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Selective Laser Melting and Sintering Technique of the Cobalt-Chromium

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Selective Laser Melting and Sintering Technique of the Cobalt-Chromium View metadata, citation and similar papers at core.ac.uk brought to you by CORE DOI: https://doi.org/10.2298/SARH190706112S provided by School of dental Medicine dIgitaL archivE 664 UDC: 615.46:616.314-7 ORIGINAL ARTICLE / ОРИГИНАЛНИ РАД Selective laser melting and sintering technique of the cobalt-chromium dental alloy Dejan Stamenković1, Kosovka Obradović-Đuričić1, Rebeka Rudolf2, Rajko Bobovnik3, Dragoslav Stamenković4 1University of Belgrade, School of Dental Medicine, Belgrade, Serbia; 2University of Maribor, Faculty of Mechanical Engineering, Maribor, Slovenia; 3Faculty of Polymers Technology, Slovenj Gradec, Slovenia; 4Serbian Medical Society, Academy of Medical Sciences, Belgrade, Serbia SUMMARY Introduction/Objective The objective of this paper is to describe the microstructure and mechanical properties of sintered Co-Cr alloy and to emphasize its advantages and disadvantages with respect to the microstructure and mechanical properties of cast Co-Cr alloy. Methods Base Co-Cr alloy, EOSint M EOS Co-Cr SP2 (EOS GmbH, Munch, Germany), was used for the purpose of this research as the base material for sintering metal structures of metal-ceramic restorations. Metal sintering was conducted by using EOSint M 280 device of German origin in a stream of neutral gas – argon. After that, the alloy was heated over a period of 20 minutes at the temperature of 800°C. The chemical composition of the alloy was determined by energy dispersive spectroscopy. Microstructure of the tested alloy samples was examined under an optical metallographic and scanning electron microscope. Physical and mechanical properties were measured in a universal testing machine. The samples were prepared according to the standard ISO 527-1:1993. Results Chemical composition of the sintered Co-Cr alloy, determined by applying energy dispersive spectroscopy, indicated the same qualitative but different quantitative composition compared to cast Co-Cr alloys. The microstructure of the sintered Co-Cr alloy is lamellar in nature, with two dominant phases: ξ-Co and/or ξ-Cr (fcc – face-centered cubic) and γ-Co (hcp – hexagonal close-packed). Mechanical properties of the Co-Cr alloy obtained by applying selective laser melting technology compared to the cast Co-Cr alloy are superior or approximately the same. Conclusion Selective laser melting of the Co-Cr alloy is a good example of new technologies based on digitization. Together with other digitized procedures, this technology is an introduction to a new era in dentistry popularly called Dentistry 4.0. The advantages of the selective laser melting technology with respect to the conventional technology of casting Co-Cr alloy metal structures are precise metal structure fitting and eco-friendly technology. Keywords: selective laser melting; sintering technique; Co-Cr alloy INTRODUCTION (3D imaging, intraoral scans, computer-aided design and computer-aided manufacturing – Dental alloys represent a very dynamic field of CAD-CAM, cone-beam computer tomography dentistry. Changes that occur in this area, in – CBCT, computer-aided implantology). The fact, reflect the developments of basic scientific transition from the third to the fourth industrial technologies. Mechanical and biological prop- revolution, i.e. to Dentistry 4.0, was barely no- erties of the same alloy are largely dependent ticeable. Dentistry 4.0 is not a completely new on the technological processes of forming the technology. In this case new system solutions alloy into dental restorations. The process of were created on the platform (infrastructure) alloy melting and casting for dental purposes originating from the previous digital revolution. Received • Примљено: has been known for centuries and melting This revolution introduced greater automation July 6, 2019 and casting conditions have been constantly in dental laboratory procedures, i.e. diagnostic Revised • Ревизија: October 7, 2019 improved – from primitive alloy melting by and therapeutic procedures in dental offices. Accepted • Прихваћено: applying naked flame and open-air casting, to The selective laser melting (SLM) and compact- October 8, 2019 melting by applying induced current in vacuum ing (sintering) of metal powder particles is a Online first: October 28, 2019 or neutral gases. Nevertheless, even the perfect step forward in the modern dental practice. This casts have certain flaws. technology plunged us into the fourth industrial A completely new approach to forming revolution, i.e. Dentistry 4.0 [1, 2, 3]. dental restorations appeared with the third, The process of making dental restorations by Correspondence to: and soon after, with the fourth industrial revo- sintering dental alloys basically includes three Dejan STAMENKOVIĆ lution. The third industrial revolution, also steps: digital impression, designing virtual res- 23 Deligradska Str. known in the field of dentistry as Digital Den- toration, and 3D printing [4–7]. Belgrade 11000 Serbia tistry or Dentistry 3.0, introduced numerous Digital impression, suitable for further com- [email protected] new procedures based on digital technologies puter processing, can be obtained by direct 3D Selective laser melting and sintering technique of the cobalt-chromium dental alloy 665 Figure 1. Outline of schematic functioning principles of selec- tive laser melting [7] digitization in the patient’s mouth (intraoral scanners) and ducted by using EOSint M 280 device of German origin indirect 3D digitization of the gypsum model (extraoral in a stream of neutral gas – argon. After that, the alloy scanners) [7, 8]. Nowadays, these scanners, in addition to was thermally treated over a period of 20 minutes at the precision, ensure great comfort for both the doctor and temperature of 800°C. the patient. The chemical composition of the alloy was determined Designing virtual restoration, i.e. computerized mod- by energy dispersive spectroscopy (EDS analysis). Micro- eling framework of crown, bridge or removable partial structure of the tested samples was examined under an denture, represents the second step in advanced technol- optical metallographic microscope (MM) and scanning ogy of making dental restorations. Virtual restorations are electron microscope (SEM) in the Materials Testing Labo- designed by using commercial computer packages, which ratory at the Faculty of Mechanical Engineering in Mari- are computer tools that facilitate and speed up the design bor, Slovenia. Physical and mechanical properties were process [9]. Virtual restoration files (STL files) are sent measured in a universal testing machine in the Materials directly to the software of the machines designed to make Testing Laboratory at the Faculty of Polymer Technology, metal frameworks of dental restorations. At this stage, one Slovenj Gradec, Slovenia. Six samples were prepared ac- intermediate step is also possible. If the design control is cording to the ISO standard 527-1:1993. required in real space, obtained STL files are sent to 3D printers, which print out a model of the future fixed or mobile dental prostheses in polymer or, less often, wax. RESULTS The detected defects can still be remedied. The third step is laser melting of the Co-Cr alloy pow- Chemical composition of the sintered Co-Cr alloy, deter- der and producing the metal framework by sintering. In mined by applying EDS, indicated the same qualitative the process of selective laser sintering, the object is printed composition as for cast Co-Cr alloys. However, there were by successive addition of thin, horizontal layers. Each layer certain differences in the quantitative composition of the is printed by applying a thin layer of alloy powder over alloy, with the values for W, Si, and O being higher (Figure previously made object, which is then melted with a laser 2, Table 1). beam in the form of the following layer [10, 11, 12]. Upon cooling, the melted metal powder is bonded hori- zontally (thus forming a new layer) and vertically (bond- ing with previously made layer). The form of each layer is determined by a computer, based on a virtual restoration model (obtained STL files). The process of powder melting is governed by cross-sections determined in such manner. The process of sintering only ensures the bond of the laser melted powder (Figure 1) [7]. The objective of this paper is to describe the microstruc- ture and mechanical properties of the sintered Co-Cr alloy and to emphasize its advantages and disadvantages with respect to the microstructure and mechanical properties of the cast Co-Cr alloy. METHODS Base Co-Cr alloy, EOSint M EOS Co-Cr SP2 (EOS GmbH, Munch, Germany), was used for the purpose of this re- search as the base material for sintering metal structures Figure 2. Results of the energy dispersive spectroscopy of EOS Co-Cr of metal-ceramic restorations. Metal sintering was con- SP2 alloy after sintering and thermal treating Srp Arh Celok Lek. 2019 Nov-Dec;147(11-12):664-669 www.srpskiarhiv.rs 666 Stamenković D. et al. Table 1. Numerical values of elements of energy dispersive spectros- of EOS Co-Cr SP2 alloy determined based on EDS analysis, copy of EOS Co-Cr SP2 alloy after sintering and thermal treating but also based on X-ray diffractometry analysis (XRD) per- Intensity Error Atomic Elt. Line Conc. (c/s) 2-sig % formed by other authors [13, 14]. Chemical compositions 2.578 wt% of alloys differ slightly depending on the manufacturer and 0.947 8.878 O Ka 22.40 2.342 wt% the surface that the analysis was performed on. 1.324 4.954 Si Ka 43.85 26.932 wt% 4.024 27.583 The microstructure of the sintered Co-Cr alloy is lamel- Cr Ka 404.78 58.110 4.525 54.323 Co Ka 511.87 wt% lar in nature, with two dominant phases: ε-Co and/or ε-Cr 1.447 2.813 Mo La 52.34 4.899 wt% (fcc – face-centered cubic) and γ-Co (hcp – hexagonal 1.323 1.810 W Ma 43.78 6.038 wt% close-packed). This structure was determined based on 100.000 100.000 wt% XRD analysis [15, 16, 17].
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