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Oxide and spatter powder formation during laser powder bed fusion of Hastelloy X
Powder Technology 354 (2019) 333–337
Contents lists available at ScienceDirect
Powder Technology journal homepage: www.elsevier.com/locate/powtec
Short Communication
Oxide and spatter powder formation during laser powder bed fusion of Hastelloy X A.N.D. Gasper a, D. Hickman a, I. Ashcroft a, S. Sharma c, X. Wang d, B. Szost d, D. Johns d, A.T. Clare a,b,⁎ a
Centre for Additive Manufacturing, The University of Nottingham, NG7 2RD, UK Department of Mechanical, Materials and Manufacturing Engineering, Faculty of Science and Engineering, University of Nottingham China, 199 Taikang East Road, University Park, Ningbo 315100, China c Oerlikon AM US Inc., 2200 Interstate N. Dr, 28206, Charlotte, NC, United States d Oerlikon AM GmbH, AM Munich Research Institute GmbH, Kapellenstrasse 12, 856222, Feldkirchen, Germany b
a r t i c l e
i n f o
Article history: Received 18 February 2019 Received in revised form 31 May 2019 Accepted 7 June 2019 Available online 12 June 2019 Keywords: Laser powder bed fusion Spatter Hastelloy X Oxides Recyclability Additive manufacturing
a b s t r a c t Improvement to the integrity of laser powder bed fusion (PBF-LB) processed Nickel-base superalloys is crucial for their wider adoption in industrial aerospace applications. The nature of PBF-LB, essentially an extended powder micro-welding process, allows opportunity for material contamination and oxidisation. This study reports how oxide formation occurs in Hastelloy X corresponding to ASTM composition, and in particular, in the material ejected from the process zone during fabrication (spatter). It is shown that oxidation occurs despite processing in an Ar atmosphere with b0.2% O2, and moreover, this can result in oxide inclusions in built parts. The spatter particles are also associated with part defects. Depending on the sieve size used, 30–60% of oxidised spatter particles are within the size range to be recycled and reprocessed in subsequent uses. This impacts the processing of powder in PBF-LB, powder quality in use in the technology, and powder manufacture in the industry. © 2019 Elsevier B.V. All rights reserved.
1. Introduction In laser powder bed fusion (PBF-LB), a laser beam with a spot size of ~50–200 μm is rastered to selectively melt areas of a powder bed in a layer-by-layer method. PBF-LB has a discrepancy of time scales; the melting of the powder is a micro-welding process which occurs over the nano to micro-scale time range, whilst a typical part build will take hours to days [1]. The micro-welds of the feedstock powder combine to produce a full part, but allow for potential inconsistencies and defects to occur. One potential source of defects is oxide formation, which occurs in the nano to micro-scale time range and can result in inclusions in the part, as demonstrated by Thijs et al. [2], and in spatter, by Simonelli et al. [3]. Spatter particles are a by-product of the high energy density laser-powder bed interactions, in which material is ejected from the process zone. Anwar & Pham [4] and Ladewig et al. [5] showed that many spatter particles are redistributed back into the powder bed where they have the potential to cause defects. They have also been identified as potential causes of porosity, lack of fusion, and inclusion defects by Tang & Pistorius [6] and Andani et al. [7], but the full effect of this phenomenon on macro-scale properties remains unknown. It may be postulated, however, that these defects may act as stress ⁎ Corresponding author at: Centre for Additive Manufacturing, The University of Nottingham, NG7 2RD, UK E-mail address: [email protected] (A.T. Clare).
https://doi.org/10.1016/j.powtec.2019.06.004 0032-5910/© 2019 Elsevier B.V. All rights reserved.
concentrators, and particularly as crack initiation/propagation locations for fatigue, and therefore need to be controlled for parts to be used in aerospace applications [8,9]. Our previous work, on another aerospace alloy, Inconel 718 [10], demonstrated that aluminium and titanium oxide formation occurs in the process zone and such oxides are seen in the spatter particles. In the melt pool, these oxides tend to float and remain at the surface and are then seen within parts as inclusions concentrated at layer interfaces. Previous investigations by Tomus et al. [11,12] and Harrison et al. [13] have showed the level of microalloying elements of Hastelloy X can have significant effects on cracking and the quality of parts produced. In this work, the processing and oxidation of Hastelloy X, including the both spatter particles and built parts is investigated, in order to better understand the role of alloying elements in oxidation and spatter formation and, more generally, to highlight the quality control requirements required to enable such alloys to be used in aerospace engineering applications. 2. Materials & methods A ReaLizer SLM50 PBF-LB system was used to manufacture samples, using parameters developed to produce specimen with N99% density, including a laser power of 100 W, 395 mm.s−1 scan speed, 60 μm hatch distance, 200 °C substrate temperature, and a 40 μm layer thickness. The system operates in an inert environment, with an oxygen level b 0.2% and a continuous flow of Ar across the powder bed. A
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conventional 15–45 μm gas-atomised Hastelloy X powder with nominal composition in line with AMS5390 nominal ranges for the alloy, composition in Table 1, was provided by Oerlikon AM. The powder was dried in an oven for 3 h at 125 °C before processing to remove any moisture. Samples of ‘pure’ spatter powder were collected near the gas outlet in the build chamber as in previous studies [10]. In these areas there is no powder prior to processing and during powder recoating, therefore, the powder present after processing is a by-product of processing. The spatter was collected from builds of 10x10x10mm cubes, built on supports with no post-processing, and the surfaces of these cubes were analysed. A single track, 10 mm in length and 10 layers high, was processed to assess if the oxide formation initiates in the melt pool during processing. Backscattered-electron scanning electron microscopy (BSE-SEM) and energy dispersive X-ray spectroscopy (EDS) were used to investigate the formation of oxides on the collected spatter, single tracks, and cube surfaces, compared to the virgin material. A combination of image processing software FUJI/ImageJ and MATLAB was used to analyse multiple powder samples and to stitch and analyse multiple part surface BSE-SEM micrographs to provide quantitative information on oxidation [14,15]. A Kratos AXIS ULTRA DLD X-ray Photoelectron Spectrometer (XPS) was used to analyse the surface chemistry of the powders. 3. Results & discussion Fig. 1a&b show that spatter particles produced during the PBF-LB processing were dissimilar from the virgin powder particles. They differed in size, shape, morphology, and the presence of oxides. Image analysis was used to characterise the particles by their size as a proportion of all spatter particles, for various common sieving sizes used for PBF-LB powders in industry. They were characterised in terms of their minimum particle width, corresponding to passing through an equivalent sized mesh, and for the particles' circular equivalent diameter, a parameter calculated in laser diffraction measurements and commonly used in industry. The circular equivalent is defined as the diameter of a circle with the equivalent area to the measured particle. The results, shown in Fig. 1d, indicate that, based on the minimum width of particles, 51.7% of particles are larger than the virgin powder specification. It can be seen in Fig. 1b that the spatter includes more agglomerates and irregular particles, which contributes to the differences between the minimum particle width and approximated circular equivalent values. Finally, the surface of many particles were visually different to virgin powder due to the formation of oxides. The BSE-SEM images showed differing chemical compositions represented by different contrasts, the dark patches on the particles representing oxides. Image analysis approximated that these oxidised particles comprised 7.4% of the spatter particles. Fig. 1b contains a higher proportion of oxidised particles than average, 24.8%, but is useful in demonstrating the range of oxidised particles and morphologies produced and the difference to the virgin material. The oxides present on the spatter particles seen in Fig. 1b were Cr and Si oxides, as shown in Fig. 2b&d. EDS quantification indicated the oxides were likely dominated by Cr2O3 and SiO2. A small proportion of the oxidised particles, 10.5% (or 0.8% of the total spatter population), displayed characteristic Al2O3 oxide spots, as seen in Fig. 2a&c, as also seen in the processing of Inconel 718 [10]. XPS analysis of the surface chemistry is presented in Fig. 1c and shows a 5.3% increase of Si in the spatter particle compared to virgin powder, and a 39.7% increase in Cr, due to the oxides present on the surface.
Oxides were also present on the top surface of the Hastelloy X cubes, as can be seen in Fig. 3a. Image analysis indicated that oxides comprised 2.8% of the cube surface. The large oxide formation in Fig. 3c-e was 207 μm long and 47 μm wide. EDS mapping in Fig. 3e shows that this is mainly Cr and Si oxides with some smaller Al oxides. The cube surfaces showed a range of oxide streaks, comprised of Cr and Si, and oxide spots comprising of Al and Si, with the largest Al oxides present on the surface being up to 29 μm in diameter. In the single tracks, as shown in Fig. 3f-h, Al and Si oxides were evident but there was no clear indication of Cr oxides. In this case oxides appear as spots, with the largest spot on the 10 mm track being 19 μm in diameter. Unlike the cubes, oxide streaks and large formations were not observed. For Inconel 718 [10], oxide formation was established to occur at the melt pool from oxygen within the chamber and/or the powder. Only Al and Ti oxides were present and remained at the surface of the part and weld tracks. These were mixed into the part during solidification due to Marangoni effects, or ejected as spatter where they then remained on the surface of the spatter particle. It should be noted that such spatter particles could then be incorporated back into the part if they land on the powder bed. For this Hastelloy X alloy, the oxidation witnessed on the single tracks, cube surfaces, and spatter particles had separate characteristics. The single track showed Al and Si oxide spots along its length indicating, again, that oxidation initiates at the melt pool level. The larger oxide formations on the cube surface, with larger Si and Cr oxides formed as a slag, indicate the initial oxides may act as nucleation for larger oxides to develop over many rasters and layers, as witnessed in Fig. 3d. The presence of Al and Si oxides on the single track, and lack of Cr oxides, is likely due to the higher affinity of Al and then Si for oxygen, as indicated in the Ellingham diagram [16]. The difference between the oxide species on the single track and cube surface may be due to lower Al and Si concentrations in the alloy, 0.1 and 0.5 wt% respectively, and higher affinity for oxygen meaning they are depleted preferentially, followed by the development of more Cr rich oxides. The Inconel 718 studied previously contained 0.44 and 0.9 wt% Al and Ti respectively, principal oxides formed being Al2O3, and TiO2 secondarily [10]. With Hastelloy X, the 0.1 wt% Al oxidised preferentially at the melt pool surface, followed by the 0.5 wt% Si, with the Cr, at 21.5 wt% eventually dominating oxide formation. The allowable Si content according to the ASTM standard, as shown in Table 1, is twice what is present in the alloy used in this study [17]. This shows that even small concentrations of elements with high oxidation potential can play an important role and will oxidise in O2 concentrations b0.2% during PBF-LB processing, and previous investigations have shown oxide formation in systems with O2 concentrations b0.1% [10]. Therefore, tighter tolerance is required for these elements. Oxide formation in the spatter particles appears to be of a different nature to that observed in the melt pool and on the surface of parts. There were some oxides on the cube surface that appear similar to those from the spatter particle, however, these are likely attributable to spatter particle inclusion, as evidenced by Fig. 3a inset. The spatter particles are covered by unique oxide formations that appear different to those generally observed on the tracks and cubes. Oxides formed on the tracks and cube tend to appear in isolation, in the form of spots or slag, or occasionally as a streak through the material, with a small amount of mixing, but are mostly separate from the bulk matrix material. The oxide formation on the spatter particles, however, appears to form in-situ with the bulk matrix of the particle, as evidenced by the inset in Fig. 1b and the particles in Fig. 2d-f, and there seems to be a complex mechanism in the formation of the particle surface. The
Table 1 Nominal range for composition for Hastelloy X (UNS N06002) according to AMS5390 [14] and the composition of the powder used. Ni
Fe
Cr
Co
Mo
W
C
Si
Mn
Al
Ti
O
Bal. Bal.
17.0–20.0 18.2
20.5–23.0 21.5
0.5–2.5 2.1
8.0–10.0 9.4
0.2–1.0 0.9
0.10 max 0.04
1.00 max 0.5
1.00 max 0.1
– 0.1
– 0.1
– 0.02
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Fig. 1. BSE-SEM micrograph of a) virgin gas-atomised Hastelloy X particles; b) Hastelloy X spatter particles with dark contrast oxides; c) XPS surface chemistry compositions of powders for O, Cr, and Si, the remainder of elements detected were C, N, Ni, and Fe; d) summary of image analysis results of the spatter particles collected, characterised by proportion of all spatter particles, for common sieving sizes for the recycling of PBF-LB powders.
oxide formations in the spatter are of a wide variety of shape, size, structure, and covering, which may be due to the unique thermal history of each particle, and potentially, factors such as inhomogeneous chemical composition in the melt pool [18]. The difference between the spatter and melt pool oxides could be due to the higher cooling rate of the
spatter particles from their lower mass and high surface area, leading to the formation of more crystalline and faceted features, and interaction with more O2 as it travels through the chamber. As they mostly form as single spherical particles larger than the input powder particles, these are most likely produced via melt ejection [10], with the elements
Fig. 2. Oxide formation on Hastelloy X spatter particle surface: a&c) BSE-SEM micrograph and EDS map showing oxide spots with concentration of Al and O in areas of dark contrast; b&d) BSE-SEM micrograph and EDS map of particle with characteristic oxide spots showing concentration of Cr, Si, and O; e-g) particles depicting some of the range of different with Si and Cr oxide formations.
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Fig. 3. BSE-SEM micrograph of Hastelloy X surfaces with: a) highlighting types of oxides present on the surface and b) showing the distribution of the oxides over the surface and the image analysis used to assess this – artefacts from particles fused to the surface, circled in red, were removed from the image analysis; c,d) detail of large oxide formation; e) EDS map of (d) identifying oxide regions; f,g) surface of single melt track; and h) EDS map of (g) identifying oxide formations. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
with high oxygen affinity bonding with oxygen present in the chamber atmosphere and shielding gas. However, further investigation is required to fully understand the mechanisms of oxide formation in spatter, particularly the conditions which allow the variety of structures produced that are so different to those formed in the melt pool. Previous investigations into PBF-LB processing of Hastelloy X by Tomus et al. [11,12] showed that the levels of Si and Mn contributed to cracking in the parts produced due to micro-segregation of the elements from the Nickel γ matrix. Harrison et al. [13], working with LPW Technology, were able to reduce the level of cracking by reducing the content of these elements and tailoring the composition via thermal dynamic simulations. The alloys used in those studies, as that presented here, are within the ASTM standards for the alloy. This demonstrates the need for bespoke alloy design tailored for PBF-LB, with reduced Si and Mn content for Hastelloy X, and tighter tolerance for the allowable composition range of elements. Powder manufacturers, such as Oerlikon AM and LPW Technology, are now providing Hastelloy X and other alloys tailored and optimised for the PBF-LB process to address these issues. Oxidation during processing presents an issue for the AM industry. Fig. 1d shows that, depending on the sieve specification, between 30 and 60% of the oxidised spatter particles will remain in the recycled powder after sieving. Spatter particles have also been observed landing on the powder bed during processing indicating that even if only virgin powder is used spatter may still be incorporated into parts. There is a drive by the industry for tight tolerance and control in powder specifications and quality [19,20]. This is leading to the use of even more advanced powder manufacturing methods such as vacuum and electrode induction gas-atomisation (EIGA and VIGA), and plasma rotating electrode process (PREP) which are more expensive than the conventional gas-atomisation process most commonly used in AM, which is
still prohibitively expensive for many applications [21]. These processes are being used for the most critical applications and reactive alloys, mainly Ti based alloys and Nickel-base superalloys, such as Hastelloy X. However, melt ejection spatter is a gas-atomisation process within the PBF-LB process, generating a secondary powder system, which we have established is dissimilar to the virgin powder and may enter recycled feedstock or land on the powder bed during processing. These more advanced and expensive powder-manufacturing processes are, hence, possibly made redundant if the PBF-LB processing is not to the same standards or O2 control. If we are to control the powders used in the system, then understanding the spatter powder generated for each material and the effect of micro-alloying elements is crucial to help design bespoke PBF-LB alloys. Acknowledgements This work was funded through the EPSRC (Engineering and Physical Sciences Research Council, UK) Centre for Doctoral Training in Additive Manufacturing and 3D Printing (EP/L01534X/1) and supported by Oerlikon AM. References [1] W.E. King, A.T. Anderson, R.M. Ferencz, N.E. Hodge, C. Kamath, S.A. Khairallah, A.M. Rubenchik, Laser powder bed fusion additive manufacturing of metals; physics, computational, and materials challenges, Appl. Phys. Rev. 2 (2015), 041304. https://doi.org/10.1063/1.4937809. [2] L. Thijs, J. Van Humbeeck, K. Kempen, E. Yasa, J. Kruth, M. Rombouts, Investigation on the inclusions in maraging steel produced by selective laser melting, Innov. Dev. Virtual Phys. Prototyp. (2011) 297–304, https://doi.org/10.1201/b11341-48. [3] M. Simonelli, C. Tuck, N.T. Aboulkhair, I. Maskery, I. Ashcroft, R.D. Wildman, R. Hague, A study on the laser spatter and the oxidation reactions during selective
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Contents lists available at ScienceDirect
Powder Technology journal homepage: www.elsevier.com/locate/powtec
Short Communication
Oxide and spatter powder formation during laser powder bed fusion of Hastelloy X A.N.D. Gasper a, D. Hickman a, I. Ashcroft a, S. Sharma c, X. Wang d, B. Szost d, D. Johns d, A.T. Clare a,b,⁎ a
Centre for Additive Manufacturing, The University of Nottingham, NG7 2RD, UK Department of Mechanical, Materials and Manufacturing Engineering, Faculty of Science and Engineering, University of Nottingham China, 199 Taikang East Road, University Park, Ningbo 315100, China c Oerlikon AM US Inc., 2200 Interstate N. Dr, 28206, Charlotte, NC, United States d Oerlikon AM GmbH, AM Munich Research Institute GmbH, Kapellenstrasse 12, 856222, Feldkirchen, Germany b
a r t i c l e
i n f o
Article history: Received 18 February 2019 Received in revised form 31 May 2019 Accepted 7 June 2019 Available online 12 June 2019 Keywords: Laser powder bed fusion Spatter Hastelloy X Oxides Recyclability Additive manufacturing
a b s t r a c t Improvement to the integrity of laser powder bed fusion (PBF-LB) processed Nickel-base superalloys is crucial for their wider adoption in industrial aerospace applications. The nature of PBF-LB, essentially an extended powder micro-welding process, allows opportunity for material contamination and oxidisation. This study reports how oxide formation occurs in Hastelloy X corresponding to ASTM composition, and in particular, in the material ejected from the process zone during fabrication (spatter). It is shown that oxidation occurs despite processing in an Ar atmosphere with b0.2% O2, and moreover, this can result in oxide inclusions in built parts. The spatter particles are also associated with part defects. Depending on the sieve size used, 30–60% of oxidised spatter particles are within the size range to be recycled and reprocessed in subsequent uses. This impacts the processing of powder in PBF-LB, powder quality in use in the technology, and powder manufacture in the industry. © 2019 Elsevier B.V. All rights reserved.
1. Introduction In laser powder bed fusion (PBF-LB), a laser beam with a spot size of ~50–200 μm is rastered to selectively melt areas of a powder bed in a layer-by-layer method. PBF-LB has a discrepancy of time scales; the melting of the powder is a micro-welding process which occurs over the nano to micro-scale time range, whilst a typical part build will take hours to days [1]. The micro-welds of the feedstock powder combine to produce a full part, but allow for potential inconsistencies and defects to occur. One potential source of defects is oxide formation, which occurs in the nano to micro-scale time range and can result in inclusions in the part, as demonstrated by Thijs et al. [2], and in spatter, by Simonelli et al. [3]. Spatter particles are a by-product of the high energy density laser-powder bed interactions, in which material is ejected from the process zone. Anwar & Pham [4] and Ladewig et al. [5] showed that many spatter particles are redistributed back into the powder bed where they have the potential to cause defects. They have also been identified as potential causes of porosity, lack of fusion, and inclusion defects by Tang & Pistorius [6] and Andani et al. [7], but the full effect of this phenomenon on macro-scale properties remains unknown. It may be postulated, however, that these defects may act as stress ⁎ Corresponding author at: Centre for Additive Manufacturing, The University of Nottingham, NG7 2RD, UK E-mail address: [email protected] (A.T. Clare).
https://doi.org/10.1016/j.powtec.2019.06.004 0032-5910/© 2019 Elsevier B.V. All rights reserved.
concentrators, and particularly as crack initiation/propagation locations for fatigue, and therefore need to be controlled for parts to be used in aerospace applications [8,9]. Our previous work, on another aerospace alloy, Inconel 718 [10], demonstrated that aluminium and titanium oxide formation occurs in the process zone and such oxides are seen in the spatter particles. In the melt pool, these oxides tend to float and remain at the surface and are then seen within parts as inclusions concentrated at layer interfaces. Previous investigations by Tomus et al. [11,12] and Harrison et al. [13] have showed the level of microalloying elements of Hastelloy X can have significant effects on cracking and the quality of parts produced. In this work, the processing and oxidation of Hastelloy X, including the both spatter particles and built parts is investigated, in order to better understand the role of alloying elements in oxidation and spatter formation and, more generally, to highlight the quality control requirements required to enable such alloys to be used in aerospace engineering applications. 2. Materials & methods A ReaLizer SLM50 PBF-LB system was used to manufacture samples, using parameters developed to produce specimen with N99% density, including a laser power of 100 W, 395 mm.s−1 scan speed, 60 μm hatch distance, 200 °C substrate temperature, and a 40 μm layer thickness. The system operates in an inert environment, with an oxygen level b 0.2% and a continuous flow of Ar across the powder bed. A
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conventional 15–45 μm gas-atomised Hastelloy X powder with nominal composition in line with AMS5390 nominal ranges for the alloy, composition in Table 1, was provided by Oerlikon AM. The powder was dried in an oven for 3 h at 125 °C before processing to remove any moisture. Samples of ‘pure’ spatter powder were collected near the gas outlet in the build chamber as in previous studies [10]. In these areas there is no powder prior to processing and during powder recoating, therefore, the powder present after processing is a by-product of processing. The spatter was collected from builds of 10x10x10mm cubes, built on supports with no post-processing, and the surfaces of these cubes were analysed. A single track, 10 mm in length and 10 layers high, was processed to assess if the oxide formation initiates in the melt pool during processing. Backscattered-electron scanning electron microscopy (BSE-SEM) and energy dispersive X-ray spectroscopy (EDS) were used to investigate the formation of oxides on the collected spatter, single tracks, and cube surfaces, compared to the virgin material. A combination of image processing software FUJI/ImageJ and MATLAB was used to analyse multiple powder samples and to stitch and analyse multiple part surface BSE-SEM micrographs to provide quantitative information on oxidation [14,15]. A Kratos AXIS ULTRA DLD X-ray Photoelectron Spectrometer (XPS) was used to analyse the surface chemistry of the powders. 3. Results & discussion Fig. 1a&b show that spatter particles produced during the PBF-LB processing were dissimilar from the virgin powder particles. They differed in size, shape, morphology, and the presence of oxides. Image analysis was used to characterise the particles by their size as a proportion of all spatter particles, for various common sieving sizes used for PBF-LB powders in industry. They were characterised in terms of their minimum particle width, corresponding to passing through an equivalent sized mesh, and for the particles' circular equivalent diameter, a parameter calculated in laser diffraction measurements and commonly used in industry. The circular equivalent is defined as the diameter of a circle with the equivalent area to the measured particle. The results, shown in Fig. 1d, indicate that, based on the minimum width of particles, 51.7% of particles are larger than the virgin powder specification. It can be seen in Fig. 1b that the spatter includes more agglomerates and irregular particles, which contributes to the differences between the minimum particle width and approximated circular equivalent values. Finally, the surface of many particles were visually different to virgin powder due to the formation of oxides. The BSE-SEM images showed differing chemical compositions represented by different contrasts, the dark patches on the particles representing oxides. Image analysis approximated that these oxidised particles comprised 7.4% of the spatter particles. Fig. 1b contains a higher proportion of oxidised particles than average, 24.8%, but is useful in demonstrating the range of oxidised particles and morphologies produced and the difference to the virgin material. The oxides present on the spatter particles seen in Fig. 1b were Cr and Si oxides, as shown in Fig. 2b&d. EDS quantification indicated the oxides were likely dominated by Cr2O3 and SiO2. A small proportion of the oxidised particles, 10.5% (or 0.8% of the total spatter population), displayed characteristic Al2O3 oxide spots, as seen in Fig. 2a&c, as also seen in the processing of Inconel 718 [10]. XPS analysis of the surface chemistry is presented in Fig. 1c and shows a 5.3% increase of Si in the spatter particle compared to virgin powder, and a 39.7% increase in Cr, due to the oxides present on the surface.
Oxides were also present on the top surface of the Hastelloy X cubes, as can be seen in Fig. 3a. Image analysis indicated that oxides comprised 2.8% of the cube surface. The large oxide formation in Fig. 3c-e was 207 μm long and 47 μm wide. EDS mapping in Fig. 3e shows that this is mainly Cr and Si oxides with some smaller Al oxides. The cube surfaces showed a range of oxide streaks, comprised of Cr and Si, and oxide spots comprising of Al and Si, with the largest Al oxides present on the surface being up to 29 μm in diameter. In the single tracks, as shown in Fig. 3f-h, Al and Si oxides were evident but there was no clear indication of Cr oxides. In this case oxides appear as spots, with the largest spot on the 10 mm track being 19 μm in diameter. Unlike the cubes, oxide streaks and large formations were not observed. For Inconel 718 [10], oxide formation was established to occur at the melt pool from oxygen within the chamber and/or the powder. Only Al and Ti oxides were present and remained at the surface of the part and weld tracks. These were mixed into the part during solidification due to Marangoni effects, or ejected as spatter where they then remained on the surface of the spatter particle. It should be noted that such spatter particles could then be incorporated back into the part if they land on the powder bed. For this Hastelloy X alloy, the oxidation witnessed on the single tracks, cube surfaces, and spatter particles had separate characteristics. The single track showed Al and Si oxide spots along its length indicating, again, that oxidation initiates at the melt pool level. The larger oxide formations on the cube surface, with larger Si and Cr oxides formed as a slag, indicate the initial oxides may act as nucleation for larger oxides to develop over many rasters and layers, as witnessed in Fig. 3d. The presence of Al and Si oxides on the single track, and lack of Cr oxides, is likely due to the higher affinity of Al and then Si for oxygen, as indicated in the Ellingham diagram [16]. The difference between the oxide species on the single track and cube surface may be due to lower Al and Si concentrations in the alloy, 0.1 and 0.5 wt% respectively, and higher affinity for oxygen meaning they are depleted preferentially, followed by the development of more Cr rich oxides. The Inconel 718 studied previously contained 0.44 and 0.9 wt% Al and Ti respectively, principal oxides formed being Al2O3, and TiO2 secondarily [10]. With Hastelloy X, the 0.1 wt% Al oxidised preferentially at the melt pool surface, followed by the 0.5 wt% Si, with the Cr, at 21.5 wt% eventually dominating oxide formation. The allowable Si content according to the ASTM standard, as shown in Table 1, is twice what is present in the alloy used in this study [17]. This shows that even small concentrations of elements with high oxidation potential can play an important role and will oxidise in O2 concentrations b0.2% during PBF-LB processing, and previous investigations have shown oxide formation in systems with O2 concentrations b0.1% [10]. Therefore, tighter tolerance is required for these elements. Oxide formation in the spatter particles appears to be of a different nature to that observed in the melt pool and on the surface of parts. There were some oxides on the cube surface that appear similar to those from the spatter particle, however, these are likely attributable to spatter particle inclusion, as evidenced by Fig. 3a inset. The spatter particles are covered by unique oxide formations that appear different to those generally observed on the tracks and cubes. Oxides formed on the tracks and cube tend to appear in isolation, in the form of spots or slag, or occasionally as a streak through the material, with a small amount of mixing, but are mostly separate from the bulk matrix material. The oxide formation on the spatter particles, however, appears to form in-situ with the bulk matrix of the particle, as evidenced by the inset in Fig. 1b and the particles in Fig. 2d-f, and there seems to be a complex mechanism in the formation of the particle surface. The
Table 1 Nominal range for composition for Hastelloy X (UNS N06002) according to AMS5390 [14] and the composition of the powder used. Ni
Fe
Cr
Co
Mo
W
C
Si
Mn
Al
Ti
O
Bal. Bal.
17.0–20.0 18.2
20.5–23.0 21.5
0.5–2.5 2.1
8.0–10.0 9.4
0.2–1.0 0.9
0.10 max 0.04
1.00 max 0.5
1.00 max 0.1
– 0.1
– 0.1
– 0.02
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Fig. 1. BSE-SEM micrograph of a) virgin gas-atomised Hastelloy X particles; b) Hastelloy X spatter particles with dark contrast oxides; c) XPS surface chemistry compositions of powders for O, Cr, and Si, the remainder of elements detected were C, N, Ni, and Fe; d) summary of image analysis results of the spatter particles collected, characterised by proportion of all spatter particles, for common sieving sizes for the recycling of PBF-LB powders.
oxide formations in the spatter are of a wide variety of shape, size, structure, and covering, which may be due to the unique thermal history of each particle, and potentially, factors such as inhomogeneous chemical composition in the melt pool [18]. The difference between the spatter and melt pool oxides could be due to the higher cooling rate of the
spatter particles from their lower mass and high surface area, leading to the formation of more crystalline and faceted features, and interaction with more O2 as it travels through the chamber. As they mostly form as single spherical particles larger than the input powder particles, these are most likely produced via melt ejection [10], with the elements
Fig. 2. Oxide formation on Hastelloy X spatter particle surface: a&c) BSE-SEM micrograph and EDS map showing oxide spots with concentration of Al and O in areas of dark contrast; b&d) BSE-SEM micrograph and EDS map of particle with characteristic oxide spots showing concentration of Cr, Si, and O; e-g) particles depicting some of the range of different with Si and Cr oxide formations.
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Fig. 3. BSE-SEM micrograph of Hastelloy X surfaces with: a) highlighting types of oxides present on the surface and b) showing the distribution of the oxides over the surface and the image analysis used to assess this – artefacts from particles fused to the surface, circled in red, were removed from the image analysis; c,d) detail of large oxide formation; e) EDS map of (d) identifying oxide regions; f,g) surface of single melt track; and h) EDS map of (g) identifying oxide formations. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
with high oxygen affinity bonding with oxygen present in the chamber atmosphere and shielding gas. However, further investigation is required to fully understand the mechanisms of oxide formation in spatter, particularly the conditions which allow the variety of structures produced that are so different to those formed in the melt pool. Previous investigations into PBF-LB processing of Hastelloy X by Tomus et al. [11,12] showed that the levels of Si and Mn contributed to cracking in the parts produced due to micro-segregation of the elements from the Nickel γ matrix. Harrison et al. [13], working with LPW Technology, were able to reduce the level of cracking by reducing the content of these elements and tailoring the composition via thermal dynamic simulations. The alloys used in those studies, as that presented here, are within the ASTM standards for the alloy. This demonstrates the need for bespoke alloy design tailored for PBF-LB, with reduced Si and Mn content for Hastelloy X, and tighter tolerance for the allowable composition range of elements. Powder manufacturers, such as Oerlikon AM and LPW Technology, are now providing Hastelloy X and other alloys tailored and optimised for the PBF-LB process to address these issues. Oxidation during processing presents an issue for the AM industry. Fig. 1d shows that, depending on the sieve specification, between 30 and 60% of the oxidised spatter particles will remain in the recycled powder after sieving. Spatter particles have also been observed landing on the powder bed during processing indicating that even if only virgin powder is used spatter may still be incorporated into parts. There is a drive by the industry for tight tolerance and control in powder specifications and quality [19,20]. This is leading to the use of even more advanced powder manufacturing methods such as vacuum and electrode induction gas-atomisation (EIGA and VIGA), and plasma rotating electrode process (PREP) which are more expensive than the conventional gas-atomisation process most commonly used in AM, which is
still prohibitively expensive for many applications [21]. These processes are being used for the most critical applications and reactive alloys, mainly Ti based alloys and Nickel-base superalloys, such as Hastelloy X. However, melt ejection spatter is a gas-atomisation process within the PBF-LB process, generating a secondary powder system, which we have established is dissimilar to the virgin powder and may enter recycled feedstock or land on the powder bed during processing. These more advanced and expensive powder-manufacturing processes are, hence, possibly made redundant if the PBF-LB processing is not to the same standards or O2 control. If we are to control the powders used in the system, then understanding the spatter powder generated for each material and the effect of micro-alloying elements is crucial to help design bespoke PBF-LB alloys. Acknowledgements This work was funded through the EPSRC (Engineering and Physical Sciences Research Council, UK) Centre for Doctoral Training in Additive Manufacturing and 3D Printing (EP/L01534X/1) and supported by Oerlikon AM. References [1] W.E. King, A.T. Anderson, R.M. Ferencz, N.E. Hodge, C. Kamath, S.A. Khairallah, A.M. Rubenchik, Laser powder bed fusion additive manufacturing of metals; physics, computational, and materials challenges, Appl. Phys. Rev. 2 (2015), 041304. https://doi.org/10.1063/1.4937809. [2] L. Thijs, J. Van Humbeeck, K. Kempen, E. Yasa, J. Kruth, M. Rombouts, Investigation on the inclusions in maraging steel produced by selective laser melting, Innov. Dev. Virtual Phys. Prototyp. (2011) 297–304, https://doi.org/10.1201/b11341-48. [3] M. Simonelli, C. Tuck, N.T. Aboulkhair, I. Maskery, I. Ashcroft, R.D. Wildman, R. Hague, A study on the laser spatter and the oxidation reactions during selective
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