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Magnetic properties of in-situ synthesized FeNi3 by selective laser melting Fe-80%Ni powders
Journal of Magnetism and Magnetic Materials 336 (2013) 49–54
Contents lists available at SciVerse ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Magnetic properties of in-situ synthesized FeNi3 by selective laser melting Fe-80%Ni powders Baicheng Zhang n, Nour-Eddine Fenineche, Hanlin Liao, Christian Coddet LERMPS, Universite´ de Technologie de Belfort-Montbe´liard, Site de Se´venans, Belfort Cedex 90010, France
a r t i c l e i n f o
abstract
Article history: Received 4 March 2012 Received in revised form 22 January 2013 Available online 21 February 2013
In this work, the FeNi3 intermetallic compound was in-situ synthesized by selective laser melting (SLM) using Fe-80%Ni mixed powder in order to obtain ferromagnetic materials. The microstructure and magnetic properties of the Fe-80%Ni alloy samples were investigated by scanning electron microscope, X-ray diffraction and hysteresis measurements. The FeNi3 intermetallic with fcc structure phase can be identified by X-ray diffraction. Meanwhile, the larger lattice parameter and small grain size could be obtained when low laser velocity was used. The effects of SLM process parameters were studied on magnetic properties of FeNi3 considering the role of melting microstructure. The relatively low coercivity (30–40 Oe) and high saturation magnetization (80–100 emu/g) can be carried by optimized laser parameters. & 2013 Elsevier B.V. All rights reserved.
Keywords: Selective laser melting (SLM) Fe–Ni alloy Soft magnetic material Magnetic property
1. Introduction Fe–Ni soft magnetic materials are of prime importance in magnetic materials research nowadays due to their extraordinary magnetic, mechanical, electrical characteristics [1]. Especially, the permalloy, which has attracted significant interest in new materials design due to their high magnetoconductivity, which was usually used in weak magnetic field. They play a key role in sensors, transformers, inductive device, electric motor, etc. [2]. However, the consolidation of magnetic system from two or three type of powders to bulk forming components preserving magnetism has always been a challenge [3,4]. Conventional powder metallurgy techniques such as high-temperature sintering and thermal spraying may lead to a loss of magnetic properties due to the excessive grain growth under high temperature conditions [5,6]. On the other hand, there is a big difficulty to obtain a precise component elaborated by the conventional methods using magnetic powder. Accordingly, it is necessary to use a post-treatment method in order to develop magnetic bulk materials. Selective laser melting (SLM), as a typical solid freeform fabrication (SFF) process, enables the quick production of complex shaped three-dimensional (3D) parts directly from metal powder [7,8]. The SLM process creates parts in a layer-by-layer way by selectively melting and consolidation of thin layer of the powder with a scanning laser beam. The SLM, due to the flexibility of feedstock and shapes, can be used for producing complex shaped parts, which
n
Corresponding author. Tel.: þ33 0384583243; fax: þ33 0384583286. E-mail addresses: [email protected], [email protected] (B. Zhang).
0304-8853/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmmm.2013.02.014
cannot be realized by other conventional methods [9,10]. Furthermore, in-situ synthesize with layer addition way could diminish the magnetocrystalline anisotropy of material. The extremely rapid melting/cooling process during SLM leads to a fine grain microstructure [11]. Thus, SLM technology has a great potential to elaborate complex part with promising magnetic properties. Base on the authors’ results, the maximal permeability of Fe– Ni binary alloy can be obtained when the weight proportion of Ni
Fig. 1. Schema of laser melting machine: MCP 250.
50
B. Zhang et al. / Journal of Magnetism and Magnetic Materials 336 (2013) 49–54
is about 80% [1]. In this experiment, Fe-80%wt Ni mixed powder was chosen as feedstock material in order to synthesis permalloy possesses highest permeability by SLM technology. The composition and magnetic properties of the SLM processed Fe–Ni alloy were evaluated, and the role of laser irradiation in the microstructure features of bulk materials was also elucidated. 2. Experimental section
Fig. 2. Laser parameters map for SLM Fe-80%Ni alloy.
Fig. 3. XRD pattern of samples with different laser scanning velocity and original powder.
In this study, iron powder with an average particle size (D50) of 35 mm was used, which was blended with nickel powder with an average article size (D50) of 30 mm in a Tumbling mixer for 45 min. The powder was prepared layer by layer to specimens with dimensions of 5 5 5 mm using MCP Realize IIslm machine (MCP HEK Tooling GmbH, Germany). Fig. 1 shows the process of SLM schema. The investigated laser melting conditions were laser power P¼50–100 W, scanning rate n ¼0.1–1.6 m/s, layer thickness o ¼ 0.05 mm. An alternative scanning pattern from layer to layer with equal line spacing in the X and Y directions was used. The diameter (d) of the laser beam was 50 mm. The building process was performed under argon atmosphere. The powder bed temperature was kept constant at 80 1C during laser melting. X-ray investigations were performed on an X-ray spectroscopy (XRD, D/mas-2400, Rigaku, Japan) in continuous scanning mode using Cu Ka radiation (l ¼0.154056 nm). The lines were measured in the range of 40–801 with a step of 0.011 for 10 s. The changes of the lattice parameters were calculated from the shift of the high angle diffraction line. To calculate the crystallite size D and the lattice parameter a, the Williamson–Hall method was adopted [12]. The mean crystallite size D and the lattice parameter a were determined with an accuracy of 71.5 nm and 5 10 5 nm. The microstructure of the SLM Fe-80%Ni alloy samples were characterized by a scanning electron microscope (SEM, JEOL, JSM5800LV, Japan), coupled with an energy dispersive spectrometer (EDS). The magnetic property measurements were realized using a hysteresimeter Bull M 2000/2010, which enabled to measure magnetic properties, namely coercivity and saturation magnetization. All the patterns were cut into slices with cross-section direction. Then these slices were ground to obtain a thickness of 1 mm. At last
Fig. 4. Fe–Ni phase diagram.
B. Zhang et al. / Journal of Magnetism and Magnetic Materials 336 (2013) 49–54
three slices from different parts (middle and sides) of each sample were performed to process magnetic test and the result of the magnetic measurements was expressed using an average value.
51
clearly show the variety phenomena during the melting process, a process-map was established base on the formation quality of the samples. There are three different processing windows defined with the entire range of laser power and scanning velocity, as shown in Fig. 2.
3. Results and discussion 3.1. Laser parameters map In the present experiment, a series of laser melting samples were elaborated with different processing parameters. In order to
(I) Complete formation zone: In this laser parameters area, the delivered energy density of laser beam is extremely high. Thus, the relatively high absolution of iron and nickel element combines with high laser energy that can lead to
Element Wt%
At%
Ni
80.15 79.35
Fe
19.85 20.65
Element Wt%
At%
Ni
80.15 79.35
Fe
19.85 20.65
Element Wt%
At%
Ni
80.15 79.35
Fe
19.85 20.65
Element Wt%
At%
Ni
80.15 79.35
Fe
19.85 20.65
Fig. 5. SEM images of samples microstructure and EDS results of samples with parameters: (a) 0.1 m/s, (b) 0.2 m/s, (c) 0.3 m/s and (d) 0.4 m/s.
52
B. Zhang et al. / Journal of Magnetism and Magnetic Materials 336 (2013) 49–54
a stable molten pool. So that, the relatively high density pieces with full-sizes can be obtained with SLM process. (II) Zone of failed formation: Compare with full-sizes of formed parts, all parts with these laser parameters possess various defects such as delamination, brittle failure and high porosity. According to the relatively low laser power with all scanning velocity, a significantly low energy input density results in a surface melting of particulate during laser melting. It can lead to a poor bond between the particulates. Thus, it cannot be avoided that the defects happened. (III) Zone of not forming: The parts with the laser parameters in this area cannot be formed as bulk material. The samples looked like a pile of powder without any mechanical strength. With a low laser power and a relatively high scanning velocity, the energy input density cannot reach the melting threshold values of Fe/Ni powder.
Base on the above research results, four samples from zone (I) were picked up to process phase and microstructure analysis. The following discussions are regarding these samples. 3.2. Phase and composition research Fig. 3 shows the XRD pattern of all samples melted from Fe80%Ni mixed powder. It can be found that the FeNi3 phase with fcc structure was well synthesized in samples with 0.1 to 0.3 m/s. From the Fe–Ni phase diagram, the FeNi3 phase can be combined with Ni in range from 70% to 90%, as shown in Fig. 4. During the melting process of Fe–Ni system, the Fe–Ni binary liquids can be locally formed instantaneously to a molten pool when laser irradiates on the powder layer (about 10 ms) [13], in which the temperature exceed 1425 1C. The intermetallic reaction Fe þ3Ni¼ FeNi3 can take place in relatively low reaction temperature like the molten pool was formed in which the temperature was just higher than melting point due to the negative Gibbs free energies of this reaction. All the powder under the effect of irradiation continuous can be involved into reaction with the laser energy input continuously. The cooling course of molten pool was extraordinary faster than other conventional combination method which can reach about 107 K/s [14], when laser beam moves away from its actuating range due to the relative different volume between melted pool and substrate. With the temperature of molten pool constantly decrease through eutectic point 516 1C, according to the atom proportion of Fe and Ni (1:4), intermetallic compound FeNi3 was formed. In addition, the additional Ni element will be present with the formation of solid solution in it. Based on the above analysis, the additional Ni is confirmed to play an important role in the formation of FeNi3 intermetallic compound and its corresponding properties. Besides that, the impurity like other Fe–Ni intermetallic compounds or Fe–Ni simple substance cannot be found in samples. In case of high scanning velocity 0.4 m/s, the main phase of sample is corresponded Fe7Ni3, and Ni single-phase can be also obtained in this sample. The high scanning velocity leads to a low laser energy density, so that the reaction of Fe–Ni cannot be processed completely. Furthermore, Fe7Ni3 phase possesses a lower Gibbs energy than that of FeNi3 during the laser melting. Thus, the phase composition of sample with 0.4 m/s is quite different of the samples with low scanning velocity.
addition, the holes cause by corrosive solution can be found in all samples. By more careful observation, it can be observed that the amount of these holes decrease with the laser scanning velocity increase (Fig. 5a–d). It is believed that the corrosive holes are attributed to the different formation of Fe–Ni structure in variable Fe–Ni alloy. In case of samples with 0.1 m/s to 0.3 m/s, the formation process of Fe–Ni alloy can be described as following: the Ni powder reaches the melting point firstly (about 1455 1C) and melts as liquid phase under the effect of laser irradiation, then surrounds the unmelted Fe powder. The Fe–Ni system absorbs heat until the temperature reaches the Fe molten point (1535 1C) under the effect of heat transmission by Ni liquid. As the laser beam moves away, the liquid system undergoes a rapid solidification process. Due to a relatively high mutual solubility of Fe and Ni (33.3 at% at 516 1C) (Fig. 4), a relatively homogenous distribution of the Fe element in the Ni matrix in the form of intermetallic compound FeNi3 can be obtained after solidification. By comparison, when the laser scanning velocity increases to 0.4 m/s, the Fe–Ni powder system cannot absorb enough laser energy during the laser irradiation to form a two phase liquid in molten pool. Thus, liquid–solid system instead of Fe–Ni metal liquid during the melting process. Furthermore, since the thermal cycle in SLM process is extremely short, the time for the homogenizing of the Fe and Ni element is too short, thereby causing the non-uniform distribution of the Fe element in the Ni matrix. Combine with the XRD, SEM and EDS results, it can be reasonably concluded that the intense of present etching solution can etch FeNi3 intermetallic compound, but it is weaken for the composites of Fe–Ni alloy and Ni element formed with high laser scanning velocity in Fig. 5(d), due to the relatively high anti-oxidation of Ni element. Fig. 6 presents the evolution of crystallite size D as a function of laser scanning parameter for laser melting Fe-80%Ni alloy. It was observed that the grain sizes increased under the conditions of the increase of laser scanning velocity. The value of grain size D increases from D ¼58 71.5 nm to 115 71.5 nm when the laser scanning velocity increased from 0.1 m/s to 0.4 m/ s. The change of crystallite sizes can be explained by the cooling rate of SLM process. It is believed that high laser scanning velocity can lead to a short molten pool preservation time. During the cooling of the molten pool, the grain size increases with the reservation time decrease. According to the velocity increase to 0.4 m/s, the grain size of Fe–Ni alloy is similar as the grain size of Fe–Ni mixed powder. Fig. 7 presents the change of lattice parameter a versus different laser beam parameters for Fe-80%Ni alloy. The value of
3.3. Composition analysis The variations of the microstructure after etching with different scanning velocity of laser beam are shown in Fig. 5. A pretty dense melting structure without any porosity can be observed. In
Fig. 6. Crystallite size of Fe-80%Ni versus laser scanning velocity.
B. Zhang et al. / Journal of Magnetism and Magnetic Materials 336 (2013) 49–54
the lattice parameter decreases from a ¼0.28757570.00005 to 0.28717870.00005 nm with increasing the velocity of laser scanning from 0.1 m/s to 0.4 m/s. The lattice parameter for pure iron was 0.28662 70.00005 nm and for pure nickel was 0.35288 70.00005 nm. It can be thought that the lower lattice parameter is caused by the higher laser scanning velocity. On the other hand, the lower laser power input is also contributed to lower lattice parameter, which can be explained by a partial substitution of Fe sites by Ni. The low laser scanning velocity can
53
lead to a strong Marangoni flow [15] in molten metal. Fe and Ni can be blended together under the metal liquid flow. In the process of solid solution, as a dissolvent, Fe atom would be displaced by solute Ni atom in crystal lattice under high temperature and long reaction time. Therefore, the lattice parameter of FeNi3 intermetallic compound with Ni solid solution is larger than that of pure Fe. Moreover, the decrease of lattice parameter leads to the decrease of solid solution formation due to shorter solidification time and lower laser power input. 3.4. Magnetic properties research
Fig. 7. Lattice parameter of Fe-80%Ni alloy versus laser scanning velocity.
Fig. 8 presents hysteresis loops of SLM Fe-80%Ni alloy with scanning velocity 0.1–0.4 m/s. It is observed that the SLM Fe80%Ni alloy have low coercivity and high saturation magnetization. These results agree with Fe–Ni alloy behavior produced by casting/ sintering process [16,17]. The values of saturation induction were shown in Fig. 9, which illustrated that the investigated alloy with saturation induction 99 emu/g decrease to 95 emu/g when the scanning velocity increase from 0.1 m/s to 0.4 m/s. Based on the literature, the FeNi3 phase possesses a high saturation magnetization. Moreover, combine the XRD and EDS results, the reason of saturation curve decrease can be explained by the content of FeNi3formed from Fe–Ni powder during the SLM process. So that, the sample with 0.4 m/s was composed by FeNi3 and Fe–Ni elementary substance, which possess less FeNi3 content in all four samples. Thus, it leads to a relatively low saturation magnetization, as shown in Fig. 9.
Fig. 8. Series of hysteresis loops of laser melted Fe-80%Ni alloy with laser velocity 0.1 m/s, 0.2 m/s, 0.3 m/s and 0.4 m/s.
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B. Zhang et al. / Journal of Magnetism and Magnetic Materials 336 (2013) 49–54
was aimed at producing good quality alloy and improving the magnetic characteristics by a laser melting parameter optimization. The XRD results indicated that more FeNi3 phase can be found when the laser scanning speed below 0.4 m/s is used, which can contribute to higher saturation magnetization. The similar results can be further verified by EDS, and the SEM photos show a fine crystalline structure under a relatively low laser scanning speed. The magnetic measure shows that the magnetic properties of laser melted Fe-80%Ni alloys can be influence by laser parameter.
Acknowledgments
Fig. 9. Saturation magnetization and coercivity curves for Fe-80%Ni alloy with laser scanning velocity.
On the other hand, the evolution of coercivity values is also shown in Fig. 9. It can be found that the coercivity of the samples is higher than that of samples in the cold compacted way or sintering way. It can be attributed that the alloy elaborated by SLM possess a very fine structure and a single domain structure which lead to higher coercivity values than the consolidated one which has a strong particle–particle interaction decreasing the coercivity. It also can be found that the coercivity values increase when use high scanning velocity. The origin of this behavior may be found in the competition among different changes occurring in the evolution of the structure. Likely, at the beginning of melting, a small grain size and poor homogeneity of FeNi alloy; while at higher temperatures and upon increasing the exposition time of melted pool, grain growth and Fe–Ni interdiffusion are responsible for the increase of Hc.
4. Conclusion In summary, the laser melted Fe–Ni alloys for magnetic applications were systematically investigated in this work. It
Authors would like to thank Mr Eric AUBRY (Nipson Belfort France), for his assistance concerning magnetic measurements.
References [1] I. Chicinas, O. Geoffroy, O. Isnard, V. Pop, Journal of Magnetism and Magnetic Materials 290-291 (2005) 1531–1534. [2] J.M.D. Coey, The Journal of Alloys and Compounds 326 (2001) 2. [3] F.E. Luborsky, Amorphous Metallic Alloys, Butterworths, London, 1983. [4] P. Ripka., Journal of Magnetism and Magnetic Materials 320 (2008) 2466–2473. [5] A. Hamler, V. Gorican, B. Sustarsic, A. Sirc., Journal of Magnetism and Magnetic Materials 304 (2006) e816–e819. [6] N.E. Fenineche, M. Cherigui, H. Aourag, C. Coddet, Materials Letters 58 (2004) 1797–1801. [7] P. Ficher, V. Romano, H.p. Weber, Acta Materialia 51 (2003) 1651–1662. [8] A. Simchi, H. Pohl, Materials Science and Engineering: A 359 (2003) 119–128. [9] B. Zhang, H. Liao, C. Coddet, Journal of Materials and Design 34 (2012) 753–758. [10] A. Simchi, F. Petzoldt, H. Pohl, Journal of Materials Processing Technology 141 (2003) 319. [11] Baicheng Zhang, Nour-Eddine Fenineche, Lin Zhu, Hanlin Liao, Christian Coddet, Journal of Magnetism and Magnetic Materials 324 (2012) 495–500. [12] G.K. Williamson, W.H. Hall, Acta Metallurgica 1 (1953) 22–31. [13] A. Simchi, Materials Science and Engineering: A 428 (2006) 148–158. [14] D. Gu, Y. Shen, Journal of Alloys and Compounds 432 (2007) P163. [15] Simchi, H. Pohl, Materials Science and Engineering: A 359 (2003) 119–128. [16] M.L. Trudeau, Nanostructured Materials 12 (1999) 55–60. [17] Y.J. Liu, I.T.H. Chang, M.R. Lees, Scripta Materialia 44 (2001) 2729–2734.
Contents lists available at SciVerse ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Magnetic properties of in-situ synthesized FeNi3 by selective laser melting Fe-80%Ni powders Baicheng Zhang n, Nour-Eddine Fenineche, Hanlin Liao, Christian Coddet LERMPS, Universite´ de Technologie de Belfort-Montbe´liard, Site de Se´venans, Belfort Cedex 90010, France
a r t i c l e i n f o
abstract
Article history: Received 4 March 2012 Received in revised form 22 January 2013 Available online 21 February 2013
In this work, the FeNi3 intermetallic compound was in-situ synthesized by selective laser melting (SLM) using Fe-80%Ni mixed powder in order to obtain ferromagnetic materials. The microstructure and magnetic properties of the Fe-80%Ni alloy samples were investigated by scanning electron microscope, X-ray diffraction and hysteresis measurements. The FeNi3 intermetallic with fcc structure phase can be identified by X-ray diffraction. Meanwhile, the larger lattice parameter and small grain size could be obtained when low laser velocity was used. The effects of SLM process parameters were studied on magnetic properties of FeNi3 considering the role of melting microstructure. The relatively low coercivity (30–40 Oe) and high saturation magnetization (80–100 emu/g) can be carried by optimized laser parameters. & 2013 Elsevier B.V. All rights reserved.
Keywords: Selective laser melting (SLM) Fe–Ni alloy Soft magnetic material Magnetic property
1. Introduction Fe–Ni soft magnetic materials are of prime importance in magnetic materials research nowadays due to their extraordinary magnetic, mechanical, electrical characteristics [1]. Especially, the permalloy, which has attracted significant interest in new materials design due to their high magnetoconductivity, which was usually used in weak magnetic field. They play a key role in sensors, transformers, inductive device, electric motor, etc. [2]. However, the consolidation of magnetic system from two or three type of powders to bulk forming components preserving magnetism has always been a challenge [3,4]. Conventional powder metallurgy techniques such as high-temperature sintering and thermal spraying may lead to a loss of magnetic properties due to the excessive grain growth under high temperature conditions [5,6]. On the other hand, there is a big difficulty to obtain a precise component elaborated by the conventional methods using magnetic powder. Accordingly, it is necessary to use a post-treatment method in order to develop magnetic bulk materials. Selective laser melting (SLM), as a typical solid freeform fabrication (SFF) process, enables the quick production of complex shaped three-dimensional (3D) parts directly from metal powder [7,8]. The SLM process creates parts in a layer-by-layer way by selectively melting and consolidation of thin layer of the powder with a scanning laser beam. The SLM, due to the flexibility of feedstock and shapes, can be used for producing complex shaped parts, which
n
Corresponding author. Tel.: þ33 0384583243; fax: þ33 0384583286. E-mail addresses: [email protected], [email protected] (B. Zhang).
0304-8853/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmmm.2013.02.014
cannot be realized by other conventional methods [9,10]. Furthermore, in-situ synthesize with layer addition way could diminish the magnetocrystalline anisotropy of material. The extremely rapid melting/cooling process during SLM leads to a fine grain microstructure [11]. Thus, SLM technology has a great potential to elaborate complex part with promising magnetic properties. Base on the authors’ results, the maximal permeability of Fe– Ni binary alloy can be obtained when the weight proportion of Ni
Fig. 1. Schema of laser melting machine: MCP 250.
50
B. Zhang et al. / Journal of Magnetism and Magnetic Materials 336 (2013) 49–54
is about 80% [1]. In this experiment, Fe-80%wt Ni mixed powder was chosen as feedstock material in order to synthesis permalloy possesses highest permeability by SLM technology. The composition and magnetic properties of the SLM processed Fe–Ni alloy were evaluated, and the role of laser irradiation in the microstructure features of bulk materials was also elucidated. 2. Experimental section
Fig. 2. Laser parameters map for SLM Fe-80%Ni alloy.
Fig. 3. XRD pattern of samples with different laser scanning velocity and original powder.
In this study, iron powder with an average particle size (D50) of 35 mm was used, which was blended with nickel powder with an average article size (D50) of 30 mm in a Tumbling mixer for 45 min. The powder was prepared layer by layer to specimens with dimensions of 5 5 5 mm using MCP Realize IIslm machine (MCP HEK Tooling GmbH, Germany). Fig. 1 shows the process of SLM schema. The investigated laser melting conditions were laser power P¼50–100 W, scanning rate n ¼0.1–1.6 m/s, layer thickness o ¼ 0.05 mm. An alternative scanning pattern from layer to layer with equal line spacing in the X and Y directions was used. The diameter (d) of the laser beam was 50 mm. The building process was performed under argon atmosphere. The powder bed temperature was kept constant at 80 1C during laser melting. X-ray investigations were performed on an X-ray spectroscopy (XRD, D/mas-2400, Rigaku, Japan) in continuous scanning mode using Cu Ka radiation (l ¼0.154056 nm). The lines were measured in the range of 40–801 with a step of 0.011 for 10 s. The changes of the lattice parameters were calculated from the shift of the high angle diffraction line. To calculate the crystallite size D and the lattice parameter a, the Williamson–Hall method was adopted [12]. The mean crystallite size D and the lattice parameter a were determined with an accuracy of 71.5 nm and 5 10 5 nm. The microstructure of the SLM Fe-80%Ni alloy samples were characterized by a scanning electron microscope (SEM, JEOL, JSM5800LV, Japan), coupled with an energy dispersive spectrometer (EDS). The magnetic property measurements were realized using a hysteresimeter Bull M 2000/2010, which enabled to measure magnetic properties, namely coercivity and saturation magnetization. All the patterns were cut into slices with cross-section direction. Then these slices were ground to obtain a thickness of 1 mm. At last
Fig. 4. Fe–Ni phase diagram.
B. Zhang et al. / Journal of Magnetism and Magnetic Materials 336 (2013) 49–54
three slices from different parts (middle and sides) of each sample were performed to process magnetic test and the result of the magnetic measurements was expressed using an average value.
51
clearly show the variety phenomena during the melting process, a process-map was established base on the formation quality of the samples. There are three different processing windows defined with the entire range of laser power and scanning velocity, as shown in Fig. 2.
3. Results and discussion 3.1. Laser parameters map In the present experiment, a series of laser melting samples were elaborated with different processing parameters. In order to
(I) Complete formation zone: In this laser parameters area, the delivered energy density of laser beam is extremely high. Thus, the relatively high absolution of iron and nickel element combines with high laser energy that can lead to
Element Wt%
At%
Ni
80.15 79.35
Fe
19.85 20.65
Element Wt%
At%
Ni
80.15 79.35
Fe
19.85 20.65
Element Wt%
At%
Ni
80.15 79.35
Fe
19.85 20.65
Element Wt%
At%
Ni
80.15 79.35
Fe
19.85 20.65
Fig. 5. SEM images of samples microstructure and EDS results of samples with parameters: (a) 0.1 m/s, (b) 0.2 m/s, (c) 0.3 m/s and (d) 0.4 m/s.
52
B. Zhang et al. / Journal of Magnetism and Magnetic Materials 336 (2013) 49–54
a stable molten pool. So that, the relatively high density pieces with full-sizes can be obtained with SLM process. (II) Zone of failed formation: Compare with full-sizes of formed parts, all parts with these laser parameters possess various defects such as delamination, brittle failure and high porosity. According to the relatively low laser power with all scanning velocity, a significantly low energy input density results in a surface melting of particulate during laser melting. It can lead to a poor bond between the particulates. Thus, it cannot be avoided that the defects happened. (III) Zone of not forming: The parts with the laser parameters in this area cannot be formed as bulk material. The samples looked like a pile of powder without any mechanical strength. With a low laser power and a relatively high scanning velocity, the energy input density cannot reach the melting threshold values of Fe/Ni powder.
Base on the above research results, four samples from zone (I) were picked up to process phase and microstructure analysis. The following discussions are regarding these samples. 3.2. Phase and composition research Fig. 3 shows the XRD pattern of all samples melted from Fe80%Ni mixed powder. It can be found that the FeNi3 phase with fcc structure was well synthesized in samples with 0.1 to 0.3 m/s. From the Fe–Ni phase diagram, the FeNi3 phase can be combined with Ni in range from 70% to 90%, as shown in Fig. 4. During the melting process of Fe–Ni system, the Fe–Ni binary liquids can be locally formed instantaneously to a molten pool when laser irradiates on the powder layer (about 10 ms) [13], in which the temperature exceed 1425 1C. The intermetallic reaction Fe þ3Ni¼ FeNi3 can take place in relatively low reaction temperature like the molten pool was formed in which the temperature was just higher than melting point due to the negative Gibbs free energies of this reaction. All the powder under the effect of irradiation continuous can be involved into reaction with the laser energy input continuously. The cooling course of molten pool was extraordinary faster than other conventional combination method which can reach about 107 K/s [14], when laser beam moves away from its actuating range due to the relative different volume between melted pool and substrate. With the temperature of molten pool constantly decrease through eutectic point 516 1C, according to the atom proportion of Fe and Ni (1:4), intermetallic compound FeNi3 was formed. In addition, the additional Ni element will be present with the formation of solid solution in it. Based on the above analysis, the additional Ni is confirmed to play an important role in the formation of FeNi3 intermetallic compound and its corresponding properties. Besides that, the impurity like other Fe–Ni intermetallic compounds or Fe–Ni simple substance cannot be found in samples. In case of high scanning velocity 0.4 m/s, the main phase of sample is corresponded Fe7Ni3, and Ni single-phase can be also obtained in this sample. The high scanning velocity leads to a low laser energy density, so that the reaction of Fe–Ni cannot be processed completely. Furthermore, Fe7Ni3 phase possesses a lower Gibbs energy than that of FeNi3 during the laser melting. Thus, the phase composition of sample with 0.4 m/s is quite different of the samples with low scanning velocity.
addition, the holes cause by corrosive solution can be found in all samples. By more careful observation, it can be observed that the amount of these holes decrease with the laser scanning velocity increase (Fig. 5a–d). It is believed that the corrosive holes are attributed to the different formation of Fe–Ni structure in variable Fe–Ni alloy. In case of samples with 0.1 m/s to 0.3 m/s, the formation process of Fe–Ni alloy can be described as following: the Ni powder reaches the melting point firstly (about 1455 1C) and melts as liquid phase under the effect of laser irradiation, then surrounds the unmelted Fe powder. The Fe–Ni system absorbs heat until the temperature reaches the Fe molten point (1535 1C) under the effect of heat transmission by Ni liquid. As the laser beam moves away, the liquid system undergoes a rapid solidification process. Due to a relatively high mutual solubility of Fe and Ni (33.3 at% at 516 1C) (Fig. 4), a relatively homogenous distribution of the Fe element in the Ni matrix in the form of intermetallic compound FeNi3 can be obtained after solidification. By comparison, when the laser scanning velocity increases to 0.4 m/s, the Fe–Ni powder system cannot absorb enough laser energy during the laser irradiation to form a two phase liquid in molten pool. Thus, liquid–solid system instead of Fe–Ni metal liquid during the melting process. Furthermore, since the thermal cycle in SLM process is extremely short, the time for the homogenizing of the Fe and Ni element is too short, thereby causing the non-uniform distribution of the Fe element in the Ni matrix. Combine with the XRD, SEM and EDS results, it can be reasonably concluded that the intense of present etching solution can etch FeNi3 intermetallic compound, but it is weaken for the composites of Fe–Ni alloy and Ni element formed with high laser scanning velocity in Fig. 5(d), due to the relatively high anti-oxidation of Ni element. Fig. 6 presents the evolution of crystallite size D as a function of laser scanning parameter for laser melting Fe-80%Ni alloy. It was observed that the grain sizes increased under the conditions of the increase of laser scanning velocity. The value of grain size D increases from D ¼58 71.5 nm to 115 71.5 nm when the laser scanning velocity increased from 0.1 m/s to 0.4 m/ s. The change of crystallite sizes can be explained by the cooling rate of SLM process. It is believed that high laser scanning velocity can lead to a short molten pool preservation time. During the cooling of the molten pool, the grain size increases with the reservation time decrease. According to the velocity increase to 0.4 m/s, the grain size of Fe–Ni alloy is similar as the grain size of Fe–Ni mixed powder. Fig. 7 presents the change of lattice parameter a versus different laser beam parameters for Fe-80%Ni alloy. The value of
3.3. Composition analysis The variations of the microstructure after etching with different scanning velocity of laser beam are shown in Fig. 5. A pretty dense melting structure without any porosity can be observed. In
Fig. 6. Crystallite size of Fe-80%Ni versus laser scanning velocity.
B. Zhang et al. / Journal of Magnetism and Magnetic Materials 336 (2013) 49–54
the lattice parameter decreases from a ¼0.28757570.00005 to 0.28717870.00005 nm with increasing the velocity of laser scanning from 0.1 m/s to 0.4 m/s. The lattice parameter for pure iron was 0.28662 70.00005 nm and for pure nickel was 0.35288 70.00005 nm. It can be thought that the lower lattice parameter is caused by the higher laser scanning velocity. On the other hand, the lower laser power input is also contributed to lower lattice parameter, which can be explained by a partial substitution of Fe sites by Ni. The low laser scanning velocity can
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lead to a strong Marangoni flow [15] in molten metal. Fe and Ni can be blended together under the metal liquid flow. In the process of solid solution, as a dissolvent, Fe atom would be displaced by solute Ni atom in crystal lattice under high temperature and long reaction time. Therefore, the lattice parameter of FeNi3 intermetallic compound with Ni solid solution is larger than that of pure Fe. Moreover, the decrease of lattice parameter leads to the decrease of solid solution formation due to shorter solidification time and lower laser power input. 3.4. Magnetic properties research
Fig. 7. Lattice parameter of Fe-80%Ni alloy versus laser scanning velocity.
Fig. 8 presents hysteresis loops of SLM Fe-80%Ni alloy with scanning velocity 0.1–0.4 m/s. It is observed that the SLM Fe80%Ni alloy have low coercivity and high saturation magnetization. These results agree with Fe–Ni alloy behavior produced by casting/ sintering process [16,17]. The values of saturation induction were shown in Fig. 9, which illustrated that the investigated alloy with saturation induction 99 emu/g decrease to 95 emu/g when the scanning velocity increase from 0.1 m/s to 0.4 m/s. Based on the literature, the FeNi3 phase possesses a high saturation magnetization. Moreover, combine the XRD and EDS results, the reason of saturation curve decrease can be explained by the content of FeNi3formed from Fe–Ni powder during the SLM process. So that, the sample with 0.4 m/s was composed by FeNi3 and Fe–Ni elementary substance, which possess less FeNi3 content in all four samples. Thus, it leads to a relatively low saturation magnetization, as shown in Fig. 9.
Fig. 8. Series of hysteresis loops of laser melted Fe-80%Ni alloy with laser velocity 0.1 m/s, 0.2 m/s, 0.3 m/s and 0.4 m/s.
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was aimed at producing good quality alloy and improving the magnetic characteristics by a laser melting parameter optimization. The XRD results indicated that more FeNi3 phase can be found when the laser scanning speed below 0.4 m/s is used, which can contribute to higher saturation magnetization. The similar results can be further verified by EDS, and the SEM photos show a fine crystalline structure under a relatively low laser scanning speed. The magnetic measure shows that the magnetic properties of laser melted Fe-80%Ni alloys can be influence by laser parameter.
Acknowledgments
Fig. 9. Saturation magnetization and coercivity curves for Fe-80%Ni alloy with laser scanning velocity.
On the other hand, the evolution of coercivity values is also shown in Fig. 9. It can be found that the coercivity of the samples is higher than that of samples in the cold compacted way or sintering way. It can be attributed that the alloy elaborated by SLM possess a very fine structure and a single domain structure which lead to higher coercivity values than the consolidated one which has a strong particle–particle interaction decreasing the coercivity. It also can be found that the coercivity values increase when use high scanning velocity. The origin of this behavior may be found in the competition among different changes occurring in the evolution of the structure. Likely, at the beginning of melting, a small grain size and poor homogeneity of FeNi alloy; while at higher temperatures and upon increasing the exposition time of melted pool, grain growth and Fe–Ni interdiffusion are responsible for the increase of Hc.
4. Conclusion In summary, the laser melted Fe–Ni alloys for magnetic applications were systematically investigated in this work. It
Authors would like to thank Mr Eric AUBRY (Nipson Belfort France), for his assistance concerning magnetic measurements.
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