Gas porosities appear in the bodies of pressure-cast parts, which are round or oval, have a smooth surface, and a light white or light yellow color. After surface treatment, they can be identified through visual inspection, while areas not accessible visually must be detected using X-ray inspection.
Causes: a) Hydrogen released from the molten metal alloy. The higher the melting temperature, the greater the solubility of hydrogen in the molten metal alloy. As the pressure casting cools and solidifies, the solubility of hydrogen decreases, and it is released from the molten metal, leading to the formation of gas porosity. Sources of hydrogen:
Cause b) Gases emitted during the filling of the molten metal alloy. In the pressure casting process, molten metal fills the mold with high pressure and high speed. If the molten metal cannot flow smoothly and steadily, it causes turbulence and gas entrapment. Turbulence is caused by:
Causec) Gases from lubricants. Lubricants on the molds decompose and produce gases when heated by the molten metal, or excessive use on the mold causes them to evaporate.
Solutions:
Shrinkage defects run parallel to the mold opening to a certain depth and appear as scratch-like strips on the surfaces of castings. Cold shuts (misruns) occur when the molten metal improperly sticks to the mold, resulting in areas of the casting with excess or insufficient material. Both defects can be identified through visual inspection.
Causes:
Blisters appear on the surfaces of die-cast parts as bulges of varying sizes. These bulges are caused by the expansion of gas beneath the surface. Blisters can be identified through visual inspection.
Cracks appear on the surface of die-cast parts as linear or irregular patterns that tend to propagate under external force. Cracks can be classified as cold cracks (no oxidation at the crack site) and hot cracks (oxidation is present at the crack site). This defect can be identified through visual inspection.
The geometric shape of the die-cast part does not conform to the drawing, and general or localized deformation occurs. The deformation can be identified through gauge inspection or visual inspection.
On the surface of the casting, there are stripes aligned with the flow direction of the molten metal, as well as clearly visible, non-directional lines that differ in color from the base material of the casting. These stripes and lines do not tend to extend further. Flow marks can be identified through visual inspection.
On the surface of die-cast parts, there are distinct, irregular, and concave linear marks.These lines are small and narrow, with smooth intersection edges, and they may extend under external force. Cold shuts can be identified through visual inspection.
On the surface of the casting, there are mesh-like, hairline protrusions or depressions. As die casting time increases, these protrusions or depressions expand and continuously extend. Turtle cracks (or network cracks) can be identified through visual inspection.
In thick-walled areas on the surface of die-cast parts, depressions (sink marks) can be observed. This defect can be identified through visual inspection.
A portion of material is missing on part of the casting surface. This defect can be identified through visual inspection.
Thin, irregularly shaped metal flashes appear along the parting line of the casting. Flash can be identified through visual inspection.
Irregularly shaped holes containing inclusions are present on the surface or inside the casting. Surface inclusions can be identified through visual inspection, while internal inclusions must be detected using ultrasonic testing (UT) or X-ray inspection.
Interlayer defects are sandwich-like layered flaws on the surface of die-cast parts, typically visible along the parting line. This defect can be identified through visual inspection.