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J. Mater. Sci. Technol., 2013, 29(8), 757e760

Microstructure and Magnetic Properties of FeeNi Alloy Fabricated by

Selective Laser Melting Fe/Ni Mixed Powders

Baicheng Zhang*, Nour-Eddine Fenineche, Hanlin Liao, Christian CoddetLERMPS, Université de Technologie de Belfort-Montbéliard, Site de Sévenans, Belfort Cedex 90010, France

[Manuscript received June 22, 2012, in revised form November 20, 2012, Available online 22 May 2013]

* Correspbaicheng1005-03JournalLimited.http://dx

FeeNi alloy, as a widely applied ferromagnetic material, is synthesized using selective laser melting (SLM). Thechemical compositions and microstructure of the SLM FeeNi alloy are characterized by X-ray diffraction (XRD),energy dispersive X-ray spectroscopy and scanning electron microscopy. It was found that the samples exhibitedfine grains with homogenous distribution when a low laser scanning velocity was used. Moreover, the magneticproperties of the samples with different laser parameters are also measured. It shows that the SLM Fee30%Nialloy possesses a low coercivity and high saturation magnetization. It also can be obtained that SLM is analternative faster method to prepare soft magnetic material with complex shapes. Moreover, the magneticproperties can be influenced by the laser parameters.

KEY WORDS: Selective laser melting (SLM); FeeNi alloy; Soft magnetic materials; Magnetic property

1. Introduction

FeeNi alloy, as a soft magnetic material with high perme-ability and low coercivity in weak magnetic field, is widelyapplied in industry. However, the conventional processingmethod of FeeNi alloy fabrication is hard to elaborate parts withcomplex shape and to obtain perfect magnetic properties due towork hardening of Ni element[1e3]. Recently, processing ofcomposite materials by laser fabrication such as selective lasermelting/selective laser sintering (SLM/SLS) has attracted interestdue to the process potential in free-form fabrication of intricatearticles with a reduced production cycle. Especially, SLM is acommercial technique for elaborating complex-shape partsdirectly from CAD data with metal powders[4,5]. Many kinds offunctional materials, such as wear resistant material, ultra-lightalloy and hydrogen storage material have been successfullysynthesized. For instance, SLM of TiNeTi5Si3[6], TieAleC[7],Mge7Mg alloy[8], Ale7Sie0.3 Mg alloy[9] have been reported.However, so far, laser synthesized FeeNi alloy, as ferromagneticmaterial has been rarely reported. In this work, SLM of the Fee30%Ni mixed powders is performed to prepare Fee30%Ni alloy.The microstructural features of the Fee30%Ni alloy underdifferent laser processing parameters were characterized.

onding author. Ph.D.; Tel.: þ33 384583243; E-mail address:.zhang@utbm.fr (B. Zhang).02/$e see front matter Copyright� 2013, The editorial office ofof Materials Science & Technology. Published by ElsevierAll rights reserved..doi.org/10.1016/j.jmst.2013.05.001

Moreover, the reasonable synthesis mechanisms behind micro-structural developments were explained.

2. Experimental

The characteristics of the materials used in the presentexperiment are summarized in Table 1. Nickel powder with anatomic fraction of 30% was added into the iron powder.Then, the mixed powders were blended in a Tumbling mixerfor 45 min. The powder mixture was processed layer bylayer to form cubic specimens with dimensions of 5 mm �5 mm � 5 mm using an MCP Realize Ⅱmachine (MCP 250 HEKTooling GmbH, Germany) which mainly consisted of a Nd:YAGlaser source with wavelength of 1064 nm. By optimizing laserparameters, the investigated laser melting power was fixed at110 W. In order to reveal the evolution of physical properties, thescanning velocity n was picked by 0.1 m/s, 0.4 m/s, and 1.6 m/s.The diameter (d) of the laser beam was 0.05 mm. The elaborationprocess was performed under argon atmosphere, and the powderbed temperature was kept constant at 80 �C during laser pro-cessing. Fig. 1 shows the working chamber of laser melting andthe obtained samples after processing.After removing the melted specimens from the building plate,

the porosity rate of the samples was measured by Archimedesmethod[10]. Then the samples were cut and ground for metallo-graphic examination according to standard procedures. A 6%nitric solution was taken as an etching agent with an etching timeof 15 s. The microstructure was characterized by scanningelectron microscopy (SEM, JEOL, JSM-5800LV, Japan),coupled with an energy dispersive spectrometer (EDS). The

Table 1 Characteristics of powders

Materials Particlesize (mm)

Meanvalue (mm)

Particleshape

Supplier

Fe <60 35 Spherical LERMPSNi <58 30 Spherical Medicoat AG,

Switzerland

Fig. 2 X-ray diffraction patterns of the laser-melted samples and

758 B. Zhang et al.: J. Mater. Sci. Technol., 2013, 29(8), 757e760

magnetic measurements were performed using a hysteresimeterBull M 2000/2010, which allowed to measure coercivity andsaturation magnetization.

3. Results and Discussion

starting powder with parameters: laser melting power 110 Wand scanning velocity 0.1 m/se1.6 m/s.

3.1. Phase

Fig. 2 shows the XRD patterns of the Fee30%Ni alloy withscanning velocities of 0.1 m/s, 0.4 m/s, and 1.6 m/s; laser powderof 110 W. The strong diffraction peaks corresponding to Fe7Ni3and Fe3Ni2 are clearly observed in the samples, while noapparent diffraction peaks for the initial Fe and Ni phases aredetected. In addition, from the intensity of the diffraction peaks,it can be observed that the content of these intermetallic com-pounds is different in these samples: the low laser scanningvelocity leads to a weak intensity of diffraction peaks for Fe3Ni2.Thus, it can be preliminarily considered that the synthesis re-action of FeeNi system by SLM can be accomplished by twosteps:

3Feþ 2Ni ¼ Fe3Ni2 (1)

3Fe3Ni2 þ 6Fe ¼ 2Fe7Ni3 (2)

In the initial phase of the SLM process, the reaction (1) cantake place at relatively low reaction temperature as the moltenpool was formed in which the temperature was just higher thanmelting point due to the negative Gibbs free energies of thisreaction[11]. Moreover, the temperature of molten pool can beincreased with the laser energy input continuously. Thus, theremained Fe element can be involved into reaction (2) becausethe Gibbs free energy of this reaction is much less than reaction

Fig. 1 A view of laser melting for Fee30%N

(1). Therefore, the reaction (2) depends on whether there is astrong driving force and a long reaction time. By comparingXRD results of the three samples, the Fe7Ni3 phase can be foundin all the samples. Only the phase in the sample with scanningvelocity of 0.1 m/s corresponded to Fe7Ni3. Thus, it is reasonableto conclude that the low laser scanning velocity is suited to yieldpromising Fee30%Ni alloy due to the relatively high operationtemperature and long reaction time.

3.2. Microstructures and compositions

Fig. 3 shows the microstructural characteristics of the sampleson the etched section. Fine dendritic grains can be found in thesample with scanning velocity of 0.1 m/s, and the coarse grainsare distributed within fine grain matrix with scanning velocitiesof 0.4 m/s and 1.6 m/s. In order to further determine theelemental distributions, EDS point measurements were per-formed. On the one hand, EDS results reveal that the Fe and Nielements in the sample with scanning velocity of 0.1 m/s isnearly equal to atomic proportion of 7:3 within these grains. TheXRD results show that the sample with low scanning velocity iscomposed of pure Fe7Ni3. On the other hand, the EDS resultsshow that the Fe and Ni elements within the matrix (Point 1,Fig. 3(b) and (c)) with scanning velocities of 0.4 m/s and 1.6 m/sare also about 7:3. However, the disorder and gross grain withinthe matrix is composed of the Fe and Ni elements with atomic

i alloy (a) and samples after melting (b).

Fig. 3 SEM micrographs showing the microstructure of laser melting samples with scanning velocity (a) 0.1 m/s, (b) 0.4 m/s, (c) 1.6 m/s and lasermelting power 110 W.

Fig. 4 Saturation magnetization and coercivity curves for FeeNi alloywith different laser parameters.

B. Zhang et al.: J. Mater. Sci. Technol., 2013, 29(8), 757e760 759

ratio of about 3:2 (Point 2, Fig. 3(b) and (c)). Thus, it isreasonable to consider that the Fe3Ni2 grains are completelyextracted via deep etching and dissolution of the metallic matrixFe7Ni3, which is presented in XRD results. Furthermore, theformation of fine grains originates from high undercooling dur-ing the solidification process which can be attributed to theintense heat absorption and elimination of heat between moltenpool and substrate. Moreover, the Marangoni flow[12] formed inthe molten pool can lead to a high nucleation rate in solidifica-tion. Once the temperature of molten pool decreased to meltingpoint, the nucleation growth process accelerates instantly andforms fine grains finally as shown in Fig. 3(a). Therefore, theintensity of Marangoni flow from upside to downside decreasesin the molten pool when a high scanning velocity is used, whichleads to a relatively low nucleation rate. Then, the relativelygross grains can be found in Fig. 3(b) and (c). Combined withthe XRD, SEM, and EDS results, it can be confirmed that theFee30%Ni composites are successfully prepared by SLM ofFeeNi mixed powders.

3.3. Research on magnetic performance

Fig. 4 shows the evolution of saturation magnetization andcoercivity via different laser scanning velocity. It can be foundthat the saturation magnetization of FeeNi alloy using scanningvelocity of 0.1 m/s is nearly similar to that of conventionalcasting Fee30%Ni alloy[13]. Furthermore, the effect of anisot-ropy on the magnetic properties cannot be ignored. According tothe XRD results, high laser scanning velocity can lead to aFe3Ni2 <111> phase which possesses low anisotropy constantK. Thus, a high saturation magnetization can be obtained whenusing the middle scanning velocity of 0.4 m/s. Contrarily, Thecombination of Fe7Ni3 and Fe3Ni2 with a relatively high porosityrate can lead to a relatively low saturation magnetization whenscanning velocity of 1.6 m/s is used due to a relatively highanisotropy constant K. It can be reasonably concluded that the

coercivity of soft magnetic materials strongly depends on themicrostructural characteristics of the material, parameters such asgrain size and its distribution and the amount of defects (intra-and inter-granular holes, inclusions, vacancies, dislocations, etc).Therefore, the movement of the magnetic domain can beobstructed by defects as mentioned above under the externalmagnetic field effect. The internal stress of the samples using lowscanning velocity is higher than that of using high scanningvelocity. Thus, the internal stress becomes the principalobstruction in magnetic field. It can be obtained that the higherthe scanning velocity, the higher the coercivity. Furthermore, thegradual increase of porosity rate and the heterogenous distribu-tion of FeeNi intermetallic compound result in a pinning ef-fect[14] which intensively restrain the movement of magneticdomain during the magnetic variation. In addition, referring tothe foregoing theory, the sample with scanning velocity of 0.4 m/s possesses a low anisotropy due to higher Fe2Ni3 content andless defects of structure. Thus, the valley point of coercivitycurve happened in 0.4 m/s can be easily explained.

760 B. Zhang et al.: J. Mater. Sci. Technol., 2013, 29(8), 757e760

According to Fig. 4, the maximum MS and minimal HC valuescan be obtained as MS ¼ 550 Am2/kg and HC ¼ 75 A/m,respectively. Compared with the conventional casting Fee30%Ni alloy[13] (MS ¼ 400 Am2/kg, HC ¼ 480 A/m), the SLM Fee30%Ni with scanning velocity of 0.4 m/s and laser power 110 Wpossesses a better magnetic property.

4. Conclusion

With the introduction of SLM in FeeNi alloy process, theFeeNi soft magnetic materials were realized. The magneticproperties of SLM Fee30%Ni alloy showed a strong micro-structure dependence, with the favorable values ofMS ¼ 550 Am2/kg, HC ¼ 75 A/m, corresponding to a micro-structures with a fine grain size and low porosity rate.

AcknowledgmentAuthors thank Mr. Eric AUBRY (Nipson Belfort France), for

his assistance concerning magnetic measurements.

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