Multi-material powder feeding through a dual-channel coaxial nozzle for laser metal deposition: Numerical simulation and experimental study

Imagine a world where a single machine could build a part that is hard and wear-resistant on the outside, yet tough and flexible on the inside, all in one continuous process. This is the promise of a technology called laser metal deposition. In this process, a high-powered laser melts metal powder as it is blown into a tiny pool of molten metal, building a component layer by layer directly from a digital design. For years, engineers have used this method to create parts from a single type of metal. However, the real world is rarely so simple. A turbine blade might need to withstand extreme heat at its tip while remaining flexible near its base; a medical implant might require a surface that bonds well with bone but a core that supports heavy loads. To meet these needs, scientists have long tried to mix different metals during the printing process. The challenge has always been control: how do you blend two different powders so perfectly that they mix instantly and evenly before they melt, without one type of powder flying off course or clumping together?

A team of researchers at Nanchang University in China has tackled this problem by designing a new kind of nozzle, the device that directs the metal powder into the laser beam. Instead of mixing the powders together in a hopper before they enter the machine, their new design keeps two different powders separate until the very last moment. They built a dual-channel coaxial nozzle, which essentially means a tube within a tube. The inner tube carries one type of metal powder, while the outer ring carries a different type. Both streams are fired simultaneously toward the same spot. The goal was to see if these two distinct streams, which naturally behave differently because of their weight and shape, could be forced to converge and mix uniformly right before they hit the molten metal.

To figure out the best way to build this nozzle, the researchers first turned to computer simulations. They created a virtual model of the gas and powder flow to test how different shapes and angles would affect the streams. They discovered that the size of the tubes and the angle at which they point are critical. If the tubes are too wide, the powder spreads out too much and loses its focus. If the angles are wrong, the two streams might miss each other or converge at different distances, leading to a messy mix. Through these simulations, they determined that the inner channel should be angled at 70 degrees and the outer channel at 60 degrees, with both tubes having a diameter of 0.8 millimeters. They also calculated that the inner tube's exit should be positioned 3 millimeters from the center of the nozzle, and the outer tube should be just 1 millimeter away from the inner one. These specific dimensions, they found, would create the tightest, most focused point where the two powders meet.

Once the computer model was optimized, the team built a physical prototype of the nozzle and put it to the test. They chose three very different metals to see if the design could handle the challenge: a titanium alloy known as TC4, molybdenum, and copper. These materials vary significantly in density and shape. The titanium and molybdenum powders were nearly perfect spheres, while the copper powder was irregular and jagged. Using high-speed cameras, the researchers watched the powders fly out of the nozzle. The footage showed that despite their differences, the two streams of powder traveled together and met at the exact same point in space. When they mixed the titanium and copper powders, the distinct colors of the metals allowed them to see the mixing in real time. The silvery-grey titanium and the reddish-brown copper blended together seamlessly at the convergence point, with no signs of one layering over the other or clumping into separate piles.

To prove that this mixing worked in a real manufacturing setting, the researchers used the nozzle to build a sample part. They fed the titanium alloy through the inner channel and the molybdenum through the outer channel, creating a single track that was ten layers high. After the part was built, they sliced it open and examined the cross-section under a microscope. The results were clear: the molybdenum was distributed evenly throughout the entire structure, just as the titanium was. There were no dark or bright patches indicating that one metal had settled to the bottom or floated to the top. This confirmed that the powders had mixed thoroughly in the air before melting, creating a uniform blend of two very different materials.

The study demonstrates that by carefully engineering the path of the powder streams, it is possible to deliver dissimilar metals to a laser beam and achieve a perfect mix without pre-blending them. This approach offers a new level of control for creating complex components with tailored properties. The researchers showed that with the right nozzle geometry, the natural differences between heavy, light, round, or jagged powders do not have to be a barrier. Instead, they can be managed to produce a consistent, high-quality material that combines the best traits of its ingredients, opening the door to more advanced and adaptable metal parts in the future.