Initial optimization of l-pbf printing parameters of super duplex stainless steel powder

26th International Seminar of Ph.D. Students SEMDOK 2023 Western Tatras – Zuberec, Slovak Republic February 1 – February 3, 2023 70 INITIAL OPTIMIZATION OF L-PBF PRINTING PARAMETERS OF SUPER DUPLEX STAINLESS STEEL POWDER Mengistu Dagnaw Jemberu1*, Zbigniew Brytan1, Jana Bidulska2, Mariusz Król1, Ewa Jonda1, Filip Pastorek3, Daniel Kajanek3 1 Silesian University of Technology, Faculty of Mechanical Engineering, Department of Engineering Materials and Biomaterials, ul. Konarsiego 18a, 44-100, Gliwice, Poland 2 Faculty of Materials, Metallurgy and Recycling, Institute of Materials and Quality Engineering, Department of Plastic Deformation and Simulation Processes, Technical University of Kosice, Vysokoskolska 4, 04200 Kosice, Slovakia 3 University of Žilina Research Centre, Univerzitná 8215/1 Str, 010 08 Žilina, Slovakia *Corresponding author: [email protected] 1. Introduction The additive manufacturing (AM) industry is growing, with more markets focusing on the implementation of this technology. Today, academic and industrial research on additive manufacturing of metal materials has attracted attention. With the possibility of producing elements directly in their desired geometry and with the corresponding mechanical properties in a fast, efficient, and safe manner, it opens the possibility of implementing a digital warehouse for spare parts in the form of computer-generated models. In addition, prototypes or custom parts can be made faster and more efficiently than with conventional methods. This technology reliably reproduces parts with expected mechanical properties and desired shapes. Research around the world focuses on 3D printing methods and printing machines to produce materials with the most faithful properties, making the process more economically and practically beneficial [1,2]. Duplex stainless steels (DSS) are an essential and versatile class of steel currently in use around the world. They have a two-phase microstructure consisting of roughly equal proportions of ferrite and austenite. DSSs are notable for their high corrosion properties compared to more traditional austenitic stainless steels and have about twice the strength. Moreover, the resistance to pitting corrosion and stress corrosion cracking (SCC) of DSSs tends to be superior to single-phase austenitic stainless steels. This is the result of the chemical composition, particularly the amount of chromium, molybdenum, and nitrogen. Super-DSS alloys with an increased alloy content with respect to the DSS group are intended for the most severe corrosive environments. Super-DSSs are widely used in the marine, petrochemical, oil, and gas industries because of their superior strength and corrosion resistance. Additionally, they can be cost-effective due to high strength as less material may be used for steel structures. Although DSSs have been used since the 1930s, researchers are always looking for innovative methods, such as alternative production procedures for certain components. However, the accurate thermomechanical processing of DSS-wrought products has led to higher costs. Therefore, it is essential to examine the proper production processes for the fabrication of Super-DSS structural parts. Super-DSS is more difficult to process than other types of stainless steel, and the 3D printing process is still not adequately developed for this group of materials. The demand for 3D-printed Super-DSS is increasing, and therefore industries want to grow further and mature additive manufacturing processes for these materials. Taking into account ecological aspects, 26th International Seminar of Ph.D. Students SEMDOK 2023 Western Tatras – Zuberec, Slovak Republic February 1 – February 3, 2023 71 stainless steel is referred to as an environmentally friendly material. This is due to the long service life, the possibility of complete recycling and the low carbon footprint during production, especially compared to other metallic and plastic materials [3,4]. In recent years, a powder of Super-DSS grade 2507 (EN 1.4410) was developed dedicated to additive manufacturing, which confirms the industry's interest of the industry in the production of printed components from high alloy stainless steels, like DSS [5]. However, the material properties of the printed Super-DSS have not been fully investigated so far, and recommendations for its heat treatment immediately after printing have not been specified. For this reason, it is advisable to undertake research work in the field of structure and properties analysis of these DSS grades manufactured by AM. At the moment, there are no recommended printing parameters dedicated to various 3D printers. Manufacturers provide data only for selected printers for which they have performed the printing process themselves. There are already several works in this field, but optimal printing parameters are still being sought. [6-10]. For this reason, research has begun on the initial optimization of printing parameters for the AM125 Renishaw 3D printer. 1. Materials and methods Super duplex stainless steel, grade 2507 (EN 1.4410), manufactured by Sandvik Osprey Ltd, with the chemical composition presented in Table 1, was used to print basic samples (cubes 10x10x10mm) in the Laser beam - Powder Bed Fusion (L-PBF) process. The 2507 powder is a gas-atomized powder with particle diameters range 15 - 45 µm. Tab. 1 Chemical composition of 2507, EN 1.4410 powder Elements Fe Cr Ni Mo Mn Si N Cu C P S [wt. %] Bal. 25.0 7.0 4.0 <1.2 <0.8 0.30 <0.50 <0.030 <0.035 <0.015 The L-PBF printing process was performed on an AM125 RENISHAW printer with variable printing parameters. This printer is characterized by an ytterbium (Yb) fiber laser with a max. laser power of 200 W, a scan speed of 2 000 mm/s, and a wavelength of 1.074 nm. The components were manufactured on a mild steel platform under an atmosphere of Ar inert gas at an oxygen level below 10 ppm. Preheating of the substrate is not required for this type of material but was applied for sets of printing parameters described as P1 to P6, where the base plate was heated to 150°C. A meander scanning strategy was used following a rotation of 67° after every layer was laid. For each condition, the energy density was calculated according to the formula: 𝐸𝑑=𝑃 𝑣∙ℎ∙𝑡 (1) where P is the laser power, V is the scan speed, h is the hatch distance, and t is the layer thickness. Process parameters were chosen in order to express entire range of the energy 26th International Seminar of Ph.D. Students SEMDOK 2023 Western Tatras – Zuberec, Slovak Republic February 1 – February 3, 2023 72 density as it is a crucial parameter in the selective laser melting technique. Table 2 lists sample designations and related energy density of the printing process. To evaluate the porosity of the printed elements, computer image analysis was applied, using Image Pro Plus software. The metallographic samples were prepared in a horizontal plane (parallel to the base plate on which the elements were printed) in the nonetched state on optical microscope. The analysis was performed on at least 10 images at 100x magnification. The porosity and its standard deviation were determined. Tab. 2. The L-PBF process conditions. Sample Designation P1 P=200W P2 P=190W P3 P=180W P4 P=170W P5 P=160W P6 P=150W v=300mm/s, t=30µm, h=100µm Energy density, Ed, J/mm3 222.22 211.11 200.00 188.89 177.78 166.67 Designation A1 v=250mm/s A2 v=300mm/s A3 v=375mm/s A4 v=500mm/s – – P=180W, t=30µm, h=120µm Ed, J/mm3 200.00 166.67 133.33 100.00 – – Designation B1 v=391mm/s B2 v=470mm/s B3 v=587mm/s B4 v=783mm/s – – Ed, J/mm3 127.66 106.38 85.11 63.83 – – Designation C1 v=525mm/s C2 v=630mm/s C3 v=787mm/s C4 v=1050mm/s – – Ed, J/mm3 95.24 79.37 63.49 47.62 – – Designation D1 v=666mm/s D2 v=800mm/s D3 v=1000mm/s D4 v=1333mm/s – – Ed, J/mm3 75.00 62.50 50.00 37.50 – – 2. Results and Discussion Figure 1 illustrates selected light optical micrographs of the cross sections of samples obtained at P1 to P6 printing parameters, while Figure 2 for A, B, C and D set of parameters. In the image analysis, at least 10 micrographs were taken for analysis and a medium value of the pores share was calculated. For this reason, the photos do not show representative porosity for a given sample, but only a selected analysis site. On the cross sections, a typical defect of AM materials can be seen, such as keyholes, caves, and gas pores. 26th International Seminar of Ph.D. Students SEMDOK 2023 Western Tatras – Zuberec, Slovak Republic February 1 – February 3, 2023 73 a) b) c) d) e) f) Fig. 1 Light optical micrographs of the cross sections of samples obtained at a) P1 to f) P6 printing parameters respectively Porosity analysis of the tested samples allows to determine relationship between laser energy density and resulting porosity (Fig. 3). From this relationship, it is clear that an increase in the laser energy density (Ed) reduces the porosity of printed DSS powder. Increase in the laser energy density above 160 J/cm3 results in a porosity level below 1%, while porosity <0.5% can be obtained at laser energy density above 188 J/cm3. In case of printing strategy where preheating was applied a slightly better results were obtained, when comparing the same level of laser energy density, i.e., Ed=200 J/cm3 (P3 and A1), but in case of lower energy Ed=166.67 J/cm3 (i.e., P6 and A2) it was not confirmed. A1 A2 A3 A4 B1 B2 B3 B4 C1 C2 C3 C4 D1 D2 D3 D4 Fig. 2 Light optical micrographs of the cross sections of samples obtained at the A, B, C and D set of printing parameters 26th International Seminar of Ph.D. Students SEMDOK 2023 Western Tatras – Zuberec, Slovak Republic February 1 – February 3, 2023 74 Fig. 3 The relation between laser energy density and porosity for L-PBF printed DSS powder 2507 (EN 1.4410) When the porosity morphology mainly melting-related defects, characterized by irregular shape, were observed [11]. In case of printing parameters, where low laser energy density level was adopted, the porosity is related to the lack of fusion defects, recognized with larger pores. Meanwhile, at high laser energy density level the porosity is mostly related to incomplete melting-induced porosity, which disappears or becomes less intense as the laser power increases. Some small spherical pores could be attributed to the entrapment of argon gases in the feedstock particles, but their contribution to the total porosity of the finished parts was negligible. 3. Conclusions The effect of the L-PBF printing parameters on the porosity of a novel super duplex stainless steel 2507 (EN 1.4410) was investigated. From the results presented, the following conclusions can be drawn: 1. The increase in laser energy density (Ed) reduces the porosity of the printed DSS powder. Increase in the laser energy density above 160 J/cm3 results in a porosity level below 1%, while porosity <0.5% can be obtained at laser energy density above 188 J/cm3. 2. The preheating of the base plate during the printing process slightly influenced the porosity reduction, but for comparable results (with the same Ed level), its influence is not clear and should be studied in detail. Apart from that, the lowest porosity level was obtained for the highest values of Ed when preheating of the base plate was applied. 3. DSS shows structural defects typical for AM technology, where incomplete meltinginduced porosity dominates and disappears or becomes less intense as the laser power increases. Moreover, very small spherical pores, attributed to argon gas entrapment in 26th International Seminar of Ph.D. Students SEMDOK 2023 Western Tatras – Zuberec, Slovak Republic February 1 – February 3, 2023 75 the feedstock particles, were observed, but their contribution to the total porosity of the parts was negligible. Acknowledgement The research presented in this work was carried out within the framework of the International Visegrad Fund scholarship. Moreover, this study received also great support from the EU -project H2020 -MSCA -RISE -2018 Number 823786 , i-Weld and international project co-financed by the program of the Ministry of Science and Higher Education entitled "PMW" in the years 2020 -2023; contract No. 5107/H2020/2020/2. References [1] Bai Y, Zhao C, Yang J, Hong R, Weng C, Wang H., Microstructure and machinability of selective laser melted high-strength maraging steel with heat treatment. J Mater Process Technol 2021;288:116906. 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