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1 The influence of the cutting density on the magnetic properties of non-oriented electrical steels cut through mechanical punching and water jet technologies V. Paltanea, G. Paltanea, H. Gavrila, D. Popovici, and G. Jiga Citation: AIP Conference Proceedings 1809, (2017); View online: View Table of Contents: Published by the American Institute of Physics Articles you may be interested in Magnetic anisotropy in grain oriented steels cut through mechanical punching and electro-erosion technologies AIP Conference Proceedings 1809, (2017); / Evaluation for elimination of methylene-orange from aqueous media by using membrane AIP Conference Proceedings 1809, (2017); / Investigation of the martensitic transformation of (Cu-Zn-Ni) shape memory alloys AIP Conference Proceedings 1809, (2017); / Production of organic nanoparticles by using nanoporous membranes AIP Conference Proceedings 1809, (2017); / FeSiAl soft magnetic composites with NiZn ferrite coating produced via solvothermal method AIP Advances 7, (2017); / Water resistance and thermal properties of polyvinyl alcohol-starch fiber blend film AIP Conference Proceedings 1809, (2017); /

2 The Influence of the Cutting Density on the Magnetic Properties of Non-oriented Electrical Steels Cut through Mechanical Punching and Water Jet Technologies V. Paltanea 1,*, G. Paltanea 1, H. Gavrila 1, D. Popovici 1 and G. Jiga 2 1 POLITEHNICA University from Bucharest, Electrical Engineering Department, 313 Splaiul Independentei, Bucharest, Romania 2 POLITEHNICA University from Bucharest, Strength of Materials Department, 313 Splaiul Independentei, Bucharest, Romania *Corresponding author: m1vera2@yahoo.com Abstract. The use of high quality non-oriented electrical steel and of an innovative design for the magnetic cores of the electrical machines are very important, in order to minimize the value of the total energy losses. The energy losses are strongly influenced by the cutting technologies, and the producers of the electrical machines want to minimize the deterioration of the magnetic properties during the manufacturing process. The influence of the cutting density on the magnetic permeability and energy losses was analyzed and one can notice that these magnetic properties are strongly influenced by the cutting technologies. There were tested sheet samples of M400-50A and M700-50A industrial steel grades (thickness of 0.50 mm), cut through mechanical and water jet technologies. All samples have the length equal to 300 mm and the width of 30, 15, 10, 7.5 and 5 mm. The magnetic characterization was performed using a laboratory single strip tester, which can make measurements on samples with an area of mm 2. In order to have the standard width of 30 mm, there were put together side by side 2, 3, 4 and 6 pieces with different widths. The magnetic properties were analyzed at 1000 mt in the frequency range Hz. It was observed that the processing conditions must be controlled and optimized, in order to maintain a low deterioration of the magnetic properties of the non-oriented steels. In the case of water jet technology an increase of the cutting speed will be useful for the introduction of this method in the large scale manufacturing of the electrical machines. Keywords: Non-oriented silicon iron steels; Energy losses; Mechanical punching; Water jet cutting technology; Magnetic permeability. PACS: Bb, Ej, Ch, d, Nt. INTRODUCTION The most important property of the non-oriented silicon iron (FeSi NO) alloys is the total energy losses. This property characterizes the energy, which is dissipated when the steel is magnetized cyclically in an alternating magnetic field and after that demagnetized. The total energy losses are usually divided in three components: hysteresis, classical and anomalous (excess) energy losses. The hysteresis losses are calculated as the area of an experimentally measured hysteresis cycle at 0 Hz; the classical energy losses are due to the eddy currents and are determined according to [1] Proceedings of the 6th International Advances in Applied Physics and Materials Science Congress & Exhibition AIP Conf. Proc. 1809, ; doi: / Published by AIP Publishing /$

3 and the excess losses are the difference between the total energy losses and the other two components. The anomalous losses are due to the micro eddy currents, which are formed in the neighborhood of the magnetic domain walls [1, 2]. The magnetic permeability of the non-oriented electrical steel is also very important, because the real part of the relative magnetic permeability represents the ability of the material to store the magnetic energy and the imaginary part of it is associated with the magnetic energy losses. High magnetic permeability and low energy losses have been recently required at the industrial level, to achieve higher efficiency and hence energy savings. The worldwide applications of the FeSi NO alloys are the electrical machines [3]. It is well known that an important part of the electric energy, which is produced in all the European countries, is generated by rotating electrical machines and more than half of this energy is used to drive the electrical motors. Improvement of energy efficiency of electrical machines leads to a large influence on energy consumption that has become an important part of the worldwide trend toward saving energy and environmental protection [1, 2]. Today efficiency classes of three-phase, cage induction electrical machines are according to [4]: base standard IE1, high efficiency IE2 and premium efficiency IE3. The international standard refers also to a future level above IE3 that is called IE4 super premium efficiency. From 2015 all rotating electrical machines sold and installed in Europe should comply with the IE3 standard and it is predicted that, by replacing gradually almost 30 million existing industrial motors, will result in a 5.5 TWh/year reduction and a corresponding decrease of carbon dioxide emissions of 3.4 Mt/year. The IE3 rotating electrical machines use low-loss grades of non-oriented silicon iron steels for the magnetic core and more active material (steel sheets, copper and aluminum) than the IE2 standard [5]. Electrical machines are produced by cutting the NO FeSi alloys into a proper shape, then laminating and fixing the cut sheets. It is known that every cutting technology influences the magnetic properties of the cut zone [1, 3]. The mechanical cutting induces a plastic deformation, visible near the cut line. During the cutting process microstructural defects appear that are pinning sites for the domain walls. This results in a local modification of the microstructure (dislocations, internal stresses and grain morphology) that has an influence on the magnetic and mechanical properties of the alloys [6, 7]. Mechanical cutting is a worldwide method and it is preferred by many machines manufactures, because it is very easy and cheap. The water-jet technology does not have thermal related problems like recast layer and thermal distortion and it exerts a minimal force on the work material. This technology is achieved by producing high velocity stream of water with or without abrasives (sand garnet, corundum aluminum oxide), which removes material mainly by erosion mechanism. In electrical steels the separation process is mainly due to the splitting and removal of material, preferably along the grain boundaries. Material removal takes place when the steel is rather hard and brittle and upon impact causes outbreaks which are removed by the water. EXPERIMENTAL PROCEDURE Two industrial non-oriented steels M400-50A and M700-50A were used for the experimental measurements. The strips were cut with different widths equal to 30, 15,

4 7.5 and 5 mm. The length of the samples was 300 mm and the thickness 0.5 mm, in the case of both industrial grades. In order to have the standard Single Strip Tester width (30 mm) there were put together side by side 2, 3, 4 and 6 pieces. The samples were cut through mechanical punching and water jet technologies. For the mechanical cutting of the samples it was developed an in house punch with length equal to 500 mm, width of 250 mm and height of 230 mm. This punch allows to cut samples with the following area: mm 2, mm 2, mm 2 and mm 2. The punch has two principal parts: one fixed and one with possibility of movement. The distance between these parts is equal to 0.7 mm. One can cut only samples with thickness of 0.65 mm and 0.5 mm. The punch is assembled on a chaser of 40 tones. Before every cut operation the mechanical punch must be edged. The investigated steels were cut with a MAXIEM 1530 abrasive water jet machine. In this type of device, the water is supplied at a very high pressure from MPa to MPa and provides a very good cutting process and a proper surface finishing [2]. The MAXIEM 1530 starts with a water pump. In the abrasive water jet machining, there is a mixture of Garnet 80 mesh (abrasive particles) and water, which helps to cut the material very fast and in a simple way. The materials have been tested between 10 Hz and 400 Hz by means of a laboratory Single Sheet Tester with digital control of the sinusoidal magnetic flux waveform according to the measuring standard IEC [8]. The form factor of the secondary voltage was kept at all frequencies within the interval ± 0.4%. The primary winding (173 turns) was supplied by a NF HSA4101 power amplifier, driven by an Agilent 33210A arbitrary function generator. The secondary winding (101 turns) was made directly around the surface of the samples. The physical properties of M400-50A and M700-50A alloys are presented in Table 1. Other physical parameters, used in the characterization of the electrical steel samples, are the density dm and the electrical resistivity ρ that have the following values: for M400-50A strips (dm = 7.7 g/cm 3, ρ = Ωm) and in the case of M700-50A strips (dm = 7.80 g/cm 3, ρ = Ωm). Table 1. Physical and geometrical properties of M400-50A and M700-50A samples. Cutting technology Number of Width [mm] M400-50A M700-50A samples Mass [g] Mass [g] Mechanical cutting Water jet cutting RESULTS and DISCUSSION In Fig. 1 are presented the normal magnetization curves, defined as the geometrical place of the (H, J) maximum point values, extracted from the symmetric hysteresis

5 loops, extending from the demagnetized state to saturation [9 12]. These dependencies are obtained in the case of the two industrial grade samples, cut through mechanical and water jet technologies. In the case of both steels, cut through water jet method, higher values of the magnetic polarization, which indicate an easier magnetization way are achieved for the same magnetic field strength. The differences between cutting technologies are more pronounced in the case of M400-50A, which is a high quality steel, because of the low non-magnetic impurity content. For M700-50A alloy, the water jet cutting method leads to a decrease of the saturation point value. For both materials and cutting technologies the increase of the cutting density determines a higher magnetization force. a) b) Figure 1. Normal magnetization curves in the case of the two electrical steels at 50 Hz: a) M400-50A grade; b) M700-50A grade

6 The total energy losses, determined in the case of water jet cut samples, are lower than those, measured for the punching technology, because water jet is a procedure that does not influence the magnetic properties of the silicon iron alloy (Fig. 2). In the case of M400-50A mechanical cut sheets the total energy losses vary between 20 mj/kg for the 1 30 mm width sample and 115 mj/kg for the 6 5 mm width strip. For M700-50A alloy the values of the total energy losses are in the range of 40 mj/kg (1 30 mm width sample) and 160 mj/kg (6 5 mm width sample). For the water jet technology, the losses are between 14 mj/kg and 83 mj/kg (M400-50A), and between 36 mj/kg and 125 mj/kg (M700-50A). In FeSi NO alloys the hysteresis energy losses are usually associated with the Barkhausen jumps (BJ). After the metallurgical manufacture, the electrical steel sheet contains a number of pinning centers (dislocations, grain boundaries, voids, precipitates) that hinder the free domain wall motion. The Barkhausen jumps are related to the irreversible domain wall motion in the case of low magnetic polarization values. a) b) Figure 2. Total energy losses as a function of the frequency in the case of the two electrical steels at 1000 mt: a) M400-50A grade; b) M700-50A grade

7 a) b) Figure 3. Hysteresis energy losses as a function of the frequency in the case of the two electrical steels at 1000 mt: a) M400-50A grade, b) M700-50A grade. In the range of medium to high magnetic polarization values, the BJ are associated with nucleation and annihilation of 180 and 90 domain walls. At high enough magnetic field strength levels the magnetization of the sample evolves mainly through the irreversible rotation magnetization processes [1, 2]. It can be observed, in the case of both steels, from Fig. 3 that the hysteresis losses are proportional to the cutting density, with higher values in the case of mechanical cut samples. This is due to the additional pinning centers, generated by the deformation of the crystalline structure near the edge of the sample, during the mechanical cutting process. In Fig. 4 are presented the excess energy loss values in the case of both types of cutting technologies. The excess energy losses, which are due to the small eddy currents formed in the neighborhood of the domain walls, and to the irregularities of the crystalline structure, are higher in the case of mechanical punching. Better results are obtained in the case of water jet cutting technology

8 The microstructural changes and the appearance of residual stresses, due to punching, affect the magnetizing behavior in different way. Local changes of grain size and texture result in local changes of the critical field for domain wall movement, which conduct to an increased value of the hysteresis and excess losses. a) b) Figure 4. Real part of the relative magnetic permeability as a function of the frequency in the case of the two electrical steels at 1000 mt: b) M700-50A grade; b) M700-50A grade

9 a) b) Figure 5. Relative magnetic permeability as a function of the frequency in the case of the two electrical steels at 1000 mt: a) M400-50A grade; b) M700-50A grade. One can observe from Fig. 5, that the water jet technology leads to three times higher values of the magnetic permeability than in the case of punching for M400-50A steel. In the case of M700-50A alloy the water jet cutting determines a small increase of the permeability, by comparing these values with those, obtained for mechanical punching. The increase of the cutting density determines a decrease of the relative magnetic permeability, for both materials and technologies. CONCLUSION In this paper it was analyzed the influence of the cutting density on the relative magnetic permeability, the total and hysteresis energy losses, and on the normal magnetization curve for two fully processed, non-oriented electrical steel strips of grades M400-50A and M700-50A, cut through mechanical and water jet methods. From the experimental data it can be concluded that the increase of the cutting density has a small influence on the energy losses in the case of water jet method

10 In order to have a low energy loss steel the NO FeSi alloys should be as free as possible of precipitates, voids, and lattice defects, and at the same time they should have a favorable distribution of grain orientations. The limitation of the impurities number could be made through water jet cut technology, because this method does not damage the crystallographic structure of the material and does not generate supplementary pinning centers. The punching method is industrial, while the water jet technology is a nonconventional method, which is used in the manufacture of the prototypes. It cannot yet be applied at industrial level, because it is expensive, unless the electrical machine producers want to develop special devices with low magnetic losses and high magnetic permeability. ACKNOWLEDGEMENT The work was supported by a grant of the Romanian Ministry of National Education, UEFISCDI, project number PN-II-PT-PCCA REFERENCES 1. V. Manescu (Paltanea), G. Paltanea, H. Gavrila, A. Nicolaide, Rev. Roum. Sci. Techn. Electrotechn. Et. Energ., 60, 2, (2015). 2. G. Paltanea, V. Manescu (Paltanea), H. Gavrila, A. Nicolaide, Rev. Roum. Sci. Techn. Electrotechn. Et. Energ., 61, 1, (2016). 3. I. Petryshynets, F. Kovac, V. Stoyka, J. Boruta, Acta Physica Polonica, 118 (5), (2010). 4. Rotating Electrical Machines Efficiency Classes of Line Operated AC Motors, IEC (2014). 5. V. Manescu (Paltanea), G. Paltanea, H. Gavrila, G. Scutaru, Rev. Roum. Sci. Techn. Electrotechn. Et. Energ., 60, 1, (2015). 6. A. J. Moses, N. Derebasi, G. Loisos, A. Shoppa, J. Magn. Magn. Mater., , (2000). 7. G. Crevecoeur, P. Sergeant, L. Dupre, L. Vandenbossche, R. Van de Walle, IEEE Trans. Magn., 44 (11), (2008). 8. J. Fuzer, Z. Bircakova, A. Zelenakova, P. Hrubovcak, P. Kollar, M. Predmersky, J. Hunady, Acta Physica Polonica, 118, 5, (2010). 9. M. Lauda, J. Fuzer, J. Fuzerova, P. Kollar, M. Streckova, M. Faberova, Acta Physica Polonica, 126 (1), (2014). 10. F. I. Hănţilă, M. Vasiliu, A. Moraru, M. Maricaru, Rev. Roum. Sci. Techn. Electrotechn. et Energ., 55 (2), (2010). 11. V. Manescu (Paltanea), G. Paltanea, H. Gavrila, Rev. Roum. Sci. Techn. Electrotechn. et Energ., 59 (3), (2014). 12. A. Nicolaide, S. Öner, Rev. Roum. Sci. Techn. Electrotechn et Energ., 56 (4), (2011)