Iron Div. wanted to quantify the effect of cooling rates on the mechanical properties of ferritic ductile iron.

Process: Various section sizes of cast-to-shape tensile specimen patterns were poured in keel block molds made of green sand and in Y-block molds made of chemically bonded sand. Some cast-to-shape tensile specimens were tested in the as-cast condition and other cast-to-shape test bars were tested after machining (Fig. 3).

Results: Yield strength and ultimate strength increased linearly as the surface area-to-volume ratio increases for a ferritic ductile iron casting. At the same time, elongation decreases linearly. The linear relation between surface area-to-volume ratio and mechanical properties also depends on the molding media. In this investigation, the effect of green sand on the mechanical properties was less pronounced than the effect of chemically bonded sand.

Fluid flow had an influence on cooling rate. In this investigation, changing the gating system changed the relationship between the properties of castings at different positions in the mold. Understanding how metal flow and solidification patterns affect mechanical properties of a casting means a casting can be designed to take that into account.

Note: The effect of fluid flow on the cooling rate in a mold requires additional study if the relationship between cooling rate and mechanical properties is to be established to compete in traditional forging markets that require predictable reliability. The effect of fluid flow is a particularly important factor for establishing casting parameters for ductile iron castings with both thick and thin sections. For more information, see paper 02-137 at the 2002 ABS Casting Congress.

"Monday Morning" Iron

Background: When a base iron is held in a furnace too long (overnight, over the weekend, etc.), it is called Monday morning iron. This iron becomes difficult to inoculate, which leads to undesirable primary carbides forming in the castings.

Goal: Canada Centre for Mineral and Energy Technology (CANMET) has quantified the effects of undercooling versus holding time for base iron with Gray Iron Research Committee (5-H) sponsorship. This project explored the effect of holding temperature and time on the inoculation potential of cast iron and compares different techniques to restore, or even improve, the nucleation potential. Equilibrium holding temperatures are used in Europe to control the nucleation potential.

Process: Previous work demonstrated that oxygen present as suspended silica particles correlates with the melt nucleation potential (where silica phases start to form).

Results: Changes in microstructure while holding at different temperatures were correlated with the thermal analysis results. Figure 4 shows how the undercooling changes during holding at each temperature for the base iron (carbon equivalent of 3.9[+ or -]0.1%), The slopes of the trend lines increase with higher holding temperatures, and undercooled Type D and E graphite will form earlier in the holding period.

High undercooling causes a delay in the beginning of eutectic freezing, leading to more austenite precipitation. After inoculation, the undercooling [approximately 77F (25C)] was almost the same for all the samples during the holding time. The effect of inoculation on the eutectic cell count was greater at the end of the longest holding period studied (8 hr).

Foundries can find the best technique to lessen results by understanding the fundamental source of the nucleation potential. Through this understanding, they can find the most cost-effective approach for their operations. For more information, see paper 02-158 at the 2002 AFS Casting Congress.

Integrated Casting Production, Inspection

Background: AMC is running a new program, Procurement Readiness Optimization--Advanced Casting Technology (PRO-ACT), for the next four years. It will move the DLA casting acquisition baseline from an average of 52 wk to a commercial baseline of 12 wk, through the development and application of innovative casting design, manufacturing and buying methods for Dept. of Defense needs.

Goal: One PRO-ACT project is to improve aluminum casting engineering standards while addressing improved consistency and quality. This approach will expand the use of cast components as equipment manufacturers gain confidence to switch from more traditional and expensive manufacturing options such as forging and fabricated mill products to castings.

Process: A full factorial experiment was designed by the Univ. of Alabama-Birmingham as a part of the study to determine the effect of grain refinement, eutectic modification and pouring temperature on casting soundness and tensile properties of A356 aluminum alloy. Unrisered plates were used to ensure the presence of shrink porosity as an experiment.

Results: Pouring at 1450F (788C) produced test plates with a better X-ray rating than plates poured at 1350F (732G). This result may have occurred because the gates stayed open longer and partially fed the casting from the runner. The severity of the sponge shrinkage increased as the plate thickness increased.

An addition of titanium-boron grain refiner improved the X-ray rating of the castings. An addition of a eutectic modifier had no observable effect on the X-ray rating. However, the combination of a grain refiner and a eutectic modifier addition improved the X-ray rating.

The plates also were examined with ultrasound signals at frequencies of 2.25 and 5 MHz, and signal attenuation, velocity and pulse echo reflections were recorded. Tensile specimens were removed from selected plates and their properties were measured to produce histograms.

A histogram with a left side tail indicates the presence of material anomalies (Fig. 5a). The presence of material anomalies means designers effectively ignore the peak in the histogram to assure reliability. Correcting the fundamental cause for the anomaly changes the histogram into a symmetric distribution where the peak in the curve at higher strength levels is not ignored (Fig 5b). This approach eliminates scrap and provides customers with a higher design stress for engineering analysis.

Note: The researchers also noted that yield strength, ultimate strength and elongation decreased with increases in ultra-sound attenuation and X-ray rating.

For more information, contact AFS at 800/537-4237 for the proceedings from the 6"' International APS Conference on Molten Aluminum Processing 2001. MC

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For More Information

To read reviews of all 50 research projects, visit www.moderncasting.com.

RELATED ARTICLE: Check Out More Research Projects on the Horizon

Magnesium Castings in Automotive Applications

USCAR and the Transportation Technology Office of DOE teamed with AFS to improve Structural Cast Magnesium Development (SCMD) to address critical issues that inhibit large-scale application of magnesium castings in automotive applications. The project will run in conjunction with a concurrent chassis program proposed for EUCAR on road wheels.

Activities will focus on developing the science and technology necessary to implement front and rear structural cradles. These components represent all of the engineering issues, including manufacturing casting process (high pressure die, semisolid, low pressure, squeeze, etc), joining and service environment challenges (corrosion, fatigue, and stress relaxation associated with fasteners).

Within 3.5 yr, a full size magnesium front-cradle cast from production-type tooling will be in a testing schedule.

The program will develop a cost model that compares cast magnesium chassis component costs to other materials and processing techniques and develop improved casting processes through scientific understanding of magnesium alloys. In addition, a comprehensive database and design guidelines should significantly increase magnesium usage in the auto industry to reduce 100 kg of vehicle mass, emissions by 5% and fuel consumption by approximately 1.0 mpg.

For more information or to participate in the project, contact AFS at 800/537-4237.

Iron Fatigue Design Properties

Without appropriate strain-life fatigue data, cast iron can become virtually "invisible" to designers who won't want to use a material without information on its properties. The lack of a comprehensive strain-life database imperils the expansion of cast iron markets.

The DOE Office of Industrial Technology has partnered with CMC in a 2-yr program to generate strain-life testing on selected materials and to compile data from cast iron suppliers and end-users. Data from these sources will be used to develop an industry representative for cast iron.

Under the guidance of the AFS Gray and Ductile Cast Iron Research Committees (5-H and 5-I), the ASTM grades of austempered ductile iron, ductile iron, compacted graphite iron and gray iron have been selected as near term priorities for completion in 2002. These grades have been divided into subcategories based on other important characteristics such as cast section size and heat treatment. The core of the program is fatigue testing. The resources of the current phase of the program allow 18 materials to be tested.

For more information, visit the website for Climax Research Services, the program manager for the Cast Iron Research committees, at www.climaxresearch .com.

Metalcasting Research in the Space Station

The SDCC is formed with the NASA Marshall Space Flight Center and administered by Auburn Univ. This program takes advantage of the extended periods of micro-gravity and vacuum available in the International Space Station to address solidification phenomena empirically that cannot be answered definitively with ground-based studies. Research is focused in three areas: thermo-physical and related properties, solidification design and control, and computational tools. It looks to establish a quantitative understanding among metalcasting process variables and design properties so that manufacturing engineers can confidlently meet new demanding engineering design requirements with advanced concepts developed in space.

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