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Fig. 18. Effect of heat treatment on the CV in the mechanical properties. The main hypothesis in this study was that standardized heat treatments can be used to reduce the variability in mechanical properties of AM 316L. Fig. 18 shows the coefficient of variation (CV) within each heat treatment condition calculated for all the studied mechanical properties. HIP significantly reduced the CV between the conditions for all other properties except elongation. SA reduced CV in R,, and density, but the CV increased compared to the SR condition for Rpo2, impact toughness, and elongation. This indicates that the used SA cycle had little-to-no impact on reducing the CV. It is also important to notice that while heat treatments can be used to reduce variability in the properties, partial or complete re-crystallization of the AM 316L microstructure during high temperature heat treatments (SA, HIP) will also reduce the tensile strength and toughness due to the loss of the extremely fine, cellular microstructure of the as-built eanditinn

Figure 18 Effect of heat treatment on the CV in the mechanical properties. The main hypothesis in this study was that standardized heat treatments can be used to reduce the variability in mechanical properties of AM 316L. Fig. 18 shows the coefficient of variation (CV) within each heat treatment condition calculated for all the studied mechanical properties. HIP significantly reduced the CV between the conditions for all other properties except elongation. SA reduced CV in R,, and density, but the CV increased compared to the SR condition for Rpo2, impact toughness, and elongation. This indicates that the used SA cycle had little-to-no impact on reducing the CV. It is also important to notice that while heat treatments can be used to reduce variability in the properties, partial or complete re-crystallization of the AM 316L microstructure during high temperature heat treatments (SA, HIP) will also reduce the tensile strength and toughness due to the loss of the extremely fine, cellular microstructure of the as-built eanditinn

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Materials & Design ‘his is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cove age and metadata, and formatting for readability, but it is not yet the definitive version of record. This versiot jill undergo additional copyediting, typesetting and review before it is published in its final form, but we ar roviding this version to give early visibility of the article. Please note that, during the production process, error: 1ay be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Materials & Design ‘his is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cove age and metadata, and formatting for readability, but it is not yet the definitive version of record. This versiot jill undergo additional copyediting, typesetting and review before it is published in its final form, but we ar roviding this version to give early visibility of the article. Please note that, during the production process, error: 1ay be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Fig. 1. The layout of test specimens on EOS M 290 (left) and SLM 125 (right) building platforms. Two L-PBF machines, SLM 125 HL from SLM Solutions GmbH and EOS M 290 from EOS GmbH, were used to produce identical sets of tensile and impact test specimens. Both machines are equipped with one Ytterbium fiber laser having a maximum nominal output power of 400 W and having approximately 80 um spot diameter at the focus. The maximum building volume in the SLM 125 HL is 125x125x125 mm and in the EOS M 290 250x250x325 mm. The part layout on the building platform is shown in Fig. 1. The build included in total ten pieces of 57 mm x 7 mm x 7 mm square bars for machining impact specimens, 16 pieces of 2 11 mm x 110 mm cylinder bars for machining tensile specimens, and one powder container (located in the middle of the platform) for measuring the effective packing density of the powder bed. The powder container design and methodology were adapted from [21]. The test bars were manufactured in a vertical orientation directly onto the building platform, without additional support structures.
Fig. 1. The layout of test specimens on EOS M 290 (left) and SLM 125 (right) building platforms. Two L-PBF machines, SLM 125 HL from SLM Solutions GmbH and EOS M 290 from EOS GmbH, were used to produce identical sets of tensile and impact test specimens. Both machines are equipped with one Ytterbium fiber laser having a maximum nominal output power of 400 W and having approximately 80 um spot diameter at the focus. The maximum building volume in the SLM 125 HL is 125x125x125 mm and in the EOS M 290 250x250x325 mm. The part layout on the building platform is shown in Fig. 1. The build included in total ten pieces of 57 mm x 7 mm x 7 mm square bars for machining impact specimens, 16 pieces of 2 11 mm x 110 mm cylinder bars for machining tensile specimens, and one powder container (located in the middle of the platform) for measuring the effective packing density of the powder bed. The powder container design and methodology were adapted from [21]. The test bars were manufactured in a vertical orientation directly onto the building platform, without additional support structures.
Fig. 3. SEM images showing the particle morphology of EOS (left) and SLM (right) powders.
Fig. 3. SEM images showing the particle morphology of EOS (left) and SLM (right) powders.
Fig. 2. Measured PSD for EOS and SLM powders.
Fig. 2. Measured PSD for EOS and SLM powders.
Fig. 4. Polished cross sections of EOS (left) and SLM powder (right).
Fig. 4. Polished cross sections of EOS (left) and SLM powder (right).
3.2. Additive manufacturing
3.2. Additive manufacturing
Table 2. Chemical compositions of printed samples, as measured with OES and powder composition and stated in tl material certificates [23,24] (the values are in wt%).
Table 2. Chemical compositions of printed samples, as measured with OES and powder composition and stated in tl material certificates [23,24] (the values are in wt%).
Fig. 5. Discontinuities in parts produced with EOS machine using SLM powder due to a temporary stoppage of the process.
Fig. 5. Discontinuities in parts produced with EOS machine using SLM powder due to a temporary stoppage of the process.
3.4. Tensile testing The measured samples generally had very low porosity values of < 0.07% (Fig. 6). The margin of error for the image analysis method was estimated to be approximately 0.015—0.02%, as indicated by the error bars. The results clearly show that the lower porosity values for the HIP’d samples compared to the other heat treatments were as expected. The results also indicate that the EOS machine produces more repeatable results regardless of the powder, whereas the EOS powder in the SLM machine resulted in higher porosities.
3.4. Tensile testing The measured samples generally had very low porosity values of < 0.07% (Fig. 6). The margin of error for the image analysis method was estimated to be approximately 0.015—0.02%, as indicated by the error bars. The results clearly show that the lower porosity values for the HIP’d samples compared to the other heat treatments were as expected. The results also indicate that the EOS machine produces more repeatable results regardless of the powder, whereas the EOS powder in the SLM machine resulted in higher porosities.
Fig. 7. Effect of machine-powder combination on Rpo»2, Rm and A in the a) SR, b) SA and c) HIP condition. state. HIP reduced the R,, by 3-8% and the Rpo.» by 43-47%, and it increased A by 17-32% compared to the SR condition. All the tested specimens in each studied condition met and exceeded the minimum requirements laid out in ASTM F3184-16 for AM 316L, namely that Rpo.=205 MPa, R,,=515 MPa, and A=30%. Within the batches of five tensile specimens for each studied condition, the standard deviations were small, 0.6—-7.7 MPa for Rpo2, 0.6-3.0 MPa for Rn, and 0.6—-3.2% for A, compared to deviations of 31-50 MPa, 16-55 MPa, and 5—20%, respectively, given in material data sheets for the as-built condition for these processes [25,26].
Fig. 7. Effect of machine-powder combination on Rpo»2, Rm and A in the a) SR, b) SA and c) HIP condition. state. HIP reduced the R,, by 3-8% and the Rpo.» by 43-47%, and it increased A by 17-32% compared to the SR condition. All the tested specimens in each studied condition met and exceeded the minimum requirements laid out in ASTM F3184-16 for AM 316L, namely that Rpo.=205 MPa, R,,=515 MPa, and A=30%. Within the batches of five tensile specimens for each studied condition, the standard deviations were small, 0.6—-7.7 MPa for Rpo2, 0.6-3.0 MPa for Rn, and 0.6—-3.2% for A, compared to deviations of 31-50 MPa, 16-55 MPa, and 5—20%, respectively, given in material data sheets for the as-built condition for these processes [25,26].
Fig. 8. Effect of machine-powder combination and heat treatment on impact toughness. 1U70 TH SMO WI det LOUSMTCSs Uldil UOSe VUE WIth LUO OLIVE Tanne, Lou et al. [27] have reported similar impact toughness (Charpy- V-notch) values for the AM 316L at room temperature They reported an impact toughness values of approximately 150 J for vertically oriented samples that had been HIP’d anc SA’d using the same heat treatment parameters as in this study, except for the quench medium after SA (water). Impact toughness values for wrought 316L ranged from 200 J to 350 J, and the authors concluded that the lower values for the AM specimens compared to the wrought alloy were due to a large number of oxide inclusions observed on the fracture surfaces. Controlling the oxygen content in the L-PBF process is difficult, which can result in a higher oxygen conten: and related oxide inclusions in finished components compared to wrought parts. The difference in oxygen content in the feedstock powders and the systems’ build chambers could therefore lead to a difference in the amount of oxide inclusion: in the build parts. Therefore, the fracture surfaces of the impact specimens were further analyzed.
Fig. 8. Effect of machine-powder combination and heat treatment on impact toughness. 1U70 TH SMO WI det LOUSMTCSs Uldil UOSe VUE WIth LUO OLIVE Tanne, Lou et al. [27] have reported similar impact toughness (Charpy- V-notch) values for the AM 316L at room temperature They reported an impact toughness values of approximately 150 J for vertically oriented samples that had been HIP’d anc SA’d using the same heat treatment parameters as in this study, except for the quench medium after SA (water). Impact toughness values for wrought 316L ranged from 200 J to 350 J, and the authors concluded that the lower values for the AM specimens compared to the wrought alloy were due to a large number of oxide inclusions observed on the fracture surfaces. Controlling the oxygen content in the L-PBF process is difficult, which can result in a higher oxygen conten: and related oxide inclusions in finished components compared to wrought parts. The difference in oxygen content in the feedstock powders and the systems’ build chambers could therefore lead to a difference in the amount of oxide inclusion: in the build parts. Therefore, the fracture surfaces of the impact specimens were further analyzed.
Fig. 9. SEM-BSE images of specimen cross section in SR, SA, and HIP conditions from the SLM-SLMp combination. The L-PBF build growth direction is horizontal towards right side.
Fig. 9. SEM-BSE images of specimen cross section in SR, SA, and HIP conditions from the SLM-SLMp combination. The L-PBF build growth direction is horizontal towards right side.
Fig. 10. EBSD images of specimen cross section in SR, SA, and HIP conditions from the SLM-SLMp combination. The L-PBF build growth direction is horizontal towards the right side.
Fig. 10. EBSD images of specimen cross section in SR, SA, and HIP conditions from the SLM-SLMp combination. The L-PBF build growth direction is horizontal towards the right side.
Fig. 11. SEM-SE images of the fractography of Charpy-tested specimens in the SR condition from the different machine- powder combinations. The vertical upward direction is the final rupture site. dimple fracture in all conditions, which is similar to that for wrought stainless steels. In the SLUM-SLMp and SLM-EOSp combinations, a stronger formation of macrovoid and secondary cracking on the fracture surface was observed, as seen in Fig. 11 (a, c) and Fig. 12 (a). Moreover, ductile fracture more likely propagated via tearing ridges rather than microvoid coalescence in the SLM-SLMp and SLM-EOSp combinations, just as in the EOS-EOSp and EOS-SLMp combinations. Tearing ridges can be observed in Fig. 11 (b, d), Fig. 12 (d), and Fig. 13 (b). Nanometer-sized dimples can be observed in Fig. 11 (f, h), Fig. 12 (b, d, f, h), and Fig. 13 (d, f, h). In general, the dimples illustrate a size difference in the three heat treatment conditions: HIP > SA > SR, which implies the coarsening of the sub-grain cellular structure and inclusions. Intergranular-like fracture topography can be observed in Fig. 13 (a, g), with SLM-SLMp and EOS-SLMp combinations in the HIP condition.
Fig. 11. SEM-SE images of the fractography of Charpy-tested specimens in the SR condition from the different machine- powder combinations. The vertical upward direction is the final rupture site. dimple fracture in all conditions, which is similar to that for wrought stainless steels. In the SLUM-SLMp and SLM-EOSp combinations, a stronger formation of macrovoid and secondary cracking on the fracture surface was observed, as seen in Fig. 11 (a, c) and Fig. 12 (a). Moreover, ductile fracture more likely propagated via tearing ridges rather than microvoid coalescence in the SLM-SLMp and SLM-EOSp combinations, just as in the EOS-EOSp and EOS-SLMp combinations. Tearing ridges can be observed in Fig. 11 (b, d), Fig. 12 (d), and Fig. 13 (b). Nanometer-sized dimples can be observed in Fig. 11 (f, h), Fig. 12 (b, d, f, h), and Fig. 13 (d, f, h). In general, the dimples illustrate a size difference in the three heat treatment conditions: HIP > SA > SR, which implies the coarsening of the sub-grain cellular structure and inclusions. Intergranular-like fracture topography can be observed in Fig. 13 (a, g), with SLM-SLMp and EOS-SLMp combinations in the HIP condition.
Fig. 12. SEM-SE images of the fractography of Charpy-tested specimens in the SA condition from the different machine- powder combinations. The vertical upward direction is the final rupture site.
Fig. 12. SEM-SE images of the fractography of Charpy-tested specimens in the SA condition from the different machine- powder combinations. The vertical upward direction is the final rupture site.
Fig. 13. SEM-SE images of the fractography of Charpy-tested specimens in the HIP condition from the different machine- powder combinations. The vertical upward direction is the final rupture site.
Fig. 13. SEM-SE images of the fractography of Charpy-tested specimens in the HIP condition from the different machine- powder combinations. The vertical upward direction is the final rupture site.
EDS area mapping and the corresponding images from the SLM-SLMp combination in the HIP condition are shown in Fig. 17. The areas revealed oxides enriched in Si content. In addition, Mo-Mn-Cr signal in the angular regions were discovered. The area mapping confirms the angular, spherical, whiter looking Mo-Mn-Fe spinel and the spherical, darker looking silicate.
EDS area mapping and the corresponding images from the SLM-SLMp combination in the HIP condition are shown in Fig. 17. The areas revealed oxides enriched in Si content. In addition, Mo-Mn-Cr signal in the angular regions were discovered. The area mapping confirms the angular, spherical, whiter looking Mo-Mn-Fe spinel and the spherical, darker looking silicate.
Fig. 15. SEM-SE images of the inclusions on the Charpy-tested specimens in the SA condition from the different machine- powder combinations. Fig. 14. SEM-SE images of the inclusions on the Charpy-tested specimens in the SR condition from the different machine- powder combinations.
Fig. 15. SEM-SE images of the inclusions on the Charpy-tested specimens in the SA condition from the different machine- powder combinations. Fig. 14. SEM-SE images of the inclusions on the Charpy-tested specimens in the SR condition from the different machine- powder combinations.
Fig. 16. SEM-SE images of the inclusions on the Charpy-tested specimens in the HIP condition from the different machine-powder combinations.
Fig. 16. SEM-SE images of the inclusions on the Charpy-tested specimens in the HIP condition from the different machine-powder combinations.
Fig. 17. EDS area analyses of the inclusions in the HIP condition from the SLM-SLMp combination.
Fig. 17. EDS area analyses of the inclusions in the HIP condition from the SLM-SLMp combination.
The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
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Quality ManagementPowder MetallurgyLean ManufacturingPRODUCTION ENGINEERINGAdditive ManufacturingMetallurgy and Materials EngineeringHeat treatment of metals and alloys3d PrinterAdditive Manufacturing and 3D printing316L stainless steel
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