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Processing–Structure–Properties Relationship in Metal Additive Manufacturing
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.ORCID iD: 0000-0001-8500-1632
2021 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The last three decades have seen the transition of additive manufacturing, from applications exclusively in rapid-prototyping to an emerging production method in the manufacturing industry that is rapidly gaining more relevance. 

Within additive manufacturing methods, selective laser melting (SLM) is one of the most widely used and mature technologies and is the focus of this thesis. In particular, this work aims at characterizing novel microstructures and/or alloys produced with SLM and to understand how the process parameters influence the microstructure and properties.

Hitherto, the most prevalent material selection approach for SLM has been the use and optimization of well-known alloys, such as steels, Ni- and Ti-based alloys, among others. Favorable microstructures are usually achieved with a combination of appropriate parameters and post-processing techniques. Another approach, especially interesting from a research perspective, is the exploration of materials and microstructures suited for the inherent characteristics of SLM. In alignment with the latter strategy, three types of materials are successfully produced and analyzed in this work: the amorphous Zr-based AMZ4 alloy, 316L stainless steel with strong preferential orientation (i.e., similar orientation of the crystalline structure of the grains) and the intermetallic MnAl(C) with strong preferential orientation. The latter contains a ferromagnetic phase with potential applications as a permanent magnet.

SLM was found to be an effective method to produce the amorphous phase in the Zr59.3Cu28.8Al10.4Nb1.5 system (AMZ4). The laser power and oxygen impurities were found to have a central role in the formation of crystalline particles in the amorphous matrix. These crystalline particles and the oxygen impurities reduced the thermal stability of the alloy in comparison to specimens fabricated by suction casting. For the more conventional 316L stainless steel, it was demonstrated that the scan strategy can be used to influence the type of texture, with a notable effect on the mechanical properties. In the case of MnAl(C), it was established that the high temperature polymorph – ε-phase, can be retained during the printing process. This phase can be subsequently transformed to the ferromagnetic τ-phase with annealing procedures. It was observed that a strong preferred orientation of the ε-phase can be achieved, although it did not translate into a strong texture in the τ-phase (after the heat treatments). 

The research methodology used in this thesis and the findings regarding the processing–structure–properties relationship in SLM provide an important reference for future studies of novel materials and microstructures produced by additive manufacturing.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2021. , p. 83
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 2070
Keywords [en]
additive manufacturing, selective laser melting, preferential orientation, neutron imaging, metallic glasses
National Category
Materials Engineering
Identifiers
URN: urn:nbn:se:uu:diva-452292ISBN: 978-91-513-1286-6 (print)OAI: oai:DiVA.org:uu-452292DiVA, id: diva2:1591049
Public defence
2021-10-22, Häggsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 09:15 (English)
Opponent
Supervisors
Funder
Swedish Foundation for Strategic Research , GSn15-0008Available from: 2021-09-30 Created: 2021-09-05 Last updated: 2021-10-19
List of papers
1. Development of process parameters for selective laser melting of a Zr-based bulk metallic glass
Open this publication in new window or tab >>Development of process parameters for selective laser melting of a Zr-based bulk metallic glass
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2020 (English)In: Additive Manufacturing, ISSN 2214-8604, E-ISSN 2214-7810, Vol. 33, article id 101124Article in journal (Refereed) Published
Abstract [en]

Parameters for selective laser melting of Zr59.3Cu28.8Al10.4Nb1.5 (trade name AMZ4), allowing crack-free bulk metallic glass with low porosity, have been developed. The phase formation was found to be strongly influenced by the heating power of the laser. X-ray amorphous samples were obtained with laser power at and below 75 W. The as-processed bulk metallic glass was found to devitrify by a two-stage crystallization process within which the presence of oxygen was concluded to play an essential role. At laser powers above 75 W, the observed crystallites were found to be a cubic phase (Cu2Zr4O). The hardness and Young’s modulus in the as-processed samples was found to increase marginally with increased fraction of the crystalline phase.

Keywords
Selective laser melting, AMZ4, Bulk metallic glass
National Category
Manufacturing, Surface and Joining Technology
Identifiers
urn:nbn:se:uu:diva-409729 (URN)10.1016/j.addma.2020.101124 (DOI)000539269900006 ()
Funder
Swedish Foundation for Strategic Research , GMT14-0048Swedish Foundation for Strategic Research
Available from: 2020-04-27 Created: 2020-04-27 Last updated: 2021-09-08Bibliographically approved
2. Thermal stability and crystallization of a Zr-based metallic glass produced by suction casting and selective laser melting
Open this publication in new window or tab >>Thermal stability and crystallization of a Zr-based metallic glass produced by suction casting and selective laser melting
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2020 (English)In: Journal of Alloys and Compounds, ISSN 0925-8388, E-ISSN 1873-4669, Vol. 825, article id 153995Article in journal (Refereed) Published
Abstract [en]

The thermal stability and crystallization mechanism of the Zr59.3Cu28.8Al10.4Nb1.5 (at%) metallic glass produced through selective laser melting SLM (from industrial grade material) was studied and compared with the same alloy produced by suction casting (from laboratory grade material of high purity). Oxygen- and Al-rich particles of a cubic phase (Fd (3) over barm) with a size of up to 200 nm are detected in the as-built selective laser melted samples by transmission electron microscopy. The crystallization process of the cast and SLM samples is investigated by in-situ X-ray diffraction experiments. In the cast samples, the initial crystallization occurs via the formation of a metastable tetragonal phase (Al2Zr3), together with tetragonal CuZr2 and hexagonal Al3Zr4 type structures, while the SLM samples initially crystallize through the formation of the metastable, oxygen- and Al-rich, cubic phase already present before annealing. The main phases present at the end of the crystallization for both type of samples are the same, mainly CuZr2 and Al3Zr4. The differences in the crystallization paths are attributed to differences in the oxygen levels. In general, the higher oxygen content (similar to 1 at%) of the SLM samples results in a decrease of the thermal stability of the alloy and promotes the formation of an oxygen-rich, metastable cubic phase. 

Place, publisher, year, edition, pages
ELSEVIER SCIENCE SA, 2020
Keywords
Metallic glass, Additive manufacturing, Selective laser melting, Laser beam powder bed fusion, Crystallization, Thermal stability
National Category
Metallurgy and Metallic Materials Materials Chemistry
Identifiers
urn:nbn:se:uu:diva-407503 (URN)10.1016/j.jallcom.2020.153995 (DOI)000514848600109 ()
Funder
Swedish Foundation for Strategic Research , GSn15-0008Swedish Foundation for Strategic Research , GMT14-0048
Available from: 2020-03-26 Created: 2020-03-26 Last updated: 2021-09-08Bibliographically approved
3. Transient nucleation in selective laser melting of Zr-based bulk metallic glass
Open this publication in new window or tab >>Transient nucleation in selective laser melting of Zr-based bulk metallic glass
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2020 (English)In: Materials & design, ISSN 0264-1275, E-ISSN 1873-4197, Vol. 195, article id 108958Article in journal (Refereed) Published
Abstract [en]

The crystallization rate during selective laser melting (SLM) of bulk metallic glasses (BMG) is a critical factor in maintaining the material's amorphous structure. To increase the understanding of the interplay between the SLM process and the crystallization behavior of BMGs, a numerical model based on the classical nucleation theory has been developed that accounts for the rapid temperature changes associated with SLM. The model is applied to SLM of a Zr-based BMG and it is shown that the transient effects, accounted for by the model, reduce the nucleation rate by up to 15 orders of magnitude below the steady-state nucleation rate on cooling, resulting in less nuclei during the build process. The capability of the proposed modelling approach is demonstrated by comparing the resulting crystalline volume fraction to experimental findings. The agreement between model predictions and the experimental results clearly suggests that transient nucleation effects must be accounted for when considering the crystallization rate during SLM processing of BMGs.

Place, publisher, year, edition, pages
ELSEVIER SCI LTD, 2020
Keywords
Additive manufacturing, Selective laser melting, Metallic glass, Crystallization, Classical nucleation theory, Transient nucleation
National Category
Energy Engineering
Identifiers
urn:nbn:se:uu:diva-423589 (URN)10.1016/j.matdes.2020.108958 (DOI)000576532400003 ()
Funder
Swedish Foundation for Strategic Research , GMT14-0048Vinnova, 2017-05200
Available from: 2020-10-30 Created: 2020-10-30 Last updated: 2021-09-05Bibliographically approved
4. Crystallization of a Zr-based metallic glass produced by laser powder bed fusion and suction casting
Open this publication in new window or tab >>Crystallization of a Zr-based metallic glass produced by laser powder bed fusion and suction casting
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2021 (English)In: Journal of Non-Crystalline Solids, ISSN 0022-3093, E-ISSN 1873-4812, Vol. 571, article id 120891Article in journal (Refereed) Published
Abstract [en]

The crystallization behavior during low-temperature annealing of samples of the Zr59.3Cu28.8Al10.4Nb1.5 (at%) bulk metallic glass produced by suction casting and the laser powder bed fusion (LPBF) process was studied with small-angle neutron scattering (SANS), X-ray diffraction, and scanning electron microscopy. The in-situ SANS measurements during isothermal annealing reveal that the phase separation in the LPBF processed material proceeds at a smaller characteristic length-scale than the cast material. Quantitative analysis of the SANS data shows that, while the crystallization process in both materials proceeds through rapid nucleation followed by diffusion-limited growth, the LPBF processed material crystallizes with a smaller cluster size and at a higher rate. The smaller cluster size is attributed to the elevated oxygen content in the LPBF processed material which reduces the nucleation barrier and thus the thermal stability.

Place, publisher, year, edition, pages
Elsevier, 2021
Keywords
AMZ4, Additive manufacturing, Small angle scattering, Crystallization, Oxygen
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:uu:diva-452041 (URN)10.1016/j.jnoncrysol.2021.120891 (DOI)000685499600007 ()
Funder
Swedish Foundation for Strategic Research , GMT14-0048Vinnova, 2020-04526Swedish Foundation for Strategic Research , GSn150008
Available from: 2021-09-02 Created: 2021-09-02 Last updated: 2024-01-15Bibliographically approved
5. The effect of laser scanning strategies on texture, mechanical properties, and site-specific grain orientation in selective laser melted 316L SS
Open this publication in new window or tab >>The effect of laser scanning strategies on texture, mechanical properties, and site-specific grain orientation in selective laser melted 316L SS
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2020 (English)In: Materials & design, ISSN 0264-1275, E-ISSN 1873-4197, Vol. 193, article id 108852Article in journal (Refereed) Published
Abstract [en]

Selective laser melting has been used to demonstrate the striking effect of laser scanning strategies on the crystalline texture in 316L SS. The aligned crystal orientation along the tensile direction (Z-axis) could be varied using the scanning strategy. A strong 〈100〉 single crystalline-like texture is obtained in the direction of the laser scan and a 〈110〉 texture was observed in the build direction when using a bidirectional scan without rotation. Fiber texture was observed along the tensile direction when the bi-directional laser scanning vectors were rotated by 67° (Rot-scan) for each layer. The study highlights a correlation between laser scanning strategies with resulting textures, microstructure, and mechanical properties in as-printed bulk 316L SS. The hardness, Young's modulus, and ultimate tensile strength were significantly influenced by the final microstructure, crystallographic texture, and porosity. Furthermore, the applied laser scanning strategies made it possible to tailor crystallographic textures locally within the component. This was demonstrated by printing characters with a fiber texture, in a matrix with ⟨100⟩ texture parallel to the Z-axis.

Keywords
Selective laser melting, Texture, Austenitic stainless steel, Solidification microstructures
National Category
Materials Engineering
Identifiers
urn:nbn:se:uu:diva-412903 (URN)10.1016/j.matdes.2020.108852 (DOI)000568760100003 ()
Funder
Swedish Foundation for Strategic Research , GMT14-0048
Available from: 2020-06-11 Created: 2020-06-11 Last updated: 2021-09-08Bibliographically approved
6. Microstructure and crystallographic texture variations induced by the scan strategy in laser powder-bed fusion
Open this publication in new window or tab >>Microstructure and crystallographic texture variations induced by the scan strategy in laser powder-bed fusion
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(English)Manuscript (preprint) (Other academic)
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:uu:diva-452276 (URN)
Funder
Swedish Foundation for Strategic Research , GSn15 - 0008
Available from: 2021-09-04 Created: 2021-09-04 Last updated: 2021-09-05
7. Preferential orientation in the MnAl(C) alloy produced by additive manufacturing
Open this publication in new window or tab >>Preferential orientation in the MnAl(C) alloy produced by additive manufacturing
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(English)Manuscript (preprint) (Other academic)
National Category
Materials Engineering
Identifiers
urn:nbn:se:uu:diva-452277 (URN)
Available from: 2021-09-04 Created: 2021-09-04 Last updated: 2021-09-05

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