Research on Method and Technology of Dual-Beam Selective Laser Melting Forming of Porous Stainless Steel

Imagine you are trying to build a delicate, sponge-like castle out of molten metal using a super-hot laser. This is the world of Selective Laser Melting (SLM), a high-tech version of 3D printing where layers of metal powder are fused together to create complex shapes. Scientists love this method because it can make lightweight, porous structures that are perfect for things like filters, heat exchangers, or even medical implants that let bone grow inside them. But there's a catch: when you zap metal with a laser, it gets incredibly hot, and when it cools down, it shrinks. Because the metal cools so unevenly—hot in the middle, cold on the edges—it gets twisted and stressed, like a rubber band that's been stretched too tight. This "residual stress" can make the final object crack or warp, ruining the perfect sponge structure. To fix this, researchers are looking for ways to control the heat, much like a chef trying to keep a soufflé from collapsing by managing the oven's temperature just right.

In this study, a team of researchers from Fujian University of Technology decided to try a clever trick: instead of using just one laser, they used two. They set up a "main" laser to melt the metal powder and a "helper" (or auxiliary) laser to gently warm up the area just ahead of the main beam. Think of it like a main chef searing a steak while a sous-chef uses a warm towel to gently preheat the meat, ensuring the temperature changes smoothly rather than shocking the metal. By simulating this process on a computer and running real-world experiments, the team discovered that this dual-laser approach acts like a thermal buffer. It reduces the sharp temperature differences that cause stress, effectively "softening" the metal's reaction to the heat. Their simulations and tests showed that this method successfully lowered the internal stress in the printed stainless steel, proving that sometimes, adding a little extra heat in the right way is the best way to keep things cool and calm.

The Experiment: Two Lasers vs. One

The researchers focused on making 316L porous stainless steel, a material known for being strong yet full of tiny holes. They built a detailed computer model of the process, treating the metal powder as a continuous block and simulating how heat moves through it. To make sure their computer model was telling the truth, they first printed a sample using a real dual-beam machine (a Dimetal-280 system) and measured the tiny "melt pools"—the little puddles of molten metal left behind. The computer's predictions were incredibly accurate, matching the real-world measurements within a 5% margin of error. This gave them the confidence to use the simulation to test hundreds of different scenarios without wasting expensive metal powder.

The "Goldilocks" Zone of Heat

The team ran a series of tests to see how different settings changed the outcome. They treated the process like a recipe, tweaking five key ingredients: the power of the helper laser, the size of its spot, how fast the laser moved, the distance between laser lines, and the time gap between the two lasers firing.

They found that the relationship between these settings and the final stress wasn't a simple "more is better" or "less is better" story. Instead, it was a delicate balancing act:

• Helper Laser Power: If the helper laser was too weak, it didn't warm the metal enough. If it was too strong, it caused the metal to overheat and expand too much, creating new stress. There was a "sweet spot" where it reduced stress perfectly.
• Spot Size: A tiny spot on the helper laser didn't warm enough area. A giant spot cooled the metal too much in some places. The right size created a smooth, warm zone that prevented sharp temperature drops.
• Speed and Spacing: Moving the laser too slowly dumped too much heat, while moving too fast or spacing the lines too far apart left cold gaps. The team found that a scanning speed of 1200 mm/s and a spacing of 100 μm worked best.
• Time Interval: This was the timing between the helper laser warming the spot and the main laser melting it. If they fired at the exact same time, it was too chaotic. If they waited too long, the metal cooled down completely before the main laser arrived. The perfect timing was a tiny 0.3 ms gap, just enough to let the preheating do its job without losing momentum.

The Winning Combination

After running through an "orthogonal test" (a smart way to test many combinations at once), the researchers identified the ultimate recipe for the least amount of stress. The ideal settings were:

• Main Laser Power: 150 W
• Main Laser Spot Size: 40 μm
• Helper Laser Power: 10 W
• Helper Laser Spot Size: 160 μm
• Scanning Speed: 1200 mm/s
• Scanning Spacing: 100 μm
• Time Interval: 0.3 ms

When they used this specific combination, the residual stress in the metal dropped to its lowest point. In fact, the group with the best settings (Group 3) had a stress level of 282 MPa, which was significantly lower than the worst-performing group (Group 9) at 384 MPa. This proved that the temperature gradient—the difference in temperature between the hot melt pool and the cold surroundings—was the main culprit behind the stress. By smoothing out that temperature difference, they smoothed out the stress.

The Final Showdown: One Beam vs. Two

To really prove their point, the team compared their best dual-beam setup against a standard single-beam setup. They kept the total energy roughly the same but split it between two lasers for the dual-beam version. The results were clear:

• Single-Beam: Produced a peak temperature of 1580 ℃, a temperature gradient of 41,522 ℃/mm, and a residual stress of 293 MPa.
• Dual-Beam: Produced a slightly lower peak temperature of 1525 ℃, a gentler temperature gradient of 39,162 ℃/mm, and a lower residual stress of 282 MPa.

The dual-beam method reduced the residual stress by approximately 3.75%. While that number might seem small, in the world of precision engineering, it's a significant improvement. The reason it worked is that the helper laser acted like a pre-heater, raising the temperature of the powder bed before the main laser arrived. This meant the metal didn't have to deal with such a sudden, shocking drop in temperature as it cooled, preventing the "twisting" that causes cracks.

What This Means

The study confirms that using two lasers in a coordinated dance is a viable way to make stronger, less stressed porous metal parts. The researchers showed that by carefully tuning the power, speed, and timing of the helper laser, you can control the thermal history of the metal. They didn't just guess; they simulated the physics, verified it with real experiments, and found a specific set of numbers that works best. While the stress reduction was modest, the method provides a solid theoretical foundation for future improvements. It suggests that if we want to print even more complex and reliable metal structures in the future, we might need to stop thinking of lasers as just "heat guns" and start thinking of them as a team, where one prepares the stage and the other does the heavy lifting.