- Email: [email protected]
TiB2 composite produced by selective laser melting
Optics & Laser Technology 80 (2016) 186–195
Contents lists available at ScienceDirect
Optics & Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Full length article
Microhardness and microstructure evolution of TiB2 reinforced Inconel 625/TiB2 composite produced by selective laser melting Baicheng Zhang n, Guijun Bi, Sharon Nai, Chen-nan Sun, Jun Wei SIMTech-Singapore Institute of Manufacturing Technology,x 71 Nanyang Dr, 638075, Singapore
art ic l e i nf o
a b s t r a c t
Article history: Received 22 September 2015 Received in revised form 23 November 2015 Accepted 4 January 2016
In this study, micron-size TiB2 particles were utilized to reinforce Inconel 625 produced by selective laser melting. Exceptional microhardness 600–700 HV0.3 of the composite was obtained. In further investigation, the microstructure and mechanical properties of Inconel 625/TiB2 composite can be significantly influenced by addition of TiB2 particles during SLM. It was found that the long directional columnar grains observed from SLM-processed Inconel 625 were totally changed to fine dendritic matrix due to the addition of TiB2 particles. Moreover, with laser energy density (LED) of 1200 J/m, a Ti, Mo rich interface around TiB2 particles with fine thickness can be observed by FESEM and EDS. The microstructure evolution can be determined by different laser energy density (LED): under 1200 J/m, γ phase in dendrite grains; under 600 J/m, γ phase in combination of dendritic and acicular grains; under 400 J/ m, γ phase acicular grains. Under optimized LED 1200 J/m, the dynamic nanohardness (8.62 GPa) and elastic modulus (167 GPa) of SLM-processed Inconel 625/TiB2 composite are higher compared with those of SLM-processed Inconel 625 (3.97 GPa and 135 GPa, respectively). & 2016 Elsevier Ltd. All rights reserved.
Keywords: Selective laser melting Inconel 625 TiB2 particle Microstructure Composite alloy
1. Introduction In recent year, the demands on high performance nickel based superalloys are increasing with the development of aerospace and oil industry [1–3]. Inconel series alloys, as a Ni–Cr based austenite superalloy, are featured by an improved balance of creep performance, fatigue strength, tensile properties, and oxidation and corrosion resistance, making them as attractive choices for diverse industrial applications. Especially, Inconel 625 alloy has been widely applied in aerospace, marine, chemical and petrochemical industries due to its superior mechanical and corrosion resistant properties. However, the precision machining of Inconel 625 alloy is still a challenge [4], which hinders the further applications in industry. Selective laser melting (SLM) is one of newly developed additive manufacturing (AM) processes which especially applicable to hardto-machine metallic materials [5]. In the process, a high energy laser beam selectively scans a powder bed under a protective atmosphere and melts the particles which solidify to form a solid layer using a computer controlled system [6]. A new layer of powders is deposited on top of the previously formed solid layer and the process continues until the part is completed [7]. Compare to conventional manufacturing techniques, SLM provides a broader n
Corresponding author. E-mail address: [email protected] (B. Zhang).
http://dx.doi.org/10.1016/j.optlastec.2016.01.010 0030-3992/& 2016 Elsevier Ltd. All rights reserved.
range of advantages, namely rapid manufacturing of objects almost without geometric constraints, high material utilization rate and direct fabrication based on CAD model [8,9]. The previous studies of the SLM-processing Inconel series alloys have shown promising results, such as high surface quality, desirable mechanical properties. Moreover, the metal matrix composites (MMC) produced by addition of ceramic particles give a great potential to increase not only hardness, but also high temperature mechanical properties [10]. However, the reinforcement mechanism by additive ceramic particles is still not clear based on present research work [11]. Some researchers have given some value research: G. Bi used self-developed laser cladding system to fabricate Inconel 625-nano TiC composite [12]; C Hong and D Gu proposed a systematic research about Inconel 625 with TiC composite by SLM [13]; J. Michael Wilson conducted a study of Inconel 690 with TiC by laser cladding [14]. A further and systemic investigation of TiB2 reinforcement of MMC for SLM process is necessary. The micron-size TiB2 particles possess a high thermalstability during metallurgy process with super hardness, which is suitable for SLM process. At present, the research of TiB2 reinforced MMC produced by additive manufacturing is quite novel. The related publications are quite rare in AM research area. Results of this research may introduce a great potential for surface modification and fabrication of 3D components applications. In the present work, SLM technology was applied to produce the TiB2 particle reinforced Inconel 625 matrix composite. Process characteristics were systematically explored. Influence of the SLM
B. Zhang et al. / Optics & Laser Technology 80 (2016) 186–195
process parameters on density was studied. Moreover, the effect of TiB2 on hardness and microstructure evolution of the fabricated Inconel 625-MMC was investigated.
187
Table 2 Chemical compositions of Inconel TiB2 powder (wt.%). Ti
B
Other
4 67.5
4 30.5
o 2.5
2. Experimental procedure 2.1. Powder preparation In this study, gas atomized spherical Inconel 625 powders with particle size distribution of 11–45 μm were used. A cubic-shaped TiB2 powders (4 97.5% purity) with the particle size distribution 5–12 μm were adopted. The nominal chemical compositions of Inconel 625 and TiB2 powders are provided in Table 1 and Table 2, respectively. 95 wt.% Inconel 625 powders and 5 wt.% TiB2 powders were mixed in a tumbling mixer for 4 h before use. 2.2. Selective laser melting process The SLM experiments were carried out using a MCP Realize Ⅱ machine (MCP 250 HEK Tooling GmbH, Germany), which mainly consists a Nd:YAG laser with wavelength of 1064 nm. The laser beam diameter was fixed at 46 mm. The hatch distance was 100 mm for an optimized 50% overlapped. The powder layer thickness was 50 mm. Cubic form specimens with dimensions of 5 5 5 mm3 were fabricated by SLM. An alternate scanning pattern with equal line spacing in the X and Y directions was used and shown in Fig. 1. The SLM process was performed under argon atmosphere, and the powder bed temperature was kept constant at 80 °C. The laser power was optimized for 80 W and 120 W. The scanning velocity was set at 100, 200, 300 mm/s by the SLM control program. Thus, except the low quality sample with 80 W laser power and 200 and 300 mm/s scanning speed, four different “laser energy densities” (LED) of 1200, 800, 600 and 400 J/m, which defined by LED¼ P/v [16], were used to assess the laser energy input to the powder layer being processed. 2.3. Characterization of microstructure and hardness After the deposition process, density evaluation was processed by ‘Archimedes-method’, which consists in weighting the samples in air and subsequently in demineralized water. Then, the samples were cut, grinded and polished according to the standard procedures for metallographic examinations. And then, the samples were etched by auto electrolytic preparation system (Struers LectroPol-5) with voltage 5 V for 25 s. The microstructure was characterized using a scanning electron microscope (SEM, JEOL, JSM5800LV, Japan). The chemical composition was examined by an energy dispersive spectrometer (EDS). The Vickers hardness test was carried out using a microhardness tester (Matsuzawa MMTX3, Japan) with load of 300 g and dwell time of 15 s. Microhardness was tested at least 10 times at different locations on the sample cross-section. The average value was applied as microhardness value. Nano-indentation tests on the polished sections of parts were performed using a Nano-indentation tester (Nano test Micro Material Ltd. Wrexham, UK) under a fixed angle of 25°. A loading-unloading test mode was used with maximum test force of 100 mN, loading speed of 1.0 mN/s, and dwell time of 10 s.
Fig. 1. Schematic diagram of selective laser melting scanning strategy [15].
3. Results and discussions 3.1. Density behavior The microstructure evolution of SLM-processed Inconel 625/TiB2 composite samples with different laser parameters are shown in Fig. 2(a–c). It can be observed that the irregular TiB2 particles were distributed in Inconel 625 matrix for all laser parameters. As a comparison, the microstructure of SLM-processed pure Inconel 625 with the same laser parameters is shown in Fig. 2(d–f). The density of both Inconel 625/TiB2 composite and Inconel 625 is shown in Fig. 3. For Inconel 625 produced with high LED 1200 J/m, the cross-section shows nearly fully dense with tiny pores, but free of any cracks, as shown in Fig. 2(d). The density of SLM Inconel 625 is 8.43 g/cm3, and the corresponding relative density is about 98.3%. With addition of TiB2, the relative density was decreases to 96.1%. The main deviation is caused by micronsize pores with size of 10-20 μm, as shown in Fig. 2(a). As the LED decreases to 600 J/m and 800 J/m, the relative density of SLM-processed Inconel 625 can decrease to 89.7% and 92.5%, respectively. With addition of TiB2 reinforcement particles, the density of SLM-processed parts continuously decreases to 86.5% and 87.2%, respectively. The cross-section of the parts shows a relatively heterogeneous matrix combined of TiB2 particles with the formation of the irregular shaped pores in a scale of 20–40 μm, as shown in Fig. 2(b). Moreover, a small amount of un-melted powders and inter-layer micro-cracks can be found in the crosssection. For low energy density input with LED of 400 J/m, a severe delamination phenomenon occurred, due to severe lack of laser energy, as shown in Fig. 2(c). Density of the sample with TiB2 particles is relatively low (73.1%), compared with the sample without TiB2 addition (79.3%). 3.2. Microhardness evolution
Table 1 Chemical compositions of Inconel 625 powder (wt.%). Ni
Cr
Mo
Al
Ti
Fe
Nb
Balance
21.5
9
0.3
0.4
2.5
3.5
Fig. 4 depicts the average microhardness measured in the cross-sections of SLM-processed Inconel 625 and Inconel 625/TiB2 parts. For all laser parameter LED from 400 J/m to 1200 J/m, the average microhardness is in the range of 300–350 HV0.3. However,
188
B. Zhang et al. / Optics & Laser Technology 80 (2016) 186–195
Fig. 2. Microstructure of SLM Inconel 625/TiB2 composite with LED 1200 J/m (a); 600 J/m (b), 400 J/m (c); SLM pure Inconel 625 parts with 1200 LED J/m (d); 600 J/m (e), 400 J/ m (f).
Fig. 3. Density of the samples produced with different laser parameters with/ without TiB2 particles.
the mean microharndess increases to 600–700 HV0.3 when TiB2 reinforcement particles were used. Therefore, the addition of TiB2 particles can significantly improve the microhardness of SLM Inconel 625 alloy. The microhardness evolution is mainly determined by the microstructure formation. It will be discussed in the next section. 3.3. Microstructural evolution Fig. 5 illustrates the microstructural features of both SLM-processed Inconel 625 and Inconel 625/TiB2 composite examined in horizontal and vertical cross-sections. Under condition of LED 1200 J/m, typical cellular and columnar grain morphology can be found in SLM-processed Inconel 625 in horizontal and vertical
Fig. 4. Micro-hardness of fabricated samples with/without TiB2 particles using different laser parameters.
cross-sections, respectively, as shown in Fig. 5(a). The grains grew along the building direction with a mean diameter of 1.2 μm. However, the microstructural feature of SLM-processed Inconel 625/TiB2 composite is very different from SLM-processed Inconel 625 due to addition of TiB2 reinforcement particles, as shown in Fig. 5(b–d). Instead of columnar grains in SLM processed Inconel 625, fine and homogenous dendritic grains can be found in SLMprocessed Inconel 625/TiB2 composite, which correspond γ phase [17]. Furthermore, it can be found that the growth morphology and dispersion state of SLM-processed Inconel 625/TiB2 composite were significantly influenced by laser energy density input LED. At a relatively high LED of 1200 J/m, the microstructure exhibited a full dendritic morphology. A mean length of dendrite trunk and arms can be found as 5.4 and 3.6 μm, respectively as shown in
B. Zhang et al. / Optics & Laser Technology 80 (2016) 186–195
189
Fig. 5. Microstructure evolution of SLM-processed Inconel 625 (a), and Inconel 625/TiB2 composite under 1200 J/m (b); 600 J/m (c); 400 J/m(d) in horizontal and vertical cross-section.
190
B. Zhang et al. / Optics & Laser Technology 80 (2016) 186–195
A
B
C
D
Fig. 6. FESEM and EDS results of SLM-processed Inconel 625/TiB2 composite under 1200 J/m.
B. Zhang et al. / Optics & Laser Technology 80 (2016) 186–195
A
B
C
D
Fig. 7. FESEM and EDS results of SLM Inconel 625/TiB2 composite under 600 J/m.
191
192
B. Zhang et al. / Optics & Laser Technology 80 (2016) 186–195
Fig. 8. FESEM and EDS results of SLM-processed Inconel 625/TiB2 composite under 400 J/m.
B. Zhang et al. / Optics & Laser Technology 80 (2016) 186–195
Fig. 9. Nano-indentation results of SLM-processed Inconel 625/TiB2 composite and Inconel 625 with different laser parameters.
Fig. 5(b). As LED decreases to 600 J/m, the dendritic grains of SLMprocessed Inconel 625/TiB2 were refined. Therefore, it is also observed that the present Inconel 625/TiB2 typically contained two different morphologies, as show in Fig. 5(c). On one hand a reduced dendritic structure with 7.3 μm of trunk length and 4.2 μm of arm length can be observed. On the other hand, the acicular grains with diameter of 200–500 nm can also be identified in the matrix. When an even lower LED 400 J/m was used, a very homogeneous dispersion of significantly refined acicular grains without dendritic microstructural feature was formed, as shown in Fig. 5(d). 3.4. Analysis of boundary reinforcement by TiB2 FESEM analysis illustrates the influence of different laser energy density LED 1200 J/m, 600 J/m and 400 J/m on the characteristic morphologies of interfacial microstructure around TiB2 particle in SLM-processed Inconel 625 composite, as shown in Figs. 6–8, respectively. In order to carefully quantify the element content distribution in composite matrix, element analysis was carried out in four different feature zones from A to D respectively. The quantitative elemental determination by EDS spot-scan indicated that the dark area contained Ti and B elements, which can be confirmed as TiB2 particle in SLM-processed composite. From EDS element analysis results, the Mo and Ni elements can be detected in blank area, among which the Ni element demonstrated a relatively homogenous distribution in matrix. When a relatively high LED of 1200 J/m was applied, as shown in Fig. 6, an obvious interfacial layer (zone B and C) can be distinguished from TiB2 particle (zone A) and matrix (zone D). A decreased gradient of Ti content can be found from zone A (58 wt%) to zone D (0.8 wt%). Moreover, the concentration of Mo element exhibited a significant increase in interfacial layer (zone B). During the SLM process, the TiB2 particle can be heated to an extending temperature due to relatively high laser energy absorption ratio [18]. Thus, the Ti and B element from overheated ceramic particle could react with Mo from Inconel 625 matrix, which resulted in the formation of Ti, Mo rich compound interfacial layer around TiB2 particles. The formation of the hard intermetallic resulted in the high hardness at LED 1200 J/m, as shown in Fig. 4. At a decreased LED of 600 J/m, as shown in Fig. 7, the reinforcement particle boundary can be easily distinguished from the matrix without the obvious interfacial layer, as shown in Fig. 6.
193
The matrix microstructure is very different from that with LED of 1200 J/m. In the area B, homogenous dendritic grains with random orientation can be found. Base on the results of Section 3.2, the microstructure close to TiB2 particle can be concluded to γ phase. In this case, element analysis was also carried out from TiB2 particle to matrix in order to determine the distribution of various elements. It shows that the Ti element distribution is very homogenous in matrix around the reinforcement TiB2 particles. The matrix area was formed by Ni, Mo element. Under this condition, the Ti and B elements from ceramic particles cannot react with Inconel 625 due to insufficient laser energy. However, the nucleation process of dendritic grains could be influenced by different thermal conductive between TiB2 and Inconel 625 alloy. The temperature gradient led to a driving force from TiB2 particles to γ-Ni matrix in molten pool, which gave isolated crystallization around TiB2 particles. Therefore, the homogenous microstructure with certain orientation can be formed along with the temperature gradient from γ-Ni matrix to TiB2 particles. When a relatively low LED of 400 J/m was applied, as shown in Fig. 8, the reinforcement particles were dispersed in γ-Ni matrix with a quite sharp boundary. Under this laser parameter, the interlayer cannot be found between the reinforcement particles and the matrix. The spot-scan results show that the Ti distribution is stable in the matrix. Meanwhile, a different Mo and Ni element distribution can be found in element map: high Mo content can be detected on blank area of matrix; high Ni element mainly distributed on the dark area of the matrix. The grain refinement effect was dominant, which contributed to the increasing micro-hardness for LED from 800 J/m to 400 J/m, as shown in Fig. 3. 3.5. Nano-indentation analysis Fig. 9 depicted the load-depth curves of nano-indentation conducted on the polished cross-section of SLM-processed Inconel 625 and Inconel/TiB2 composite. Under the load of 100 mN the indentation depth of the SLM-processed Inconel 625 was 1141 nm, which is obviously deeper than that of the SLM-processed Inconel 625/TiB2 composite, which is about 814.5–840.5 nm. As shown in Table 3, the nanohardness (Hd) of SLM-processed Inconel 625/TiB2 composite is from 8.62 to 10.7 GPa, and the elastic modulus (Er) ranges from167.61 to 273.65 GPa. As a comparison, the nanohardness and the elastic modulus of the SLM-processed Inconel 625 are 3.97 GPa and 135.51 GPa, respectively. Furthermore, different applied LED can lead different mechanical properties, as show in Table 3. The maximum dynamic nanohardness can be found as 10.7 GPa under 400 J/m. Meanwhile, the highest elastic modulus as 273.65 GPa can be achieved by 600 J/m. As show in Fig. 6, the TiB2 particle with Ti, Mo rich interlayer and dendritic matrix can be found in samples with LED 1200 J/m. In this case, a low cooling rate can be generated caused by high laser power and low scanning velocity. Thus, the Inconel 625 alloy grains can grow large under a low cooling rate during the solidification process which leads to a relative long dendritic structure. In which, this kind of microstructure of matrix shows a high elasticity and low hardness. Moreover, the Ti, Mo rich interlayer also decrease the enforcement effect of TiB2 particle. Thus, a Table 3 Nano-indentation results. Hardness, Hd, GPa Young's modulus, Er, GPa Inconel Inconel Inconel Inconel
625 LED, 1200 J/m 625/TiB2, LED 1200 J/m 625/TiB2, LED 600 J/m 625/TiB2, LED 400 J/m
3.97 8.62 70.82 8.82 70.34 10.707 0.43
135.51 167.617 11.16 273.65 7 7.67 255.34 7 9.44
194
B. Zhang et al. / Optics & Laser Technology 80 (2016) 186–195
Fig. 10. Schematic diagrams showing the mechanism of SLM-processed Inconel 625 and Inconel 625/TiB2 composite.
relatively low hardness and Young's modulus can be obtained in sample with 1200 J/m. With the decrease of laser energy input of 600 and 400 J/m, an increase of cooling rate can be generate during solidification. A combination of acicular and dendritic microstructure can be found in sample with 600 J/m, as shown in Fig. 7. In this case, the fine acicular structure can decrease the material ductility and lead to a high hardness. Thus, a medium hardness and relatively high Young's modulus can be found in samples with LED 600 J/m. With continue decrease of LED to 400 J/ m, the single phase of fine acicular grain can be found in sample, as show in Fig. 8. Thus, the highest hardness and low elasticity can be obtained in samples with LED 400 J/m. 3.6. Mechanism of SLM-processed Inconel 625/TiB2 composite Fig. 10 depicted the different solidification mechanisms between pure Inconel 625 (a–c) and Inconel 625/TiB2 (d–f) composite processed by SLM. During the SLM process, the laser beam provided a Gaussian energy distibution on powder layer. The Inconel 625 powders reached melting point and formed the molten pool in a very short time interval [19], as shown in Fig. 10(a) and (d). After laser beam moved from the molten pool, a significant high cooling rate can be generated from the bottom to surface of melted Inconel 625, as shown Fig. 10(b). Therefore, the normal long dendritic grains with certain orientation can be found in SLM-processed Inconel 625 in all laser parameters, as shown in Fig. 10(c). However, the TiB2 particles are stable in molten pool due to the difference of melting point of Inconel 625 and TiB2 (1350 °C of Inconel 625 versus 3230 °C of TiB2). As a comparison, the TiB2 particles were relocated in molten pool due to the Marangoni convention [20], and privoded a chemical reaction with Mo element in Inconel 625 under high laser energy density, as shown in Fig. 10(e). Thus, a driving force can be radiatedly generated from particles to the matrix in molten pool. The integral themal gradient from molten pool bottom to top surface during processing Inconel 625 was changed to isolated area around TiB2 particles during procssing Inconel/TiB2 composite. Thus, dendritic grains with random orientation can be generated
from γ-Ni matrix to TiB2 particles after solidification, as shown in Fig. 10(f). At low laser energy density, this effect is quite obvious.
4. Conclusions In this study, a comparison of Inconel 625 and Inconel 625/TiB2 composite processed by SLM was presented. The main conclusions can be drawn as follows: [1] Addition of TiB2 particles can decrease the density due to formation of pores and cracks. [2] The microstructure of SLM-processed Inconel 625/TiB2 composite can be influenced by addition of TiB2 particles, a fine Ti, Mo rich interfacial layer around TiB2 can be observed. [3] The composite microstructure can be influenced by different laser energy density: fine dendritic grains (1200 J/m); a combination of dendritic and acicular grains (600 J/m); and single phase of acicular grains (400 J/m). [4] The re-formation of particles surface and fine matrix structure generally favored the increase of the nano/micro hardness and Young's modulus. [5] The addition of TiB2 particles in Inconel 625 altered the heating and cooling effect during SLM process. This resulted in the formation of intermetallics around the particles, and modification of microstructures. All these factors contributed to the reinforcement effect of TiB2 particles on the Inconel 625 processed by SLM.
Reference [1] Miroslav Zetek, Ivana Česáková, Vojtěch Švarc, Procedia Eng. 69 (2014) 1115–1124. [2] Alenka Vesel, Aleksander Drenik, Kristina Elersic, Miran Mozetic, Janez Kovac, Tomaz Gyergyek, Jan Stockel, Jozef Varju, Radomir Panek, BalatPichelin Marianne, Appl. Surf. Sci. 305 (2014) 674–682. [3] G. Casalino, S.L. Campanelli, N. Contuzzi, A.D. Ludovico, Opt. Laser Technol. 65 (2015) 151–158. [4] Ravindra I. Badiger, S. Narendranath, M.S. Srinath, J. Manuf. Process. 18 (2015) 117–123.
B. Zhang et al. / Optics & Laser Technology 80 (2016) 186–195
[5] Shuai Li, Qingsong Wei, Yusheng Shi, Zicheng Zhu, Danqing Zhang, J. Mater. Sci. Technol. 31 (2015) 946–952. [6] Baicheng Zhang, Lucas Dembinski, Christian Coddet, Mater. Sci. Eng.: A 584 (1) (2013) 21–31. [7] Lore Thijs, Karolien Kempen, Jean-Pierre Kruth, Jan Van Humbeeck, Acta Mater. 61 (5) (2013) 1809–1819. [8] P. Edwards, M. Ramulu, Mater. Sci. Eng.: A 598 (26) (2014) 327–337. [9] Baicheng Zhang, Hanlin Liao, Christian Coddet, Mater. Des. 34 (2012) 753–758. [10] J.C. Heigel, P. Michaleris, T.A. Palmer, J. Mater. Process. Technol. 220 (2015) 135–145. [11] Pengpeng Yuan, Dongdong Gu, Donghua Dai, Mater. Des. 82 (5) (2015) 46–55. [12] G. Bi, C.N. Sun, M.L. Nai, J. Wei, Phys. Procedia 41 (2013) 828–834. [13] Chen Hong, Dongdong Gu, Donghua Dai, Moritz Alkhayat, Wolf Urban, Pengpeng Yuan, Sainan Cao, Andres Gasser, Andreas Weisheit, Ingomar Kelbassa, Minlin Zhong, Reinhart Poprawe, Mater. Sci. Eng.: A 635 (2015) 118–128.
195
[14] J. Michael Wilson, Yung C. Shin, Surf. Coat. Technol. 207 (2012) 517–522. [15] Waldemar A. Monteiro, Light Metal Alloys Applications, 2014, ISBN 978-95351-1588-5. [16] Dongdong Gu, Hongqiao Wang, Donghua Dai, Pengpeng Yuan, Wilhelm Meiners, Reinhart Poprawe, Scr. Mater. 96 (2015) 25–28. [17] K. Devendranath Ramkumar, R. Jagat Sai, V. Santhosh Reddy, Sridhar Gundla, T. Harsha Mohan, Vimal Saxena, N. Arivazhagan, J. Manuf. Process. 18 (2015) 23–45. [18] B. Vrancken, L. Thijs, J.-P. Kruth, J. Van Humbeeck, Acta Mater. 68 (15) (2014) 150–158. [19] P. Krakhmalev, I. Yadroitsev, Intermetallics 46 (2014) 147–155. [20] Deepak K. Pattanayak, A. Fukuda, T. Matsushita, M. Takemoto, S. Fujibayashi, K. Sasaki, N. Nishida, T. Nakamura, T. Kokubo, Acta Biomater. 7 (3) (2011) 1398–1406.
Contents lists available at ScienceDirect
Optics & Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Full length article
Microhardness and microstructure evolution of TiB2 reinforced Inconel 625/TiB2 composite produced by selective laser melting Baicheng Zhang n, Guijun Bi, Sharon Nai, Chen-nan Sun, Jun Wei SIMTech-Singapore Institute of Manufacturing Technology,x 71 Nanyang Dr, 638075, Singapore
art ic l e i nf o
a b s t r a c t
Article history: Received 22 September 2015 Received in revised form 23 November 2015 Accepted 4 January 2016
In this study, micron-size TiB2 particles were utilized to reinforce Inconel 625 produced by selective laser melting. Exceptional microhardness 600–700 HV0.3 of the composite was obtained. In further investigation, the microstructure and mechanical properties of Inconel 625/TiB2 composite can be significantly influenced by addition of TiB2 particles during SLM. It was found that the long directional columnar grains observed from SLM-processed Inconel 625 were totally changed to fine dendritic matrix due to the addition of TiB2 particles. Moreover, with laser energy density (LED) of 1200 J/m, a Ti, Mo rich interface around TiB2 particles with fine thickness can be observed by FESEM and EDS. The microstructure evolution can be determined by different laser energy density (LED): under 1200 J/m, γ phase in dendrite grains; under 600 J/m, γ phase in combination of dendritic and acicular grains; under 400 J/ m, γ phase acicular grains. Under optimized LED 1200 J/m, the dynamic nanohardness (8.62 GPa) and elastic modulus (167 GPa) of SLM-processed Inconel 625/TiB2 composite are higher compared with those of SLM-processed Inconel 625 (3.97 GPa and 135 GPa, respectively). & 2016 Elsevier Ltd. All rights reserved.
Keywords: Selective laser melting Inconel 625 TiB2 particle Microstructure Composite alloy
1. Introduction In recent year, the demands on high performance nickel based superalloys are increasing with the development of aerospace and oil industry [1–3]. Inconel series alloys, as a Ni–Cr based austenite superalloy, are featured by an improved balance of creep performance, fatigue strength, tensile properties, and oxidation and corrosion resistance, making them as attractive choices for diverse industrial applications. Especially, Inconel 625 alloy has been widely applied in aerospace, marine, chemical and petrochemical industries due to its superior mechanical and corrosion resistant properties. However, the precision machining of Inconel 625 alloy is still a challenge [4], which hinders the further applications in industry. Selective laser melting (SLM) is one of newly developed additive manufacturing (AM) processes which especially applicable to hardto-machine metallic materials [5]. In the process, a high energy laser beam selectively scans a powder bed under a protective atmosphere and melts the particles which solidify to form a solid layer using a computer controlled system [6]. A new layer of powders is deposited on top of the previously formed solid layer and the process continues until the part is completed [7]. Compare to conventional manufacturing techniques, SLM provides a broader n
Corresponding author. E-mail address: [email protected] (B. Zhang).
http://dx.doi.org/10.1016/j.optlastec.2016.01.010 0030-3992/& 2016 Elsevier Ltd. All rights reserved.
range of advantages, namely rapid manufacturing of objects almost without geometric constraints, high material utilization rate and direct fabrication based on CAD model [8,9]. The previous studies of the SLM-processing Inconel series alloys have shown promising results, such as high surface quality, desirable mechanical properties. Moreover, the metal matrix composites (MMC) produced by addition of ceramic particles give a great potential to increase not only hardness, but also high temperature mechanical properties [10]. However, the reinforcement mechanism by additive ceramic particles is still not clear based on present research work [11]. Some researchers have given some value research: G. Bi used self-developed laser cladding system to fabricate Inconel 625-nano TiC composite [12]; C Hong and D Gu proposed a systematic research about Inconel 625 with TiC composite by SLM [13]; J. Michael Wilson conducted a study of Inconel 690 with TiC by laser cladding [14]. A further and systemic investigation of TiB2 reinforcement of MMC for SLM process is necessary. The micron-size TiB2 particles possess a high thermalstability during metallurgy process with super hardness, which is suitable for SLM process. At present, the research of TiB2 reinforced MMC produced by additive manufacturing is quite novel. The related publications are quite rare in AM research area. Results of this research may introduce a great potential for surface modification and fabrication of 3D components applications. In the present work, SLM technology was applied to produce the TiB2 particle reinforced Inconel 625 matrix composite. Process characteristics were systematically explored. Influence of the SLM
B. Zhang et al. / Optics & Laser Technology 80 (2016) 186–195
process parameters on density was studied. Moreover, the effect of TiB2 on hardness and microstructure evolution of the fabricated Inconel 625-MMC was investigated.
187
Table 2 Chemical compositions of Inconel TiB2 powder (wt.%). Ti
B
Other
4 67.5
4 30.5
o 2.5
2. Experimental procedure 2.1. Powder preparation In this study, gas atomized spherical Inconel 625 powders with particle size distribution of 11–45 μm were used. A cubic-shaped TiB2 powders (4 97.5% purity) with the particle size distribution 5–12 μm were adopted. The nominal chemical compositions of Inconel 625 and TiB2 powders are provided in Table 1 and Table 2, respectively. 95 wt.% Inconel 625 powders and 5 wt.% TiB2 powders were mixed in a tumbling mixer for 4 h before use. 2.2. Selective laser melting process The SLM experiments were carried out using a MCP Realize Ⅱ machine (MCP 250 HEK Tooling GmbH, Germany), which mainly consists a Nd:YAG laser with wavelength of 1064 nm. The laser beam diameter was fixed at 46 mm. The hatch distance was 100 mm for an optimized 50% overlapped. The powder layer thickness was 50 mm. Cubic form specimens with dimensions of 5 5 5 mm3 were fabricated by SLM. An alternate scanning pattern with equal line spacing in the X and Y directions was used and shown in Fig. 1. The SLM process was performed under argon atmosphere, and the powder bed temperature was kept constant at 80 °C. The laser power was optimized for 80 W and 120 W. The scanning velocity was set at 100, 200, 300 mm/s by the SLM control program. Thus, except the low quality sample with 80 W laser power and 200 and 300 mm/s scanning speed, four different “laser energy densities” (LED) of 1200, 800, 600 and 400 J/m, which defined by LED¼ P/v [16], were used to assess the laser energy input to the powder layer being processed. 2.3. Characterization of microstructure and hardness After the deposition process, density evaluation was processed by ‘Archimedes-method’, which consists in weighting the samples in air and subsequently in demineralized water. Then, the samples were cut, grinded and polished according to the standard procedures for metallographic examinations. And then, the samples were etched by auto electrolytic preparation system (Struers LectroPol-5) with voltage 5 V for 25 s. The microstructure was characterized using a scanning electron microscope (SEM, JEOL, JSM5800LV, Japan). The chemical composition was examined by an energy dispersive spectrometer (EDS). The Vickers hardness test was carried out using a microhardness tester (Matsuzawa MMTX3, Japan) with load of 300 g and dwell time of 15 s. Microhardness was tested at least 10 times at different locations on the sample cross-section. The average value was applied as microhardness value. Nano-indentation tests on the polished sections of parts were performed using a Nano-indentation tester (Nano test Micro Material Ltd. Wrexham, UK) under a fixed angle of 25°. A loading-unloading test mode was used with maximum test force of 100 mN, loading speed of 1.0 mN/s, and dwell time of 10 s.
Fig. 1. Schematic diagram of selective laser melting scanning strategy [15].
3. Results and discussions 3.1. Density behavior The microstructure evolution of SLM-processed Inconel 625/TiB2 composite samples with different laser parameters are shown in Fig. 2(a–c). It can be observed that the irregular TiB2 particles were distributed in Inconel 625 matrix for all laser parameters. As a comparison, the microstructure of SLM-processed pure Inconel 625 with the same laser parameters is shown in Fig. 2(d–f). The density of both Inconel 625/TiB2 composite and Inconel 625 is shown in Fig. 3. For Inconel 625 produced with high LED 1200 J/m, the cross-section shows nearly fully dense with tiny pores, but free of any cracks, as shown in Fig. 2(d). The density of SLM Inconel 625 is 8.43 g/cm3, and the corresponding relative density is about 98.3%. With addition of TiB2, the relative density was decreases to 96.1%. The main deviation is caused by micronsize pores with size of 10-20 μm, as shown in Fig. 2(a). As the LED decreases to 600 J/m and 800 J/m, the relative density of SLM-processed Inconel 625 can decrease to 89.7% and 92.5%, respectively. With addition of TiB2 reinforcement particles, the density of SLM-processed parts continuously decreases to 86.5% and 87.2%, respectively. The cross-section of the parts shows a relatively heterogeneous matrix combined of TiB2 particles with the formation of the irregular shaped pores in a scale of 20–40 μm, as shown in Fig. 2(b). Moreover, a small amount of un-melted powders and inter-layer micro-cracks can be found in the crosssection. For low energy density input with LED of 400 J/m, a severe delamination phenomenon occurred, due to severe lack of laser energy, as shown in Fig. 2(c). Density of the sample with TiB2 particles is relatively low (73.1%), compared with the sample without TiB2 addition (79.3%). 3.2. Microhardness evolution
Table 1 Chemical compositions of Inconel 625 powder (wt.%). Ni
Cr
Mo
Al
Ti
Fe
Nb
Balance
21.5
9
0.3
0.4
2.5
3.5
Fig. 4 depicts the average microhardness measured in the cross-sections of SLM-processed Inconel 625 and Inconel 625/TiB2 parts. For all laser parameter LED from 400 J/m to 1200 J/m, the average microhardness is in the range of 300–350 HV0.3. However,
188
B. Zhang et al. / Optics & Laser Technology 80 (2016) 186–195
Fig. 2. Microstructure of SLM Inconel 625/TiB2 composite with LED 1200 J/m (a); 600 J/m (b), 400 J/m (c); SLM pure Inconel 625 parts with 1200 LED J/m (d); 600 J/m (e), 400 J/ m (f).
Fig. 3. Density of the samples produced with different laser parameters with/ without TiB2 particles.
the mean microharndess increases to 600–700 HV0.3 when TiB2 reinforcement particles were used. Therefore, the addition of TiB2 particles can significantly improve the microhardness of SLM Inconel 625 alloy. The microhardness evolution is mainly determined by the microstructure formation. It will be discussed in the next section. 3.3. Microstructural evolution Fig. 5 illustrates the microstructural features of both SLM-processed Inconel 625 and Inconel 625/TiB2 composite examined in horizontal and vertical cross-sections. Under condition of LED 1200 J/m, typical cellular and columnar grain morphology can be found in SLM-processed Inconel 625 in horizontal and vertical
Fig. 4. Micro-hardness of fabricated samples with/without TiB2 particles using different laser parameters.
cross-sections, respectively, as shown in Fig. 5(a). The grains grew along the building direction with a mean diameter of 1.2 μm. However, the microstructural feature of SLM-processed Inconel 625/TiB2 composite is very different from SLM-processed Inconel 625 due to addition of TiB2 reinforcement particles, as shown in Fig. 5(b–d). Instead of columnar grains in SLM processed Inconel 625, fine and homogenous dendritic grains can be found in SLMprocessed Inconel 625/TiB2 composite, which correspond γ phase [17]. Furthermore, it can be found that the growth morphology and dispersion state of SLM-processed Inconel 625/TiB2 composite were significantly influenced by laser energy density input LED. At a relatively high LED of 1200 J/m, the microstructure exhibited a full dendritic morphology. A mean length of dendrite trunk and arms can be found as 5.4 and 3.6 μm, respectively as shown in
B. Zhang et al. / Optics & Laser Technology 80 (2016) 186–195
189
Fig. 5. Microstructure evolution of SLM-processed Inconel 625 (a), and Inconel 625/TiB2 composite under 1200 J/m (b); 600 J/m (c); 400 J/m(d) in horizontal and vertical cross-section.
190
B. Zhang et al. / Optics & Laser Technology 80 (2016) 186–195
A
B
C
D
Fig. 6. FESEM and EDS results of SLM-processed Inconel 625/TiB2 composite under 1200 J/m.
B. Zhang et al. / Optics & Laser Technology 80 (2016) 186–195
A
B
C
D
Fig. 7. FESEM and EDS results of SLM Inconel 625/TiB2 composite under 600 J/m.
191
192
B. Zhang et al. / Optics & Laser Technology 80 (2016) 186–195
Fig. 8. FESEM and EDS results of SLM-processed Inconel 625/TiB2 composite under 400 J/m.
B. Zhang et al. / Optics & Laser Technology 80 (2016) 186–195
Fig. 9. Nano-indentation results of SLM-processed Inconel 625/TiB2 composite and Inconel 625 with different laser parameters.
Fig. 5(b). As LED decreases to 600 J/m, the dendritic grains of SLMprocessed Inconel 625/TiB2 were refined. Therefore, it is also observed that the present Inconel 625/TiB2 typically contained two different morphologies, as show in Fig. 5(c). On one hand a reduced dendritic structure with 7.3 μm of trunk length and 4.2 μm of arm length can be observed. On the other hand, the acicular grains with diameter of 200–500 nm can also be identified in the matrix. When an even lower LED 400 J/m was used, a very homogeneous dispersion of significantly refined acicular grains without dendritic microstructural feature was formed, as shown in Fig. 5(d). 3.4. Analysis of boundary reinforcement by TiB2 FESEM analysis illustrates the influence of different laser energy density LED 1200 J/m, 600 J/m and 400 J/m on the characteristic morphologies of interfacial microstructure around TiB2 particle in SLM-processed Inconel 625 composite, as shown in Figs. 6–8, respectively. In order to carefully quantify the element content distribution in composite matrix, element analysis was carried out in four different feature zones from A to D respectively. The quantitative elemental determination by EDS spot-scan indicated that the dark area contained Ti and B elements, which can be confirmed as TiB2 particle in SLM-processed composite. From EDS element analysis results, the Mo and Ni elements can be detected in blank area, among which the Ni element demonstrated a relatively homogenous distribution in matrix. When a relatively high LED of 1200 J/m was applied, as shown in Fig. 6, an obvious interfacial layer (zone B and C) can be distinguished from TiB2 particle (zone A) and matrix (zone D). A decreased gradient of Ti content can be found from zone A (58 wt%) to zone D (0.8 wt%). Moreover, the concentration of Mo element exhibited a significant increase in interfacial layer (zone B). During the SLM process, the TiB2 particle can be heated to an extending temperature due to relatively high laser energy absorption ratio [18]. Thus, the Ti and B element from overheated ceramic particle could react with Mo from Inconel 625 matrix, which resulted in the formation of Ti, Mo rich compound interfacial layer around TiB2 particles. The formation of the hard intermetallic resulted in the high hardness at LED 1200 J/m, as shown in Fig. 4. At a decreased LED of 600 J/m, as shown in Fig. 7, the reinforcement particle boundary can be easily distinguished from the matrix without the obvious interfacial layer, as shown in Fig. 6.
193
The matrix microstructure is very different from that with LED of 1200 J/m. In the area B, homogenous dendritic grains with random orientation can be found. Base on the results of Section 3.2, the microstructure close to TiB2 particle can be concluded to γ phase. In this case, element analysis was also carried out from TiB2 particle to matrix in order to determine the distribution of various elements. It shows that the Ti element distribution is very homogenous in matrix around the reinforcement TiB2 particles. The matrix area was formed by Ni, Mo element. Under this condition, the Ti and B elements from ceramic particles cannot react with Inconel 625 due to insufficient laser energy. However, the nucleation process of dendritic grains could be influenced by different thermal conductive between TiB2 and Inconel 625 alloy. The temperature gradient led to a driving force from TiB2 particles to γ-Ni matrix in molten pool, which gave isolated crystallization around TiB2 particles. Therefore, the homogenous microstructure with certain orientation can be formed along with the temperature gradient from γ-Ni matrix to TiB2 particles. When a relatively low LED of 400 J/m was applied, as shown in Fig. 8, the reinforcement particles were dispersed in γ-Ni matrix with a quite sharp boundary. Under this laser parameter, the interlayer cannot be found between the reinforcement particles and the matrix. The spot-scan results show that the Ti distribution is stable in the matrix. Meanwhile, a different Mo and Ni element distribution can be found in element map: high Mo content can be detected on blank area of matrix; high Ni element mainly distributed on the dark area of the matrix. The grain refinement effect was dominant, which contributed to the increasing micro-hardness for LED from 800 J/m to 400 J/m, as shown in Fig. 3. 3.5. Nano-indentation analysis Fig. 9 depicted the load-depth curves of nano-indentation conducted on the polished cross-section of SLM-processed Inconel 625 and Inconel/TiB2 composite. Under the load of 100 mN the indentation depth of the SLM-processed Inconel 625 was 1141 nm, which is obviously deeper than that of the SLM-processed Inconel 625/TiB2 composite, which is about 814.5–840.5 nm. As shown in Table 3, the nanohardness (Hd) of SLM-processed Inconel 625/TiB2 composite is from 8.62 to 10.7 GPa, and the elastic modulus (Er) ranges from167.61 to 273.65 GPa. As a comparison, the nanohardness and the elastic modulus of the SLM-processed Inconel 625 are 3.97 GPa and 135.51 GPa, respectively. Furthermore, different applied LED can lead different mechanical properties, as show in Table 3. The maximum dynamic nanohardness can be found as 10.7 GPa under 400 J/m. Meanwhile, the highest elastic modulus as 273.65 GPa can be achieved by 600 J/m. As show in Fig. 6, the TiB2 particle with Ti, Mo rich interlayer and dendritic matrix can be found in samples with LED 1200 J/m. In this case, a low cooling rate can be generated caused by high laser power and low scanning velocity. Thus, the Inconel 625 alloy grains can grow large under a low cooling rate during the solidification process which leads to a relative long dendritic structure. In which, this kind of microstructure of matrix shows a high elasticity and low hardness. Moreover, the Ti, Mo rich interlayer also decrease the enforcement effect of TiB2 particle. Thus, a Table 3 Nano-indentation results. Hardness, Hd, GPa Young's modulus, Er, GPa Inconel Inconel Inconel Inconel
625 LED, 1200 J/m 625/TiB2, LED 1200 J/m 625/TiB2, LED 600 J/m 625/TiB2, LED 400 J/m
3.97 8.62 70.82 8.82 70.34 10.707 0.43
135.51 167.617 11.16 273.65 7 7.67 255.34 7 9.44
194
B. Zhang et al. / Optics & Laser Technology 80 (2016) 186–195
Fig. 10. Schematic diagrams showing the mechanism of SLM-processed Inconel 625 and Inconel 625/TiB2 composite.
relatively low hardness and Young's modulus can be obtained in sample with 1200 J/m. With the decrease of laser energy input of 600 and 400 J/m, an increase of cooling rate can be generate during solidification. A combination of acicular and dendritic microstructure can be found in sample with 600 J/m, as shown in Fig. 7. In this case, the fine acicular structure can decrease the material ductility and lead to a high hardness. Thus, a medium hardness and relatively high Young's modulus can be found in samples with LED 600 J/m. With continue decrease of LED to 400 J/ m, the single phase of fine acicular grain can be found in sample, as show in Fig. 8. Thus, the highest hardness and low elasticity can be obtained in samples with LED 400 J/m. 3.6. Mechanism of SLM-processed Inconel 625/TiB2 composite Fig. 10 depicted the different solidification mechanisms between pure Inconel 625 (a–c) and Inconel 625/TiB2 (d–f) composite processed by SLM. During the SLM process, the laser beam provided a Gaussian energy distibution on powder layer. The Inconel 625 powders reached melting point and formed the molten pool in a very short time interval [19], as shown in Fig. 10(a) and (d). After laser beam moved from the molten pool, a significant high cooling rate can be generated from the bottom to surface of melted Inconel 625, as shown Fig. 10(b). Therefore, the normal long dendritic grains with certain orientation can be found in SLM-processed Inconel 625 in all laser parameters, as shown in Fig. 10(c). However, the TiB2 particles are stable in molten pool due to the difference of melting point of Inconel 625 and TiB2 (1350 °C of Inconel 625 versus 3230 °C of TiB2). As a comparison, the TiB2 particles were relocated in molten pool due to the Marangoni convention [20], and privoded a chemical reaction with Mo element in Inconel 625 under high laser energy density, as shown in Fig. 10(e). Thus, a driving force can be radiatedly generated from particles to the matrix in molten pool. The integral themal gradient from molten pool bottom to top surface during processing Inconel 625 was changed to isolated area around TiB2 particles during procssing Inconel/TiB2 composite. Thus, dendritic grains with random orientation can be generated
from γ-Ni matrix to TiB2 particles after solidification, as shown in Fig. 10(f). At low laser energy density, this effect is quite obvious.
4. Conclusions In this study, a comparison of Inconel 625 and Inconel 625/TiB2 composite processed by SLM was presented. The main conclusions can be drawn as follows: [1] Addition of TiB2 particles can decrease the density due to formation of pores and cracks. [2] The microstructure of SLM-processed Inconel 625/TiB2 composite can be influenced by addition of TiB2 particles, a fine Ti, Mo rich interfacial layer around TiB2 can be observed. [3] The composite microstructure can be influenced by different laser energy density: fine dendritic grains (1200 J/m); a combination of dendritic and acicular grains (600 J/m); and single phase of acicular grains (400 J/m). [4] The re-formation of particles surface and fine matrix structure generally favored the increase of the nano/micro hardness and Young's modulus. [5] The addition of TiB2 particles in Inconel 625 altered the heating and cooling effect during SLM process. This resulted in the formation of intermetallics around the particles, and modification of microstructures. All these factors contributed to the reinforcement effect of TiB2 particles on the Inconel 625 processed by SLM.
Reference [1] Miroslav Zetek, Ivana Česáková, Vojtěch Švarc, Procedia Eng. 69 (2014) 1115–1124. [2] Alenka Vesel, Aleksander Drenik, Kristina Elersic, Miran Mozetic, Janez Kovac, Tomaz Gyergyek, Jan Stockel, Jozef Varju, Radomir Panek, BalatPichelin Marianne, Appl. Surf. Sci. 305 (2014) 674–682. [3] G. Casalino, S.L. Campanelli, N. Contuzzi, A.D. Ludovico, Opt. Laser Technol. 65 (2015) 151–158. [4] Ravindra I. Badiger, S. Narendranath, M.S. Srinath, J. Manuf. Process. 18 (2015) 117–123.
B. Zhang et al. / Optics & Laser Technology 80 (2016) 186–195
[5] Shuai Li, Qingsong Wei, Yusheng Shi, Zicheng Zhu, Danqing Zhang, J. Mater. Sci. Technol. 31 (2015) 946–952. [6] Baicheng Zhang, Lucas Dembinski, Christian Coddet, Mater. Sci. Eng.: A 584 (1) (2013) 21–31. [7] Lore Thijs, Karolien Kempen, Jean-Pierre Kruth, Jan Van Humbeeck, Acta Mater. 61 (5) (2013) 1809–1819. [8] P. Edwards, M. Ramulu, Mater. Sci. Eng.: A 598 (26) (2014) 327–337. [9] Baicheng Zhang, Hanlin Liao, Christian Coddet, Mater. Des. 34 (2012) 753–758. [10] J.C. Heigel, P. Michaleris, T.A. Palmer, J. Mater. Process. Technol. 220 (2015) 135–145. [11] Pengpeng Yuan, Dongdong Gu, Donghua Dai, Mater. Des. 82 (5) (2015) 46–55. [12] G. Bi, C.N. Sun, M.L. Nai, J. Wei, Phys. Procedia 41 (2013) 828–834. [13] Chen Hong, Dongdong Gu, Donghua Dai, Moritz Alkhayat, Wolf Urban, Pengpeng Yuan, Sainan Cao, Andres Gasser, Andreas Weisheit, Ingomar Kelbassa, Minlin Zhong, Reinhart Poprawe, Mater. Sci. Eng.: A 635 (2015) 118–128.
195
[14] J. Michael Wilson, Yung C. Shin, Surf. Coat. Technol. 207 (2012) 517–522. [15] Waldemar A. Monteiro, Light Metal Alloys Applications, 2014, ISBN 978-95351-1588-5. [16] Dongdong Gu, Hongqiao Wang, Donghua Dai, Pengpeng Yuan, Wilhelm Meiners, Reinhart Poprawe, Scr. Mater. 96 (2015) 25–28. [17] K. Devendranath Ramkumar, R. Jagat Sai, V. Santhosh Reddy, Sridhar Gundla, T. Harsha Mohan, Vimal Saxena, N. Arivazhagan, J. Manuf. Process. 18 (2015) 23–45. [18] B. Vrancken, L. Thijs, J.-P. Kruth, J. Van Humbeeck, Acta Mater. 68 (15) (2014) 150–158. [19] P. Krakhmalev, I. Yadroitsev, Intermetallics 46 (2014) 147–155. [20] Deepak K. Pattanayak, A. Fukuda, T. Matsushita, M. Takemoto, S. Fujibayashi, K. Sasaki, N. Nishida, T. Nakamura, T. Kokubo, Acta Biomater. 7 (3) (2011) 1398–1406.