Journal of Materials Processing Technology 112 (2001) 199±204
Core loss reduction in grain-oriented silicon steels by
excimer laser scribing
Part I: experimental work
Satish V. Ponnaluri, Ramachand Cherukuri*, P.A. Molian
Department of Mechanical Engineering, Iowa State University, Ames, IA 50014, USA
Received 25 August 2000; received in revised form 25 August 2000; accepted 2 March 2001
Abstract
In the fabrication route of core laminations used in motors and transformers, a laser is often considered to scribe the steel surfaces after
cold-rolling and annealing in order to reduce the energy losses associated with hysteresis and eddy currents. In this work, a 248 nm
wavelength, 23 ns pulsed excimer laser was used to scribe the grain-oriented electrical steel grade M-4. A core loss reduction of 26%
maximum has been achieved under a speci®c set of laser parameters. This is substantially higher than normally possible (10%) with the
traditional Nd:YAG and CO2 lasers. The improved core loss reduction was attributed to the bene®cial thermal stress distributions developed
during short-pulsed excimer laser scribing process that in turn re®ned the magnetic domains and reduced the eddy current losses. Part II will
describe a comprehensive, ®nite-difference thermal model that is capable of predicting thermal stresses and correlating the stress
distributions with laser parameters. The model will facilitate obtaining the optimum laser parameters that will further reduce the core
losses because there is potential for reducing the core losses up to 70% in grain-oriented steels. # 2001 Elsevier Science B.V. All rights
reserved.
Keywords: Core loss; Internal stresses; Magnetic domains; Laser scribing
1. Introduction
Electrical steels that contain silicon have the largest
market in the ®eld of soft magnetic materials. These steels
are used for numerous core-laminated products, ranging
from small, inexpensive electrical clock motors to large,
power demanding distribution transformers. When these
steels are magnetized in motors and transformers under
alternating current conditions, heat or the so-called core
loss is produced. Core loss consists of two main components: hysteresis loss and apparent eddy current loss. Apparent eddy current loss is a combination of classical eddy
current loss and domain-dependent eddy loss [1]. Core loss
is strongly dependent on silicon content, thickness, impurities, resistivity, permeability, and domain structure of the
steel (a domain is a region in which the magnetic moments
of all the atoms are aligned in a particular direction).
Electrical steels contain silicon in the range 0.5±5%.
Addition of silicon decreases both eddy-current and hyster-
*
Corresponding author. Tel.: ‡1-515-292-9580; fax: ‡1-515-294-3261.
E-mail address: ramcc@iastate.edu (R. Cherukuri).
esis losses and reduces the density of the steel but increases
its brittleness. The impurities, including carbon, in the steel
severely degrade the soft magnetic characteristics, hence,
they should be kept as low as possible. Electrical steels can
be divided into two categories, non-oriented and grainoriented. Non-oriented steels, considered to be isotropic
in nature, contain 0±3% of silicon, the best grades having
the highest silicon content [2]. These steels are mostly used
for rotating electrical machinery, including motors. Other
applications include welding transformers, medium generators, reactors, and recti®ers. The core loss in these steels,
ranging from 0.5 to 2.0 W/kg at 60 Hz and 1.5 T [3], is
dominated by the hysteresis loss, which constitutes about
60±70% of the total loss. Non-oriented steel grades commonly available in today's market are M-13, M-15, M-19,
M-22, and M-36 [3]. Grain-oriented steels, which are anisotropic in nature, contain 3±3.8% of silicon. These steels
are much more ef®cient but are useful only in stationary
electrical machinery. They exhibit superior magnetic properties along the rolling direction. Typical applications are
distribution transformers and large generators.
Grain-oriented steels are developed by an intelligent
process design that involves a combination of heat treatment
0924-0136/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved.
PII: S 0 9 2 4 - 0 1 3 6 ( 0 1 ) 0 0 5 7 3 - 8
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S.V. Ponnaluri et al. / Journal of Materials Processing Technology 112 (2001) 199±204
and cold-rolling processes to facilitate a speci®c crystallographic texture, accounting for the excellent magnetic
properties in the rolling direction [2]. The heat treatment
step causes secondary recrystallization, which permits the
grains to grow to a large size (up to 10 mm) along the rolling
direction but suppresses the growth of grains in other
directions. If a high proportion of grains have crystallographic [0 0 1] directions close to the rolling direction and
(1 1 0) planes close to the plane of the sheet, then the core
loss reduction and permeability [2] can be substantially
higher. However, not all the grains have the ideal
[0 0 1](1 1 0) texture, but most are within 68 to the rolling
direction. The core loss in these steels range from 0.3 to
0.5 W/kg at 60 Hz and 1.5 T. About 75% of the total loss in
these steels is due to classical and anomalous eddy current
losses. These steels are available as M-2, M-3, M-4, and M-6
[3]. The factors that affect the core loss in these steels
include orientation and size of the grains, inclusions and
impurities, internal stresses, silicon content, thickness, and
degree of domain re®nement [4].
Cores in transformers and motors are constructed of
laminations so as to decrease eddy current losses. It is
important that the laminations be separated or insulated
from each other so that the eddy currents will not ¯ow from
one to the other. The presence of natural oxide ®lm is not
adequate to provide this interlamination resistance. As a
result, the grain-oriented steels are coated externally with
phosphate coatings.
A key problem that prevents grain-oriented steels from
achieving minimum core loss is the presence of internal
stresses. The sources of internal stresses are impurities, cold
rolling, and insulative coating that induce residual stresses
and distortion. For example, the phosphate coating results in
internal tensile stresses estimated at 1.2 MPa in the rolling
direction, which causes problems such as core loss and
magnetostriction [2].
The objective of this work is to use a laser scribing process
to reduce core losses in grain-oriented steels by eliminating
unfavorable internal stresses and providing favorable stresses that re®ne the magnetic domains. If the stress is tensile in
the rolling direction, it can re®ne the domain wall spacing
and reduce core loss. On the other hand, if the stress is
compressive, then it tends to increase core loss. For example,
a compressive stress of 6 MPa has been shown to increase
core loss by 30% in grain-oriented steels [2].
2. Domain re®nement techniques
Researchers have sought methods to produce better magnetic properties in grain-oriented steels. Consequently,
newer techniques known as domain re®nement techniques
(DRT) are being investigated to reduce the core losses in
grain-oriented steels by reducing the domain size in the
material. DRT techniques include mechanical scratching,
plasma irradiation, spark ablation, and laser scribing. While
all these techniques improve the core loss reduction, laser
scribing is the most attractive because of its ef®ciency, ease
of implementation, non-contact nature, and relative lack of
damage to the surface coating. Armco, an US based steel
manufacturer, has already implemented laser scribing in its
production line.
In 1924, Haynes and Wolford [5] ®rst demonstrated that
mechanical scratching of steels to produce lines at regular
intervals and perpendicular to the rolling direction reduced
the core losses. In the 1970s, Pepperhoff and Fieldler [6]
obtained a core loss reduction of 40% by cutting ®ne grooves
at regular intervals perpendicular to the rolling direction.
The mechanical scratching technique was most effective in
large grain materials. However, this process was too slow to
be successfully implemented in practice. It also damaged the
insulation coating and caused tool wear because of the
extreme hardness of the phosphate coating [7].
Another technique, called the spark ablation method, was
developed to produce a favorable stress pattern that would
reduce the core losses. Snell and Lockhart [8] using the
spark ablation technique achieved a core loss reduction of
up to 15% by producing controlled lines of spots, 0.05 mm
in and 0.5 mm apart. The spots were produced by high
surface energy sparks generated by 5±10 kV source. A postre®nement operation, application of a thin, inorganic
coating, was required to compensate for the damage to
the coating [7].
The major breakthrough to enjoy commercial success
came when Nippon Steel employed lasers for scribing steels.
Luchi et al. [9] used a Q-switched Nd:YAG laser to perform
laser scribing on grain-oriented silicon steels. The laser
beam was focused to a spot size of 0.15 mm for pulse
energy of 3.75 mJ. Scribing was done perpendicular to
the rolling direction, with a scribe spacing of 5 mm. Core
loss was reduced about 10% under optimum laser irradiation
conditions at 1.7 T and 50 Hz. However, damage was caused
to the surface coating because of high intensities associated
with Q-switched lasers.
Neisheisel and Schoen [10] used Nd:YAG lasers for
scribing grain-oriented 3% silicon steels. Three types of
Nd:YAG lasers Ð continuous wave, normal pulsed, and Qswitched Ð were used. A maximum core loss reduction of
10% was again reported. The continuous wave laser did not
damage the coating suf®cient enough to warrant a replacement. Krause et al. [11] used a 32 W continuous CO2 laser to
study laser scribing of silicon steels and observed a core loss
reduction of 9% at 1.7 T. However, CO2 lasers are not
acceptable because of the high absorption of glass-like
materials, used as surface coatings, of the 10.6 mm radiation
[1].
The DRTs described so far are applicable to stack core
laminations but not suitable to wound magnetic cores, which
requires annealing to remove the winding and bending
stresses. In such cases, laser irradiation followed by nitric
acid etching has been shown to create re®nement that is
stable after recoating and stress relief annealing.
S.V. Ponnaluri et al. / Journal of Materials Processing Technology 112 (2001) 199±204
201
2.1. Rationale for research
3.2. Excimer laser
Although laser scribing is technically and economically
meritorious, two issues warrant further research. One is the
percentage of core loss reduction. To dates scribing results
show a reduction of about 10% in core loss. It is theoretically
possible to achieve a core loss reduction up to 70% if the
process is better understood and a better laser tool is used.
The other issue is the damage of coating by the laser
irradiation. A continuous wave, low power laser is normally
suggested in order to minimize the coating damage. However, if a laser of suitable wavelength that can transmit the
light through the glassy, silicate coating can be found, then
coating damage can be completely suppressed. In this
investigation, we chose an excimer laser operating at a
wavelength of 248 nm that is capable of superior scribing
and, consequently, offers enhanced core loss reduction. In
addition, the insulation coatings on the grain-oriented steels
are transparent to the 248 nm laser beam.
An excimer laser of wavelength 248 nm and a pulse width
of 23 ns was used. Laser scribing requires tight beam
focusing at the work surface and precise control of pulse
power and shape. The key to improving scribing ef®ciency
and quality is not the level of absolute power a laser
produces, but the spatial and temporal pro®le of the laser
beam at the work surface [13]. It has also been proven that
lasers with higher wavelength would damage the surface
coatings [1]. Excimer lasers with a wavelength lower than
that of Nd:YAG …l ˆ 1064 nm† and CO2 l ˆ 10,600 nm,
small pulse width, and the capability to be focused to very
small spot sizes would be a good choice for scribing the steel
without damaging the surface coating. The speci®cations of
the excimer laser (Lambda Physik, Model LPX 110) used for
the experiments are given in Table 1.
3. Experimental details
3.1. Samples
The samples used in this work, 0.3 mm thick M-4 steels
insulated by AISI …C2 ‡ C5† coatings, were supplied by
Pohong Iron and Steel, Korea. After being manufactured,
these steels were annealed to a temperature of 8008C for 2 h
to relieve the stresses that were generated in the process of
shearing and punching [12].
3.3. Laser scribing
Laser scribing refers to partial cutting or perforation by
creating a line of blind holes. Grain-oriented silicon steel
samples, 300 mm 30 mm 0:3 mm, were prepared and
cleaned prior to scribing. A schematic of the experimental
setup for laser scribing is shown in Fig. 1. The beam delivery
system for steering the beam from the laser to the work plane
consisted of two telescope lenses (200 and 100 mm focal
lengths), a variable aperture, a turn mirror, and an image
lens. The telescope lenses, set in a confocal con®guration,
were used to reduce the original beam size to half. The
variable aperture consisted of two sets of knife-edges, which
Table 1
Speci®cations of excimer laser
Medium
Wavelength (nm)
Pulse energy (mJ)
Repetition rate (Hz)
Pulse width (ns)
Beam size (mm)
KrF
248
0±300
1±100
23
10 20
Fig. 1. Schematic representation of the experimental setup for excimer laser scribing.
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S.V. Ponnaluri et al. / Journal of Materials Processing Technology 112 (2001) 199±204
were moved to change the spot size on the sample. The
turning mirror was used to redirect the laser beam on to the
sample surface. The 100 mm focal length image lens was
used to demagnify the aperture (object) size on the sample
surface. The spot size was calculated using the demagni®cation ratio M ˆ O=I, where O is the distance between the
aperture and the image lens, and I the distance between the
image lens and the sample.
The samples were mounted onto a computer-controlled
X±Y table and moved at variable speeds (12±38 mm/s).
Scribing was performed with pulse energies at the sample
surfaces varying from 20 to 100 mJ. The pulse repetition
rates were set at 10, 20 and 50 Hz. Two spot sizes,
1:9 mm 0:7 mm and 1:2 mm 0:7 mm, were used. The
scribe spacing (distance between two successive laser scans)
was made constant at 4 mm, which had been found optimum
in a previous study [1]. During scribing, a gas stream at the
imaging lens was used to protect it from the debris evolved
from vaporization. The samples were scribed along the
rolling direction (longitudinal) and perpendicular to the
rolling direction (transverse). The core losses were measured using Epstein test apparatus (ASTM standard A34 and
A343) at 50 Hz along the longitudinal and transverse directions at 1 and 1.7 T.
4. Results and discussion
Table 2 and Fig. 2 present the data on the effects of pulse
energy on the core loss reduction of samples laser-scribed
along the rolling direction (longitudinal). The core loss was
also measured in the same direction. At low pulse energies,
there is a negative effect on the core loss reduction possibly
due to surface roughening effects. However, as the pulse
energy increases, the core losses decrease and tends to
saturate at about 76 mJ. The data shows that the best
achievable core loss reduction (2.7%) is only marginal
and suggests that it is not a prudent approach to laser scribe
in the rolling direction.
Table 3 and Fig. 3 show the core loss data of the samples
scribed in the transverse direction (perpendicular to the
rolling direction). When the core loss was measured in
the longitudinal direction, a core loss reduction of as much
as 10% was obtained. This is a signi®cant improvement
compared to the scribed samples in the longitudinal
Fig. 2. Variation of core loss reduction with pulse energy for longitudinally scribed samples (scan velocity ˆ 20 mm=s, repetition rate ˆ
10 Hz, spot size ˆ 1:9 mm 0:7 mm).
Table 3
Core losses in longitudinally and transversally measured samples at 20 Hz
Scan velocity
(mm/s)
Core loss (W/kg)
Before scribing
After scribing
Longitudinally measured
12.7
1.006
19.3
1.024
25.4
0.973
31.75
1.005
37.5
1.047
0.988
0.932
0.906
0.922
0.943
Transversally measured
19.3
4.53
31.75
4.74
45.2
4.47
3.722
3.620
3.735
% reduction
in core loss
1.79
8.97
6.9
8.27
9.94
17.8
23.7
16.5
Table 2
Core losses before and after longitudinal scribing
Energy (mJ)
77
64
51
38
Core loss at 1.0 T (W/kg)
Before scribing
After scribing
0.352
0.367
0.336
0.358
0.344
0.359
0.336
0.359
% reduction
in core loss
2.27
2.18
0
0.28
Fig. 3. Variations of core loss reduction with scan velocity for
transversally scribed samples (energy ˆ 64 mJ, repetition rate ˆ 20 Hz,
spot size ˆ 1:2 mm 0:7 mm).
S.V. Ponnaluri et al. / Journal of Materials Processing Technology 112 (2001) 199±204
203
Table 4
Core losses in longitudinally and transversally measured samples at 50 Hz
Scan velocity
(mm/s)
Core loss (W/kg)
Before scribing
After scribing
% reduction in
core loss
Longitudinally measured
19.3
1.013
31.75
1.04
45.2
1.018
1.052
1.049
1.004
3.8
0.9
1.4
Transversally measured
19.3
4.7
31.75
4.73
45.2
4.87
3.402
3.514
3.541
27.7
25.8
27.4
direction and is simply attributed to cutting across the grain
where a large number of domains exist. This result is also
about the same as obtained with the Nd:YAG lasers [1].
When measured in transverse direction, core loss reduction
of 24% is obtained. Two distinct observations can be made
with reference to Fig. 3. One is the increase in core loss
reduction with an increase in scan velocity. Second is that
the core loss reduction was greater when the material is
measured in transverse direction. In an attempt to further
enhance the effect of laser scribing, a higher pulse repetition
rate was employed. Table 4 and Fig. 4 provide the data that
show the effects of higher repetition rate. In the case of
longitudinally measured samples, high repetition rate had a
detrimental effect on the core loss. The pulse separation
Fig. 4. Variation of core loss reduction with scan velocity for transversally
scribed samples (energy ˆ 64 mJ, repetition rate ˆ 50 Hz, spot size ˆ
1:2 mm 0:7 mm).
distance, given by the ratio of scan velocity and repetition
rate, was made smaller by increasing the pulse repetition
rate. When the pulses are nearby, the stresses generated by
one pulse are relieved by the consecutive pulse and, consequently, the bene®cial effects are nulli®ed. It is further
supported by the observation that high repetition rate did not
effect the core loss reduction in transversally measured
samples.
Fig. 5. Domain structure of silicon steel after laser scribing (sample speed ˆ 31:75 mm=s, repetition rate ˆ 50 Hz, spot size ˆ 1:9 mm 0:7 mm).
204
S.V. Ponnaluri et al. / Journal of Materials Processing Technology 112 (2001) 199±204
It is evident from the results that the samples have to be
scribed in the transverse direction in order to obtain the best
results. The investigation shows that there is interplay
among pulse energy, pulse repetition rate, scan velocity,
and core loss measuring direction in affecting the core loss.
It is clear that higher pulse energies are not bene®cial.
Higher pulse repetition rates appear to have a favorable
effect on the transverse-direction core loss as opposed to
longitudinal-direction core loss. The scan velocity increases
the core loss reduction in both directions.
Fig. 5 shows the magnetic domain structure of the material before and after laser treatment. The magnetic domains
were observed using a Bitter technique. In this technique, a
drop of colloidal dispersion of magnetic iron oxide is placed
on the surface of a magnetic material. The colloidal gets
attracted by the stray ®elds and outlines the domain structure
[14]. It was found that the 180o domain wall spacings were
re®ned by the laser induced spot arrays.
The reduction in core loss when a material is subjected to
laser scribing is mainly attributed to domain re®nement.
Grain-oriented steels have large grain size, resulting in large
magnetic domains. These large magnetic domains give rise
to a large loss as the domain walls move back and forth
under the action of alternating ®eld. If the domain size is
kept small, the walls do not move as far and less energy is
lost in moving them. When the material is subjected to laser
scribing, the laser beam imparts a thermal shock to the
surface of the material. This thermal input is localized to a
small spot on the surface and is delivered in a very short time
frame. Because the surrounding material is not affected, the
heated area is constrained to react only by expanding out of
the plane of the material surface. This rapid surface displacement launches a dilatational wave that propagates the
stress distribution throughout the depth of the material. The
resultant strain is believed to cause a slip in the plane that
provides the new wall boundaries [1].
5. Conclusions
Improvements are needed in the energy ef®ciency of
grain-oriented steels used as laminations in distribution
transformers and power transformers. In this work, excimer
laser scribing was demonstrated to be superior to Nd:YAG
and CO2 lasers in reducing core losses. The bene®cial effect
of laser scribing is attributed to the re®nement of domains
due to the stresses induced in the process.
Acknowledgements
The authors acknowledge the ®nancial support provided
for this research by the Power Af®liate Program of Electrical
Power Research Center at Iowa State University.
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