When working with materials such as aluminum and stainless steel, it’s possible to encounter problems with porosity. This is particularly an issue when welding. It’s critical if you encounter issues with porosity that you identify the source of the problem as soon as possible and find a solution.
In many cases, porosity is a pretty straightforward problem that can be dealt with by using proper welding methods and materials. However, sometimes even experienced welders find they are stumped. At times such as this, it pays to work with material suppliers who have experience with these issues.
What is porosity?
In general terms, porosity refers to a measure of empty spaces in a material. It can also be referred to as the void fraction. This is because in engineering, porosity is measured as the fraction of the volume of voids over the total volume. It is usually written as a fraction between 0 and 1, although it can also be noted as a percentage.
While porosity is a common term in metallurgy and engineering, many other fields may refer to it as well, such as pharmaceutics, ceramics, hydrology, the earth sciences and soil mechanics. When measuring the porosity, it’s important to know whether it was tested by measuring just the accessible void, which is the total amount of the void that can be reached from the surface, or if some other technique was used, such as a CT scan, x-ray or ultrasound, that might be able to probe deeper than just the surface.
A specific type of porosity can arise when welding metals. When gas gets trapped in the weld metal during solidification, the result is cavities in the metal that can be very difficult to fix. It’s not hard to spot the signs of porosity, and it is usually characterized by the presence of holes in the weld location. Depending on the application, a certain amount of porosity may be tolerated, so it’s important to know what specifications are required.
Fortunately, in most cases, porosity can be avoided with the proper tools and techniques.
What are the common types of porosity?
When welding gases get trapped in the work piece, bubbles will form. The type of porosity is usually referred to by the shape of these bubbles. For example, when the holes left behind by the bubbles are rounded, it is known as spherical porosity. If these holes are elongated, it is called wormhole porosity.
Another type of porosity is known as crater pipe porosity. This occurs due to shrinkage during the weld pool solidification. It is likely to take place anytime there are conditions that exaggerate the liquid to solid volume change, and is often associated with the switching off of the welding current.
Also a factor in identifying the cause of porosity is whether it is distributed or not. Distributed porosity is characterized by fine pores found throughout the weld bead. When these holes can be seen breaking through the surface of the weld, then it is known, appropriately enough, as surface-breaking porosity.
It should be noted that you’re likely to encounter different types of problems when working with aluminum versus stainless steel. Broadly speaking, aluminum poses more of a welding challenge, but stainless steel will also show evidence of porosity if the correct welding techniques aren’t used.
What are the possible causes of porosity and how can it be prevented?
Identifying the cause of your porosity problem could be as simple as knowing what type of welding tools you were using with what alloy, or it could take quite a bit of sleuthing. The reality is there are many possible explanations for weld porosity and the cause may not be immediately obvious.
One of the most common causes is simple: your cylinder is out of gas. Funnily enough, this happens a lot more than you might think. Anytime you’re getting unexpected porosity, your first check should be whether your welding tank is empty.
Another factor that may not be immediately apparent if you aren’t an experienced welder is the affect that air movement can have on your weld. When there is a breeze or draft in the area that interferes with the shielding gas, it can result in porosity. Even indoors, if there are powerful fans or an HVAC system, it can impact the gas delivery. As a rule of thumb, any airflow more powerful than 4 to 5 miles per hour should be avoided.
Another no-no for your weld site is moisture. Whether it’s morning condensation or water from adjacent machinery, even a small amount can lead to porosity. It’s imperative that you make sure there’s no moisture on the work pieces to be welded, such as by preheating the metal to create evaporation.
Many problems with porosity are the result not of equipment failure, but user error. Some factors to be on the look out for include a plugged or restricted gun nozzle, holding the weld nozzle too far away, holding the welder at an incorrect angle and the gas flow being too high.
Of course, while the above are problems that we see repeatedly, not every problem is immediately obvious or common. For example, a contaminated or constricted gas hose could be to blame. In one bizarre example, an insect had built a nest in a gas hose, leading to an interruption of the gas supply.
Another problem arises when a contaminant, such as grease, oil or glue, gets onto the filler metal. This can lead to the release of gases when heated. For instance, dirty work gloves have been known to pollute the weld and lead to porosity.
There are too many potential causes of porosity to list them all here. Many of the causes are unexpected and will only be recognized with experience. That’s why working with a professional supplier who’s committed to helping their clients through every step of the material selection process can pay big dividends.
At Clinton Aluminum, we pride ourselves on having a sales team and warehouse managers that average nearly 13 years of experience with Clinton. That means we’ve got a combined knowledge base that can directly benefit our customers. Contact us today to learn more.
