Gas porosity is a common issue in aluminum casting, and it can significantly affect the quality and strength of the final product. Of course, there are plenty of other issues that can arise in aluminum casting, like shrinkage, inclusions, hot tearing, cold shuts, misruns, oxide formations, and mold shifting. We’ll cover all of those in a later blog. Today we want to focus only on gas porosity.
We should mention that if you are going to pour medical-grade castings or any casting that demands strict structural integrity and that requires an x-ray, then you need to go way beyond these common solutions listed below. But for general practice, let’s talk about a few methods to reduce gas porosity in aluminum casting.
What is Gas Porosity?
Gas porosity occurs when gases, particularly hydrogen, become trapped in molten aluminum during the melting process. As aluminum melts, it absorbs hydrogen from the moisture in the atmosphere or the materials used. When the metal solidifies, the gas gets trapped, forming small bubbles or voids within the casting. These voids weaken the metal and affect the quality of the final product.
To prevent this, aluminum alloys must undergo a process called degassing while still in the furnace. This involves introducing inert gases like argon, which help to encapsulate and remove hydrogen from the molten aluminum. Degassing also aids in bringing impurities like slag and dross to the surface, where they can be removed before pouring.
However, effective degassing requires attention to several factors. The temperature at which the metal is melted plays a critical role. Foundries may be tempted to melt at higher temperatures to improve metal flow into molds, especially for thin-walled castings, but this can lead to increased gas absorption.
Here at Precision Enterprises, we take a proactive approach to ensure minimal gas porosity. Before each heat is released, we pour a test chip from the degassed metal and solidify it under vacuum. This method amplifies any gas-related voids, making them easily visible during inspection. By polishing and analyzing these chips, we can identify and address porosity issues before proceeding with casting. Let’s dive into each of these steps in detail.
Degassing
As mentioned, degassing is the first step of many. This involves removing dissolved gases from the molten aluminum before casting. Common methods for degassing include using rotary degassing equipment or adding degassing tablets to the molten metal.
- Rotary Degassing (Agron Injection)
In this method, an inert gas like argon or nitrogen is injected and stirred into the molten aluminum dispersing tiny bubbles throughout. These bubbles collect hydrogen atoms dissolved in the aluminum, allowing the hydrogen to rise to the surface and escape. The constant agitation caused by the rotary degasser ensures that the gas removal is thorough and efficient. - Degassing Tablets
Another common method is using degassing tablets, which are composed of compounds that release gases like chlorine or nitrogen when submerged in molten aluminum. Similarly to the argon and nitrogen in the rotary degassing, these gases interact with the dissolved hydrogen, causing it to form bubbles and rise to the surface. Degassing tablets are often used for smaller operations or when rotary equipment is unavailable. - Best Practice! Vacuum Testing of Test Chips
Before each heat is released, a vacuum chamber can be used to test the quality of the molten metal by pouring a test chip from the degassed aluminum and solidifying it under vacuum. This step allows foundries to inspect the metal for any residual gas content before the casting process.
Here’s how it works:
– A small sample or test chip of the molten aluminum is poured into a mold and placed inside a vacuum chamber.
– The vacuum is applied, making it easier to see any porosity issues as they expand and form larger voids.
– After the test chip solidifies, it is sectioned and polished for inspection.
By solidifying the metal under vacuum, any gas present in the aluminum will form exaggerated voids, making it easier to detect gas porosity. The inspection of this polished chip provides a clear indication of the gas levels in the molten aluminum. If excessive porosity is detected, adjustments to the degassing process can be made before proceeding with the full casting run.
Here at Precision Enterprises, we use this vacuum test chip method before each heat is released. By pulling a vacuum on the test chip, we ensure that any residual gas in the molten aluminum is exposed and easy to detect. This allows us to confidently move forward with casting only after verifying that the metal is free from significant gas content, ensuring the highest quality for our cast products.
Fluxing
Fluxes are chemical compounds, typically a mixture of salts, that are added to the molten metal to help eliminate non-metallic inclusions (like oxides and dross) and minimize gas content. Adding fluxes to the molten aluminum can help remove impurities and reduce the amount of dissolved gases. There are two main types of fluxes used in aluminum casting, cleaning and cover. Cleaning fluxes are designed to remove impurities from the molten aluminum by bringing them to the top to be skimmed off, and cover fluxes are designed to form a protective layer to prevent additional impurities from getting in.
In addition to removing solid impurities, fluxing plays a role in reducing the number of dissolved gases in the molten metal. Some fluxes are designed to release gases like chlorine or fluorine, which help strip hydrogen from the aluminum. The hydrogen gas is then carried to the surface of the molten metal, where it can escape, helping to minimize the risk of gas porosity.
Proper Melting Practices
This one is important. We touched on it earlier, that some foundries can be tempted to melt at a higher temperature than normal to help the feed of the metal into the mold. Overheating the aluminum is a risky way to add gas. We talk extensively about the proper temperatures of melting aluminum here, when it’s OK to overheat, and the risks of overheating here.
When heating aluminum, make sure that the melting furnace is clean and free of contaminants. This can help minimize the amount of gas absorbed by the molten metal. On top of introducing gas, overheating the metal can destroy important characteristics inherent in the metal.
Vacuum Casting
To further reduce gas porosity in aluminum casting, some foundries use a vacuum during the casting process. It works by creating a vacuum through which gases—particularly hydrogen—can be efficiently removed from the molten aluminum or mold cavity, leading to castings with fewer voids and greater structural integrity.
- Vacuum-Assisted Casting
In vacuum-assisted casting, a vacuum is applied to the mold during the pouring of molten aluminum. By lowering the pressure in the mold, the vacuum helps draw out any gases that might be trapped in the molten metal, preventing gas bubbles from forming as the metal solidifies. The reduced pressure within the mold also encourages better metal flow, especially in complex or thin-walled castings, resulting in a more uniform and defect-free structure.
Vacuum-assisted casting not only minimizes gas porosity but also improves the overall density and mechanical properties of the cast component. It is often used for high-precision or high-strength aluminum castings where minimal defects are critical.
If the temperature of the metal is constantly monitored, and the moisture of the sand is tightly controlled, Vacuum Casting is an added procedure not necessarily needed to produce a consistently solid casting.
Hydrogen Measurement
This one seems like a no-brainer to those with experience in aluminum casting. But we can’t go without saying that regularly measuring the hydrogen content in the molten aluminum can help identify and address potential gas porosity issues before casting. Since hydrogen is the primary gas responsible for porosity in aluminum, regular monitoring helps foundries to take corrective measures during the casting process. There are four main ways to measure hydrogen in molten aluminum:
- Hydrogen Measurement via the Reduced Pressure Test (RPT)
- Hydrogen Measurement Using a Hydrogen Analyzer (Degas Analysis)
- Hydrogen Measurement Using Solidification Under Vacuum (Vacuum Solidification)
- Aluminum Quality Control Standards
70+ Years of Aluminum Foundry Experience
With several causes of porosity not mentioned here, you can count on the 70+ years of experience and expertise of Precision Enterprises for your aluminum casting needs. If you have more questions about how to reduce gas porosity in your aluminum casting or anything else around sand and aluminum casting, we’re here for you. Reach out today for a quote!
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