Can't tell the difference between shrinkage cavities and gas holes in casting defects, cold shuts and wrinkles?

In castings, various defects are often encountered. If one is not familiar with or has no understanding of them before, it is easy to get confused just by looking at the names of some defects. For example, the four defects we are going to talk about today: shrinkage cavities, gas holes, cold laps and wrinkles. Do you know how they are formed and what the differences are? Let's take a look together today.

Let's first take a look at shrinkage cavities and gas holes. It will be easier to understand with pictures.

Shrinkage cavities, which we generally also call shrinkage porosity, are caused by the volume shrinkage of materials during the solidification process, resulting in a volume gap. If the gap volume is not adequately replenished, it will occur. Since the solidification of metallic liquids usually starts from the surface, shrinkage cavities or porosity often occur inside the part material, in areas where cooling is relatively slow, typically in areas where the material is thicker. Visually, the edges of shrinkage cavities or porosity are usually determined by the dendrite growth structure and are mostly irregular in shape. As shown on the left side of the above picture.

Porosity is usually caused by the fact that the gases present in the casting cannot be discharged in time. The formation of these gases is related to the vacuum degree of the cavity, the use of adhesives and release agents, as well as the vaporization of water condensed on the surface. They often form in areas where bubbles tend to accumulate, such as near the core, beneath the surface of the casting, or at the surface of the casting. From the appearance, pores are usually spherical and have a smooth surface. The comparison between it and shrinkage porosity is shown in the above figure.

Let's take a look at the cold skin barrier and the wrinkled skin. Let's first post two pictures for everyone to compare.

The defect on the left is cold lap, which refers to the surface discontinuity formed when two streams of molten metal converge but fail to fuse well. It often occurs in areas far from the gate. Its main feature is obvious, irregular, and concave linear lines. At the junctions, the edges are usually smooth, but there may also be fractures. This kind of defect is usually caused by improper temperature control, too slow pouring speed or turbulence within the mold that disrupts the metal flow.

Wrinkling, as the name suggests, refers to the presence of many wrinkles on the surface of a casting. In most cases, it is caused by the high temperature of the alloy and the low temperature of the mold. At this time, the alloy will rebound from the colder hot front, thus causing wrinkles. Sometimes, if a solid, slightly hard oxide film forms on the surface of liquid metal, this kind of wrinkled skin will also occur after molding. If you look at wrinkles when magnified, they are usually rough and porous.