M. SAHIN: OPTIMIZING THE PARAMETERS FOR FRICTION WELDING STAINLESS STEEL TO COPPER PARTS
109–115
OPTIMIZING THE PARAMETERS FOR FRICTION WELDING
STAINLESS STEEL TO COPPER PARTS
OPTIMIRANJE PARAMETROV PRI TORNEM VARJENJU
NERJAVNEGA JEKLA NA BAKRENE DELE
Mumin Sahin
Department of Mechanical Engineering, Trakya University, 22180 Edirne, Turkey
mumins@trakya.edu.tr
Prejem rokopisa – received: 2015-01-25; sprejem za objavo – accepted for publication: 2015-02-13
doi:10.17222/mit.2015.023
St-Cu (stainless steel and copper) parts were friction welded with the aim to optimize the process parameters in the present
study. The joints obtained with various process-parameter combinations were subjected to a tensile test. Empirical relationships
were developed to predict the strength of the joints using RSM (the response-surface methodology) and the coherency of the
model was tested. The tensile properties, microhardness variations, SEM, the EDS analysis and X-ray diffraction (XRD)
analysis of the welded specimens were evaluated. It was found, with an ANOVA analysis, that the friction pressure/friction time
relation has the largest influence on the tensile strength of the joints followed by the rotational speed. However, it was also
found that the formation of intermetallics at the interface is responsible for a higher hardness and lower tensile strength of the
friction-welded stainless steel-copper joints.
Keywords: friction welding, metallurgy, response-surface methodology, tensile strength
V predstavljenem delu so bili deli St-Cu (nerjavno jeklo in baker) torno varjeni z namenom optimizacije procesnih parametrov.
Spoji, dobljeni z razli~nimi procesnimi parametri, so bili preizku{eni z nateznim preizkusom. Razvite so bile empiri~ne
odvisnosti za napovedovanje trdnosti spojev s pomo~jo RSM (Metodologija odgovora povr{ine) in izvr{ena je bila koherenca
modela. Ocenjene so bile natezne lastnosti, spreminjanje mikrotrdote, SEM, EDS analiza in rentgenska difrakcija (XRD)
zvarjenih vzorcev. Iz ANOVA analize je bilo ugotovljeno, da ima torni tlak/~as trenja najve~ji vpliv na natezno trdnost spojev,
sledi pa mu hitrost vrtenja. Ugotovljeno je bilo, da je ve~ja trdota in manj{a natezna trdnost torno varjenih spojev posledica
nastanka intermetalne zlitine na stiku nerjavno jeklo-baker.
Klju~ne besede: torno varjenje, metalurgija, metodologija odgovora povr{ine, natezna trdnost
1 INTRODUCTION
Parts made of different materials are known to be
cost-effective. The life cycle of the materials, especially
in corrosive media, is prolonged. Many ferrous and non-
ferrous alloys can be friction welded. Friction welding
can be used to join metals of widely different thermal
and mechanical properties. The combinations that can be
friction welded cannot be joined with other welding
techniques because of the formation of brittle phases that
make the joint poor with respect to mechanical pro-
perties. Friction welding prevents distortion of the mate-
rials, as heat is not applied.
The welding technology is widely used in manufac-
turing. The development of new welding methods gained
importance along with the developing technology.1–4
Welding of different metals and their alloys is a common
application in engineering solutions. Fusion welding is
almost impossible in such cases due to incompatible
physical characteristics and chemical compositions of
different metals and alloys. As a result, friction welding
was developed. Several researches worked on the heat in
friction welding.5–7 In friction welding, heat is generated
at the interface of the workpieces since mechanical ener-
gy is dissipated as heat during the rotation under
pressure. Friction welding is a solid-state welding pro-
cess, using the heat generated through the mechanical
friction with a moving workpiece, with an addition of an
upsetting pressure to plastically displace the material.
Friction welding is generally used to join the parts that
are axially symmetrical and have circular cross-sections.
However, it can be easily used to join parts without cir-
cular cross-sections, with the aid of automation devices
and computerized control facilities.8 It is an energy saver
since heat is not applied.
The friction time and pressure, the upset time and
pressure and the speed of rotation are the principal
variables in friction welding.9–12 There are two types of
friction-welding techniques: continuous-drive friction
welding and inertia friction welding. Different metals
have different hardness values and different melting
points. Interface activity during friction welding forms
brittle intermetallic phases or eutectics with low melting
points. Clean welding surfaces are also of prime import-
ance.13–15 In welded St-Cu joints, the joint strength in-
creases with the increasing upset pressure up to the
critical value. An increase in the friction time causes a
lower strength of a St-Cu joint compared to the Cu base
metal.16 A deformation of the material during friction
welding is generally due to the diffusion involving a
Materiali in tehnologije / Materials and technology 50 (2016) 1, 109–115 109
UDK 621.791.1:669.14.018.8:669.3:519.61/.64 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 50(1)109(2016)
migration of lattice defects, which can be influenced by
an external electric field.17 Sintered powder metallurgical
preforms have a low mass, high stiffness and, therefore,
their natural frequency is high. Having inherent porosity,
they can also be good dampeners besides possessing the
latent lubricant.18 Maalekian19 found that the formation
of hard interlayers, such as intermetallic phases, when
joining dissimilar materials may cause a joint to become
brittle. Further, Sahin et al.20,21 showed that the interme-
tallic phases formed in the interface cause a decrease in
the strength of the joints.
However, based on the literature review, Murti and
Sundaresan22 carried out a study about a parameter opti-
mization using a statistical approach based on factorial-
experiment-design friction welding of dissimilar
materials. The response-surface methodology (RSM) is a
collection of mathematical and statistical techniques that
are useful for designing a set of experiments, developing
a mathematical model, analyzing the optimum combina-
tion of the input parameters and graphically expressing
the values.23 To obtain the maximum strength, it is essen-
tial to have complete control over the relevant process
parameters as demonstrated.24
Therefore, in this work, an attempt was made to opti-
mize the process parameters of continuous-drive friction
welding to achieve the maximum tensile strength of
stainless steel-copper parts using the response-surface
methodology. Tensile tests were performed on the
welded test parts. A microstructure analysis, EDS anal-
ysis, XRD analysis and microhardness variations were
also carried out on the test parts.
2 EXPERIMENTAL WORK
In the experiments, AISI 304 austenitic stainless-steel
and copper parts having a diameter of 10 mm were made
using the continuous-drive friction-welding process
parameters. The chemical composition and mechanical
properties of the stainless-steel and copper parts are
presented in Tables 1 and 2, respectively, as given in25.
Different combinations of the process parameters
were used to carry out the trial runs. Process parameters
were tested by varying one of the factors while keeping
the rest of them at constant values. The working range of
each process parameter was determined for a smooth
appearance without any observable defects. The selected
levels of the process parameters and design matrix with
their units and notations are presented in Tables 3 and 4.
However, in order to examine the intermetallic phases
formed at the interface of the joints, SEM (scanning
electron microscopy) and EDS (energy-dispersive X-ray
spectroscopy) were applied to the joints. Examinations
were carried out with an SEM-JEOL JSM 5410 LV
microscope and in the field of 200 kV. In addition, the
weld zones of the joints were analyzed in this work since
an XRD analysis of the phase constituents in the weld
zone is of a great importance.
Then, the strength of the joints was related to the
hardness variation within the HAZ. The hardness vari-
ations across the welding regions of the joints were
measured using a 0.3 kg load Vickers microhardness test.
3 RESULTS AND DISCUSSION
3.1 Empirical relationships and the optimization
The responses, the tensile-strength (TS) values of
friction-welded joints, are the functions of the friction-
welding parameters such as the friction pressure per
second (F), the forging pressure per second (D) and the
rotational speed per second (N) and they can be ex-
pressed as:
M. SAHIN: OPTIMIZING THE PARAMETERS FOR FRICTION WELDING STAINLESS STEEL TO COPPER PARTS
110 Materiali in tehnologije / Materials and technology 50 (2016) 1, 109–115
Table 1: Chemical composition of austenitic stainless steel used in the experiment25
Tabela 1: Kemijska sestava avstenitnega nerjavnega jekla, uporabljenega pri preizkusu25
Material % C % P % S % Mn % Si % Cr % Ni Tensile strength(MPa)
AISI 304
(X5CrNi1810) < 0.07 < 0.045 < 0.030 < 2.0 < 1.0 17–19 8.5–10.5 825
Table 2: Chemical composition of copper used in the experiment
Tabela 2: Kemijska sestava bakra, uporabljenega pri preizkusu
Copper % Sn % Pb % Zn % P % Mn % Fe % Ni % Si % Mg % Al % Bi % S % Sb % Cu Tensile strength(MPa)
0.00222 <0.0020 <0.0010 0.00137 <0.0005 0.0381 <0.0010 0.00745 0.00376 0.00500 <0.0005 0.00251 <0.0020 99.93 300
Table 3: Feasible working limits of friction-welding parameters
Tabela 3: Obmo~je delovnih parametrov pri tornem varjenju
Parameter Notation Unit
Level
–1.68 (–1) (0) (+1) +1.68
Friction pressure/Friction time F MPa/s 3.78 5.82 8.82 11.82 13.86
Upset pressure/Upset time D MPa/s 2.96 5 8 11 13.04
Rotational speed/sec N s–1 18.46 20.5 23.5 26.5 28.54
TS = f {F, D, N} (1)
The second-order polynomial (regression) equation
used to represent the response surface Y (TS) is as
follows:
Y = b0 + bixi + bii xi
2+ bij xi xj 2)
and for three factors, the selected polynomial could be
expressed as:
TS = b0 + b1(F) + b2(D) + b3(N) + b12(FD) + b13(FN) +
+ b23(DN) + b11(F
2) + b22(D
2) + b33(N
2) (3)
Regression coefficients are b1, b2, b3, … b44 where b0
is the average of the responses and they depend on the
respective linear, interaction and squared terms of the
factors as shown in26,27. The significance of each
coefficient was determined with a t-test and p-values,
listed in Table 5.
The value of a coefficient was calculated using the
Design-Expert software. The values of the probability>F
of less than 0.05 indicate that the model terms are
significant. In this case, F, D, N, FD, FN, DN, F2, D2 and
N2 are significant model terms. The values greater than
0.1 point out that the model terms are not significant.
The results of multiple linear regression coefficients for
the second-order response surface model are given in
Table 6.
The final empirical relationship was obtained using
only these coefficients, and the developing final empiri-
cal relationship for the tensile strength is given below:
TS = [221.36 + 18.16F + 10.90D + 13.05N – 6.62FD –
9.12FN + 2.88DN – 14.74F2 – 12.80D2 – 6.08N2] (4)
The ANOVA (analysis of variance) technique was
used to check the adequacy of the developing empirical
relationship. In this investigation, the desired level of
confidence was taken to be 95 %. The relationship is
considered adequate if the calculated F-value of the
model developed does not go over the standard tabulated
F-value and the calculated R-value of the developed
relationship exceeds the standard tabulated R-value for a
desired level of confidence. It was found that the above
model is adequate. In the same way, interactions FD, FN,
DN had significant effects. A lack of fit was not signi-
ficant though it was desired. The normal probability plot
of the residuals for the tensile strength is shown in
Figure 1. It reveals that the residuals are on a straight
line, which means that the errors are distributed nor-
mally. Each predicted value matches well its experimen-
tal value, as shown in Figure 2. The response-surface
methodology (RSM) was used to optimize the friction-
welding parameters in this study. The response contours
can assist in the prediction of the response for any zone
in the experimental field as observed in24,25.
M. SAHIN: OPTIMIZING THE PARAMETERS FOR FRICTION WELDING STAINLESS STEEL TO COPPER PARTS
Materiali in tehnologije / Materials and technology 50 (2016) 1, 109–115 111
Table 4: Design matrix and the corresponding output response
Tabela 4: Postavljena matrika in ustrezni dobljeni odgovori
Standard
order
Run
order
Original value Tensile
strength
(MPa)F D N
16 1 8.82 8 23.5 223
18 2 8.82 8 23.5 222
4 3 11.82 11 20.5 190
9 4 3.78 8 23.5 150
20 5 8.82 8 23.5 218
6 6 11.82 5 26.5 210
15 7 8.82 8 23.5 223
1 8 5.82 5 20.5 130
7 9 5.82 11 26.5 213
5 10 5.82 5 26.5 180
8 11 11.82 11 26.5 215
12 12 8.82 13.04 23.5 217
11 13 8.82 2.96 23.5 160
10 14 13.86 8 23.5 216
3 15 5.82 11 20.5 150
19 16 8.82 8 23.5 222
17 17 8.82 8 23.5 219
13 18 8.82 8 18.46 200
14 19 8.82 8 28.54 215
2 20 11.82 5 20.5 195
Table 5: ANOVA test results for the response of the tensile strength
Tabela 5: Rezultati ANOVA preizkusa za odgovore natezne trdnosti
Sum of
squares
Mean
square F-value
P-value
si
gn
if
ic
an
t
Source df prob > F
Model 14726.69 9 1636.30 13.39 0.0002
A-F 4503.47 1 4503.47 36.85 0.0001
B-D 1622.62 1 1622.62 13.28 0.0045
C-N 2325.93 1 2325.93 19.03 0.0014
AB 351.12 1 351.12 2.87 0.1209
AC 666.12 1 666.12 5.45 0.0417
BC 66.13 1 66.13 0.54 0.4789
A^2 3132.15 1 3132.15 25.63 0.0005
B^2 2360.38 1 2360.38 19.31 0.0013
C^2 532.80 1 532.80 4.36 0.0633
Residual 1222.11 10 122.21
Lack of fit 1199.28 5 239.86 52.52 0.0003
Pure error 22.83 5 4.57
Cor. total 15948.80 19
Std. dev. 11.05 R-squared 0.9234
Mean 198.40 Adj. R-squared 0.8544
Table 6: Estimated regression coefficients
Tabela 6: Ocenjeni regresijski koeficienti
Factor
Estimated regression
coefficients
Tensile strength (MPa)
Intercept 221.36
F–friction force/friction time 18.16
D–upset force/upset time 10.90
N–rotational speed 13.05
FD –6.62
FN –9.12
DN 2.88
F2 –14.74
D2 –12.80
N2 –6.08
The end of the response plot shows the maximum
achievable tensile strength. Figures 3 and 4 show that
the tensile strength increases with the increasing friction
pressure/time relation and rotational speed and then it
decreases.
The maximum tensile strength of the friction-welded
joints was attained under the following welding con-
ditions: a friction pressure/time relation of 8.82 MPa/s (a
friction pressure of 75 MPa and a friction time of 8.5 s),
an upset pressure/time relation of 8 MPa/s (an upset
pressure of 160 MPa and an upset time of 20 s) and a
rotational speed of and 23.5 s–1, showing the accuracy of
the model.
During the welding processes, the strength of the
welds obtained with dissimilar materials strongly de-
pends on the temperature attained by each substrate.
Differences in the mechanical and thermophysical
properties and behaviour of the substrates at the interface
influence the quality of the joints during the welding as
reported in20,21.
3.2 Metallurgical analysis
The macrophotography of the joints is given in Fig-
ure 5. There is no evidence of cracking or other defects
in the joints. Due to the variations in the strength of the
materials, an appreciable variation in the width of the
HAZ (heat-affected zone) region is evident from the
joints. However, the microstructure of stainless steel is
characterized by equiaxed grains, in the austenitic-grain
structure being the natural structure of this type of steel
at room temperature (Figure 6).
M. SAHIN: OPTIMIZING THE PARAMETERS FOR FRICTION WELDING STAINLESS STEEL TO COPPER PARTS
112 Materiali in tehnologije / Materials and technology 50 (2016) 1, 109–115
Figure 4: Contour plots of the process parameters for the tensile
strength
Slika 4: Prikaz obrisov procesnih parametrov na natezno trdnost
Figure 2: Correlation graph of the response
Slika 2: Prikaz korelacije odziva
Figure 3: Response plots of the process parameters for the tensile
strength
Slika 3: Prikaz odziva procesnih parametrov na natezno trdnost
Figure 1: Normal probability plot of residuals
Slika 1: Normalna verjetnost izrisa ostankov
However, copper is formed of eutectic particles,
having dark points indicating that it is a mixture of pure
copper and cuprous oxide, dispersed in the ground
copper (Figure 7). The effect of melting was minimal at
the interface because the heat-affected zone (HAZ) was
small (Figure 8).
It is also observed that the joints have larger deforma-
tions on the Cu side compared to the steel side (Figure
5). Welding flashes occur on the copper side of the inter-
face because the melting temperature of copper is lower
than the melting temperature of steel. However, stainless
steel does not undergo an extensive deformation while
copper undergoes an extensive melting because of the
high generated and concentrated frictional heat.
Since copper has a higher thermal conductivity than
steel, the heat-affected zone on the copper side is wider
M. SAHIN: OPTIMIZING THE PARAMETERS FOR FRICTION WELDING STAINLESS STEEL TO COPPER PARTS
Materiali in tehnologije / Materials and technology 50 (2016) 1, 109–115 113
Figure 9: EDS analysis of the intermetallic-phase zone of a joint: a)
SEM, b) EDS spectrum
Slika 9: EDS-analiza intermetalne faze na stiku: a) SEM, b) EDS
spekter
Figure 7: Microstructure of copper
Slika 7: Mikrostruktura bakra
Figure 8: Image of the interface of a joint
Slika 8: Posnetek stika v spoju
Figure 5: Macrophotography of a joint
Slika 5: Makroposnetek spoja
Figure 6: Microstructure of stainless steel
Slika 6: Mikrostruktura nerjavnega jekla
than that of the steel side. There is no change in the grain
size on the steel side. The presence of small particles on
the copper side reveals hardening on this side. There are
equiaxed  grains and Cu2O particles on the copper side.
The interface elements of both materials diffused along
the interface and some intermetallic phases were formed
at the interface as reported in20,21.
The EDS analysis performed at a defined zone of the
interface showed that the interface was formed of 2.70 %
C, 0.98 % Cr, 2.96 % Fe and 93.36 % Cu (Table 7).
Thus, the presence of intermetallic phases at the inter-
face is obvious. Copper-oxide films were broken into
pieces due to an excessive deformation at the interface
caused by the rotation (Figure 9). According to Figure
10, the X-ray diffraction results for friction-welded
stainless steel-copper joints indicated that FeCu4 and
Cu2NiZn intermetallics were formed in the welding
zone. The thickness of the layer containing the inter-
metallic phases varied between 8.72 μm and 17.53 μm
(Figure 11).
3.3 Microhardness measurement
The microhardness of a joint was measured across
the weld region and the values were plotted as shown in
Figure 12. The microhardness is maximum at the inter-
face; this may be due to the formation of brittle inter-
metallics, and it is one of the reasons for a lower tensile
strength of dissimilar joints.
4 CONCLUSIONS
Stainless-steel and copper parts were successfully
friction joined in this work. The following important
conclusions were obtained from this investigation:
• Empirical relationships were developed to predict the
tensile strength of the friction-welded stainless-steel
and copper parts incorporating process parameters at
a 95 % confidence level. The friction-welding para-
meters were optimized with the response-surface
methodology to attain the maximum tensile strength.
• The maximum tensile strength of 223 MPa was
attained in the friction-welded joints under the
following welding conditions: a friction pressure/
time relation of 8.82 MPa/s, an upset pressure/time
relation of 8 MPa/s and a rotational speed of 23.5 s–1.
• The friction pressure/friction time relation was found
to have the greatest influence on the tensile strength
of the joints, followed by the rotational speed.
• Various intermetallic phases such as FeCu4 and
Cu2NiZn occurred at the interface. The formation of
intermetallics at the interface is responsible for the
M. SAHIN: OPTIMIZING THE PARAMETERS FOR FRICTION WELDING STAINLESS STEEL TO COPPER PARTS
114 Materiali in tehnologije / Materials and technology 50 (2016) 1, 109–115
Figure 12: Microhardness variation across the joint
Slika 12: Spreminjanje mikrotrdote preko spoja
Figure 10: XRD results for the welding zone of a joint
Slika 10: Rezultati rentgenske analize (XRD) v zvaru stika
Table 7: EDS analysis of the defined zone at the interface
Tabela 7: EDS-analiza ozna~enega podro~ja na stiku
Element (w/%) (x/%)
C 2.70 12.72
Cr 0.98 1.07
Fe 2.96 3.00
Cu 93.36 83.21
Total 100.00
Figure 11: Thicknesses of the intermetallic phases at the interface
Slika 11: Debelina intermetalnih faz na stiku
higher hardness and lower tensile strength of the
friction-welded stainless steel-copper joints.
• The intermetallic phases at the interface are also
expected to play a role in the hardness variations.
Acknowledgement
The author would like to thank Trakya Univer-
sity/Edirne–Turkey, Hema Industry/Çerkezköy–Turkey
and TUBITAK MRC/Gebze–Turkey for the help in the
experimental part of the study.
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M. SAHIN: OPTIMIZING THE PARAMETERS FOR FRICTION WELDING STAINLESS STEEL TO COPPER PARTS
Materiali in tehnologije / Materials and technology 50 (2016) 1, 109–115 115