Coupled Phase-Field and Lattice Boltzmann Modeling of Dendritic Growth and Bubble Entrapment during Solidification of an Al–4.5 wt.% Cu Alloy

When molten metal cools and turns into a solid, it shrinks. This is a fundamental truth of physics that anyone who has ever watched water freeze into ice can observe, though the effect is far more dramatic in a furnace. As aluminum alloys transform from liquid to solid, they often trap pockets of gas that were dissolved in the melt. If these gas pockets cannot escape, they become tiny holes, or pores, inside the finished metal. These defects can weaken the material, making it prone to cracking under stress. For engineers who build aircraft or automobiles, understanding exactly how these holes form and where they end up is critical. The challenge lies in the fact that the metal does not freeze all at once; instead, it grows in tree-like structures called dendrites. As these microscopic branches reach out to fill the space, they can block the path of rising gas bubbles, trapping them in the final solid. To see this process clearly, researchers must look at a scale too small for the naked eye and a speed too fast for standard cameras to capture.

To solve this puzzle, a team of researchers at Xi'an University of Science and Technology and the Suzhou Laboratory turned to a powerful combination of computer simulation and real-world observation. They focused on an aluminum alloy containing 4.5 percent copper, a material widely used in industry. Their goal was to map the complex dance between growing metal crystals and rising gas bubbles. They built a sophisticated computer model that could track two things at once: the shape of the solidifying metal and the movement of the gas bubbles within the liquid. This model allowed them to watch, in slow motion, how the metal branches grew and how the bubbles tried to rise through the thickening liquid. To ensure their digital world matched reality, they also performed physical experiments. They melted the same aluminum alloy, controlled the amount of hydrogen gas inside it, and let it cool in a specific way. They then sliced the resulting metal bars and examined them with high-powered microscopes and 3D X-ray scanners to see exactly where the holes ended up.

The researchers discovered that the behavior of a gas bubble depends entirely on the type of metal structure it encounters. In one scenario, the metal grows in a random, star-like pattern known as equiaxed growth. Here, the branches shoot out in all directions, quickly forming a tangled net. As this net tightens, it squeezes the liquid channels between the branches until they are too narrow for a bubble to pass. The bubble gets stuck, trapped in a small pocket of liquid that eventually freezes around it. Because the metal grows equally in all directions, the trapped bubble remains roughly round, sitting isolated at the junction where several metal grains meet. The computer simulations showed this clearly, and the physical experiments confirmed it: the X-ray scans revealed round, scattered holes with no specific direction, just as the model predicted.

In a different scenario, the metal grows in long, parallel columns, a pattern called columnar growth. This happens when heat is pulled in one specific direction, forcing the metal branches to grow like a forest of straight trees. In this environment, the gas bubbles have a clear path to travel. They rise upward, following the channels between the columns. However, as the columns grow longer and thicker, they eventually catch up to the bubbles. The advancing tips of the metal branches push the bubbles along, stretching them out like taffy. Instead of staying round, the bubbles elongate into long, worm-like shapes that line up perfectly with the direction the metal was growing. The simulations showed these bubbles getting trapped in a chain-like formation, and the physical experiments matched this perfectly. The X-ray images showed long, ribbon-like pores running parallel to the growth direction, confirming that the metal had indeed captured and stretched the gas as it solidified.

The study also revealed how the speed of the liquid metal affects the shape of the bubbles before they get trapped. When the liquid is still, surface tension acts like a tight skin, keeping the bubble round. But as the liquid moves or the bubble rises faster, the force of the moving liquid begins to flatten the bubble. If the surface tension is weak, the bubble can stretch so much that it breaks apart into two smaller bubbles, which then rise separately and might merge again later. The researchers found that the size and shape of the final pore are determined by a tug-of-war between the buoyancy pushing the bubble up and the resistance of the thickening metal slowing it down. They also noted that the presence of these bubbles slightly slows down the growth of the metal itself, as the bubbles take up space that the solid metal would otherwise occupy.

By combining these digital simulations with physical proof, the team provided a clear picture of how defects form in aluminum castings. They showed that the shape of a pore is not random; it is a direct record of how the metal grew around it. If the metal grew in a random net, the holes are round and scattered. If the metal grew in straight lines, the holes are long and aligned. This understanding gives engineers a new way to predict and control the quality of metal parts. By knowing how the metal grows, they can adjust the cooling process to either prevent bubbles from forming or ensure that if they do form, they end up in a shape and location that is least harmful to the final product. The work bridges the gap between the invisible world of microscopic physics and the tangible reality of strong, reliable metal structures.