AFS Research Report-Theory Translates to Practical Results.

With a focus on practical solutions to common foundry problems, AFS' research program reports project highlights from the past year and ongoing efforts for 2001.

A historic strength of AFS is its committee system, which continuously identifies and responds to changing industry needs. All 10 technical divisions have active research projects meant to increase practical knowledge of casting processes for today and tomorrow. Currently, there are 50 active projects valued at more than $14 million that are either directly funded by AFS or supported by its partners.

This article outlines a few examples of how AFS research is re-inventing the industry, offering casting data and searching for methods to achieve better casting quality and properties. Covered are the results of projects that have been completed in 2000 and the progress of research that will be close to completion in the months ahead.

Heat Transfer in Al Alloys

The Engineering Div.'s Process Design and Modeling Committee (1-F) has been working toward achieving validated heat transfer coefficient values for solidification modeling. The heat transfer coefficients must be understood in terms of mold coatings, melt oxide film thickness and air gap formation, To reach this goal, the committee is working to develop instrumentation and test methodology to measure the heat transfer coefficient in aluminum, iron, copper and steel on an ongoing basis. The long-term goal of this initiative is to include permanent mold, sand and investment casting as well as low-pressure and high-pressure diecasting for all engineering casting alloys.

With funding from the Dept. of Energy (DOE) through the Cast Metals Coalition (CMC), the Univ. of Michigan has developed a framework for a Heat Transfer Coefficient Evaluator. The goal is to produce an algorithm that commercial solidification modeling suppliers can incorporate in their packages. It provides the solidification modeler with sound estimates of heat transfer coefficients on various surfaces of a solidifying casting.

A commercial solidification package was used to simulate heat transfer coefficient due to air gap for a wheel-like permanent mold. Separate heat transfer coefficient values were defined for each of six segments (Fig. 1). In assigning heat transfer coefficient values, it was assumed that there is no air gap between the bottom mold and casting due to gravity and that an air gap between the top core and casting is formed as it contracts. The overall heat transfer coefficient dropped from 1765 W/[m.sup.2] K to about 500 W/[m.sup.2] K with a maximum air gap of 70mm over 360 sec.

A test casting of a gear blank shape produced by vertical squeeze casting was validated. The secondary dendrite arm spacing was correlated with a specific cooling rate for a modified A356 alloy. During solidification, the interfacial heat transfer coefficient was relatively uniform over the casting. On reaching critical solidification pressure, the heat transfer coefficient was 4700 W/[m.sup.2] K. The report indicates that even higher coefficients can be reached at higher pressures and with no mold coating.

Over the next 6 months, plant trials will be completed and the data from the work will be analyzed.

AI Permanent Mold Gating

The Aluminum Div.'s Permanent Mold Practices Committee (2-F) has been looking at ways to optimize the introduction of metal into the mold. In a preliminary study funded by the DOE-Office of Industrial Technology (OIT) and industry sponsors, Case Western Reserve Univ. examined the gating of vertically parted aluminum permanent mold castings through a combination of experiments and computer simulations. This work is aimed at developing improved gating designs that promote higher casting yields, lower scrap rates, lower defect contents with fewer initial design changes.

This program used real-time X-ray radiography to visualize the filling of molten aluminum. Experiments also were performed with the same gating designs in instrumented permanent molds to measure the thermal history of the casting during mold filling and solidification. The combination of radiography and thermal measurements produced a set of benchmarks to verify a computer model of the permanent mold casting process. This heat and fluid-flow model was based on a commercial finite element computational fluid dynamics code. This validated model was modified to examine a number of gating and process variations. Much of this work is applicable to vertically parted sand molds as well, although the heat transfer conditions do vary from a permanent mold (Fig. 2).

In Table 1, the smaller runner and ingate for the mold designation M22 entrained the most bubbles and showed a tendency to increase flow turbulence forthe metallostatic head used. When the ingate and runner were enlarged in mold designation M13, the relative amount of trapped gas was reduced. The least entrapped gas was found in the mold with a large vertical riser connected by a web, M51. Unfortunately, the yield with a vertical riser is lower (38%) than systems with smaller ingates and runners, M22. Hence, the use of filters provides a utilitarian compromise butguide-lines must be based on further investigation.

Clean Metal Casting

With funding through the DOE-OIT, the Advanced Casting Research Center sponsors at Worcester Polytechnic Institute developed technology for clean aluminum processing. The program had four technical tasks: the development of melt cleanliness assessment technology, melt contamination avoidance technology, high-temperature phase separation technology, and the establishment of a correlation between the level of melt cleanliness and as-cast mechanical properties.

A standardized reduced pressure test was developed as an AFS-recommended practice (Table 2). A melt cleanliness sensor based on the principles of electromagnetic separation was developed. An industrial partner is continuing development to commercialize a cost-effective robust cleanliness sensor based on these principles.

Environmentally friendly fluxes that do not contain fluorine were developed within the context of melt contamination avoidance technology, and these fluxes also will be commercialized. Rotary degassing was modeled, and the model predictions were verified with experimental data within the context of the development of high-temperature phase separation technology. This model is being used to optimize the performance of commercial industrial rotary degassers.

Age-Strengthening of Gray Iron Castings

The Gray Iron Div.'s Research Committee (5-H) has worked to understand and control age-strengthening in gray iron to consistently increase the strength-to-hardness ratio for improved machinability. Phase I and Phase II research sponsored by AFS at Tri-State Univ. statistically proved the phenomenon of age-strengthening in gray cast iron.

Phase III, which is sponsored by DOE-OIT and industry sponsors, will identify the age-strengthening mechanism in gray cast iron, quantify the parameters that control the process, measure properties to develop a predictive model, and quantify the relationship between aging and machinability. It will begin with the 30-day room temperature aging and strive to develop sufficient understanding to allow a modified time-temperature-composition combination aimed at substantially reducing 30-day work-in-progress. Section size reductions in strength-limiting applications is another opportunity.

This project may increase yield by facilitating higher strengths at higher carbon equivalent values. Some preliminary results on the artificial aging study are given in Fig. 3.

Lead-Free Copper Alloy Design

With support from the DOE-OIT, the Copper Div.'s Permanent Mold Committee (3-E) has been working to produce permanent mold design and mechanical properties standards.

The mechanical, fracture toughness, impact and fatigue properties of 13 lead-free copper alloys (aluminum bronzes, yellow brass, high-strength yellow brass, silicon brass, manganese yellow brasses and the high-copper alloys) were measured. For the first time, comprehensive data on mechanical, impact, fatigue, fracture toughness, and wear and corrosion properties (selected alloys from the group) are available.

The mechanical, fracture toughness, impact and fatigue properties of these alloys are strongly dependent on the chemical composition. The nominal composition does not always provide the best combination of strength and ductility. To achieve optimum properties for a given application, a narrower composition range than in the current practice should be targeted, especially for those elements with the greatest effect on properties. It is critical for design engineers to have mechanical properties that are representative of the industry capability for component design. The mechanical properties were adopted by the Copper Development Assn. and will be proposed for ASTM specifications.

Al Bronze Impurity Limits

The Copper Alloy Div. Research Committee (3-C) has sought to quantify the effects of impurity elements on the mechanical properties, heat treatment and weldability of aluminum bronze alloys. The alloys selected for the study were aluminum bronze (C95400) and nickel-aluminum bronze (C95800), and the impurity elements investigated were lead, zinc, tin, bismuth, selenium, chromium, silicon and beryllium. These elements were added in one-, two-, and three-element combinations.

The ultimate tensile strength and yield strength of alloy C95800 was not reduced by the addition of impurity elements. Instead, these elements improved the strength in both as-cast and heat-treated conditions. However, the ductility of the alloy was adversely affected by the impurity element addition. The work on the weldability of alloy C95800 indicated that only lead and bismuth were detrimental and caused cracking in the heat-affected zone. The new limits for the impurity elements in alloy C95800 are shown in Table 3.

The results from the work on alloy C95400 indicate that more emphasis should be placed on maximum aluminum content. The existing standards call for a nominal aluminum content of 10.8%, which is too high to achieve the required ductility. The optimum aluminum content was 10.6%. Only two elements (lead and bismuth) among those investigated were detrimental to properties after heat treatment. Each of these elements should be controlled below 0.02% since they cause severe embrittlement. This work was funded in-part by the DOE-OIT.

Metallic Recovery

Metallic charge materials represent the largest cost in the electric furnace production of ferrous castings, but little has been published on metallic recovery and the impact of surface conditions on alloy loss. There is up to a 50% increase in metallic loss as surface oxidation increases due to pre-heating (from 44-72 lb of slag/ton melted). The Melting Methods and Materials Div.'s Charge Materials Committee (8-G) has worked to determine the impact of the quality of charge materials on metallic yield and alloy recovery.

A baseline study was launched to develop quantitative information on various metallic charges. This work conducted at Tri-State Univ. will give the electric melter the ability to more accurately determine the impact of charge material quality on metallic yield and alloy recovery. Knowing the impact of quality on metallic yield and alloy recovery is critical in determining the actual and lowest cost materials for production. All computer models using "least cost charging" require this information.

The final report for this study is being reviewed by the Steering Committee.

Beneficial Reuse

The Environmental Control Div. is concerned that there will be significant disposal price increases due to key disposal facility closures and state efforts to limit interstate disposal of solid wastes. DOE-OIT is supporting work at Pennsylvania State Univ. and the Univ. of Wisconsin at Madison through the CMC to help develop technical specifications and performance standards for flowable fill and hot-mix asphalt. Experiments to measure the effect of by-product additions at different levels on the mechanical properties of asphalt and flowable fill is expected to aid foundries in beneficially reusing 75% of their byproducts.

In addition, Foundry Industry Recycling Starts Today (FIRST), an industry-initiated collaborative research and technology transfer program to create sustainable markets for foundry industry by-products, is attacking market entry barriers. Technical information for marketplace applications (asphalt, concrete, flowable fill, landfill, Portland cement, soil amendments and structural fill) are available at www.foundryrecycling.org.

Manufactured Radioscopic Standards

Under the U.S. Council for Automotive Research "Design for Product Optimization" project administered by AFS, DOE's Lawrence Livermore National Laboratory has worked to develop a set of manufactured standards for radioscopy (real-time and digital radiography).

Present American Society for Testing and Materials (ASTM) image quality indicators, contrast sensitivity gauges and radioscopic system practices are used to qualify systems. However, these are not always adequate to judge system performance for detection and measurement of particular types of discontinuities that are of interest in aluminum and magnesium. In addition, only film images are available to judge discontinuity grades, and they don't necessarily represent the range, contrast and resolution of digital radioscopic systems.

The manufactured standards have been related to the size, distributions, and discontinuity classifications of the ASTM E155 and E505 film radiography standards and are intended as image quality indicators for online periodic qualification of radioscopic inspection systems and acceptance of products.

A draft standard for design and manufacture use of these standards have been submitted to ASTM. Reproducible manufactured Representative Quality Indicators (RQI) are necessary for verifying radioscopic system performance and obtaining system-produced radioscopic images of standards for grading discontinuities.

The RQI developed under the "Design for Product Optimization" project is a basic set to represent shrinkage "sponge." Nevertheless, the design can accommodate representations of other types of discontinuities, such as shrinkage cavity and porosity (round or elongated). The shrinkage sponge RQI consists of a set of perforated foils captured between solid metal blocks. The blocks are selected to represent section thickness of the part to be inspected, and the perforations represent material loss from a discontinuity. Perforation areas in foil layers cover circular regions of successively larger diameters for each layer set to represent different levels of material loss with different areas of extent.

The design, manufacture and material classifications for the shrinkage sponge RQI is contained in a draft standard that has been submitted to ASTM. An example of a radioscopic image is provided in Fig. 4.

                   Sprue, Runner, Riser and Filter Test
                Data Used to Examine Permanent Mold Gating
   Mold                                                     Pore
Designation              Description                       Volume
                                                   [x10.sup.-3] [in.sup.2]
    M22           Straight horizontal runner
                    and large sprue basin                   4.50
    M13             Enlarged: sprue well,
                 horizontal runner and ingate
             with a tapered horizontal runner end           0.50
    M31        Vertical filters near the sprue
             well placed in the horizontal runner           0.13
    M41        Horizontal filter at the bottom
                  ingate of the mold cavity                 0.06
    M51      Vertical riser and web gate between
            the vertical riser and the mold cavity          0.03
   Mold          Sprue       Cross Section Ratio:
Designation      Choke       Sprue Base: Runner:
            Area, [mm.sup.2]        Ingate
    M22
                  129             1:2.4:4.9
    M13
                  129             1:6.3:6.5
    M31
                   78             1:4.6:10.7
    M41
                   78             1:9.0:10.0
    M51
                   78             1:2.5:5.0
                             (3-mm web thickness)

Procedure for the Reduced Pressure Test for Aluminum

TAKING THE SAMPLE

1. RPT test equipment should be placed near the metal being tested to assure rapid application of vacuum before solidification begins.

2. Before conducting a test and with the vacuum chamber cover in place, turn on the pump and check the pressure reading. Adjust as necessary to desired pressure. Release vacuum and remove chamber cover.

3. Coat crucible with graphite spray. Using dry tongs, place crucible in molten metal and heat to cherry red.

4. Skim the surface of the melt with the underside of the crucible then dip the crucible to allow the clean metal to fill the crucible approximately 2/3 full. Caution: Do not let the tongs touch the metal in the crucible as this may introduce hydrogen into the sample.

5. Lift sample from melt carefully to avoid disturbing the sample and carry quickly to the vacuum chamber. Remove any metal drips or films from the crucible and do not allow them to fall into sample.

6. Place sample crucible on refractory disc, cover and turn on the vacuum pump. Set timer for 5 mm. Check that pressure is correct, adjust if necessary.

7. When timer sounds or sample is solidified, turn off pump and release pressure. Remove sample from crucible, chill in water if desired.

EVALUATING THE SAMPLE--VISUAL EVALUATION

1. Saw sample in two. Sand sample using 120-grit abrasive on a belt or disc sander until flat. Sample may be lightly blasted with abrasive grit to improve view of porosity.

2. Compare sample to visual chart such as the Stahl Comparative Standards Chart.

SPECIFIC GRAVITY MEASUREMENT

The following additional equipment is required for specific gravity testing: A pan scale accurate to 0.01g, thermometer, wire basket and a water container to weigh sample in water.

1. Zero scale, place sample on the pan, record the weight ([M.sub.air])

2. Suspend basket from scale, place basket in water, zero scale.

3. Place sample in the basket and record weight in water ([M.sub.water])

4. Record water temperature. Use chart to find water density ([C.sub.wt]).

5. Calculate specific gravity using the following formula: S.G. = ([M.sub.air]*[C.sub.wt])/([M.sub.air] -- [M.sub.water])

                     Impurity Element Limits in Ni-Al
                              Bronze (C95800)
Element  Limit   Element  Limit
  Lead   0.01%    Zinc    1.00%
  Tin    0.10%   Bismuth  0.03%
Selenium 0.035%  Silicon  0.10%
Chromium 0.10%  Beryllium 0.10%