J. KRANER et al.: A REVIEW OF ASYMMETRIC ROLLING
731–743
A REVIEW OF ASYMMETRIC ROLLING
OSNOVNI PREGLED ASIMETRI^NEGA VALJANJA
Jakob Kraner
1
, Toma` Smolar
2
, Darja Vol{ak
2
, Peter Cvahte
2
, Matja` Godec
1
,
Irena Paulin
1*
1
Institute of Metals and Technology, IMT, Lepi pot 11, 1000 Ljubljana, Slovenia
2
Impol Aluminium Industry, Partizanska 38, 2310 Slovenska Bistrica, Slovenia
Prejem rokopisa – received: 2020-08-12; sprejem za objavo – accepted for publication: 2020-08-30
doi:10.17222/mit.2020.158
The metal-forming industry is increasing its array of products every year. A connection between scientific research and practical
examples from industry keeps improving the rolling processes. A historical overview proves that asymmetric rolling has been an
interesting topic of many studies for more than six decades. Once performed only on steels and aluminium alloys, this process
has outgrown the basic types and its application was extended to other metal alloys. This paper presents a basic review of asym-
metric rolling focusing on activities in the roll gap and special mechanisms in deformation zone. Moreover, the effect of asym-
metric rolling on the rolling force, torque, reduction and deformation was explained and supported with different results from
scientific research. The microstructure and texture differences in comparison to the conventional (symmetric) rolling were dis-
cussed as an improvement of the mechanical properties. Besides the comparative study between simulations and experimental
rolling, regarding the undesirable ski-effect, the discussion in this paper is always reinstating the possibilities of using asymmet-
ric rolling in industry.
Keywords: rolling, types, deformation zone, ski-effect, characterization
Obseg valjanih proizvodov se v metalur{ko-predelovalni industriji iz leta v leto pove~uje. Ob povezovanju znanstvenih raziskav
z dobrimi primeri iz industrije se tudi valjarni procesi in postopki vedno znova izbolj{ujejo. Zgodovinski pregled dokazuje, da je
asimetri~no valjanje zanimiva tematika {tevilnih raziskav `e ve~ kot {est desetletij. Nekaj osnovnih postopkov asimetri~nega
valjanja je v tem ~asu preraslo v posebne na~ine asimetri~nega valjanja, ki so se najprej izvajali na jeklih in aluminijevih
zlitinah, nato pa prehajali na {tevilne druge kovinske zlitine. Pregledni ~lanek predstavlja osnovni pregled asimetri~nega
valjanja, kjer je podrobno razlo`eno dogajanje v valj~ni re`i in so predstavljene posebnosti mehanizmov v deformacijskem
obmo~ju. Nadalje so razlage vplivov na silo in moment valjanja, na redukcijo ter deformacijo podkrepljene z razli~nimi rezultati
opravljenih raziskav. Mikrostrukturna in teksturna razlikovanja v primerjavi s klasi~nim (simetri~nim) valjanjem so obravnavana
kot vzrok za izbolj{anje mehanskih lastnosti. Ob primerjalni {tudiji simulacij in realnega valjanja za ne`elen upogib
obdelovanca se razprava pri ~lanku vedno znova vra~a na mo`nosti uporabe asimetri~nega valjanja v industrijskih obratih.
Klju~ne besede: valjanje, na~ini, deformacijsko obmo~je, upogib valjanca, karakterizacija
1 INTRODUCTION
The importance of activities in the metal forming in-
dustry is nowadays not only due to economic reasons,
but rather due to environmental and social impacts. The
automotive industry represents the main branch of form-
ing industry
1
and is forced to accommodate an increasing
environmental aspect, to a greater extent. Just a few spe-
cial deformation techniques for the improvement of me-
chanical properties with the creation of ultrafine grained
microstructure are known.
2
The equal channel angular
pressing (ECAP)
3
and high pressure torsion (HPT),
4
which are more laboratory limited processes, and on the
other hand, asymmetric rolling, which can be suitable for
use in industrial plants.
2
The same as conventional or
symmetric rolling, asymmetric rolling was successfully
investigated at elevated
5
, as well as at low
6
temperatures.
7
Besides many different metals and their alloys, the desir-
able, positive effects of asymmetric rolling were devel-
oped on products with different shapes.
8
Not only on
thick
9
and thin
10
sheets, it was also performed on strips,
11
wires,
12
billets
13
and foils.
14
2 HISTORY
The theory of asymmetric rolling was already known
and well explained in 1941.
15
More than ten years later,
in 1957, the first laboratory or industrial experiments
were performed and technological results were pub-
lished.
16
With new ideas and developments in 1966, the
researches of single-drive rolling
17
and rolling with dif-
ferent roller diameters were the most intensive.
18
Later,
in 1972, there was more emphasis on different impacts
on the ski effect, as a unique phenomenon.
19
One of the
first models and the finite-element approach for asym-
metric rolling occurred in 1994. In the same year, the
ideas of introducing asymmetric rolling into industry ap-
peared.
20
In 1997 a very important distribution of asym-
metric rolling types and cases was made.
21
With each
year, through decades, asymmetric rolling was becoming
more and more developed, but new challenges and stud-
ies of asymmetric rolling remain.
Materiali in tehnologije / Materials and technology 54 (2020) 5, 731–743 731
UDK 67.017:621.771.8:620.174 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 54(5)731(2020)
*Corresponding author's e-mail:
irena.paulin@imt.si (Irena Paulin)

3 CATEGORIES OF ASYMMETRIC ROLLING
The distribution of asymmetric rolling in most cases
covers four major rolling types. These four as well as
some other special types, are influenced by the velocity,
geometry and friction.
22
In every asymmetric rolling type
one parameter has the highest influence, nevertheless
that each type has an impact on all three mentioned pa-
rameters.
21,23,24
3.1 Different rotation speeds of the rollers
Probably the most appropriate asymmetric rolling
type for industry is rolling with different rotation speeds
of the rollers.
23
The rollers’ dimensions are in that case
the same. The greatest impact on the deformation zone is
the speed difference on the workpiece velocity. The dis-
cussed asymmetric rolling type (Figure 1) can be per-
formed with a faster driven upper roller,
25
or a faster
driven lower roller,
26
which has a different impact on the
friction. Higher friction in the deformation zone has ap-
peared in the area of the faster roller. All the described
effects already arise at very small speed differences, but
more significant asymmetric effect contributions are cre-
ated with 5 m·s
–1
rotation speed difference.
27,28
The factor
of asymmetry f
a
for asymmetric rolling with different ro-
tation speeds of the rollers v was calculated with the
Equation (1):
f
v
v
a
faster
slower
= (1)
3.2 Different diameters of the rollers
The rolling process where asymmetry is provided
with different dimensions of the upper and lower work
rollers, presented in Figure 2, has great impact on the
geometry and results in a strong curvature of the
workpiece. More significant bending is visible at diame-
ter differences of 10 mm, where the lower geometrical ir-
regularities of the workpiece have already appeared in
the case of a lower dimensional difference.
29,30
The speed
of rollers was, for this type of asymmetric rolling, the
same for the upper and lower rollers. A smaller roller,
where the friction is higher, can be installed on the
upper
31
or on the lower
32
position of the rolling mill.
33,34
The factor of asymmetry f
a
was for asymmetric rolling
with different diameters of rollers D, calculated with
Equation (2):
f
D
D
a
larger
smaller
= (2)
3.3 Single-drive roller
Similar to the asymmetric rolling with different rota-
tion speeds, but with greater friction differences in defor-
mation zone, single-drive rolling can be performed (Fig-
ure 3). This type of asymmetric rolling has a higher
impact on the friction and workpiece velocity. In this
case of rolling type the rollers’ diameters are the same.
Higher friction appeared in the contact between the
workpiece and the driven roller. In most cases, the upper
work roller is idle, but it also works the other way
around.
35
A common issue with this kind of asymmetric
rolling is gripping of the workpiece.
36
The presented
asymmetric rolling type is possible to perform with a big
enough grabbing angle and friction on entry of the defor-
mation zone.
37
To calculate the factor of asymmetry f
a
for
single drive roller with the set speed 0 m·s
–1
of one roller
cannot be taken into account. With the as performed roll-
ing set up, the mentioned factor will always be 0 or 1,
which is incorrect. For the calculation of the factor of
asymmetry, the speed v of the idle roller is used, as pre-
sented in the equation:
f
v
v
a
driven
idle
= (3)
J. KRANER et al.: A REVIEW OF ASYMMETRIC ROLLING
732 Materiali in tehnologije / Materials and technology 54 (2020) 5, 731–743
Figure 3: Asymmetric rolling with a single drive roller
Figure 1: Asymmetric rolling with different rotation speeds of the
rollers
Figure 2: Asymmetric rolling with different diameters of the rollers

3.4 Differently lubricated roller surfaces
During the rolling process, lubrication has the great-
est impact on the friction (Figure 4). The roller diame-
ters and rolling speeds are the same with differently lu-
bricated roller surfaces asymmetric rolling type.
38
The
reduction in friction will appear in the case of using dif-
ferent lubricants, as well as in the case of partial lubrica-
tion.
39
A drawback of different lubricants usage is the
possibility of surface contamination.
27,40
The factor of
asymmetry f
a
can be, for different lubricated rollers sur-
faces, calculated as the difference between the friction
coefficients μ:
f
a
higher
lower
=
  (4)
3.5 Special asymmetric rolling types
The special asymmetric rolling types also include
combinations of four basic types, described above. More
cases of rolling with different diameters and different
speeds of rollers are known, as well as the reduction of
friction for a single-drive roller with differently lubri-
cated roller surfaces. Further, the asymmetric rolling
type with displaced axis was performed.
41,42
Also, the
dead block as the interface between the roller and the
workpiece was used.
1,43
Thermal experiments with asym-
metric cryo-rolling were also conducted.
44
Besides new
ways of electro plastic deformations,
45
the mechanical
specifics were studied also on multirole asymmetric roll-
ing.
46,47
Some experiments were also conducted as a
combination of single-drive rolling and equal-channel
angular pressing, the two process for material production
with a fine-grained microstructure.
48
4 ROLL GAP AND MECHANICS IN THE
DEFORMATION ZONE
Due to the introduced asymmetry in the rolling pro-
cess the reduction of material from the upper and lower
rollers are not the same. A higher reduction from the
lower roller will be created, when the mentioned roller is
driven faster than the upper roller.
49
On the other hand, if
the speed of the upper roller is higher, a greater reduction
appears in the area of the upper roller. The speed differ-
ences between the work rollers are the result of a differ-
ent workpiece velocity during the plastic deformation
zone. On entry to the deformation zone, the workpiece
has a lower velocity than both rollers. After passing that
zone, the workpiece has a higher velocity than the slower
roller, but a lower velocity than the faster roller. Upon
exit, the velocity of the workpiece is higher than both the
driven work rollers. Among all the presented velocity
rates, the workpiece is in contact with both rollers.
50
The
contact length L for asymmetric rolling with different ro-
tation speeds of rollers, presented in Figure 5, was cal-
culated with the Equation (5):
LRhh
hh
=⋅−−
−
⎛ ⎝ ⎜ ⎞ ⎠ ⎟ ()
()
12
12
2
2
(5)
where R is the radius of the rollers, h
1
and h
2
present the
entry and exit thicknesses of the workpiece.
23
In combi-
nation with the results of the presented equation, the ap-
proximation of the important parameter  for the end
dimension of workpiece is shown in the Equation (6):
Δ= ≈
⋅−
h
L
h
Rh h
2
12
()
(6)
where all the symbols for the parameters are the same as
in the previous equation.
51
With variations of different
rolling parameters, we can influence the geometric, ki-
netic and mostly friction properties of the workpiece in
the deformation zone. According to the presented un-
equal velocity of the workpiece and rollers, the resulting
rolling forces F, stresses  and strains  have worked dif-
ferently. In the case of the asymmetric rolling, the ten-
sile and compressive forces, and consequently the
stresses and strains, that are also present at the symmet-
ric rolling, produce more of the shear components of
quantities mentioned above. The factor of asymmetry
has a major impact on the creation of a higher share of
the shear quantities with all the known asymmetric roll-
ing types.
40,49
The difference between the forces and
their associated stresses and strains for the symmetric
and asymmetric rolling are presented with the matrix in
Figure 6. The states of the shear-orientated stresses  J. KRANER et al.: A REVIEW OF ASYMMETRIC ROLLING
Materiali in tehnologije / Materials and technology 54 (2020) 5, 731–743 733
Figure 5: Deformation zone of asymmetric rolling with different rota-
tion speed of rollers
Figure 4: Asymmetric rolling with different lubricated roller surfaces

and strains in the matrix depend on the asymmetric
rolling type and the rolling direction.
52,53
F
F
FF
F
(,)
(,)
(,) (,)
(,


  
 00
000
00
0
000
−
⎡ ⎣ ⎢ ⎢ ⎤ ⎦ ⎥ ⎥ ±
±  )( , )
() ()
0 −
⎡ ⎣ ⎢ ⎢ ⎤ ⎦ ⎥ ⎥ F
ab
Figure 6: Matrix of force, stress and strain condition for: a) symmet-
ric rolling, (b) asymmetric rolling
52,53
Because of the unchanged energy balance during the
rolling process, regarding the kinetics and friction differ-
ences, the vertical position of the neutral points in the de-
formation zone has changed. The neutral points, com-
pared to the symmetric rolling, have changed their
vertical positions depending on the asymmetric rolling
type and their kinetics, geometrics or friction differences.
The relationship between the position of the upper neu-
tral point N
u
and the lower neutral point N
l
for asymmet-
ric rolling with different rotation speeds of the rollers are
given by the Equation (7):
NN
h
R
u l
=⋅+−
⎛ ⎝ ⎜ ⎞ ⎠ ⎟ ⋅
!
!
!
!
 "
 "
2
2
1 (7)
where !
2
and !
1
are the peripheral speeds of the faster
and slower roller, R is the radius of the rollers and h
2
is
the exit thickness of workpiece. A delay of the neutral
points has created three zones in the plastic deformation
zone, which are also in conjunction with the speed rela-
tionship between the workpiece and rollers, as well as
with the area of the shear orientated forces. In zone I on
entry and zone III on exit the forces with their belonging
stresses and strains are normal. Zone II, in the middle,
where the velocity of the workpiece is faster than one
roller and slower than the other roller, the area of more
shear directed forces, stresses and strains was cre-
ated.
49,51
The distribution of the plastic deformation
zone on three zones is suitable for asymmetric rolling
with different rotation speeds of the rollers,
49,50
as well
as for asymmetric rolling with different diameters of the
rollers.
29,54,55
In a special case of asymmetric rolling with different
geometric dimensions of the work rollers or with the
roller’s axis displacement, the deformation zone can be
divided into four zones (Figure 7). For that kind of dis-
tribution, the bonding point x
b
must be introduced. Zone
I is in that case from the entry to bonding point x
b
. From
there to the neutral point of the smaller roller x
n1
is zone
II. The continued zone III has taken an area between a
neutral point of a smaller roller x
n1
and a neutral point of
a bigger roller x
n2
. Last but not least, zone IV is a contin-
uation of the previous zone to the exit of the deformation
zone. The length of the deformation zone L for the asym-
metric rolling with different diameters of rollers can be
calculated with the Equation (8):
LRhr
i
=⋅⋅
eq
(8)
where the equivalent work roller radius R
eq
can be deter-
mined by the Equation (9):
R
RR
RR
eq
=
⋅
+
2
12
12
()
(9)
furthermore, h
i
is a present entry thickness of a
workpiece and r is the reduction of the material. R
1,
in
second equation, presents the radius of the smaller roller
and R
2
is the radius of the bigger roller.
41,56
5 TECHNICAL PARAMETERS
Variations of kinetics, geometrics and friction param-
eters change the longitudinal speed of the workpiece in
the deformation zone. Consequential processes in the
area of the upper and lower work roller have an impact
on the critical angle and the forward or backward slip,
which improves the productivity of the rolling operation
in the reduction of the rolling force, pressure and torque,
increases the reduction of material and improves the sur-
face properties of the workpiece. Unfortunately, for the
same reasons we have the unwanted bent part of work-
piece, known as the ski effect.
49,57
5.1 Rolling force and torque
The asymmetric rolling is based mainly on a direct
action being applied on the workpiece in the deformation
zone, where the occurring longitudinal tensile stresses,
owing to the asymmetry in the forming process, have the
effect of reducing the magnitude of unit pressure in the
roll gap. Furthermore, the equalization of the non-uni-
form distribution of the rolled workpiece was created as
a consequence of the mentioned effect. The regulated
distribution over the workpiece shape in thickness,
length, flatness and the cross-section results in the reduc-
tion of the total rolling force.
58
The rolling force applied
across the constant contact length and the workpiece
width present the normal contact stress. This can be di-
J. KRANER et al.: A REVIEW OF ASYMMETRIC ROLLING
734 Materiali in tehnologije / Materials and technology 54 (2020) 5, 731–743
Figure 7: Special distributed deformation zone of asymmetric rolling
with different diameters of the rollers

vided into the deformation resistance of the material and
the longitudinal supporting stress. The reduction of the
longitudinal supporting stresses is, as mentioned, possi-
ble with the creation of longitudinal tensile stresses with
types of asymmetric rolling and is more compatible as a
reduction of the deformation resistance, which depends
on the strain, strain rate and specific temperature.
59
In
simple terms, torque is a product of force and distance,
which are in rolling process the contact length. The roll-
ing force and torque are reduced during the asymmetric
rolling, but each type has its own particular manner
which also depends on the efficiency of the rolling force
and the torque reduction. The impact of the diameter dif-
ference will be apparent in the lower pressures and the
smaller contact distance. With larger geometric differ-
ences, both mentioned quantities decrease more. Accord-
ing to the presented changes in Figure 8a, a decrease of
the rolling force and rolling torque, which need to be
summed as the torque of upper and lower rollers, is pre-
sented. On the other hand, due to impact of the speed dif-
ference, the contact distance stays more-or-less un-
changed. Nevertheless, pressure decreases with a higher
speed difference, similar to the diameter difference. The
rolling force decreases according to the decreased pres-
sure, and the torque of the slower roller consequently in-
creases (Figure 8b). The rolling torque of the faster
roller can be negative and the same as with the diameter
difference summed to the torque of the other roller.
40,60
Compared to symmetric hot rolling, the rolling force
during asymmetric hot rolling of austenitic steel de-
creased with increasing rolling temperature and descend-
ing reduction. The trends of rolling force decrease with
the increasing diameter differences also occurred. Com-
paring the symmetric hot rolling with the asymmetric hot
rolling of AA 5182 aluminium alloy, the rolling forces
were reduced by5%to30%.
61–63
5.2 Reduction and deformation
The decreased magnitude of the rolling force has at
the same time a direct impact on the elastic deflection of
the whole rolling mill and an indirect effect on the roll
gap shape that determines the exit dimensions of the
workpiece.
58
In conjunction with the smaller elastic de-
flection, also the jump of the rollers was less significant.
Overall, the exit thickness of the workpiece will be
closer to the set height of the roll gap. Moreover, the
higher reductions and strains for each rolling pass were
achieved. Besides that, a decrease of the pressure and
better distribution of them throughout the workpiece
with asymmetric rolling enables the metal forming pro-
cess, where higher strains with a lower rolling force were
created. The economic advantages in industry are also in
more rolling passes. In this case, reversed rolling can be
considered. Due to higher strains having been reached,
fewer passes in the rolling schedule are needed to
achieve the desired exit dimensions. Consequently, the
operating costs as well as the wear of the rollers de-
creased.
42,64,65
5.3 Ski effect
Rolling with asymmetric conditions causes bending
of the rolled workpiece towards the direction of one
roller. This technical phenomenon is referred to as the
ski effect. Generally, it is declared as an undesired bend-
ing effect, because further material transport on proceed-
ing process can be obstructed and might damage the roll-
ing mill’s components.
66
Different rolling parameters
have bigger or smaller impacts on the creation of the ski
effect. Besides all the introduced asymmetries, also the
temperature as well as entry and exit thickness of
workpiece influences the creation of the ski effect.
57,66
If
different interface frictions are applied, the curvature of
the workpiece, or the ski effect, always goes towards the
roller with the greater friction. The increased magnitude
of the ski effect is shown with increased draft and fric-
tion difference.
67
With asymmetric rolling, where the
speed difference of rollers is applied, the bending is
more significant, the thicker the workpiece.
68
To reduce
or to eliminate ski effect, the angle of the workpiece at
the entry to the deformation zone needs to be changed.
The inclined entry can be done with a rolling table ad-
justment. The curvature can be also simulated or calcu-
lated on different modes.
40,69
Each calculation or predic-
tion has some special features and other influencing
factors.
70,71
Therefore, the simulations’ approximations
can be very precise or totally mistaken. The type of
J. KRANER et al.: A REVIEW OF ASYMMETRIC ROLLING
Materiali in tehnologije / Materials and technology 54 (2020) 5, 731–743 735
Figure 8: Decrease of rolling force and torque with asymmetric roll-
ing: a) different diameters of rollers (R
l
is diameter of lower roller and
R
u
is diameter of upper roller), b) different rotation speed of rollers (v
l
is rotation speed of lower roller and v
u
is rotation speed of upper
roller)
40

asymmetry has a strong impact on the accuracy and rele-
vance of the predictions and calculations.
72
Simplifica-
tion of the specifics of the ski effect can be explained
with two effects that contribute to the bending of the
workpiece. The first is due to the difference in the axial
strains at the rollers’ surface and the second is due to the
difference in the shear strains at the same inter-surface
area.
50
The calculated and simulated ski effect for labora-
tory rolling is presented in Figure 9a.
56
Figure 9b pres-
ents the real ski effect of a steel workpiece in industry.
24
6 MATERIALS
Among extensive investigations of asymmetric roll-
ing performed on many different metal alloys and com-
pounds, asymmetric rolling was also performed on
low-carbon steels,
73
silicon steels,
74
duplex stainless
steels,
75
austenitic steels,
61
high-strength low-alloy steels
(HSLA)
32
and some others.
76,77
Asymmetric rolling is for
further investigations, besides work hardening alu-
minium alloys from series AA 5xxx
11,36,78
and AA
3xxx,
79
also interesting for some other aluminium alloys.
For example, for the alloys from series AA 6xxx, where
the heat treatment process is important.
80,81
A lot of re-
search was done on pure aluminium.
82
The most often
used for asymmetric rolling is also the magnesium alloy
AZ31.
83,84
Besides the mentioned alloys, the effects of
asymmetric rolling were also studied on high-entropy al-
loys (HEA),
85
tantalum,
86
copper,
87
different titanium
sheets,
88
as well as on zinc alloy sheet.
89
7 MICROSTRUCTURES
To analyse the improvement of the mechanical prop-
erties, a comparison between symmetric and asymmetric
rolling needs to be made.
80,90,91
In Figure 10 the
microstructure of the same material with symmetric and
asymmetric rolling is shown.
92
The microstructure of the
symmetrically rolled material consists of more-or-less
elongated grains, fragmented into equiaxed sub-grains.
With applied asymmetry, significant changes in the
microstructure were observed. A characteristic band
structure appeared.
80,90,91
Compared to symmetric rolling,
the asymmetric rolling leads to a refinement of the
microstructure with very elongated bands fragmented
into sub-grains (Figure 11a). Just like the average crys-
J. KRANER et al.: A REVIEW OF ASYMMETRIC ROLLING
736 Materiali in tehnologije / Materials and technology 54 (2020) 5, 731–743
Figure 9: Ski effect: a) simulation, b) real example in the steel industry (not with the same rolling parameters)
24,56
Figure 10: Microstructure of symmetric and asymmetric rolled aluminium

tal grains size is reduced, the image quality factor is
smaller with an increased factor of asymmetry (Figure
11b). In contrast, the disorientation was increased with a
higher factor of asymmetry (Figure 11c).
90
The ratio of
the grain refinement depends on the material itself, the
temperature of the metal-forming process, the achieved
strain, as well as the asymmetric rolling type and the fac-
tor of asymmetry. Table 1 presents all the mentioned im-
pacts for seven different cases. For all the compared
microstructures, the major purpose of smaller or greater
grain refinement was reached with asymmetric roll-
ing.
5,40,80,90,91,93,94
With the asymmetric rolling, also higher
homogeneity of grain size distribution throughout the
cross-section appeared.
95
When the relations between the
symmetric and asymmetric rolled average grain sizes are
defined, the comparison of the microstructure changes
between the different asymmetric rolling types and the
different factors of asymmetry is open for further investi-
gations.
77,96–98
Besides, the heat-treatment before and af-
ter the deformation process is investigated as key impact
for the microstructure changes.
13,78
8 TEXTURES
Crystallographic texture engineering is a powerful
and effective tool for obtaining differences in the defor-
mation and recrystallization mechanisms. Just like other
microstructure characteristics, the texture has several im-
pacts on the mechanical properties.
99
With asymmetric
rolling the heterogeneity of texture is a desired phenome-
non.
100,101
To understand the differences and effects of
them, the major created texture components and fibres
with conventional deformation processes, for most of
steels with body-centred cubic (bcc), aluminium alloys
and some other non-ferrous metals with face-centred cu-
bic (fcc) and magnesium alloys with hexagonal
close-packed (hcp), need to be known.
99–101
Hot and cold rolling of steel (bcc) has produced tex-
tures with increased  -fibre n||RD ({001}<110> to
{111})<110> and -fibre {111}||ND ({111}<110> to
{111}<112>). After recrystallization, a decrease of  -fi-
bre appeared, so that only -fibre {111}||ND remain.
Asymmetry in the rolling process displaces the -fibre
{111}||ND with shear components in the orientation
space. This displacement is in the opposite direction to
the one where the shear components are reversed.
9,102
In the case of aluminium and aluminium alloys (fcc)
different textures with different texture components are
created with hot and cold rolling. With hot rolling the
most prominent fcc recrystallization texture components
appeared. The cube component {001}<100> has charac-
teristic shifts or scatterings of the orientation peak. Dur-
ing cold rolling, a typical fcc rolling texture develops.
With low strains the connection between the G (Goss)
{110}<001> texture component and B (brass)
{011}<211> as  -fibre is created. More frequently, with
greater strains the connection between C (copper)
{112}<111>, S {123}<634> and B (brass) {011}<211>
as  -fibre is created (Figure 12). The major idea and
purpose of asymmetric rolling is the creation of shear
J. KRANER et al.: A REVIEW OF ASYMMETRIC ROLLING
Materiali in tehnologije / Materials and technology 54 (2020) 5, 731–743 737
Figure 11: Microstructure changes: a) average grain size, b) image quality, c) kernel average misorientation
90
Table 1: Grain refinement of different materials with asymmetric rolling
5,40,80,90,91,93,94
Material
Temperature of
metal forming
process
Strain
(%)
Average grain size
SYMMETRIC rolling
(μm)
Average grain size
ASYMMETRIC
rolling (μm)
Type of asymmetric
rolling
(factor of asymmetry)
Aluminium Alloy 6061 COLD 36 9.0 4.0 Speed difference (1.5)
Aluminium Alloy 6111 COLD 50 25.0 20.0 Diameter difference (1.5)
Aluminium Alloy 6016 HOT 20 90.0 32.0 Diameter difference (1.5)
AR-CMnNb steel HOT 50 7.1 5.8 Diameter difference (1.5)
C-Mn steel HOT 35 14.7 13.6 Idle roller (/)
Low C steel (0.07 w/% C) COLD 50 82.5 78.8 Speed difference (1.3)
Pure Mg COLD 50 2.3 2.1 Diameter difference (1.5)

texture components. Most of them are with a {111}||ND
orientation.
8,99,103–106
In contrast to aluminium and steel, which can be cold
deformed to the desired scales, the reductions to the final
dimensions of magnesium alloys (hcp) have to be per-
formed at elevated temperatures between 300 °C and 450
°C. At ambient temperature, the plastic deformation of
magnesium is limited to two main deformation mecha-
nisms, {0001} basal slip and {10-12} mechanical
twinning. Other slip systems, such as {10-11} prismatic
slip and {10-11} pyramidal slip, have the same < >-slip
direction in common, but they require a larger critical re-
solved shear stress for activation.
99
AZ31 is a typical
magnesium alloy sheet in most investigated cases. It ex-
hibits strong basal-type textures with grain orientations
having basal planes parallel to the sheet plane. The stron-
ger intensity of the basal texture on the surface than in
the centre layer is reduced with asymmetric roll-
ing.
99,107,108
After the general texture is determined, further inves-
tigation can be made on specific texture components and
the ratios between them.
109,110
Throughout the history of
metallurgical processes, the material changes texture.
Nevertheless, some texture components can originate
from the start and cannot be removed. The stability of
the texture is also important.
52,73,111
9 MECHANICAL PROPERTIES
The general purposes of asymmetric rolling are pre-
sented to improve the weak mechanical properties of the
chosen material.
112
For example, the toughness wants to
be improved with asymmetric rolling for HSLA steel
32
as
well as a superior combination of strength and ductility
at HEAs.
85
The most stress was therefore directed to-
wards improving the formability of aluminium alloys.
82
At the same time, the increase of the strength and hard-
ness was observed with asymmetric rolling. Due to prob-
lems with curvature of the workpiece (ski effect), rolling
with more passes is difficult. The idea is to use asymmet-
ric rolling only for the last rolling pass and so create
better mechanical properties.
113
9.1 Strength and hardness
Tensile and yield strength have reached higher values
with asymmetric rolling, with either the same or even
higher strain. The improvement of the properties is, as
well as grain sizes and textures, influenced by the de-
formed material and the asymmetric rolling type.
11,36
In
Figure 13 the stress-strain curves for symmetric rolling
and asymmetric rolling types with different speeds of the
rollers are presented. It is noticeable that the tensile
stresses are more than 50 MPa higher during asymmetric
rolling than during symmetric rolling. It is also important
to note, that with the highest factor of asymmetry, the
tensile strength is not the highest.
114
This can be a conse-
quence of the maximum achieved shear stress and strain
components.
9
The hardness value is not as important as
is its evenly distributed throughout the cross-section of
the workpiece. With asymmetric rolling, higher strains
were achieved, with the same set roll gap and conse-
quently also a higher hardness.
79,90
J. KRANER et al.: A REVIEW OF ASYMMETRIC ROLLING
738 Materiali in tehnologije / Materials and technology 54 (2020) 5, 731–743
Figure 13: Stress–strain curves for symmetric and asymmetric
rolling
114
Figure 12: Major texture components and connected fibres presented in Euler space and with a pole figure

9.2 Formability
To determine the formability, the Erichsen cupping
test for thin samples or plastic strain ratio test for thicker
samples are performed. Besides the general deep
drawability, the anisotropy is very important. That is why
the properties need to be measured in different direc-
tions. The dependence of the deep drawability and aniso-
tropy can be expressed with the Lankford factor (R),
which is a combination of the elongation and shrinkage
of the tested sample. According to the mentioned factor,
the normal anisotropy (R) can be calculated with the
Equation (10):
R
RRR
=
++
04 59 0
2
4
(10)
and the planar anisotropy  R with the equation:
ΔR
RRR
=
−+
04 59 0
2
2
(11)
where R
0
, R
45
, and R
90
are the Lankford factors in differ-
ent directions according to rolling the direction (Fig-
ure 14). With asymmetric rolling and texture heteroge-
neity, the value increases and the  R value
decreases.
64,82,115–117
10 SIMULATIONS
Nowadays, computer modelling and simulating
makes possible almost anything. Asymmetric rolling is
no exception.
118
Simulations of the temperature
119
and
mechanical
120
different rolling processes can save a lot of
time and reduce the costs of laboratory investigation.
With the simulation of asymmetric rolling it is possible
to predict the ski effect,
70,71
microstructure,
42
texture
113,121
and mechanical properties.
53
However, many simulated
asymmetric processes are backed up with experimental
works,
115,122–124
but the real industrial conditions with all
the impact factors are very unlikely to be taken into ac-
count.
125–127
That is why the results of asymmetric rolling
simulations are staying for now as only very good ap-
proximations.
11 CONCLUSIONS
Asymmetric rolling is a specific metal-forming pro-
cess divided into different types regarding the velocity of
rollers, the rollers’ geometry and the friction changes, as
directly influenced parameters. Deliberate asymmetry in
the rolling process can be created with different rotation
speeds of the rollers, different diameters of the rollers, a
single-drive roller and with differently lubricated roller
surfaces. Besides the listed options, the asymmetric roll-
ing includes combinations of basic methods and a wide
range of special asymmetric rolling types.
The behaviour in the deformation zone is, with asym-
metric rolling, different than with symmetric (conven-
tional) rolling. The delay of neutral points creates addi-
tional areas between the entry and exit zone, where the
larger shear forces, strains and stresses appear. The de-
formation zone has an effect on the technical parameters,
such as the rolling force and the torque. Especially the
rolling force reduction is distinguished with asymmetric
rolling, as it has an additional option to influence the re-
duction and strain during the technological process.
Higher achieved strains with asymmetric rolling enable
fewer passes in the rolling schedule and as a conse-
quence contributes to an improvement of the rolling pro-
cess’ economy. The bending of the workpiece during
rolling, also known as the ski effect, presents a major
problem for the widespread use of asymmetric rolling in
industry because it can cause damage to the rolling mill.
With asymmetric rolling, different mechanical prop-
erties can be improved due to significant microstructure
changes. Grain size refinement is most often presented as
the main advantage of asymmetric rolling in comparison
with symmetric rolling, as the pronounced refinement al-
most always appears with a higher factor of asymmetry.
The higher achieved tensile strength, yield strength,
hardness and elongation are closely related to the
microstructure changes with asymmetric rolling. The
changes and differences in the crystallographic texture
are directly connected to the improved formability and
decreased anisotropy. The heterogeneity of the texture is
a desired phenomenon and is due to more shear texture
components, easier to reach with asymmetric rolling than
with symmetric rolling.
Despite the adjustability of asymmetric rolling simu-
lations, where the technological and mechanical proper-
ties can be predicted in the details, the reality from in-
dustry rolling is still overlooked in a lot of aspects.
Asymmetric rolling represents many different advan-
tages and improvements of material properties, benefi-
cial to the metal-forming industry, but the problem of its
implementation and realisation already exists for several
decades. The primary challenges are in mechanical engi-
neering to solve the problems with appropriate construc-
tion of rolling mills in industry for asymmetric rolling.
J. KRANER et al.: A REVIEW OF ASYMMETRIC ROLLING
Materiali in tehnologije / Materials and technology 54 (2020) 5, 731–743 739
Figure 14: Schematic presentation of asymmetric rolling and the sam-
ple directions for the mechanical properties

Acknowledgements
This work was performed with help of many years of
good cooperation between the Impol Aluminium Indus-
try and the Institute of Metals and Technology. We grate-
fully acknowledge the financial support of the Republic
of Slovenia – Ministry of Education, Science and Sport
and of the European Union – the European Regional De-
velopment Fund, which enabled the MARTIN
programme (grant number OP20.03531), in framework
of which the presented work was carried out. The au-
thors also acknowledge the financial support from the
Slovenian Research Agency, research core funding No.
P2-0132.
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