Bifilm generation in metal handling and degassing: Total numbers, sizes and damage

In the world of metal casting, the strength of a finished part often depends on what cannot be seen. When molten aluminum is poured into a mold, it is constantly exposed to the air, and this contact creates a thin, invisible skin of oxide on the surface. If the liquid metal is stirred or poured carelessly, this skin can get folded over onto itself, trapping a tiny pocket of air between the two layers. Engineers call these folded skins "bifilms." They are not just empty holes; they are like microscopic, double-layered cracks that act as weak points inside the metal. While traditional testing methods look for these defects by checking how much the metal shrinks or how many visible bubbles form, these tests often miss the vast majority of the damage. The real question facing manufacturers is not just whether these flaws exist, but how many are created during the messy, turbulent process of moving and treating the molten metal, and how much harm they actually cause.

A team of researchers at Jönköping University has taken a fresh look at this problem, moving beyond simple observation to build a detailed picture of how these hidden defects are born. They focused on two specific moments in the production line where the metal is most likely to be damaged: when it is poured from one container to another, and when it is treated with a spinning rod to remove gases. By combining mathematical models of fluid motion with high-tech imaging of actual metal samples, they discovered that the way we currently handle molten aluminum is creating far more damage than anyone realized, and that the most dangerous step is not the gas removal, but the simple act of pouring.

The researchers began by modeling the physics of what happens when liquid metal is poured. Imagine a stream of water falling from a height into a bucket; the impact creates ripples and splashes that trap air. In the same way, when molten aluminum is poured from a furnace into a ladle, or from a ladle into a holding furnace, the falling stream hits the surface of the pool below. This impact creates a specific type of turbulence that tears the surface oxide skin and folds it into the liquid. The team calculated that under typical industrial conditions, involving two such transfers, this process alone generates a minimum of 1,577 of these folded oxide defects in every cubic millimeter of metal. They found that the size of these defects can reach up to 83 micrometers, which is roughly the width of a human hair.

Next, they examined the process of rotary degassing. In this step, a spinning rod is dipped into the molten metal while an inert gas is bubbled through it to remove unwanted hydrogen. The spinning action creates a vortex that helps the gas bubbles rise, carrying impurities to the surface. However, the researchers found that this spinning motion also breaks the gas bubbles into smaller pieces, and each of these tiny bubbles becomes a new site for an oxide film to form. Their model predicted that this process adds another 92 defects per cubic millimeter. While this number is smaller than what is created by pouring, the researchers noted that the damage from degassing is highly sensitive to how fast the rod spins. If the rotation is too fast, the turbulence becomes violent enough to create a massive number of tiny, hidden defects, making the degassing step more harmful than intended.

To see if their calculations matched reality, the scientists turned to a powerful technique called focused ion beam serial sectioning. Instead of just looking at the surface of a metal sample, they used a beam of ions to slice away the material layer by layer, creating a three-dimensional map of the inside. What they found was startling. While their models predicted a minimum of roughly 1,577 defects per cubic millimeter, the actual samples contained approximately 187,500 hidden oxide defects in the same volume. This is more than two orders of magnitude higher than the prediction. Even when the researchers adjusted their models to account for the fact that bubbles can break apart into smaller "daughter" bubbles—potentially raising the predicted number to between 6,000 and 8,000 per cubic millimeter—the gap remained enormous.

This huge discrepancy led the team to a critical conclusion: the pouring and degassing processes, while damaging, are not the sole source of the problem. The sheer number of defects found in the samples suggests that a significant portion of these oxide films are already present in the raw aluminum before it even reaches the casting plant. They likely originate during the primary production of the metal, a process that dates back to the early days of aluminum smelting. This means that even if a foundry perfects its pouring technique, it cannot eliminate all defects because the material itself arrives with a hidden history of damage.

The researchers also developed a way to measure how dangerous these defects are, not just by counting them, but by considering their size and where they sit inside the metal. They found that the defects created during pouring are generally larger and more damaging than those created during degassing. In fact, the damage caused by a single pour from a height of just 0.3 meters is roughly equivalent to six minutes of vigorous degassing. This finding challenges current industry practices, which often focus heavily on optimizing the degassing step while paying less attention to the transfer of the metal. The study suggests that the most effective way to improve the quality of cast aluminum is to minimize the turbulence and height of the pour, rather than just trying to clean the metal afterward.

Ultimately, this work reshapes our understanding of why aluminum parts fail. It is not enough to look for the large, visible holes that form when gas expands; the real threat lies in the millions of tiny, folded oxide films that are invisible to the naked eye and to standard tests. These films act as the starting points for cracks that can weaken the metal under stress. By realizing that the raw material arrives with a heavy load of these hidden flaws, and that pouring adds even more, engineers can begin to rethink how they handle molten metal. The goal is no longer just to remove defects, but to understand that the metal's journey from the furnace to the mold is a continuous process of damage accumulation, where the simplest actions, like pouring, can have the most profound impact on the final strength of the part.