| 1. |
- Hulme-Smith, Christopher, 1989-, et al.
(author)
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Spreadability Testing of Powder for Additive Manufacturing
- 2020
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In: Berg- und Huttenmännische Monatshefte (BHM). - : Springer Nature. - 0005-8912 .- 1613-7531. ; 166:1, s. 9-13
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Journal article (peer-reviewed)abstract
- The spreading of powders into thin layers is a critical step in powder bed additive manufacturing, but there is no accepted technique to test it. There is not even a metric that can be used to describe spreading behaviour. A robust, image-based measurement procedure has been developed and can be implemented at modest cost and with minimal training. The analysis is automated to derive quantitative information about the characteristics of the spread layer. The technique has been demonstrated for three powders to quantify their spreading behaviour as a function of layer thickness and spreading speed.
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| 2. |
- Marchetti, Lorenzo, et al.
(author)
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On Metal Powder Tribocharging and Humidity Adsorption
- 2022
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In: World PM 2022 Congress Proceedings. - : European Powder Metallurgy Association (EPMA).
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Conference paper (peer-reviewed)abstract
- In this work, three characterization techniques were applied to a set of alloys in fine powder form (TiAl6V4, AlSi10Mg, AlSi7, 316L, ferritic stainless steel, martensitic stainless steel and WC-Co-mixes). We sought deeper understanding of response to humidity and flowability as a result of the powder alloy. Slight tribo-charging (induced and measured using a GranuCharge setup) against metal surfaces was found to occur for all alloys. Although the accumulated charge was small and dissipated quickly. Greater charging occurs if the environment is humid, and if the powder slides against plastic surfaces. Dynamic Vapor Sorption (DVS) was employed to understand the adsorption capacity of powders. It showed that WC-Co-mixes adsorbed much more humidity than the other materials. Some alloys retained some of the adsorbed mass when humidity returned to normal conditions. RPA was tested on powders during exposure to 20-98% RH, which above 80% RH caused declining flowability.
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| 3. |
- Mellin, Pelle, et al.
(author)
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Moisture in Metal Powder and Its Implication for Processability in L-PBF and Elsewhere
- 2020
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In: Berg- und Huttenmännische Monatshefte (BHM). - 0005-8912 .- 1613-7531. ; 166:1, s. 33-39
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Journal article (peer-reviewed)abstract
- The impact of moisture on flowability and spread- ability is discussed. More notably we also present research on the impact of moisture on built nickel-base material. One lot of a newly opened Hastelloy X (HX) L-PBF powder was split into two equal batches. One batch was moisturized using a programmable climate chamber, the other was un- treated. We built bars with both batches for mechanical testing, in an EOS M100, with a cold build plate. The mois- ture content of the two powder batches, before and after the build-jobs, were determined using Karl Fischer titration (KF). Regarding the periodical monitoring of moisture con- tent, it is not needed according to the findings of this paper. More moisture contributes to a slightly higher O-content in the built material, and in turn, a very slight reduction in im- pact toughness. If a newly purchased powder exhibits poor flowability or high oxygen content, the analysis using oven- desorption followed by KF is recommended. If the moisture content is high, a drying of the powder is recommended
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| 4. |
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| 5. |
- Nilsson Åhman, Hanna, et al.
(author)
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An Enhanced Understanding of the Powder Bed Fusion-Laser Beam Processing of Mg-Y-3.9wt%-Nd-3wt%-Zr-0.5wt% (WE43) Alloy through Thermodynamic Modeling and Experimental Characterization
- 2022
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In: Materials. - : MDPI AG. - 1996-1944. ; 15:2
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Journal article (peer-reviewed)abstract
- Powder Bed Fusion-Laser Beam (PBF-LB) processing of magnesium (Mg) alloys is gaining increasing attention due to the possibility of producing complex biodegradable implants for improved healing of large bone defects. However, the understanding of the correlation between the PBF-LB process parameters and the microstructure formed in Mg alloys remains limited. Thus, the purpose of this study was to enhance the understanding of the effect of the PBF-LB process parameters on the microstructure of Mg alloys by investigating the applicability of computational thermodynamic modelling and verifying the results experimentally. Thus, PBF-LB process parameters were optimized for a Mg WE43 alloy (Mg-Y-3.(9wt%)-Nd-3wt%-Zr-0.5wt%) on a commercially available machine. Two sets of process parameters successfully produced sample densities >99.4%. Thermodynamic computations based on the Calphad method were employed to predict the phases present in the processed material. Phases experimentally established for both processing parameters included alpha-Mg, Y2O3, Mg3Nd, Mg24Y5 and hcp-Zr. Phases alpha-Mg, Mg24Y5 and hcp-Zr were also predicted by the calculations. In conclusion, the extent of the applicability of thermodynamic modeling was shown, and the understanding of the correlation between the PBF-LB process parameters and the formed microstructure was enhanced, thus increasing the viability of the PBF-LB process for Mg alloys.
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| 6. |
- Nilsson Åhman, Hanna, et al.
(author)
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Higher Laser power improves strength but reduces corrosion resistance of Mg WE43 processed by powder bed fusion
- 2024
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In: Materials Today Communications. - : Elsevier. - 2352-4928. ; 39
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Journal article (peer-reviewed)abstract
- Powder bed fusion – laser beam (PBF-LB) of Mg alloys provides new possibilities for the production of complex structures with optimized designs, both for weight reduction in aerospace applications, as well as for patient-specific implants in orthopedic applications. However, even though numerous studies have been carried out on the topic, the influence of the individual PBF-LB process parameters on the microstructure and resulting material properties of Mg alloys remains ambiguous. Thus, this study aims to investigate the influence of laser power on the surface roughness, microstructure and resulting key material properties, namely corrosion resistance and mechanical performance. Samples were produced by PBF-LB from gas atomized Mg-4%Y-3%Nd-0.5%Zr (WE43) alloy powder, using three different laser powers: 60 W, 80 W, and 90 W. Contrary to expectation, the 90 W samples exhibited the highest degradation rate, while 60 W samples had the lowest, despite the latter having highest surface roughness and large internal pores. The higher degradation rate for the 90 W samples was instead found to stem from the near-surface microstructure. The higher energy input and subsequently reduced grain size, resulted in an increased amount of second phase precipitates than for the 60 W samples, thereby increasing the tendency for pitting via microgalvanic corrosion. For the tensile strength and elongation at break, the opposite trend was observed. Here, a reduction in grain size and an increase in precipitates for the 90 W samples were found to be beneficial. In conclusion, a definite influence of laser power on the formation of microstructure was observed, ultimately impacting the resulting corrosion and tensile properties of WE43. Future work should investigate the influence of other PBF-LB process parameters, with the aim of establishing an optimum balance between corrosion resistance and mechanical properties.
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| 7. |
- Nilsson Åhman, Hanna, et al.
(author)
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Influence of Hot Isostatic Pressing on the corrosion resistance of Mg-4wt%Y-3wt%Nd processed by Laser - Powder Bed Fusion
- 2021
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Conference paper (other academic/artistic)abstract
- Magnesium alloys have recently gained increased attention as a biodegradable metal implant. They can be biocompatible and have superior mechanical properties compared to the biodegradable polymeric implants used today. However, poor formability and excessive corrosion rates have limited the clinical implementation of Mg alloys to an extruded Mg-Y-Nd alloy .Additive manufacturing through Laser - Powder Bed Fusion (L-PBF) allows for the production of patient specific implant designs. Hot Isostatic Pressing (HIP) is a common method applied after printing to obtain fully dense materials for improved mechanical and corrosion properties. However, the amount of work done on L-PBF of Mg alloys remains limited. Esmaily et al found an improved corrosion resistance after HIP of a Mg-Y-Nd alloy processed by L-PBF, as evidenced by surface activity and H2 evolution over 24h. However, the influence of HIP on the long term corrosion properties and the part morphology after corrosion have not been evaluated. Herein we show that HIP can be highly detrimental to the long-term corrosion properties of a Mg alloy processed by L-PBF.
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| 8. |
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| 9. |
- Nilsson Åhman, Hanna, et al.
(author)
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Microstructural Origins of the Corrosion Resistance of a Mg-Y-Nd-Zr Alloy Processed by Powder Bed Fusion - Laser Beam
- 2022
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In: Frontiers in Bioengineering and Biotechnology. - : Frontiers Media S.A.. - 2296-4185. ; 10
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Journal article (peer-reviewed)abstract
- Magnesium alloys are biocompatible, biodegradable and have the ability to promote bone ingrowth, making them ideal candidate materials for replacing auto- and allografts in future treatments of large bone defects. Powder bed fusion-laser beam (PBF-LB) additive manufacturing of these alloys would further allow for the production of complex structures, optimized for bone grafting. However, the corrosion rates of structures processed by PBF-LB remain too high. An improved understanding of the influence of the microstructure generated during PBF-LB on the corrosion properties is considered key to their future implementation in implants. In this study, the effect of PBF-LB processing and subsequent hot isostatic pressing (HIP) on the microstructure and texture in different sample directions was studied and related to the corrosion behavior of a Mg-Y-Nd-Zr alloy. The results were compared with an extruded Mg-Y-Nd-Zr alloy. A higher amount of secondary phases resulted in a higher rate of localized corrosion for the PBF-LB processed material compared to that for the extruded one. Due to growth of the secondary phases, the corrosion rate was further increased after HIP. Moreover, a strong texture was observed in the PBF-LB material, and it was also enhanced in the HIP material. While this affected the electrochemical activity as measured by potentiodynamic polarization tests, any texture effect appeared to be masked by the contribution of the secondary phases in the longer-term mass change and hydrogen evolution tests. Future work should look further into the influence of individual process parameters on the microstructure and the resulting corrosion behavior of the material, to further clarify its interdependence.
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| 10. |
- Nilsson Åhman, Hanna
(author)
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Powder Bed Fusion – Laser Beam of Mg alloy WE43 : Establishing the process – structure – properties relationship
- 2023
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Doctoral thesis (other academic/artistic)abstract
- Powder bed fusion - laser beam (PBF-LB) of Mg alloy WE43 (Mg-4wt%Y-3wt%RE-Zr) has great potential for the development of future biodegradable metal implants, as well as aerospace lightweight materials. However, the work published thus far has mainly focused on obtaining a fully dense material, and the understanding of the relationship among the PBF-LB process parameters, structure and the resulting material properties remains limited. Thus, the aim of the thesis was to relate the main PBF-LB processing parameters to the formation of key microstructural features in WE43, and their effect on corrosion and tensile test properties. The work was carried out on PBF-LB processing units EOS M100 and EOS M290, and the investigated process parameters included laser power, laser scanning speed, hatch distance and sample wall thickness. Depending on the resulting thermal conditions, two main microstructural regions were observed. For process parameters resulting in warmer processes, such as higher laser powers and shorter scan lengths, mainly equiaxed dendritic grains were observed. The grains measured up to 10 µm in maximum diameter and exhibited a weak texture, with the inter-dendritic regions rich in Mg-RE intermetallic compunds. For process parameters resulting in conductive mode melting, mainly a lamellar structure was observed. The lamellar structure consisted in large grains with basal texture, and an intragranular structure where lines of Mg-RE intermetallic compunds precipitated parallel to the melt pool boundary. The larger grains had a maximum diameter of around 60 µm to 100 µm in the build direction, and up to 250 µm in the transverse direction, with a preferential growth along the melt pool.A larger number of dendritic grains was detrimental to the corrosion properties but resulted in higher tensile strength. The result was ascribed to the higher amount of Mg-RE intermetallics and the smaller grains, strengthening the material, but also causing microgalvanic corrosion. Hot isostatic pressing also resulted in growth of the secondary phases and was thus also detrimental to corrosion properties. While a change in hatch distance (40-60 mm) did not cause any dendritic structure to form, a higher hatch distance resulted in improved corrosion properties, but had minor effect on tensile properties, showing the possibilities of applying hatch distance variations to balance corrosion and tensile properties.In conclusion, the findings presented here show the possibilities of controlling the microstructure and thus the material properties by changing some of the key PBF-LB process parameters, and the major importance of understanding the relationship among process, structure and material properties of PBF-LB processed WE43.
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