- Email: [email protected]
Notch fatigue behavior of Inconel 718 produced by selective laser melting
Available online at www.sciencedirect.com Available online at www.sciencedirect.com
ScienceDirect ScienceDirect
Structuralonline Integrity 00 (2019) 000–000 Available atProcedia www.sciencedirect.com Structural Integrity Procedia 00 (2019) 000–000
ScienceDirect
www.elsevier.com/locate/procedia www.elsevier.com/locate/procedia
Procedia Structural Integrity 17 (2019) 138–145
ICSI 2019 The 3rd International Conference on Structural Integrity ICSI 2019 The 3rd International Conference on Structural Integrity
Notch fatigue behavior of Inconel 718 produced by selective laser Notch fatigue behavior of Inconel 718 produced by selective laser melting melting Radomila Konečnáaa, Gianni Nicolettobb *, Enrica Rivabb Radomila Konečná , Gianni Nicoletto *, Enrica Riva
University of Žilina, Faculty of Mechanical Engineering, Department of Materials Engineering, Univerzitná 8215/1, 010 26 Žilina, Slovakia Universityb University of Žilina, Faculty of Mechanical Department of Materials Univerzitná 26 Žilina, of Parma, department Engineering, of Engineering and Architecture, ParcoEngineering, Area della Scienze 181/A,8215/1, 43124 010 Parma, Italy Slovakia b University of Parma, department of Engineering and Architecture, Parco Area della Scienze 181/A, 43124 Parma, Italy
a a
Abstract Abstract
Inconel 718 is widely used in hot structures of jet engines because of its excellent mechanical properties at high Inconel 718Recently, is widely customized used in hot structures of jet engines because its excellentfabricated mechanical high temperatures. parts of complex geometry are ofincreasingly byproperties SelectiveatLaser temperatures. customized partsThis of complex geometry are increasingly fabricated by Selective Laser Melting (SLM)Recently, of Inconel 718 powder. contribution investigates the still largely unexplored topic of the Melting of Inconel 718andpowder. contribution investigates still largely topic of the combined(SLM) influence of notches as-builtThis surfaces on the fatigue strengththe of SLM Inconel unexplored 718 parts. An innovative combined notches and as-built surfaces on the tested fatigueinstrength SLMbending Inconelis718 parts. An innovative fatigue testinfluence method of using miniature notched specimens cyclic of plane adopted. Four sets of fatigue test method using miniature notched specimens tested in cyclic plane bending is adopted. Four specimens, each with a different orientation of the notch surface with respect to the build axis, are fabricatedsets withofa specimens, each with a different orientation of the notch surface with respect to the build axis, are fabricated with commercial SLM system, heat treated and fatigue tested. The fatigue results show the directional nature of the as-a commercial SLM system, treatedsurface and fatigue tested. Thenotched fatiguespecimens results show directional nature of the asbuilt notch effect. The linkheat between quality of the andthe their layer-wise fabrication is built notch by effect. The link between surface quality of the notched specimens and their layer-wise fabrication is determined a metallographic investigation. determined by a metallographic investigation. © 2019 The Authors. Published by Elsevier B.V. © 2019 Published by Elsevier B.V. B.V. © 2019The TheAuthors. Authors. Published by Peer-review under responsibility of Elsevier the ICSI organizers. Peer-review under responsibility of the ICSI 2019 2019 organizers. Peer-review under responsibility of the ICSI 2019 organizers. Keywords: selective laser melting SLM, Inconel 718, notch fatigue, as-built surface quality, miniature specimen. Keywords: selective laser melting SLM, Inconel 718, notch fatigue, as-built surface quality, miniature specimen.
1. Introduction 1. Introduction Selective laser melting (SLM) is an additive manufacturing technology that uses 3D CAD part data as a digital Selective laser melting (SLM) is an powder additivewithout manufacturing that uses 3D CAD partsystems data as currently a digital source and fabricates it from a metal the needtechnology for part-specific tooling. SLM source and fabricates it from a metal powder without the need for part-specific tooling. SLM systems currently
* Corresponding author. Tel.: +39-0521-905884; fax: +39-0521-905715. * E-mail Corresponding Tel.: +39-0521-905884; fax: +39-0521-905715. address:author. [email protected] E-mail address: [email protected] 2452-3216 © 2019 The Authors. Published by Elsevier B.V. 2452-3216 2019responsibility The Authors. of Published Elsevier B.V. Peer-review©under the ICSIby 2019 organizers. Peer-review under responsibility of the ICSI 2019 organizers.
2452-3216 2019 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the ICSI 2019 organizers. 10.1016/j.prostr.2019.08.019
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Radomila Konečná et al. / Procedia Structural Integrity 17 (2019) 138–145 Author name / Structural Integrity Procedia 00 (2019) 000–000
139
produce near fully-dense parts with limited presence of internal defects. Inconel 718 is a nickel–chromium alloy extensively used in gas turbines, rocket motors, space crafts, etc. that is now successfully processed by SLM, Reed (2006), Clark et al. (2018). The microstructure of SLM Inconel 718 is substantially different from that of conventionally manufactured materials because of the typical rapid solidification. The static mechanical properties of SLM Inconel 718 are comparable if not better of their conventionally manufactured counterparts, Popovich et al. (2015). On the other hand, in the presence of fatigue loading, the as-built surfaces of SLM Inconel 718 parts are a source of weakness, Wells (2016). In general, the fatigue properties of SLM metals vary considerably according to the processing parameters because they affect the microstructure, porosity content, residual stresses and relatively rough part surfaces. Quantification and understanding of this fatigue gap is a fundamental step in SLM process qualification, Yadollahi et al. (2017). Part performance in fatigue is therefore significantly lower compared to machined counterparts. In topologically optimized SLM parts, the effect is further enhanced by many local stress concentrations at geometrical notches and cross-sectional variations, Gorelik (2017). The presence of regions of high stress concentration in actual SLM components, contribute to fatigue in ways that may not be predictable based solely on standard smooth specimens. Therefore, notched fatigue specimens should be used to evaluate the influence of stress concentrators. The recent work of Witkin et al. (2018) is noteworthy because the current lack of data on notch fatigue behavior of as-built SLM Inconel 718. Notched specimens designed with three different notch geometry were printed in both the vertical and horizontal direction, so that the notched section was oriented either in-plane (vertical specimens) or vertically (horizontal specimens). Fatigue tests were performed on samples with as-produced notch surfaces and machined notches. The results not only show that the surfaces of SLM metal parts influence the fatigue behavior in the presence of a macroscopic designed notch, but also that the final as-built notch dimensions are dependent on both the notch geometry and specimen orientation. This contribution also investigates the link between surface quality, directional material fabrication and the resulting notch fatigue behavior of SLM Inconel 718. An innovative fatigue test method using miniature notched specimens tested in cyclic plane bending is adopted instead of the thin notched plates in traction of Witkin et al. (2018). Four sets for specimens, each with a different orientation of the notch surface with respect to the build axis, were produced out of Inconel 718 powder by SLM processing and fatigue tested. Since the as-built specimens show the directional nature of the notch fatigue response, a metallographic investigation examines the link between surface quality of the notched specimens and the layer-wise fabrication of SLM process. 2. Experimental details 2.1. Material and SLM process The material of this study is gas atomized Inconel 718 alloy powder with spherical particles in the diameter range from 15 to 45 µm. The chemical composition was determined by spectrometry and was the following: Table 1. Chemical composition of IN 718 powder. Element
Ni
Cr
Fe
Nb
Mo
Co
Ti
Al
Cu
Wt. %
51.56
17.9
18.2
5.23
3.21
0.15
1.14
2.19
0.05
A SLM Solution 280HL system (SLM Solutions, Germany) was used to manufacture four sets of fatigue specimens by service provider BEAM-IT (Fornovo Taro, Italy). A layer thickness of 50 µm was used together with a fluence F = 54.82 J/ mm3 given by a laser power P = 250 W, hatch spacing h = 0.12 mm and scan speed v = 760 mm/s. The layer-wise powder transformation by selective laser melting was carried out in an Argon atmosphere with built plate temperature maintained at 200 °C. After SLM fabrication the post-processing steps were: heat treatment before specimen removal from the base plate and a two-step heat treatment after removal given by: i) stress relief (solution with heating to 970 °C for 1 hour followed by cooling in Argon atmosphere) ii) age hardening by double aging (heating to 710 °C and holding for 8 hours, further aged at 610 °C for 8 hours and final cooling to room temperature in Argon).
Radomila Konečná et al. / Procedia Structural Integrity 17 (2019) 138–145 Author name / Structural Integrity Procedia 00 (2019) 000–000
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2.2. Material characterization Metallographic specimens were prepared according to standard techniques and then observed using the Neophot 32 light microscope and Tescan LYRA 3 XMU FEG/SEM with EDX analysis system. Microstructure was analyzed after etching with Kalling's reagent (2 g of CuCl2, 40 ml of HCl, 80 ml of methanol). 2.3. Fatigue testing A special miniature prismatic specimen geometry introduced by Nicoletto (2017) was adopted to investigate the directional effect of the as-built surface quality on the notch fatigue strength of as-built SLM Inconel 718. The present test methodology has been already extensively used on DMLS Ti6Al4V, Nicoletto (2018). Fig. 1 shows the side view of the specimen subjected to bending and the associated principal stress distribution obtained by elastic finite element analysis. The size of the nominal minimum section is 5 x 5 mm2, the radius of the semicircular notch is 2 mm and the specimen length is 22 mm. A mild stress concentration factor Kt = max,FEA/ nom = 1.63 was obtained using nom = M/W and W is the section modulus of the notched section. The specimen loading is cyclic bending with a stress ratio R = 0 and a frequency of 20 Hz. Test run-out was fixed at 2x106 cycles.
Fig. 1. Side view of notched miniature specimen subjected to bending and elastic stress distribution.
Fig. 2 shows the different specimen directions with respect to build direction (i.e. Z directions) and their respective denomination.
Fig. 2. Orientation of the notched miniature specimens on the build plate and denomination (Z is build direction).
The surface quality of the notches is affected by the specimen orientation due to the layer-wise SLM fabrication. In addition, the direction of applied stress at the notch root may be parallel or orthogonal to the material layers. Namely, the semicircular notch geometry of Type B specimens is not influenced by the fabrication and the applied stress is parallel to the layers. The semicircular geometry of Type C specimen at its notch root is weakly influenced by the layer-wise fabrication and the applied stress is perpendicular to the layers. The circularity of the notch roots of
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Type A+ and Type A- specimens are strongly affected by the layer-wise fabrication (i.e. stair stepping, see Gebhard et al. (2014)). In addition, the surface of Type A- notches is classified as down-skin while the surface of Type A+ notches is up-skin. It is well known that down-skin surfaces are of lower quality (i.e. higher roughness poor geometrical accuracy) than up-skin surfaces. Applied stress in both Type A specimens is parallel to the layers. This qualitative description of the interaction of SLM fabrication and surface quality of the notches suggest that the fatigue behavior will depend on specimen directions. 3. Results and discussion 3.1. Directional notch fatigue behavior Fig. 3 shows the directional notch fatigue effect obtained in the four sets of the present mildly notched miniature specimens of SLM Inconel 718 with as-built surfaces. The independent variable is the maximum nominal stress of the bending load cycle with R = 0 and the dependent variable is number of cycles to failure. Four different and well defined S/N plots are determined for the different directions of fabrication. Considering Fig. 1, the Type B specimens shows the best fatigue performance while the Type A specimens the worst fatigue performance. Type C specimens determine an intermediate behavior. As Inconel 718 is face-centered cubic it may not exhibit a true fatigue limit, Reed (2006), a nominal fatigue strength at 2x106 cycles can be considered for comparison. So the fatigue strength varies from a maximum value of 360 MPa for Type B notched specimens to a minimum of 200 MPa for the Type A- specimens with a down-skin notch. These data will be assessed against limited published data, i.e. Witkin et al. (2018), in a later section. However, the relatively large differences in the respective S/N plots suggest that the fatigue behavior is not primarily controlled by the notch geometry (i.e. theoretically by Kt) but rather by the as-built surface quality, which on the other hand depend on process parameters and surface orientation. .
Fig. 3. Directional notch fatigue data for as-built SLM Inconel 718 (Kt = 1.63).
3.2. Directional quality of notch surfaces This section presents the characterization of the notch surfaces of the four specimen types of Fig. 2 using magnified optical microscopy. The aim is the qualitative comparison of the actual notch geometry from the theoretical semicircular geometry of 2-mm-radius. Not the entire profile is however relevant for the notch fatigue
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behavior. Since the peak stress and fatigue crack initiation occur at the notch root, the geometrical deviations between design intent and actual geometry are examined there. Fig. 4 compares the semicircular notch with the notch of Type B specimens. The build direction does not affect the entire notch geometry because it is obtained by sequential layer contouring. Contour width, process parameters and system accuracy may explain slight deviations observed.
Fig. 4. Effective semicircular notch in Type B specimen (white dotted curve is theoretical 2-mm radius circle).
Fig. 5 groups together three notch views taken from specimens that share the common build direction shown by the arrow. In these cases, large global deviations from notch circularity are apparent, the most evident being for the Type C and Type A- specimens while the Type A+ specimen is macroscopically close to circular.
Fig. 5. Effective semicircular notch in three types of specimens (white broken curve is theoretical 2 mm radius circle).
As mentioned before, however, it is the local quality at the notch root that is critical for the fatigue behavior. Therefore, Type C notch is globally poor but its notch root is not too different from the theoretical geometry because the layer-by-layer fabrication locally does not introduce significant stair-stepping of the surface, see also the magnified view in Fig. 6a.
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The notch root conditions are different for Type A+ specimens because the local surface roughness is strongly affected by stair stepping associated to the finite layer thickness, Gebhard et al. (2014). The magnified view of the notch root in Fig. 6b demonstrates the stair stepping introduced by the 50 m thick layers (broken lines).
a)
b)
Fig. 6. Effective notch root profile of a) Type C specimen and b) Type A- specimen. Dashed lines are 50 m-thick-layers.
Finally, the notch of Type A- specimens in Fig. 5 shows the worst quality of all. Specifically, the notch root combines the negative impact of the stair stepping effect of Type A+ notch and the un-supported down-skin surface orientation. The latter generates dross formation, high roughness and lack of geometrical accuracy. The optical inspection of the respective as-built notch roots of the SLM Inconel 718 specimens demonstrated the significant difference in effective quality. The ranking of the quality (from best i.e. Type B to worst i.e. Type A-) matches the experimental ranking obtained by fatigue testing of the four sets of fatigue specimens with as-built notch surfaces. Therefore, the S/N curves shown in Fig. 3 quantify the impact of the SLM technology on notch fatigue behavior. 3.3. Assessment of the notch fatigue behavior The notch fatigue behavior of SLM Inconel 718 shown in Fig. 3 has been obtained with a non-standard specimen configuration and test methodology, Nicoletto (2017). An assessment of the experimental data is important to support further use of the innovative method. Unfortunately, notch fatigue data of SLM Inconel 718 are scarce in the literature, an important and recent exception being the work of Witkin et al. (2018) that published fatigue data obtained with notched specimens fabricated on a Concept Laser M2 Cusing (layer thickness 30 μm), subsequently HIPped, solution treated and aged per industry standards. Axial tensile pulsating loading was performed in compliance with ASTM standard E466. Flat fatigue specimens printed in the horizontal direction and with a mild notch effect and as-built surfaces are considered here. The notched specimens of Witkin et al. (2018) and the present miniature specimens are compared in Fig. 7. The respective thicknesses were 2.5 mm for Witkin’s specimens and 5 mm for the present miniature specimens. Both specimen geometries and loading are characterized by a mild notch severity (i.e. K t = 1.93 vs. Kt = 1.63). Fig. 7 demonstrates the considerable difference in overall size and the same 2-mm radius of the notches. Since two notches were present in Witkin’s specimens, it is the worst-quality notch (i.e. down-skin) that controls fatigue crack initiation. On the other hand, the method of Nicoletto (2017) tested one notch at the time. So the two Type A miniature specimens not only characterized in fatigue the horizontal fabrication direction but also, selectively, the notch surface orientation (i.e. up-skin notch of Type A+ vs. down-skin notch of Type A-).
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The fatigue results for the as-built horizontal double notched specimens of SLM Inconel 718 by Witkin et al. (2018) are compared to the fatigue results of the as-built Type A+ and Type A- miniature specimens in Fig. 8. The three sets of data share the slope of the S/N curves but are slightly shifted one with respect to the others. In this case, however, the fatigue strength of about 150 MPa for the doubly-notched specimens is very close to the response of the Type A- miniature specimen (the specimen with the down-skin notch) while the Type A+ specimen data are unequivocally better.
Fig. 7. Comparison of double notched tensile plate of Witkin et al. (2018) and the two miniature bending specimens used here.
Fig. 8 Notched fatigue behavior of horizontal double notched plates vs. Type A+ and Type A- miniature specimens.
4. Conclusions This study was aimed at the experimental investigation of fatigue notch sensitivity of as-built SLM Inconel 718 in the presence of different surface orientations with respect to the build direction using the test methodology recently introduced by Nicoletto (2017). The main conclusions are the following: - The test methodology using miniature fatigue specimens efficiently provided original insight into the dependence of the fatigue behavior on the directionality of the notch fabrication by SLM processing. - Of the four directional specimens investigated and shown in Fig. 2, Type A- showed the worst notch fatigue performance, Type B the best and Type C and Type A+ intermediate notch fatigue performances.
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Optical inspection of the notch roots where fatigue crack initiation occurs clearly identified different surface qualities in accordance the experimental ranking in terms of fatigue strength. The present conclusions support the recommendation by Witkin et al. (2018) that the influence of both rough surfaces on fatigue behavior and the tendency of the SLM process to build features that deviate from design intent should be considered by designers and analysts when assessing the HCF performance of parts containing stress concentrations subject to cyclic loads. -
Acknowledgements Part of the research was supported by the project Slovak VEGA grant No. 1/0463/2019. Specimen fabrication by the technological partner BEAM-IT, Fornovo Taro (Italy) is gratefully acknowledged.
References Clark, D., Bache, M.R., Whittaker, M.T., 2008. Shaped metal deposition of a nickel alloy for aero engine applications. Journal of Materials Processing Technology 203, 39-448. Gebhardt, A., Hötter, J., Ziebura, D., 2014. Impact of SLM build parameters on the surface quality. RTejournal - Forum für Rapid Technologie, Vol. 2014 https://www.rtejournal.de/ausgabe11/3852 (accessed 2019/05/09) Gorelik, M., 2017. Additive manufacturing in the context of structural integrity. International Journal of Fatigue, 94, 168 ‐ 177. Nicoletto, G., 2017. Anisotropic high cycle fatigue behavior of Ti-6Al-4V obtained by powder bed laser fusion. International Journal of Fatigue, 94, pp. 255-262. Nicoletto, G., 2018, Directional and notch effects on the fatigue behavior of as-built DMLS Ti6Al4V, International Journal of Fatigue, 106, pp.124-131. Popovich, A.A., Sufiiarov, V.Sh., Polozov, I.A., Borisov, E.V., 2015. Microstructure and Mechanical Properties of Inconel 718 Produced by SLM and Subsequent Heat Treatment, Key Engineering Materials, doi ={10.4028/ www.scientific.net/ KEM.651-653.665} Reed, R.C., 2006. The Superalloys. Fundamentals and Applications, Cambridge University Press, pp. 372. Wells, D., 2016. Overview of fatigue and damage tolerance performance of powder bed fusion alloy N07718. ASTM/NIST Workshop on Mechanical Behavior of Additive Manufactured Components, San Antonio, Texas, NASA Report No. 20160007069. Witkin, D.B., Patel, D. N., Bean, G.E., 2018. Notched fatigue testing of Inconel 718 prepared by selective laser melting, Fatigue Fract Eng Mater Struct, 1 – 12. DOI: 10.1111/ffe.12880 Yadollahi, A., Shamsaei, N., 2017. Additive manufacturing of fatigue resistant materials: Challenges and opportunities, International Journal of Fatigue, 9814–31.
ScienceDirect ScienceDirect
Structuralonline Integrity 00 (2019) 000–000 Available atProcedia www.sciencedirect.com Structural Integrity Procedia 00 (2019) 000–000
ScienceDirect
www.elsevier.com/locate/procedia www.elsevier.com/locate/procedia
Procedia Structural Integrity 17 (2019) 138–145
ICSI 2019 The 3rd International Conference on Structural Integrity ICSI 2019 The 3rd International Conference on Structural Integrity
Notch fatigue behavior of Inconel 718 produced by selective laser Notch fatigue behavior of Inconel 718 produced by selective laser melting melting Radomila Konečnáaa, Gianni Nicolettobb *, Enrica Rivabb Radomila Konečná , Gianni Nicoletto *, Enrica Riva
University of Žilina, Faculty of Mechanical Engineering, Department of Materials Engineering, Univerzitná 8215/1, 010 26 Žilina, Slovakia Universityb University of Žilina, Faculty of Mechanical Department of Materials Univerzitná 26 Žilina, of Parma, department Engineering, of Engineering and Architecture, ParcoEngineering, Area della Scienze 181/A,8215/1, 43124 010 Parma, Italy Slovakia b University of Parma, department of Engineering and Architecture, Parco Area della Scienze 181/A, 43124 Parma, Italy
a a
Abstract Abstract
Inconel 718 is widely used in hot structures of jet engines because of its excellent mechanical properties at high Inconel 718Recently, is widely customized used in hot structures of jet engines because its excellentfabricated mechanical high temperatures. parts of complex geometry are ofincreasingly byproperties SelectiveatLaser temperatures. customized partsThis of complex geometry are increasingly fabricated by Selective Laser Melting (SLM)Recently, of Inconel 718 powder. contribution investigates the still largely unexplored topic of the Melting of Inconel 718andpowder. contribution investigates still largely topic of the combined(SLM) influence of notches as-builtThis surfaces on the fatigue strengththe of SLM Inconel unexplored 718 parts. An innovative combined notches and as-built surfaces on the tested fatigueinstrength SLMbending Inconelis718 parts. An innovative fatigue testinfluence method of using miniature notched specimens cyclic of plane adopted. Four sets of fatigue test method using miniature notched specimens tested in cyclic plane bending is adopted. Four specimens, each with a different orientation of the notch surface with respect to the build axis, are fabricatedsets withofa specimens, each with a different orientation of the notch surface with respect to the build axis, are fabricated with commercial SLM system, heat treated and fatigue tested. The fatigue results show the directional nature of the as-a commercial SLM system, treatedsurface and fatigue tested. Thenotched fatiguespecimens results show directional nature of the asbuilt notch effect. The linkheat between quality of the andthe their layer-wise fabrication is built notch by effect. The link between surface quality of the notched specimens and their layer-wise fabrication is determined a metallographic investigation. determined by a metallographic investigation. © 2019 The Authors. Published by Elsevier B.V. © 2019 Published by Elsevier B.V. B.V. © 2019The TheAuthors. Authors. Published by Peer-review under responsibility of Elsevier the ICSI organizers. Peer-review under responsibility of the ICSI 2019 2019 organizers. Peer-review under responsibility of the ICSI 2019 organizers. Keywords: selective laser melting SLM, Inconel 718, notch fatigue, as-built surface quality, miniature specimen. Keywords: selective laser melting SLM, Inconel 718, notch fatigue, as-built surface quality, miniature specimen.
1. Introduction 1. Introduction Selective laser melting (SLM) is an additive manufacturing technology that uses 3D CAD part data as a digital Selective laser melting (SLM) is an powder additivewithout manufacturing that uses 3D CAD partsystems data as currently a digital source and fabricates it from a metal the needtechnology for part-specific tooling. SLM source and fabricates it from a metal powder without the need for part-specific tooling. SLM systems currently
* Corresponding author. Tel.: +39-0521-905884; fax: +39-0521-905715. * E-mail Corresponding Tel.: +39-0521-905884; fax: +39-0521-905715. address:author. [email protected] E-mail address: [email protected] 2452-3216 © 2019 The Authors. Published by Elsevier B.V. 2452-3216 2019responsibility The Authors. of Published Elsevier B.V. Peer-review©under the ICSIby 2019 organizers. Peer-review under responsibility of the ICSI 2019 organizers.
2452-3216 2019 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the ICSI 2019 organizers. 10.1016/j.prostr.2019.08.019
2
Radomila Konečná et al. / Procedia Structural Integrity 17 (2019) 138–145 Author name / Structural Integrity Procedia 00 (2019) 000–000
139
produce near fully-dense parts with limited presence of internal defects. Inconel 718 is a nickel–chromium alloy extensively used in gas turbines, rocket motors, space crafts, etc. that is now successfully processed by SLM, Reed (2006), Clark et al. (2018). The microstructure of SLM Inconel 718 is substantially different from that of conventionally manufactured materials because of the typical rapid solidification. The static mechanical properties of SLM Inconel 718 are comparable if not better of their conventionally manufactured counterparts, Popovich et al. (2015). On the other hand, in the presence of fatigue loading, the as-built surfaces of SLM Inconel 718 parts are a source of weakness, Wells (2016). In general, the fatigue properties of SLM metals vary considerably according to the processing parameters because they affect the microstructure, porosity content, residual stresses and relatively rough part surfaces. Quantification and understanding of this fatigue gap is a fundamental step in SLM process qualification, Yadollahi et al. (2017). Part performance in fatigue is therefore significantly lower compared to machined counterparts. In topologically optimized SLM parts, the effect is further enhanced by many local stress concentrations at geometrical notches and cross-sectional variations, Gorelik (2017). The presence of regions of high stress concentration in actual SLM components, contribute to fatigue in ways that may not be predictable based solely on standard smooth specimens. Therefore, notched fatigue specimens should be used to evaluate the influence of stress concentrators. The recent work of Witkin et al. (2018) is noteworthy because the current lack of data on notch fatigue behavior of as-built SLM Inconel 718. Notched specimens designed with three different notch geometry were printed in both the vertical and horizontal direction, so that the notched section was oriented either in-plane (vertical specimens) or vertically (horizontal specimens). Fatigue tests were performed on samples with as-produced notch surfaces and machined notches. The results not only show that the surfaces of SLM metal parts influence the fatigue behavior in the presence of a macroscopic designed notch, but also that the final as-built notch dimensions are dependent on both the notch geometry and specimen orientation. This contribution also investigates the link between surface quality, directional material fabrication and the resulting notch fatigue behavior of SLM Inconel 718. An innovative fatigue test method using miniature notched specimens tested in cyclic plane bending is adopted instead of the thin notched plates in traction of Witkin et al. (2018). Four sets for specimens, each with a different orientation of the notch surface with respect to the build axis, were produced out of Inconel 718 powder by SLM processing and fatigue tested. Since the as-built specimens show the directional nature of the notch fatigue response, a metallographic investigation examines the link between surface quality of the notched specimens and the layer-wise fabrication of SLM process. 2. Experimental details 2.1. Material and SLM process The material of this study is gas atomized Inconel 718 alloy powder with spherical particles in the diameter range from 15 to 45 µm. The chemical composition was determined by spectrometry and was the following: Table 1. Chemical composition of IN 718 powder. Element
Ni
Cr
Fe
Nb
Mo
Co
Ti
Al
Cu
Wt. %
51.56
17.9
18.2
5.23
3.21
0.15
1.14
2.19
0.05
A SLM Solution 280HL system (SLM Solutions, Germany) was used to manufacture four sets of fatigue specimens by service provider BEAM-IT (Fornovo Taro, Italy). A layer thickness of 50 µm was used together with a fluence F = 54.82 J/ mm3 given by a laser power P = 250 W, hatch spacing h = 0.12 mm and scan speed v = 760 mm/s. The layer-wise powder transformation by selective laser melting was carried out in an Argon atmosphere with built plate temperature maintained at 200 °C. After SLM fabrication the post-processing steps were: heat treatment before specimen removal from the base plate and a two-step heat treatment after removal given by: i) stress relief (solution with heating to 970 °C for 1 hour followed by cooling in Argon atmosphere) ii) age hardening by double aging (heating to 710 °C and holding for 8 hours, further aged at 610 °C for 8 hours and final cooling to room temperature in Argon).
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2.2. Material characterization Metallographic specimens were prepared according to standard techniques and then observed using the Neophot 32 light microscope and Tescan LYRA 3 XMU FEG/SEM with EDX analysis system. Microstructure was analyzed after etching with Kalling's reagent (2 g of CuCl2, 40 ml of HCl, 80 ml of methanol). 2.3. Fatigue testing A special miniature prismatic specimen geometry introduced by Nicoletto (2017) was adopted to investigate the directional effect of the as-built surface quality on the notch fatigue strength of as-built SLM Inconel 718. The present test methodology has been already extensively used on DMLS Ti6Al4V, Nicoletto (2018). Fig. 1 shows the side view of the specimen subjected to bending and the associated principal stress distribution obtained by elastic finite element analysis. The size of the nominal minimum section is 5 x 5 mm2, the radius of the semicircular notch is 2 mm and the specimen length is 22 mm. A mild stress concentration factor Kt = max,FEA/ nom = 1.63 was obtained using nom = M/W and W is the section modulus of the notched section. The specimen loading is cyclic bending with a stress ratio R = 0 and a frequency of 20 Hz. Test run-out was fixed at 2x106 cycles.
Fig. 1. Side view of notched miniature specimen subjected to bending and elastic stress distribution.
Fig. 2 shows the different specimen directions with respect to build direction (i.e. Z directions) and their respective denomination.
Fig. 2. Orientation of the notched miniature specimens on the build plate and denomination (Z is build direction).
The surface quality of the notches is affected by the specimen orientation due to the layer-wise SLM fabrication. In addition, the direction of applied stress at the notch root may be parallel or orthogonal to the material layers. Namely, the semicircular notch geometry of Type B specimens is not influenced by the fabrication and the applied stress is parallel to the layers. The semicircular geometry of Type C specimen at its notch root is weakly influenced by the layer-wise fabrication and the applied stress is perpendicular to the layers. The circularity of the notch roots of
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Type A+ and Type A- specimens are strongly affected by the layer-wise fabrication (i.e. stair stepping, see Gebhard et al. (2014)). In addition, the surface of Type A- notches is classified as down-skin while the surface of Type A+ notches is up-skin. It is well known that down-skin surfaces are of lower quality (i.e. higher roughness poor geometrical accuracy) than up-skin surfaces. Applied stress in both Type A specimens is parallel to the layers. This qualitative description of the interaction of SLM fabrication and surface quality of the notches suggest that the fatigue behavior will depend on specimen directions. 3. Results and discussion 3.1. Directional notch fatigue behavior Fig. 3 shows the directional notch fatigue effect obtained in the four sets of the present mildly notched miniature specimens of SLM Inconel 718 with as-built surfaces. The independent variable is the maximum nominal stress of the bending load cycle with R = 0 and the dependent variable is number of cycles to failure. Four different and well defined S/N plots are determined for the different directions of fabrication. Considering Fig. 1, the Type B specimens shows the best fatigue performance while the Type A specimens the worst fatigue performance. Type C specimens determine an intermediate behavior. As Inconel 718 is face-centered cubic it may not exhibit a true fatigue limit, Reed (2006), a nominal fatigue strength at 2x106 cycles can be considered for comparison. So the fatigue strength varies from a maximum value of 360 MPa for Type B notched specimens to a minimum of 200 MPa for the Type A- specimens with a down-skin notch. These data will be assessed against limited published data, i.e. Witkin et al. (2018), in a later section. However, the relatively large differences in the respective S/N plots suggest that the fatigue behavior is not primarily controlled by the notch geometry (i.e. theoretically by Kt) but rather by the as-built surface quality, which on the other hand depend on process parameters and surface orientation. .
Fig. 3. Directional notch fatigue data for as-built SLM Inconel 718 (Kt = 1.63).
3.2. Directional quality of notch surfaces This section presents the characterization of the notch surfaces of the four specimen types of Fig. 2 using magnified optical microscopy. The aim is the qualitative comparison of the actual notch geometry from the theoretical semicircular geometry of 2-mm-radius. Not the entire profile is however relevant for the notch fatigue
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behavior. Since the peak stress and fatigue crack initiation occur at the notch root, the geometrical deviations between design intent and actual geometry are examined there. Fig. 4 compares the semicircular notch with the notch of Type B specimens. The build direction does not affect the entire notch geometry because it is obtained by sequential layer contouring. Contour width, process parameters and system accuracy may explain slight deviations observed.
Fig. 4. Effective semicircular notch in Type B specimen (white dotted curve is theoretical 2-mm radius circle).
Fig. 5 groups together three notch views taken from specimens that share the common build direction shown by the arrow. In these cases, large global deviations from notch circularity are apparent, the most evident being for the Type C and Type A- specimens while the Type A+ specimen is macroscopically close to circular.
Fig. 5. Effective semicircular notch in three types of specimens (white broken curve is theoretical 2 mm radius circle).
As mentioned before, however, it is the local quality at the notch root that is critical for the fatigue behavior. Therefore, Type C notch is globally poor but its notch root is not too different from the theoretical geometry because the layer-by-layer fabrication locally does not introduce significant stair-stepping of the surface, see also the magnified view in Fig. 6a.
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The notch root conditions are different for Type A+ specimens because the local surface roughness is strongly affected by stair stepping associated to the finite layer thickness, Gebhard et al. (2014). The magnified view of the notch root in Fig. 6b demonstrates the stair stepping introduced by the 50 m thick layers (broken lines).
a)
b)
Fig. 6. Effective notch root profile of a) Type C specimen and b) Type A- specimen. Dashed lines are 50 m-thick-layers.
Finally, the notch of Type A- specimens in Fig. 5 shows the worst quality of all. Specifically, the notch root combines the negative impact of the stair stepping effect of Type A+ notch and the un-supported down-skin surface orientation. The latter generates dross formation, high roughness and lack of geometrical accuracy. The optical inspection of the respective as-built notch roots of the SLM Inconel 718 specimens demonstrated the significant difference in effective quality. The ranking of the quality (from best i.e. Type B to worst i.e. Type A-) matches the experimental ranking obtained by fatigue testing of the four sets of fatigue specimens with as-built notch surfaces. Therefore, the S/N curves shown in Fig. 3 quantify the impact of the SLM technology on notch fatigue behavior. 3.3. Assessment of the notch fatigue behavior The notch fatigue behavior of SLM Inconel 718 shown in Fig. 3 has been obtained with a non-standard specimen configuration and test methodology, Nicoletto (2017). An assessment of the experimental data is important to support further use of the innovative method. Unfortunately, notch fatigue data of SLM Inconel 718 are scarce in the literature, an important and recent exception being the work of Witkin et al. (2018) that published fatigue data obtained with notched specimens fabricated on a Concept Laser M2 Cusing (layer thickness 30 μm), subsequently HIPped, solution treated and aged per industry standards. Axial tensile pulsating loading was performed in compliance with ASTM standard E466. Flat fatigue specimens printed in the horizontal direction and with a mild notch effect and as-built surfaces are considered here. The notched specimens of Witkin et al. (2018) and the present miniature specimens are compared in Fig. 7. The respective thicknesses were 2.5 mm for Witkin’s specimens and 5 mm for the present miniature specimens. Both specimen geometries and loading are characterized by a mild notch severity (i.e. K t = 1.93 vs. Kt = 1.63). Fig. 7 demonstrates the considerable difference in overall size and the same 2-mm radius of the notches. Since two notches were present in Witkin’s specimens, it is the worst-quality notch (i.e. down-skin) that controls fatigue crack initiation. On the other hand, the method of Nicoletto (2017) tested one notch at the time. So the two Type A miniature specimens not only characterized in fatigue the horizontal fabrication direction but also, selectively, the notch surface orientation (i.e. up-skin notch of Type A+ vs. down-skin notch of Type A-).
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The fatigue results for the as-built horizontal double notched specimens of SLM Inconel 718 by Witkin et al. (2018) are compared to the fatigue results of the as-built Type A+ and Type A- miniature specimens in Fig. 8. The three sets of data share the slope of the S/N curves but are slightly shifted one with respect to the others. In this case, however, the fatigue strength of about 150 MPa for the doubly-notched specimens is very close to the response of the Type A- miniature specimen (the specimen with the down-skin notch) while the Type A+ specimen data are unequivocally better.
Fig. 7. Comparison of double notched tensile plate of Witkin et al. (2018) and the two miniature bending specimens used here.
Fig. 8 Notched fatigue behavior of horizontal double notched plates vs. Type A+ and Type A- miniature specimens.
4. Conclusions This study was aimed at the experimental investigation of fatigue notch sensitivity of as-built SLM Inconel 718 in the presence of different surface orientations with respect to the build direction using the test methodology recently introduced by Nicoletto (2017). The main conclusions are the following: - The test methodology using miniature fatigue specimens efficiently provided original insight into the dependence of the fatigue behavior on the directionality of the notch fabrication by SLM processing. - Of the four directional specimens investigated and shown in Fig. 2, Type A- showed the worst notch fatigue performance, Type B the best and Type C and Type A+ intermediate notch fatigue performances.
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Optical inspection of the notch roots where fatigue crack initiation occurs clearly identified different surface qualities in accordance the experimental ranking in terms of fatigue strength. The present conclusions support the recommendation by Witkin et al. (2018) that the influence of both rough surfaces on fatigue behavior and the tendency of the SLM process to build features that deviate from design intent should be considered by designers and analysts when assessing the HCF performance of parts containing stress concentrations subject to cyclic loads. -
Acknowledgements Part of the research was supported by the project Slovak VEGA grant No. 1/0463/2019. Specimen fabrication by the technological partner BEAM-IT, Fornovo Taro (Italy) is gratefully acknowledged.
References Clark, D., Bache, M.R., Whittaker, M.T., 2008. Shaped metal deposition of a nickel alloy for aero engine applications. Journal of Materials Processing Technology 203, 39-448. Gebhardt, A., Hötter, J., Ziebura, D., 2014. Impact of SLM build parameters on the surface quality. RTejournal - Forum für Rapid Technologie, Vol. 2014 https://www.rtejournal.de/ausgabe11/3852 (accessed 2019/05/09) Gorelik, M., 2017. Additive manufacturing in the context of structural integrity. International Journal of Fatigue, 94, 168 ‐ 177. Nicoletto, G., 2017. Anisotropic high cycle fatigue behavior of Ti-6Al-4V obtained by powder bed laser fusion. International Journal of Fatigue, 94, pp. 255-262. Nicoletto, G., 2018, Directional and notch effects on the fatigue behavior of as-built DMLS Ti6Al4V, International Journal of Fatigue, 106, pp.124-131. Popovich, A.A., Sufiiarov, V.Sh., Polozov, I.A., Borisov, E.V., 2015. Microstructure and Mechanical Properties of Inconel 718 Produced by SLM and Subsequent Heat Treatment, Key Engineering Materials, doi ={10.4028/ www.scientific.net/ KEM.651-653.665} Reed, R.C., 2006. The Superalloys. Fundamentals and Applications, Cambridge University Press, pp. 372. Wells, D., 2016. Overview of fatigue and damage tolerance performance of powder bed fusion alloy N07718. ASTM/NIST Workshop on Mechanical Behavior of Additive Manufactured Components, San Antonio, Texas, NASA Report No. 20160007069. Witkin, D.B., Patel, D. N., Bean, G.E., 2018. Notched fatigue testing of Inconel 718 prepared by selective laser melting, Fatigue Fract Eng Mater Struct, 1 – 12. DOI: 10.1111/ffe.12880 Yadollahi, A., Shamsaei, N., 2017. Additive manufacturing of fatigue resistant materials: Challenges and opportunities, International Journal of Fatigue, 9814–31.