Short Answer:

Porosity defect is a casting flaw characterized by small holes or voids inside or on the surface of a casting. It occurs due to trapped gases or shrinkage during solidification of molten metal.

Porosity weakens the casting, reduces structural integrity, and may lead to failure under stress. Proper mold design, degassing of molten metal, and controlled pouring help minimize porosity defects and ensure strong, high-quality castings.

Detailed Explanation :

Porosity Defect

Porosity is a common casting defect that appears as small cavities or holes in a casting. It can occur on the surface or inside the metal and significantly reduces the strength and durability of the casting. Porosity is caused by gas entrapment, improper feeding of molten metal, or contraction during solidification.

Causes of Porosity

Trapped Gas – Gases like hydrogen, oxygen, or nitrogen can dissolve in molten metal and get trapped during solidification, forming pores.
Moisture in Mold – Water or moisture in the mold sand or core releases steam when molten metal is poured, causing porosity.
Inadequate Venting – Poor vent design prevents gases from escaping, leading to air pockets.
Shrinkage Porosity – If molten metal does not properly feed areas that are shrinking during solidification, voids can form.
Turbulent Flow – Fast or uneven pouring traps air in the molten metal, causing bubbles.

Types of Porosity

Surface Porosity – Visible holes or pitting on the casting surface.
Internal Porosity – Voids inside the casting, often detected using X-ray or ultrasonic inspection.
Blind Porosity – Closed cavities that are partially exposed inside the casting.
Gas Porosity – Caused mainly by trapped gases during pouring and solidification.
Shrinkage Porosity – Resulting from contraction of molten metal during cooling without proper feeding.

Prevention of Porosity

Proper Mold Venting – Ensures trapped gases can escape during pouring.
Dry Mold and Cores – Eliminates moisture that can vaporize and form gas bubbles.
Degassing Molten Metal – Removing dissolved gases before pouring reduces gas porosity.
Controlled Pouring – Smooth and slow pouring reduces turbulence and air entrapment.
Use of Risers – Ensures proper feeding of metal to prevent shrinkage porosity.

Importance of Controlling Porosity

Porosity reduces mechanical strength, causes leaks in pressure components, and compromises dimensional accuracy. It is critical to control porosity in high-stress applications like engine parts, valves, and structural components to ensure safety and longevity.

Conclusion

Porosity defect is a casting flaw caused by trapped gases, shrinkage, or improper metal flow during solidification. It appears as surface or internal cavities and weakens the casting. Proper venting, dry molds, degassing, controlled pouring, and riser design are essential to prevent porosity, ensuring durable and high-quality castings.