| Abstract | Dentalne Co-Cr legure se uglavnom proizvode tradicionalnim postupcima lijevanja i kovanja
dok se u posljednji desetak godine sve učestalije koristi tehnologija selektivnoga laserskoga
taljenja. Selektivnim laserskim taljenjem se mogu proizvoditi konstrukcije fiksnoprotetskih
nadomjestaka individualno prilagođeni svakom pacijentu koji posjeduju poboljšana svojstva i
mogućnost proizvodnje tankih stijenki što doprinosi smanjenju mase i opterećenja na čeljusti.
Promjenom parametara selektivnoga laserskoga taljenja značajno se utječe na strukturu
proizvedenog materijala, a time i na njegova svojstva što se je proučilo u ovom doktorskom
radu.
U ovom doktorskom radu je ispitan utjecaj glavnih parametara SLM-a: snage lasera, brzine
skeniranja i temperature predgrijavanja radne podloge na strukturu i svojstva dentalne legure
EOS CoCr SP2. Centralno-kompozitnim planom pokusa definirano je 17 kombinacija
parametra pomoću kojih su se SLM postupkom proizveli uzorci Co-Cr legure za analizu
mikrostrukture i ispitivanje mehaničkih svojstava. Svi uzorci su analizirani pomoću svjetlosne
i elektronske mikroskopije čime su određeni kvalitativni i kvantitativni mikrostrukturni
parametri te postojanje poroziteta, uključina, pukotina i drugih grešaka u mikrostrukturi. EDS
analizom je provedena analiza kemijskog sastava materijala dok je XRD i EBSD metodama
određena prisutnost pojedinih mikrostrukturnih faza i provedena analiza kristalografske
orijentacije pojedinih zrna. Provedeno je statičko vlačno ispitivanje za određivanje
konvencionalne granice razvlačenja i istezanja, ispitivanje udarnog rada loma za određivanje
žilavosti i ispitivanje savijanja u tri točke za određivanje savojne čvrstoće. Rezultati dobiveni
mehaničkim ispitivanjima su statistički obrađeni te su modelirane odzivne površine i
matematički modeli pomoću kojih se utvrdila značajnost pojedinih parametara na ispitivana
svojstva. Određeni su optimalni parametri za dobivanje maksimalnih vrijednosti pojedinog
svojstva i provedena je validacija optimalnih parametara za dobivanje maksimalnih vrijednosti
žilavosti. Na temelju provedenih analiza i mehaničkih ispitivanja dani su zaključci i prijedlog
budućih smjerova istraživanja dentalnih Co-Cr legura. |
| Abstract (english) | Cobalt-chromium (Co-Cr) alloys are most often used in the production of metal parts of
prosthetic replacements for fixed dental prostheses such as crowns and bridges and various
orthopaedic implants because they possess an excellent combination of mechanical properties,
wear and corrosion resistance combined with good biocompatibility. Traditional methods of
casting and milling supported by computer systems (CAD-CAM) with subsequent processing
(heat treatment, grinding, etc.) are used in production of fixed prosthetic replacements.
Additive technologies, especially selective laser melting (SLM), have been trying to overcome
the shortcomings of traditional production processes in the last few years. Therefore, today
there are large investments in the development of technologies and devices for additive
manufacturing intended specifically to produce dental parts, and devices for additive
manufacturing using SLM. The SLM process is complex because it contains many parameters
that affect the structure and properties of the consolidated material.
The main purpose of this scientific research was to determine the influence of SLM parameters
on the structure and properties of the EOS CoCr SP2 dental alloy. The research aims to
contribute to the understanding of the influence of the parameters of the SLM procedure on the
structure and properties of the dental Co-Cr alloy and the following hypotheses are assumed:
Through laboratory tests and material characterization is possible to determine the changes and
connection of the structure and properties of the dental Co-Cr alloy depending on the used
parameters of the SLM procedure by applying the central composite design. It is possible to
quantify the significance of the parameters of the SLM procedure on the structure and
properties of the dental Co-Cr alloy, the magnitude of their action and interaction.
The research conducted in this doctoral thesis is described in eight chapters.
The influence of certain input parameters of SLM on the structure and properties of
consolidated dental Co-Cr alloys and their interaction is an important area for research, which
includes methods of modern scientific research work.
The preparation of the research includes the definition of all scientific research elements
necessary for the successful implementation of the experiment. Primarily, it refers to the type
and composition of the powder material from which the samples were made, the definition and
characterization of the structure that was carried out and provided information on the
relationship between structure and properties. Special emphasis is placed on the definition and selection of standards for the testing of manufactured samples by SLM, and the types of
laboratory tests, shapes, and dimensions of the samples were defined.
The number of samples for testing was determined by applying a central composite design,
which developed a model that describes the process if the studied factors affect the process,
and if the results of the test were of acceptable accuracy and precision. A centrally composite
design was applied to obtain as much information as possible about the installed system with
minimal costs.
The experimental part of the work includes the production of samples on the SLM device of
the German manufacturer Aconity 3D GmbH, AconityMINI with complete freedom of access
to all production parameters. The most significant input parameters of SLM that are varied are
laser beam power, scanning speed, and base plate preheating temperature.
Two groups of samples were made: one for testing the mechanical properties, and the other for
structure characterization. Characterization of the structure was done in the laboratories of the
Faculty of Mechanical Engineering and Naval Architecture in Zagreb and the Institute of Metal
Materials and Technology in Ljubljana. The characterization also included metallographic
preparation of samples and macro and microstructure analysis. These analyses include light
and electron microscopy, which determined qualitative and quantitative microstructural
parameters and the possible existence of porosity, inclusions, cracks, or other defects in the
structure. Using other modern methods of characterizing the structure of the material,
specifically Energy Dispersive Spectroscopy (EDS) or Wavelength-Dispersive Spectroscopy
(WDS) analysis, Electron Backscatter Diffraction (EBSD) method, a microanalysis of the
chemical composition of the material and an analysis of the crystallographic orientation was
carried out. Testing of other properties of the consolidated Co-Cr alloy is focused on properties
important for dental applications, which include static tensile tests to determine conventional
yield strength Rp0,2 and elongation ε, impact fracture tests to determine toughness KVa, and
three-point bending tests to determine flexural strength Rms.
Analyses of the obtained results determined the significance of the variable factors on the tested
properties for which a central composite design was applied. The significance of the
contribution of individual variables was determined as well as their mutual influence. A special
emphasis in the analysis of the results is on determining the correlation of the production
parameters with the obtained properties of the tested samples. The conclusions are qualitative
and quantitative forms, for which available software packages were used. The scientific research in this doctoral thesis has resulted in scientific contributions that can be
confirmed based on the obtained results:
• The influence of certain SLM parameters on changes in the structure and properties of the
used Co-Cr alloy was determined.
• SLM parameters were selected for the production of a dental Co-Cr alloy with improved
properties compared to conventionally produced alloys.
Directions for future research are also suggested. |
