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Laser sintered materials with Non-equilibrium structures
Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK). (Zhijian Shen)
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis is focused on achieving materials with non-equilibrium structures fabricated by high-energy laser sintering. The chosen precursor materials have rigid and inert structures like high-melting point ceramics or metals. It was necessary to use real-time monitoring of temperature and spectrum profiles for selecting the optimal laser parameters for the laser sintering process. This monitoring was done by an off-axial setup that also controls the surface morphologies during the laser irradiation process. The laser focal spot receives very high temperatures and subsequent extreme cooling rates within a short time period. New non-equilibrium structures will emerge ruled by kinetics, huge temperature gradients or stresses and freeze by quenching in solid state. These material structures were found to form at different length scales from nano- to macro-level, frequently by a hierarchical ordering. This opens a method to engineer materials with both hierarchical and non-equilibrium structures by a single operation in both metal and ceramics by laser sintering. In the Co-Cr-Mo alloy system, structures on three levels of lengths were observed, namely i) nano-level structures dominated by the grain boundary segregation; ii) micron-level structures characterized by the interlocked clusters of columns; and iii) macro-level structures defined by the selected laser scan patterns. The non-equilibrium structures of the Co-Cr-Mo alloy are related to mechanical, corrosion and bio-compatibility properties. In ZrO2 ceramics, the final product had a non-equilibrium nano- and micron-sized structure created by uneven absorption of laser energy and rupture. The structure inside the micron-sized grains is formed through ordered coalescence of nano-crystals. Properties of the laser sintered materials were established and related to the observed structures. The materials properties might be tailored by controlling the structures in different levels and potential applications of the new materials will be given.

Place, publisher, year, edition, pages
Stockholm: Department of Materials and Environmental Chemistry, Stockholm University , 2014. , 61 p.
Keyword [en]
Laser sintering, Materials, Non-equilibrium structures, microstructure, hierarchical structure
National Category
Materials Engineering
Research subject
Inorganic Chemistry
Identifiers
URN: urn:nbn:se:su:diva-101096ISBN: 978-91-7447-866-2 (print)OAI: oai:DiVA.org:su-101096DiVA: diva2:698924
Public defence
2014-03-31, Magnélisalen, Kemiska övningslaboratoriet, Svante Arrhenius väg 16 B, Stockholm, 13:00 (English)
Opponent
Supervisors
Note

At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 2: Accepted. Paper 3: Accepted. Paper 4: Submitted. Paper 5: Manuscript.

Available from: 2014-03-09 Created: 2014-02-25 Last updated: 2017-05-11Bibliographically approved
List of papers
1. Laser sintering of ceramics
Open this publication in new window or tab >>Laser sintering of ceramics
2013 (English)In: Journal of Asian Ceramic Societies, ISSN 2187-0764, Vol. 1, no 4, 315-321 p.Article, review/survey (Refereed) Published
Abstract [en]

The progress of laser sintering of ceramics is reviewed by focusing on the principles and the potentials ofthis new technique as one member of a family of additive manufacturing technologies. After a brief intro-duction of the technology and the interaction between laser beam and ceramics, the characters of lasersintered ceramics are discussed in detail. The unique opportunities of achieving hierarchically structuredheterogeneities and non-equilibrium phase assemblages in bulk ceramics are illustrated. It reveals thatthe laser sintering is not only a feasible technology allowing net shape production of customized 3D partsin a single manufacturing operation but also a promising way for developing ceramics with structuresthat can hardly be made by conventional processing methods.

Keyword
Laser sintering, Ceramics, Microstructure, Heterogeneity
National Category
Materials Engineering
Research subject
Inorganic Chemistry
Identifiers
urn:nbn:se:su:diva-101052 (URN)10.1016/j.jascer.2013.08.004 (DOI)
Available from: 2014-02-24 Created: 2014-02-24 Last updated: 2014-02-26Bibliographically approved
2. Monitoring of temperature profiles and surface morphologies during lasersintering of alumina ceramics
Open this publication in new window or tab >>Monitoring of temperature profiles and surface morphologies during lasersintering of alumina ceramics
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(English)In: Journal of Asian Ceramic Societies, ISSN 2187-0764Article in journal (Refereed) In press
Abstract [en]

Additive manufacturing of alumina by laser is a delicate process and small changes of processing parame-ters might cause less controlled and understood consequences. The real-time monitoring of temperatureprofiles, spectrum profiles and surface morphologies were evaluated in off-axial set-up for controllingthe laser sintering of alumina ceramics. The real-time spectrometer and pyrometer were used for rapidmonitoring of the thermal stability during the laser sintering process. An active illumination imagingsystem successfully recorded the high temperature melt pool and surrounding area simultaneously. Thecaptured images also showed how the defects form and progress during the laser sintering process. Allof these real-time monitoring methods have shown a great potential for on-line quality control duringlaser sintering of ceramics.

Keyword
Monitoring, Laser sintering, Ceramics, Alumina
National Category
Materials Engineering
Research subject
Inorganic Chemistry
Identifiers
urn:nbn:se:su:diva-101053 (URN)
Available from: 2014-02-24 Created: 2014-02-24 Last updated: 2014-02-26Bibliographically approved
3. In-vitro biocompatibility of CoCrMo dental alloys fabricated by selective laser melting
Open this publication in new window or tab >>In-vitro biocompatibility of CoCrMo dental alloys fabricated by selective laser melting
Show others...
2014 (English)In: Dental Materials, ISSN 0109-5641, E-ISSN 1879-0097, Vol. 30, no 5, 525-534 p.Article in journal (Refereed) Published
Abstract [en]

Objectives: Selective laser melting (SLM) is increasingly used for the fabrication of customized dental components made of metal alloys such as CoCrMo. The main aim of the present study is to elucidate the influence of the non-equilibrium microstructure obtained by SLM on corrosion susceptibility and extent of metal release (measure of biocompatibility).Methods: A multi-analytical approach has been employed by combining microscopic and bulk compositional tools with electrochemical techniques and chemical analyses of metals in biologically relevant fluids for three differently SLM fabricated CoCrMo alloys and one cast CoCrMo alloy used for comparison.Results: Rapid cooling and strong temperature gradients during laser melting resulted in the formation of a fine cellular structure with cell boundaries enriched in Mo (Co depleted), and suppression of carbide precipitation and formation of a martensitic ε (hcp) phase at the surface. These features were shown to improve the corrosion and metal release susceptibility of the SLM alloys compared with the cast alloy. Unique textures formed in the pattern of the melting pools of the three different laser melted CoCrMo alloys predominantly explain observed small, though significant, differences. The susceptibility for corrosion and metal release increased with an increased number (area) of laser melt pool boundaries.Significance: This study shows that integrative and interdisciplinary studies of microstructural characteristics, corrosion, and metal release are essential to combine and consider during the design and fabrication of CoCrMo dental components of optimal biocompatibility. The reason is that metal release from CoCrMo alloys is dependent on the fabrication procedure.

Keyword
Selective laser melting, additive manufacturing, CoCrMo, cobalt, chromium, protein, metal release, corrosion, microstructure
National Category
Materials Engineering
Research subject
Inorganic Chemistry
Identifiers
urn:nbn:se:su:diva-101055 (URN)10.1016/j.dental.2014.02.008 (DOI)000334585200010 ()
Available from: 2014-02-24 Created: 2014-02-24 Last updated: 2017-12-05Bibliographically approved
4. Assembled nano-structures from micron-sized precursors
Open this publication in new window or tab >>Assembled nano-structures from micron-sized precursors
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2014 (English)In: RSC Advances, ISSN 2046-2069, E-ISSN 2046-2069, Vol. 4, no 58, 30754-30757 p.Article in journal (Refereed) Published
Abstract [en]

Oriented assembly of small crystals to form larger structures are common in nature and full understanding and control of such process would be beneficial for the forthcoming nano-technologies. Such bottom-up assembly of nano-structures most often requires a precursor of nano-sized crystals. Here we demonstrate a novel laser sintering process that can rupture a submicron-sized zirconia precursor into nano-sized crystals, followed by a rapid oriented assembly of these small crystals. A detailed structure study has been carried out to understand the complex process and a model has been proposed. Such a process sheds light on a new way of fabricating nano-materials using commercialized oxide materials.

National Category
Chemical Sciences
Research subject
Inorganic Chemistry
Identifiers
urn:nbn:se:su:diva-107448 (URN)10.1039/c4ra03191a (DOI)000340500300042 ()
Note

AuthorCount:5;

Available from: 2014-09-15 Created: 2014-09-15 Last updated: 2017-12-05Bibliographically approved
5. Defect-tolerant characteristics of laser sintered CoCrMo alloy ensured by strong structural hierarchies
Open this publication in new window or tab >>Defect-tolerant characteristics of laser sintered CoCrMo alloy ensured by strong structural hierarchies
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

Laser sintering of a CoCrMo alloy have been found to introduce intergrown structural features on several length scales from nano- to macro-levels, i.e. structural hierarchies. The quenching of the small laser formed melting pool, when the laser focus spot moves away, will preserve high temperature phases or microstructures. A micron-sized cellular microstructure will form with Mo being enriched in a nanometer thin zone at the cell boundaries and carbide formation is suppressed. Clusters of elongated cells (crystallites) join along one common crystal structure direction forming larger up to 100 µm bundles and these grow in different intercrossing directions in the microstructure. The interlocked framework prohibits mechanical creep or deformation by sliding along weak structure planes and consumes the energy of a propagating crack. A macro-scale “weld line” structure can be formed in 3-dimensions by the bottom-up approach; the sintering of layer by layer with a tailored scan track. The sintered alloy contained three types of defects comprising micron sized voids or cracks occurring at the cluster boundaries and larger cracks at the “weld line” boundaries. Unexpected good mechanical properties are achieved despite the observed microstructural defects and the laser sintered Co-Cr-Mo alloy seems very defect tolerant. This positive effect is ascribed to the strong structural hierarchies found in the laser sintered CoCrMo alloy. 

Keyword
laser sintereing, Alloy, mechanical property, microstrcture
National Category
Materials Engineering
Identifiers
urn:nbn:se:su:diva-101095 (URN)
Available from: 2014-02-25 Created: 2014-02-25 Last updated: 2014-02-26Bibliographically approved

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