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Laser 3D printing of Zr-based bulk metallic glass
Journal of Manufacturing Processes 39 (2019) 102–105
Contents lists available at ScienceDirect
Journal of Manufacturing Processes journal homepage: www.elsevier.com/locate/manpro
Laser 3D printing of Zr-based bulk metallic glass a
a,⁎
a
b,⁎
T
Huidong Xu , Yunzhuo Lu , Zhenghong Liu , Gang Wang a b
School of Materials Science and Engineering, Dalian Jiaotong University, Dalian, 116028, People’s Republic of China School of Mechanical and Automotive Engineering, Anhui Polytechnic University, Wuhu, 241000, People’s Republic of China
A R T I C LE I N FO
A B S T R A C T
Keywords: Laser 3D printing Additive manufacturing Bulk metallic glass
Bulk metallic glasses (BMGs) have attracted increasing attention and tremendous interest owing to their extraordinary mechanical and chemical properties. However, the limited dimensions and simple geometries of the BMGs fabricated by traditional techniques severely restrict their widespread applications. In the present study, a large-size Zr50Ti5Cu27Ni10Al8 (Zr50) BMG is successfully fabricated by laser 3D printing technique. The SEM and XRD results reveal that the microstructure of printed Zr50 BMG consists of both amorphous and crystalline phases. The printed Zr50 alloy possessed high percentage of amorphous phase and dose not crystallize more seriously as the deposited sample becomes thicker.
1. Introduction Bulk metallic glasses (BMGs), which are thought to be promising structural and functional materials, have attracted increasing attention and tremendous interest owing to their excellent mechanical and chemical properties, such as superior strength and high hardness, high corrosion resistance, and near-theoretical elastic limits [1,2]. These exceptional properties are primarily attributed to the special atomic configurations that consist of short-range order and long-range disorder atomic arrangement. However, there are still some problems and bottlenecks remaining to be solved regarding the preparation and processing of BMGs [3]. One of the most serious problems is the limited critical dimensions of BMGs fabricated by the copper mold casting technique, which is the common method used in laboratories and factories [4]. At present, the largest BMG Pd40Cu30Ni10P20 fabricated by this method is only 72 mm in diameter [5]. In addition, the method of copper mold casting is also exceedingly difficult to meet the requirement of machining BMG components with complex shapes. Therefore, how to breakthrough the size and geometry limitations has become the key to extend the applications of BMGs. With the deepening of the research on the processing of the BMGs, the connection technologies have been paid more attentions to solve the above problems. Some researchers attempted to weld small BMGs prepared by copper mold casting method into large sizes [6]. The welding methods of BMGs can be roughly divided into two types: the first type of the method is liquid phase welding, for example, electron beam, laser welding, pulse current. [7–9]. Among these methods, because of a higher welding temperature than the melting point of
⁎
metallic glasses, it is difficult to avoid the structural relaxation, phase separation and crystallization in the welding process. Another kind is solid state welding method, including friction welding, friction stir welding, explosion welding and diffusion welding, etc. [10–12]. Among these methods, the heat input is comparatively low and the crystallization can be avoided. However, complex shapes are still not easily achieved by these methods. Thus, adopting new methods to integrally fabricate large complex BMG component has great demand. Laser 3D printing, which is an additive manufacturing technology and enables the one-piece production of complex shaped metallic components, provides a new opportunity for the preparation of largesized BMGs [13–19]. Laser 3D printing is a process that builds metallic components point by point and layer by layer directly from a computer model without specific tooling and manual intervention. Since the spot diameter of laser beam is small, the molten pool formed by focusing the laser beam on the substrate is small. The heat of the molten pool diffuses quickly through the deposited part and the substrate. The cooling rate of the small molten pool formed by laser as a heat source can reach on the order of 103-104 K/s, which is observably higher than the critical cooling rate of most metallic glasses forming amorphous state [20]. This provides a way of large scale fabricating metallic glasses without the need to simultaneously quenching the entire part. Recently, some researchers have attempted to produce metallic glasses by using this technology [21,22]. For instance, Yang et al. have fabricated a 1.4-mm thickness Zr55Al10Ni5Cu30 metallic glass with the pre-laid powder method by laser processing. The deposit consists amorphous and crystalline phases. The volume fraction of amorphous phase in the deposit is about 92.44% [23]. Zheng et al. have produced an
Corresponding authors. E-mail addresses: [email protected] (Y. Lu), [email protected] (G. Wang).
https://doi.org/10.1016/j.jmapro.2019.02.020 Received 1 August 2018; Received in revised form 18 November 2018; Accepted 18 February 2019 1526-6125/ © 2019 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved.
Journal of Manufacturing Processes 39 (2019) 102–105
H. Xu, et al.
atmosphere in the controlled-environment glove box to prevent oxidation. Meanwhile, argon was used as the shielding gas for laser head. The laser power and laser scan speed was 450 W and 1000 mm/min, respectively. To obtain continuous deposited layers, the overlapping fraction of the parallel tracks portions overlaps by 30%. During multilayer deposition process, the layer thickness in z direction was set as 0.8 mm. The No. 45 steel plates with the size of 50 mm × 50 mm × 10 mm were chosen as the substrate. Prior to the laser deposition experiments, the substrates were surface ground with 80 grit SiC paper and cleaned with acetone to remove surface contaminants that could adversely affect the GFA of the metallic glass. The No. 45 steel substrate was glued on a water-cooled copper plate by thermally conductive silica gel to improve the heat dissipation efficiency. The microstructure of deposited samples was characterized using X-ray diffraction (XRD) and scanning electron microscope (SEM). An XRD equipped with a Cu Kα X-ray source was utilized to examine the phase of deposited samples. The XRD patterns were taken at 2θ angle ranging 20～100° at a scanning rate of 5°/min. The samples were polished following the standard polishing procedures before SEM observation. SYSWELD, a professional finite-element simulation software, is applied to calculate the thermal cycle curves in the present work. The FEM model was generally meshed into cubic elements. A denser meshing was applied in the heat affected zone where the detailed thermal information was expected. During the laser 3D printing process, a laser beam is used as a thermal energy source to induce the fusion of the material. In the present study, the laser beam is modeled as a Gaussian distribution of heat flux from a moving heat source with conical shape.
Fe58Cr15Mn2B16C4Mo2Si1W1Zr1 metallic glass by laser 3D printing with coaxial powder feeding method. The crystalline phase also found in the deposited Fe-based metallic glass. The fraction of amorphous phases decreases with the increasing deposited height. There was a serious crystallization in the deposited sample with the thickness of 2 mm [24]. Pauly et al. have used the selective laser melting method to implement complex geometries and components, including cylinder, the corners of the scaffold as well as ribbon, from an Fe74Mo4P10C7.5B2.5Si2 metallic glass. High volume fractions of crystalline phases are observed in the samples of cylinder and the corners of the scaffold. Only the simple ribbon appears to be fully amorphous [18]. From the above reported results, we can notice that the deposited metallic glasses with the thicknesses of only a few millimeters crystallize seriously. Then how to produce fully or nearly fully metallic glasses is the major issue that should be solved firstly. In this work, a large-size Zr50Ti5Cu27Ni10Al8 (Zr50) BMG is fabricated with success by laser 3D printing technique. The microstructure of printed Zr50 BMG is analyzed by the SEM and XRD, which reveal that the microstructure of printed Zr50 BMG is comprised of both amorphous and crystalline phases. When the sample is deposited thicker, the printed Zr50 alloy has high percentage of amorphous phase and dose not crystallize more severely. 2. Materials and methods Argon atomized Zr50 powder in the size range of 20˜50 μm was used for laser 3D printing process. The laser 3D printing machine consists of a 6 kW fiber laser, a four-nozzle coaxial powder feed system, a controlled environment glove box, and a motion control system [25]. A schematic of this equipment is shown in Fig. 1. The laser beam was focused on the substrate surface to a spot of approximately 3 mm in diameter. The powder-delivering nozzles are designed and arranged in such a way that the powder streams converge at the focal point of the laser beam. To make a 3D printing deposit, a CAD model is sliced firstly into a series of layers of finite thickness using computer software. Each of these layers is then translated into a series of line patterns in order to deposit the layers. The laser beam is used as a heat source to create a molten pool on the substrate, and the powder is injected into the melt pool by an inert gas flowing through the powder feed system. The first layer of the component is bonded to the substrate. The substrate, together with the component under fabrication, is moved along the line patterns in the plane of current layer with the motion control system. After completing a layer, the laser focal point and powder-delivering nozzles are incremented upward in the height direction in an amount of the layer thickness. A new layer is subsequently deposited onto the previous layer until the component is fully constructed in the layer-bylayer fashion. The substrate can be removed when the final product is fabricated. The laser 3D printing processing is performed in an argon
3. Results and discussion Fig. 2 shows the SEM micrograph of gas-atomized Zr50 alloy powder. Observations of the alloy powder with SEM reveals that most of the alloy powder is spherical in shape. Some large powders have small satellites with a size of 5˜10 μm. The XRD pattern of the as-fabricated Zr50 powder is shown in Fig. 3. Clearly, the diffraction patterns consists a broad peak in 2 theta region of 30˜45° without any detectable crystalline Bragg peaks, indicating its fully amorphous structure. Fig. 4(b) shows the outer appearance image of a multi-layer Zr50 alloy sample printed from the CAD model shown in Fig. 4(a) (c) shows the cross-section microstructure of the printed Zr50 BMGs perpendicular to the laser scanning direction. Further observations show that the light gray regions are surrounded by thin dark gray bands. The light gray regions are the melt zones that are heated by laser irradiation
Fig. 1. Schematic of a coaxial powder feeding laser cladding system.
Fig. 2. SEM micrographs of gas-atomized Zr50 amorphous alloy powder. 103
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and growth rates during crystallization [27]. Our simulation results have found that the heating rates of the thin dark gray bands, which are determined by the heat transferred from the melt zones, are lower than the heating rate required to abstain crystallization [20]. Therefore, the thin dark gray bands surrounding the melt zones are crystalline state. The XRD pattern of this sample is presented in Fig. 3, which exhibits a few sharp crystal peaks superimposed on a broad amorphous peak, further demonstrating that the light and dark gray thin bands are amorphous and crystalline, respectively. We can also find from Fig. 4(c) that the thin gray bands take about 10% of the total cross area, marking that the printed Zr50 alloy contains about 90% amorphous phase. The volume fraction of amorphous phase estimated from the SEM is very close to that calculated from the XRD profile shown in Fig. 3. The volume fraction of amorphous phase obtained by comparing the area of amorphous peak with that of the sum of all peaks in the XRD pattern is 91.2%. Since the heat generated by laser irradiation is mainly dispersed through the already deposited Zr50 alloy, the heat would disperse more slowly and there is a risk that the printed Zr50 alloy may crystallize more severely as the deposited sample becomes thicker. From Fig. 4(c) we can inspiringly find that the widths of dark gray crystalline bands are nearly constant with increasing the deposited layers. This may be due to the excellent thermal stability of the Zr50 BMG. Therefore, the Zr50 metallic glass without shape or size limitation can be produced by using the present parameters. Though the deposited metallic glass does not crystallize seriously as the deposited layers increasing, the heating process is inhomogeneous in the printed metallic glass along the building direction. To investigate this inhomogeneity, we calculate the thermal cycle curves of five typical points located at five adjacent layers along the building direction during the laser 3D printing process of a 6-layer Zr50 metallic glass by FEM simulation. Each layer consists of 5 tracks. The relative positions of the multiple tracks are shown in the schematic of Fig. 5(a). The red thin bands and yellow regions refer to the crystalline and amorphous phase, respectively. The thermal cycle curves of the five typical points are presented in Fig. 5(b). Clearly, the cooling curves of first tracks for all the five points are almost the same, the deposited metallic glass is cooled from the molten liquid. No crystallization takes place at these first tracks, since the cooling rates of first tracks are fast enough to vitrify the molten liquid [20]. After the first track, the subsequent tracks
Fig. 3. XRD patterns of the printed Zr50 sample and gas-atomized Zr50 alloy powder.
during the laser 3D printing process. In our recently published work [20], we have investigated the crystallization mechanism of the Zr50 BMG during the laser 3D printing process basing on the TTT diagrams of the Zr50 BMG and the thermal cycle curves obtained by FEM analysis. It is noted that the cooling rates of the molten liquids in the melt zones are high enough to continue to keep amorphous structure [20]. Then the light gray regions shown in Fig. 4(c) are amorphous state [26]. Differently, the thin dark gray bands surrounding the melt zones maintain the solid state during the whole laser 3D printing process. In these regions, the phase state was controlled by transferring the heat from the neighboring melt zones. The results of many studies have suggested that the crystallization behavior of BMG between cooling from the liquid state into the amorphous and heating from the amorphous solid state was obviously asymmetric [27]. For example, the critical cooling rate Rc of Zr41.2Ti13.8Cu12.5Ni10.0Be22.5 BMG (Vit1) is approximately 1 K/s, however, the critical heating rate Rh to prevent crystallization of Vit1 happening is two orders of magnitude larger than Rc, which is about 200 K/s [28]. The discrepancy between Rc and Rh of BMGs is so large, which is due to the significant difference in nucleation
Fig. 4. (a) CAD model, (b) outer appearance image of a multi-layer Zr50 alloy sample printed from the CAD model, (c) typical microstructure of the printed Zr50 alloy perpendicular to the laser travel direction. 104
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Fig. 5. (a) A schematic of the cross section for the laser 3D printing of a six-layer Zr50 metallic glass. (b) Thermal cycle curves of the five typical points show in (a).
will accumulate the structural relaxation for the Zr50 metallic glass formed at the first track through the heat transfer. Then from Fig. 5(b) we can note that the points located at different layers experience different thermal history. For instance, the point in the first layer undergoes more thermal cycle curves than the points in the upper layers, indicating that the structural relaxation of the bottom layers is more serious than the upper layers.
[3] Wang G, Huang Y, Cao W, Huang Z, Huttula M, Su Y, et al. Microstructure and crystallization mechanism of Ti-based bulk metallic glass by electron beam welding. J Manuf Process 2017;32:93–9. [4] Cole KM, Kirk DW, Singh CV, Thorpe SJ. Optimizing electrochemical micromachining parameters for Zr-based bulk metallic glass. J Manuf Process 2017;25:227–34. [5] Inoue A, Nishiyama N, Kimura H. Preparation and thermal stability of bulk amorphous Pd40Cu30Ni10P20 alloy cylinder of 72 mm in diameter. Mater Trans JIM 1997;38:179–83. [6] Zhang H, Lu Y, Huang Y, Feng A, Qin Z, Lu X. Joining of Zr51Ti5Ni10Cu25Al9 BMG to aluminum alloy by friction stir welding. Vacuum 2015;120:47–9. [7] Shao L, Datye A, Huang J, Ketkaew J, Sohn SW, Zhao S, et al. Pulsed laser beam welding of Pd43Cu27Ni10P20 bulk metallic glass. Sci Rep 2017;7(1):7989. [8] Chen B, Shi TL, Li M, Yang F, Yan F, Liao GL. Laser welding of annealed Zr55Cu30Ni5Al10 bulk metallic glass. Intermetallics 2014;46(3):111–7. [9] Wang D, Li N, Liu L. Magnetic pulse welding of a Zr-based bulk metallic glass with aluminum plate. Intermetallics 2018;93:180–5. [10] Wang G, Huang YJ, Makhanlall D, Shen J. Resistance spot welding of Ti40Zr25Ni3Cu12Be20 bulk metallic glass: experiments and finite element modeling. Rare Met 2017;36(2):1–6. [11] Jamili-Shirvan Z, Haddad-Sabzevar M, Vahdati-Khaki J, Chen N, Shi Q, Yao KF. Microstructure characterization and mechanical properties of Ti-based bulk metallic glass joints prepared with friction stir spot welding process. Mater Des 2016;100:120–31. [12] Jiang MQ, Huang BM, Jiang ZJ, Lu C, Dai LH. Joining of bulk metallic glass to brass by thick-walled cylinder explosion. Scr Mater 2015;97:17–20. [13] Deng L, Wang S, Wang P, Kühn U, Pauly S. Selective laser melting of a Ti-based bulk metallic glass. Mater Lett 2018;212:346–9. [14] Ouyang D, Li N, Xing W, Zhang J, Liu L. 3D printing of crack-free high strength Zr-based bulk metallic glass composite by selective laser melting. Intermetallics 2017;90:128–34. [15] Li XP. Additive manufacturing of advanced multi‐component alloys: bulk metallic glasses and high entropy alloys. Adv Eng Mater 2017. [16] Li XP, Roberts MP, O’Keeffe S, Sercombe TB. Selective laser melting of Zr-based bulk metallic glasses: processing, microstructure and mechanical properties. Mater Des 2016;112:217–26. [17] Sun H, Flores KM. Spherulitic crystallization mechanism of a Zr-based bulk metallic glass during laser processing. Intermetallics 2013;43(12):53–9. [18] Pauly S, Löber L, Petters R, Stoica M, Scudino S, Kühn U, et al. Processing metallic glasses by selective laser melting. Mater Today 2013;16(1-2):37–41. [19] Shen Y, Li Y, Chen C, Tsai HL. 3D printing of large, complex metallic glass structures. Mater Des 2017;117:213–22. [20] Lu Y, Zhang H, Li H, Xu H, Huang G, Qin Z, et al. Crystallization prediction on laser threedimensional printing of Zr-based bulk metallic glass. J Non-Cryst Solids 2017;461:12–7. [21] Li N, Zhang J, Xing W, Ouyang D, Liu L. 3D printing of Fe-based bulk metallic glass composites with combined high strength and fracture toughness. Mater Des 2018;143:285–96. [22] Zhang C, Wang W, Li YC, Yang YG, Yue W, Lin L. 3D printing of Fe-based bulk metallic glasses and composites with large dimension and enhanced toughness by thermal spraying. J Mater Chem A 2018;6(16):6800–5. [23] Yang G, Lin X, Liu F, Hu Q, Ma L, Li J, et al. Laser solid forming Zr-based bulk metallic glass. Intermetallics 2012;22(3):110–5. [24] Zheng B, Zhou Y, Smugeresky JE, Lavernia EJ. Processing and behavior of Fe-based metallic glass components via laser-engineered net shaping. Metall Mater Trans A 2009;40(5):1235–45. [25] Balu P, Leggett P, Kovacevic R. Parametric study on a coaxial multi-material powder flow in laser-based powder deposition process. J Mater Process Technol 2012;212(7):1598–610. [26] Sun H, Flores K. Laser deposition of a Cu-based metallic glass powder on a Zr-based glass substrate. J Mater Res 2008;23(10):2692–703. [27] Schroers J, Johnson WL. History dependent crystallization of Zr41Ti14Cu12Ni10Be23 melts. J Appl Phys 2000;88(1):44–8. [28] Schroers J, Busch R, Bossuyt S, Johnson WL. Crystallization behavior of the bulk metallic glass forming Zr41Ti14Cu12Ni10Be23 liquid. Mater Sci Eng A 2001;304:287–91.
4. Conclusions In summary, a large-size Zr50 BMG was successfully fabricated by laser 3D printing technique at the laser power of 450 W and laser scanning speed of 1000 mm/min. The SEM and XRD results revealed that the microstructure of printed sample consisted of both amorphous and crystalline phases. The printed Zr50 alloy possessed high percentage of amorphous phase and did not crystallize more seriously as the deposited sample becomes thicker. Author contributions Y.L. conceived and designed the experiments; H.X. and Z.L. performed the experiments; Y.L. and G.W. analyzed the data; H.X. and Y.L. wrote the paper. Conflicts of interest The authors declare no conflict of interest. Acknowledgements The authors would like to acknowledge the financial supports from the National Natural Science Foundation of China under Grant Nos. 51671042, 51401041, 51671043, the China Postdoctoral Science Foundation under Grant No. 2015M570242, and the Basic Research Program of the Key Lab in Liaoning Province Educational Department under Grant No. LZ2015011, the Liaoning Natural Science Foundation under Grant No. 201602126, the Key Research and Development Plan of Anhui Province under Grant No 1704a0902056. References [1] Wang WH, Dong C, Shek CH. Bulk metallic glasses. Mater Sci Eng R Rep 2004;44:45–89. [2] Wang G, Huang Z, Xiao P, Zhu X. Spraying of Fe-based amorphous coating with high corrosion resistance by HVAF. J Manuf Process 2016;22:34–8.
105
Contents lists available at ScienceDirect
Journal of Manufacturing Processes journal homepage: www.elsevier.com/locate/manpro
Laser 3D printing of Zr-based bulk metallic glass a
a,⁎
a
b,⁎
T
Huidong Xu , Yunzhuo Lu , Zhenghong Liu , Gang Wang a b
School of Materials Science and Engineering, Dalian Jiaotong University, Dalian, 116028, People’s Republic of China School of Mechanical and Automotive Engineering, Anhui Polytechnic University, Wuhu, 241000, People’s Republic of China
A R T I C LE I N FO
A B S T R A C T
Keywords: Laser 3D printing Additive manufacturing Bulk metallic glass
Bulk metallic glasses (BMGs) have attracted increasing attention and tremendous interest owing to their extraordinary mechanical and chemical properties. However, the limited dimensions and simple geometries of the BMGs fabricated by traditional techniques severely restrict their widespread applications. In the present study, a large-size Zr50Ti5Cu27Ni10Al8 (Zr50) BMG is successfully fabricated by laser 3D printing technique. The SEM and XRD results reveal that the microstructure of printed Zr50 BMG consists of both amorphous and crystalline phases. The printed Zr50 alloy possessed high percentage of amorphous phase and dose not crystallize more seriously as the deposited sample becomes thicker.
1. Introduction Bulk metallic glasses (BMGs), which are thought to be promising structural and functional materials, have attracted increasing attention and tremendous interest owing to their excellent mechanical and chemical properties, such as superior strength and high hardness, high corrosion resistance, and near-theoretical elastic limits [1,2]. These exceptional properties are primarily attributed to the special atomic configurations that consist of short-range order and long-range disorder atomic arrangement. However, there are still some problems and bottlenecks remaining to be solved regarding the preparation and processing of BMGs [3]. One of the most serious problems is the limited critical dimensions of BMGs fabricated by the copper mold casting technique, which is the common method used in laboratories and factories [4]. At present, the largest BMG Pd40Cu30Ni10P20 fabricated by this method is only 72 mm in diameter [5]. In addition, the method of copper mold casting is also exceedingly difficult to meet the requirement of machining BMG components with complex shapes. Therefore, how to breakthrough the size and geometry limitations has become the key to extend the applications of BMGs. With the deepening of the research on the processing of the BMGs, the connection technologies have been paid more attentions to solve the above problems. Some researchers attempted to weld small BMGs prepared by copper mold casting method into large sizes [6]. The welding methods of BMGs can be roughly divided into two types: the first type of the method is liquid phase welding, for example, electron beam, laser welding, pulse current. [7–9]. Among these methods, because of a higher welding temperature than the melting point of
⁎
metallic glasses, it is difficult to avoid the structural relaxation, phase separation and crystallization in the welding process. Another kind is solid state welding method, including friction welding, friction stir welding, explosion welding and diffusion welding, etc. [10–12]. Among these methods, the heat input is comparatively low and the crystallization can be avoided. However, complex shapes are still not easily achieved by these methods. Thus, adopting new methods to integrally fabricate large complex BMG component has great demand. Laser 3D printing, which is an additive manufacturing technology and enables the one-piece production of complex shaped metallic components, provides a new opportunity for the preparation of largesized BMGs [13–19]. Laser 3D printing is a process that builds metallic components point by point and layer by layer directly from a computer model without specific tooling and manual intervention. Since the spot diameter of laser beam is small, the molten pool formed by focusing the laser beam on the substrate is small. The heat of the molten pool diffuses quickly through the deposited part and the substrate. The cooling rate of the small molten pool formed by laser as a heat source can reach on the order of 103-104 K/s, which is observably higher than the critical cooling rate of most metallic glasses forming amorphous state [20]. This provides a way of large scale fabricating metallic glasses without the need to simultaneously quenching the entire part. Recently, some researchers have attempted to produce metallic glasses by using this technology [21,22]. For instance, Yang et al. have fabricated a 1.4-mm thickness Zr55Al10Ni5Cu30 metallic glass with the pre-laid powder method by laser processing. The deposit consists amorphous and crystalline phases. The volume fraction of amorphous phase in the deposit is about 92.44% [23]. Zheng et al. have produced an
Corresponding authors. E-mail addresses: [email protected] (Y. Lu), [email protected] (G. Wang).
https://doi.org/10.1016/j.jmapro.2019.02.020 Received 1 August 2018; Received in revised form 18 November 2018; Accepted 18 February 2019 1526-6125/ © 2019 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved.
Journal of Manufacturing Processes 39 (2019) 102–105
H. Xu, et al.
atmosphere in the controlled-environment glove box to prevent oxidation. Meanwhile, argon was used as the shielding gas for laser head. The laser power and laser scan speed was 450 W and 1000 mm/min, respectively. To obtain continuous deposited layers, the overlapping fraction of the parallel tracks portions overlaps by 30%. During multilayer deposition process, the layer thickness in z direction was set as 0.8 mm. The No. 45 steel plates with the size of 50 mm × 50 mm × 10 mm were chosen as the substrate. Prior to the laser deposition experiments, the substrates were surface ground with 80 grit SiC paper and cleaned with acetone to remove surface contaminants that could adversely affect the GFA of the metallic glass. The No. 45 steel substrate was glued on a water-cooled copper plate by thermally conductive silica gel to improve the heat dissipation efficiency. The microstructure of deposited samples was characterized using X-ray diffraction (XRD) and scanning electron microscope (SEM). An XRD equipped with a Cu Kα X-ray source was utilized to examine the phase of deposited samples. The XRD patterns were taken at 2θ angle ranging 20～100° at a scanning rate of 5°/min. The samples were polished following the standard polishing procedures before SEM observation. SYSWELD, a professional finite-element simulation software, is applied to calculate the thermal cycle curves in the present work. The FEM model was generally meshed into cubic elements. A denser meshing was applied in the heat affected zone where the detailed thermal information was expected. During the laser 3D printing process, a laser beam is used as a thermal energy source to induce the fusion of the material. In the present study, the laser beam is modeled as a Gaussian distribution of heat flux from a moving heat source with conical shape.
Fe58Cr15Mn2B16C4Mo2Si1W1Zr1 metallic glass by laser 3D printing with coaxial powder feeding method. The crystalline phase also found in the deposited Fe-based metallic glass. The fraction of amorphous phases decreases with the increasing deposited height. There was a serious crystallization in the deposited sample with the thickness of 2 mm [24]. Pauly et al. have used the selective laser melting method to implement complex geometries and components, including cylinder, the corners of the scaffold as well as ribbon, from an Fe74Mo4P10C7.5B2.5Si2 metallic glass. High volume fractions of crystalline phases are observed in the samples of cylinder and the corners of the scaffold. Only the simple ribbon appears to be fully amorphous [18]. From the above reported results, we can notice that the deposited metallic glasses with the thicknesses of only a few millimeters crystallize seriously. Then how to produce fully or nearly fully metallic glasses is the major issue that should be solved firstly. In this work, a large-size Zr50Ti5Cu27Ni10Al8 (Zr50) BMG is fabricated with success by laser 3D printing technique. The microstructure of printed Zr50 BMG is analyzed by the SEM and XRD, which reveal that the microstructure of printed Zr50 BMG is comprised of both amorphous and crystalline phases. When the sample is deposited thicker, the printed Zr50 alloy has high percentage of amorphous phase and dose not crystallize more severely. 2. Materials and methods Argon atomized Zr50 powder in the size range of 20˜50 μm was used for laser 3D printing process. The laser 3D printing machine consists of a 6 kW fiber laser, a four-nozzle coaxial powder feed system, a controlled environment glove box, and a motion control system [25]. A schematic of this equipment is shown in Fig. 1. The laser beam was focused on the substrate surface to a spot of approximately 3 mm in diameter. The powder-delivering nozzles are designed and arranged in such a way that the powder streams converge at the focal point of the laser beam. To make a 3D printing deposit, a CAD model is sliced firstly into a series of layers of finite thickness using computer software. Each of these layers is then translated into a series of line patterns in order to deposit the layers. The laser beam is used as a heat source to create a molten pool on the substrate, and the powder is injected into the melt pool by an inert gas flowing through the powder feed system. The first layer of the component is bonded to the substrate. The substrate, together with the component under fabrication, is moved along the line patterns in the plane of current layer with the motion control system. After completing a layer, the laser focal point and powder-delivering nozzles are incremented upward in the height direction in an amount of the layer thickness. A new layer is subsequently deposited onto the previous layer until the component is fully constructed in the layer-bylayer fashion. The substrate can be removed when the final product is fabricated. The laser 3D printing processing is performed in an argon
3. Results and discussion Fig. 2 shows the SEM micrograph of gas-atomized Zr50 alloy powder. Observations of the alloy powder with SEM reveals that most of the alloy powder is spherical in shape. Some large powders have small satellites with a size of 5˜10 μm. The XRD pattern of the as-fabricated Zr50 powder is shown in Fig. 3. Clearly, the diffraction patterns consists a broad peak in 2 theta region of 30˜45° without any detectable crystalline Bragg peaks, indicating its fully amorphous structure. Fig. 4(b) shows the outer appearance image of a multi-layer Zr50 alloy sample printed from the CAD model shown in Fig. 4(a) (c) shows the cross-section microstructure of the printed Zr50 BMGs perpendicular to the laser scanning direction. Further observations show that the light gray regions are surrounded by thin dark gray bands. The light gray regions are the melt zones that are heated by laser irradiation
Fig. 1. Schematic of a coaxial powder feeding laser cladding system.
Fig. 2. SEM micrographs of gas-atomized Zr50 amorphous alloy powder. 103
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and growth rates during crystallization [27]. Our simulation results have found that the heating rates of the thin dark gray bands, which are determined by the heat transferred from the melt zones, are lower than the heating rate required to abstain crystallization [20]. Therefore, the thin dark gray bands surrounding the melt zones are crystalline state. The XRD pattern of this sample is presented in Fig. 3, which exhibits a few sharp crystal peaks superimposed on a broad amorphous peak, further demonstrating that the light and dark gray thin bands are amorphous and crystalline, respectively. We can also find from Fig. 4(c) that the thin gray bands take about 10% of the total cross area, marking that the printed Zr50 alloy contains about 90% amorphous phase. The volume fraction of amorphous phase estimated from the SEM is very close to that calculated from the XRD profile shown in Fig. 3. The volume fraction of amorphous phase obtained by comparing the area of amorphous peak with that of the sum of all peaks in the XRD pattern is 91.2%. Since the heat generated by laser irradiation is mainly dispersed through the already deposited Zr50 alloy, the heat would disperse more slowly and there is a risk that the printed Zr50 alloy may crystallize more severely as the deposited sample becomes thicker. From Fig. 4(c) we can inspiringly find that the widths of dark gray crystalline bands are nearly constant with increasing the deposited layers. This may be due to the excellent thermal stability of the Zr50 BMG. Therefore, the Zr50 metallic glass without shape or size limitation can be produced by using the present parameters. Though the deposited metallic glass does not crystallize seriously as the deposited layers increasing, the heating process is inhomogeneous in the printed metallic glass along the building direction. To investigate this inhomogeneity, we calculate the thermal cycle curves of five typical points located at five adjacent layers along the building direction during the laser 3D printing process of a 6-layer Zr50 metallic glass by FEM simulation. Each layer consists of 5 tracks. The relative positions of the multiple tracks are shown in the schematic of Fig. 5(a). The red thin bands and yellow regions refer to the crystalline and amorphous phase, respectively. The thermal cycle curves of the five typical points are presented in Fig. 5(b). Clearly, the cooling curves of first tracks for all the five points are almost the same, the deposited metallic glass is cooled from the molten liquid. No crystallization takes place at these first tracks, since the cooling rates of first tracks are fast enough to vitrify the molten liquid [20]. After the first track, the subsequent tracks
Fig. 3. XRD patterns of the printed Zr50 sample and gas-atomized Zr50 alloy powder.
during the laser 3D printing process. In our recently published work [20], we have investigated the crystallization mechanism of the Zr50 BMG during the laser 3D printing process basing on the TTT diagrams of the Zr50 BMG and the thermal cycle curves obtained by FEM analysis. It is noted that the cooling rates of the molten liquids in the melt zones are high enough to continue to keep amorphous structure [20]. Then the light gray regions shown in Fig. 4(c) are amorphous state [26]. Differently, the thin dark gray bands surrounding the melt zones maintain the solid state during the whole laser 3D printing process. In these regions, the phase state was controlled by transferring the heat from the neighboring melt zones. The results of many studies have suggested that the crystallization behavior of BMG between cooling from the liquid state into the amorphous and heating from the amorphous solid state was obviously asymmetric [27]. For example, the critical cooling rate Rc of Zr41.2Ti13.8Cu12.5Ni10.0Be22.5 BMG (Vit1) is approximately 1 K/s, however, the critical heating rate Rh to prevent crystallization of Vit1 happening is two orders of magnitude larger than Rc, which is about 200 K/s [28]. The discrepancy between Rc and Rh of BMGs is so large, which is due to the significant difference in nucleation
Fig. 4. (a) CAD model, (b) outer appearance image of a multi-layer Zr50 alloy sample printed from the CAD model, (c) typical microstructure of the printed Zr50 alloy perpendicular to the laser travel direction. 104
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Fig. 5. (a) A schematic of the cross section for the laser 3D printing of a six-layer Zr50 metallic glass. (b) Thermal cycle curves of the five typical points show in (a).
will accumulate the structural relaxation for the Zr50 metallic glass formed at the first track through the heat transfer. Then from Fig. 5(b) we can note that the points located at different layers experience different thermal history. For instance, the point in the first layer undergoes more thermal cycle curves than the points in the upper layers, indicating that the structural relaxation of the bottom layers is more serious than the upper layers.
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4. Conclusions In summary, a large-size Zr50 BMG was successfully fabricated by laser 3D printing technique at the laser power of 450 W and laser scanning speed of 1000 mm/min. The SEM and XRD results revealed that the microstructure of printed sample consisted of both amorphous and crystalline phases. The printed Zr50 alloy possessed high percentage of amorphous phase and did not crystallize more seriously as the deposited sample becomes thicker. Author contributions Y.L. conceived and designed the experiments; H.X. and Z.L. performed the experiments; Y.L. and G.W. analyzed the data; H.X. and Y.L. wrote the paper. Conflicts of interest The authors declare no conflict of interest. Acknowledgements The authors would like to acknowledge the financial supports from the National Natural Science Foundation of China under Grant Nos. 51671042, 51401041, 51671043, the China Postdoctoral Science Foundation under Grant No. 2015M570242, and the Basic Research Program of the Key Lab in Liaoning Province Educational Department under Grant No. LZ2015011, the Liaoning Natural Science Foundation under Grant No. 201602126, the Key Research and Development Plan of Anhui Province under Grant No 1704a0902056. References [1] Wang WH, Dong C, Shek CH. Bulk metallic glasses. Mater Sci Eng R Rep 2004;44:45–89. [2] Wang G, Huang Z, Xiao P, Zhu X. Spraying of Fe-based amorphous coating with high corrosion resistance by HVAF. J Manuf Process 2016;22:34–8.
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