Numerical Simulation and Process Parameters for Laser Cleaning of Oxide Films on TA15 Titanium Alloy

Imagine you are trying to clean a dirty window, but instead of a rag and some soap, you are using a super-fast, super-hot beam of light. This is the world of laser cleaning, a high-tech method that uses pulses of light to blast away grime without touching the surface. It's like using a tiny, precise snowblower made of energy to clear a path. But here's the tricky part: if the snowblower is too weak, the snow stays; if it's too strong, it might melt the window itself. Scientists have been trying to figure out the "Goldilocks zone"—the exact settings where the light is strong enough to vaporize dirt but gentle enough to leave the material underneath perfectly safe. This is especially important for materials like titanium alloys, which are the super-strong, lightweight metals used to build airplanes and rockets. These metals are great, but they naturally grow a crusty "oxide film" (like rust, but for titanium) when they sit in the air. If you try to weld these metals together without cleaning off that crust first, the weld will be weak and full of holes, which is a disaster for anything flying at high speeds.

So, a team of researchers from Harbin Aircraft Industry Group and the Harbin Institute of Technology decided to play the role of "laser chefs" to find the perfect recipe for cleaning these metals. They didn't just guess; they built a computer simulation first. Think of this simulation as a video game where they could test thousands of laser settings instantly to see how the heat would move through the metal without actually melting their lab equipment. They created a digital model of the TA15 titanium alloy (a specific type of metal used in aviation) and its oxide crust, programming it to react to laser heat just like the real thing. They assumed the laser acts like a heat beam that makes the crust get hot, melt, and then boil away into thin air.

Once their computer model was ready, they moved to the real world. They took actual pieces of TA15 titanium that had sat around long enough to grow their natural oxide films and started blasting them with a nanosecond pulsed laser. A "nanosecond" is a billionth of a second, so these pulses are incredibly fast—like a camera flash that happens a billion times a second. They tested different "knobs" on their machine: how much power the laser had, how fast it fired pulses, how fast the laser beam moved across the metal, and how fast the whole cleaning head moved.

Here is what they discovered. First, their computer model was spot-on. It predicted that the metal surface would heat up and cool down in a flash (within microseconds), and the experiments proved this was true. They found that if the laser power was too low (like 70 W or 140 W), the oxide film would just get warm and maybe melt a little, but it wouldn't disappear. It was like trying to boil water with a candle; it just gets lukewarm. However, when they cranked the power up to 200 W, the surface got hot enough to reach the boiling point of the oxide film (around 3673 K), and the crust simply vaporized and vanished.

But power isn't the only thing that matters. The team found that the speed of the laser pulses and the movement of the laser mattered just as much. If they fired the laser too fast (like 30 kHz), the individual pulses didn't have enough energy to do the job, and the cleaning failed. It's like trying to break a rock with a hammer: if you swing too fast, you don't hit hard enough. They also found that if they moved the laser too slowly, they burned the metal, creating tiny cracks. If they moved it too fast, they missed spots, leaving the oxide film behind.

After testing all these combinations, they found the perfect recipe for cleaning TA15 titanium: a laser power of 200 W, firing at a repetition rate of 10 kHz, with the laser beam scanning at 3000 mm/s and the cleaning head moving at 7.5 mm/s. With these settings, the oxide film was completely removed, leaving a smooth, clean surface ready for welding.

Interestingly, they noticed something cool happening when they pushed the laser power even higher (above 200 W). Instead of just cleaning, the laser started to create tiny, repeating patterns on the metal surface, almost like a microscopic landscape of hills and valleys. While this wasn't the goal for simple cleaning, the researchers suggest this could be useful for other things, like making the surface better at holding water or reducing friction.

In short, this paper didn't just say "lasers clean metal." It provided a detailed map of exactly how to do it without breaking the metal. They proved through both computer simulations and real-world experiments that 200 W is the sweet spot for power, and that moving the laser at the right speed is crucial to avoid cracks or missed spots. They ruled out the idea that just any old setting works, showing that too much speed or too little power leads to failure. While they didn't solve every cleaning problem in the universe, they gave engineers a reliable, tested guide for cleaning the specific titanium alloys used in our most advanced aircraft.