Two laser-based methods have taken precedence in additive manufacturing during the past several decades. In the first, the laser builds a larger 3D structure out of 2D contours in a powder bed. In the second, a material is deposited on a surface and melted by the laser to create a new surface. As is necessary of other laser processes, it is critical to strike a balance between precision and process speed. 3D metal printing in record time: A computer numerical control machine modified for the extreme high-speed laser material deposition (EHLA) technique performs highly dynamic and precise tool movements in the transverse direction. With a rotary and tilting table, it is suitable for additive manufacturing as well as for coating freeform surfaces. Courtesy of Fraunhofer ILT. Such a compromise between parameters, as it turns out, is an effective gauge to differentiate the processes. The first method was named laser powder bed fusion (LPBF) by its developers at Fraunhofer Institute for Laser Technology ILT (Fraunhofer ILT) when they received a basic patent for the innovation in the 1990s. Since then, members from industry have subsequently coined many different names for this process, which typically refer to various highly similar methods. These include selective laser melting (SLM, Nikon SLM solutions); direct metal laser sintering (EOS); laser cusing (Concept Laser); and laser metal fusion (TRUMPF and Sisma 3D). Still, these variations include direct metal printing as well as LPBF of metals. The basic procedure underlying LPBF is relatively simple: A laser beam hits a point in an even powder bed and melts it while moving along a contour. Next, a new layer of powder is added, and the laser starts again. Over time, the laser builds a 3D shape out of the 2D contours. The excess powder is removed when the 3D shape is completed. Post-processing can then be applied to remove support structures or achieve a perfect surface. The second method is directed energy deposition (DED), or laser metal deposition (LMD), and it is sometimes referred to as laser cladding. In this process, the laser creates a molten pool on the surface of the workpiece, into which the powder- or wire-based filler material is continuously introduced and melted. The laser melts both the substrate and the filler material, resulting in a fusion-metallurgical bond between the coating and the carrier component. This method is normally chosen to improve the mechanical properties of the surface or to harden it against corrosion. Both methods offer strengths and weaknesses. In Figure 1, LMD, for example, excels with buildup rates — at limited precision — while LPBF achieves better structural resolution, typically at a slower pace. LPBF is a standard process to produce complex parts in small quantities, while LMD helps to economically repair worn surfaces, such as of turbine components. Figure 1. Within additive manufacturing, there is always a trade-off between buildup time and structural resolution. This is noticeable in laser powder bed fusion (LPBF) versus laser metal deposition (LMD) as well as when considering extreme high-speed laser material deposition (EHLA). Courtesy of Fraunhofer ILT. High-speed laser material deposition A few years ago, German researchers from Fraunhofer ILT and Rheinisch-Westfälische Technische Hochschule (RWTH) Aachen developed a method to replace procedures such as poisonous chromium hard plating and thermal spraying. This advancement occurred after European regulations established bans in 2017 on hazardous technologies, including chromium hard plating. The newly devised method enabled rapid coating of rotational symmetric components with metallic alloys for corrosion and wear resistance. The primary objective of this method, called extreme high-speed laser material deposition (EHLA, in German), is to use a nozzle to deposit metal powder on the surface of a workpiece and melt this powder with a laser beam before it hits the surface. The mechanism differs significantly from conventional LMD, in which the filler material is melted in the melt pool on the workpiece. EHLA achieves process speeds between 20 and 500 m/min and coating rates >5 m2/h. It also solves the problem of layer thickness. Using legacy thermal spraying technology, layers are typically required to be between 500- and 1000-µm thick. The EHLA process allows for layers that measure as small as 25 to 250 µm. Additionally, the individual layers are nonporous, and they bond firmly to the substrate. The EHLA process uses ~90% of the powder materials for the coating deposition. This makes the process far more efficient. Plus, the small weld pool, thin layers, and a minimum heat affected zone of ~10 µm enable users to process difficult-to-weld materials and material pairings, such as alloys of iron (Fe), nickel (Ni), cobalt, and copper as well as metallic glasses and high-entropy alloys. Today, EHLA is an established method for various industrial applications. These include the manufacture of brake disks, pistons and cylinders, and bearings for the automotive sector. Major manufacturers currently market machines that apply this process, which are currently in production in several countries in Europe, the Americas, and Asia. Nozzles and optics A coaxial supply of powder and laser power is crucial for the quality of any LMD process. This is especially true for a rapid process such as EHLA. Therefore, the team at Fraunhofer ILT has developed a series of custom nozzles and optics. For the powder nozzle, the team addressed two challenges: In the first, the powder gas jet should be made adjustable to optimize the injection of the powder into the laser beam. Secondly, the powder gas jet should be dense to maximize powder efficiency. To meet the demands on the nozzle component, the researchers developed a powder gas-jet canal for homogeneous powder distribution. In combination with a protective gas stream, they adapted the powder gas stream to the laser beam caustic and, at the same time, optimized the powder focus. By surface finishing the powder flow surfaces, the researchers significantly lengthened the service life of the process. Further, for areas that are difficult to access, they developed a lateral powder nozzle, which directs the powder jet laterally into the laser beam. In a regular setting, the laser beam is centered in the nozzle construction and the powder gas jet enters it conically to ensure direction-independent processing. Since a wire cannot be fed to be conical, it must be in the center, and the laser beam itself must be conical to avoid directional dependencies in-process. This principle raises a question: How can a wire be fed into the laser focus without interrupting the laser beam? The Fraunhofer ILT scientists overcame the bottleneck using special beam shaping optics. First, the laser beam is transformed into a ring shape, which is then split into two semicircles; these are focused onto the workpiece and reunited in the final laser focus (Figure 2). These optics can currently be purchased from laser company Precitec. Additional wire-based processing optics follow a similar approach in which several discrete laser beams are arranged coaxially around the wire. These systems are also available from several companies, including Meltio and Oscar PLT. Figure 2. Custom optics create a circular focus coaxially onto a central wire. The solution aims to avoid interrupting and/or distorting the laser beam without hindering the extreme high-speed laser material deposition (EHLA) operation. Courtesy of Fraunhofer ILT. EHLA offers a dynamic solution for rotational symmetric parts. But, as it is the tendency of engineers to anticipate advancements, Fraunhofer ILT began to focus on kinematics. Since 2019, Fraunhofer ILT has been working simultaneously on two systems engineering approaches to transfer the EHLA coating technology to additive manufacturing and freeform machining. The first is a tripod kinematics (vmax = 200 m/min) approach, in cooperation with manufacturing solutions firm ponticon GmbH, which uses stationary processing optics. The second initiative is a modified 5-axis computer numerical control system (vmax = 30 m/min), in cooperation with tool manufacturer Makino Asia Pte Ltd. The workpiece moves in the first method, whereas the laser optics are in motion in this second approach. Importantly, the 3D approach(es) retain the benefits of the original EHLA solution: low heat transfer and effective powder utilization (>90%). These advantages are combined with high 3D productivity. Regarding structural resolution, original LMD, with its 500- to 2000-μm thick layers, is comparable to the structurally targeted, precise buildup of LPBF with 30- to 100-μm thick layers. EHLA 3D is in the mid-range of these values, with layers ranging 50- to 300-μm thick. Figure 3. The extreme high-speed laser material deposition (EHLA) method has been transferred into a modified 5-axis computer numerical control system in which the nozzle moves (above). Courtesy of Fraunhofer ILT. Figure 4. The extreme high-speed laser material deposition (EHLA) 3D process pioneered by Fraunhofer Institute for Laser Technology ILT (Fraunhofer ILT) is currently used in various applications (right). Beyond the 5-axis computer numerical control system, the method has additionally been transferred into a tripod kinematics with a stationary nozzle. Courtesy of Fraunhofer ILT. Fabrication yielded solid volumes that were crack-free, with relative densities of >99.5%. And, to date, numerous materials have been validated. This includes materials that are Fe-based (316L, M2); Ni-based (IN625, IN718, and IN738); aluminum-based (AlSi10Mg, AlSi12, AlMg, among others); as well as the materials Ti64, CuSn12Ni2, and aluminum-bronze. Recycled powders have also been tested. The EHLA 3D process has been validated and is currently used in various applications. Most are covered by nondisclosure agreements — though it can be said that the buildup of thin-walled parts from aluminum is among them. The repair of difficult-to-grind materials in aerospace technology, for example, is a field for which EHLA 3D has raised interest. Additive manufacturing has advanced from a research topic to a mature solution. Several procedures are currently available, differing in structural precision and throughput. Following the technical progress, the favored method should be chosen based on the task of an application rather than the availability of a certain technology. Meet the authors Min-Uh Ko is group leader in additive manufacturing and repair laser metal deposition (LMD) at Fraunhofer ILT. He is responsible for the research activities involving various additive manufacturing and repair applications with LMD; email: min-uh.ko@ilt.fraunhofer.de. Andreas Thoss, Ph.D., is a laser physicist, founder of THOSS Media, and a contributing editor to Photonics Spectra. He has been writing and editing technical texts, with a focus on the field of photonics, for two decades; email: th@thoss-media.de.
