Microstructure engineering of additively manufactured materials by powder bed fusion-laser beam: Cases for ferritic stainless steels and medium entropy alloys
Doctoral thesis, 2024
Powder bed fusion laser beam (PBF-LB) is a powder bed fusion additive manufacturing process that is one of the most established and widely used techniques for manufacturing near net shape components. The unique characteristics of PBF-LB, such as the high cooling rates and local directional heat transfer, lead to the formation of unique microstructures. These microstructures can be tailored to achieve desired properties by controlling the process parameters. This thesis studies the influence of PBF-LB processing conditions on the microstructure and resulting properties of two different types of alloys: high alloyed ferritic stainless steels and medium entropy alloys.
Due to the rapid melting and solidification, the resulting microstructure typically consists of columnar grains due to the conditions favourable for the epitaxial growth. Ways to address this include, by either manipulating the process parameters to limit the favourable conditions for epitaxial growth, or by utilizing the alloy design approach to promote the columnar to equiaxed transition. The first part of the thesis focuses on ferritic stainless steels. The influence of inoculation on heterogeneous nucleation and as-printed microstructure is studied in ferritic stainless steels with and without inoculants. The results show that inoculation can promote equiaxed grain growth and significantly reduce the epitaxial growth by altering the solidification conditions.
The second part of the thesis focuses on medium entropy alloys. The influence of interstitial solid solution strengthening on the mechanical properties of CoCrNi-N medium entropy alloys in as-printed and heat-treated conditions is studied. The results show that interstitial solid solution strengthening can improve the mechanical properties of the alloys and also provide significant microstructural stability by delaying the onset of recrystallization and grain growth as compared to the nitrogen free CoCrNi. In addition, the influence of processing conditions, specifically the high cooling rates on the as-printed microstructure, mechanical properties, and TRIP behaviour of a Co45Cr25(FeNi)30 metastable medium entropy alloy is investigated. The results show that processing conditions can significantly affect the microstructure and phase stability which in turn influence the resulting deformation behaviour of the alloy.
The results of this thesis demonstrate that PBF-LB is a versatile process that can be used to produce high-performance materials with tailored microstructure and properties. The findings of this research will be valuable to researchers and engineers who are interested in developing novel alloys and materials by and for additive manufacturing.
Due to the rapid melting and solidification, the resulting microstructure typically consists of columnar grains due to the conditions favourable for the epitaxial growth. Ways to address this include, by either manipulating the process parameters to limit the favourable conditions for epitaxial growth, or by utilizing the alloy design approach to promote the columnar to equiaxed transition. The first part of the thesis focuses on ferritic stainless steels. The influence of inoculation on heterogeneous nucleation and as-printed microstructure is studied in ferritic stainless steels with and without inoculants. The results show that inoculation can promote equiaxed grain growth and significantly reduce the epitaxial growth by altering the solidification conditions.
The second part of the thesis focuses on medium entropy alloys. The influence of interstitial solid solution strengthening on the mechanical properties of CoCrNi-N medium entropy alloys in as-printed and heat-treated conditions is studied. The results show that interstitial solid solution strengthening can improve the mechanical properties of the alloys and also provide significant microstructural stability by delaying the onset of recrystallization and grain growth as compared to the nitrogen free CoCrNi. In addition, the influence of processing conditions, specifically the high cooling rates on the as-printed microstructure, mechanical properties, and TRIP behaviour of a Co45Cr25(FeNi)30 metastable medium entropy alloy is investigated. The results show that processing conditions can significantly affect the microstructure and phase stability which in turn influence the resulting deformation behaviour of the alloy.
The results of this thesis demonstrate that PBF-LB is a versatile process that can be used to produce high-performance materials with tailored microstructure and properties. The findings of this research will be valuable to researchers and engineers who are interested in developing novel alloys and materials by and for additive manufacturing.
ferritic stainless steels
recrystallization
interstitial solid solution strengthening
phase transformation
inoculation
medium entropy alloys
Laser-based powder bed fusion
microstructural characterization
Virtual Development Laboratory (VDL-room), Chalmers Tvärgata 4C, Chalmers University of Technology, Gothenburg
Opponent: Professor Iain Todd, The University of Sheffield, Sheffield, United Kingdom
Author
Bala Malladi
Chalmers, Industrial and Materials Science, Materials and manufacture
Grain refinement in additively manufactured ferritic stainless steel by in situ inoculation using pre-alloyed powder
Scripta Materialia,;Vol. 194(2021)
Journal article
Single track versus bulk samples: Understanding the grain refinement in inoculated ferritic stainless steels manufactured by powder bed fusion-laser beam
Materialia,;Vol. 32(2023)
Journal article
Sri Bala Aditya Malladi, Dmitri Riabov, Sheng Guo, Lars Nyborg. Additive Manufacturing of interstitial nitrogen strengthened CoCrNi medium entropy alloy.
Corrosion behaviour of additively manufactured 316L and CoCrNi
Surface and Interface Analysis,;Vol. 55(2023)p. 404-410
Journal article
Laser-based Powder Bed Fusion of dispersion strengthened CoCrNi by ex-situ addition of TiN
Procedia CIRP,;Vol. 111(2022)p. 368-372
Paper in proceeding
Sri Bala Aditya Malladi, R. Gholizadeh, N. Tsuji, Sheng Guo, Lars Nyborg. Nitrogen-Mediated Retardation of Recrystallization Kinetics in CoCrNi-N Medium Entropy Alloys Fabricated by Powder Bed Fusion-Laser Beam.
Sri Bala Aditya Malladi, Tatiana Mishurova, Vishnu Anilkumar, Bharat Mehta, Alexander Evans, Kumar Babu Surreddi, Malte Blankenburg, Giovanni Bruno, Sheng Guo, Lars Nyborg. Understanding phase transformation and mechanical behaviour of an additively manufactured Co45Cr25(FeNi)30 metastable medium entropy alloy.
Imagine creating durable, lightweight, and customizable metal parts with the precision of a 3D printer. That's the promise of Powder Bed Fusion Laser Beam (PBF-LB), a cutting-edge additive manufacturing technique that transforms metal powders into intricate three-dimensional structures. However, PBF-LB also introduces unique microstructures that can dramatically influence the properties of the final product.
My PhD research delves into the intricate relationship between PBF-LB processing conditions and the microstructure and properties of two promising alloy systems: ferritic stainless steels and medium entropy alloys. For ferritic stainless steels, we discovered that a simple technique called inoculation – adding substances that promote heterogeneous nucleation – can significantly reduce the undesirable columnar grain structure and enhance mechanical properties.
Venturing into the domain of medium entropy alloys, our focus shifts to the realm of interstitial solid solution strengthening, a mechanism where atoms strategically occupy minute spaces within the crystal lattice. This strengthening tactic not only elevates mechanical properties but also introduces a layer of resilience by delaying recrystallization and grain growth, fortifying the alloys against temporal challenges.
Furthermore, we investigated the impact of high cooling rates on a metastable medium entropy alloy. This alloy exhibits a unique transformation-induced plasticity behaviour, where a phase transformation enhances strength-ductility synergy. Our findings revealed that processing conditions can influence the microstructure and phase stability, directly influencing the alloy's deformation behaviour.
The culmination of our research underscores the versatility of the PBF-LB process in precision-engineering material properties to meet specific engineering requirements. This breakthrough not only advances our understanding of alloys but also charts a course for developing innovative materials, poised to redefine engineering applications across aerospace, automotive, and medical domains. The engineering frontier is evolving, and PBF-LB is a transformative force shaping the future of materials engineering.
My PhD research delves into the intricate relationship between PBF-LB processing conditions and the microstructure and properties of two promising alloy systems: ferritic stainless steels and medium entropy alloys. For ferritic stainless steels, we discovered that a simple technique called inoculation – adding substances that promote heterogeneous nucleation – can significantly reduce the undesirable columnar grain structure and enhance mechanical properties.
Venturing into the domain of medium entropy alloys, our focus shifts to the realm of interstitial solid solution strengthening, a mechanism where atoms strategically occupy minute spaces within the crystal lattice. This strengthening tactic not only elevates mechanical properties but also introduces a layer of resilience by delaying recrystallization and grain growth, fortifying the alloys against temporal challenges.
Furthermore, we investigated the impact of high cooling rates on a metastable medium entropy alloy. This alloy exhibits a unique transformation-induced plasticity behaviour, where a phase transformation enhances strength-ductility synergy. Our findings revealed that processing conditions can influence the microstructure and phase stability, directly influencing the alloy's deformation behaviour.
The culmination of our research underscores the versatility of the PBF-LB process in precision-engineering material properties to meet specific engineering requirements. This breakthrough not only advances our understanding of alloys but also charts a course for developing innovative materials, poised to redefine engineering applications across aerospace, automotive, and medical domains. The engineering frontier is evolving, and PBF-LB is a transformative force shaping the future of materials engineering.
Additive Manufacturing using Metal Pilot Line (MANUELA)
European Commission (EC) (EC/H2020/820774), 2018-10-01 -- 2022-09-30.
Design av nya material och processer för nästa generations additiv tillverkning
VINNOVA (2018-00803), 2018-05-16 -- 2021-05-15.
Areas of Advance
Production
Subject Categories
Manufacturing, Surface and Joining Technology
Metallurgy and Metallic Materials
ISBN
978-91-7905-989-7
Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 5455
Publisher
Chalmers
Virtual Development Laboratory (VDL-room), Chalmers Tvärgata 4C, Chalmers University of Technology, Gothenburg
Opponent: Professor Iain Todd, The University of Sheffield, Sheffield, United Kingdom