There are many types of casting defects, but they are generally divided into four categories, namely:
• Metallurgical defects
• Thermal defects
• Mold material defects
• Casting shape defects
Metallurgical defects
These defects occur mainly due to problems related to the metal material. There are five different types of metallurgical defects, including:
1. Porosity defects
These are internal die casting defects that are usually difficult to find. You will usually find them where there are small holes, voids or bubbles in the metal. Usually, porosity occurs during the casting process whenever air is trapped in the metal. As a result, gaps may appear at the top of the mold, or the mold may fill too slowly. As a result, the resulting part may have weakened structural resistance. There are two main types of porosity defects:
Porosity
When the cast metal solidifies inside the mold, it cannot retain as much gas as it does when it is liquid. Therefore, the metal tends to release gas. The inability of the gas to pass through the mold can easily lead to bubbles being trapped inside the metal. These bubbles appear as:
• Pinholes. These are small holes that appear on the upper part of the die casting. You will usually see them in groups near the surface of the casting (through visual inspection).
• Porosity. They are larger holes that appear inside the cast workpiece. Unlike pinholes, they are not visible to the naked eye. Therefore, you will need X-ray, ultrasonic, harmonic or magnetic analysis to detect them.
• Open Porosity. These are the types of pores that appear on the surface of the workpiece. They are caused by air entrapment when the metal is poured into the mold. Scars are shallow open holes.
These defects appear as round or oval shapes on the surface of the casting. They usually have a bright yellow or bright white smooth surface. As a result, you can identify porosity defects in castings by visual inspection after the casting process. On the other hand, you need an X-ray device to detect this defect on non-machined surfaces.
Causes
• Gases involved in metal alloy filling. Metal casting involves filling the mold with molten metal alloy at high pressure and speed. Therefore, the inability of the molten alloy to flow steadily and orderly may lead to turbulence involving gases. Turbulence often occurs due to blind spots in the gating system, unreasonable pouring parameters, improper runner design, etc. Therefore, you may notice some porosity defects due to these turbulences.
• Hydrogen release from the molten alloy. This is one of the main reasons for the formation of pores in castings. Moisture in the air or smelting tools may enter the molten metal or steel alloy and decompose into hydrogen. When the casting cools and solidifies, the solubility of hydrogen decreases, which is released to form pores.
• Gases from mold release agents. Upon contact, molten metal alloys usually heat the mold release agents. As a result, they decompose and produce gases that eventually lead to porosity defects. Similarly, using too much mold release agent can cause gas volatilization.
Remedies
• Make sure to use clean and dry metal alloy ingots to prevent the formation of hydrogen.
• Use appropriate casting parameters, including injection speed and pressure.
• Control the melting temperature and prevent overheating.
• Use sprues and runners with a length greater than 50 mm. Thus, you will ensure a stable and sufficient airflow.
• Release agents should be of the highest quality and in controlled quantity.
