In the 1990s at the Ford Windsor Aluminum Plant, Windsor, Ontario, Canada, (now Nemak Canada, Windsor Aluminum Plant) clogging problems began to develop with the electromagnetic pump used to deliver molten 319 aluminum to the mold in the Cosworth casting process the plant was utilizing. The clogging was due to solid deposits that precipitated from the melt.
An investigation initially revealed that the problem could be partially attributed to the precipitation of insoluble aluminum-titanium-silicon (AlTiSi crystals from the melt, which could be corrected through the lowering of Ti levels to 0.10 wt%). Unfortunately, solving the problem of crystal formation only marginally improved pump performance.
The buildup of solid deposits continued to restrict the efficiency of melt flow through the pump passages. On average, the 319 alloy pump would become unusable after 500,000 lb of aluminum (10 days of casting for the plant, 6000 castings). When this occurred, the pump would be removed from the melt and rebuilt, increasing costs and reducing productivity. These conditions weren't acceptable and the foundry looked for a new solution.
The Cosworth Process for casting uses low-pressure filled, precision sand molds to produce high-strength castings. An electromagnetic pump with a computer-controlled flow rate is employed to fill the mold. The pump is designed to draw melt from the middle of the furnace, avoiding the entrapment of oxides and precipitates from the furnace and improving casting quality.
The original Cosworth Process employed by Cosworth Castings, Worcester, England, pumped melt through the bottom of a zircon mold until the filling process was complete. Pump pressure would be maintained until solidification was complete. At its research facility in Windsor, Ford engineers developed a high production process using a rollover technique in which the zircon mold is rotated 180 degrees after filling so that it can be removed from the filling station while the metal is still molten.
In order to determine the reasons for the pump clogging, the foundry set up an experiment to test the variables and establish a new shop-floor procedure to ensure smooth and efficient production.
A castable ceramic "brick" was removed from the orifice of the clogged electromagnetic pump. This brick was cut into several sections and photographed from a variety of angles. Visual inspection revealed that the passage running through the brick was clogged with an unknown material. It also revealed that some of the clogging residue had diffused through the brick's boron nitride (BN) coating. The BN coating itself was not damaged.
The surface of the clogging residue was examined with a scanning electron microscope (SEM) equipped with an energy dispersive spectrometer (EDS) and a light optical microscope (LOM). The cleanliness of the 319 aluminum alloy melt at Windsor also was evaluated using the Porous Disk Filtration Apparatus (PoDFA) technique.

2011-04-02

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