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Effect of Surface Roughness on Fatigue Strength of Ti-6Al-4V Alloy Manufactured by Additive Manufacturing
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Procedia Structural Integrity 19 (2019) 294–301
Fatigue Design 2019 Fatigue Design 2019
Effect of Surface Roughness on Fatigue Strength of Ti-6Al-4V Effect of Surface Roughness on Fatigue Strength of Ti-6Al-4V Alloy Manufactured by Additive Manufacturing Alloy Manufactured by Additive Manufacturing Masanori Nakatani a,a, *, Hiroshige Masuo bb, Yuzo Tanaka aa, Yukitaka Murakami a,c Masanori Nakatani *, Hiroshige Masuo , Yuzo Tanaka , Yukitaka Murakami a,c a Kobe Material Testing Laboratory Co. Ltd., Harima-Town, Hyogo 675-0135, Japan a Kobe Material Laboratory Co.Ebina, Ltd., Harima-Town, Hyogo 675-0135, Japan b MetalTesting Technology Co. Ltd., Kanagawa, 243-0424, Japan b Metal Technology Co. Ltd., Ebina,Fukuoka Kanagawa, 243-0424, Japan c Kyushu University, Nishi-ku, 819-0395, Japan c Kyushu University, Nishi-ku, Fukuoka 819-0395, Japan
Abstract Abstract This study focuses on the effects of surface roughness on the fatigue strength of Ti-6Al-4V alloy manufactured by additive This study focuses onelectron the effects surface(EBM) roughness on themetal fatigue strength of (DMLS) Ti-6Al-4V alloy manufactured by pressing additive manufacturing (AM); beamofmelting and direct laser sintering methods. The hot isostatic manufacturing (AM); electron beam and direct metal laser sintering (DMLS) methods. The hot Fatigue isostaticlimit pressing (HIP) was applied to some series of melting polished(EBM) specimens and as-built specimens to eliminate internal defects. was (HIP) was by applied to some series of polished specimensHIP andincreased as-built specimens eliminate internal was evaluated rotating bending fatigue tests. Although the fatigueto limits slightly, the defects. fatigue Fatigue limits oflimit as-built evaluated by rotating bending fatigue30% tests. HIP increased the fatigue slightly, the fatigue limits as-built specimens with HIP were only about of Although the ideal fatigue limit expected from limits the Vickers hardness. On the otherofhand, the specimensspecimens with HIPwith wereHIP only about 30% oflimit the ideal limitupper expected from thefatigue Vickers hardness. On the atother hand,sites the polished had the fatigue closefatigue to the ideal bound. The crack was initiated multiple polished specimens with HIP had the fatigue limit close to the ideal upper bound. Theonfatigue crack was initiated at multiple sites on the surface in as-built specimens. Thus, surface roughness has crucial influence fatigue strength of AM component. The on the surface in as-built specimens. Thus,issurface roughness hascomplex. crucial influence on fatigue strength ofdefect AM component. The surface morphology of as-build specimen three-dimensionally The evaluation of equivalent size for surface surface morphology ofby as-build specimen model is three-dimensionally complex. The evaluation of equivalent defect size forsize surface roughness was attempt √area parameter assuming the surface roughness is a small defect. The estimated defect was roughness was attempt by √area model assuming the surfacethat roughness is a small defect.ofThe estimated defectshould size was over the applicable range of theparameter √area parameter model, suggesting the surface roughness as-built specimen be over the applicable range of the √area parameter model, suggesting that the surface roughness of as-built specimen should be considered as long crack. considered as long crack. © 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 Fatigue Design 2019 Organizers. Peer-review under responsibility of the Fatigue Design 2019 Organizers. Peer-review under responsibility of the Fatigue Design 2019 Organizers. Keywords: Additive manufacturing, Fatigue, Surface roughness, Titanium alloy, Hot isostatic pressing, √area parameter model, Keywords: Additive manufacturing, Fatigue, Surface roughness, Titanium alloy, Hot isostatic pressing, √area parameter model,
* Corresponding author. Tel.: +81-79-435-5010; fax: +81-79-435-5102. * Corresponding Tel.: +81-79-435-5010; fax: +81-79-435-5102. E-mail 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 by Elsevier B.V. Organizers. Peer-review©under the Fatigue Design 2019 Peer-review under responsibility of the Fatigue Design 2019 Organizers.
2452-3216 © 2019 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the Fatigue Design 2019 Organizers. 10.1016/j.prostr.2019.12.032
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1. Introduction The additive manufacturing (AM) is a group of processes that join materials to make objects from threedimensional model data, usually layer by layer. AM is expected as an innovative manufacturing process for various parts with complex geometry that cannot fabricated by traditional process. Many review papers on AM have been also published (e.g. Frazier (2014), Lewandowski (2016), Yadollahi (2017), Liu (2019)). However, AM still remains some problems to be overcome; such as cost, size, quality and reliability. One of the problems is low fatigue strength in comparison with conventional material. Many researchers emphasizes that the tensile strength and hardness of the AM material are the same level as those of the conventional material (e.g. Baufeld (2010), Hrabe (2013), Hayes (2017), Wysocki (2017)). However, it is reported that the fatigue strength of as-built AM material is significantly low because of defects (Günther (2017), Romano (2018), Masuo (2018), Yamashita (2018)). The defects are caused by gas pore and spatter during melting process of metal powder. The occurrence of these defects is not solved completely though the defects decrease with a development of AM technology. The existence of defects disturbs the application of AM to structural components that need fatigue strength. We previously conducted the fatigue tests of Ti-6Al-4V alloy fabricated by AM and revealed that fatigue limit can be quantitatively evaluated using the √area parameter model proposed by Murakami (1994) (Masuo (2018)). Moreover, we also pointed out that hot isostatic pressing (HIP) is effective to improve the fatigue strength of AM material in the same literature. Another problem is rough surface of as-built AM components. It has been known that surface roughness is one of detrimental factors affecting fatigue strength. In some literatures (Chan (2013), Whcisk (2014), Masuo (2018)), the fatigue strength of as-built AM specimen is lower than that of AM specimen with machined surface. However, the relationship between surface roughness and fatigue limit has not been discussed quantitatively. The surface roughness depends on process parameter such as powder diameter, scan rate, layer thickness and so on. As same as the defects, the surface quality of AM components will become better by improvement of building software. To determine a final goal of surface quality, it is important to evaluate the effect of surface roughness on the fatigue strength of AM component quantitatively. In this study, the effect of surface roughness on the fatigue strength of a Ti-6Al-4V alloy manufactured by AM was investigated. We conducted rotary bending fatigue tests for as-built specimen with different surface roughness. The method of quantitative evaluation of surface roughness for AM component was discussed. Nomenclature a b HV Nf Sa Sz R RSm
depth of an infinite row of circumferential crack pitch of an infinite row of circumferential crack Vickers hardness number of cycles to failure arithmetic surface roughness obtained by area measurement maximum height obtained by area measurement stress ratio mean width of profile elements obtained by line measurement a stress amplitude w fatigue limit w,ideal ideal fatigue limit of a material without defects K stress factor intensity range Kth threshold stress factor intensity range applied stress range √𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎 square root of projection area of defect √𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎 R equivalent crack size of surface roughness
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2. Experimental procedures The round bars and specimens were fabricated by AM processes; electron beam melting (EBM) and direct metal laser sintering (DMLS) methods. The particle sizes used in AM processes are about 80 m for EBM and about 40 m for DMLS. The bars and specimens were built in the direction of the specimen axis. For EBM process, preheating was applied in every layer. On the other hand, stress relief heat treatment was applied to specimens built by DMLS. To separate the effects of defect and surface roughness on the fatigue strength, the HIP was applied to several series of specimens. HIP was carried out at 920 °C and 100 MPa for 2 hours and the specimens were cooled in the furnace. For as-built specimen with/without HIP, only chuck part was machined. The polished specimen with HIP was machined from AM round bar after HIP and the surface was polished with #600 emery paper. Table 1 shows the mechanical properties of the Ti-6Al-4V alloys manufactured by AM. Tensile strength and Vickers hardness are almost the same between EBM and DMLS. The AM specimens were softened slightly by HIP treatment. Figure 1 shows the microstructures fabricated by DMLS process. HIP eliminated the internal defects and made the grain size large. On the other hand, the surface roughness remained after HIP. The similar microstructures were also observed in DMLS specimen. To investigate the difference of as-built surface between EBM and DMLS, the surface roughness and morphology were measured using an optical three-dimensional profilometer (KEYENCE, VR-5000). As explained later, the surfaces of as-built specimen have irregular and three-dimensionally complex morphology. Thus, the area roughness was measured at 10 areas of 4.5 × 6.0 mm to evaluate the maximum value of roughness parameter. In this study, the maximum height Sz and the arithmetical mean height Sa were focused on. In addition, mean width of profile elements RSm was measured from roughness profiles parallel to building direction. Figure 2 shows the shape and dimension of fatigue test specimens for rotating bending fatigue. Rotating bending fatigue tests with a stress ratio R of -1 were carried out at 60Hz. The test were run out at 10 7 cycles. After fatigue tests, the fracture surface was observed by a scanning electron microscope. Table 1. Mechanical properties.
AM process
EBM
DMLS
HIP
No
Yes
No
Yes
Tensile strength (MPa)
1046
986
1176
980
Elongation (%)
20
22
14
22
369
345
378
340
Vickers hardness HV0.3
(kgf/mm2)
Fig. 1. Microstructure of Ti-6Al-4V alloy fabricated by EBM process.
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Fig. 2. Shape and dimensions (mm) of rotating bending fatigue test specimen. Table 2. Surface roughness of as-built specimen.
AM process
Sa
Sz
RSm
Max.
Ave.
Max.
Ave.
Max.
Ave.
EBM
45
32
320
211
978
696
DMLS
20
12
128
88
907
535
Fig. 3. SEM images of surface of as-built specimens.
3. Results and discussion 3.1. Surface roughness and morphology Table 2 summarizes the average and maximum values of measured surface roughness. Here, the surface roughness of as-built specimen with/without HIP was not distinguished because HIP does not affect the surface morphology. The Sa value of as-built EBM specimen is 2 times higher than that of as-built DMLS specimen. The Sz values have also the same tendency. The difference of particle size and building layer thickness influence the surface roughness of as-built specimen. Figures 3 and 4 show SEM images and bird-eye height images of as-built specimen respectively. It can be seen that many unfused particles remain on surface. The existence of unfused particles enlarges the scatter of surface roughness value. The as-built surface has not only periodic but also three-dimensionally complex surface morphology. 3.2. S-N curves Figure 5 shows S-N curves for EBM and DMLS specimens. The dotted lines in Fig. 5 indicate the ideal fatigue limits for EBM and DMLS specimens with HIP. It is well known that there is a very good correlation between
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fatigue limit and Vickers hardness HV up to HV=~400. There is a robust empirical formula (Eq. (1)) for steels between the fatigue limit w,ideal (unit: MPa) and HV (unit: kgf/mm2) for HV<400. 𝜎𝜎w,ideal = 1.6𝐻𝐻𝐻𝐻 ± 0.1𝐻𝐻𝐻𝐻
(1)
The S-N curves for both AM process showed common trend. The fatigue limit of polished specimens with HIP was close to ideal fatigue limit expected from HV. On the other hand, the fatigue limits of as-built specimens without HIP was only about 30% of ideal fatigue limit. For as-built specimen, HIP treatment increased the fatigue strength slightly. These results indicate that the surface roughness have a significant influence on the fatigue strength even if internal defects are eliminated by HIP treatment. The polished specimens with HIP in both process have almost the same fatigue strength. However, the fatigue strength of as-built EBM specimen with HIP was lower than that of as-built DMLS specimen with HIP. This would be caused by the difference of surface roughness because the rougher surface induces higher stress concentration.
Fig. 4. Bird-eye height images of as-built specimens. The measured area is 4.5 × 6.0 mm.
Fig. 5. S-N curves. The dotted lines indicate the ideal fatigue limits estimated from HV of specimen with HIP.
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3.3. Fracture surface Figure 6 shows the FE-SEM images of fatigue fracture surface near crack initiation site. As shown in Fig. 6(a), radial marks were observed and this suggests that the fracture was caused by a single crack in polished specimen with HIP. On the other hand, it is difficult to define the crack initiation site for the surface roughness of as-built specimen with/without HIP. The existence of ratchet mark indicates that the multiple cracks were simultaneously initiated at the rough surface in the case of as-built specimens. It is considered that the surface roughness of as-built AM contributes to fatigue fracture as shallow and wide defect or circumferential notch. In addition, the defect was not observed on subsurface near crack initiation in the as-build specimen without HIP. This suggests that the surface roughness is a determinant factor in a degradation of fatigue strength for as-built specimen irrespective of HIP. It is noted that the defect also should contribute to fatigue fracture if there is defect near subsurface. 3.4. Effect of surface roughness In this study, it was revealed that the surface roughness of three-dimensionally complex surface morphology influences the fatigue strength of as-built AM specimen. Here, we discuss the evaluation method for the effect of the surface roughness in AM. As described firstly, the effect of small defect on the fatigue limit can be evaluated using √area parameter model (Murakami (1994), Murakami (2002)). It has been reported that the √area parameter model can be adopt to the fatigue limit prediction for machined specimen with artificial surface roughness (Murakami (2002)). The maximum height values in that literature ranged from 20.5 to 74 m. On the other hand, the Sz values of as-built specimen fabricated by EBM and DMLS are over 88 m. Therefore, the applicability range of √area parameter model is necessary to be confirmed. Some of authors conducted the fatigue tests for polished specimen without HIP which fabricated by EBM and DMLS as same as this study and investigated the relationship between K and √area as shown in Fig. 7 (Masuo (2018)). K value for a crack with arbitrary shape was calculated using a followed equation. Δ𝐾𝐾 = 0.65Δσ√𝜋𝜋√𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎
(2)
Here, is stress range (= 2a, unit: MPa), √area is square root of projection area of defect observed at crack initiation site (unit: m). The specimens which have runout for N = 107 cycles were tested again at a higher stress and the size of defects at fracture origins was identified. The data agreed with the √area parameter model indicated by a solid line. In Fig. 7, the dashed horizontal line corresponds to the threshold stress intensity factor range for long crack in a forged Ti-6Al-4V alloy; Kth = 10 MPam1/2 (Oberwinkler (2010)). Thus, the upper limit of √area parameter model in tested sample are expected to be about 230 m. On the other hand, the √area calculated from w and HV using √area parameter model (Eq. (3)) are listed in Table 3. 1/6
𝜎𝜎w = 1.43(𝐻𝐻𝐻𝐻 + 120)/(√𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎)
Fig. 6 FESEM images of fatigue fracture surface near crack initiation site.
(3)
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Fig. 7. Relationship between Kth (R = -1) and defect size √area in Ti-6Al-4V alloy fabricated by AM (Masuo (2018)). Table 3. Estimated defect size and equivalent crack size.
AM process EBM
DMLS
w
HIP
HV (kgf/mm2)
(MPa)
No
374
140
Yes
349
195
No
380
155
Yes
342
210
Surface roughness Sz RSm (m) (m) 646
696
260
535
√𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎 calculated using Eq. (3) (m) 16505 1655 9635 970
Equivalent crack size √𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎R (Eq. (4) or (5)) (m) 264
203
Estimated Kth (Eq. (6)) (MPam1/2) 5.2 7.3 5.1 6.9
Here, the units are w: MPa, HV: kgf/mm2, √area : m. In the any case, the calculated √area is much larger than upper limit which √area parameter model can apply. This result indicates that √area parameter model cannot deal with the surface roughness of as-built specimen. Therefore, the surface roughness of as-built specimen should be considered as long crack. Murakami (2002) proposed an evaluation method of equivalent defect size, √𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎 R , for periodic surface notches as artificial surface roughness with a depth of a (the vertical distance of root-to-peak) and a pitch of 2b (the horizontal distance of peak-to-peak) as followed. √𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎R 2𝑏𝑏 √𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎R 2𝑏𝑏
𝑎𝑎 2
𝑎𝑎
𝑎𝑎 3
= 2.97 ( ) − 3.51 ( ) − 9.74 ( ) , for 2𝑏𝑏
≈ 0.38, for
𝑎𝑎
2𝑏𝑏
2𝑏𝑏
≥ 0.195
2𝑏𝑏
𝑎𝑎
2𝑏𝑏
≤ 0.195
(4) (5)
Here, maximum value of maximum height Sa and mean width of profile elements RSm were substituted for a depth of a and a pitch of 2b respectively. The estimated√𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎 R are also summarized in Table 3. Kth was calculated using Eq. (6). Δ𝐾𝐾th = 0.65 × 2σ𝑤𝑤 √𝜋𝜋√𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎 R
(6)
The calculated Kth was lower than the Kth value for a forged Ti-6Al-4V alloy; about 10 MPam1/2. The Kth value of AM Ti-6Al-4V alloy has a tendency to be low compared with that of conventional Ti-6Al-4V alloy (Liu (2019)). In addition, it is reported that Kth value for R = 0.1 ranges 2 to 5 MPam1/2 in AM Ti-6Al-4V alloy. Thus, the
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estimated results may be reasonable. Further investigation and discussion are necessary to evaluate the effect of surface roughness quantitatively. As described previously, the fatigue strength of as-built AM sample is quite low compared with the ideal fatigue strength, preventing AM process from applying to actual mechanical components. Thus, it is significantly important to decrease or eliminate the surface roughness using some technique such as chemical etching, barrel polishing and so on. We are investigating the effect of polishing on the fatigue strength in Ti-6Al-4V alloy fabricated by AM process. The results will be also introduced in our presentation. 4. Conclusions The effect of surface roughness on the fatigue strength of Ti-6Al-4V alloys fabricated by two AM processes was investigated. As-built specimen has extremely rough surface with three-dimensional complex morphology. The surface roughness was dependent on the particle size used in AM process. The fatigue strength of as-built specimens decreased to the level of 1/3 of the fatigue strength of the ideal fatigue strength. Though HIP can eliminate the internal defects, the surface roughness was not improved by HIP. Therefore, surface roughness has a detrimental influence on fatigue strength. Thus, it is significantly important to decrease or eliminate the surface roughness. The evaluation method for the effect of surface roughness was also discussed. Even DMLS as-built specimen which surface roughness is relatively small does not fit the √area parameter model. This suggests that the surface roughness of as-built specimen should be considered to be long crack rather than small defect. References Baufeld, B., Biest, O.V., Gault, R., 2010. Additive manufacturing of Ti–6Al–4V components by shaped metal deposition: Microstructure and mechanical properties. Materials & Design 31, S106-S111. Frazier, W.E., 2014. Metal Additive Manufacturing: A Review. Journal of Materials Engineering and Performance 23, 1917-1928. Günther, J., Krewerth, D., Lippmann, T., Leuders, S., Tröster, T., Weidner, A., Biermann, H., Niendorf, T., 2017. Fatigue life of additively manufactured Ti-6Al-4V in the very high cycle fatigue regime. International Journal of Fatigue 94, 236–245. Hayes, B.J., Martin, B.W., Welk, B., Kuhr, S.J., Ales, T.K., Brice, D.A., Ghamarian, I., Baker, A.H., Haden, C.V., Harlow, D.G., Fraser, H.L., Collins, P.C., 2017. Predicting tensile properties of Ti–6Al–4V produced via directed energy deposition. Acta Materiaria 133, 120–133. Lewandowski, J.J., Mohsen Seifi, M., 2016. Metal Additive Manufacturing: A Review of Mechanical Properties. Annual Review of Materials Research 46, 151-186. Liu, S. Yung C. Shin, Y. C., 2019. Additive manufacturing of Ti6Al4V alloy: A review. Materials and Design 164, 107552. Chan, K.S., Koike, M., Mason, R.L., Okabe, T., 2013. Fatigue Life of Titanium Alloys Fabricated by Additive Layer Manufacturing Techniques for Dental Implants. Metallurgical and Materials Transactions A 44, 1010-1022. Hrabe, N., Quinn, T., 2013. Effects of processing on microstructure and mechanical properties of a titanium alloy (Ti–6Al–4V) fabricated using electron beam melting (EBM). part 2: energy input, orientation, and location. Materials Science and Engineering A 573, 271-277. Masuo, H., Tanaka, Y., Morokoshi, S., Yagura, H., Uchida, T., Yamamoto, Y., Murakami, Y., 2018 Influence of defects, surface roughness and HIP on the fatigue strength of Ti-6Al-4V manufactured by additive manufacturing. International Journal of Fatigue 117, 163-179. Murakami, Y., Endo, M., 1994. Effects of defects, inclusions and inhomogeneities on fatigue strength. International Journal of Fatigue 16, 163– 182. Murakami, Y., 2002. Metal fatigue: effects of small defects and nonmetallic inclusions. Elsevier, 57-71. Murakami, Y., 2002. Metal fatigue: effects of small defects and nonmetallic inclusions. Elsevier, 305-312. Oberwinkler, B., Riedler, M., Eichlseder, W., 2010. Importance of local microstructure for damage tolerant light weight design. International Journal of Fatigue 32, 808–814. Romano, S., Brückner-Foit, A., Brandão, A., Gumpinger, J., Ghidini, T., Beretta, S., 2018. Fatigue properties of AlSi10Mg obtained by additive manufacturing: Defect-based modelling and prediction of fatigue strength. Engineering Fracture Mechanics 187, 165-189. Wycisk, E., Solbach, A., Siddique, S., Herzog, D., Walther, F., Emmelmann, C., 2014. Effects of Defects in Laser Additive Manufactured Ti-6Al4V on Fatigue Properties, Physics Procedia 56, 371-378. Wysocki, B., Maj, P., Sitek, R., Buhagiar, J., Kurzydłowski, K., Święszkowski, W., 2017. Laser and electron beam additive manufacturing methods of fabricating titanium bone implants, Applied Science 7, 657. Yadollahi, A., Shamsaei, N., Additive manufacturing of fatigue resistant materials: Challenges and opportunities. International Journal of Fatigue 98, 14-31. Yamashita, Y., Murakami, T., Mihara, R., Okada, M., Murakami, Y., 2018. Defect analysis and fatigue design basis for Ni-based superalloy 718 manufactured by selective laser melting. International Journal of Fatigue 117, 485-495.
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Procedia Structural Integrity 19 (2019) 294–301
Fatigue Design 2019 Fatigue Design 2019
Effect of Surface Roughness on Fatigue Strength of Ti-6Al-4V Effect of Surface Roughness on Fatigue Strength of Ti-6Al-4V Alloy Manufactured by Additive Manufacturing Alloy Manufactured by Additive Manufacturing Masanori Nakatani a,a, *, Hiroshige Masuo bb, Yuzo Tanaka aa, Yukitaka Murakami a,c Masanori Nakatani *, Hiroshige Masuo , Yuzo Tanaka , Yukitaka Murakami a,c a Kobe Material Testing Laboratory Co. Ltd., Harima-Town, Hyogo 675-0135, Japan a Kobe Material Laboratory Co.Ebina, Ltd., Harima-Town, Hyogo 675-0135, Japan b MetalTesting Technology Co. Ltd., Kanagawa, 243-0424, Japan b Metal Technology Co. Ltd., Ebina,Fukuoka Kanagawa, 243-0424, Japan c Kyushu University, Nishi-ku, 819-0395, Japan c Kyushu University, Nishi-ku, Fukuoka 819-0395, Japan
Abstract Abstract This study focuses on the effects of surface roughness on the fatigue strength of Ti-6Al-4V alloy manufactured by additive This study focuses onelectron the effects surface(EBM) roughness on themetal fatigue strength of (DMLS) Ti-6Al-4V alloy manufactured by pressing additive manufacturing (AM); beamofmelting and direct laser sintering methods. The hot isostatic manufacturing (AM); electron beam and direct metal laser sintering (DMLS) methods. The hot Fatigue isostaticlimit pressing (HIP) was applied to some series of melting polished(EBM) specimens and as-built specimens to eliminate internal defects. was (HIP) was by applied to some series of polished specimensHIP andincreased as-built specimens eliminate internal was evaluated rotating bending fatigue tests. Although the fatigueto limits slightly, the defects. fatigue Fatigue limits oflimit as-built evaluated by rotating bending fatigue30% tests. HIP increased the fatigue slightly, the fatigue limits as-built specimens with HIP were only about of Although the ideal fatigue limit expected from limits the Vickers hardness. On the otherofhand, the specimensspecimens with HIPwith wereHIP only about 30% oflimit the ideal limitupper expected from thefatigue Vickers hardness. On the atother hand,sites the polished had the fatigue closefatigue to the ideal bound. The crack was initiated multiple polished specimens with HIP had the fatigue limit close to the ideal upper bound. Theonfatigue crack was initiated at multiple sites on the surface in as-built specimens. Thus, surface roughness has crucial influence fatigue strength of AM component. The on the surface in as-built specimens. Thus,issurface roughness hascomplex. crucial influence on fatigue strength ofdefect AM component. The surface morphology of as-build specimen three-dimensionally The evaluation of equivalent size for surface surface morphology ofby as-build specimen model is three-dimensionally complex. The evaluation of equivalent defect size forsize surface roughness was attempt √area parameter assuming the surface roughness is a small defect. The estimated defect was roughness was attempt by √area model assuming the surfacethat roughness is a small defect.ofThe estimated defectshould size was over the applicable range of theparameter √area parameter model, suggesting the surface roughness as-built specimen be over the applicable range of the √area parameter model, suggesting that the surface roughness of as-built specimen should be considered as long crack. considered as long crack. © 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 Fatigue Design 2019 Organizers. Peer-review under responsibility of the Fatigue Design 2019 Organizers. Peer-review under responsibility of the Fatigue Design 2019 Organizers. Keywords: Additive manufacturing, Fatigue, Surface roughness, Titanium alloy, Hot isostatic pressing, √area parameter model, Keywords: Additive manufacturing, Fatigue, Surface roughness, Titanium alloy, Hot isostatic pressing, √area parameter model,
* Corresponding author. Tel.: +81-79-435-5010; fax: +81-79-435-5102. * Corresponding Tel.: +81-79-435-5010; fax: +81-79-435-5102. E-mail 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 by Elsevier B.V. Organizers. Peer-review©under the Fatigue Design 2019 Peer-review under responsibility of the Fatigue Design 2019 Organizers.
2452-3216 © 2019 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the Fatigue Design 2019 Organizers. 10.1016/j.prostr.2019.12.032
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1. Introduction The additive manufacturing (AM) is a group of processes that join materials to make objects from threedimensional model data, usually layer by layer. AM is expected as an innovative manufacturing process for various parts with complex geometry that cannot fabricated by traditional process. Many review papers on AM have been also published (e.g. Frazier (2014), Lewandowski (2016), Yadollahi (2017), Liu (2019)). However, AM still remains some problems to be overcome; such as cost, size, quality and reliability. One of the problems is low fatigue strength in comparison with conventional material. Many researchers emphasizes that the tensile strength and hardness of the AM material are the same level as those of the conventional material (e.g. Baufeld (2010), Hrabe (2013), Hayes (2017), Wysocki (2017)). However, it is reported that the fatigue strength of as-built AM material is significantly low because of defects (Günther (2017), Romano (2018), Masuo (2018), Yamashita (2018)). The defects are caused by gas pore and spatter during melting process of metal powder. The occurrence of these defects is not solved completely though the defects decrease with a development of AM technology. The existence of defects disturbs the application of AM to structural components that need fatigue strength. We previously conducted the fatigue tests of Ti-6Al-4V alloy fabricated by AM and revealed that fatigue limit can be quantitatively evaluated using the √area parameter model proposed by Murakami (1994) (Masuo (2018)). Moreover, we also pointed out that hot isostatic pressing (HIP) is effective to improve the fatigue strength of AM material in the same literature. Another problem is rough surface of as-built AM components. It has been known that surface roughness is one of detrimental factors affecting fatigue strength. In some literatures (Chan (2013), Whcisk (2014), Masuo (2018)), the fatigue strength of as-built AM specimen is lower than that of AM specimen with machined surface. However, the relationship between surface roughness and fatigue limit has not been discussed quantitatively. The surface roughness depends on process parameter such as powder diameter, scan rate, layer thickness and so on. As same as the defects, the surface quality of AM components will become better by improvement of building software. To determine a final goal of surface quality, it is important to evaluate the effect of surface roughness on the fatigue strength of AM component quantitatively. In this study, the effect of surface roughness on the fatigue strength of a Ti-6Al-4V alloy manufactured by AM was investigated. We conducted rotary bending fatigue tests for as-built specimen with different surface roughness. The method of quantitative evaluation of surface roughness for AM component was discussed. Nomenclature a b HV Nf Sa Sz R RSm
depth of an infinite row of circumferential crack pitch of an infinite row of circumferential crack Vickers hardness number of cycles to failure arithmetic surface roughness obtained by area measurement maximum height obtained by area measurement stress ratio mean width of profile elements obtained by line measurement a stress amplitude w fatigue limit w,ideal ideal fatigue limit of a material without defects K stress factor intensity range Kth threshold stress factor intensity range applied stress range √𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎 square root of projection area of defect √𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎 R equivalent crack size of surface roughness
Masanori Nakatani et al. / Procedia Structural Integrity 19 (2019) 294–301 Author name / Structural Integrity Procedia 00 (2019) 000–000
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2. Experimental procedures The round bars and specimens were fabricated by AM processes; electron beam melting (EBM) and direct metal laser sintering (DMLS) methods. The particle sizes used in AM processes are about 80 m for EBM and about 40 m for DMLS. The bars and specimens were built in the direction of the specimen axis. For EBM process, preheating was applied in every layer. On the other hand, stress relief heat treatment was applied to specimens built by DMLS. To separate the effects of defect and surface roughness on the fatigue strength, the HIP was applied to several series of specimens. HIP was carried out at 920 °C and 100 MPa for 2 hours and the specimens were cooled in the furnace. For as-built specimen with/without HIP, only chuck part was machined. The polished specimen with HIP was machined from AM round bar after HIP and the surface was polished with #600 emery paper. Table 1 shows the mechanical properties of the Ti-6Al-4V alloys manufactured by AM. Tensile strength and Vickers hardness are almost the same between EBM and DMLS. The AM specimens were softened slightly by HIP treatment. Figure 1 shows the microstructures fabricated by DMLS process. HIP eliminated the internal defects and made the grain size large. On the other hand, the surface roughness remained after HIP. The similar microstructures were also observed in DMLS specimen. To investigate the difference of as-built surface between EBM and DMLS, the surface roughness and morphology were measured using an optical three-dimensional profilometer (KEYENCE, VR-5000). As explained later, the surfaces of as-built specimen have irregular and three-dimensionally complex morphology. Thus, the area roughness was measured at 10 areas of 4.5 × 6.0 mm to evaluate the maximum value of roughness parameter. In this study, the maximum height Sz and the arithmetical mean height Sa were focused on. In addition, mean width of profile elements RSm was measured from roughness profiles parallel to building direction. Figure 2 shows the shape and dimension of fatigue test specimens for rotating bending fatigue. Rotating bending fatigue tests with a stress ratio R of -1 were carried out at 60Hz. The test were run out at 10 7 cycles. After fatigue tests, the fracture surface was observed by a scanning electron microscope. Table 1. Mechanical properties.
AM process
EBM
DMLS
HIP
No
Yes
No
Yes
Tensile strength (MPa)
1046
986
1176
980
Elongation (%)
20
22
14
22
369
345
378
340
Vickers hardness HV0.3
(kgf/mm2)
Fig. 1. Microstructure of Ti-6Al-4V alloy fabricated by EBM process.
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Fig. 2. Shape and dimensions (mm) of rotating bending fatigue test specimen. Table 2. Surface roughness of as-built specimen.
AM process
Sa
Sz
RSm
Max.
Ave.
Max.
Ave.
Max.
Ave.
EBM
45
32
320
211
978
696
DMLS
20
12
128
88
907
535
Fig. 3. SEM images of surface of as-built specimens.
3. Results and discussion 3.1. Surface roughness and morphology Table 2 summarizes the average and maximum values of measured surface roughness. Here, the surface roughness of as-built specimen with/without HIP was not distinguished because HIP does not affect the surface morphology. The Sa value of as-built EBM specimen is 2 times higher than that of as-built DMLS specimen. The Sz values have also the same tendency. The difference of particle size and building layer thickness influence the surface roughness of as-built specimen. Figures 3 and 4 show SEM images and bird-eye height images of as-built specimen respectively. It can be seen that many unfused particles remain on surface. The existence of unfused particles enlarges the scatter of surface roughness value. The as-built surface has not only periodic but also three-dimensionally complex surface morphology. 3.2. S-N curves Figure 5 shows S-N curves for EBM and DMLS specimens. The dotted lines in Fig. 5 indicate the ideal fatigue limits for EBM and DMLS specimens with HIP. It is well known that there is a very good correlation between
Masanori Nakatani et al. / Procedia Structural Integrity 19 (2019) 294–301 Author name / Structural Integrity Procedia 00 (2019) 000–000
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fatigue limit and Vickers hardness HV up to HV=~400. There is a robust empirical formula (Eq. (1)) for steels between the fatigue limit w,ideal (unit: MPa) and HV (unit: kgf/mm2) for HV<400. 𝜎𝜎w,ideal = 1.6𝐻𝐻𝐻𝐻 ± 0.1𝐻𝐻𝐻𝐻
(1)
The S-N curves for both AM process showed common trend. The fatigue limit of polished specimens with HIP was close to ideal fatigue limit expected from HV. On the other hand, the fatigue limits of as-built specimens without HIP was only about 30% of ideal fatigue limit. For as-built specimen, HIP treatment increased the fatigue strength slightly. These results indicate that the surface roughness have a significant influence on the fatigue strength even if internal defects are eliminated by HIP treatment. The polished specimens with HIP in both process have almost the same fatigue strength. However, the fatigue strength of as-built EBM specimen with HIP was lower than that of as-built DMLS specimen with HIP. This would be caused by the difference of surface roughness because the rougher surface induces higher stress concentration.
Fig. 4. Bird-eye height images of as-built specimens. The measured area is 4.5 × 6.0 mm.
Fig. 5. S-N curves. The dotted lines indicate the ideal fatigue limits estimated from HV of specimen with HIP.
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3.3. Fracture surface Figure 6 shows the FE-SEM images of fatigue fracture surface near crack initiation site. As shown in Fig. 6(a), radial marks were observed and this suggests that the fracture was caused by a single crack in polished specimen with HIP. On the other hand, it is difficult to define the crack initiation site for the surface roughness of as-built specimen with/without HIP. The existence of ratchet mark indicates that the multiple cracks were simultaneously initiated at the rough surface in the case of as-built specimens. It is considered that the surface roughness of as-built AM contributes to fatigue fracture as shallow and wide defect or circumferential notch. In addition, the defect was not observed on subsurface near crack initiation in the as-build specimen without HIP. This suggests that the surface roughness is a determinant factor in a degradation of fatigue strength for as-built specimen irrespective of HIP. It is noted that the defect also should contribute to fatigue fracture if there is defect near subsurface. 3.4. Effect of surface roughness In this study, it was revealed that the surface roughness of three-dimensionally complex surface morphology influences the fatigue strength of as-built AM specimen. Here, we discuss the evaluation method for the effect of the surface roughness in AM. As described firstly, the effect of small defect on the fatigue limit can be evaluated using √area parameter model (Murakami (1994), Murakami (2002)). It has been reported that the √area parameter model can be adopt to the fatigue limit prediction for machined specimen with artificial surface roughness (Murakami (2002)). The maximum height values in that literature ranged from 20.5 to 74 m. On the other hand, the Sz values of as-built specimen fabricated by EBM and DMLS are over 88 m. Therefore, the applicability range of √area parameter model is necessary to be confirmed. Some of authors conducted the fatigue tests for polished specimen without HIP which fabricated by EBM and DMLS as same as this study and investigated the relationship between K and √area as shown in Fig. 7 (Masuo (2018)). K value for a crack with arbitrary shape was calculated using a followed equation. Δ𝐾𝐾 = 0.65Δσ√𝜋𝜋√𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎
(2)
Here, is stress range (= 2a, unit: MPa), √area is square root of projection area of defect observed at crack initiation site (unit: m). The specimens which have runout for N = 107 cycles were tested again at a higher stress and the size of defects at fracture origins was identified. The data agreed with the √area parameter model indicated by a solid line. In Fig. 7, the dashed horizontal line corresponds to the threshold stress intensity factor range for long crack in a forged Ti-6Al-4V alloy; Kth = 10 MPam1/2 (Oberwinkler (2010)). Thus, the upper limit of √area parameter model in tested sample are expected to be about 230 m. On the other hand, the √area calculated from w and HV using √area parameter model (Eq. (3)) are listed in Table 3. 1/6
𝜎𝜎w = 1.43(𝐻𝐻𝐻𝐻 + 120)/(√𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎)
Fig. 6 FESEM images of fatigue fracture surface near crack initiation site.
(3)
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Fig. 7. Relationship between Kth (R = -1) and defect size √area in Ti-6Al-4V alloy fabricated by AM (Masuo (2018)). Table 3. Estimated defect size and equivalent crack size.
AM process EBM
DMLS
w
HIP
HV (kgf/mm2)
(MPa)
No
374
140
Yes
349
195
No
380
155
Yes
342
210
Surface roughness Sz RSm (m) (m) 646
696
260
535
√𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎 calculated using Eq. (3) (m) 16505 1655 9635 970
Equivalent crack size √𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎R (Eq. (4) or (5)) (m) 264
203
Estimated Kth (Eq. (6)) (MPam1/2) 5.2 7.3 5.1 6.9
Here, the units are w: MPa, HV: kgf/mm2, √area : m. In the any case, the calculated √area is much larger than upper limit which √area parameter model can apply. This result indicates that √area parameter model cannot deal with the surface roughness of as-built specimen. Therefore, the surface roughness of as-built specimen should be considered as long crack. Murakami (2002) proposed an evaluation method of equivalent defect size, √𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎 R , for periodic surface notches as artificial surface roughness with a depth of a (the vertical distance of root-to-peak) and a pitch of 2b (the horizontal distance of peak-to-peak) as followed. √𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎R 2𝑏𝑏 √𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎R 2𝑏𝑏
𝑎𝑎 2
𝑎𝑎
𝑎𝑎 3
= 2.97 ( ) − 3.51 ( ) − 9.74 ( ) , for 2𝑏𝑏
≈ 0.38, for
𝑎𝑎
2𝑏𝑏
2𝑏𝑏
≥ 0.195
2𝑏𝑏
𝑎𝑎
2𝑏𝑏
≤ 0.195
(4) (5)
Here, maximum value of maximum height Sa and mean width of profile elements RSm were substituted for a depth of a and a pitch of 2b respectively. The estimated√𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎 R are also summarized in Table 3. Kth was calculated using Eq. (6). Δ𝐾𝐾th = 0.65 × 2σ𝑤𝑤 √𝜋𝜋√𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎 R
(6)
The calculated Kth was lower than the Kth value for a forged Ti-6Al-4V alloy; about 10 MPam1/2. The Kth value of AM Ti-6Al-4V alloy has a tendency to be low compared with that of conventional Ti-6Al-4V alloy (Liu (2019)). In addition, it is reported that Kth value for R = 0.1 ranges 2 to 5 MPam1/2 in AM Ti-6Al-4V alloy. Thus, the
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estimated results may be reasonable. Further investigation and discussion are necessary to evaluate the effect of surface roughness quantitatively. As described previously, the fatigue strength of as-built AM sample is quite low compared with the ideal fatigue strength, preventing AM process from applying to actual mechanical components. Thus, it is significantly important to decrease or eliminate the surface roughness using some technique such as chemical etching, barrel polishing and so on. We are investigating the effect of polishing on the fatigue strength in Ti-6Al-4V alloy fabricated by AM process. The results will be also introduced in our presentation. 4. Conclusions The effect of surface roughness on the fatigue strength of Ti-6Al-4V alloys fabricated by two AM processes was investigated. As-built specimen has extremely rough surface with three-dimensional complex morphology. The surface roughness was dependent on the particle size used in AM process. The fatigue strength of as-built specimens decreased to the level of 1/3 of the fatigue strength of the ideal fatigue strength. Though HIP can eliminate the internal defects, the surface roughness was not improved by HIP. Therefore, surface roughness has a detrimental influence on fatigue strength. Thus, it is significantly important to decrease or eliminate the surface roughness. The evaluation method for the effect of surface roughness was also discussed. Even DMLS as-built specimen which surface roughness is relatively small does not fit the √area parameter model. This suggests that the surface roughness of as-built specimen should be considered to be long crack rather than small defect. References Baufeld, B., Biest, O.V., Gault, R., 2010. Additive manufacturing of Ti–6Al–4V components by shaped metal deposition: Microstructure and mechanical properties. Materials & Design 31, S106-S111. Frazier, W.E., 2014. Metal Additive Manufacturing: A Review. 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