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ferrous crucible. If overheating does occur, the attack rate will increase dramatically at temperatures above 842F (450C). Excessive iron contamination depletes the aluminum content of zinc alloys, exacerbating the iron pickup and causing the formation of interme- tallic particles. In severe cases, these particles will appear after polishing as star-shaped impressions on the surface. Te defect is often referred to as a hard spot or inclusion and is virtually impossible to remove by grinding or polishing. Te phenomena also will appear after plating or painting, often

developing as a sharp, grit-like feature on the surface of the casting. Care should be exercised when remelting castings with the defect. After melting, remove the heat source and allow the melt to stand before removing the dross and oxide on the surface. Con- tinued application of heat moves the metal within the crucible, causing the particles to circulate in the current and enlarging the intermetallic particles. Teir “fir tree” configuration allows them to lock together with iron and aluminum in the melt, further deplet- ing the aluminum and increasing the attack rate on the iron crucible.

Low levels of lead, tin and cad-

mium have a severe effect on zinc casting quality and must be controlled down to a maximum of 50 ppm or less. Te elements sit at the grain boundar- ies of zinc and promote inter-granular corrosion through the formation of a minute galvanic cell stimulat- ing the zinc to sacrifice itself at the grain boundaries. Tis can lead to a total collapse of the casting structure, converting it to oxide dust. In less severe cases, corrosion stress cracks are formed, promoting premature part failure. Te presence of these impuri- ties is often indicated by hot shortness cracks during casting. However, these cracks also can be caused by other process-based problems. In all instances, control of metal

chemistry, regular furnace analysis and proper operating procedures by adequately trained personnel will limit the risk of defects.

Fig. 1. The surface of this black powder coated automotive part shows a blister (circled) due to entrapped gas under the surface of the casting.

Fig. 2. The picture shows a cavity under the surface of the casting due to a blister. The thickness of the zinc layer directly beneath the paint is approximately 0.45 mm. The areas circled show the separation cracks between the inner surface of the laminated blister and the basic casting substrate.

Melting Practice Te zinc diecasting process per-

Fig. 3. Numerous large pores (approximately 1 mm in diameter) are evident in this example of gas porosity.

Fig. 4. Shown is an enlarged view of micro- porosity directly under a plated surface.

forms optimally when all features are consistent. Tis particularly applies to melt temperatures. Low melt tempera- tures, large variations in melt tem- peratures and low metal levels in the furnace are potential sources of defects that should be eradicated by process control procedures. Te diecaster should attempt to maintain a minimum pot temperature appropriate for the alloy being used. To avoid excessive variations, the diecaster should use appropriate furnace technol- ogy, avoid charging ingots directly into the melt and ensure the maximum level of metal is present. Tis prevents dross entry into the goose neck and helps maintain temperature stability. Low levels of alloy allow the pot

Fig. 5. This photograph shows an enlarged view of an impingement defect on the surface of a casting.

Fig. 6. Sub-surface porosity has a significant ef- fect on fused powder paint finishes. This common defect, known as cissing, indicates the presence of minor gas, air or steam pockets below the surface of the casting.

26 | MODERN CASTING September 2012

temperature to increase and necessitate large additions of cooler material. Tis promotes instability and inconsistency in the process. Te use of two cell furnaces eliminates this problem, but the diecaster always should be aware of the problems associated with machine furnace operation. Where single furnace crucibles are

in use, the diecaster should avoid feed- ing scrap, sprues and runners directly into the melt. Sprues and runners may

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