Titanium alloys, such as Ti-6Al-4V, are particularly susceptible to oxidation, which is why their processing in the laser-based powder bed fusion process is carried out conventionally in a protective gas atmosphere. However, this atmosphere still contains critical residual oxygen levels, which are to be eliminated as part of a new approach. This approach envisages doping the argon protective gas atmosphere with small amounts of
the highly reactive gas silane (ratio < 1:1000). The residual oxygen content is particularly critical in filigree and thin-walled structures that have a high surface-area-to-volume ratio and are a typical field of application for this additive manufacturing process. Therefore, this work focuses on the manufacturing of Ti-6Al-4V structures with different surface-area-to-volume ratios in conventional argon (< 200 ppm residual oxygen) and argon-silane atmospheres (< 10-14 ppm residual oxygen) on an innovative laboratory
machine. After processing, the specimens are analyzed for surface topography, microstructure, and Vickers hardness. In addition, energy-dispersive X-ray spectroscopy and X-ray diffraction measurements are carried out to further investigate the chemical composition and present phases in the as-built specimens. The influence of the different atmospheres and their residual oxygen content, the surface-to-volume ratio, and possible interactions between them are discussed

Bernhard, R.

Emminghaus, Nicole

Hermsdorf, J.

Kaierle, S.

Overmeyer, L.

English

Publikationsserver der Technischen Universität Clausthal

Laser-Based Powder Bed Fusion of Ti-6Al-4V Structures with Different Surface-Area-to-Volume Ratios in Oxygen-Reduced and Oxygen-Free En-vironment  N. Emminghaus1, R. Bernhard1, J. Hermsdorf1, L. Overmeyer2, S. Kaierle1,2 1 Laser Zentrum Hannover e. V. (LZH) 2 Leibniz Universität Hannover, Institut für Transport- und Automatisierungstechnik (ITA) n.emminghaus@lzh.de  Abstract Titanium alloys, such as Ti-6Al-4V, are particularly susceptible to oxidation, which is why their processing in the laser-based powder bed fusion process is carried out con-ventionally in a protective gas atmosphere. However, this atmosphere still contains crit-ical residual oxygen levels, which are to be eliminated as part of a new approach. This approach envisages doping the argon protective gas atmosphere with small amounts of the highly reactive gas silane (ratio < 1:1000). The residual oxygen content is particu-larly critical in filigree and thin-walled structures that have a high surface-area-to-vol-ume ratio and are a typical field of application for this additive manufacturing process. Therefore, this work focuses on the manufacturing of Ti-6Al-4V structures with differ-ent surface-area-to-volume ratios in conventional argon (< 200 ppm residual oxygen) and argon-silane atmospheres (< 10-14 ppm residual oxygen) on an innovative laboratory machine. After processing, the specimens are analyzed for surface topography, micro-structure, and Vickers hardness. In addition, energy-dispersive X-ray spectroscopy and X-ray diffraction measurements are carried out to further investigate the chemical com-position and present phases in the as-built specimens. The influence of the different at-mospheres and their residual oxygen content, the surface-to-volume ratio, and possible interactions between them are discussed.  1 Introduction The laser-based powder bed fusion process (PBF-LB) gains increasing interest from both academia and industry. Due to the layerwise buildup of parts, the manufacturing of intricate shapes is possible. Parts with high surface-area-to-volume ratios (SVR), e.g. heat exchangers or lattice structures for implants, are typical applications for the PBF-LB process. In the case of bone implants, the high surface-area-to-volume ratio enhances bone ingrowth [1]. In heat exchangers, the high SVR allows superior heat transfer [2]. However, PBF-LB and the processed powders are susceptible to impurity pick-up dur-ing the process. It is known that the amount of oxygen pick-up is influenced by the implemented energy input, as shown by Tan et al. for the PBF-LB of TiNi lattice struc-tures [3]. Regarding the manufacturing process of thin structures itself, it has to be taken into account that the heat conduction conditions are different compared to the manufac-turing of bulk parts. With less solid material and more surrounding powder, less heat can be transferred toward the build platform that acts as a heat sink. Additionally, it is hypothesized that with an increasing SVR, the atmospheric impurity pick-up also in-creases and that it alters the part and powder properties. Oxygen pick-up due to the residual oxygen in the processing atmosphere is especially critical when it comes to the processing of highly reactive materials like titanium alloys. For example, the most used titanium alloy, Ti-6Al-4V, has a high oxygen solubility of up to 14.3 wt-% in the alpha phase [4]. This in turn is by far exceeding most specifications that regard a limit of 0.2 wt-% as critical [5,6]. Besides an oxygen increase in the total amount of powder, also single oxidized particles are critical. Due to their altered chemical composition, they can represent chemical and mechanical flaws in the fabricated parts [7]. In common industrial machines, argon is used as an inert processing atmosphere to prevent oxida-tion during processing. These machines typically achieve residual oxygen contents of 1000 ppm [8, 9]. However, as demonstrated by Dietrich et al., already at a residual ox-ygen content of 2 ppm, the oxygen content increases from virgin powder to the pro-cessed part [8]. Consequently, it is necessary to establish an even lower residual oxygen content in the PBF-LB processing atmosphere.  The implementation of a vacuum is expensive and can lead to increased spattering since there is no counteracting pressure to the vaporization pressure [10]. A novel approach to reduce the residual oxygen is to use small additions of a reactive gas to the inert gas. In the case of the Collaborative Research Center 1368 “Oxygen-free Production” monosilane (SiH4, hereinafter abbreviated to silane) is used. In the processing atmos-phere, it reacts with the residual oxygen to form hydrogen and silicon dioxide (SiO2). In this work, specimens with different surface-area-to-volume ratios are built using the PBF-LB process under a silane-doped atmosphere. The study aims to evaluate the effect of the used process atmosphere on the fundamental part properties including the surface roughness, hardness, and microstructure.  2 Materials and Methods 2.1 Experimental Material The investigated powder material was Ti-6Al-4V Grade 12 powder, supplied by ECKART TLS GmbH. It has a specified size range of 20–53 µm. As shown in the SEM (scanning electron microscopy) image in Fig. 1, the powder particles are overall spher-ical with a smooth surface and some satellite particles. These could have a negative impact on powder flowability.   Fig. 1: SEM image of Ti-6Al-4V powder  2.2 Experimental Setup To establish and maintain the silane-doped processing atmosphere, a special machine setup is needed. For this purpose, a specialized laboratory machine was developed and built, which is described in more detail in an earlier publication by the authors [11]. The optical setup inside the machine consists of a continuous wave ytterbium fiber laser (YLR-500-AC by IPG Laser GmbH, Germany; wavelength of 1070 nm) and the scanner system AM-Module Next Gen by Raylase GmbH, Germany. With this, a minimum spot diameter of 38 µm can be obtained. With conventional argon flooding for 30 minutes, a residual oxygen content of 100–200 ppm can be achieved inside the machine’s build chamber. To establish an oxygen-free environment that is adequate to an extreme high vacuum (XHV) an argon-silane mixture with 1 vol-% silane is introduced in a subsequent gas-purging step. During this step, the mixture is further diluted to a mixing ratio of < 1:1000 leading to a residual oxygen content of < 10-14 ppm. For control and maintenance of the required residual oxygen content, the machine is equipped with a zirconium dioxide (lambda probe) oxygen sensor. The residual oxygen content is measured above the processing chamber. A measurement and evaluation sys-tem, that was developed at the Clausthal University of Technology and is based on a commercial lambda sensor measurement system (Mesa Industrie-Elektronik GmbH, Marl, Germany), is employed for this purpose.  2.3 Experimental Design To focus only on the influence of the different SVR and the two different atmospheres, i.e. argon and silane-doped argon, all other process parameters were kept constant. They were based on prior parameter studies by the authors as well as on literature [12]. A laser power of 200 W, a scanning speed of 1100 mm/s, a hatch spacing of 100 µm, and a layer thickness of 30 µm were used. No platform heating was applied. The hatching strategy consisted of bidirectional cross-hatching with a 67° rotation between adjacent layers. The contour was scanned after the infill and skywriting was activated to prevent over-heating at the beginning and end of the scan paths. The specimens were built on top of 3 mm support structures and had the nominal dimensions given in Tab. 1. The SVR was calculated by dividing the surface area by the volume and rounding the value to one decimal place. For each dimension, five specimens were built. The samples were placed on the build platform in a randomized manner to minimize possible position-dependent influences due to gas flow and recoater movement. Tab. 1: Nominal specimen dimensions length in mm width in mm height in mm surface area in mm² volume in mm³ surface-area-to-volume ratio in mm-1 3 4 5 94 60 1,6 2 6 5 104 60 1,7 1 12 5 154 60 2,6  2.4 Methods for Analysis The investigated response variables are the relative density, the top and side surface roughness, and the Vickers hardness. These were chosen because they represent funda-mental properties of the built parts and allow a first assessment of the part quality. After manufacturing and detachment from the build platform, the parts were rinsed with iso-propanol for cleaning. Subsequently, the top and the larger side surface topography was characterized using a confocal laser scanning microscope (VKX 1000 by Keyence). The investigated roughness metric was the arithmetic mean surface roughness Sa that was calculated for an area of (2· 0.8) mm². All specimens were embedded and polished three times parallel to the build direction. This way, three cross-sections of every specimen could be analyzed using light microscopy. On the last cross-section, Vickers hardness measurements (INNOVATEST NEXUS 4000 testing machine, HV0.1) were conducted. On each specimen, five indentations were made starting 0.5 mm above the bottom sur-face and with a 1 mm distance between them.  Finally, the cross-sections were etched using Kroll’s reagent to reveal the as-built mi-crostructure. The microstructure was investigated using the light microscopy function of the confocal laser scanning microscope. JMP® 15 statistical software (SAS Institute Inc.) was used for statistical analysis and graphical presentation of the experimental data.  3 Results 3.1 Roughness In Fig. 2, the top and side surface roughness Sa for the built specimens is shown for the different SVR. The plots are shaded according to the implemented process atmosphere. For the side surface, no significant influence of the atmosphere or the SVR could be observed. The measured side surface roughness values lie between 8 and 12 µm. In contrast, for the top surface, there is a significant difference between the highest SVR of 2.6 mm-1 and the two lower settings. The mean surface roughness for the SVR of 1.6 mm-1 is 7.6 µm, the one of 1.7 mm-1 is 8.8 µm and the one for an SVR of 2.6 mm-1 is 14.8 µm. The significance was confirmed by using the Tukey-Kramer test after en-suring equal variances. Higher SVR or in other words narrower scanning areas lead to shorter scan paths. The critical parts of a scan path, where it comes to excessive spatter formation and protrusions, are the beginning and the end of a scan path. In the case of short scan paths, these critical sections take up a large part of the exposure. This leads to increased top surface roughness. The side surface topography is mainly determined by adhering sintered particles. This phenomenon occurs regardless of the atmosphere or the dimensions of a built part.  Fig. 2: Boxplot of top and side surface roughness Sa in dependence on the processing atmos-phere and surface-area-to-volume ratio  3.2 Relative Density  Fig. 3: Boxplot of mean relative density in dependence on the processing atmosphere and surface-area-to-volume ratio  Regarding the mean relative density, no significant differences between the three SVR and between the different atmospheres could be observed. There is a wider spread of the measured values for the two higher SVR. This might be connected to the higher top surface roughness. In the further course of the process, surface defects due to impaired wetting or adhering spatter particles develop into internal defects, more precisely pores. However, all values are above 99.9 % relative density, and the variations are in the range of measurement uncertainty.  3.3 Hardness The boxplots in Fig. 4 show a slight trend toward higher hardness for increasing SVR. While for the smallest SVR, the mean hardness is 397.5 HV0.1 it is 405.7 HV0.1 for the highest SVR of 2.6 mm-1. However, the differences between the means are far smaller than the variances within the different groups. No statistically significant difference be-tween the atmospheres or the different SVR could be observed. This shows that the observed hardness is either independent of the residual oxygen in the processing atmos-phere in the investigated range (100–200 ppm vs. < 10-14 ppm) or the hardness-reducing effect of the elimination of oxygen is reversed by other mechanisms. One possible other mechanism introducing oxygen to the built parts is the residual moisture in the atmos-phere and the used powder. This moisture can only partly be eliminated by the silane-doped atmosphere. Additionally, in general, additively manufactured Ti-6Al-4V has higher hardness compared to conventionally fabricated parts [13].  Fig. 4: Boxplot of mean hardness HV0.1 in dependence on the processing atmosphere and surface-area-to-volume ratio 3.4 Microstructure The microstructures after etching with Kroll’s reagent for all specimen dimensions and both atmospheres are shown by the light microscopy images in Fig. 5. No difference in the visible microstructure between the two atmospheres is observable. In all cases, the typical Ti-6Al-4V as-built microstructure with fine α’-martensite needles in elongated prior-β grains can be seen. The martensite needles are also shown in more detail in the SEM image in Fig. 6. The prior-β grains are oriented in the build direction (BD) since they grow in the opposite direction of the thermal flow. For the thinnest structures, the grain growth seems to be slightly inclined toward the center of the specimen. This can be explained by the altered heat flow conditions in thin structures. There is less solid material that can promote the heat conduction compared to the thicker structures.   Fig. 5: Light microscopy images of the microstructure after etching with Kroll’s reagent, two prior-β grains marked exemplarily   Fig. 6: SEM image of the microstructure after etching with Kroll’s reagent, revealing the martensite needles  4 Conclusion and Outlook In this work, the laser-based powder bed fusion of Ti-6Al-4V structures with different surface-area-to-volume ratios ranging from 1.6 to 2.6 mm-1 under oxygen-reduced (ar-gon) and technically oxygen-free (silane-doped argon) atmosphere was investigated. The outcomes for the fundamental part properties surface roughness, relative density, hardness, and microstructure were compared. Only for the top surface roughness, there was a significant influence of the SVR with higher SVR leading to a higher top surface roughness Sa. The implemented process atmosphere did not show a significant influence on any of the investigated response variables. High mean relative densities of over 99.9 % were achieved for all settings. A high overall mean hardness of 400.8 HV0.1 was observed which can be attributed to the martensitic as-built microstructure. The results of this study show that a silane-doped atmosphere can be implemented for the PBF-LB process of thin structures without affecting the process and part qualities. However, whether this approach can actually achieve a reduced oxygen content in the built parts must be investigated further. Additionally, the investigated range of SVR was within narrow boundaries and more levels of this factor in combination with variations of the laser processing parameters need to be considered. Future research will therefore include parameter studies as well as the investigation of different scanning area sizes, scanning strategies, and their influence on the impurity pick-up during processing. Acknowledgements Funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) – Project-ID 394563137 – SFB 1368.  References [1] van Bael, S.; Chai, Y.C.; Truscello, S.; Moesen, M.; Kerckhofs, G.; van Ooster-wyck, H.; Kruth, J.-P.; Schrooten, J.: The effect of pore geometry on the in vitro biological behavior of human periosteum-derived cells seeded on selective laser-melted Ti6Al4V bone scaffolds. Acta Biomaterialia (2012) 8, pp. 2824–2834. [2] Sélo, R.R.; Catchpole-Smith, S.; Maskery, I.; Ashcroft, I.; Tuck, C.: On the ther-mal conductivity of AlSi10Mg and lattice structures made by laser powder bed fusion. Additive Manufacturing (2020) 34, 101214. [3] Tan, C.; Li, S.; Essa, K.; Jamshidi, P.; Zhou, K.; Ma, W.; Attallah, M.M.: Laser Powder Bed Fusion of Ti-rich TiNi lattice structures: Process optimisation, geo-metrical integrity, and phase transformations. International Journal of Machine Tools and Manufacture (2019) 141, pp. 19–29. [4] Murray, J.L.; Wriedt, H.A.: The O−Ti (Oxygen-Titanium) system. JPE (1987) 8, pp. 148–165. [5] Fang, Z.Z.; Paramore, J.D.; Sun, P.; Chandran, K.S.R.; Zhang, Y.; Xia, Y.; Cao, F.; Koopman, M.; Free, M.: Powder metallurgy of titanium – past, present, and future. International Materials Reviews (2018) 63, pp. 407–459. [6] O'Leary, R.; Setchi, R.; Prickett, P.; Hankins, G.: An Investigation into the Recy-cling of Ti-6Al-4V Powder Used Within SLM to Improve Sustainability. In Im-pact: The Journal of Innovation Impact (2016) 8, p. 377. [7] Williams, R.; Bilton, M.; Harrison, N.; Fox, P.: The impact of oxidised powder particles on the microstructure and mechanical properties of Ti-6Al-4V processed by laser powder bed fusion. Additive Manufacturing (2021) 46, 102181. [8] Dietrich, K.; Diller, J.; Dubiez-Le Goff, S.; Bauer, D.; Forêt, P.; Witt, G.: The in-fluence of oxygen on the chemical composition and mechanical properties of Ti-6Al-4V during laser powder bed fusion (L-PBF). Additive Manufacturing (2020) 32, 100980. [9] Pauzon, C.; Dietrich, K.; Forêt, P.; Dubiez-Le Goff, S.; Hryha, E.; Witt, G.: Con-trol of residual oxygen of the process atmosphere during laser-powder bed fusion processing of Ti-6Al-4V. Additive Manufacturing (2021) 38, 101765. [10] Di Wang; Wu, S.; Fu, F.; Mai, S.; Yang, Y.; Liu, Y.; Song, C.: Mechanisms and characteristics of spatter generation in SLM processing and its effect on the prop-erties. Materials & Design (2017) 117, pp. 121–130. [11] Emminghaus, N.; Fritsch, S.; Büttner, H.; August, J.; Tegtmeier, M.; Huse, M.; Lammers, M.; Hoff, C.; Hermsdorf, J.; Kaierle, S.: PBF-LB/M process under a silane-doped argon atmosphere: Preliminary studies and development of an inno-vative machine concept. Advances in Industrial and Manufacturing Engineering (2021) 2, 100040. [12] Promoppatum, P.; Onler, R.; Yao, S. C.: Numerical and experimental investiga-tions of micro and macro characteristics of direct metal laser sintered Ti-6Al-4V products. Journal of Materials Processing Technology (2017) 240, pp. 262-273. [13] Oliveira Campos, F. de; Araujo, A.C.; Jardini Munhoz, A.L.; Kapoor, S.G.: The influence of additive manufacturing on the micromilling machinability of Ti6Al4V: A comparison of SLM and commercial workpieces. Journal of Manu-facturing Processes (2020) 60, pp. 299–307.  Author address M. Sc. Nicole Emminghaus Laser Zentrum Hannover e. V. Hollerithallee 8 30129 Hannover  Telefon: +49 511 2788-355 Telefax: +49 511 2788-100 E-Mail: info@lzh.de  

Laser-based powder bed fusion of Ti-6Al-4V structures with different surface-area-to-volume ratios in oxygen-reduced and oxygen-free environment

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Laser-based powder bed fusion of Ti-6Al-4V structures with different surface-area-to-volume ratios in oxygen-reduced and oxygen-free environment

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