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Investigation of the heterogeneous structure and its effect on mechanical properties of electron beam melting fabricated 316L stainless steel
Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
(English)Manuscript (preprint) (Other academic)
Abstract [en]

316L stainless steel samples were prepared with Electron Beam Melting. Samples were produced to investigate what parameters and conditions govern the microstructural evolution during the process, as well as the effect of the microstructural variations on the tensile strength. The effects investigated in this study are the cooling speed of the melt, the heatwaves moving through the sample due to additional layers being melted on top, and the constant base temperature of around 800 °C. Several different samples were prepared to simulate different conditions that occur during the manufacturing of real parts. The microstructures of these samples were investigated and characterized using scanning electron microscope. The investigation showed that all these effects had different effects on the microstructure. The tensile strength was homogeneous throughout the material, with little variance in the values. The ultimate tensile strength was recorded to 562±4 MPa, the yield strength to 261±3 MPa, and the Elongation to 67.5±4.8 %. Further investigation into why the change in microstructure has no impact on tensile strength was done using transmission electron microscopy. It was revealed that the material was missing the dislocation networks that are responsible for the increased strength observed in samples prepared by the selective laser melting process.

National Category
Materials Chemistry
Research subject
Materials Chemistry
Identifiers
URN: urn:nbn:se:su:diva-176464OAI: oai:DiVA.org:su-176464DiVA, id: diva2:1376058
Available from: 2019-12-06 Created: 2019-12-06 Last updated: 2019-12-11Bibliographically approved
In thesis
1. Additive metallurgy - Thermal influences on structure and properties of stainless steel 316L
Open this publication in new window or tab >>Additive metallurgy - Thermal influences on structure and properties of stainless steel 316L
2020 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Additive manufacturing (AM) as a manufacturing process has, in recent years, become widely accepted as capable of manufacturing parts that will be used in end products. In this thesis, stainless steel grade 316L parts are manufactured using two different powder bed fusion techniques, selective laser melting (SLM) and electron beam melting (EBM). It is recognized that parts made using these processes will have unique microstructures and mechanical properties that have not been seen in bulk parts produced with other methods. The driving force behind the formation of these structures is the fast cooling speed that induces segregation of elements, forming an inhomogeneous microstructure. How these structures react to thermal treatment is less well known and an essential aspect in many applications. Parts manufactured using SLM was treated with hot isostatic pressing (HIP) to investigate if the material retains its unique features. Two different HIP cycles were used, one with 1150 °C and one with 1040 °C. In both cases, the cellular sub-grain structure fades. The cycle utilizing the high temperature is found to coarsen the grain structure and thus lowering the mechanical properties significantly. As manufactured parts show yield strength (615±1 MPa), tensile strength (725±2 MPa) and microhardness (211±10 Hv), compared to values of yield strength (284±2 MPa), tensile strength (636±1 MPa) and microhardness (178±8 Hv) after 1150 °C HIP. Using HIP at 1040 °C, the material will retain a finer grain structure resulting in higher yield strength (417±7 MPa) compared to 1150 °C HIP temperature, while the UTS and hardness have a similar value. It is also observed that the dispersed inclusions formed during SLM are still present after HIP to increase the mechanical properties compared to a conventionally annealed bar (TS: 515 MPa, YS: 205 MPa). Samples manufactured using EBM was investigated to understand the influence of the in-situ heat treatment that is present in the EBM process. The material possesses a long-range heterogeneous structure in addition to the cellular structure, where the cellular structure is present at the top and disappears further down the sample. Samples with different geometries were produced to study the effect of heat flux, cooling speed and the elevated temperature of 800 °C that is present during the EBM process. The thickness of the cell boundaries is measured in different areas, revealing that geometry and size of manufactured parts have a significant impact on the evolving microstructure. It is also revealed that the tensile strength (562±4 MPa) and microhardness (161±11 Hv) is not affected by the change in microstructure, resulting in a very homogeneous material concerning these parameters. Heat treating the material at 800 °C show that the cellular structure becomes diffuse after several hours, but the grain morphology stays the same.

Place, publisher, year, edition, pages
Stockholm: Department of Materials and Environmental Chemistry, Stockholm University, 2020
Keywords
Additive manufacturing, Selective laser melting, Electron beam melting, Hot isostatic pressing, Stainless steel, Microstructural heterogeneity
National Category
Materials Chemistry
Research subject
Materials Chemistry
Identifiers
urn:nbn:se:su:diva-176458 (URN)978-91-7797-968-5 (ISBN)978-91-7797-969-2 (ISBN)
Public defence
2020-02-07, Magnélisalen, Kemiska övningslaboratoriet, Svante Arrhenius väg 16 B, Stockholm, 09:00 (English)
Opponent
Supervisors
Note

At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 5: Manuscript. Paper 6: Manuscript.

Available from: 2020-01-15 Created: 2019-12-09 Last updated: 2020-01-17Bibliographically approved

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