ANALYSIS: ADDITIVE MANUFACTURING


or skewed surfaces into the system. As shown by the IFSW7, this


has already proven beneficial in a post-processing step to increase the accuracy and surface quality of PBF-LB/M printed parts. The in-situ application of this technology, based on what has been shown previously, will increase the possibilities even further. It can already be said


Figure 2: Samples of pure iron produced by 100 cycles of consecutive additive and subtractive laser processes to reach a height of 5mm. The orientation of the slits created in the right sample is marked with red rectangles


incorporates two separate beam sources. A schematic illustration of such a system is shown in figure 1, consisting of a custom-built powder bed, a galvanometer scanner with an f-theta lens, and two laser sources. A continuous-wave laser is used for the additive process of melting the metal powder for each new layer. The ultrafast laser is used to locally ablate material after each melting process. Both laser sources operate at the same wavelength of 1,030nm and are guided along the same optical path to the scanning and focusing optics, and finally to the workpiece. Fast and reproducible switching between the two laser sources is enabled by a pneumatically switchable flipping mirror, allowing a machine-intrinsic manufacturing process consisting of sequential additive and subtractive laser processes. The realised focal diameters of the continuous- wave and ultrafast lasers are 190±5µm and 50±5µm, respectively. For the first experimental


phase, pure iron powder with an average particle size of 35µm was used for the additive process in the following example. To melt the metal powder, the continuous-wave laser was operated at an


Figure 3: Cross-section through a sample of pure iron. 100 cycles of consecutive additive and subtractive laser processes were performed. The average width of the slits produced by the subtractive laser process is 40±10µm


average power of 400W in combination with a scanning speed of 1m/s. The height of each powder layer was set constant to 50µm. A cross jet was used to prevent spatter from reaching the f-theta lens. Nitrogen was used as shielding gas during the process. The smallest possible weld bead width was approximately 200µm.


The subtractive process was


performed as follows. With an average power of 30W and a scanning speed of 3m/s, lines with a length of 4mm were ablated after each consecutive layer of powder had been welded onto the part. The pulse energy was 100µJ with a pulse duration of approximately 8ps. With a repetition rate of 300kHz, the resulting pulse overlap was approximately 80 per cent. The number of repetitions for each line was set to 500 after each added layer. The consecutive process steps, PBF-LB/M and ablation, described above were repeated for 100 layers, resulting in cube-shaped samples with an edge length of 5mm. The samples are shown in figure 2. The orientation of the slits created in the right sample is marked with red rectangles. In figure 3 a cross-section


is shown perpendicular to the scanning direction of the ablated lines. The porosity visible in this picture could be


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explained by a less than optimal inert gas atmosphere and the high reactivity of the pure iron with oxygen. Apart from some minor


transversal bulges, the average slit width is 40±10µm. With a built height of 5mm, this results in an aspect ratio of height-to-width of about 125:1. No principal limits for the achievable aspect ratio were found at this stage of the investigations.


Outlook


This is a major achievement because it pushes the boundaries of what is possible with post-processing, even if the geometry is of great simplicity and accessibility. A second important finding of the initial experiments is that although the slit width is minimally larger than the average particle size of the powder, no residual particles were observed within the slits. This is a remarkable


discovery which is to be focused on during upcoming investigations. It indicates that to some extent, thin empty voids can be produced with this combined process and open ports for later powder removal are dispensable. Beyond the slits, future


investigations will aim to incorporate the controlled ablation process of horizontal


that combining additive and subtractive laser applications in a consecutive process is a promising approach to achieving a new level of precision in printed parts. Bringing this technology to market requires little more than a flexible laser source within currently available PBF-LB/M machines to cover both process regimes.


The initial results8 of the ADDSUB project indicate a most probable future demand. It demonstrates the enormous potential of future laser materials processing through process combinations on the way to a universal laser machine that expands the horizon for previously unimagined applications. l


Manuel Henn and Matthias Buser are research associates and Volkher Onuseit is the head of the system engineering department at the University of Stuttgart’s Institut für Strahlwerkzeuge (IFSW)


References [1]


[2]


Yasa E, Kruth J-P, Deckers J. Manufacturing by combining Selective Laser Melting and Selective Laser Erosion/laser re-melting. CIRP Annals 2011; 60(1):263–6. https://doi.org/10.1016/j. cirp.2011.03.063.


[3]


Goll D, Schuller D, Martinek G, Kunert T, Schurr J, Sinz C et al. Additive manufacturing of soft magnetic materials and components. Additive Manufacturing 2019; 27:428–39. https://doi. org/10.1016/j.addma.2019.02.021.


[4]


Goll D, Schurr J, Trauter F, Schanz J, Bernthaler T, Riegel H et al. Additive manufacturing of soft and hard magnetic materials. Procedia CIRP 2020; 94:248–53. https://doi.org/10.1016/j. procir.2020.09.047.


[5]


Li R, Liu J, Shi Y, Wang L, Jiang W. Balling behavior of stainless steel and nickel powder during selective laser melting process. The International Journal of Advanced Manufacturing Technology 2012; 59(9-12):1025–35. https://doi.org/10.1007/ s00170-011-3566-1.


[6] [7]


Ziri S, Hor A, Mabru C. Effect of powder size and processing parameters on surface, density and mechanical properties of 316L elaborated by Laser Powder Bed Fusion. ESAFORM 2021. https://doi.org/10.25518/esaform21.1563. https://www.youtube.com/watch?v=z41Jnd_I0B8


[8]


Holder D, Leis A, Buser M, Weber R, Graf T. High- quality net shape geometries from additively manufactured parts using closed-loop controlled ablation with ultrashort laser pulses. Advanced Optical Technologies 2020; 9(1-2):101–10. https://doi.org/10.1515/aot-2019-0065.


Henn M, Buser M, Onuseit V, Weber R, Graf T. Combining LPBF and ultrafast laser processing to produce parts with deep microstructures. In: Wissenschaftliche Gesellschaft Lasertechnik e.V., editor. Proceedings of the Lasers in Manufacturing Conference 2021.


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Henn et al.


Henn et al.


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