Gas porosity occurs when the metal traps gas (most often nitrogen, oxygen or hydrogen) during casting.
When the casting cools and solidifies, bubbles form because the solid form of the metal cannot hold as much gas as the liquid form. These bubbles appear on a casting as rounded, circular cavities or holes.
There are three types of casting defects related to gas porosity:
- Pinholes
Pinholes, also sometimes referred to as porosities, are very tiny holes (about 2 mm) usually found in the cope (upper) part of the mold, in poorly vented pockets.
They usually appear in large numbers together, either at the surface or just below the surface of the casting. They are always visible to the naked eye and don’t require equipment to identify.
- Subsurface blowhole
Blowholes, or simply blows, are larger cavities than pinholes.
A subsurface blowhole appears on the inside of a cast and usually isn’t visible until after machining.
Subsurface blowholes can be difficult to detect before machining, requiring harmonic, ultrasonic, magnetic or x-ray analysis.
- Open holes
These blowholes appear on the surface of the cast and are easier to detect than subsurface blowholes.
Causes and prevention of gas porosity
There are several causes of cavity defects.
- Poor venting of mold and cores
- Insufficient drying of mold and cores
How can you prevent gas porosity?
Scars are shallow blows that appear on a flat surface, while blisters are scars covered with a thin layer of metal.
- Excessive moisture content of molding sand
- Inadequate gas permeability of molding sand
Potential solutions include:
- Incorporate good fluxing and melting practices: melt metal in a vacuum, in an environment of low-solubility gases or under a flux that prevents contact with the air
- Increase gas permeability of sand: coarser sands have a higher permeability
- Increase permeability of mold and cores. Allow air and gas to escape from the mold cavity
- Dry out molds and cores before use and store dry
- Increase rate of solidification by reducing metal temperature during casting
What’s the Difference in Gas and Shrinkage Porosity?
Posted by Andy Marin on Fri, Jan 12, 2018 @ 08:57 AM
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The definition of porosity is any void or hole in a casting. But this definition does not describe or give direction on the root cause of porosity. Casting porosity can be caused by gas formation or solidification shrinkage while the metal is a liquid. If a casting needs to be pressure tight, then the porosity can allow gas and fluids to seep from the part. In addition, the porosity can weaken the casting. In this blog, we will discuss the difference between gas and shrink porosity, and the best solution to seal porosity.
Gas-Related Porosity
What is it?
Gas-related porosity is caused by trapped mold or core gases in the liquid metal. Air is present in the tool cavity, and is easily trapped as metal fills the cavity. The air dispersed as small sphere of high pressure air when the metal fills the cavity.
What does it look like?
Gas-related voids generally look like bubbles with a smooth interior. Gas porosity is always buoyant and will be near the top of the casting.
Shrinkage-Related Porosity
What is it?
When a casting solidifies inside the tool, it always shrinks in size. Shrinkage-related porosity is caused by sections of a casting that solidify later than the surrounding sections, and do not have enough metal flow into the section to completely fill.
What does it look like?
Shrinkage porosity will have a jagged or linear appearance. Shrinkage porosity usually occurs in either the cope or drag portion of the casting and below the surface.
Problems of Porosity
As the amount of porosity increases in an aluminum or iron casting, it can become inter-connected and cause a leak path. The porosity makes the casting unusable for holding pressure in applications like pumps, compressors, transmissions and plumbing fixtures.
Porosity in powdered metal parts can cause severe plating problems where the plating chemicals are trapped in the pores. The entrapped plating chemicals expand with enough force to cause spotting on the plating.
How to Stop Casting Porosity
The most economical and successful approach to stop casting porosity is through vacuum impregnation. Vacuum impregnation is a method that seals the casting resulting from porosity. The impregnating sealant is introduced into the voids within the wall thickness of the casting through vacuum and/or pressure methods. This method is a cost effective and permanent solution to casting porosity. There is no limit to the size of castings which can be impregnated. Since the impregnation process occurs within the part, the vacuum impregnation process does not distort, discolor, or affect the casting.
Vacuum impregnation achieves the results that casting engineers have sought for centuries: a leak-free and pressure tight casting that is economical and usable in all the applications required.