Influence of chill distribution in counter gravity casting of large thin wall aluminum alloy plate

The research adopts the flat plate structure, the gap gating system and the low pressure pouring. The design of plate test piece and gating system is shown in Fig. 1a. In Figure 1b, six plates (1-6) are arranged in sequence, which are divided into gating and riser system and general cold iron.

The initial thickness of the plate casting is set as 20 mm, and the size is 576 mm × 573 mm. Because of its large area, each plate is set with a gap runner, the diameter of the runner is Φ 65-70 mm, the length of the gate is 800 mm, and the initial thickness of the general cold iron is set as 15 mm. Because the typical wall thickness is thin and the riser is not easy to place, the casting is fed by lengthening the length of gap runner.

The low pressure pouring curve is shown in Fig. 2, and the differential pressure is set at 35kpa.

The results show that the plate defects are shrinkage porosity defects (marked in the figure), which are located at the large gap of cold iron.

According to the location of defects, the causes are analyzed. The maximum chill gap is more than 30mm, and these parts solidify later than the surrounding areas. The liquid metal can not be replenished during the solidification process, thus forming shrinkage porosity defects.

In view of the above problems, the actual process is accurately controlled to ensure that the gap between the cold irons is less than 5mm. If the slab is poured again, the defects produced by the plate are still shrinkage defects, as shown in Fig. 4.

Although the types of defects are similar, the location of defects is obviously different from that when the gap between cold irons is large. The distribution of shrinkage porosity defects on the plate is relatively scattered, and there are multiple distribution in the upper part, lower part, side near the vertical tube and far away from the vertical cylinder, and also in the center position of the cold iron, as shown in Fig. 5.

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