pin hole porosity in aluminum sand casting.

What is Porosity in Aluminum Sand Casting?

Porosity in castings refers to voids or holes that form within the aluminum casting during the pouring and solidification phase of the casting process. These imperfections occur as the material transitions from a liquid to a solid state. There are multiple types of porosity. Let’s talk about the most common found in aluminum and sand casting. Porosity starts at the furnace level.

Gas Porosity

The melting process introduces gases into the metal, which can contribute to porosity. Aluminum alloys must be ‘degassed’ while being melted in the furnace. This involves adding argon or other materials that will encapsulate the hydrogen gas in the molten aluminum while still in the furnace. Impurities (slag and dross) are brought to the top of the furnace by these materials and will be removed prior to pouring. However, much more is involved in degassing metals.

The temperature at which the metal is melted is also critical. Some castings have very thin walls and foundries might be tempted to melt at a higher temperature than normal to aid the feed of the metal into the mold. Melting at higher temperatures may add gas to the metal.

Here at Precision Enterprises, prior to every heat being released by the furnace tender, we pour a test chip from the degassed metal still in the furnace, solidify it under vacuum, section the chip and polish it, to inspect for porosity. Since it is solidified under vacuum, if there is any gas in the metal, the voids become very large and easy to identify.

gas porosity in aluminum casting.
air entrainment in aluminum casting.

Air Entrainment

This is often referred to as ‘turbulence’ porosity. During casting production, the metal is poured into the mold and fills the cavity created by the pattern. Just like pouring soda into a glass, if poured too fast, it can entrap air bubbles in the molten metal. The metal ‘tumbles’ through the gating system that introduces the metal into the actual mold. While the metal cools these entrapped bubbles become voids in the internal portions of the casting. Many times this forms sub-surface and you cannot see these voids until secondaries like machining uncover these areas. Depending upon the end use of the casting, this type of porosity can be judged as just a cosmetic defect and the casting can be acceptable. But if the casting needs to be totally solid, this type of porosity will be caught by QC and the casting will be rejected.

Slowing the actual pouring of the metal, adjusting the gating system, or adding choking devices all slows the metal to avoid the tumbling, smoothing and slowing the introduction of the metal flow.

Shrinkage Porosity

Shrinkage porosity is due to a combination of factors. The most common cause of this casting defect is improper gating. The gating system is how the molten metal is fed to each section of the mold. If the gating is not adequately proportioned to feed each section of the mold, the molten metal cools and solidifies too early and creates a void in the casting. The metal ‘shrinks’ away from the mold wall because there is inadequate metal flow to that specific area.

As mentioned before some casting designs have thin walls and foundries may try to pour the metal at higher than nominal temperatures to avoid shrinkage. But this may add gas porosity.

Normally, if the shrinkage is always in one area of the casting, chills are added in those sections of the mold. The metal hits the chill and solidifies normally, removing the possible shrink.

shrinkage porosity in aluminum casting.
pin hole porosity in aluminum sand casting.

Pin Hole porosity

Some say you can always find porosity somewhere in each casting. Depending upon how small you want to consider porosity, that is a true statement. On the microscopic level, yes you may find porosity. So, it is the foundry’s job to move the porosity to a section of the mold where it will cause the least damage.

Sand castings are gravity fed, meaning the metal is poured into the top of the mold and naturally follows gravity down to the bottom of the mold, filling the cavity that the pattern made in the sand. The gating system takes care of the path of the metal.

The metal is poured into a sprue on the top of the mold. The sprue introduces the molten metal into the mold vertically. The metal then flows into horizontal runners, which distributes the metal to all sections of the mold. When the mold is full, the riser will fill up on top of the mold, signaling that the mold is full…. pouring stops at this point.

However, the riser is several inches higher than the actual mold, so as the casting is solidifying, it can draw from the molten aluminum held in the riser, and that can feed the casting as it shrinks.

Aluminum naturally shrinks as it solidifies. That is why the sprue/riser design is critical to the feeding of the casting as it solidifies.

Precision Enterprises specializes in medical grade casting. Not all foundries do. There are many foundries that do not specialize in critical castings. The picture associated is of pin hole porosity that is usually allowed in most ‘non-critical’ casting.

This is something that would be scrapped by our QC department. Internal porosity that is sub-surface and cannot be readily seen will be caught in our machining facility, when the casting is cut.

70+ Years of Foundry Experience

There are several more causes of porosity not covered here. But these are the most common. We have over 70+ years of foundry experience with pouring aluminum. We have foundry engineers on staff that can advise you about your current casting project. Give us a call, we would be glad to help.

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