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1. Uneven tissue
After the liquid metal is poured into the mold, the layer of liquid metal that first contacts with the mold wall has the fastest temperature drop, so it quickly solidifies into finer grains. As the distance from the mold wall increases, the influence of the mold wall gradually weakens, and the crystals grow into columnar crystals parallel to each other along the direction perpendicular to the mold wall. In the central part of the casting, the heat dissipation has no significant directionality, and it can grow freely in all directions until they touch each other, so an equiaxed crystal region is formed. It can be seen that the structure in the casting is not uniform, and generally speaking, the grains are relatively coarse.
2. The tissue is not dense
The crystallization of liquid metal proceeds in the form of branch growth, and the liquid metal between the branches solidifies at last, but it is difficult to fill the branches with metal liquid, resulting in the general non-compactness of castings. In addition, if the liquid metal injected into the mold shrinks in volume during cooling and solidification without sufficient replenishment, it may also form loose or even shrinkage cavities. Graphite in iron castings often appears in larger flakes, spheres or other shapes, and can also be regarded as a non-dense structure.
3. Rough surface
The surface is generally rough, cannot be compared with the machined surface, and the shape is more complicated.