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Fatigue strength of notched specimens made of Ti-6Al-4V produced by Selected Laser Melting technique
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Procedia Integrity 13 StructuralStructural Integrity Procedia 00 (2018) (2016) 74–78 000–000
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ECF22 - Loading and Environmental effects on Structural Integrity ECF22 - Loading and Environmental effects on Structural Integrity
Fatigue strength of notched specimens made of Ti-6Al-4V produced Fatigue strength of notched specimens made of Ti-6Al-4V produced by Selected Laser Melting technique XV Portuguese Conference on Fracture, PCF 2016, 10-12 February 2016, Paço de Arcos, Portugal by Selected Laser Melting technique S.M.J. Razavi*, F. Berto Thermo-mechanical modeling of a high pressure turbine blade of an S.M.J. Razavi*, F. Berto Department of Mechanical and Industrial Engineering, Norwegian University of Science and Technology (NTNU), Trondheim 7491, Norway airplane gas turbine engine Department of Mechanical and Industrial Engineering, Norwegian University of Science and Technology (NTNU), Trondheim 7491, Norway P. Brandãoa, V. Infanteb, A.M. Deusc*
Abstract AbstractaDepartment of Mechanical Engineering, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1, 1049-001 Lisboa, Portugal As an b Additive Manufacturing (AM) method, Selective Laser Melting (SLM) allows fabrication of metallic components of any IDMEC, Department of Mechanical Engineering, Instituto Laser Superior Técnico, Universidade de Lisboa, Av.ofRovisco Pais, 1, 1049-001 of Lisboa, As an Additive Manufacturing (AM) method, Selective Melting (SLM) allows fabrication metallic components any desired geometry with densities greater than 99.5%. This method is usually used for geometrically optimized components with Portugal desired geometry with densities greater than 99.5%. This method is usually used for geometrically optimized components with c complex geometries including various kinds of notches. Hence, it is important to have a clear information about the fatigue CeFEMA, Department of Mechanical Engineering, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1, 1049-001 Lisboa, complex of geometries including various kinds notches.This Hence, it evaluates is important to havebehavior a clear of information fatigue behavior notched components made by SLMofmethod. paper the fatigue Ti-6Al-4Vabout blunt the V-notched Portugal behavior of notched made by Thisresults paper evaluates fatigue behavior blunt V-notched and circular notched components samples produced by SLM SLM.method. The fatigue were thenthe compared with thoseofofTi-6Al-4V the corresponding smooth and circular by SLM. Thewere fatigue results using were then compared with thoseElectron of the corresponding smooth samples. Thenotched fracturesamples surfacesproduced of the tested samples evaluated Environmental Scanning Microscopy (ESEM) Abstract samples. The fracture of the testedmechanisms samples were evaluated using Environmental Scanning Electron Microscopy (ESEM) and the crack initiationsurfaces points and fracture were identified. and the crack initiation points and fracture mechanisms were identified. their operation, modern aircraft B.V. engine components are subjected to increasingly demanding operating conditions, © During 2018 The Authors. Published by Elsevier © especially 2018 The Authors. Published byturbine Elsevier B.V. B.V. the high pressure (HPT) blades. Such conditions cause these parts to undergo different types of time-dependent © 2018 The Authors. Published by Elsevier Peer-review underresponsibility responsibility of ECF22 the ECF22 organizers. Peer-review under the organizers. degradation, oneresponsibility of which isofcreep. model using the finite element method (FEM) was developed, in order to be able to predict Peer-review under of theAECF22 organizers. the creep behaviour of HPT blades. Flight records for a notch, specific aircraft, provided by a commercial aviation Keywords: Fatigue; Notch Mechanics; Selective Laser data Melting (SLM);(FDR) Semi-circular V notch. company, were Notch used Mechanics; to obtain thermal dataSemi-circular for three different Keywords: Fatigue; Selective and Lasermechanical Melting (SLM); notch, V flight notch. cycles. In order to create the 3D model needed for the FEM analysis, a HPT blade scrap was scanned, and its chemical composition and material properties were obtained. The data that was gathered was fed into the FEM model and different simulations were run, first with a simplified 3D 1. rectangular Introduction block shape, in order to better establish the model, and then with the real 3D mesh obtained from the blade scrap. The 1. overall Introduction expected behaviour in terms of displacement was observed, in particular at the trailing edge of the blade. Therefore such a In thecan ambit of continuous of manufacturing processes, short product life cycles and the evermodel be useful in the goaldigitalization of predicting turbine blade life, given a set of modern FDR data.
In the ambit digitalization manufacturing modernproduction short product life cycles and the evergrowing need of forcontinuous high performance, lowof weight productsprocesses, with minimal needs, we face stringent growing needon for performance, weight products with minimal production needs, Inweorder faceto stringent © 2016 The Authors. Published by Elsevierlow B.V.of requirements bothhigh time and sophistication modern structural design and property prediction. digitally Peer-review under responsibility of the Scientific of PCF 2016. requirements on both time and sophistication modern conventional structural design andtechniques, property prediction. In order to methods digitally produce the advanced components of the futureofCommittee designs, design structural assessment produce the advanced components of the future designs,requirements conventional for design techniques, structural which assessment methods and production routines fail to achieve necessary structural complexities enhance the Keywords: High Pressure Turbine Creep; Finite Element Method; 3D Model; and production routines fail Blade; to achieve necessary requirements forSimulation. structural complexities which enhance the
* Corresponding author. Tel.: +47-735-93761. * Corresponding Tel.: +47-735-93761. E-mail address:author. [email protected] E-mail address: [email protected] 2452-3216 © 2018 The Authors. Published by Elsevier B.V. 2452-3216 © 2018 Authors. Published Elsevier B.V. Peer-review underThe responsibility of theby ECF22 organizers. * Corresponding Tel.: +351of218419991. Peer-review underauthor. responsibility the ECF22 organizers. E-mail address: [email protected] 2452-3216 © 2016 The Authors. Published by Elsevier B.V.
Peer-review under responsibility of the Scientific Committee of PCF 2016. 2452-3216 2018 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the ECF22 organizers. 10.1016/j.prostr.2018.12.013
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S.M.J. Razavi et al. / Procedia Structural Integrity 13 (2018) 74–78 Author name / Structural Integrity Procedia 00 (2018) 000–000
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performance of the components. AM technologies allow the direct conversion of digital designs into physical products with direct production leading to avoid the setup time and the use of tools. Dealing with AM techniques, a significant problem stand up in failure assessment. Considering the conventional manufacturing technologies (such as casting, milling, etc.), one utilizes a given material with defined and well-known material properties and removes material to obtain the desired geometry. In contrast, the material properties of additively manufactured components change during the fabrication process making it to be dependent on the geometry of the components. This means that every change in the geometry will change the way that the AM machine performs its fabrication routine affecting the properties of the resulting solid. The AM components are no longer isotropic, commonly not fully dense, the surfaces roughness is considerable and there is a high risk of inclusions, inhomogeneities and impurities, all related to the underlying manufacturing strategy, which, in turn, is a function of the input geometry (Wu et al., 2014; Lindgren et al., 2016; Todai et al., 2017). Although AM technology is undoubtedly of high potential and can fulfil the needs of modern digital manufacturing, however, due to poorly known material properties and lack of appropriate failure criteria their failure prediction is not fully guaranteed. The latest topology optimization routines can be used and developed further for better usability and better interaction with the AM manufacturing process chain fulfilling stringent requirements of aerospace, automotive and biomedical applications. To develop experimental and theoretical understanding of the structural integrity of these advanced geometric complex components is then a fundamental step for taking advantage of AM processes in structural components. Currently, no specific design criteria are presented for complex AM components considering stress concentration phenomena arising from geometrical discontinuities. Furthermore, only limited fatigue data are presented in technical literature by testing notched components made by AM metals. Hence, it is a very strategic point to fill this knowledge gap allowing future applicants to take full advantage of the unique features of AM, which will be key to integrate AM in every-day manufacturing. Among various methods of additive manufacturing, a particular attention is paid on Selective Laser Melting (SLM) which is a powder bed fusion laser method (Kruth et al., 2005). Due to excellent corrosion resistance, high specific strength, low density and low elastic modulus of Ti-6Al-4V, attracted considerable attention in aerospace and biomedical applications. Both in aerospace and biomedical applications, fatigue is the primary mechanism of rupture in components such as turbine blade, hip prosthesis and mechanical heart valve (Cherolis, 2008; Song et al., 2014; Sun et al., 2014). Hence, the fatigue strength of additive manufactured parts is widely studied in literature (Leuders et al., 2013; Spierings et al., 2013; Kasperovich and Hausmann, 2015; Riemer et al., 2015; Yadollahi and Shamsaei, 2017; Razavi et al., 2018). For example, Leuders et al. (2013) found that porosity acts as strong stress raiser and lead to failure of SLM produced TiAl6V4 samples. Reduction of porosity during fabrication process was considered by some researchers and defined to be more important than microstructure optimization for improving the fatigue strength of Ti6Al-4V samples manufactured by SLM. Kasperovich and Hausmann (2015) found a reduction in fatigue strength of SLM processed TiAl6V4 compared to the wrought alloy due to a combination of the unfavorable martensitic microstructure, unmolten particles, pores, and microcracks. They reported that the Hot Isostatic Pressing (HIP) process should be used for SLM-processed TiAl6V4 samples in order to restore the fatigue strength. The HIP process reduces porosity of printed material and subsequently enhance the fatigue life of the components. Finally, they proposed that heat treatment methods don’t give a considerable improvement on high cycle fatigue strength of AM samples. The majority of engineering components and structures contain notches of different shapes (e.g. V-, blunt V-, U-, O-, semi-circular and key-hole notches), mainly used for connecting various components together. By their utility, notches are prone to crack initiation due to the intensified stress at their neighborhood. Microcrack(s) in the vicinity of the notch root may extent somehow and lead to final failure of the component. Hence, it is commonly attempted in design of notched components to prevent or delay the crack initiation from the notch edge under static and fatigue loading (Ayatollahi et al., 2014; Ayatollahi et al., 2017). Dealing with AM notched components, no specific design criteria have been proposed so far to take into account stress concentration phenomena arising from geometrical discontinuities/features. In this context, this paper aims to contribute to the fundamental understanding of the fatigue behavior of additively manufactured Ti-6Al-4V specimens weakened by circular notches and blunt V notches. For this aim, the fatigue data related to notched AM samples available in two recently published researches by the authors (Razavi et al., 2017; Razavi et al., in press) are presented and compared. In the first part of the paper, details of the manufacturing method that was used for AM fabrication of the samples are described. Afterwards, the fatigue results of notched samples are presented and compared.
S.M.J. Razavi et al. / Procedia Structural Integrity 13 (2018) 74–78 Author name / Structural Integrity Procedia 00 (2018) 000–000
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3
2. Experimental procedure The Ti-6Al-4V samples were fabricated using the SLM technique according to the optimized process parameters that guaranteed a density greater than 99.7%. Afterwards, test samples were sandblasted at 6 bar using corundum sand with a mean grain size of 220 m followed by stress relief heat-treatment in non-controlled atmosphere (heating rate: 10.8 °C/min; holding time: 3h at 650 °C; cooling rate: 2 °C/min) to provide a better surface finish with lower residual stresses. Then the samples were cut off the base plate and re-sandblasted at 6 bar using corundum sand. A layer thickness of 60 m was considered for all specimens. The geometries of smooth, semi-circular and blunt V- notched specimens are illustrated in Fig. 1. The thickness of all testing samples was 3 mm. The notch tip radius of semi-circular notches and blunt V notches were 5 mm and 1mm, respectively. The Fatigue tests were carried out using a universal MTS machine (250 kN). All tests have been carried out under load control, using a sinusoidal signal in uniaxial tension with a frequency of 10 Hz and load ratio R = 0 (Razavi et al., 2017; Razavi et al., in press). a
b
c
Fig. 1. Geometries of the test samples, built axis (Z) (dimensions in mm); (a) smooth sample, (b) circular notched sample, and (c) blunt V notched sample.
3. Results and discussion Figure 2 show the fatigue test results for smooth and notched samples. Values of stress amplitude related to a survival probability of 50%, the slope of the Wöhler curve and the scatter index T which is the ratio between the stress amplitudes corresponding to 10% and 90% of survival probability are reported in the above-mentioned figures. Specimens survived over 1 million cycles are considered as run out and marked up with an arrow. It can be noted that the difference between the Wöhler curves are related only to the mean value of the stress amplitude at one million cycles, but not to the scatter index T. The fatigue strength of smooth samples was 243 MPa, while fatigue strengths 213 MPa and 144 MPa were respectively reported for the double circular notched specimens and blunt V notched specimens at 1 million cycles. By comparing the results from SLM semi-circular notched and smooth specimens it is observed a reduction of about 12% of the mean value of the stress range at 1 million cycles. While for the blunt V-notched specimens this reduction was about 41%. The inverse slope of the three curves is also significantly different, k = 4.74 for the smooth specimens, k = 4.88 for the semi-circular specimens and k = 4.15 for the blunt V-notched specimens. In all cases the scatter index is limited, with T σ = 1.328 for smooth specimens, Tσ = 1.332 for semi-circular specimens and Tσ = 1.158 for blunt V-notch specimens. From the high values for inverse slope k and fatigue stress, it is evident that SLM additively manufactured specimens demonstrate a comparable fatigue strength compared to the traditionally produced specimens (Berto et al., 2015). It should be mentioned that the notch sensitivity of the SLM Ti6Al4V samples were quite small for semi-circular specimens which can be related to the rough surface of the AM specimens and also to the presence of surface defects.
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S.M.J. Razavi et al. / Procedia Structural Integrity 13 (2018) 74–78 Author name / Structural Integrity Procedia 00 (2018) 000–000
b
1000
k=4.74
100 1.0E+04
1.0E+05 1.0E+06 Number of cycle, N (log)
1.0E+07
1000
Max. stress, σmax [MPa] (log)
c
1000
Max. stress, σmax [MPa] (log)
Smooth, R=0.01, f=10 Hz Scatter index Tσ = 1.328 Dσ50% = 243 MPa (N = 1 ∙ 106)
Max. stress, σmax [MPa] (log)
a
77
Circular notch, R=0.01, f=10 Hz Scatter index Tσ = 1.332 Dσ50% = 213 MPa (N = 1 ∙ 106)
k=4.88
100 1.0E+04
1.0E+05 1.0E+06 Number of cycle, N (log)
1.0E+07
Blunt V-notch, R=0.01, f=10 Hz Scatter index Tσ = 1.158 Dσ50% = 144 MPa (N = 1 ∙ 106)
k=4.15
100 1.0E+04
1.0E+05 1.0E+06 1.0E+07 Number of cycle, N (log) Fig. 2. Fatigue life curved for the tested specimens; (a) smooth samples, (b) circular notched samples, and (c) blunt V notched samples.
2D numerical simulations under plain stress condition were carried out to calculate the stress concentration factor, Kt. Young’s modulus and Poisson’s coefficient were set equal to 110 GPa and 0.34, respectively. By using the obtained experimental data, the notch sensitivity, q can be obtained using Eq. 1. q
K f 1 Kt 1
(1)
where Kf is the fatigue notch factor and Kt is the stress concentration factor. The stress concentration factor defined by maximum local stress to nominal stress ratio and the fatigue notch factor defined by the ratio between fatigue strength of smooth and notched samples were calculated and presented in Table 1. Notch sensitivities of 0.458 and 0.538 were obtained for semi-circular and V notched samples. According to the fatigue results, semi-circular samples with lower notch sensitivity had higher scatter index and V notched samples with higher notch sensitivity had lower scatter index. This can be due to this fact that the samples with lower stress concentration factor are more sensitive to the surface condition meaning that the presence of any defect or high roughness on the surface results in a different fatigue life that expected. This increases the scatter index for semi-circular samples. Generally, two categories of global discontinuities (notch geometries) and local discontinuities (such as defects and surface roughness) are available in materials (including AM materials). For the notched specimens with high stress concentration factor, it is assumed that the global discontinuities govern the failure. While for the notch specimens with low stress concentration factor, the local discontinuities can govern the fatigue failure. The stress concentration factor, the surface condition and the number of defects are the key parameters in defining the governing failure mechanisms. Further experiments are required to illustrate a clear pattern among these parameters.
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Table 1. Notch sensitivity results for the tested samples. Δσ50%* (MPa) Kt Unnotched 243 1.073 Semi-circular notch 213 1.308 V notch 144 2.279 * fatigue strength at 106 cycles.
Kf 1.141 1.688
5
q 0.458 0.538
4. Conclusions This work aimed to improve the basic understanding of the fatigue behavior of the notched specimens produced with AM techniques by comparing the fatigue data of two different notched specimens produced by Selective Laser Melting technique. The fatigue resistance of SLM produced Ti-6Al-4V notched samples were presented and compared for different notch geometries. Results were compared with those corresponding to smooth samples. Despite the fatigue specimens were weakened by the notches, a low notch sensitivity was observed, which was reported to be correlated to the surface roughness of the AM samples. References Ayatollahi, M.R., Razavi, S. M. J., Sommitsch, C., Moser, C., 2017. Fatigue life extension by crack repair using double stop-hole technique. Materials Science Forum 879, 3-8. Ayatollahi, M.R., Razavi, S.M.J., Chamani, H.R., 2014. A numerical study on the effect of symmetric crack flank holes on fatigue life extension of a SENT specimen. Fatigue and Fracture of Engineering Materials and Structures 37(10), 1153-1164. Berto, F., Campagnolo, A., Lazzarin, P., 2015. Fatigue strength of severely notched specimens made of Ti–6Al–4V under multiaxial loading. Fatigue and Fracture of Engineering Materials and Structures 38(5), 503-517. Cherolis, N. E., 2008. Fatigue in the Aerospace Industry: Striations. Journal of Failure Analysis and Prevention 8(3), 255–258. Kasperovich, G., Hausmann, J., 2015. Improvement of fatigue resistance and ductility of Ti-6Al-4V processed by selective laser melting. Journal of Materials Processing Technology 220, 202-214. Kruth, J. P., Mercelis, P., Van Vaerenbergh, J., Froyen, L., Romboust, M., 2005. Binding mechanism in selective laser sintering and selective laser melting. Rapid Prototyping Journal 11(1), 26-36. Leuders, S., Thöne, M., Riemer, A., Niendorf, T., Tröster, T., Richard, H. A., Maier, H. J., 2013. On the mechanical behaviour of titanium alloy TiAl6V4 manufactured by selective laser melting: fatigue resistance and crack growth performance. International Journal of Fatigue 48, 300307. Lindgren, L. E., Lundbäck, A., Fisk, M., Pederson, R., Andersson, J., 2016. Simulation of additive manufacturing using coupled constitutive and microstructure models. Additive Manufacturing 12, 144–158. Razavi, S.M.J., Ferro, P., Berto, F., 2017. Fatigue Assessment of Ti–6Al–4V Circular Notched Specimens Produced by Selective Laser Melting. Metals 7(8), 291. Razavi, S.M.J., Bordonaro, G.G., Ferro, P., Torgersen, J., Berto, F., 2018. Fatigue Behavior of Porous Ti-6Al-4V Made by Laser-Engineered Net Shaping. Materials 11(2), 284. Razavi, S.M.J., Ferro, P., Berto, F., Torgersen, J., (in press). Fatigue strength of blunt V-notched specimens produced by selective laser melting of Ti-6Al-4V. Theoretical and Applied Fracture Mechanics. (DOI: 10.1016/j.tafmec.2017.06.021). Riemer, A., Richard, H.A., Bruggemann, J.P., Wesendahl, J.N., 2015. Fatigue crack growth in additive manufactured products. Frattura ed Integrità Strutturale 34, 437-446. Song, X., Wang, L., Niinomi, M., Nakai, M., Liu, Y., Zhu, M., 2014. Microstructure and fatigue behaviors of a biomedical Ti–Nb–Ta–Zr alloy with trace CeO2 additions. Materials Science and Engineering: A 619, 112–118. Spierings, A.B., Starr, T.L., Wegener, K., 2013. Fatigue performance of additive manufactured metallic parts. Rapid Prototyping Journal 19(2), 88-94. Sun, Z., Chemkhi, M., Kanoute, P., Retraint, D., 2014. Fatigue properties of a biomedical 316L steel processed by surface mechanical attrition. Materials Science and Engineering 63, 012021. (DOI:10.1088/1757-899X/63/1/012021) Todai, M., Nakano, T., Liu, T., Yasuda, H. Y., Hagihara, K., Cho, K., Ueda, M., Takeyama, M., 2017. Effect of building direction on the microstructure and tensile properties of Ti-48Al-2Cr-2Nb alloy additively manufactured by electron beam melting. Additive Manufacturing 13, 61–70. Wu, X., Liang, J., Mei, J., Mitchell, C., Goodwin, P. S., Voice, W., 2014. Microstructures of laser-deposited Ti-6Al-4V. Materials & Design 25(2), 137-144. Yadollahi, A., Shamsaei, N., 2017. Additive manufacturing of fatigue resistant materials: Challenges and opportunities. International Journal of Fatigue 98, 14–31.
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Procedia Integrity 13 StructuralStructural Integrity Procedia 00 (2018) (2016) 74–78 000–000
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ECF22 - Loading and Environmental effects on Structural Integrity ECF22 - Loading and Environmental effects on Structural Integrity
Fatigue strength of notched specimens made of Ti-6Al-4V produced Fatigue strength of notched specimens made of Ti-6Al-4V produced by Selected Laser Melting technique XV Portuguese Conference on Fracture, PCF 2016, 10-12 February 2016, Paço de Arcos, Portugal by Selected Laser Melting technique S.M.J. Razavi*, F. Berto Thermo-mechanical modeling of a high pressure turbine blade of an S.M.J. Razavi*, F. Berto Department of Mechanical and Industrial Engineering, Norwegian University of Science and Technology (NTNU), Trondheim 7491, Norway airplane gas turbine engine Department of Mechanical and Industrial Engineering, Norwegian University of Science and Technology (NTNU), Trondheim 7491, Norway P. Brandãoa, V. Infanteb, A.M. Deusc*
Abstract AbstractaDepartment of Mechanical Engineering, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1, 1049-001 Lisboa, Portugal As an b Additive Manufacturing (AM) method, Selective Laser Melting (SLM) allows fabrication of metallic components of any IDMEC, Department of Mechanical Engineering, Instituto Laser Superior Técnico, Universidade de Lisboa, Av.ofRovisco Pais, 1, 1049-001 of Lisboa, As an Additive Manufacturing (AM) method, Selective Melting (SLM) allows fabrication metallic components any desired geometry with densities greater than 99.5%. This method is usually used for geometrically optimized components with Portugal desired geometry with densities greater than 99.5%. This method is usually used for geometrically optimized components with c complex geometries including various kinds of notches. Hence, it is important to have a clear information about the fatigue CeFEMA, Department of Mechanical Engineering, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1, 1049-001 Lisboa, complex of geometries including various kinds notches.This Hence, it evaluates is important to havebehavior a clear of information fatigue behavior notched components made by SLMofmethod. paper the fatigue Ti-6Al-4Vabout blunt the V-notched Portugal behavior of notched made by Thisresults paper evaluates fatigue behavior blunt V-notched and circular notched components samples produced by SLM SLM.method. The fatigue were thenthe compared with thoseofofTi-6Al-4V the corresponding smooth and circular by SLM. Thewere fatigue results using were then compared with thoseElectron of the corresponding smooth samples. Thenotched fracturesamples surfacesproduced of the tested samples evaluated Environmental Scanning Microscopy (ESEM) Abstract samples. The fracture of the testedmechanisms samples were evaluated using Environmental Scanning Electron Microscopy (ESEM) and the crack initiationsurfaces points and fracture were identified. and the crack initiation points and fracture mechanisms were identified. their operation, modern aircraft B.V. engine components are subjected to increasingly demanding operating conditions, © During 2018 The Authors. Published by Elsevier © especially 2018 The Authors. Published byturbine Elsevier B.V. B.V. the high pressure (HPT) blades. Such conditions cause these parts to undergo different types of time-dependent © 2018 The Authors. Published by Elsevier Peer-review underresponsibility responsibility of ECF22 the ECF22 organizers. Peer-review under the organizers. degradation, oneresponsibility of which isofcreep. model using the finite element method (FEM) was developed, in order to be able to predict Peer-review under of theAECF22 organizers. the creep behaviour of HPT blades. Flight records for a notch, specific aircraft, provided by a commercial aviation Keywords: Fatigue; Notch Mechanics; Selective Laser data Melting (SLM);(FDR) Semi-circular V notch. company, were Notch used Mechanics; to obtain thermal dataSemi-circular for three different Keywords: Fatigue; Selective and Lasermechanical Melting (SLM); notch, V flight notch. cycles. In order to create the 3D model needed for the FEM analysis, a HPT blade scrap was scanned, and its chemical composition and material properties were obtained. The data that was gathered was fed into the FEM model and different simulations were run, first with a simplified 3D 1. rectangular Introduction block shape, in order to better establish the model, and then with the real 3D mesh obtained from the blade scrap. The 1. overall Introduction expected behaviour in terms of displacement was observed, in particular at the trailing edge of the blade. Therefore such a In thecan ambit of continuous of manufacturing processes, short product life cycles and the evermodel be useful in the goaldigitalization of predicting turbine blade life, given a set of modern FDR data.
In the ambit digitalization manufacturing modernproduction short product life cycles and the evergrowing need of forcontinuous high performance, lowof weight productsprocesses, with minimal needs, we face stringent growing needon for performance, weight products with minimal production needs, Inweorder faceto stringent © 2016 The Authors. Published by Elsevierlow B.V.of requirements bothhigh time and sophistication modern structural design and property prediction. digitally Peer-review under responsibility of the Scientific of PCF 2016. requirements on both time and sophistication modern conventional structural design andtechniques, property prediction. In order to methods digitally produce the advanced components of the futureofCommittee designs, design structural assessment produce the advanced components of the future designs,requirements conventional for design techniques, structural which assessment methods and production routines fail to achieve necessary structural complexities enhance the Keywords: High Pressure Turbine Creep; Finite Element Method; 3D Model; and production routines fail Blade; to achieve necessary requirements forSimulation. structural complexities which enhance the
* Corresponding author. Tel.: +47-735-93761. * Corresponding Tel.: +47-735-93761. E-mail address:author. [email protected] E-mail address: [email protected] 2452-3216 © 2018 The Authors. Published by Elsevier B.V. 2452-3216 © 2018 Authors. Published Elsevier B.V. Peer-review underThe responsibility of theby ECF22 organizers. * Corresponding Tel.: +351of218419991. Peer-review underauthor. responsibility the ECF22 organizers. E-mail address: [email protected] 2452-3216 © 2016 The Authors. Published by Elsevier B.V.
Peer-review under responsibility of the Scientific Committee of PCF 2016. 2452-3216 2018 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the ECF22 organizers. 10.1016/j.prostr.2018.12.013
2
S.M.J. Razavi et al. / Procedia Structural Integrity 13 (2018) 74–78 Author name / Structural Integrity Procedia 00 (2018) 000–000
75
performance of the components. AM technologies allow the direct conversion of digital designs into physical products with direct production leading to avoid the setup time and the use of tools. Dealing with AM techniques, a significant problem stand up in failure assessment. Considering the conventional manufacturing technologies (such as casting, milling, etc.), one utilizes a given material with defined and well-known material properties and removes material to obtain the desired geometry. In contrast, the material properties of additively manufactured components change during the fabrication process making it to be dependent on the geometry of the components. This means that every change in the geometry will change the way that the AM machine performs its fabrication routine affecting the properties of the resulting solid. The AM components are no longer isotropic, commonly not fully dense, the surfaces roughness is considerable and there is a high risk of inclusions, inhomogeneities and impurities, all related to the underlying manufacturing strategy, which, in turn, is a function of the input geometry (Wu et al., 2014; Lindgren et al., 2016; Todai et al., 2017). Although AM technology is undoubtedly of high potential and can fulfil the needs of modern digital manufacturing, however, due to poorly known material properties and lack of appropriate failure criteria their failure prediction is not fully guaranteed. The latest topology optimization routines can be used and developed further for better usability and better interaction with the AM manufacturing process chain fulfilling stringent requirements of aerospace, automotive and biomedical applications. To develop experimental and theoretical understanding of the structural integrity of these advanced geometric complex components is then a fundamental step for taking advantage of AM processes in structural components. Currently, no specific design criteria are presented for complex AM components considering stress concentration phenomena arising from geometrical discontinuities. Furthermore, only limited fatigue data are presented in technical literature by testing notched components made by AM metals. Hence, it is a very strategic point to fill this knowledge gap allowing future applicants to take full advantage of the unique features of AM, which will be key to integrate AM in every-day manufacturing. Among various methods of additive manufacturing, a particular attention is paid on Selective Laser Melting (SLM) which is a powder bed fusion laser method (Kruth et al., 2005). Due to excellent corrosion resistance, high specific strength, low density and low elastic modulus of Ti-6Al-4V, attracted considerable attention in aerospace and biomedical applications. Both in aerospace and biomedical applications, fatigue is the primary mechanism of rupture in components such as turbine blade, hip prosthesis and mechanical heart valve (Cherolis, 2008; Song et al., 2014; Sun et al., 2014). Hence, the fatigue strength of additive manufactured parts is widely studied in literature (Leuders et al., 2013; Spierings et al., 2013; Kasperovich and Hausmann, 2015; Riemer et al., 2015; Yadollahi and Shamsaei, 2017; Razavi et al., 2018). For example, Leuders et al. (2013) found that porosity acts as strong stress raiser and lead to failure of SLM produced TiAl6V4 samples. Reduction of porosity during fabrication process was considered by some researchers and defined to be more important than microstructure optimization for improving the fatigue strength of Ti6Al-4V samples manufactured by SLM. Kasperovich and Hausmann (2015) found a reduction in fatigue strength of SLM processed TiAl6V4 compared to the wrought alloy due to a combination of the unfavorable martensitic microstructure, unmolten particles, pores, and microcracks. They reported that the Hot Isostatic Pressing (HIP) process should be used for SLM-processed TiAl6V4 samples in order to restore the fatigue strength. The HIP process reduces porosity of printed material and subsequently enhance the fatigue life of the components. Finally, they proposed that heat treatment methods don’t give a considerable improvement on high cycle fatigue strength of AM samples. The majority of engineering components and structures contain notches of different shapes (e.g. V-, blunt V-, U-, O-, semi-circular and key-hole notches), mainly used for connecting various components together. By their utility, notches are prone to crack initiation due to the intensified stress at their neighborhood. Microcrack(s) in the vicinity of the notch root may extent somehow and lead to final failure of the component. Hence, it is commonly attempted in design of notched components to prevent or delay the crack initiation from the notch edge under static and fatigue loading (Ayatollahi et al., 2014; Ayatollahi et al., 2017). Dealing with AM notched components, no specific design criteria have been proposed so far to take into account stress concentration phenomena arising from geometrical discontinuities/features. In this context, this paper aims to contribute to the fundamental understanding of the fatigue behavior of additively manufactured Ti-6Al-4V specimens weakened by circular notches and blunt V notches. For this aim, the fatigue data related to notched AM samples available in two recently published researches by the authors (Razavi et al., 2017; Razavi et al., in press) are presented and compared. In the first part of the paper, details of the manufacturing method that was used for AM fabrication of the samples are described. Afterwards, the fatigue results of notched samples are presented and compared.
S.M.J. Razavi et al. / Procedia Structural Integrity 13 (2018) 74–78 Author name / Structural Integrity Procedia 00 (2018) 000–000
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2. Experimental procedure The Ti-6Al-4V samples were fabricated using the SLM technique according to the optimized process parameters that guaranteed a density greater than 99.7%. Afterwards, test samples were sandblasted at 6 bar using corundum sand with a mean grain size of 220 m followed by stress relief heat-treatment in non-controlled atmosphere (heating rate: 10.8 °C/min; holding time: 3h at 650 °C; cooling rate: 2 °C/min) to provide a better surface finish with lower residual stresses. Then the samples were cut off the base plate and re-sandblasted at 6 bar using corundum sand. A layer thickness of 60 m was considered for all specimens. The geometries of smooth, semi-circular and blunt V- notched specimens are illustrated in Fig. 1. The thickness of all testing samples was 3 mm. The notch tip radius of semi-circular notches and blunt V notches were 5 mm and 1mm, respectively. The Fatigue tests were carried out using a universal MTS machine (250 kN). All tests have been carried out under load control, using a sinusoidal signal in uniaxial tension with a frequency of 10 Hz and load ratio R = 0 (Razavi et al., 2017; Razavi et al., in press). a
b
c
Fig. 1. Geometries of the test samples, built axis (Z) (dimensions in mm); (a) smooth sample, (b) circular notched sample, and (c) blunt V notched sample.
3. Results and discussion Figure 2 show the fatigue test results for smooth and notched samples. Values of stress amplitude related to a survival probability of 50%, the slope of the Wöhler curve and the scatter index T which is the ratio between the stress amplitudes corresponding to 10% and 90% of survival probability are reported in the above-mentioned figures. Specimens survived over 1 million cycles are considered as run out and marked up with an arrow. It can be noted that the difference between the Wöhler curves are related only to the mean value of the stress amplitude at one million cycles, but not to the scatter index T. The fatigue strength of smooth samples was 243 MPa, while fatigue strengths 213 MPa and 144 MPa were respectively reported for the double circular notched specimens and blunt V notched specimens at 1 million cycles. By comparing the results from SLM semi-circular notched and smooth specimens it is observed a reduction of about 12% of the mean value of the stress range at 1 million cycles. While for the blunt V-notched specimens this reduction was about 41%. The inverse slope of the three curves is also significantly different, k = 4.74 for the smooth specimens, k = 4.88 for the semi-circular specimens and k = 4.15 for the blunt V-notched specimens. In all cases the scatter index is limited, with T σ = 1.328 for smooth specimens, Tσ = 1.332 for semi-circular specimens and Tσ = 1.158 for blunt V-notch specimens. From the high values for inverse slope k and fatigue stress, it is evident that SLM additively manufactured specimens demonstrate a comparable fatigue strength compared to the traditionally produced specimens (Berto et al., 2015). It should be mentioned that the notch sensitivity of the SLM Ti6Al4V samples were quite small for semi-circular specimens which can be related to the rough surface of the AM specimens and also to the presence of surface defects.
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S.M.J. Razavi et al. / Procedia Structural Integrity 13 (2018) 74–78 Author name / Structural Integrity Procedia 00 (2018) 000–000
b
1000
k=4.74
100 1.0E+04
1.0E+05 1.0E+06 Number of cycle, N (log)
1.0E+07
1000
Max. stress, σmax [MPa] (log)
c
1000
Max. stress, σmax [MPa] (log)
Smooth, R=0.01, f=10 Hz Scatter index Tσ = 1.328 Dσ50% = 243 MPa (N = 1 ∙ 106)
Max. stress, σmax [MPa] (log)
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Circular notch, R=0.01, f=10 Hz Scatter index Tσ = 1.332 Dσ50% = 213 MPa (N = 1 ∙ 106)
k=4.88
100 1.0E+04
1.0E+05 1.0E+06 Number of cycle, N (log)
1.0E+07
Blunt V-notch, R=0.01, f=10 Hz Scatter index Tσ = 1.158 Dσ50% = 144 MPa (N = 1 ∙ 106)
k=4.15
100 1.0E+04
1.0E+05 1.0E+06 1.0E+07 Number of cycle, N (log) Fig. 2. Fatigue life curved for the tested specimens; (a) smooth samples, (b) circular notched samples, and (c) blunt V notched samples.
2D numerical simulations under plain stress condition were carried out to calculate the stress concentration factor, Kt. Young’s modulus and Poisson’s coefficient were set equal to 110 GPa and 0.34, respectively. By using the obtained experimental data, the notch sensitivity, q can be obtained using Eq. 1. q
K f 1 Kt 1
(1)
where Kf is the fatigue notch factor and Kt is the stress concentration factor. The stress concentration factor defined by maximum local stress to nominal stress ratio and the fatigue notch factor defined by the ratio between fatigue strength of smooth and notched samples were calculated and presented in Table 1. Notch sensitivities of 0.458 and 0.538 were obtained for semi-circular and V notched samples. According to the fatigue results, semi-circular samples with lower notch sensitivity had higher scatter index and V notched samples with higher notch sensitivity had lower scatter index. This can be due to this fact that the samples with lower stress concentration factor are more sensitive to the surface condition meaning that the presence of any defect or high roughness on the surface results in a different fatigue life that expected. This increases the scatter index for semi-circular samples. Generally, two categories of global discontinuities (notch geometries) and local discontinuities (such as defects and surface roughness) are available in materials (including AM materials). For the notched specimens with high stress concentration factor, it is assumed that the global discontinuities govern the failure. While for the notch specimens with low stress concentration factor, the local discontinuities can govern the fatigue failure. The stress concentration factor, the surface condition and the number of defects are the key parameters in defining the governing failure mechanisms. Further experiments are required to illustrate a clear pattern among these parameters.
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Table 1. Notch sensitivity results for the tested samples. Δσ50%* (MPa) Kt Unnotched 243 1.073 Semi-circular notch 213 1.308 V notch 144 2.279 * fatigue strength at 106 cycles.
Kf 1.141 1.688
5
q 0.458 0.538
4. Conclusions This work aimed to improve the basic understanding of the fatigue behavior of the notched specimens produced with AM techniques by comparing the fatigue data of two different notched specimens produced by Selective Laser Melting technique. The fatigue resistance of SLM produced Ti-6Al-4V notched samples were presented and compared for different notch geometries. Results were compared with those corresponding to smooth samples. Despite the fatigue specimens were weakened by the notches, a low notch sensitivity was observed, which was reported to be correlated to the surface roughness of the AM samples. References Ayatollahi, M.R., Razavi, S. M. J., Sommitsch, C., Moser, C., 2017. Fatigue life extension by crack repair using double stop-hole technique. Materials Science Forum 879, 3-8. Ayatollahi, M.R., Razavi, S.M.J., Chamani, H.R., 2014. A numerical study on the effect of symmetric crack flank holes on fatigue life extension of a SENT specimen. Fatigue and Fracture of Engineering Materials and Structures 37(10), 1153-1164. Berto, F., Campagnolo, A., Lazzarin, P., 2015. Fatigue strength of severely notched specimens made of Ti–6Al–4V under multiaxial loading. Fatigue and Fracture of Engineering Materials and Structures 38(5), 503-517. Cherolis, N. E., 2008. Fatigue in the Aerospace Industry: Striations. Journal of Failure Analysis and Prevention 8(3), 255–258. Kasperovich, G., Hausmann, J., 2015. Improvement of fatigue resistance and ductility of Ti-6Al-4V processed by selective laser melting. Journal of Materials Processing Technology 220, 202-214. Kruth, J. P., Mercelis, P., Van Vaerenbergh, J., Froyen, L., Romboust, M., 2005. Binding mechanism in selective laser sintering and selective laser melting. Rapid Prototyping Journal 11(1), 26-36. Leuders, S., Thöne, M., Riemer, A., Niendorf, T., Tröster, T., Richard, H. A., Maier, H. J., 2013. On the mechanical behaviour of titanium alloy TiAl6V4 manufactured by selective laser melting: fatigue resistance and crack growth performance. International Journal of Fatigue 48, 300307. Lindgren, L. E., Lundbäck, A., Fisk, M., Pederson, R., Andersson, J., 2016. Simulation of additive manufacturing using coupled constitutive and microstructure models. Additive Manufacturing 12, 144–158. Razavi, S.M.J., Ferro, P., Berto, F., 2017. Fatigue Assessment of Ti–6Al–4V Circular Notched Specimens Produced by Selective Laser Melting. Metals 7(8), 291. Razavi, S.M.J., Bordonaro, G.G., Ferro, P., Torgersen, J., Berto, F., 2018. Fatigue Behavior of Porous Ti-6Al-4V Made by Laser-Engineered Net Shaping. Materials 11(2), 284. Razavi, S.M.J., Ferro, P., Berto, F., Torgersen, J., (in press). Fatigue strength of blunt V-notched specimens produced by selective laser melting of Ti-6Al-4V. Theoretical and Applied Fracture Mechanics. (DOI: 10.1016/j.tafmec.2017.06.021). Riemer, A., Richard, H.A., Bruggemann, J.P., Wesendahl, J.N., 2015. Fatigue crack growth in additive manufactured products. Frattura ed Integrità Strutturale 34, 437-446. Song, X., Wang, L., Niinomi, M., Nakai, M., Liu, Y., Zhu, M., 2014. Microstructure and fatigue behaviors of a biomedical Ti–Nb–Ta–Zr alloy with trace CeO2 additions. Materials Science and Engineering: A 619, 112–118. Spierings, A.B., Starr, T.L., Wegener, K., 2013. Fatigue performance of additive manufactured metallic parts. Rapid Prototyping Journal 19(2), 88-94. Sun, Z., Chemkhi, M., Kanoute, P., Retraint, D., 2014. Fatigue properties of a biomedical 316L steel processed by surface mechanical attrition. Materials Science and Engineering 63, 012021. (DOI:10.1088/1757-899X/63/1/012021) Todai, M., Nakano, T., Liu, T., Yasuda, H. Y., Hagihara, K., Cho, K., Ueda, M., Takeyama, M., 2017. Effect of building direction on the microstructure and tensile properties of Ti-48Al-2Cr-2Nb alloy additively manufactured by electron beam melting. Additive Manufacturing 13, 61–70. Wu, X., Liang, J., Mei, J., Mitchell, C., Goodwin, P. S., Voice, W., 2014. Microstructures of laser-deposited Ti-6Al-4V. Materials & Design 25(2), 137-144. Yadollahi, A., Shamsaei, N., 2017. Additive manufacturing of fatigue resistant materials: Challenges and opportunities. International Journal of Fatigue 98, 14–31.