Casting Defects of Metal Materials, part 3

The main defects of casting products are segregation, pores, shrinkage and shrinkage, inclusions, cracks, cold shut and other defects.

Shrinkage

During the solidification process, the metal shrinks in volume, and the melt cannot be replenished in time, and shrinkage holes appear in the last solidified place, which are called shrinkage holes.

Large and concentrated shrinkage holes are called concentrated shrinkage holes, and small and dispersed shrinkage holes are called shrinkage porosity. Among them, shrinkage holes that appear at grain boundaries and dendrites with the help of a microscope are called micro-shrinkage holes.

The surface of shrinkage holes is mostly uneven and nearly sawtooth-shaped. The shrinkage holes between grain boundaries and dendrites are mostly angular. Some shrinkage holes are often filled with precipitated gas, and the hole walls are relatively smooth. At this time, the shrinkage holes are also pores, and low-melting-point substances are often associated with the shrinkage holes.

Shrinkage holes all appear in the central area of the cross section. Shrinkage holes at the head are mostly conical, with uneven inner surfaces or coarse crystalline structures.

Intermittent shrinkage holes in the middle are mostly irregularly shaped holes of different sizes, which are sometimes filled with gas precipitated during metal solidification. The surface is relatively smooth, and it is often difficult to weld in subsequent processing, forming stratification and bubbles.

Shrinkage cavities are also prone to stress concentration and cracks during processing. Shrinkage cavities are often distributed near the center of the cross section or the entire cross section, and sometimes appear near shrinkage cavities, with small and scattered holes distributed at the grain boundaries or dendrite gaps.

Some tiny shrinkage cavities are difficult to detect with the naked eye and can only be detected with the help of a microscope or water pressure test. Looseness causes the metal structure to be non-dense, thereby greatly reducing the mechanical properties and corrosion resistance of the alloy.

The size of shrinkage cavities and shrinkage areas is related to factors such as the solidification shrinkage coefficient of the alloy, the fluidity of the metal liquid, the width of the crystallization temperature range, the cross-sectional size of the ingot, the casting temperature, and the solidification conditions.

The larger the solidification shrinkage coefficient of the alloy, the larger the size of the ingot cross section, and the more serious the shrinkage cavities will be. The narrower the crystallization temperature range of the alloy and the better the fluidity, the more concentrated the shrinkage cavities; on the contrary, if the crystallization temperature of the alloy is wider and the crystallization transition zone during solidification is wider, shrinkage cavities will be more likely to form.

The main reasons for shrinkage cavities and shrinkage porosity are: unreasonable smelting process, low casting temperature, poor shrinkage compensation, and flow interruption; high cooling intensity and fast casting speed; unreasonable crystallizer design, the insulation cap is too low and humid; the alloy crystallization temperature range is wide and the fluidity is poor.