Flow-field simulation and experimental validation of ultrasonic-assisted abrasive water flow polishing for additively manufactured high-aspect-ratio internal channels

Imagine trying to clean the inside of a very long, skinny straw that has a sharp 90-degree bend in the middle. Now, imagine that straw is made of metal, built layer-by-layer by a 3D printer, and its walls are as rough as a sand dune. This is the challenge engineers face with the fuel nozzles in jet engines. These parts are the heartbeats of the plane, but the 3D printing process leaves them with a bumpy, jagged interior that can mess up how fuel sprays out. To fix this, scientists use a method called "abrasive flow polishing." Think of it like forcing a thick, gritty sludge (water mixed with tiny sand grains) through the straw. The sand grains scrape against the walls, smoothing them out like sandpaper on wood. However, just like trying to push water through a long, bent pipe, the sand doesn't always hit the walls evenly. The corners get hit too hard in some spots and not enough in others, leaving the "bumpy" parts still bumpy.

This is where a new trick comes in: adding a "shaker" to the mix. By attaching an ultrasonic vibrator to the pipe, the scientists shake the gritty water at a super-fast frequency. This shaking creates tiny, invisible bubbles that pop with incredible force, acting like microscopic hammers that help the sand grains do their job even better. The big question was: Can this shaking trick smooth out those tricky, hard-to-reach corners in these long, 3D-printed metal tubes without making a mess of the rest of the pipe?

In this study, researchers Zhe Yan and Yuli Sun from Nanjing University of Aeronautics and Astronautics decided to find out. They didn't just guess; they built a digital twin of the problem first. Using computer simulations, they watched how the water and sand moved inside a 90-degree bent channel made of AlSi10Mg aluminum alloy (a common metal for 3D printing). They discovered that without help, the sand grains tended to rush through the middle and miss the outer corners, while the inner corners got hammered too hard. They also simulated how the ultrasonic shaking changed the game. They found that shaking the fluid at an amplitude of 25 μm and a frequency of 20 kHz created a massive amount of those helpful popping bubbles, especially in the tricky corner areas.

After running the digital simulations, they built a real machine to test it. They took their 3D-printed metal tubes and ran three different cleaning tests: pushing the gritty water through once (unidirectional), pushing it back and forth (bidirectional), and pushing it back and forth while shaking it with ultrasonic vibrations (ultrasonic-assisted bidirectional). They measured the surface roughness, which is a number that tells you how bumpy the surface is (lower is smoother).

The results were clear. The simple "push-through" method left the entrance and the middle of the bend looking rough. The "back-and-forth" method was better at making the entrance and exit look similar, but it still struggled with the sharp corners. However, when they added the ultrasonic shaking, the magic happened. The combination of the shaking bubbles and the sand grains worked together to smooth out the difficult corners that the other methods missed. After just 20 minutes of this ultrasonic-assisted cleaning, the surface was smoother than what they got after 30 minutes of the non-shaking methods. In fact, the roughness at the corner dropped from a very bumpy Ra 13.725 μm down to a much smoother Ra 1.730 μm. The entrance and exit sections became incredibly smooth, reaching Ra 0.529 μm.

The study suggests that this ultrasonic-assisted method is a powerful way to fix the bumpy insides of complex 3D-printed parts. It makes the cleaning process faster—cutting the time needed by half to get the same result—and makes sure the whole tube, even the tricky corners, gets polished evenly. While the researchers noted that if they kept polishing for too long (over 40 minutes), the surface could actually get scratched again, they found that 30 minutes was the sweet spot for the best results. This work offers a promising new tool for making high-tech engine parts that are not only strong but also have perfectly smooth insides.