finite element analysis of friction welding process...
TRANSCRIPT
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Chapter 8
FINITE ELEMENT ANALYSIS OF FRICTION WELDING
PROCESS FOR DISSIMILAR MATERIALS
8.1 INTRODUCTION
In friction welding process heat is produced by conversion of
mechanical energy to thermal energy at the interfaces of the two work
pieces. In order to simulate friction welding process, combination of
thermal and mechanical effects needs to be considered. The finite
element analysis helps in better understanding of friction welding
process and it is important to calculate temperature and stress fields
during welding process. The knowledge of temperature distribution
helps in predicting the tendency of intermetallic compounds formation
as they strongly depends on local temperature attained during welding
process. Finite element analysis also helps to determine optimum
parameters and design of special purpose friction welding machines.
Most of the previous studies on finite element analysis of
friction welding process are related to friction stir welding. Some
amount of work is carried out on finite element analysis of inertia
friction welding and very limited work is carried out on finite element
analysis of direct drive friction welding process. Jolanta Zimmerman,
Wlosinski, Zdzislaq R. and Lindemann [16] have presented modelling
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of friction welding of elastic plastic metals. L. W. Zhang, C.D Liu, W.H
Zhu, S.Qu and J.H. wang [69] have developed the coupled FEM
analysis of the transient temperature field during inertia welding of
GH4169.
In the present research work a thermo-mechanical model is
developed using ANSYS and ABAQUS software to predict weld
interface temperature, Von Mises stress and deformation during
friction welding process for two dissimilar materials combinations (Al
6061- SS 304 and Al 5052-SS 304). Temperature dependent material
properties were considered for analysis.
8.2 EXPERIMENTAL PROCEDURE:
The weld combination of Al 6061 to SS 304 rods and Al
5052 to SS 304 are friction welded with regular and new joint
geometry, diameter 25 mm and length 50 mm were considered.
The optimum weld parameters selected for welding of Al 6061 to
SS 304 were rpm 1400, friction pressure 50 MPa, friction time 1
second, forging pressure 100 MPa and forging time 6 second.
The optimum weld parameters selected for welding of Al 5052 to
SS 304 were rpm 1400, friction pressure 39 MPa, friction time 1
second, forging pressure 160 MPa and forging time 6 seconds
The temperature at the interface is recorded with
thermocouple and infrared sensor. The resultant variation of rpm,
friction pressure, forging pressure, material consumption and torque
are measured by data acquisition system.
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8.3 THERMAL MODELLING
The governing equation for thermal model for friction welding
process is given as [108],
K [∂2T/∂x2 + ∂2T/∂y2 + ∂2T/∂z2] + G = ρ c ∂T/∂t (8.1)
where k is thermal conductivity, T is the temperature, G is the
heat generation rate, c is the specific heat, ρ is the density t is the
time, and x, y, z are spatial coordinates.
The ρ, c, and K are functions of temperature, which is important for
accurate thermal modelling. Both T and G are function of x, y, z and t.
Friction welding process consist of the heat generation by the
friction between the two work pieces qf, and heating from irreversible
plastic deformation of both the work piece, qp
Heat generation rate G is given as
G = qf + qp (8.2)
This study assumes that friction between both the work pieces
follows Coulomb‟s friction law.
The friction force, Ff, is directly proportional to normal force, Fn,
by the coefficient of friction, μ, i.e.,
Ff= μFn (8.3)
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The friction heat generation rate, qf , is equal to Ff times the
surface velocity of the work piece, V , at the local constant point with
work piece radius, R,
V=2ΠRN, (8.4)
Where, N is the work piece rotational speed.
The friction heat generation rate, qf , can be formulated as
qf = 2 ΠRNμFn (8.5)
The heat generation rate due to plastic deformation, qp is
qp = ησέ (8.6)
Where, η is the inelastic heat fraction, σ is the effective stress and έ is
the plastic straining rate.
Alternatively, Friction heat generation can also be calculated using
machine torque as given below
qf = (2ΠN τ / 60 A) * η (8.7)
Where, τ is torque, A is cross section area and „η‟ is inelastic heat
fraction.
8.4 FINITE ELEMENT ANALYSIS OF AL 6061 TO SS 304
The physical set up of friction welding is shown Figure 8.1. The
simplified finite element model for the same is shown in Figure 8.2.
The numerical simulation of the process is considered as axis
180
symmetric. ANSYS 11.0 finite element package was used for analysis
purpose. The finite element model consists of 5979 nodes and 5784
elements.
First transient thermal analysis was done to determine the
temperature distribution and then structural analysis is carried out
using the temperature distributions, which were obtained from the
transient thermal analysis. Thermal analysis was conducted using
plane 55 element which has two dimensional thermal conduction
capability and four nodes with a single degree of freedom at each
node. Initial temperature of 25 °C and convection coefficient of 40
w/m2 °C was considered.
Heat loss due to radiation was ignored due to very small value.
Temperature dependent material properties considered for SS 304 and
Al 6061 are as shown in Table 8.1 and Table 8.2 respectively. Thermal
conductivity and density are considered for thermal analysis and
temperature dependent material properties like modules of elasticity,
poisons ratio and thermal expansion are considered for elasto-plastic
analysis. Plane 182 was used for structural analysis. Very fine mesh
was created near the weld zone and element size increases as distance
from weld increases.
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Figure 8.1 Physical set up of friction welding process
Figure 8.2 Finite element model
The model is semi empirical as experimentally measured torque
is used as input for the heat flux. The heat flux qf, generated due to
friction at weld interface is calculated by using equation (8.7). The
change in heat flux depends on change in pressure distribution,
temperature dependent friction coefficient and relative velocity. The
change in temperature is due to conduction, convection and radiation.
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Modulus of elasticity (K N/mm2)
20 °C 100 °C 200 °C 300 °C 400 °C 500 °C
200 194 186 179 172 165
Mean coefficient of thermal expansion (10-6 x K-1) between 20C and :
100 °C 200 °C 300 °C 400 °C 500 °C
16 16.5 17 17.5 18
Poisson's Ratio
150 °C 260 °C 370 °C 480 °C 590 °C 700 °C 820 °C
0.28 0.3 0.32 0.28 0.29 0.28 0.25
Density Kg/m3
20 °C 90 °C 200 °C 320 °C 430 °C 540 °C 650 °C 760 °C 870 °C
7910 7880 7840 7790 7740 7690 7640 7590 7540
Thermal Expansion 10-6 x C-1
100 °C 200 °C 300 °C 400 °C 500 °C 600 °C 700 °C 800 °C
16.3 16.7 17.1 17.6 18 18.3 19 20
Thermal Conductivity kcal / m.hr.deg C
200 °C 400 °C 600 °C
15 17.5 18
Specific Heat J/kg.K
20 °C 90 °C 200 °C 320 °C 430 °C 540 °C 650 °C 760 °C 870 °C
456 490 532 557 574 586 599 620 645
Table 8.1 Temperature dependent Material Properties of SS 304
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Temperature(°C) 37.8 93.3 149 204 260 316 371 427
Thermal Conductivity(W/m °C) 162 177 184 192 201 207 217 223
Heat Capacity (J/Kg °C) 945 978 1000 1030 1052 1080 1100 1130
Density (Kg/m3) 2690 2690 2670 2660 2660 2630 2630 2600
Young‟s Modulus(GPa) 68.5 66.2 63.1 59.2 54.0 47.5 40.3 31.7
Yield Strength(MPa) 274 265 248 219 160 66.2 34.5 17.9
Thermal Expansion(1/°C)*10-6 23.5 24.6 25.7 26.6 27.6 28.5 29.6 30.7
Table 8.2 Temperature dependent material properties of Al 6061
8.5 RESULTS AND DISCUSSIONS FOR AL 6061-SS 304 ANALYSIS
8.5.1 TEMPERATURE DISTRIBUTION IN AL 6061-SS 304
The temperature distribution during friction welding is
calculated for new joint geometry using ANSYS software. The amount
of heat transferred to the Al 60610 and SS 304 can be calculated by
their thermal conductivity. Most of the heat generated at the interface
is transferred to Al 6061 because of its higher thermal conductivity.
Figure 8.3, Figure 8.4, Figure 8.5 and Figure 8.6 shows the variation
of temperature for friction time of 1 second, 0.8 second, 0.4 second
and 0.2 second respectively, cases along the length at the outer
periphery. Based on energy balance, the ratio of heat partition into the
Al 6061 was determined by,
qAl 6061= KAl 6061 / (KAl 6061 + KSS 304) (8.8)
Where, KAl 6061 and KSS 304 are thermal conductivities of Al 6061 and SS
304 respectively.
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In friction welding of Al 6061 and SS 304, only Al 6061 is
consumed in the form of flash due to softer material and also due to
higher thermal conductivity, as most of the heat generated at the
interface is transferred to Al 6061. Deformation of SS 304 is negligible
due to its higher hardness value, lower thermal conductivity and
higher melting point. The temperature generated at the weld interface
is not sufficient enough to plasticize SS 304. Heat affected zone in Al
6061 is higher than SS 304 because higher amount of heat is
transferred to Al 6061. Maximum temperature of 223.89°C was
observed at the weld interface, which closely matches with
experimental value.
Figure 8.3 Variation of temperature profile
for Al 6061 and SS 304 weld at friction time=1 second
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Figure 8.4 Variation of temperature profile
for Al 6061 and SS 304 weld at friction time=0.8 second
Figure 8.5 Variation of temperature profile
for Al 6061 and SS 304 weld at friction time=0.4 second
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Figure 8.6 Variation of temperature profile
For Al 6061 and SS 304 weld at friction time=0.2 second
Figure 8.7 Variation of temperature during friction welding
(In Al 6061 and SS 304)
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Figure 8.7 shows the evolution of temperature with time
adjacent to weld line (as indicated by symbol x on Al 6061 and SS 304
side). Temperature observed in Al 6061 is higher than SS 304 due to
higher thermal conductivity of Al 6061. The temperature observed by
finite element analysis closely matches with experimental value.
8.5.2 COMPARISON OF EXPERIMENTAL AND FEA VALUES FOR
VARIATION OF FRICTION TIME
The variation of experimental weld interface temperature with
friction time for Al 6061-SS 304 is shown in Figure 8.8. The weld
interface temperature increases as friction time increases. The FEA
weld interface temperature closely matches with the experimental
values. The weld interface temperature plays a very important role.
The tendency of intermetallic compounds formation increases as
temperature increases above a threshold value. In welding of Al 6061-
SS 304 the tendency of intermetallic compounds increases if the weld
interface temperature exceeded 270 ° C.
Figure 8.8 Variation of weld interface temperature with friction time
for Al 6061 – SS 304
0
100
200
300
400
0 1 2 3 4
Tem
pe
ratu
re °
C
Friction Time
Experimental Values
FEA Values
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8.5.3 COMPARISON OF WELD INTERFACE TEMPERATURE FOR
REGULAR JOINT GEOMETRY AND NEW JOINT GEOMETRY
The variation of temperature during friction welding for regular
and new joint geometry for friction time 1 second is shown in Figure
8.9.
Figure 8.9 Variation of temperature during friction welding for Al 6061
– SS 304
From Figures 8.9, the maximum temperature of 218°C and 402°C
were observed at the weld interface of new joint geometry and regular
joint geometry respectively. The weld interface temperature generated
in new joint geometry was less than regular joint geometry therefore
the tendency of intermetallic layer formation in new joint geometry is
less when compared to regular joint geometry. The analysis results are
closely matching with experimental results.
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8.5.4 VON MISES STRESS IN AL 6061-SS 304
Maximum finite element Von Mises stress observed at the weld
interface was 115 MPa and 172 MPa for new joint geometry and
regular and joint geometry respectively. The stress decreases as
distance from weld increases.
8.5.5 DEFORMATION
The maximum deformation observed was 7 mm.
8.6 FINITE ELEMENT ANALYSIS OF AL 5052 TO SS 304
Finite element analysis is carried out for Al 5052 and SS 304
combination using ABAQUS code. Temperature dependent material
properties are considered for analysis purpose as shown in Figure
8.10. The C3D8RT element type was used. The general contact
algorithm was used for modeling. An additional algorithm was used to
express the contact definition of the process. The friction welding
process has two steps. First, the friction phase where SS 304 is
rotated against Al 5052 under the compressive force for predefined
time and the second step is forging phase where the rotation is
stopped and the compressive force is increases to complete the weld.
The adoptive mesh is considered to adjust the grain refinement during
the contact deformation process. The thermal effect was assumed to
be adiabatic.
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Figure 8.10 Temperature dependent material properties of Al 5052
(a. Thermal properties and b. Mechanical Properties)
8.7 RESULTS AND DISCUSSIONS FOR AL 5052-SS304
The temperature observed from finite element analysis (with
new joint geometry) at weld interface is 204°C and the experimental
value was 212°C. It can be noticed that the finite element analysis
weld interface temperature is closely matching with experimental
values.
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It is observed that maximum heat generated at weld interface is
transferred to Al 5052 due to high thermal conductivity and maximum
stress of 267 MPa was observed at weld interface.
Figure 8.11 Finite element model of Al 5052-SS 304
Figure 8.12 Physical welded part of Al 5052-SS 304
It can be observed from the above two figures that the shape of
FEA analysis model closely matches with physical weld specimen. The
deformation observed was 8mm which closely matches with
experiment value.