On the use of elemental powders to process Fe-50Co alloys by powder injection molding

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1 On the use of elemental powders to process Fe-50Co alloys by powder injection molding P. A. P. Wendhausen¹, A. Silva¹, André L. Slaviero, R. Machado² ¹Universidade Federal de Santa Catarina, Departamento de Engenharia Mecânica, Laboratório de Materiais, Caixa Postal Campus Universitário, , Florianópolis, SC, Brasil. ²Lupatech S.A, Divisão Steelinject, Caixa Postal 1215, , Caxias do Sul, RS, Brasil Keywords: FeCo alloys, powder injection molding, magnetic saturation, densification Abstract: Aiming to obtain components with higher density, this work evaluated the technical and economical viabilities to replace the pre-alloyed Fe49Co2V by an elemental powder alloy of iron and cobalt (Fe-50Co). Using an elemental alloy could increase the density of the final material due to the driving force created by the chemical gradient between the powders. The results showed that is possible to achieve higher densities in an elemental powder Fe-50Co alloy sinterized at the same temperature and in shorter times than the Fe49Co2V alloy. The analysis of economical viability showed that the replacement of the alloys have advantages as the pre-alloyed powder price is higher than the elemental. Introduction In particular components fabricated by the powder injection molding process, specific magnetic properties are attempted to achieve. An example of magnetic alloy commercially used is the Fe49Co2V. This alloy is processed in pre-alloyed form to the fabrication of components for fuel injection systems of internal combustion motors. In these components, as higher the material density better are the magnetic properties that are important to their proper functioning. The present work has the objective to verify the possibility to replace the pre-alloyed Fe49Co2V by the elemental Fe-50Co alloy for the reason that is expected to have an increase in the densification when using elemental powders as the chemical gradient is a higher driving force than the small particle size and also because Fe-50Co has a superior saturation magnetization. The difficulty to process by conventional ways due the fragility made that the alloy Fe-50Co to be substituted by the FeCo2V alloy, which was more ductile but had lower saturation magnetization. The desire to have elevated magnetic saturation is because it allows the material to sustain a higher magnetic flux in a smaller area. The Fe-Co alloys, particularly near equiatomic composition, tend to be brittle because of the ordering of the α-phase and the presence of trace amounts of C, H and O [08]. Therefore, the use Fe-50Co had limitation in the conventional process like foundry followed by lamination. However, in the powder injection molding process the material will not be deformed, as it already has the final shape in the end of the process and is easier to control the impurities of the final part, so is expected that the fragility of the alloy will not be a problem in this kind of process.

2 Literature review Iron-cobalt alloys near the equiatomic composition are technologically important because of their high saturation magnetization, low magnetocrystalline anisotropy and associated high permeability [08]. It is known that an increase in the density increases the saturation magnetization. The principal step of densification in the metal injection molding process is the sintering stage. The densification will depend on many factors of the processing parameters and powder characteristics (morphology, average size, surface area, and chemical composition). Alteration in the sintering mechanism will cause changes in the microstructure and, consequently, in the densification. Treatment to increase the sintering can be categorized as methods to alter the kinetic or driving force associated with the mass transport. In this study was analyzed how the changes in the chemical composition and the particle size could interfere in the sintering step. The small particle size increases the sintering because of stress created in the small curvature radius, also the total contact area between the particles is enhanced and the superficial energy is higher than in the large particles. All these aspects increase the mass transport. When exists elemental powders mixtures, and if the there is solubility between the particles, the chemical gradient will dominated the sintering as it is predominant when compared with the effects of the superficial energy of the small particles. In these cases the sintering will occur by the reactivity between the powders due the high heat generated in the exothermic reaction during the thermal cycle. With an increase in the chemical activity more the sintering is controlled by the solubility and by the chemical reactions between the powders [2]. Methodology The elemental alloy was formed by commercial powders, 50% of carbonyl iron powder (average particle size 4,0µm) and 50% of cobalt powder (average particle size 1,4µm). The feedstock formed had approximately 13% in weight of binder. The pre-alloyed Fe49Co2V (average particle size 10µm) was prepared with approximately 8% in weight of binder. This difference in the quantity of binder is due the size and shape of the particles of each material. As demonstrated in the Figure 1, the samples were prepared in a toroid shape for posterior magnetic characterizations of the alloys. Figure 1 Demonstration of the parts after the first step of the process and after sintering for Fe49Co2V and Fe50Co.

3 Both alloys were submitted to the same debinder. First they were chemical debinded in vapour and in liquid of hexano at 65ºC, after the samples were thermally debinded in temperature of 980ºC for 14 hours with hydrogen atmosphere. The sintering temperature for both was 1330ºC with atmosphere formed by 90% of argon and 10% of hydrogen and partial pressure of 0,8torr, but the routes were different. For the Fe49Co2V this temperature was kept for 4 hours and the cooling was controlled (cooled until 840ºC and kept in this temperature for 2 hours, and then cooled with closed furnace). For the Fe-50Co alloy, the temperature of 1330ºC was kept for 3 hours and was cooled without a control, with closed furnace. This difference in the sintering routes was tested to verify if was real necessary to control the cooling when using a Fe-50Co alloy processed by powder injection molding. With the samples were analysed: - Density, as it could change the magnetic properties; - Quantity of carbon to verify if the debinder was carried through successfully as the presence of the carbon could affect the magnetic properties; - Hardness as the alloy to the specific function needs to have a minimal hardness of 84HRb. Results After the chemical debinding, both alloys showed a good extraction of binder with no problems with shape distortions or cracks as presented in the Figure 2. Figure 2 Part of Fe-50Co alloy after the thermal debinding. The results found after the thermal debinding are presented in the Table 1, and is possible to verify that the alloys are already pre-sintered but with small grain size as showed in the Figure 3. Table 1 - Results found after thermal debinding for Fe-50Co and Fe49Co2V alloys. Properties Fe-50Co Fe49Co2V Density (g/cm³) 7,63 7,59 Carbon (%) 0 0,015

4 Figure 3 Microstructure of Fe-50Co after thermal debinding (Amplification of 100x). The samples of Fe-50Co alloy showed the best values of density after sintering as presented in the Table 2, the quantity of carbon and oxides was acceptable in both alloys and the hardness were similar. Table 2 - Results found after sintering for Fe-50Co and Fe49Co2V alloys. Properties Fe-50Co Fe49Co2V Density (g/cm³) 8,03 +/- 0,1 7,82+/-0,1 Carbon (%) 0,005 0,015 Porosity 2,07 5,3 oxides No No Hardness Rockwell B (transformed from HV0,2) 98 +/ /- 10 It is possible to confirm the theory of the higher driving force of chemical gradient by observing the microstructure of both alloys. For the Fe-50Co the grain size was so big in the end of the process that was not possible to have a picture of the grain size with 100 times of amplification as showed in the Figure 4, the grain had the same diameter of the toroid prepared to measure the magnetic properties as demonstrated in the Figure 5. While the Fe49Co2V showed a much smaller grain size than the Fe-50Co (Figure 4). Figure 4 - Microstructure of Fe-50Co (on the left) and the Fe49Co2V (on the right) after sintering (Amplification of 100x).

5 Figure 5 - Microstructure of Fe-50Co after sintering showing the grain size with the same diameter of the toroid (amplification of 25x). Discussion As expected, the results showed the better values of density for the elemental powders than for the pre-alloyed powders. With these results, probably the saturation magnetization and the permeability will be higher for the Fe-50Co as the density and the grain size is much higher than the Fe49Co2V. Experiments using dilatometer also are being done to have a complete knowledge of the materials behaviour during the sintering. There was no difficult to process the Fe-Co alloy by the powder injection molding because of its fragility. The parts produced showed no problems of distortions or cracks. The minimal hardness required for the specific application was reached and the impurities were in an acceptable level. The best part of the research until now is that the economical analyses showed that beyond the best results of density of the elemental alloy, the replacement of the pre-alloyed Fe49Co2V by the elemental alloy Fe-50Co have advantages, as the pre-alloyed powder price is 35% higher than the elemental. Acknowledgements The authors would like to thanks to Lupatech S.A. for research support and for the research co-operation program. References [1] GERM7AN, R. M. Powder Injection Molding, Metal Powder Industries Federation, Princeton, New Jersey, [2] GERMAN, R. M., Sintering Theory and Practice, Innovative Material Solutions, Inc., Pennsylvania, [3] RODRIGUES, D.; LANDGRAF, F.J.G.; CONCILIO, V.G.; Uma contribuição ao estudo da sinterização das ligas de Fe-50Co, Divisão de metalurgia do IPT, São Paulo, Brasil. [4] SORESCUA, M.; GRABIASA, A., Structural and Magnetic Properties of Fe50Co50 System, Institute of Electronics Materials Technology, Poland, 2001.

6 [5] SOURMAIL, T., Near equiatomic FeCo alloys: constitution, mechanical and magnetic properties, Department of Materials Science and Metallurgy, University of Cambridge, Cambridge, [6] SUNDAR, R.S.; DEEVI, S.C.; Influence of alloying elements on the mechanical properties of FeCo V alloys, Chrysalis Technologies Incorporated, Richmond, VA, USA [7] TORRES. F.W; Estudo da Sinterização de Ligas Permendur (Fe-Co-V) Obtidas Através do Processo de Moldagem por Injeção, Trabalho de conclusão de curso de engenharia de materiais, Universidade Federal de Santa Catarina, Florianópolis, [8] CHIN, G.Y.; WERNICK, J.H., Ferromagnetic materials Handbook on the properties of magnetically ordered substances, vol.2, edited by, E.P. wohlfarth, North-Holland Physics Publishing, 1986.