High-entropy alloys are known for their nearly equiatomic mixture of multiple metal elements. They can simultaneously perform well in terms of strength, toughness, high temperature resistance, and corrosion resistance. They are regarded as the next generation of key structural materials for the aerospace and energy fields. However, their preparation has always faced problems such as uneven mixing and "mottling" of the structure.A research team from the National Institute of Standards and Technology (NIST) recently proposed a new laser path control method for metal 3D printing. By "microscopic stirring" of the molten pool during the printing process, it successfully improved the mixing efficiency of high-entropy alloys at the atomic scale, while directly printing parts with complex structures.
Traditional alloys often use a single metal as the matrix, supplemented by a small amount of other elements to improve performance. For example, adding a small amount of carbon to iron can produce steel with significantly improved strength, and adding nickel and chromium can form stainless steel with good corrosion resistance. As the demand for engineering applications continues to increase, especially in turbines, gas turbines, spacecraft and other scenarios where the comprehensive requirements for strength, durability and high temperature resistance are becoming more and more stringent, high-entropy alloy systems relying on five or more metals with roughly similar proportions have begun to receive widespread attention. However, different metals have huge differences in density, melting point and solidification behavior. Even if they can be temporarily fused at high temperatures, they can easily separate during the cooling process, forming zones with significantly different properties and weakening the overall performance of the material. As NIST physicist Fan Zhang, who participated in the study, emphasized, for high-entropy alloys to take advantage of their advantages, they must achieve sufficient and uniform mixing at the atomic scale, which places higher requirements on the manufacturing process.
Currently, common routes for preparing high-entropy alloys in laboratories include arc melting, powder metallurgy, etc. They can obtain research samples or simple ingots, but it is difficult to directly manufacture final parts with complex inner cavities and adjustable local compositions. The laser selective fusion (Laser Powder Bed Fusion) technology in metal additive manufacturing can theoretically mix a variety of metal powders in a powder bed and build components with complex geometric shapes through layer-by-layer melting and stacking. Therefore, it is regarded as a potential path to realize complex components of high-entropy alloys. In a conventional process, a high-power laser moves along a linear scanning trajectory on the surface of a thin powder layer to form a short-lived tiny molten pool, which is then rapidly cooled and solidified. This process is usually sufficient for a single metal or simple alloy to ensure performance. However, for high-entropy alloys that require full mixing of multiple elements, the residence time of the molten pool is too short and the internal flow is insufficient, making it difficult to uniformly disperse the various metal components.
The solution proposed by the NIST team refers directly to the flow and stirring process inside the molten pool: actively "stirring" the metal molten pool during the printing process to mix multiple elements as fully as possible before solidification. Instead of significantly modifying the equipment at the hardware level, they chose to re-plan the laser movement method at the software level, rewriting the traditional straight-line scanning trajectory into a "loop" path composed of tiny elliptical closed curves, allowing the laser to repeatedly draw loops in an extremely small space. This laser trajectory is equivalent to transforming the laser from a simple heat source into a microscopic "stirring tool", creating stronger convection and stirring effects inside the molten pool, forcing different metals to be mixed more fully and uniformly in a short time. The research team developed new tool path software to generate these complex elliptical scan patterns because existing commercial metal 3D printing software does not yet have similar capabilities.
To verify the effectiveness of this idea, the researchers chose an extremely difficult material combination for the test: placing the high-density refractory high-entropy alloy RHEA-19 and the lightweight titanium alloy side by side, and letting the laser scan across the boundary of the two materials along an elliptical trajectory. The two alloys have distinct differences in density and thermal properties. They are easily phase separated under conventional molten pool conditions and are difficult to form a uniform new alloy. Therefore, they are very suitable for "strict examination questions." With this arrangement, the team hopes to observe whether the molten pool can mix the two materials into a new uniform alloy at the boundary under the action of laser "stirring", rather than just forming a two-phase structure with a clear interface.
To understand what is happening inside the molten pool, it is not enough to observe the solidified sample after the fact, because the melting and solidification process occurs on a time scale of less than a second, and the high-density metal is opaque to visible light, making it difficult for conventional imaging methods to "see through" the interior. To this end, the researchers relied on the Advanced Photon Source (Advanced Photon Source) large synchrotron radiation facility at Argonne National Laboratory. This stadium-sized circular accelerator can provide extremely bright X-ray beams, suitable for penetrating metal samples and obtaining internal structural information. The team used X-ray diffraction technology to record the scattering patterns of X-rays inside the material in real time during the melting and solidification processes, from which they analyzed the evolution trajectories of the atomic arrangement at different stages, and constructed a time-series image of the dynamic structure of the molten pool. At the same time, they also used electron microscopes to conduct detailed observations of the final solidified material to confirm whether the alloy structure achieved the expected uniformity and performance potential.
Experimental evidence shows that the laser "stirring" strategy does improve an otherwise difficult-to-mix material combination, with boundary regions forming new alloy structures that are more uniformly mixed, rather than simply layered or chunked. More importantly, research shows that the design of the laser path not only affects the forming geometry, but can also be used as a key process parameter to control the alloy formation method and promote the mixing of multiple elements. This provides a new control dimension for the development of new alloy systems using additive manufacturing methods. Taken together, the technical solution proposed by the team uses the existing laser powder bed fusion platform to simultaneously achieve high-entropy alloy raw material preparation and complex final part forming in the same process through software-defined trajectory control.
From a longer-term perspective, the impact of this work goes beyond printing a certain "tricky" high-entropy alloy. At present, metal 3D printing often relies on a single powder that is pre-alloyed. Making different alloys means preparing a variety of corresponding powders, which is difficult in cost, logistics and process adaptation. The "laser mixing" idea proposed by NIST points to another possibility: putting relatively basic metal powders in the same equipment, and mixing them on demand inside the equipment through laser path and process parameter control, similar to a color printer that mixes a few inks to produce rich colors, making the additive manufacturing platform an alloy factory that integrates on-site "formulation" and on-site forming. Once mature applications are realized, printing equipment can not only reduce powder types and inventory costs, but also realize gradient design of components within a single part - for example, using more heat-resistant alloy formulas in high-temperature areas of turbine blades, and using formulas that balance strength and density in structural load-bearing or weight-reducing areas, without the need to weld or mechanically connect different material components.
Of course, this technology is still in the research and verification stage and is not a ready-to-use industrial solution. The behavior of different alloy systems in the molten pool is very different. Mixing is only one link. In engineering applications, multiple variables such as crack tendency, pore defects, residual stress, cooling rate, powder quality and subsequent heat treatment must also be controlled simultaneously. In addition, commercial software ecosystem and equipment control systems also need to follow up to support such complex laser tool paths and alloy mixing strategies on a regular basis in industrial scenarios. Relevant research results have been published in the journal "Additive Manufacturing", providing a new process direction with an empirical basis for the future additive manufacturing of high-entropy alloys and complex structural parts.