During the solidification process, the volume of the metal shrinks, the melt cannot be replenished in time, and the shrinkage holes appear in the final solidification place, which is called shrinkage holes or shrinkage porosity. Large and concentrated shrinkage pores are called concentrated shrinkage pores, small and dispersed shrinkage pores are called shrinkage porosity, which appears in the grain boundaries and dendrites with the aid of a microscope is called microshrinkage porosity.
The surface of the shrinkage holes is mostly uneven, approximately serrated, and the shrinkage holes between the grain boundaries and dendrites are often angular. Some shrinkage holes are often filled with precipitated gas, and the wall of the holes is relatively smooth. At this time, the shrinkage holes are also porosity, and low melting point objects are often associated with the shrinkage holes.
The shrinkage holes all appear in the central area of the section. The shrinkage holes located in the head are mostly conical, and the inner surface is uneven or has coarse crystalline structure. The intermittent shrinkage holes located in the middle are mostly irregular sized holes, and the interior is sometimes filled with gas precipitated when the metal solidifies, and the surface is smooth, which is often difficult to weld and form layers and bubbles in the future processing. The vicinity of the shrinkage hole is also easy to cause stress concentration and crack in the processing.
Shrinkage is usually distributed near the center of the section or the entire section, and sometimes appears near the shrinkage holes, which are small dispersed holes distributed in the grain boundaries or dendrite gaps. Some small contractions are difficult to see with the naked eye and can only be detected with the help of a microscope or hydraulic test. The porosity causes the metal structure to be less dense, which greatly reduces the mechanical properties and corrosion resistance of the alloy.
The size of the shrinkage hole and the shrinkage area is related to the solidification shrinkage coefficient of the alloy, the fluidity of the metal liquid, the width of the crystallization temperature range, the section size of the ingot, the casting temperature and solidification conditions. The larger the solidification shrinkage coefficient of the alloy, the larger the size of the ingot section, the more serious the shrinkage holes will be. The narrower the crystallization temperature range and the better the fluidity of the alloy, the more concentrated the shrinkage holes are. On the contrary, the wider the crystallization temperature of the alloy, the wider the crystallization transition zone during solidification, the easier it is to form shrinkage.
The main causes of shrinkage and porosity are: unreasonable melting process, low casting temperature, poor feeding and flow interruption; The cooling strength is large, the casting speed is fast; The mold design is unreasonable, the thermal cap is too low and wet; The alloy has wide crystallization temperature range and poor fluidity.
The microstructure of shrinkage porosity and shrinkage porosity are shown in FIG. 3 and FIG. 4.
Figure 3: At the end of the casting process, the filling is poor and a shrinkage hole is formed in the head, and the gas in the shrinkage hole is not escaped
Figure 4: There are scattered shrinkage holes in the cross section, accompanied by shrinkage loosens and small cracks