Can sophisticated metal components also be 3D printed productively and reproducibly in series? Researchers in Aachen have answered this question in the affirmative: at the Fraunhofer ILT, they have transferred the two-dimensional EHLA process to a modified 5-axis CNC system for the additive manufacturing of complex components. By extending the EHLA process to the third dimension, difficult-to-weld materials such as tool steels, titanium, aluminum and nickel-based alloys can be 3D printed.
For decades, two laser processes dominated the printing and coating of metal components. The dominant technology for direct industrial, metallic 3D printing is the laser-based powder bed fusion (LPBF) process patented 26 years ago by the Fraunhofer Institute for Laser Technology (ILT). The laser beam melts a small part of the base material and converts the powder into a fusible metallurgically adhesive layer. In this way, a 3D component grows layer by layer from the powder bed.
Laser material deposition (LMD) has also proven to be an efficient surface technology of a special kind. In laser material deposition, a molten pool is formed on the surface of the component, into which the filler material is continuously introduced and melted, for example using wire or powder. This melts both the substrate and the filler material, resulting in a fusion-metallurgical bond between the coating and the carrier component. The economic potential lies firstly in the possibility of upgrading basic components with a functional layer or making local, additive component modifications. The second important area of application for LMD is repair welding - i.e. saving expensive components, for example from the aerospace industry or toolmaking. Worn or defective components are now indispensable for metal-based additive manufacturing after a local LMD and LPBF due to the process-specific advantages: LMD impresses with its high productivity, while LPBF can be used for 3D printing of extremely delicate and complex components. Fraunhofer ILT and the Chair of Digital Additive Production DAP at RWTH Aachen University broke completely new ground in 2012 with the development of extreme high-speed laser cladding (EHLA). In this patented process, a laser melts the powder particles above the melt pool. This allows the process speed to be increased from the previous 0.5 to 2.0 (LMD) to up to 200 m/min and the layer thickness to be reduced from 500 to 10 μm. Up to five square meters can now be coated per hour. In addition, the layers have become smoother and the roughness has been reduced to a tenth of the typical value for laser cladding.
International success with fast coating
The invention was well received: Hornet Laser Cladding B.V. from Lexmond (Netherlands) integrated a laser beam source, EHLA processing head and powder feed system into its lathes for industrial use of the EHLA process. TRUMPF Laser- und Systemtechnik GmbH from Ditzingen has also included the process in its product portfolio and offers laser systems and system technology for the EHLA process. The first users include companies in the Netherlands, China, Germany and Turkey. The breakthrough came in 2015 in the offshore sector: since then, many hundreds of meter-long hydraulic cylinders have been coated with wear and corrosion-resistant alloys for worldwide use in maritime environments (Fig. 1).
After further successes in the fast and reliable coating of brake discs, pistons, cylinders and bearings, the step into the third dimension followed in 2019: Jonathan Schaible, then a research assistant at Fraunhofer ILT, was involved in the further development as part of his doctorate, in which he dealt with the question of which special requirements for machine and system technology have to be met in order to combine EHLA with high-speed 3D printing. At the same time, his successor, Min-Uh Ko, continued the process engineering investigations on a specially modified 5-axis CNC system that combines maximum precision and high feed rates for additive manufacturing, free-form coating and component repair using EHLA. EHLA 3D combines the productivity of LMD with its 500 to 2000 μm thick layers with the structurally targeted, precise build-up of LPBF with 30 to 100 μm thick layers. EHLA 3D is in the mid-range here with 50 to 300 μm.
The low mixing zone and high cooling speed also speak in favor of the process. Thanks to these properties, components made of difficult-to-weld materials and multi-material combinations can also be additively manufactured.
The process shows its strengths in real 3D printing
Fig. 2: High dynamics: with EHLA 3D, productivity stands and falls with the interplay of fly-in and fly-outTheprocess shows its strengths in real 3D printing. EHLA 3D can be used to productively manufacture components that come very close to the final contour. In addition to near-net-shaping, the process also enables the fast and precise construction and coating of free-form surfaces. Producing sophisticated shapes in record time is only possible with appropriately designed machine technology and adapted path planning of the CNC programs. Productivity here stands and falls with the interaction of the so-called fly-in, in which the laser head flies to the place of use with the laser beam switched on, and the subsequent fly-out, the decelerated fly-out from the processing zone (Fig. 2). The efficiency results from the ratio of the processing time with the laser beam switched on to the total process time. Schaible's investigations prove this: At an acceleration of 50 m/s2 and a feed rate of 50 m/min for a distance of 100 mm, the efficiency M-PDE (Machine-Related Powder Deposition Efficiency) is around 80 percent. At an acceleration of 10 m/s2, the M-PDE is approx. 40 percent.
The effort put into the further development of the EHLA process has paid off, as a look at the first successful demonstrations shows. At the "AKL'22 - International Laser Technology Congress" in Aachen in spring 2022, scientist Ko presented the latest advances in EHLA 3D technology. In a video, he demonstrated the productive, additive manufacturing of a molded component whose printing time was reduced by a factor of 2 compared to LMD. Further advantages result from the reduction in the amount of finishing work required.
The 3D printing of components made from the aerospace material Inconel 718, which was produced at a deposition rate of more than 2 kg/h with a density of over 99.5 percent, is also characterized by high efficiency. Excitingly, the Aachen team also investigated how the characteristic values change when they work with recycled metal powder instead of new metal powder. In both cases, the tensile strength Rm was around 1300 MPa. In both cases, the tensile strength was just as good as that of the cast material. Good results were also achieved by scientist Schaible, whose work included the EHLA 3D process development of components made from 316L stainless steel and aluminum-silicon alloys. Here too, the mechanical properties achieved correspond to those specified in the literature for conventionally produced samples. The currently possible structural resolution of thin-walled aluminum components manufactured using EHLA 3D is around 500 μm. The CNC system at the Fraunhofer ILT is a specially adapted prototype that can perform reliable, precise and highly dynamic tool movements.
Photos: Fraunhofer ILT, Aachen
