M. VUKOTI] et al.: MAGNETIC PROPERTIES OF AS-CAST IRON-COBALT ALLOYS
657–662
MAGNETIC PROPERTIES OF AS-CAST IRON-COBALT ALLOYS
MAGNETNE LASTNOSTI LITIH @ELEZO-KOBALTOVIH ZLITIN
Mario Vukoti}
1*
, Alen Ali}
1
, Urban Rupnik
1
, Damijan Miljavec
1
, Jaka Burja
2
1
University of Ljubljana, Faculty of Electrical Engineering, Ljubljana, Slovenia
2
Institute of Metals and Technology, Ljubljana, Slovenia
Prejem rokopisa – received: 2024-06-26; sprejem za objavo – accepted for publication: 2024-09-04
doi:10.17222/mit.2024.1228
The structural and magnetic properties of iron-cobalt alloys (FeCoV) were compared to those of electrical steel alloys. The latter
are widely used for the ferromagnetic cores of electric motors. However, the as-cast and rolled FeCoV alloys, which were ana-
lysed, showed enhanced magnetic properties compared to electrical steel alloys. Moreover, since they can be cast directly into
the final shape of the rotor’s main structure (excluding the pole shoes, which must be laminated) for a wound-rotor synchronous
electric motor, the production becomes more cost-effective and energy-efficient. In addition, the performance of the electric mo-
tor is improved in terms of mechanical output power and energy efficiency.
Keywords: iron-cobalt alloys, ferromagnetic materials, electromagnetic energy conversion
Strukturne in magnetne lastnosti `elezo-kobaltove zlitine (FeCoV) smo primerjali z zlitino elektro plo~evine. Slednja se pogosto
uporablja za feromagnetna jedra v elektromotorjih. Vendar pa so ulite in valjane zlitine FeCoV, ki smo jih analizirali, pokazale
izbolj{ane magnetne lastnosti v primerjavi z zlitinami elektro plo~evine. Poleg tega jih je mogo~e ulivati neposredno v kon~no
obliko in s tem formirati glavni del rotorja (razen polovih ~evljev, ki morajo biti laminirani) sinhronskega elektromotorja z
navitim rotorjem, zato je proizvodnja cenej{a, porabljena energija pa manj{a. Mehanska mo~ na gredi elektromotorja ter
energijski izkoristek motorja se prav tako pove~ata.
Klju~ne besede: `elezo-kobaltove zlitine, feromagnetni materiali, elektromagnetna pretvorba energije
1 INTRODUCTION
Soft-magnetic cores of electromagnetic energy con-
verters, such as electric motors, transformers, and actua-
tors, are commonly built from electrical steel alloys.
These alloys achieve levels of saturation magnetization
at approximately 1.8 T,
1
allowing compact designs of
magnetic circuits. On the other hand, iron-cobalt alloys
(FeCoV) have superior magnetic properties compared to
electrical steel, opening a new path for electromagnetic
structures with ultra-high compactness. The saturation
magnetization of binary iron-cobalt alloys reaches 2.4 T,
which is an approximately 30-% increase from electri-
cal-steel saturation levels, and consequently a significant
reduction in a device’s volume.
2–7
However, a binary
iron-cobalt alloy is extremely brittle and in order to
avoid this as well as improve other mechanical properties
(high strength and creep resistance
8
) and electrical prop-
erties (electrical resistivity
8
), other alloying elements
must be added, e.g., V, Si, Al, Cr, Mn, etc.
8,9
Adding dif-
ferent amounts of these elements will unavoidably de-
crease the initial saturation magnetization of the binary
iron-cobalt alloy, but the decrease can be mitigated with
proper heat treatment between 600 °C and 900 °C. Heat
treatment also improves mechanical properties by in-
creasing strength and ductility, and enhances some mag-
netic properties by decreasing coercivity and core
losses.
8
The temperature of the heat treatment can
enhance either mechanical or magnetic properties, e.g.,
annealing at 600 °C shows the best creep resistance,
8
while annealing at 800 °C provides a good balance be-
tween saturation magnetization and coercivity (2.019 T
and 194 A/m, respectively).
9
FeCoV alloys have shown
the potential for usage in internal combustion engines as
high-temperature actuators with high dynamic perfor-
mance,
9
in electric vehicles as high-efficiency traction
motors,
10–13
and particularly in aerospace applications,
where they can be used in compact and low-weight gen-
erators and actuators.
14–21
What these applications have
in common is their ability to justify the higher initial cost
of components, arising from the high content of the ex-
pensive material (cobalt), due to the energy savings dur-
ing their lifetime.
The focus of this study was on the magnetic proper-
ties of the as-cast ferromagnetic materials for high-per-
formance electric motors, e.g., for the automotive indus-
try, intended for easier and energy-saving production.
FeCoV alloys were used for magnetic cores to enhance
the motor mechanical performance. Alternative produc-
tion methods such as casting, additive manufacturing
(3D printing)
22–24
and powder metallurgy
25
were not used
in this study due to the additional step required for pow-
der production. Moreover, in order to maintain the intrin-
sic material properties, all rough mechanical reshaping,
such as stamping, causing material deterioration
26,27
had
to be avoided. Thus, in the study, only directly as-cast
Materiali in tehnologije / Materials and technology 58 (2024) 5, 657–662 657
UDK 52-334.7:669.15’24’25-196 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 58(5)657(2024)
*Corresponding author's e-mail:
mario.vukotic@fe.uni-lj.si (Mario Vukoti})

magnetic materials and additionally rolled FeCoV alloy
were considered. Such a production procedure could be
considered for a wound-rotor synchronous motor
(WRSM). The WRSM has a DC magnetic field in the ro-
tor, which rotates synchronously with the rotating mag-
netic field (AC) on the stator. Due to the time-invariable
magnetic field there is no electromagnetic induction that
would cause eddy-current losses in the rotor. Therefore,
the internal part of the rotor (shown in turquoise in Fig-
ure 1) could be die-cast as a single component, while the
pole shoes (shown in blue in Figure 1) had to remain
laminated due to the presence of the stator slots. Namely,
the latter causes time-varying higher harmonic compo-
nents in the magnetic field.
For the purposes of magnetic comparison of the ma-
terials, a widely used ferromagnetic material, i.e., electri-
cal steel, was first produced. Its acquired properties
served as the internal benchmark. Then, a custom initial
FeCoV alloy with 49 % Fe and Co, and 2 % V was pro-
duced. Vanadium was added for hardening
28
. Addition-
ally, the influence of rolling on the magnetic properties
of FeCoV was analysed by measuring these properties.
2 EXPERIMENTAL PART
The materials were made from pure elements includ-
ing electrolytic Fe, electrolytic Co, pure V, metallurgi-
cally pure Si, and primary Al. 10-kg charges were melted
in a vacuum induction melting furnace with an alumina
crucible. The metals were cast into 60 × 60 mm ingots,
which were roughly 400 mm long. The as-cast electrical
steel (Fe-Si-Al) served as the introductory sample. Then
a part of the as-cast Fe-Co-V ingot was cut and rolled at
1200 °C to a 20-mm thickness. Metallographic samples
were ground and polished, followed by 5 % Nital etch-
ing. Optical micrographs were taken with a Nikon
Microphot FXA light optical microscope. The chemical
compositions of the alloys were analysed with LECO CS
and ICP-OES. They are given in Table 1.
Three samples in the form of a rod were produced:
as-cast electrical steel, as-cast FeCoV and rolled FeCoV.
They were machined (ground across whole length) to a
diameter of 10.005 ± 0.001 mm in order to accurately
determine their cross-sectional areas. Precise information
regarding the cross-section was needed because the mag-
netic flux density B for the samples was calculated from
the measured magnetic flux. Using the MagnetPhysik
Remagraph, the relation between the magnetic field
strength H (Figure 2a) and magnetic flux density B (Fig-
ure 2b) was obtained as a quasi-static DC hysteresis
loop, in accordance with the IEC 60404-4 standard.
29,30
Each sample was demagnetized prior to each B-H mea-
M. VUKOTI] et al.: MAGNETIC PROPERTIES OF AS-CAST IRON-COBALT ALLOYS
658 Materiali in tehnologije / Materials and technology 58 (2024) 5, 657–662
Figure 2: Temporal measurement of: a) magnetic field strength H; b) magnetic flux density B
Table 1: Chemical compositions of the electrical steel and FeCoV alloy (w/%)
Al Co Si V C Fe
Electrical steel 0.43 / 2.40 / 0.004 Bal.
As-cast/rolled FeCoV / 48.95 / 1.99 0.002 Bal.
Figure 1: a) WRSM design (one magnetic pole); b) current density of
eddy currents in the cast part of the rotor

surement. Then, the DC hysteresis measurement was
performed on the samples by varying the current excita-
tion in the standardized yoke, changing the magnetic
field strength from –5000 A/m to +5000 A/m. Such an
amplitude of the magnetic field strength was sufficient to
reach the saturation levels for all three samples. There-
fore, it represents the measurement of one cycle by the
Remagraph RE3, forming the hysteresis loop (Figure 3).
3 RESULTS
The microstructure of electrical steel consisted of
100 % ferrite (a body centred cubic (BCC) crystal lat-
tice); the average grain size was larger than 1 mm, as
seen in Figure 4; and there was no preferential orienta-
tion of the grains due to the absence of deformation, as it
was in the as-cast state.
The FeCoV alloy has a BCC crystal lattice at room
temperature. As the solidification process can be a bit
more complex, there are still remains of the dendritic
structure, as seen in Figure 5, resulting in a more uneven
grain size ranging from a couple of mm to less than
100 μm.
The rolled FeCoV alloy shows a significant grain-
size reduction, but the grains are elongated, measuring a
few 100 μm in length and several 10 μm in width, as seen
in Figure 6.
The hysteresis loops for the as-cast electrical steel,
as-cast FeCoV and rolled FeCoV are shown in Figures 7
and 8. For comparison, each of the graphs also includes
M. VUKOTI] et al.: MAGNETIC PROPERTIES OF AS-CAST IRON-COBALT ALLOYS
Materiali in tehnologije / Materials and technology 58 (2024) 5, 657–662 659
Figure 5: Microstructure of the as-cast FeCoV alloy: a) at a lower and b) at a higher magnification
Figure 3: Complete measurement of B-H hysteresis loop with the DC
method
Figure 4: Microstructure of the as-cast electrical steel: a) at a lower and b) at a higher magnification

the B-H curve of a commonly used electrical steel
(i.e., Cogent M270-35A, from a particular datasheet
31
).
The area of the hysteresis loop of the as-cast electrical
steel (Figure 7) is small, resulting in a very narrow hys-
teresis loop, hardly visible in the graph. The maximum
values of magnetic flux density reach approximately
B = 1.76 T, which is anticipated for this ferromagnetic
material. Both FeCoV samples show enhanced perfor-
mance in terms of magnetic flux density compared to the
as-cast electrical steel. The measured values exceed
B = 2.2 T (see Table 2). However, the areas of the hys-
teresis loops are visibly larger. The hysteresis loop of the
as-cast FeCoV is somewhat smaller than that of the
rolled FeCoV (Figure 8).
Table 2: Comparison of magnetic properties of the samples
B at
H = 2000 A/m
B at
H = 5000 A/m
As-cast electrical steel 1.603 T 1.762 T
As-cast FeCoV 2.076 T 2.230 T
Rolled FeCoV 2.035 T 2.212 T
4 DISCUSSION
Both FeCoV samples showed higher saturation mag-
netic flux densities compared to electrical steel. The
as-cast FeCoV sample has a smaller area of the hyster-
esis loop and slightly better magnetic performance than
the rolled FeCoV (+2 % at H = 2000 A/m and +0.8 % at
H = 5000 A/m). However, since the magnetic field in the
rotor of a synchronous motor is constant (DC), the hys-
teresis loop area has a negligible influence on the motor
performance. Moreover, it might be even desired in some
cases to have a larger hysteresis loop in order to maintain
magnetization, when the magnetization current is zero
(remanent magnetic flux density) and even more when
there is an opposing magnetic field, excited by the stator
winding. The methods for extending the area of hyster-
esis loops for a WRSM with as-cast rotor structures will
be the subject of the future investigations.
M. VUKOTI] et al.: MAGNETIC PROPERTIES OF AS-CAST IRON-COBALT ALLOYS
660 Materiali in tehnologije / Materials and technology 58 (2024) 5, 657–662
Figure 8: Hysteresis loops of the FeCoV alloys
Figure 7: Hysteresis loop of the electrical steel alloy
Figure 6: Microstructure of the rolled FeCoV alloy: a) at a lower and b) at a higher magnification

5 CONCLUSIONS
The goal of the study was to investigate the magnetic
properties of the as-cast FeCoV material. With the study,
we wished to improve the mechanical performance of the
wound-rotor synchronous motor (WRSM) as well as re-
duce the costs and energy consumption during the pro-
duction of WRSM rotors. The rotor generates a DC mag-
netic field and can be built from a massive part, i.e., there
is no need for laminations. It was shown that the as-cast
FeCoV alloy has excellent magnetic properties, com-
pared to the electrical steel alloy. Consequently, the main
part of the rotor can be made thinner, leading to a lower
mass and higher energy efficiency during the motor op-
eration. Moreover, the as-cast FeCoV alloy also has
slightly better properties than the rolled FeCoV. This is
an important fact as the main magnetic part of the rotor
can then be directly cast into the final shape without the
need for rolling and further excessive machining of the
rotor. Thus, certain steps in the production process can
be omitted, saving the time and energy, and lastly also
the costs.
Acknowledgment
The authors acknowledge that this research was fi-
nancially supported by project ID L2-50084, funded by
the Slovenian Research and Innovation Agency.
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