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Microstructural feature and mechanical property in different building directions of additive manufactured Ti6Al4V alloy
Journal Pre-proofs Microstructural feature and mechanical property in different building direc‐ tions of additive manufactured Ti6Al4V alloy Kun Chang, Enquan Liang, Wenjing Huang, Xi Zhang, Ying Chen, Jinfang Dong, Ren Zhang PII: DOI: Reference:
S0167-577X(20)30221-4 https://doi.org/10.1016/j.matlet.2020.127516 MLBLUE 127516
To appear in:
Materials Letters
Received Date: Revised Date: Accepted Date:
6 January 2020 14 February 2020 17 February 2020
Please cite this article as: K. Chang, E. Liang, W. Huang, X. Zhang, Y. Chen, J. Dong, R. Zhang, Microstructural feature and mechanical property in different building directions of additive manufactured Ti6Al4V alloy, Materials Letters (2020), doi: https://doi.org/10.1016/j.matlet.2020.127516
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2020 Published by Elsevier B.V.
Microstructural feature and mechanical property in different building directions of additive manufactured Ti6Al4V alloy Kun Chang*, Enquan Liang, Wenjing Huang, Xi Zhang, Ying Chen, Jinfang Dong, Ren Zhang* Standard and Materials Research Department, Shanghai Aircraft Design and Research Institute, Jinke Road 5188, Shanghai 201210, PR China. *Corresponding author. E-mail address: [email protected] (K. Chang); [email protected] (R. Zhang) Abstract Near-fully dense Ti6Al4V alloy was successfully fabricated by SLM additive manufacturing. The acicular martensite α′ microstructure was considerably optimized by the formation of α + β phase via annealing treatment. The annealed Ti6Al4V alloy showed a very favorable tensile strength and excellent ductility even compared with the as-built part. Although an obvious vertical fatigue scatter, the tensile yielding strength ~ 953 MPa, elongation ~ 17.7% and fatigue strength ~ 546 MPa in vertical direction were all superior to that in horizontal direction, showing a fancy anisotropy. This was completely different from previous research. The mechanism for this phenomenon was analyzed along with the fracture morphology observation. Keywords: Additive manufacturing; Selective laser melting; Microstructure; Mechanical property; Fatigue 1. Introduction As a typical additive manufacturing (AM) technology, selective laser melting (SLM) is famous for precision manufacturing and has been popularly used for fabricating metallic materials with complex geometry and high dimensional accuracy [1,2]. Amount of all the metallic materials, Ti6Al4V alloy is one of the most mature materials for additive manufacturing. The Ti6Al4V components fabricated by SLM have been applicated in aerospace, biomedicine, military due to its favorable mechanical property, biocompatibility and good corrosion resistance [3]. Due to the rapid melting/cooling rate, SLM-fabricated Ti6Al4V alloy possesses 1
significantly refined acicular α′ martensite [3]. Although the static performance is strengthened by forming this refined phase, the as-built Ti6Al4V components are in an un-balanced status because of internal residual stress and exhibit typical anisotropy in terms of microstructure and mechanical properties due to the specific metallurgical process [4,5]. Therefore, as-built Ti6Al4V components are not recommended to be used directly prior to some post heat treatments. Up to now, researches are mainly focused on exploiting an optimist process route to improve the densification, mechanical properties and obtain homogeneous microstructures by optimizing SLM processing and heat treatment parameters [4,6]. However, the discrepancy on different directions is scarcely investigated. In order to understand the microstructure and mechanical property in different directions of SLM-fabricated Ti6Al4V alloy, the microstructure features and defects were investigated in this study. The mechanical properties in terms of tensile and fatigue behaviors were also estimated. 2. Experimental details The Ti6Al4V powder in a spherical morphology with D50 of 32.8 μm and D90 of 54.8 μm was used as raw material (Fig. 1a). Rectangular samples and cylindrical specimens (Fig. 1b) were processed by S310 SLM system adopting a zigzag scanning strategy with oxygen concentration below 100 ppm. The optimized parameters were adopted: laser power 350 W, scan speed 1250 mm/s, hatching space 0.1 mm and layer thickness 0.06 mm. The annealing heat treatment was performed on the SLMfabricated samples at 800 ℃ for 2h, and then furnace cooled in vacuum environment. The cylindrical specimens were then machined into the shapes of Fig. 1c and d for fatigue and tensile testing, respectively. The microstructures were characterized using optical microscopy. The tensile tests were carried out using an Instron 5887 machine according to ASTM E8/E8M with strain rate of 0.005 mm/mm/min before yielding and displacement rate of 5 mm/min after yielding. The fatigue tests were performed on Electromagnetic resonance fatigue machine (QBG-25) according to ASTM E466 (Kt=1, R=0.1). Powder and fracture morphologies were observed by scanning electron microscope 2
(SEM). Micro-focus computed tomography (CT) machine (IPT6016B2) was employed to analyze the internal defects of annealed samples.
a
b
c
d
Fig. 1 The morphology of (a) raw powder and (b) SLM-fabricated Ti6Al4V samples. The dimensions of (c) fatigue and (d) tensile specimens. 3. Results and discussion The as-built samples were characterized by acicular α′ phase with a random dispersion (Fig. 1a and b), which was induced by the rapid cooling rate of laser melt pool [7,8]. The prior β grains could be clearly observed on both the as-built and annealed specimens. The top surfaces mainly consisted of prior near-cellular β grains (Fig.1a and c) whereas the cross-sections were featured with a columnar morphology along the building direction (Fig. 1b and e), presenting a distinct anisotropy. This difference is attributed to the different thermal diffusion manner in the parallel and perpendicular to the building direction [3]. After annealing of the SLM-fabricated samples, the acicular martensite α′ was completely replaced by lath-like α phase accompanied by the formation of β phase (Fig. 1d and f), indicating that the phase transition of α′ → α + β was occurred during annealing.
3
a
b Building direction Acicular α′
Acicular α′
c
d α β
e
f α Prior columnar β grain
β
Building direction
g
h
i
j
k
Building direction
Micro-voids at boundary
Fig. 2 The microstructure features: (a) top surface and (b) cross-section of as-built 4
sample, (c) top surface and (e) cross-section of annealed sample. The (d) and (f) are the view in a higher magnification of (c) and (e), respectively. The inset pictures in (a) and (b) are corresponding lower magnification images. (g) CT (longitudinal) image of annealed sample, (h) 3-dimension image of selected part inside of the sample, (i) 3dimension re-construction image for the dispersion of defects, and the typical morphologies (j) irregular micro-voids and (k) near-spherical impurity. As observed in Fig. 2g, the micro-voids were mainly dispersed along the edge of the sample with depth less than 300 μm. It is considered that the switching of laser beam between adjacent tracks combined with the energy damping at the end of melt track resulted in the formation of boundary micro-voids [9]. On the contrary, the internal of the sample was significantly dense. To further analyze the defect features inside of the sample, internal cuboid of 15 × 8.3 × 9.6 mm3 was selected and tested, as shown in Fig. 2h. The testing results revealed that only a few defects primary including irregular micro-voids and near-spherical impurity were formed and randomly dispersed (Fig. 2i) with a significantly low porosity of 0.0026%. The single biggest micro-void volume was 7.2 × 10-4 mm3 (Fig. 2j) and the biggest diameter of near-spherical impurity is about 57 μm (Fig. 2k), which might be large-size residual powder particle un-melted. After annealing treatment, the ultimate tensile strength (UTS) ~ 1032 MPa in horizontal direction was exactly similar with that (~ 1035 MPa) in vertical direction, showing an isotropy feature (Fig. 3a). However, both the yielding strength (YS) ~953 MPa and elongation (El.) ~ 17.7% in vertical direction were higher than the YS ~ 929.5 MPa and El. ~ 15.6 MPa in horizonal direction, revealing an anisotropy. Interestingly, this phenomenon was completely different form previous study results that generally the properties in vertical direction were lower than that in horizontal direction [10,11]. In addition, the annealed part showed a significantly improved El. and maintained a similar YS simultaneously compared with the as-built one (~ 7.4%, 952 ~ 968 MPa) [11]. The similar anisotropic result was obtained for fatigue testing, as depicted in Fig. 3b. The fatigue strength of 546 MPa in vertical direction was 5
higher than that (~ 530 MPa) in horizontal direction whereas showed a lager scatter.
b
a
c
d
Necking
Micro-voids
Dimples
Micro-voids
e
f
Laminar tearing
Cleavage plane Crack nucleation site
Crack nucleation site
Fig. 3 (a) The tensile and (b) fatigue testing results in different directions of SLMfabricated Ti6Al4V alloy after annealing treatment. Tensile fractographies in horizontal direction in (c) low magnification and (d) high magnification. Fatigue fractographies in (e) horizontal and (f) vertical directions. Tensile fractographies observation in horizontal direction revealed obvious necking and vary sized dimples (Fig. 3c and d), which were the typical symbolizes of ductile fracture. Additionally, some micro-voids could be also observed on the fracture, verifying the result of CT testing. However, the fatigue behavior was sensitive to the internal defects. It was found that the micro-voids were easily to act as the crack nucleation and induce the fatigue fracture (Fig. 3e and f). Moreover, the fatigue fractography in horizontal direction was characterized by few cleave planes (Fig. 3e), which further verified the columnar grain boundary cracking in horizontal direction at dynamic load condition, resulting in a relative lower fatigue strength (Fig. 3b). The laminar tearing feature was appeared on the fatigue fractography in the 6
vertical direction (Fig. 3f), which reflected the destruction of metallurgical bonding layers under rigorously dynamic load conditions. The large scatter of vertical fatigue behavior might be ascribed to the occurrence of break of SLM metallurgical bonding layers. It is considered that the crystallographic texture, columnar microstructure intrinsic to AM contribute to the anisotropic behavior of the fabricated Ti6Al4V samples [12-16]. Theoretically, the grain tends to grow opposite to the maximum temperature gradient at the solidification front. Generally, the primary overall heat deviation and dissipation from center to bottom of the melt pool can give rise to a large thermal gradient downward [13]. Under the condition of directional solidification, the prior β phase preferentially grows along the preferred ˂001>β orientation [13-14], which is parallel to the building direction, resulting in an obvious crystallographic texture in AM of Ti6Al4V alloy and resultant anisotropic mechanical behavior. This typical texture feature in AM-fabricated Ti6Al4V has also been proved by previous study [15]. Another reason for improved mechanical property in vertical direction is due to the formation of the columnar elongated prior β grains along building direction (Fig. 2e). The directionally solidified columnar grains have a distinct strengthen effect along their growth direction [16]. On the contrary, as the dynamic or static load perpendicular to columnar elongated grains, cracking is easier to occur at the grain boundary when the bonding energy cannot bear the dislocation energy, resulting in a relatively low strength and elongation in horizontal direction. 4. Conclusions The annealing treatment at 800 ℃ for 2 h facilitated the phase transition of α′ → α + β of SLM-fabricated Ti6Al4V alloy. Although two kinds of defects (irregular micro-voids and near-spherical impurity) were detected by Micro-CT, the near-fully dense part was still obtained with internal porosity bellow than 0.0026%. Moreover, the primary micro-voids were apt to disperse along the boundary with depth less than 300 μm. The YS ~ 953 MPa and El. ~ 17.7% in vertical direction were higher than that in horizontal direction. The fatigue behavior showed a similar anisotropy that the 7
fatigue strength ~ 546 MPa in vertical direction was higher compared to that ~ 530 MPa in horizontal direction. Meanwhile, the vertical fatigue scatter was also larger than the horizontal one. It was considered that the crystallographic texture and columnar grains played a dominant role on anisotropic mechanical property in different directions and the deposited layer-like structure intrinsic to SLM contributed to the obvious scatter of fatigue behavior in vertical direction. Acknowledgements This work was financially supported by National Key Research and Development Program of China “Additive Manufacturing and Laser Manufacturing” (No. 2016YFB1100105) and Technology Innovation Program of COMAC (No. Y16GS05-20). References [1] D. Herzog, V. Seyda, E. Wyrick, C. Emmelmann, Acta Mater. 117 (2016) 371-392. [2] W.H. Yu, S.L. Sing, C.K. Chua, C.N. Kuo, X.L. Tian, Prog. Mater. Sci. 104 (2019) 330379. [3] B. He, W. Wu, L. Zhang, L. Lu, Q. Yang, Q. Long, et al., Vacuum 150 (2018) 79-83. [4] S. Cao, R. Chu, X. Zhou, K. Yang, Q. Jia, C.V.S. Lim, et al., J. Alloys Compd. 744 (2018) 357-363. [5] Z.W. Xu, A. Liu, X.S. Wang, Mater. Sci. Eng. A 767 (2019) 138409. [6] X. Yan, R. Lupoi, H. Wu, W. Ma, M. Liu, G. O’Donnell, et al., Mater. Lett. 255 (2019) 126537. [7] J. Yang, H. Yang, H. Yu, Z. Wang, H. Wang, X. Zeng, Mater. Lett. 215 (2018) 246-249. [8] H. Attar, K.G. Prashanth, A.K. Chaubey, M. Calin, L.C. Zhang, S. Scudino, et al., Mater. Lett. 142 (2105) 38-41. [9] S.A. Khairallah, A.T. Anderson, A. Rubenchik, W.E. King, Acta Mater. 108 (2016) 3645. [10] A. Takaichi, Y. Kajima, N. Kittikundecha, H.L. Htat, H.H.W. Cho, T. Hanawa, et al., J. Mech. Behav. Biomed. Mater. 102 (2020) 103496. [11] R. Konecna, L. Kunz, A. Baca, G. Nicoletto, Eng. Fract. Mech. 185 (2017) 82-91. [12] B. AlMangour, D. Grzesiak, J.M. Yang, J. Alloys Compd. 728 (2017) 424-435. [13] L. Thijs, M.L.M. Sistiaga, R. Wauthle, Q. Xie, J.P. Kruth, J.V. Humbeeck, Acta Mater. 61 (2013) 4657-4668. [14] S. Liu, Y.C. Shin, Mater. Des. 164 (2019) 107552. [15] C. Formanoir, S. Michotte, O. Rigo, L. Germain, S. Godet, Mater. Sci. Eng. A 652 (2016) 105-119. [16] K. Kunze, T. Etter, J. Grasslin, V. Shklover, Mater. Sci. Eng. A 620 (2015) 213-222.
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Highlights:
The near-fully dense Ti6Al4V alloy was fabricated by SLM.
Two kinds of micro-defect types for SLM were displayed.
The SLM-fabricated alloy showed an excellent mechanical property.
An abnormal anisotropy feature in different building directions was found.
The texture and columnar microstructure contributed to the interesting anisotropy.
Statement for conflict of interest
The authors declare no competing interests.
Credit Author Statement:
Kun Chang: Investigation, Methodology, Writing-Original draft preparation. Enquan Liang: Investigation, Resources. Wenjing Huang: Validation, Investigation. Xi Zhang: Resources, Visualization. Ying Chen: Formal analysis. 9
Jinfang Dong: Writing-Reviewing and Editing. Ren Zhang: Writing-Reviewing and Editing, Project administration.
a
b
c
d
Fig. 1 The morphology of (a) raw powder and (b) SLM-fabricated Ti6Al4V samples. The dimensions of (c) fatigue and (d) tensile specimens.
10
a
b Building direction Acicular α′
Acicular α′
c
d α β
e
f α Prior columnar β grain
β
Building direction
11
g
h
i
j
k
Building direction
Micro-voids at boundary
Fig. 2 The microstructure features: (a) top surface and (b) cross-section of as-built sample, (c) top surface and (e) cross-section of annealed sample. The (d) and (f) are the view in a higher magnification of (c) and (e), respectively. The inset pictures in (a) and (b) are corresponding lower magnification images. (g) CT (longitudinal) image of annealed sample, (h) 3-dimension image of selected part inside of the sample, (i) 3dimension re-construction image for the dispersion of defects, and the typical morphologies (j) irregular micro-voids and (k) near-spherical impurity.
12
b
a
c
d
Necking
Micro-voids
Dimples
Micro-voids
e
f
Laminar tearing
Cleavage plane Crack nucleation site
Crack nucleation site
Fig. 3 (a) The tensile and (b) fatigue testing results in different directions of SLMfabricated Ti6Al4V alloy after annealing treatment. Tensile fractographies in horizontal direction in (c) low magnification and (d) high magnification. Fatigue fractographies in (e) horizontal and (f) vertical directions.
13
Figure Captions:
Fig. 1 The morphology of (a) raw powder and (b) SLM-fabricated Ti6Al4V samples. The dimensions of (c) fatigue and (d) tensile specimens. Fig. 2 The microstructure features: (a) top surface and (b) cross-section of as-built sample, (c) top surface and (e) cross-section of annealed sample. The (d) and (f) are the view in a higher magnification of (c) and (e), respectively. The inset pictures in (a) and (b) are corresponding lower magnification images. (g) CT (longitudinal) image of annealed sample, (h) 3-dimension image of selected part inside of the sample, (i) 3dimension re-construction image for the dispersion of defects, and the typical morphologies (j) irregular micro-voids and (k) near-spherical impurity. Fig. 3 (a) The tensile and (b) fatigue testing results in different directions of SLMfabricated Ti6Al4V alloy after annealing treatment. Tensile fractographies in horizontal direction in (c) low magnification and (d) high magnification. Fatigue fractographies in (e) horizontal and (f) vertical directions.
14
15
S0167-577X(20)30221-4 https://doi.org/10.1016/j.matlet.2020.127516 MLBLUE 127516
To appear in:
Materials Letters
Received Date: Revised Date: Accepted Date:
6 January 2020 14 February 2020 17 February 2020
Please cite this article as: K. Chang, E. Liang, W. Huang, X. Zhang, Y. Chen, J. Dong, R. Zhang, Microstructural feature and mechanical property in different building directions of additive manufactured Ti6Al4V alloy, Materials Letters (2020), doi: https://doi.org/10.1016/j.matlet.2020.127516
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2020 Published by Elsevier B.V.
Microstructural feature and mechanical property in different building directions of additive manufactured Ti6Al4V alloy Kun Chang*, Enquan Liang, Wenjing Huang, Xi Zhang, Ying Chen, Jinfang Dong, Ren Zhang* Standard and Materials Research Department, Shanghai Aircraft Design and Research Institute, Jinke Road 5188, Shanghai 201210, PR China. *Corresponding author. E-mail address: [email protected] (K. Chang); [email protected] (R. Zhang) Abstract Near-fully dense Ti6Al4V alloy was successfully fabricated by SLM additive manufacturing. The acicular martensite α′ microstructure was considerably optimized by the formation of α + β phase via annealing treatment. The annealed Ti6Al4V alloy showed a very favorable tensile strength and excellent ductility even compared with the as-built part. Although an obvious vertical fatigue scatter, the tensile yielding strength ~ 953 MPa, elongation ~ 17.7% and fatigue strength ~ 546 MPa in vertical direction were all superior to that in horizontal direction, showing a fancy anisotropy. This was completely different from previous research. The mechanism for this phenomenon was analyzed along with the fracture morphology observation. Keywords: Additive manufacturing; Selective laser melting; Microstructure; Mechanical property; Fatigue 1. Introduction As a typical additive manufacturing (AM) technology, selective laser melting (SLM) is famous for precision manufacturing and has been popularly used for fabricating metallic materials with complex geometry and high dimensional accuracy [1,2]. Amount of all the metallic materials, Ti6Al4V alloy is one of the most mature materials for additive manufacturing. The Ti6Al4V components fabricated by SLM have been applicated in aerospace, biomedicine, military due to its favorable mechanical property, biocompatibility and good corrosion resistance [3]. Due to the rapid melting/cooling rate, SLM-fabricated Ti6Al4V alloy possesses 1
significantly refined acicular α′ martensite [3]. Although the static performance is strengthened by forming this refined phase, the as-built Ti6Al4V components are in an un-balanced status because of internal residual stress and exhibit typical anisotropy in terms of microstructure and mechanical properties due to the specific metallurgical process [4,5]. Therefore, as-built Ti6Al4V components are not recommended to be used directly prior to some post heat treatments. Up to now, researches are mainly focused on exploiting an optimist process route to improve the densification, mechanical properties and obtain homogeneous microstructures by optimizing SLM processing and heat treatment parameters [4,6]. However, the discrepancy on different directions is scarcely investigated. In order to understand the microstructure and mechanical property in different directions of SLM-fabricated Ti6Al4V alloy, the microstructure features and defects were investigated in this study. The mechanical properties in terms of tensile and fatigue behaviors were also estimated. 2. Experimental details The Ti6Al4V powder in a spherical morphology with D50 of 32.8 μm and D90 of 54.8 μm was used as raw material (Fig. 1a). Rectangular samples and cylindrical specimens (Fig. 1b) were processed by S310 SLM system adopting a zigzag scanning strategy with oxygen concentration below 100 ppm. The optimized parameters were adopted: laser power 350 W, scan speed 1250 mm/s, hatching space 0.1 mm and layer thickness 0.06 mm. The annealing heat treatment was performed on the SLMfabricated samples at 800 ℃ for 2h, and then furnace cooled in vacuum environment. The cylindrical specimens were then machined into the shapes of Fig. 1c and d for fatigue and tensile testing, respectively. The microstructures were characterized using optical microscopy. The tensile tests were carried out using an Instron 5887 machine according to ASTM E8/E8M with strain rate of 0.005 mm/mm/min before yielding and displacement rate of 5 mm/min after yielding. The fatigue tests were performed on Electromagnetic resonance fatigue machine (QBG-25) according to ASTM E466 (Kt=1, R=0.1). Powder and fracture morphologies were observed by scanning electron microscope 2
(SEM). Micro-focus computed tomography (CT) machine (IPT6016B2) was employed to analyze the internal defects of annealed samples.
a
b
c
d
Fig. 1 The morphology of (a) raw powder and (b) SLM-fabricated Ti6Al4V samples. The dimensions of (c) fatigue and (d) tensile specimens. 3. Results and discussion The as-built samples were characterized by acicular α′ phase with a random dispersion (Fig. 1a and b), which was induced by the rapid cooling rate of laser melt pool [7,8]. The prior β grains could be clearly observed on both the as-built and annealed specimens. The top surfaces mainly consisted of prior near-cellular β grains (Fig.1a and c) whereas the cross-sections were featured with a columnar morphology along the building direction (Fig. 1b and e), presenting a distinct anisotropy. This difference is attributed to the different thermal diffusion manner in the parallel and perpendicular to the building direction [3]. After annealing of the SLM-fabricated samples, the acicular martensite α′ was completely replaced by lath-like α phase accompanied by the formation of β phase (Fig. 1d and f), indicating that the phase transition of α′ → α + β was occurred during annealing.
3
a
b Building direction Acicular α′
Acicular α′
c
d α β
e
f α Prior columnar β grain
β
Building direction
g
h
i
j
k
Building direction
Micro-voids at boundary
Fig. 2 The microstructure features: (a) top surface and (b) cross-section of as-built 4
sample, (c) top surface and (e) cross-section of annealed sample. The (d) and (f) are the view in a higher magnification of (c) and (e), respectively. The inset pictures in (a) and (b) are corresponding lower magnification images. (g) CT (longitudinal) image of annealed sample, (h) 3-dimension image of selected part inside of the sample, (i) 3dimension re-construction image for the dispersion of defects, and the typical morphologies (j) irregular micro-voids and (k) near-spherical impurity. As observed in Fig. 2g, the micro-voids were mainly dispersed along the edge of the sample with depth less than 300 μm. It is considered that the switching of laser beam between adjacent tracks combined with the energy damping at the end of melt track resulted in the formation of boundary micro-voids [9]. On the contrary, the internal of the sample was significantly dense. To further analyze the defect features inside of the sample, internal cuboid of 15 × 8.3 × 9.6 mm3 was selected and tested, as shown in Fig. 2h. The testing results revealed that only a few defects primary including irregular micro-voids and near-spherical impurity were formed and randomly dispersed (Fig. 2i) with a significantly low porosity of 0.0026%. The single biggest micro-void volume was 7.2 × 10-4 mm3 (Fig. 2j) and the biggest diameter of near-spherical impurity is about 57 μm (Fig. 2k), which might be large-size residual powder particle un-melted. After annealing treatment, the ultimate tensile strength (UTS) ~ 1032 MPa in horizontal direction was exactly similar with that (~ 1035 MPa) in vertical direction, showing an isotropy feature (Fig. 3a). However, both the yielding strength (YS) ~953 MPa and elongation (El.) ~ 17.7% in vertical direction were higher than the YS ~ 929.5 MPa and El. ~ 15.6 MPa in horizonal direction, revealing an anisotropy. Interestingly, this phenomenon was completely different form previous study results that generally the properties in vertical direction were lower than that in horizontal direction [10,11]. In addition, the annealed part showed a significantly improved El. and maintained a similar YS simultaneously compared with the as-built one (~ 7.4%, 952 ~ 968 MPa) [11]. The similar anisotropic result was obtained for fatigue testing, as depicted in Fig. 3b. The fatigue strength of 546 MPa in vertical direction was 5
higher than that (~ 530 MPa) in horizontal direction whereas showed a lager scatter.
b
a
c
d
Necking
Micro-voids
Dimples
Micro-voids
e
f
Laminar tearing
Cleavage plane Crack nucleation site
Crack nucleation site
Fig. 3 (a) The tensile and (b) fatigue testing results in different directions of SLMfabricated Ti6Al4V alloy after annealing treatment. Tensile fractographies in horizontal direction in (c) low magnification and (d) high magnification. Fatigue fractographies in (e) horizontal and (f) vertical directions. Tensile fractographies observation in horizontal direction revealed obvious necking and vary sized dimples (Fig. 3c and d), which were the typical symbolizes of ductile fracture. Additionally, some micro-voids could be also observed on the fracture, verifying the result of CT testing. However, the fatigue behavior was sensitive to the internal defects. It was found that the micro-voids were easily to act as the crack nucleation and induce the fatigue fracture (Fig. 3e and f). Moreover, the fatigue fractography in horizontal direction was characterized by few cleave planes (Fig. 3e), which further verified the columnar grain boundary cracking in horizontal direction at dynamic load condition, resulting in a relative lower fatigue strength (Fig. 3b). The laminar tearing feature was appeared on the fatigue fractography in the 6
vertical direction (Fig. 3f), which reflected the destruction of metallurgical bonding layers under rigorously dynamic load conditions. The large scatter of vertical fatigue behavior might be ascribed to the occurrence of break of SLM metallurgical bonding layers. It is considered that the crystallographic texture, columnar microstructure intrinsic to AM contribute to the anisotropic behavior of the fabricated Ti6Al4V samples [12-16]. Theoretically, the grain tends to grow opposite to the maximum temperature gradient at the solidification front. Generally, the primary overall heat deviation and dissipation from center to bottom of the melt pool can give rise to a large thermal gradient downward [13]. Under the condition of directional solidification, the prior β phase preferentially grows along the preferred ˂001>β orientation [13-14], which is parallel to the building direction, resulting in an obvious crystallographic texture in AM of Ti6Al4V alloy and resultant anisotropic mechanical behavior. This typical texture feature in AM-fabricated Ti6Al4V has also been proved by previous study [15]. Another reason for improved mechanical property in vertical direction is due to the formation of the columnar elongated prior β grains along building direction (Fig. 2e). The directionally solidified columnar grains have a distinct strengthen effect along their growth direction [16]. On the contrary, as the dynamic or static load perpendicular to columnar elongated grains, cracking is easier to occur at the grain boundary when the bonding energy cannot bear the dislocation energy, resulting in a relatively low strength and elongation in horizontal direction. 4. Conclusions The annealing treatment at 800 ℃ for 2 h facilitated the phase transition of α′ → α + β of SLM-fabricated Ti6Al4V alloy. Although two kinds of defects (irregular micro-voids and near-spherical impurity) were detected by Micro-CT, the near-fully dense part was still obtained with internal porosity bellow than 0.0026%. Moreover, the primary micro-voids were apt to disperse along the boundary with depth less than 300 μm. The YS ~ 953 MPa and El. ~ 17.7% in vertical direction were higher than that in horizontal direction. The fatigue behavior showed a similar anisotropy that the 7
fatigue strength ~ 546 MPa in vertical direction was higher compared to that ~ 530 MPa in horizontal direction. Meanwhile, the vertical fatigue scatter was also larger than the horizontal one. It was considered that the crystallographic texture and columnar grains played a dominant role on anisotropic mechanical property in different directions and the deposited layer-like structure intrinsic to SLM contributed to the obvious scatter of fatigue behavior in vertical direction. Acknowledgements This work was financially supported by National Key Research and Development Program of China “Additive Manufacturing and Laser Manufacturing” (No. 2016YFB1100105) and Technology Innovation Program of COMAC (No. Y16GS05-20). References [1] D. Herzog, V. Seyda, E. Wyrick, C. Emmelmann, Acta Mater. 117 (2016) 371-392. [2] W.H. Yu, S.L. Sing, C.K. Chua, C.N. Kuo, X.L. Tian, Prog. Mater. Sci. 104 (2019) 330379. [3] B. He, W. Wu, L. Zhang, L. Lu, Q. Yang, Q. Long, et al., Vacuum 150 (2018) 79-83. [4] S. Cao, R. Chu, X. Zhou, K. Yang, Q. Jia, C.V.S. Lim, et al., J. Alloys Compd. 744 (2018) 357-363. [5] Z.W. Xu, A. Liu, X.S. Wang, Mater. Sci. Eng. A 767 (2019) 138409. [6] X. Yan, R. Lupoi, H. Wu, W. Ma, M. Liu, G. O’Donnell, et al., Mater. Lett. 255 (2019) 126537. [7] J. Yang, H. Yang, H. Yu, Z. Wang, H. Wang, X. Zeng, Mater. Lett. 215 (2018) 246-249. [8] H. Attar, K.G. Prashanth, A.K. Chaubey, M. Calin, L.C. Zhang, S. Scudino, et al., Mater. Lett. 142 (2105) 38-41. [9] S.A. Khairallah, A.T. Anderson, A. Rubenchik, W.E. King, Acta Mater. 108 (2016) 3645. [10] A. Takaichi, Y. Kajima, N. Kittikundecha, H.L. Htat, H.H.W. Cho, T. Hanawa, et al., J. Mech. Behav. Biomed. Mater. 102 (2020) 103496. [11] R. Konecna, L. Kunz, A. Baca, G. Nicoletto, Eng. Fract. Mech. 185 (2017) 82-91. [12] B. AlMangour, D. Grzesiak, J.M. Yang, J. Alloys Compd. 728 (2017) 424-435. [13] L. Thijs, M.L.M. Sistiaga, R. Wauthle, Q. Xie, J.P. Kruth, J.V. Humbeeck, Acta Mater. 61 (2013) 4657-4668. [14] S. Liu, Y.C. Shin, Mater. Des. 164 (2019) 107552. [15] C. Formanoir, S. Michotte, O. Rigo, L. Germain, S. Godet, Mater. Sci. Eng. A 652 (2016) 105-119. [16] K. Kunze, T. Etter, J. Grasslin, V. Shklover, Mater. Sci. Eng. A 620 (2015) 213-222.
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Highlights:
The near-fully dense Ti6Al4V alloy was fabricated by SLM.
Two kinds of micro-defect types for SLM were displayed.
The SLM-fabricated alloy showed an excellent mechanical property.
An abnormal anisotropy feature in different building directions was found.
The texture and columnar microstructure contributed to the interesting anisotropy.
Statement for conflict of interest
The authors declare no competing interests.
Credit Author Statement:
Kun Chang: Investigation, Methodology, Writing-Original draft preparation. Enquan Liang: Investigation, Resources. Wenjing Huang: Validation, Investigation. Xi Zhang: Resources, Visualization. Ying Chen: Formal analysis. 9
Jinfang Dong: Writing-Reviewing and Editing. Ren Zhang: Writing-Reviewing and Editing, Project administration.
a
b
c
d
Fig. 1 The morphology of (a) raw powder and (b) SLM-fabricated Ti6Al4V samples. The dimensions of (c) fatigue and (d) tensile specimens.
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a
b Building direction Acicular α′
Acicular α′
c
d α β
e
f α Prior columnar β grain
β
Building direction
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g
h
i
j
k
Building direction
Micro-voids at boundary
Fig. 2 The microstructure features: (a) top surface and (b) cross-section of as-built sample, (c) top surface and (e) cross-section of annealed sample. The (d) and (f) are the view in a higher magnification of (c) and (e), respectively. The inset pictures in (a) and (b) are corresponding lower magnification images. (g) CT (longitudinal) image of annealed sample, (h) 3-dimension image of selected part inside of the sample, (i) 3dimension re-construction image for the dispersion of defects, and the typical morphologies (j) irregular micro-voids and (k) near-spherical impurity.
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b
a
c
d
Necking
Micro-voids
Dimples
Micro-voids
e
f
Laminar tearing
Cleavage plane Crack nucleation site
Crack nucleation site
Fig. 3 (a) The tensile and (b) fatigue testing results in different directions of SLMfabricated Ti6Al4V alloy after annealing treatment. Tensile fractographies in horizontal direction in (c) low magnification and (d) high magnification. Fatigue fractographies in (e) horizontal and (f) vertical directions.
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Figure Captions:
Fig. 1 The morphology of (a) raw powder and (b) SLM-fabricated Ti6Al4V samples. The dimensions of (c) fatigue and (d) tensile specimens. Fig. 2 The microstructure features: (a) top surface and (b) cross-section of as-built sample, (c) top surface and (e) cross-section of annealed sample. The (d) and (f) are the view in a higher magnification of (c) and (e), respectively. The inset pictures in (a) and (b) are corresponding lower magnification images. (g) CT (longitudinal) image of annealed sample, (h) 3-dimension image of selected part inside of the sample, (i) 3dimension re-construction image for the dispersion of defects, and the typical morphologies (j) irregular micro-voids and (k) near-spherical impurity. Fig. 3 (a) The tensile and (b) fatigue testing results in different directions of SLMfabricated Ti6Al4V alloy after annealing treatment. Tensile fractographies in horizontal direction in (c) low magnification and (d) high magnification. Fatigue fractographies in (e) horizontal and (f) vertical directions.
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