Die Implementierung von Laser Powder Bed Fusion bei Aluminiumoxidkeramiken ist aufgrund einer geringen Temperaturwechselbeständigkeit, Bauteilverdichtung, Pulverfließfähigkeit und Lichtabsorption eine große Herausforderung. In dieser Arbeit wurden diese Prob-leme mit unterschiedlichen Ansätzen adressiert. Sprühgetrocknete Aluminiumoxid Granulate wurde zur Verbesserung der Laserabsorption (über 80 % Verbesserung) mit farbigen Nano-Oxidpartikeln dotiert. Es wurden verschiedene Partikelpackungstheorien und Pulverbehand-lungen getestet, um die Pulverbettdichte und damit die Dichte des endgültigen Bauteils (Dichten bis zu 98,6 %) zu erhöhen. Die Pulverqualität wurde durch Schütt und Rütteldichte, Feuchtigkeitsgehalt, Partikelgrößenverteilung, Hausner-Verhältnis, Lawinenwinkel und Oberflächenfraktal charakterisiert. Des Weiteren wurde der Zusatz geeigneter Stoffe zur Verringerung der Rissbildung durch thermische Spannungen getestet. Die In-situ-Bildung von Phasen mit geringer und negativer Wärmeausdehnung reduzierte die Rissbildung in den lasergefertigten Oxidkeramiken stark.:1 Introduction 1
1.1 Motivation 1
1.2 State of the art . 2
1.3 Aim of the project 2
2 Literature review 5
2.1 Additive manufacturing by laser powder bed fusion 5
2.1.1 Classification and process description 5
2.1.2 Advantages against other AM processes 9
2.1.3 Challenges of laser powder bed fusion 12
2.1.4 State of the art of laser powder bed fusion of aluminum oxide based ceramics 13
2.1.4.1 Powder bed preparation and impact on the process 13
2.1.4.2 Critical rating of the powder bed preparation techniques 17
2.1.4.3 Processing methods and properties 19
2.1.4.4 Part properties 26
2.2 Theoretical and experimental considerations for powder bed preparation 35
2.2.1 Spray granulation 35
2.2.2 Particle packing theories 39
2.3 Mechanisms for particle dispersing 41
2.3.1 DLVO-theory 41
2.3.2 Surface charge and electrical double layer 43
2.4 Conceptualization of new ideas for laser powder bed fusion of aluminum oxide 45
2.4.1 Densification, powder flowability and absorption issue 46
2.4.2 Reduction of crack formation 47
3 Doped spray-dried granules to solve densification and absorption issue in laser powder bed fusion of alumina 55
3.1 Dispersing of aluminum oxide, iron oxide and manganese oxide 55
3.1.1 Experimental 55
3.1.2 Particle characterization 57
3.1.3 Saturation amount evaluation of dispersant 59
3.1.4 Particle size distributions after dispersing 62
3.1.4.1 Particle size distributions of alumina powders 62
3.1.4.2 Particle size distribution of dopant 67
3.2 Packing density increase of spray-dried granules 76
3.2.1 Experimental 77
3.2.2 Influence of solid load and particle ratio on granules 83
3.2.3 Influence of dopant shape and multimodal distributions on granules 84
3.2.4 Evolution of pH-value during slurry preparation and slurry stability after mixing of all components 85
3.2.5 Influence of slurry viscosity on yield of granules 88
3.2.6 Addition of coarse alumina to spray-dried granules 89
3.2.7 Application of Andreasen model on mixtures of ceramic particles with spray-dried granules 94
3.2.8 Thermal pre-treatment of granules 98
3.2.9 Influence of surface tension of slurry on granule size and density 110
3.3 Investigation of laser manufactured parts 114
3.3.1 Experimental 115
3.3.2 Influence of different iron oxide dopants and multimodal particle distributions within granules 118
3.3.3 Influence of coarse alumina variation 121
3.3.4 Influence of thermal pre-treatment of powders 127
3.3.5 Grain structure of laser additive manufactured parts 135
3.3.6 Thermal expansion of laser processed parts 137
3.3.7 Influence of thermal pre-treatment and laser processing on manganese amount within granules and laser additive manufactured parts 138
4 Additives to reduce crack formation in selective laser melting and sintering of alumina 143
4.1 Experimental 144
4.2 Additives to reduce thermal stresses 150
4.2.1 Selective laser melting with mullite additives 150
4.2.2 Amorphous alumina formation by rare earth oxide doping 160
4.2.3 Formation of aluminum titanate by use of reduced titanium oxide 169
4.2.3.1 Dispersing of titanium oxide nanoparticles in water 170
4.2.3.2 Thermal treatment of Al2O3/TiO2 granules under argon/hydrogen atmosphere 172
4.2.3.3 Laser manufacturing of parts 178
4.2.4 In-situ formation of negative thermal expansion materials 187
4.2.4.1 Dispersing of zirconia and tungsten oxide nanoparticles 187
4.2.4.2 Influence of spray drying process parameters 191
4.2.4.3 Preparation of final powders for laser powder bed fusion 197
4.2.4.4 Laser manufacturing of layers and parts 200
4.3 Mechanical properties of laser processed parts 205
5 Flowability and inner structure of customized granules 209
5.1 Experimental 209
5.2 Comparison of flowability in terms of Hausner ratio, Avalanche angle and surface fractal measurements 211
5.2.1 Influence of coarse alumina AA18 variation 211
5.2.2 Influence of thermal pre-treatment of powders 213
5.2.3 Influence of dopant content within granules 216
5.2.4 Flowability of zirconia-tungsten oxide granules and alumina granules with mullite or rare earth oxide addition 219
5.2.5 Flowability of titanium oxide doped alumina powders 221
5.3 Cross sections of customized granules to image inner structure 224
6 Summary, conclusions and outlook 233
6.1 Summary and conclusions 233
6.2 Outlook 241
References 245
List of Figures 260
List of Tables 269 / The implementation of laser powder bed fusion of aluminum oxide ceramics is challenging due to a low thermal shock resistance, part densification, powder flowability and light absorptance. In this work, these challenges have been addressed by different approaches. Spray-dried alumina granules were doped with colored oxide nanoparticles to improve the laser absorption (improvement by over 80%). Different particle packing theories and powder treatments were tested to increase the powder bed density and therefore, the final part density (densities up to 98.6%). The powder quality was characterized by apparent and tapped density, moisture content, particle size distribution, Hausner ratio, avalanche angle and sur-face fractal. Furthermore, the addition of suitable was tested to reduce crack formation caused by thermal stresses. The in-situ formation of low and negative thermal expansion phases strongly reduced the crack formation in the laser manufactured oxide ceramic parts.:1 Introduction 1
1.1 Motivation 1
1.2 State of the art . 2
1.3 Aim of the project 2
2 Literature review 5
2.1 Additive manufacturing by laser powder bed fusion 5
2.1.1 Classification and process description 5
2.1.2 Advantages against other AM processes 9
2.1.3 Challenges of laser powder bed fusion 12
2.1.4 State of the art of laser powder bed fusion of aluminum oxide based ceramics 13
2.1.4.1 Powder bed preparation and impact on the process 13
2.1.4.2 Critical rating of the powder bed preparation techniques 17
2.1.4.3 Processing methods and properties 19
2.1.4.4 Part properties 26
2.2 Theoretical and experimental considerations for powder bed preparation 35
2.2.1 Spray granulation 35
2.2.2 Particle packing theories 39
2.3 Mechanisms for particle dispersing 41
2.3.1 DLVO-theory 41
2.3.2 Surface charge and electrical double layer 43
2.4 Conceptualization of new ideas for laser powder bed fusion of aluminum oxide 45
2.4.1 Densification, powder flowability and absorption issue 46
2.4.2 Reduction of crack formation 47
3 Doped spray-dried granules to solve densification and absorption issue in laser powder bed fusion of alumina 55
3.1 Dispersing of aluminum oxide, iron oxide and manganese oxide 55
3.1.1 Experimental 55
3.1.2 Particle characterization 57
3.1.3 Saturation amount evaluation of dispersant 59
3.1.4 Particle size distributions after dispersing 62
3.1.4.1 Particle size distributions of alumina powders 62
3.1.4.2 Particle size distribution of dopant 67
3.2 Packing density increase of spray-dried granules 76
3.2.1 Experimental 77
3.2.2 Influence of solid load and particle ratio on granules 83
3.2.3 Influence of dopant shape and multimodal distributions on granules 84
3.2.4 Evolution of pH-value during slurry preparation and slurry stability after mixing of all components 85
3.2.5 Influence of slurry viscosity on yield of granules 88
3.2.6 Addition of coarse alumina to spray-dried granules 89
3.2.7 Application of Andreasen model on mixtures of ceramic particles with spray-dried granules 94
3.2.8 Thermal pre-treatment of granules 98
3.2.9 Influence of surface tension of slurry on granule size and density 110
3.3 Investigation of laser manufactured parts 114
3.3.1 Experimental 115
3.3.2 Influence of different iron oxide dopants and multimodal particle distributions within granules 118
3.3.3 Influence of coarse alumina variation 121
3.3.4 Influence of thermal pre-treatment of powders 127
3.3.5 Grain structure of laser additive manufactured parts 135
3.3.6 Thermal expansion of laser processed parts 137
3.3.7 Influence of thermal pre-treatment and laser processing on manganese amount within granules and laser additive manufactured parts 138
4 Additives to reduce crack formation in selective laser melting and sintering of alumina 143
4.1 Experimental 144
4.2 Additives to reduce thermal stresses 150
4.2.1 Selective laser melting with mullite additives 150
4.2.2 Amorphous alumina formation by rare earth oxide doping 160
4.2.3 Formation of aluminum titanate by use of reduced titanium oxide 169
4.2.3.1 Dispersing of titanium oxide nanoparticles in water 170
4.2.3.2 Thermal treatment of Al2O3/TiO2 granules under argon/hydrogen atmosphere 172
4.2.3.3 Laser manufacturing of parts 178
4.2.4 In-situ formation of negative thermal expansion materials 187
4.2.4.1 Dispersing of zirconia and tungsten oxide nanoparticles 187
4.2.4.2 Influence of spray drying process parameters 191
4.2.4.3 Preparation of final powders for laser powder bed fusion 197
4.2.4.4 Laser manufacturing of layers and parts 200
4.3 Mechanical properties of laser processed parts 205
5 Flowability and inner structure of customized granules 209
5.1 Experimental 209
5.2 Comparison of flowability in terms of Hausner ratio, Avalanche angle and surface fractal measurements 211
5.2.1 Influence of coarse alumina AA18 variation 211
5.2.2 Influence of thermal pre-treatment of powders 213
5.2.3 Influence of dopant content within granules 216
5.2.4 Flowability of zirconia-tungsten oxide granules and alumina granules with mullite or rare earth oxide addition 219
5.2.5 Flowability of titanium oxide doped alumina powders 221
5.3 Cross sections of customized granules to image inner structure 224
6 Summary, conclusions and outlook 233
6.1 Summary and conclusions 233
6.2 Outlook 241
References 245
List of Figures 260
List of Tables 269
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa:de:qucosa:79448 |
| Date | 04 August 2022 |
| Creators | Pfeiffer, Stefan |
| Contributors | Aneziris, Christos G., Graule, Thomas, Technische Universität Bergakademie Freiberg |
| Source Sets | Hochschulschriftenserver (HSSS) der SLUB Dresden |
| Language | English |
| Detected Language | English |
| Type | info:eu-repo/semantics/publishedVersion, doc-type:doctoralThesis, info:eu-repo/semantics/doctoralThesis, doc-type:Text |
| Rights | info:eu-repo/semantics/openAccess |
| Relation | https://doi.org/10.1002/adem.201801351, https://doi.org/10.1016/j.jeurceramsoc.2022.02.046, https://doi.org/10.1016/j.jeurceramsoc.2021.05.035, https://doi.org/10.1016/j.apsusc.2020.146304, https://doi.org/10.1016/j.oceram.2020.100007 |
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