The room temperature compressive stress-strain curves for the SMC-0, SMC-2.5 and SMC-5 samples are shown in Fig. 6.5. The addition of the TiB2 particles has resulted in a drastic increase of the yield strength (0.2 % offset), raising from 412 ± 7 MPa for SMC-0 to 600 ± 3 MPa and 845 ± 5 MPa for SMC-2.5 and SMC-5, respectively. Several factors can explain this observation. Firstly, as reported in previous studies [76,132,197], 316L stainless steel will reject the ferrite stabilizers (e.g., Cr, Mo and Si ) from the liquid along the solid- liquid interface during solidification. They are precipitated at the cell walls and grain boundaries and contribute together with the TiB2 particles in the matrix and along the boundary to cause an accumulation structure mechanism [191,198], which hinders the slip of dislocations. Secondly, as grain size decreases, the grain boundary area increases [58], as observed in the EBSD analysis; that means more obstacles able to block the dislocation movements and force the dislocations to snarl and pile up near the grain boundaries. Thirdly, the cellular microstructure commonly has subgrains, as observed from the TEM analysis; these subgrains have misorientations [171]. As grain and cell refinement occurs, more subgrains and cells are formed which retard the dislocation movement and increase the
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plastic deformation. Fourthly, the spherical nano-sized TiB2 precipitations at the subgrain boundaries, as revealed from TEM, improve the adequacy of load transfer from 316L matrix to the hard TiB2 phase. Fifthly, the precipitation of the TiB2 at grain boundaries plays a key role in increasing the concentration of high stresses which also contribute to an increased the strain hardening [191,199,200]. For all these reasons, the yield strength of the 316L matrix composite is enhanced.
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Chapter 7: Conclusions and outlook
This thesis has systematically investigated the important role played by the scanning strategy on densification, microstructure and mechanical properties of 316L steel samples. The following conclusions can be drawn:
A single-phase austenite is formed regardless of the scanning strategy used and all specimens display a cellular microstructure and random crystallographic texture.
The scanning strategy, however, considerably affects the density of the samples and the size of the characteristic microstructural features: cells and grains. Specifically, the stripe with contour style generates the smallest cell and grain sizes (610 ± 19 nm and 45 ± 3 µm, respectively), whereas the checkerboard strategy the largest (887 ± 15 nm and 64 ± 7 µm). The microstructural refinement induced by the stripe strategy leads to the best
combination of mechanical properties: yield strength and ultimate tensile strength are about 550 and 1010 MPa and plastic deformation exceeds 50 %. The findings reported here further confirm the fundamental importance of the scanning strategy for the synthesis of SLM parts with optimized microstructures and enhanced mechanical performance, and offer a possible route for the microstructural refinement of parts synthesized by selective laser melting.
In addition, the effect of annealing on the stability of phases, composition and microstructure of 316L stainless steel has been examined. The findings of the annealing study can be summed up as below:
The complex cellular microstructure with fine subgrain structures is stable up to 873 K. The cell size increases with increasing annealing temperature until the cellular microstructure can no longer be observed at high temperatures (T ≥ 1273 K).
Annealing does not change the random crystallographic orientation observed in the as- synthesized material, neither leads to phase transformation. The strength of the specimens
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decreases with increasing annealing temperature as a result of the microstructural coarsening.
The complex cellular microstructure and subgrains along with the misorientation between grains, cells, cell walls and subgrains contribute to the superior strength and good ductility of the as-SLM samples in comparison with the annealed specimens.
The findings reported here show that the optimal combination of strength and ductility for the current 316L material is already reached during SLM processing and that additional heat treatments do not improve the performance of the material, as the decrease of strength is not compensated by a corresponding increase of plastic deformation. Alternative methods that utilize the remarkable ductility of the as-SLM 316L, such as in composites where strengthening of a ductile matrix is achieved by introducing a hard second phase, appear to have higher potential for development.
Moreover, this thesis successfully optimizes the SLM processing parameters, particularly the laser scanning speed, in order to synthesize 316L/CeO2 and 316L/TiB2 matrix composites. The results obtained from these materials can be shown as below:
The laser scanning speed has been optimized in order to achieve highly-dense defect-free 316L/CeO2 and 316L/TiB2 specimens.
The addition of the CeO2 or TiB2 phase does not change the phase formation during solidification but it affects the microstructure of the composite, which is considerably refined compared with the unreinforced 316L material. The refined microstructure induces significant strengthening in the composite without deteriorating the plastic deformation.
The distribution of the TiB2 particles with a size of about 50-100 nm along the grain boundaries together with the refined microstructure contribute to the strengthening of the materials without sacrificing the plastic deformation.
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These preliminary results not only indicate that highly-dense either 316L/CeO2 or 316L/TiB2 composites with enhanced room-temperature strength can be synthesized by SLM, but also they open the possibility to extend the analysis of their properties to high temperatures, enlarging the possible applications of this type of composites.
The results of this study show the importance of the scanning strategy in determining the microstructure and mechanical properties of the 316L steel fabricated by SLM. Moreover, this study has revealed how the addition of ceramic particles to the 316L steel matrix can significantly refine the microstructure and improve the mechanical properties. The knowledge obtained from the present work can productively extend the use of 316L steel matrix composites for high temperatures applications. For further investigations, the focus will be directed to study endurance of resultant steel matrix composites during different mechanical tests, such as tensile tests at room and different temperatures, low and high cycles fatigue tests and wear tests.
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Acknowledgements
̒̒
over every possessor of knowledge is one more knowing."-Quran
First and foremost, all praises to Almighty GOD, the most merciful and compassionate who helped me to complete my journey till here brightly.
I would like to express my gratitude to Prof. J. Eckert for his constant support and giving me this priceless opportunity to do my Ph.D. in Leibniz Institute for Solid State and Materials Research (IFW Dresden) (Vielen Dank).
I do not have enough words those can express my deep thanks to Dr. S. Scudino for his continuous tolerance, patience, guidance and advice, without his support, I could not have reached this destination (molte grazie).
I am also forever grateful to Rosa Vladimirovna, Pramote Thirathipviwat, Dr. Rub Nawaz Shahid, Alina Israfilova, Olya Pertseva, Mohamed Abdelhameed, Dr. Mohammed Shosha, Ahmed El-Shorbagy, Jens Wendler, Marta Markudis, Clemens, Yesmine, Trigui, Diana Ch, Jad Khodor, Mushtak Khaleel, Tianbing HE, Dr. Onur Ertugrul, Dr. Omar Aljanbi, Rodolfo Batalha, Angelo Andreoli and Monica Manga, who have been an unstinting source of support, without them, I would not have accomplished this mission successfully and spent unforgotten nice times in Dresden or Germany.
My warmest thanks go to my lifetime friend Firas Maan who has always encouraged me to fulfil this dream of my life.
My sincere thanks for the support, help and for the nice time during the course of this thesis to Dr. Pei Wang, Dr. Parthiban, Dr. Lixia Xi, Dr. Supriya Bera, Dr Subhash Thota, Prof. Dr. Thomas Niendorf, Assoc. Prof. Dr. Prashanth Konda, Dr. Christoph Gammer, Dr. Anil Chaubey, Dr. Uta. Kuehn, Dr. A. Gebert, Dr. H. Wendrock, Dr. Thomas. Gemming, Dr. Holger Shwab, Dr.
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Tobias Gustmann, Dr. Jan Sander, Liang Deng, Dr. Simon Pauly, Dr. Mariana Calin, Dr. Konrad Kosiba, Brit Präßler-Wüstling Dr. Daniel Sopu, Dr. Junhee Han, Dr. Baran Sarac, Janett Schuster, Dr. David Geißler, Dr. Lars Giebeler, Dr. Anja Waske, Dr. Mahmoud Madian, Alexander Funk, Birgit Opitz, Harald Merker, Steffen Grundkowski and Nicole Geißler.
I gratefully acknowledge financial support from the Ministry of Higher Education & Scientific Research (MoHESR), Iraq.
Finally, my heartfelt thanks go my parents, my lovely Brother (Hamza), my lovely godmother aunt (Hanna), and my all family members, without your motivated words and support during this long journey, I could not be able to achieve this dream successfully.
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Erklärung
Hiermit versichere ich, dass ich die vorliegende Arbeit ohne unzulässige Hilfe Dritter und ohne Benutzung anderer als der angegebenen Hilfsmittel angefertigt habe; die aus fremden Quellen direkt oder indirekt übernommenen Gedanken sind als solche kenntlich gemacht. Bei der Auswahl und Auswertung des Materials sowie bei der Herstellung des Manuskripts habe ich Unterstützungsleistungen von folgenden Personen erhalten: Prof. Dr. Jürgen. Eckert und Dr. Sergio. Scudino. Weitere Personen waren an der geistigen Herstellung der vorliegenden Arbeit nicht beteiligt. Insbesondere habe ich nicht die Hilfe eines kommerziellen Promotionsberaters in Anspruch genommen. Dritte haben von mir keine geldwerten Leistungen für Arbeiten erhalten, die in Zusammenhang mit dem Inhalt der vorgelegten Dissertation stehen. Die Arbeit wurde bisher weder im Inland noch im Ausland in gleicher oder ähnlicher Form einer anderen Prüfungsbehörde vorgelegt und ist auch noch nicht veröffentlicht worden. Dissertation wurde an der TU Dresden am Institut für Werkstoffwissenschaft der Fakultät Maschinenwesen unter der wissenschaftlichen Betreuung durch Prof. Dr.-Ing. habil. Dr. h.c. Jürgen Eckert angefertigt. Die Promotionsordnung der Fakultät Maschinenwesen der TU Dresden aus dem Jahre 2001 erkenne ich an.
Omar Oday Salman Dresden, 12.12.2018
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