Y. HUANG et al.: MICROSTRUCTURAL EVOLUTION AND MATERIALS FLOW IN BUTT COLD WELDING OF COPPER
799–805
MICROSTRUCTURAL EVOLUTION AND MATERIALS FLOW IN
BUTT COLD WELDING OF COPPER
MIKROSTRUKTURNI RAZVOJ IN PLASTI^NO TE^ENJE
MATERIALA MED HLADNIM ^ELNIM VARJENJEM BAKRA
Yong Huang
1,2*
, Xiao-Juan Yan
1
, Xiao-Long Ran
1
1
Lanzhou University of Technology, State Key Laboratory of Advanced Processing and Recycling of Nonferrous Metals,
No. 287, Langongping Road, Qilihe District, Lanzhou 730050, Gansu, China
2
Lanzhou University of Technology, School of Materials Science and Engineering, Lanzhou 730050, China
Prejem rokopisa – received:2019-11-27 ; sprejem za objavo – accepted for publication: 2020-07-27
doi:10.17222/mit.2019.286
In this study, deformation behavior and microstructure evolution of pure copper during butt cold welding were investigated. The
relationship between the strain during bonding and the corresponding microstructure evolution was discussed. A finite-element
model (FEM) of butt cold welding was established to explore the relationship between the strain and metal bonding. The strain
in the extrusion centre line reaches a value of 2.6, at which bonding starts to occur in this material. An electron-backscatter-
diffraction (EBSD) analysis and optical microscopy were conducted in the corresponding regions and the microstructural
evolution with the strain was examined. Further study of the microscopic evolution revealed that the DRX degree was
significantly improved with the increase in the strain, indicating that recrystallization plays a very important role in copper cold
welding.
Keywords: copper, butt cold welding, finite-element simulation, deformation behavior, microstructure evolution
V ~lanku avtorji opisujejo raziskavo deformacijskega obna{anja in razvoja mikrostrukture ~istega bakra med hladnim ~elnim
varjenjem. Obravnavali so deformacijo nastalo med spajanjem (varjenjem) in ustreznim razvojem mikrostrukture. Izdelali so
model hladnega ~elnega varjenja na osnovi metode kon~nih elementov (FEM) zato, da so lahko raziskovali povezavo med
deformacijo in spajanje dveh kosov bakra med njunim hladnim zvarjanjem. Deformacija v sredi{~ni liniji ~elnega zvara pri
kateri je nastopilo spajanje materiala je dosegla vrednost 2,6. Z analizo na osnovi povratno sipanih elektronov (EBSD) in z
opti~no mikroskopijo so v karakteristi~nih podro~jih zvara raziskovali razvoj mikrostrukture in deformacijo. Nadaljnji {tudij
razvoja mikrostrukture je pokazal stopnjo dinami~ne rekristalizacije (DRX) in da se le ta mo~no izbolj{a z nara{~anjem
deformacije. To ka`e na to, da rekristalizacija igra pomembno vlogo med hladnim varjenjem bakra.
Klju~ne besede: baker, ~elno varjenje, simulacija na osnovi metode kon~nih elementov, deformacijsko vedenje, razvoj
mikrostrukture
1 INTRODUCTION
Copper plays an extremely significant role in many
industry areas, such as electronic vehicle and nuclear
industries, due to its excellent thermal conductivity and
favorable combination of strength and ductility. As they
are conducting materials in many applications, high
electrical conductivity and high mechanical strength are
often required simultaneously.
1
In this context, the
solid-state process (SSP) offers an ideal basis for
application in such structures with no fusion zone and no
heat-affected zone (HAZ).
2
This process has been shown
suitable for joining a variety of materials with similar or
dissimilar configurations. Cold pressure welding (CPW)
is one type of the SSP, mainly carried out in two basic
variants: spot welding and butt welding. A butt-welding
process involves surface expansion only, and the bonding
starts with the formation of small cracks in the oxide
layer during plastic deformation and proceeds with the
extrusion of the underlying oxide-free metal through the
cracks.
On the other hand, in cold pressure welding, the
surface oxide is continuously removed during the com-
pression due to the upset collar formation.
3
There are
several theories of the weld formation in CPW, which
mainly include metallurgical adhesion derived from a
significant amount of plastic deformation, diffusion
across the weld interface, local melting, mechanical
interlocking and dynamic recrystallization.
4
For similar-
material joints, dynamic recrystallization (DRX) was
found to be the main bonding mechanism. In the case of
dissimilar materials, diffusion promotes the solid-solu-
tion strengthening phase at the weld interface.
5
There has
been a considerable research effort aiming at under-
standing the origin of recrystallization. The importance
of recrystallization nucleation is that it is a crucial factor
in determining both the size and orientation of the result-
ing grains. Strain-induced grain-boundary migration
(SIBM) is a dominating mechanism, and it has been
observed in a wide variety of metals. SIBM involves the
bulging of part of the pre-existing grain boundary,
Materiali in tehnologije / Materials and technology 54 (2020) 6, 799–805 799
UDK 67.017:621.791.725:669.715 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 54(6)799(2020)
*Corresponding author's e-mail:
hyorhot@lut.cn (Yong Huang)

creating a region with a lower dislocation content behind
the migrating boundary.
6
Severe plastic deformation and high temperature
(higher than the recrystallization temperature) may cause
the formation of new crystalline structures at the inter-
face.
7
DRX can occur in the continuous or discontinuous
mode depending on the material and deformation mode.
A continuous process involves the rotation of subgrains,
which can be observed simultaneously in the entire
microstructure, whereas discontinuous dynamic recrys-
tallization (DDRX) involves an inhomogeneous
nucleation step followed by the growth of recrystallized
grains due to high-angle grain-boundary (HAGB) mig-
ration. Moreover, continuous dynamic recrystallization
(CDRX) is associated with a low stacking-fault energy
(SFE) due to an easier cross slip and recovery, whereas a
high SFE typically results in a discontinuous process.
Under the shear or shear-compression state, DRX occurs
locally through a mechanism similar to the continuous
process described above, involving the formation of
elongated subgrains or bands, which subsequently break
up into an equiaxed structure with the local rotation of
subgrain boundaries. This also takes place in medium to
high SFE materials such as copper, due to the
localization of plastic deformation into shear bands
resulting from the adiabatic heating and/or geometrical
constraints.
8
It was reported that bonding does not occur until a
certain threshold deformation is reached. After reaching
this threshold, depending on the metal combination, the
bonding strength increases rapidly with the degree of
deformation and reaches a plateau corresponding to the
strength of the base metal.
9,10
This mechanism was
confirmed
11,12
with scanning-electron-microscopy (SEM)
studies of a fractured welding surface by different re-
searchers. It was found that the surface preparation
before cold welding is an important parameter, govern-
ing the weld formation.
13
It was also found that the
welding quality is determined by the metal-flow
behavior, solid-state bonding process and microstructural
evolution.
14
Consequently, it is of great significance to understand
the parametric conditions, under which cold bonding
takes place during the divergent extrusion of butt cold
welding (BCW) of copper. In the present investigation,
the divergent extrusion of butt cold welding of pure
copper was further discussed and explored to reveal the
conditions under which cold bonding takes place.
Macro- and microscopic analyses, and FE modeling
were jointly used to reveal the details of the ma-
terial-flow pattern and deformation mode at the mating
interfaces during bonding, and establish a simple strain-
based criterion for metallic bonding. Furthermore, the
corresponding microstructure evolution was discussed
and results of the macroscopic analysis were provided to
validate the FE modeling.
2 EXPERIMENTAL PART
Commercially pure copper (T2) rods with a purity of
99.9 %, a diameter of 8.0 mm and a length of 18 mm
were prepared for bonding with butt cold welding at
room temperature, using a die as shown in Figure 1.
These welding experiments were carried out using a
YT100 four-column press with a speed of 10 mm/s. All
the experiment specimens were degreased and scratch-
brushed using a kind of efficient surface-preparation
method
9,15
prior to welding. After the surface pre-
paration, handling of these rods was performed carefully
to avoid renewed contamination. The time between the
surface preparation and compression was kept to less
than 120 s to avoid the formation of a thick and
continuous oxide layer on the bonding surfaces of these
rods.
Following the BCW process, a 1-mm welding gap
between the two positioning moulds is ensured. The
samples for the microstructural examination were
prepared by slicing the rods perpendicular to the
extrusion direction into  2.0-mm-thick slices, and then
mechanically polished until a mirror-like surface was
achieved. Finally, those slices were electro-polished for
6–8 min in a solution of 50 % distilled water, 25 %
phosphoric acid and 25 % alcohol, using the 4-V
polishing voltage. Electron-backscatter-diffraction
(EBSD) measurements were taken using a Quanta
FEG-450 high-resolution scanning-electron microscope
(SEM) equipped with a field-emission cathode. EBSD
maps were obtained with HKL (Oxford Instruments)
software package Channel 5. A representative set of
copper rods joined with BCW were cleaned and ground
to remove at least 1 mm of the outermost material to
reveal the underlying bulk-material flow pattern. The
specimens were subsequently incrementally polished
until the surface roughness reached 3 μm and then
anodized. The grain-deformation structure was studied
with an Axio Scope A1 optical microscope. The pictures
were taken at a low magnification of 50× to reveal the
entire material flow pattern within the deformation zone.
Y. HUANG et al.: MICROSTRUCTURAL EVOLUTION AND MATERIALS FLOW IN BUTT COLD WELDING OF COPPER
800 Materiali in tehnologije / Materials and technology 54 (2020) 6, 799–805
Figure 1: Schematic diagram of the butt-cold-welding mold:
1 – hydraulic-press head; 2, 3 – copper rod; 4,5–f ixedmold base;
6, 7 – positioning molds

3 RESULTS AND DISCUSSION
3.1 Finite-element analysis and simulation set-up
Finite-element (FE) modelling was used to reveal the
conditions, under which cold bonding takes place during
the divergent extrusion of a BCW process, and the
conditions were determined with the underlying-material
flow pattern and deformation mode within the die region.
The simulations were conducted using the commercial
metal-forming FE code ANSYS WORKBENCH. Defor-
mation was the controlling parameter for cold welding.
In order to save the computation time, an axial sym-
metric 2D model and the modelling domain of a half
specimen were employed. Figures 2a and 2b show the
two copper-rod configurations in FEM before and after
the deformation, respectively, and Figure 2C shows a
detail of the modelling with the finite-element method.
In the simulations, the die was treated as the rigid
isothermal object, while the copper rod was modelled as
the plastic workpiece. They were adequately represented
by a homogenous friction coefficient acting on the mould
surfaces. The interaction between the two copper rods
was also fully expressed by a uniform friction coeffi-
cient, which was set to 0.18.
16
Since the material testing
was done at room temperature to comply with the
cold-extrusion experiments, the strain-rate dependence
of the flow stress was rather weak. Relevant material
parameters from the database were used for the
calculation in the simulations.
3.2 Predicted strain distributions during divergent
extrusion
Figures 3 summarizes the FE simulation results of
the divergent-extrusion process of BCW. In Figure 3a,
the left-hand side shows a picture of the physical
specimen after BCW, while the right-hand side shows
the simulated effective strain distribution within the
two-rod specimen being extruded with a 1-mm welding
gap. There is no noticeable difference between these two
parts. The separating interface between the copper rods
to be welded is still presented in the central region, while
no clear interface exists in the fin region, as clearly
shown in Figure 3a. This is due to the fact that the
plastic flow in the central region is constrained.
Figure 3b is a locally enlarged view of the material
flow pattern at the position of the arrow in Figure 3a.
These results provide a good overall view of the
intersecting shear-deformation bands formed during the
extrusions. In the shear-deformation zone, the material is
subjected to extensive plastic deformation, forming two
intersecting bands being orientated ±45° with respect to
the longitudinal axis of the extrusion channels. However,
at the centre of the intersection of the deformation zone,
we can see that the degree of deformation is low.
Two different deformation modes that the material
undergoes during the divergent extrusion are further
highlighted in Figures 3. The rod of the workpiece
experiences a homogenous shear deformation and the
narrow zone towards the centre line is subjected to the
plane-strain compression in the primary-extrusion
direction. The resulting squeezing of the material in the
horizontal direction contributes to a large surface expan-
sion, consequently providing the necessary conditions
for cold welding. Note that the surface expansion is also
required in the cases where two oxide-free surfaces are
allowed to mate under pressure, as is the case during
divergent extrusion.
3
Part A of Figure 4 shows an optical-microscope
image of the welded specimen, while the snapshot of the
resulting effective strain distribution is provided in part
B. When comparing them, it becomes clear that the
metals in the elliptical area, indicated by the arrow, were
joined where the macroscopically effective strain
Y. HUANG et al.: MICROSTRUCTURAL EVOLUTION AND MATERIALS FLOW IN BUTT COLD WELDING OF COPPER
Materiali in tehnologije / Materials and technology 54 (2020) 6, 799–805 801
Figure 2:A,B–ask etch of butt cold welding; C – details of modelling with the finite-element method, f
1,2
= coefficient of friction in the die, f
1,1
= coefficient of friction between the two copper rods

exceeded 2.6 and reached 2.9, which is similar to the
numerical simulation of divergent extrusion under the
conditions of cold-pressure welding of commercially
pure aluminium.
17
In that study, when the effective strain
at the centre line on the two mating surfaces was high
enough, reaching 3.0, the essential conditions for cold
bonding were met.
3.3 Microstructure evolution
The bonding between the two rods was believed to be
related to the strains and microstructure evolution in the
material. In order to find these relationships, micro-
structures from different regions of the welded specimen
were investigated through an EBSD analysis. The EBSD
scanned areas are shown in Figure 5 and the corres-
ponding results are given in Figures 6 to 8. As shown in
Figure 5, zone a is subjected to a low strain, zone b is in
Y. HUANG et al.: MICROSTRUCTURAL EVOLUTION AND MATERIALS FLOW IN BUTT COLD WELDING OF COPPER
802 Materiali in tehnologije / Materials and technology 54 (2020) 6, 799–805
Figure 6: Local-misorientation mapping of the specimens from the centre line: a) centre of the sample, b) connection-boundary area, c) joint
zone
Figure 5: Electron-backscatter-diffraction (EBSD) sampling positions
and scanned areas
Figure 3: a) snapshots showing the two-bar specimen being extruded
with a 1-mm welding gap, left – result of the experiment, right – result
of the FE model; b) partially enlarged view of the material flow
pattern at the position of the arrow, left – result of the FE model, right
– photograph taken with the optical microscope
Figure 4: Snapshots showing the fin after extrusion with a 1-mm
welding gap, A – photograph taken by the optical microscope, the
elliptical area indicated by the arrow is the area where the connection
takes place, B – strain-distribution result of the FE model

the welding center with a medium strain, and zones c
and d are in the region of a high strain along the original
mating surfaces. It should be noted that only half of zone
c is in the high-strain region.
It is commonly recognized that dislocation densities
are almost uniformly distributed at the initial stages of
plastic deformation of polycrystalline materials. How-
ever, the mismatch of the slips at grain boundaries or
intragranular regions results in the gradients of plastic
strain with the increase of deformation, which sub-
sequently results in the formation of geometrically
necessary dislocations (GNDs) of the compatible
deformations of different parts of the grains.
18,19
Figure 6
shows the local-misorientation mapping of the speci-
mens from the centre line. It can be clearly seen on the
figure that the fraction of the blue zone in the interface
area between the two white lines is increased with the
increase in the strain, indicating an increased dynamic
recrystallization with fewer low-angle grain boundaries
(LABs) under a relatively high strain. As shown in Fig-
ures 6a to 6c, no bonding occurs in the centre of the
interface with the low DRX degree, while it occurs in the
centre of the interface with a high DRX degree. This
indicates that recrystallization plays a very important
role in copper cold welding.
Figure 7 shows grain/sub grain boundary (GSB)
maps with different deformation strains corresponding to
different positions. In these GSB maps, the black and
green lines represent high-angle grain boundaries
(HAGBs) and low-angle grain boundaries (LAGBs),
respectively. In this study, the grain boundaries in the
deformed microstructure are categorized as the following
two types: low-angle boundaries (LABs) and high-angle
grain boundaries (HABs). LABs are defined as the boun-
daries having a misorientation angle of less than 15°,
while HABs are considered as the boundaries having a
misorientation angle of more than 15°.
20
As shown in Figure 8, it can be easily found that the
fraction of LABs greatly decreases with the increase in
the strain. This means that the increased number of
recrystallized grains are under a relatively high strain.
New fine grains evolve with further deformation under
lower strains, while coarse grains with high angles still
exist. As shown in Figure 7a, large primary grains are
not significantly reduced and low grain boundaries are
inhomogeneously developed, accompanied by embryos
of dislocation substructures. When moderate strains
appear, an extension of large grains can be observed,
accompanied by a large deformation and large amounts
of the subgrain structure inside the grains. Grains with
high misorientations are elongated in the freeform-defor-
mation (x) direction, which is vertical to the pressing
direction. A weak fibrous microstructure consisting of
small deformation bands is thus found aligned to the
pressing direction. Although the percentage of LAGBs is
reduced, the practical amount of grains in this type is
increased. The recrystallized grains may be generated
due to the combining effect of severe plastic deformation
and local heating. New fine grains with low-angle
dislocation sub-grain boundaries are homogeneously
developed in the deformed HAGB grain interiors (as
shown Figures 7a and 7b).
When higher strains appear, we can see that the
volume fraction of dynamic-recrystallization (DRX)
grains is increased with the increase in the strains, indi-
cating that the DRX behavior occurs easily at higher
strains. Obviously, the microstructure evolution is
closely related to strains. In general, DDRX is relevant
for the bulging mechanism operating in the materials
with low or medium stacking-fault energy, involving the
nucleation of new grains and a subsequent grain growth.
CDRX is considered as a recovery process and pro-
ceeded by continuous absorption of dislocations in
subgrain boundaries, which results in the formation of
high-angle boundaries and new grains. It is found that
the DRX of the original grain boundaries becomes
serrated and bulging, as shown in the partially enlarged
view on Figure 7c, indicating the initiation of DRX
owing to the strain-induced boundary migration. Mean-
while, a few dynamically recrystallized grains are also
Y. HUANG et al.: MICROSTRUCTURAL EVOLUTION AND MATERIALS FLOW IN BUTT COLD WELDING OF COPPER
Materiali in tehnologije / Materials and technology 54 (2020) 6, 799–805 803
Figure 8: LAB-fraction evolution in the specimens under different
conditions
Figure 7: Grain boundaries (black HABs and green LABs) in the
specimens under different conditions

observed along the grain boundaries (as shown in
Figures 7c and 7d).
Lilleby et al. observed divergent extrusion of com-
mercially pure aluminum. According to the previous
finite-element (FE) simulations,
17
it was easy to confirm
that the increase in the maximum temperature due to
plastic deformation is about 110 °C, assuming adiabatic
conditions. However, the real temperature increase is
much lower and does not exceed 20 °C because of the
heat conduction in the aluminum billet and the surround-
ing tool steel. Yang et al. studied the effect of cold-roll-
ing deformation-induced heat on the interfacial bonding
strength. Their results revealed that deformation-induced
localized heating in low-thermal-conductivity metal
multilayer systems may provide opportunities for achiev-
ing successful accumulative roll bonding.
21
Since the
temperatures of the specimens were below 50 °C, dyna-
mic recrystallization occurred at a relatively low tem-
perature and the DRX grain size changed little with the
increasing strains, which was mainly related to the lower
mobility of DRX grain boundaries at the lower
deformation temperature.
22–25
Figure 9 shows the grain-size distribution in the spe-
cimens under different conditions. As can be seen, grain
sizes become gradually smaller with the increased
strains. The average grain sizes were found to be reduced
from 5.4170 μm to 1.7525 μm, as shown in Figures 9 a
to 9d, indicating a grain-refinement strengthening
effect.
26
However, as shown in Figures 9c and 9d, the
grain size changes little and this is believed to be caused
by dynamic recrystallization, which is consistent with
the increase in the volume fraction of DRX grains as the
strain increases.
4 CONCLUSIONS
The butt-cold-welding bonding occurs in the fin re-
gion because the metal flow in the central region is con-
strained. Based on the developed FEM model, a
cold-welding-process interpretation was achieved by es-
tablishing the correlation between the strain distribution
and metal connection. When the macro-effective strain
exceeds a value of about 2.6, the interface is welded.
Comparing the three different areas of the center line,
the fact that the metal connection occurs at a higher
strain can be confirmed. Further study of the microscopic
evolution reveals that the DRX degree is significantly
improved with the increasing strain, indicating that
recrystallization plays a very important role in copper
cold welding.
As the value of strain increases, a large number of
dynamically recrystallized grains begin to appear around
the original grain boundaries, gradually replacing the
original deformed grains, resulting in an increase in the
volume fraction of dynamic recrystallization. Owing to
the inadequate temperature, the size of dynamically
recrystallized grains presents an insensible change with
the subsequently increased strain.
5 REFERENCES
1
J. Ning, L. J. Zhang, A. Wang, Q. L. Bai, J. N. Yang, J. X. Zhang,
Effects of double-pass welding and extrusion on properties of fiber
laser welded 1.5-mm thick T2 copper joints, Journal of Materials
Processing Technology, 237 (2016), 75–87, doi:10.1016/j.jmatprotec.
2016.06.011
2
R. Kocich, A. Machá~ková, L. Kun~ická, F. Fojtík, Fabrication and
characterization of cold-swaged multilayered Al-Cu clad composites,
Materials and Design, 71 (2015), 36–47, doi:10.1016/j.matdes.
2015.01.008
3
A. Lilleby, Ø. Grong, H. Hemmer, T. Erlien, Experimental and finite
element studies of the divergent extrusion process under conditions
applicable to cold pressure welding of commercial purity aluminium,
Materials Science and Engineering: A, 518 (2009) 1–2, 76–83,
doi:10.1016/j.msea.2009.04.021
4
S. Shawn Lee, T. Hyung Kim, S. Jack Hu, W. W. Cai, J. A. Abell, J.
J. Li, Characterization of joint quality in ultrasonic welding of
battery tabs, Journal of Manufacturing Science and Engineering, 135
(2013) 2, 021004, doi:10.1115/1.4023364
5
G. A. Pinheiro, C. A. W. Olea, J. F. dos Santos, K. U. Kainer, Micro-
structural and mechanical behavior of friction welds in a high creep
resistance magnesium alloy, Advanced Engineering Materials, 9
(2007) 9, 757–763, doi:10.1002/adem.200700159
6
F. J. Humphreys, M. Hatherly, Recrystallization and related anneal-
ing phenomena, Elsevier, UK 2012, 254–259
7
W. Cai, G. Daehn, A. Vivek, J. J. Li, H. Khan, R. S. Mishra, M.
Komarasamy, A state-of-the-art review on solid-state metal joining,
Journal of Manufacturing Science and Engineering, 141 (2019)3 ,
031012, doi:10.1115/1.4041182
8
N. Mortazavi, N. Bonora, A. Ruggiero, M. Hörnqvist Colliander,
Dynamic recrystallization during high-strain-rate tension of copper,
Metallurgical and Materials Transactions A, 47 (2016) 6, 2555–2559,
doi:10.1007/s11661-016-3491-x
9
S. A. Hosseini, M. Hosseini, H. Danesh Manesh, Bond strength
evaluation of roll bonded bi-layer copper alloy strips in different
rolling conditions, Materials and Design, 32 (2011) 1, 76–81,
doi:10.1016/j.matdes.2010.06.032
Y. HUANG et al.: MICROSTRUCTURAL EVOLUTION AND MATERIALS FLOW IN BUTT COLD WELDING OF COPPER
804 Materiali in tehnologije / Materials and technology 54 (2020) 6, 799–805
Figure 9: Grain-diameter distribution in the specimens under different
conditions

10
H. C. Schmidt, D. Rodman, O. Grydin, C. Ebbert, W. Homberg, H. J.
Maier, G. Grundmeier, Joining with electrochemical support: Cold
pressure welding of copper – weld formation and characterization,
Advanced Materials Research, 966–967 (2014), 453–460,
doi:10.4028/www.scientific.net/AMR.966-967.453
11
S. Zenitani, N. Hayakawa, J. Yamamoto, K. Hiraoka, Y. Morikage, T.
Kubo, K. Yasuda, K. Amano, Development of new low transform-
ation temperature welding consumable to prevent cold cracking in
high strength steel welds, Science and Technology of Welding and
Joining, 12 (2013) 6, 516–522, doi:10.1179/174329307x213675
12
M. Eizadjou, H. Danesh Manesh, K. Janghorban, Investigation of roll
bonding between aluminum alloy strips, Materials and Design, 29
(2008) 4, 909–913, doi:10.1016/j.matdes.2007.03.020
13
B. W. Zhang, N. Bay, Cold welding – experimental investigation of
the surface preparation methods, Welding Journal, 76 (1997)8 ,
326–330
14
J. Q. Yu, G. Q. Zhao, Study on the welding quality in the porthole
die extrusion process of aluminum alloy profiles, Procedia Engi-
neering, 207 (2017), 401–406, doi:10.1016/j.proeng.2017.10.795
15
M. Sahin, Effect of surface roughness on weldability in aluminium
sheets joined by cold pressure welding, Industrial Lubrication and
Tribology, 60 (2008) 5, 249–254, doi:10.1108/00368790810895187
16
C. D. Tian, D. Tang, D. Y. Li, Y. H. Peng, A testing method of fric-
tion coefficient and its application in numerical simulation, Journal
of Plasticity Engineering, 15 (2008) 5, 53–56, 61, (Chinese)
17
A. Lilleby, Ø. Grong, H. Hemmer, Experimental and finite element
simulations of cold pressure welding of aluminium by divergent
extrusion, Materials Science and Engineering: A, 527 (2009) 1–2,
179–186, doi:10.1016/j.msea.2009.07.051
18
O. Rezvanian, M. A. Zikry, A. M. Rajendran, Statistically stored,
geometrically necessary and grain boundary dislocation densities:
Microstructural representation and modelling, Proceedings of the
Royal Society A: Mathematical, Physical and Engineering Sciences,
463 (2007) 2087, 2833–2853, doi:10.1098/rspa.2007.0020
19
Y. C. Lin, D. G. He, M. S. Chen, X. M. Chen, C. Y. Zhao, X. Ma, Z.
L. Long, EBSD analysis of evolution of dynamic recrystallization
grains and  phase in a nickel-based superalloy during hot
compressive deformation, Materials & Design, 97 (2016), 13–24,
doi:10.1016/j.matdes.2016.02.052
20
S. Saadatkia, H. Mirzadeh, M. J. M. Cabrera, Hot deformation
behavior, dynamic recrystallization, and physically-based constitu-
tive modeling of plain carbon steels, Materials Science &
Engineering A, 636 (2015), 196–202, doi:10.1016/j.msea.2015.
03.104
21
D. Yang, K, J. Y. Xiong, P. Hodgson, C. e. Wen, Influence of defor-
mation-induced heating on the bond strength of rolled metal multi-
layers, Materials Letters, 63 (2009) 27, 2300–2302, doi:10.1016/
j.matlet.2009.07.051
22
Z. F. Yan, D. H. Wang, X. L. He, W. X. Wang, H. X. Zhang, P. Dong,
C. H. Li, Y. L. Li, J. Zhou, Z. Liu, L. Y. Sun, Deformation behaviors
and cyclic strength assessment of AZ31B magnesium alloy based on
steady ratcheting effect, Materials Science and Engineering: A, 723
(2018), 212–220, doi:10.1016/j.msea.2018.03.023
23
H. B. Zhang, K. F. Zhang, S. S. Jiang, Z. Lu, The dynamic recrys-
tallization evolution and kinetics of Ni–18.3Cr–6.4Co–5.9W–
4Mo–2.19Al–1.16Ti superalloy during hot deformation, Journal of
Materials Research, 30 (2015) 7, 1029–1041, doi:10.1557/jmr.
2015.78
24
H. B. Zhang, K. B. Zhang, Z. Lu, C. H. Zhao, X. L. Yang, Hot
deformation behavior and processing map of a  '-hardened nickel-
based superalloy, Materials Science and Engineering: A, 604 (2015),
196–202, doi:10.1016/j.msea.2014.03.015
25
D. F. Li, Q. M. Guo, S. L. Guo, H. J. Peng, Z. G. Wu, The micro-
structure evolution and nucleation mechanisms of dynamic
recrystallization in hot-deformed Inconel 625 superalloy, Materials
& Design, 32 (2011) 2, 696–705, doi:10.1016/j.matdes.2010.07.040
26
J. L. Gu, X. S. Wang, J. Bai, J. L. Ding, S. Williams, Y. C. Zhai, K.
Liu, Deformation microstructures and strengthening mechanisms for
the wire+arc additively manufactured Al-Mg4.5Mn alloy with
inter-layer rolling, Materials Science and Engineering: A, 712
(2018), 292–301, doi:10.1016/j.msea.2017.11.113
Y. HUANG et al.: MICROSTRUCTURAL EVOLUTION AND MATERIALS FLOW IN BUTT COLD WELDING OF COPPER
Materiali in tehnologije / Materials and technology 54 (2020) 6, 799–805 805