Added value through technical development: six technology and process development projects that provide practical information on the improvement of metalcasting are highlighted.

The ability to see the big picture often is what separates successful companies from those that fail to reach the next level. Firms that recognize this are the ones that invest in technology and process development. They then examine the results and use the findings to their advantage. Through the information available today, the metalcasting industry has the freedom to explore new ideas, improve existing practices and even accomplish goals that were once thought unattainable.

This article details six research projects conducted by the AFS Copper, Steel, Aluminum, Iron and Lost Foam Casting Divisions that have provided metalcasters with practical information to improve metalcasting practices. The research is funded by industry, AFS, the Defense Logistics Agency (DLA) through the American Metalcasting Consortium (AMC) and the U.S. Dept. of Energy Industrial Technology Program (DOE-ITP) through the Cast Metals Coalition (CMC).

Grain Refinement of Copper Permanent Mold Alloys

Background--The AFS Copper Alloy Div. Research Committee (3-C) wanted to enhance the hot tear resistance of permanent mold cast copper alloys through grain refinement.

Goal--Because boron and zirconium grain refiners will fade with oxidation losses, the committee wanted to define practical operational techniques to identify when fading had occurred.

Approach--Two copper-zinc alloys (refined with boron, yellow brass and EnviroBrass III) and two copper-silicon alloys (refined with zirconium, silicon brass and silicon bronze) were induction melted for laboratory trials. Samples were poured into a graphite cup, and cooling curves were recorded. The cup casting was then sectioned, polished and macro-etched to use optical and scanning electron microscopy and wet chemical analysis.

Results--The cooling curve analysis successfully predicted the onset of fading and grain refinement in yellow brasses. Thermal analysis (Fig. 1) then was validated in two metalcasting facilities. It was determined that grain refinement by boron and iron can remain over a long period of time--for at least 72 hrs. of holding or after remelting a few times. Contrarily, refinement by zirconium is lost quite rapidly--sometimes within 1 hr. of holding. In all of the cases, it is possible to revive grain refinement by adding more of the appropriate refining element.

[FIGURE 1 OMITTED]

Machinability of Thin Wall Iron Castings

Background--High-speed machining systems used on thin wall gray and ductile iron castings do not always produce the desired results because of batch-to-batch casting variations. The variations in cast iron machinability require frequent tool changes and CNC operating parameters. Most of the "hard-to-machine" castings met all specifications, leaving no explanation for the unacceptably high tool-wear rates.

Goal--The objective of the project was to determine the metalcasting process variables that control the machinability of iron castings and demonstrate how objectionable factors can be eliminated.

Approach--A cast iron compressor piston consistently was receiving complaints about its poor machinability. An analysis revealed the presence of variable microcarbides, prompting an investigation into the effect of microcarbides on the machinability of cast iron.

Results--The research proved that excessive microcarbides (11% or more dispersed in pearlite) reduce tool life.

The machinability of gray iron is shown in Table 1. Two irons were studied and rated as the best and worst in terms of machinability. The machinability is expressed as the volume of metal removed before reaching 6.5 mils of tool flank wear. Figure 2 shows the results of a study on property and microstructure data that produced tool wear rates differing by as much as 500%.

[FIGURE 2 OMITTED]

It also was found that microcarbide formation is controlled by: elements that control carbon diffusion rate between eutectic and eutectoid temperatures; shakeout temperature; and excess concentrations of carbide-forming elements. Density, along with "first break" ultrasonic velocity at 2.25 MHz, is a good indicator of the machinability of gray iron.

Permanent Mold Casting of High Phosphorus Brass

Background--The Permanent Mold Committee (3-E) of the AFS Copper Div. was interested in alloys that can be cast at lower temperatures to increase mold life. Although copper that contains 25% zinc and 5.8% phosphorus can be cast at 1,454F (790C), phosphorus makes the alloy brittle.

Goal--Investigations sought to develop a new lead-free, low melting point, copper-base alloy to join the family of permanent mold copper-base alloys for plumbing applications.

Approach--Nickel additions can combat the adverse effect of phosphorus. Therefore, casting characteristics were determined for a range of phosphorus and nickel contents.

Results--It was found that the optimum composition for good fluidity, excellent hot-tear resistance, low machinability (25% of free machining brass) and good mechanical properties is 25% zinc, 3-4% phosphorus and 3.5-4.5% nickel (Fig. 3). Aluminum additions produced improved surface finishes and corrosion-resistant characteristics comparable to EnviroBrass III. Prototype faucets suitable for polishing were created with minimal melt losses at 1,697F (925C) and passed pressure tests up to 125 psi.

[FIGURE 3 OMITTED]

Improved Processing of Austenitic Manganese Steels

Background--Austenitic manganese steels are difficult to machine, which makes verification of thick section properties using specimen preparation for mechanical testing too costly. Alternatively, the heterogeneous distribution and variety of embrittling phases make it difficult to verify that heat treatment objectives are met.

Goal--The interpretation of carbide, microporosity and phosphide eutectic features needed to be understood in order to improve the prediction of mechanical properties.

Approach--Macrosegregation and microsegregation were to be identified with respect to section size and their effect on embrittlement. The role of phosphide eutectic embrittlement and its recovery also was included in this study.

Results--A procedure to estimate impact toughness from metallographic observations was developed based on the weighted grain boundary coverage metric (Fig. 4).

[FIGURE 4 OMITTED]

Quench times were determined from quench water temperature measurements (Fig. 5). Only the "thick" carbide films were harmful for impact toughness. They are distinguishable from the "thin" carbides by the appearance of an austenite-cementite phase boundary on both sides of the carbide film.

[FIGURE 5 OMITTED]

Macrosegregation was not a factor for embrittlement in thick sections. Microsegregation increased with section thickness, and it was discovered that phosphide eutectic embrittlement is reversible. The results suggest that new precipitation-hardened manganese steel is possible with a high yield strength.

Permanent Mold Casting of Alloy 535

Background--Aluminum-magnesium alloy 535 is susceptible to hot tearing and poor fluidity when poured in metal molds. However, it exhibits good strength and elongation and has good corrosion resistance after cooling. Further, it does not need to be reheated for solution heat treating before aging.

Goal--The Aluminum Div. was interested in overcoming the hot tearing and fluidity challenges of alloy 535 because of its good as-cast mechanical properties and excellent corrosion resistance.

Approach--Casting fluidity was studied by tilt pouring and direct pouring spiral castings in specially designed permanent molds. The hot tearing resistance was studied by pouring ring, cup and constrained rod castings in specially designed permanent molds.

Results--Alloy 535 was successfully cast in preheated (329F/200C) gravity permanent molds with adequate fluidity and without hot tearing when poured with a metal superheat of 122-212F (50-100C). The three hot tearing mold designs show different degrees of sensitivity, but all are effective in showing the susceptibility of the alloy to hot tearing (Fig. 6). Excessive metal superheat increased the alloy's tendency to form hot tear cracks, but additions of grain refiners improved the alloy's hot tearing resistance. Grain sizes ranging from 9.8-18.5 in. (250-470 mm) were achieved by individually adding 0.05% titanium, 0.085% boron, 0.2% zirconium, 0.5% scandium or 0.1% strontium. A combination 0.05% titanium and 0.01% boron also was an effective grain refinement addition.

[FIGURE 6 OMITTED]

Development of Software for Lost Foam Pattern Blowing, Steaming

Background--By understanding the fundamental fluid flow and thermal physics of EPS patterns, metalcasting facilities are able to consistently obtain desirable foam pattern characteristics.

Goal--A gas-solid computational fluid dynamics engineering program was sought to be developed for modeling the bead-blowing cycle and the fusion of lost foam patterns. The study also sought to create a user-friendly, integrated computational tool addressing all gas/particle/thermal phenomena.

Approach--Work centered on improving bead compaction, pattern swelling and pattern fusion modeling physics. New experimental data were obtained. Experiments were conducted in prototypical lost foam tooling to evaluate the 3-D filling of beads from individual guns into complex-geometry tooling.

Results--The EPS software confirmed the data, and commercial use now occurrs within the industry. The software can determine where fill guns deliver their beads, providing a physics-based model to ensure cost- and energy-effective designs. The software also determines the optimum location and number of fill guns, gun pressure/cycle parameters and tool vent sizing and distributions.

During steam fusion, the software computations revealed 3-D, transient steam flow into the packed bead bed within the tooling, the swelling of beads with a corresponding loss of porosity for steam flow as each local region reaches the "glass transition temperature," bead and steam temperatures throughout the mold and a localized fusion index at the end of the cycle.

Table 1. Effect of turning speed on volume of Class
35 gray iron removed before change-out.

              Best Avg       Worst Avg

BHN            210 (4)        215 (8)

Tensile Strength
Ksi             37.8           36.7
Mpa              260            253

Turing Speed ([in..sup.3] removed prior to change-out)
650 sfm      6.35 (0.72)    1.74 (0.80)
800 sfm      1.76 (0.71)    1.42 (0.61)
1,200 sfm    0.54 (0.53)    0.55 (0.13)

* The change-out criteria is 6.5-mils of tool flank wear

For More Information

Visit the For More Information section at www.moderncasting.com to view "Fading of Grain Refinement in Permanent Mold Cast Copper Alloys," M. Sadayappan, J.P. Thomson, R. Zavadil, M. Sahoo and H.T. Michels, 2004 AFS Transactions, Paper No. 04-112.

"Grain Refinement of Permanent Mold Cast Copper-Base Alloys," M. Sadayappan, D. Cousineau, R. Zavadil, M. Sahoo and H. Michels, 2002 AFS Transactions, Paper No. 02-108.

"Gravity Permanent Mold Casting of Al-Mg Alloy 535," F.A. Fasoyinu, J. Thomson, D. Cousineau, T. Castles and M. Sahoo, AFS Transactions 2002, Paper No. 02-145.

Joe Santner, Wayne Rasmussen and Scott Shudy, AFS, Schaumburg, Illinois