Casting aluminum/ceramic composites at Progress Castings.
In producing test castings, Progress Castings is developing the foundry practices needed to assure quality in cast aluminum/ceramic composites.
Since mid-1988, Progress Castings, Plymouth, MN, a division of Progress Casting Group, Inc and a manufacturer of premium quality aluminum castings, has been participating in a test casting program with Dural Aluminum Composites Corp. The program is aimed at developing both foundry procedures and applications for the aluminum/ceramic composite foundry ingot.
"We are pouring two composite castings today," states Robert E. Carity, Progress Castings, "a test specimen and a prototype automobile upper control arm. And with each program of pattern development and test casting, we have advanced the technology of casting the aluminum/ceramic composite material."
Both castings are made from A356-20% SiC ingot. The test speciment, shown in Fig. 1, is made by the gravity permanent mold process, in a conventional, machined iron mold. It is being used in a study to determine the optimum machinability parameters for the composite material. The control arm, shown in Fig. 2, is cast in a conventional precision dry sand mold.
According to Carity, "Early attempts to produce castings with silicon carbide reinforced material met with limited and unpredictable results. The major defects encountered were misruns, trapped air and oxides." Subsequent analysis indicated that the trouble was being caused by "large bubbles being created during the pouring process."
This bubbling characteristic, unique to this material, could not be prevented by eliminating oxygen in the mold cavity, or by filtration. A gating technique was designed to prevent the bubbles from ever entering the casting.
"Elimination of the bubbles from the gating system has to be accomplished after the sprue well, but before the first entry," says Carity. "If any filtration is to be used, the bubbles have to be removed prior to the metal entering the filter to prevent blockage."
Filtration is highly recommended to trap inclusions, because currently available methods of cleaning or fluxing cannot be used to process the molten composite material. In removing nonmetallics, fluxing will remove the silicon carbide particles from the melt as well.
A skimmer core and open riser used in combination were designed for sand and permanent molds for this purpose and are shown in Fig. 3. "The intent is to use the core to direct the bubbles into the riser and allow them to float to the surface," Carity explains.
This technique results in a heavy gating system, resulting in low yields. "But yield is not a factor at this point," notes Carity. "The primary objective for now is to keep the bubbles and oxides from entering the casting to produce a quality casting."
Also keeping yields low for now are the bubbles and oxides entrapped in the gating. Due to their presence, remelting of gating is not recommended at this time, according to Carity. R&D efforts, however, are reportedly close to providing a method of fluxing or degassing the molten material, a development which will improve foundry processing and permit remelting.
Melting & Pouring
Progress Castings melts the aluminum/ceramic material in an 1100 lb electric resistance crucible furnace. To prevent settling out of the silicon carbide particles, the melt is stirred with an air-driven variable speed motor, graphite lance and impeller head attached to the furnace.
Again, because fluxing and degassing cannot as yet be accomplished, the metal must be protected from hydrogen and oxygen pickup. The furnace is equipped with an insulated cover and argon gas is injected into the crucible to cover the melt for this purpose.
As discussed in the preceding article, temperature must not be allowed to exceed 1440F (780C). The aluminum carbide which precipitates at this temperature cannot be treated, according to Carity, and forces disposal of the melt. A thermocouple attached to the furnace assures maintenance of melt temperature at between 1300-1400F. The complete furnace setup at Progress Castings, with gas injection and stirrer in place, is shown in Fig. 4.
To avoid turbulence and subsequent reoxidation, molten metal transfer should be minimized, Carity says. Small hand ladles are used exclusively to hand dip and pour the molds, as shown in Fig. 5.
Finishing & Heat Treating
The presence of very hard silicon carbide particles dictates that carbide-tipped saw blades be used for degating. Saw blades and grinding wheels are said to have short life spans. To reduce grinding, Progress Castings gates into machined surfaces. Reportedly, diamond-tipped machine tools are required for machining.
Heat treating of the composite castings follows conventional procedures for aluminum castings. Only the times and temperatures are different. Optimum heat treat cycles are the subject of recently reported research.
Progress Castings expects to continue R&D of aluminum/ceramic composites and sees a growing interest in the material among casting users.
PHOTO : Fig. 1. The composite machinability specimen is shown.
PHOTO : Fig. 2. Shown is the finished composite upper control arm.
PHOTO : Fig. 3. Pictured is the control arm (top) and specimen casting (bottom) with gating
PHOTO : attached. Note bubbles entrapped before the filter (top).
PHOTO : Fig. 4. The resistance furnace used to melt the aluminum/ceramic composite material is
PHOTO : shown with gas injection and stirrer in place.
PHOTO : Fig. 5. Small hand ladles are used at Progress Castings to hand dip and pour the molten
PHOTO : composite material. References Carity, R., "Foundry Experience and Variables in Casting Silicon Carbide Reinforced Aluminum Alloys," paper No. 89-142, 93rd AFS Casting Congress, San Antonio, TX (May 1989). Hammond, D., "A Heat Treating Study of Cast Metal Matrix Composites," paper No. 89-178, 93rd AFS Casting Congress San Antonio, TX (May 1989).
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|Author:||Bralower, Paul M.|
|Date:||Aug 1, 1989|
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