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Correlating process parameters to thrust forces and torque in the friction stir processing of AZ31B

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Abstract

Introduced in this work are correlations that capture the behavior of thrust force and torque vs. input process parameters in friction stir processing (FSP) of twin-roll cast (TRC) AZ3 IB. The correlations are based on the findings of an experimentally validated robust 3D FE model that was used to simulate the FSP process at different values of tool rotational and traverse speeds. The findings are fitted into simple power equations relating thrust force and torque to input parameters of spindle speed and feed. An experimental test matrix was used to validate the proposed correlations. The correlation equations were found to be able to predict the experimentally measured forces during friction stir processing with good statistical significance with average estimate errors of 6.2% and 5.4% for the thrust force and torque, respectively. The thrust force and torque exhibited opposite trends with increasing tool rotational speed. The thrust force increased while the torque decreased as the tool rotational speed increased. Copyright © (2014) by the Society of Manufacturing Engineers All rights reserved.
Proceedings of NAMRI/SME, Vol. 42, 2014
Correlating Process Parameters to Thrust Forces and Torque
in the Friction Stir Processing of AZ31B
Ali H. Ammouri and Ramsey F. Hamade
Department of Mechanical Engineering
American University of Beirut
PO Box 11-0236 Beirut, Lebanon
ABSTRACT
Introduced in this work are correlations that capture the behavior of thrust force and torque vs. input process parameters in
friction stir processing (FSP) of twin-roll cast (TRC) AZ31B. The correlations are based on the findings of an experimentally
validated robust 3D FE model that was used to simulate the FSP process at different values of tool rotational and traverse
speeds. The findings are fitted into simple power equations relating thrust force and torque to input parameters of spindle
speed and feed. An experimental test matrix was used to validate the proposed correlations. The correlation equations were
found to be able to predict the experimentally measured forces during friction stir processing with good statistical
significance with average estimate errors of 6.2% and 5.4% for the thrust force and torque, respectively. The thrust force and
torque exhibited opposite trends with increasing tool rotational speed. The thrust force increased while the torque decreased
as the tool rotational speed increased.
KEYWORDS
Friction stir processing, AZ31B, thrust, torque, speed, feed
INTRODUCTION
Friction stir processing (FSP) is a microstructure
reforming processes originally proposed by Mishra [1] and
is based on the same principles of friction stir welding
(FSW). Whereas FSW welds separate plates, FSP is used to
refine the microstructure. Both processes utilize a rotating
tool that comprises a shoulder and a pin. The tool is first
plunged into the material to be processed and is, then,
traversed across areas of interest to be modified. Severe
mechanical deformation and frictional heating associated
with FSP initiates dynamic recrystallization (DRX) that is
the main mechanism behind grain refinement.
Magnesium alloy AZ31B is one of the light weight alloys
that have potential future in being adopted by the automotive
industry. FSP of magnesium AZ31B is desirable due to the
improvements it grant to the material’s mechanical
properties. These improvements are mainly achieved by
grain refinement and homogeneity that results in superplastic
behavior of alloys. Fine and more homogenized grain
structure of AZ31 was attained by friction stir processing [2].
Ultrafine-grained microstructures with an average grain size
of 100-300 nm were achieved in solution-hardened AZ31
alloy prepared by friction stir processing equipped with a
rapid heat sink [3]. The same approach was followed by
another author who used two-pass FSP to achieve an average
grain size of 85 nm [4]. A recent publication by [5] presented
AZ31 magnesium alloy prepared by friction stir processing
which exhibited 268% elongation at 723K and 10-2 s-1
indicating that high strain rate superplasticity could be
achieved.
The forces exerted on the FSP tool highly depend on the
process parameters especially the tool rotational speed and
feed. Establishing relations between the forces exerted on the
tool and the process parameters of FSP is important for
successful control of the process especially for temperature
sensitive alloys such as the AZ31B. Relationships between
FSP processing parameters and torque for several aluminum
alloys can be found in the literature [6, 7]. Other state
variables such as grain size has been previously reported [8].
In this work we present 2 correlations that relate the
thrust force and torque during FSP to the tool rotational and
traverse speeds using a multiplicative power law which is
commonly used in machining operations. The behavior of
the process thrust force and torque for the considered AZ31B
was found to be similar to what other authors reported for
Aluminum alloys [6, 7]. The correlations were constructed
from the simulation results of an experimentally validated
3D FE model that was constructed using the commercial
DEFORM 3D software. The FE model utilized the physical
based Zerilli-Armstrong material model for HCP material
which is capable of predicting accurate state variables [9].
The proposed correlations are only valid for the range of
process parameters described by the test matrix and for the
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Proceedings of NAMRI/SME, Vol. 42, 2014
geometries of the tool and workpiece adopted in the FE
model.
THE FE MODEL
A 3D thermo-mechanically coupled FE model was
developed using the commercial FEA software DEFORM-
3D™ (Scientific Forming Technologies Corporation, 2545
Farmers Drive, Suite 200, Columbus, Ohio 43235 [10]). The
meshed model shown in Figure 1 consists of a tool, a
workpiece, and backing plate. Both the tool and the backing
plate were modeled as rigid un-deformable bodies where
only heat transfer was accounted for while the workpiece
was modeled as a plastic body subject to both deformation
and heat transfer.
The considered tool had a 19 mm cylindrical shoulder
with a 6.4 mm diameter smooth unthreaded pin that extrudes
2.7 mm from the bottom of the shoulder. Both the workpiece
and the backing plate had an area of 90x40 mm2 and a height
of 3 mm. Materials used in the FEM model were H13 steel
for the tool, AISI-1025 steel for the backing plate and AZ31B
for the workpiece.
Figure 1. The meshed FEM model used in the simulations.
Selecting a proper material model for describing the
mechanical behavior of any material is key for a successful
simulation of friction stir processing where temperature,
strain, and strain rate gradients vary abruptly within, and
when moving away, from the stirring zone. The Zerilli-
Armstrong (ZA) is a physical based constitutive relation that
holds advantages over the empirical constitutive relations
that are usually used as “de-facto” in the simulation of
friction stir processes [11-13]. The ZA model is such a model
that is based on thermally activated dislocation mechanics. It
accounts for strain hardening, strain rate hardening, and
temperature softening as well as the grain size effect. The
HCP-specific ZA material model is described by
 
 
 
 
0 0 1
0 0 1
exp ln
1 exp exp ln
rr
T
T
CB
Ba

 
 

 

(1)
where
is the flow stress,
is the strain rate, T is the
temperature,
is the plastic strain, and C0, B, β0, β1, B0, εr,
α0, and α1 are determined experimentally (see [13] for
detailed explanation of the derivation of the constant terms).
Non-linear regression analysis was used to fit published
[14] experimental tensile test data for wrought twin roll cast
AZ31B into the ZA equations described in Equations 1. The
resulting fit had an R2 value of 0.917 and the corresponding
coefficients are summarized as:
Table 1. Fit coefficients for ZA of TRC wrought AZ31B.
C0
β0
β1
εr
α0
α1
0
3.6 e-3
3.6e-5
0.089
6.1e-4
1.7e-4
Tetrahedral elements were used in the FEM model with
active local re-meshing triggered by a relative interference
ratio of 70% between contacting edges. This would ensure
the integrity of the workpiece geometry during deformation.
The tool and the backing plate were meshed for thermal
analysis purposes with each containing around 6000 and
5000 elements respectively while the workpiece had around
16000 elements. To further capture the state variables at the
tool-workpiece interface, a rectangular mesh control window
was applied around the processing area of interest where
finer mesh elements were created. The conjugate gradient
iterative solver with direct iteration method was used in
deformation calculations whereas the sparse solver was used
for temperature.
Heat transfer with the environment was accounted for all
the three meshed objects with a convective heat coefficient
of 20 W/(m2 ºC) at a constant temperature of 293K. The heat
transfer coefficient between the tool-workpiece and backing
plate-workpiece interfaces was set to 11 kW/(m2 ºC) [15].
Friction at the tool-workpiece interface is a significant
factor in any FSP/FSW simulation. It is determined that 86%
of the heat generated is due to frictional forces [16].
Determination of the friction factor is a daunting challenge
due to the variation of temperature, strain rate, and stress.
Different publications found in literature investigated the
value of friction coefficient in magnesium alloys [17-19].
Most authors use the ring upsetting and compressions tests
for determining the coefficient of friction. It is agreed that
the friction factor increases with temperature [20]. However,
this increase of friction factor with temperature is valid until
the liquidus temperature of AZ31B (630°C) is reached where
the friction drops drastically. The values of experimental
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Proceedings of NAMRI/SME, Vol. 42, 2014
data [17] were entered to the model and then extrapolated by
tuning different runs and analyzing state variables. The
friction coefficient vs. temperature used in the FE model is
shown in Figure 2. This is based on experimental data [17]
as well as on sensitivity analysis for model calibration as
previously published by the authors [21].
Figure 2. Friction coefficient VS temperature as used in the
FE model; shown compared with experimental data [17].
The FE model was used to run FSP simulations for the
24 test cases shown in Table 2. The label shown under each
feed rate is used to refer to matrix test cases. To reduce
simulation times, only the traverse phase of the friction stir
process was considered. The temperature rise from the
skipped plunging phase was accounted for by adding a
dwelling phase of 1 second was to each simulation.
Table 2. The FEM test matrix.
Tool rotational speed, RPM
600
800
1000
1200
1400
1600
1800
2000
Feed rate
,mm/min
75
(A1)
100
(B1)
100
(C1)
150
(D1)
300
(E1)
350
(F1)
400
(G1)
500
(H1)
100
(A2)
125
(B2)
150
(C2)
250
(D2)
500
(E2)
550
(F2)
600
(G2)
700
(H2)
125
(A3)
150
(B3)
200
(C3)
350
(D3)
700
(E3)
750
(F3)
800
(G3)
900
(H3)
SIMULATION RESULTS
Thrust force and torque exerted on the FSP tool were
extracted from the simulation results of each of the test
matrix cells. The values of these variables highly depend on
the instantaneous contact area between the tool and
workpiece and thus a lot of noise is expected in their reported
values. A moving average filter was applied to the force and
torque signals to reduce the noise and to present the data in a
readable format. The dwelling section of each signal was
discarded and only the steady state traverse phase values
were considered. Figures 3 and 4 show a sample of the thrust
force and torque for test cases A2. A similar procedure was
repeated for each of the test cases of Table 2.
Figure 3. Sample of thrust force data resulting from the
FEM simulation of test case A2.
Figure 4. Sample of the torque data resulting from the FEM
simulation of test case A2.
Figures 5a and 5b show a summary of the resulting thrust
force and torque for the test cases of Table 2. It can be
noticed that the thrust force and torque had opposite trends
with increasing tool rotational speed. The thrust force
increased while the torque decreased as the tool rotational
speed increased. The decrease of the thrust force can be
justified by material softening due to the increase in process
temperature whereas the increase in torque could be due to
the extra sticking caused by the increased temperature.
0
0.1
0.2
0.3
0.4
200 400 600 800 1000
Friction Coefficient
Temperature (K)
Friction coefficient variation
Experimental data [17]
As used in FE model
0
4
8
12
16
20
04812 16
Thrust force, kN
Time, sec
Raw data
Resampled and averaged data
0
10
20
30
40
0 4 8 12 16
Torque, Nm
Time, sec
Raw data
Resampled and averaged data
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(a)
(b)
Figure 5. FEM simulations of the (a) Thrust force and (b)
Torque of the 24 test cases of Table 2.
PROPOSED CORRELATION EQUATIONS
The steady state thrust force (F) and torque (τ) of FSP
were related to the tool rotational speed (N) and feed (f)
according to a multiplicative power law which is commonly
used in machining operations:
12
nm
SV A P P 
(2)
where SV is the state variable to be related, P1 and P2
being the process parameters (speed and feed in this case),
with A, n, and m being the fit coefficients of the power law.
In this work, coefficients A, n, and m were determined
by non-linear regression fitting of the simulation results to
Equation 2 using MS Excel solver. The fit coefficients of the
thrust force power relation were 15.11, -0.329, and 0.261
with an R-squared value of 0.76 whereas the coefficients of
the torque were 144.22,
-0.465, and 0.165 with an R-squared value of 0.824.
Equations (3) and (4) describe the proposed correlations
between force and torque from one side and the feed and
speed process parameters from the other side using the power
law described by Equation (2).
0.329 0.261
15.11F N f
  
(3)
0.465 0.165
144.2 Nf
 
(4)
A comparison between the values of the thrust force and
torque obtained from the FEM results and those obtained
from Equations 3 and 4 is shown in Figure 6. It can be
noticed that the proposed mathematical relation accurately
captures the FEM predicted values.
(a)
(b)
Figure 6. Comparison between the results of the proposed
correlations and FEM simulations for (a) Thrust force and
(b) Torque of the 24 test cases of Table 2.
EXPERIMENTAL VALIDATION
The two proposed correlations were experimentally
validated by running single FSP passes for each of the cells
of the test matrix described in Table 2. The runs were
conducted on a HAAS VF6 vertical machining center retrofit
with external hardware to perform friction stir processes.
The tool and workpiece fixture had the same dimensions
of those used in the FEM simulations. Figure 7 shows two
samples of the processed test cases.
(a)
(b)
Figure 7. Friction stir processed samples; Test cases (a) A2
and (b) C2.
Force measurements were collected using the Kistler’s
type 9123C rotary 4-Component (Fx, Fy, Fz, and Torque)
dynamometer. The Kistler 5223B charge amplifier acquires
and amplifies the signal emanating from the dynamometer
which is then collected by a custom developed LabVIEW
0
3
6
9
A1 A2 A3 B1 B2 B3 C1 C2 C3 D1 D2 D3 E1 E2 E3 F1 F2 F3 G1 G2 G3 H1 H2 H3
Thrust force, kN
0
7
14
21
A1 A2 A3 B1 B2 B3 C1 C2 C3 D1 D2 D3 E1 E2 E3 F1 F2 F3 G1 G2 G3 H1 H2 H3
Torque, Nm
0
3
6
9
A1 A2 A3 B1 B2 B3 C1 C2 C3 D1 D2 D3 E1 E2 E3 F1 F2 F3 G1 G2 G3 H1 H2 H3
Thrust force, kN
Obtained from FEM Calculated from Equation 3
0
7
14
21
A1 A2 A3 B1 B2 B3 C1 C2 C3 D1 D2 D3 E1 E2 E3 F1 F2 F3 G1 G2 G3 H1 H2 H3
Torque, Nm
Obtained from FEM Calculated from Equation 4
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Proceedings of NAMRI/SME, Vol. 42, 2014
software through four analog input channels of a National
Instruments USB-6251 data acquisition card.
During the plunging phase in FSP, the rotating tool is
“plunged” into the workpiece at a constant feed rate until the
tool shoulder engages into the top surface of the workpiece.
Since the plunging speed was fixed for all the test cases, the
maximum thrust force and torque during the plunging phase
depended solely on the tool rotational speed which resulted
in similar peaks for the test cases with the same letter prefix.
Figures 8 and 9 show typical thrust force and torque
behaviors during FSP for two different tool speeds. It can be
clearly noticed from both figures that the plunging part
(initial part prior to the vertical dashed line) had the same
amplitude of forces and torques for the considered test cases.
(a)
(b)
Figure 8. Experimental thrust forces for test cases (a) A1
A3 and (b) H1 H3.
(a)
(b)
Figure 9. Experimental torques for test cases (a) A1, A2,
A3 and (b) H1, H2, H3.
The traverse thrust force and torque starts from the
reached plunging values and decrease to constant steady state
values. These values depend of the feed of the considered test
case. Both steady state torques and forces increased with
increasing feed at constant tool rotational speeds due to less
material softening as feed increased (less heat is generated at
faster feeds). The steady state force and torque had opposite
trends with increasing tool rotational speed. The thrust force
increased while the torque decreased as the tool rotational
speed increased. The decrease of the thrust force can be
justified by material softening due to the increase process
temperature (as determined earlier from the FEM results)
whereas the increase in torque could be due to the extra
sticking caused by the increased temperature (which
correlates to the friction factor adopted in the FEM
simulations).
Figures 10a and 10b compare the experimental steady
state traverse thrust force and torque results of the 24 cells of
the test matrix with the results of the proposed correlations
(Equations 3 and 4).
(a)
(b)
Figure 10. Comparison of the correlations’ results and the
experimental results of (a) Thrust force and (b) Torque
Good agreements can be noted between the
experimentally determined results and the values obtained
from the proposed correlations. The percent prediction error
of the correlations which is the percentage of variation of the
correlation predictions from the experimental results was
calculated for quantitative assessment. The average,
maximum, minimum, and standard deviation of errors were
6.2%, 17.7%, 0.5%, and 5.4% for the thrust forces and 5.4%,
13.3%, <0.1%, and 3.8% for torques and which indicate good
statistical agreement.
CONCLUSION
Developed in this work are correlation equations that
capture the behavior of thrust force and torque during friction
stir processing (FSP) of AZ31B as function of the process
parameters (namely tool rotational speed and traverse feed).
The equations utilized the results of 3D FEM simulation runs
of a test matrix covering a wide range of process parameters
ranging from 600 2000 RPM for tool rotational speed and
75 90 mm/min for tool traverse feed. Experimental test
cases were used to demonstrate the validity of the proposed
correlations.
Since multiplicative power law is widely used in the
modeling of fabrication operations, the correlation equation
0
5
10
15
20
020 40 60
Thrust force, kN
Time, sec
A1
A2
A3
0
5
10
15
20
020 40 60
Thrust force, kN
Time, sec
H1
H2
H3
0
15
30
020 40 60
Torque, Nm
Time, sec
A3
A2
A1
0
15
30
020 40 60
Torque, Nm
Time, sec
H3
H2
H1
0
3
6
9
A1 A2 A3 B1 B2 B3 C1 C2 C3 D1 D2 D3 E1 E2 E3 F1 F2 F3 G1 G2 G3 H1 H2 H3
Thrust force, kN
Calculated from Equation 3 Experimental
0
7
14
21
A1 A2 A3 B1 B2 B3 C1 C2 C3 D1 D2 D3 E1 E2 E3 F1 F2 F3 G1 G2 G3 H1 H2 H3
Torque, Nm
Calculated from Equation 4 Experimental
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Proceedings of NAMRI/SME, Vol. 42, 2014
was assumed to take on this form. The resulting power
equations were shown to be capable of successfully fitting
the results of the FEM simulations with R2 values of 0.76 and
0.824 for the thrust force and torque, respectively. The better
fit coefficient of the torque could be due to the lower signal
to noise ratio when compared to the thrust force. This
variation arises from the re-meshing approximation and
frequency which is utilized quite often in the modeling of
FSP due to the severe deformation undergone by the work
material during the process.
When tested, the proposed correlations were able to
predict the experimental thrust force and torque for the 24
test cases with an average estimate error (percentage of
variation of the correlation prediction to the experimental
result) of 6.2% and 5.4% with a standard deviation of 5.4%
and 3.8%, respectively..
The thrust force and torque had opposite trends with
increasing tool rotational speed. The thrust force increased
while the torque decreased as the tool rotational speed
increased. The decrease of the thrust force can be justified by
material softening due to the increase process temperature
whereas the increase in torque could be due to the extra
sticking caused by the increased temperature.
The proposed correlations are valid for the range of
process parameters described by the test matrix and for the
geometries of the tool and workpiece adopted in the FE
model.
ACKNOLEDGEMENTS
This publication was made possible by the National
Priorities Research Program (NPRP) grant #09-611-2-236
from the Qatar National Research Fund (a member of The
Qatar Foundation). The statements made herein are solely
the responsibility of the authors. The first author gratefully
acknowledges the support of Consolidated Contractors
Company (CCC) through the CCC Doctoral Fellowship in
Manufacturing.
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... Many researchers [24,26,[106][107][108][109][110][111][112][113][114][115] have opted Lagrangian approach to simulate FSW. With this approach, FSW can be simulated from inception to steady-state which is not possible with Eulerian approach. ...
... A rotating type piezoelectric Kistler dynamometer is used to measure force, and thermocouples are used for temperature measurement. Various researchers [113][114][115] have also used Lagrangian technique to simulate FSW for different materials like copper, magnesium alloys etc. They have predicted temperature and strain field and also gave an insight on the material flow using point tracking method. ...
Chapter
Full-text available
Friction Stir Welding (FSW) is a new solid-state welding technique which finds application in various industries. This chapter introduces the process, basic mechanism, application, and recent research developments. Research work in this book chapter is broadly divided in two parts: experimental-based, and finite element modeling (FEM)-based approaches of the FSW process. In the experimental studies, three recent developments are presented in this chapter: first, a unique twin-tool concept to modify the FSW process and provide alternative to multi-pass FSW; second, feasibility of using ultrasonic coupled with FSW is studied to reduce the amount of force generated during the process and improve the process efficiency; and finally, formability study of friction stir welded blank is presented. Formability of welded blank plays a vital factor for different industrial application, especially in automobile industry. In the second part, FEM method is implemented to simulate the process. Different modeling techniques are also discussed. A case study in each case is presented with sample results, to have a better understanding on the process and development.
... Many researchers [24,26,[106][107][108][109][110][111][112][113][114][115] have opted Lagrangian approach to simulate FSW. With this approach, FSW can be simulated from inception to steady-state which is not possible with Eulerian approach. ...
... A rotating type piezoelectric Kistler dynamometer is used to measure force, and thermocouples are used for temperature measurement. Various researchers [113][114][115] have also used Lagrangian technique to simulate FSW for different materials like copper, magnesium alloys etc. They have predicted temperature and strain field and also gave an insight on the material flow using point tracking method. ...
Chapter
Full-text available
Friction Stir Welding (FSW) is a new solid-state welding technique which finds application in various industries. This chapter introduces the process, basic mechanism, application, and recent research developments. Research work in this book chapter is broadly divided in two parts: experimental-based, and finite element modeling (FEM)-based approaches of the FSW process. In the experimental studies, three recent developments are presented in this chapter: first, a unique twin-tool concept to modify the FSW process and provide alternative to multi-pass FSW; second, feasibility of using ultrasonic coupled with FSW is studied to reduce the amount of force generated during the process and improve the process efficiency; and finally, formability study of friction stir welded blank is presented. Formability of welded blank plays a vital factor for different industrial application, especially in automobile industry. In the second part, FEM method is implemented to simulate the process. Different modeling techniques are also discussed. A case study in each case is presented with sample results, to have a better understanding on the process and development.
... With increase in the rotational speed from 600 rpm to 1000 rpm, welding force has reduced from 4270 N to 2523 N. This indicates that welding force decreases with increase in rotational speed of the tool, this similar behavior of welding force has also been reported in the literature [29][30][31]. Similar to axial force, spindle torque also increases in plunging phase and attains a maximum value during impingement of shoulder, followed by a drop during plunging phase and attaining a steady value during welding, as shown in Fig. 5. ...
Article
Full-text available
A three dimensional coupled thermo-mechanical finite element model (FEM) is proposed to simulate a friction stir welding (FSW) process based on Lagrangian incremental technique. Since FSW is a large deformation process, workpiece is considered as a rigid visco-plastic material. The model has been developed for predicting forces, spindle torque, temperature and plastic strain for a butt welding between two AA2024-T4 metals having thickness of 5.9 mm each. The developed model has been validated with experimental results (forces, spindle torque) obtained from literature. Maximum force is obtained during the plunging phase of the tool and this makes tool susceptible to failure. Forces and spindle torque reduce with the increase in rotational speed due to increase in heat generation rate which is also reflected in temperature distribution. Effect of welding speed and frictional boundary condition are studied. Conical pin shape produces higher material velocity as compared to cylindrical with reduced plunge force.
Article
Introduced in this work is a relation that captures the behavior of grain size with the varying process parameters in friction stir processing of AZ31B. The relation was based on the results of a 3D FE model that was used to run simulations of the process at different tool rotational and traverse speeds. The model was validated by comparing its state variable outputs to experimental results found in the literature. The coefficients of the proposed relation were determined for magnesium alloy AZ31B. This proposed relation will aid in controlling the output grain size in computerized friction stir processes.
Article
The dynamic friction properties of the extruded AZ31 magnesium alloy with the initial average grain size of 15 μm were investigated by the ring compression test at 473 and 523 K and in a strain rate range from 1.0×10−2 to 3.0 s−1. Two types of the tool, WC-Co tool (WC) and WC-Co coated with diamond like carbon tool (DLC) were used. At 523 K, few differences in terms of the friction coefficient were observed due to the difference with or without DLC. At 473 K, the friction coefficient for the sample deformed by DLC tool was smaller than that done by WC tool. The investigation of the texture near the surface of the tested work pieces with different tools reveals that the integration degree of the grains within 10 degree from 〈0001〉 direction to compressive axis in the sample deformed by the DLC tool was smaller than that done by WC tool. It was concluded that the larger friction could enhance alignment of the planes perpendicular to the compressive direction to the basal plane even if under same testing condition.
Conference Paper
Utilizing a proper material model for describing the mechanical behavior of any material is key for a successful simulation of friction stir processing (FSP) where temperature, strain, and strain rate gradients vary abruptly within, and when moving away, from the stirring zone. This work presents a comparison of how faithfully do three different constitutive equations reproduce the state variables of strain, strain rate, and temperature in an FEM simulation of a test-case FSP (1000 rpm spindle speed, and 90 mm/min feed). The three material models considered in this comparison are namely: Johnson-Cook (JC), Sellars-Tegart (ST), and Zerilli-Armstrong (ZA). Constants for these constitutive equations are obtained by fitting these equations to experimental mechanical behavior data collected under a range of strain rates and temperatures of twin-rolled cast wrought AZ31B sheets. It is widely recognized that JC-based models over predicts stress values in the stir zone whereas STbased models are incapable of capturing work hardening outside of the stir zone. Therefore, a ZA model, being a physical based-HCP specific model, is hereby investigated for its suitability as a material model that would overcome such drawbacks of JC- and ST-based models. The equations from the constitutive models under consideration are fed into an FEM model built using DEFORM 3D to simulate the traverse phases of a friction stir process. Amongst these three material models, comparison results suggest that the HCP-specific ZA model yield better predictions of the state variables: strain, strain rate, and temperature, and, consequently, the estimated values for flow stresses.
Conference Paper
Controlling the temperature in friction stir processing (FSP) of Magnesium alloy AZ31b is crucial given its low melting point and surface deformability. A numerical FEM study is presented in this paper where a thermo-mechanical-based model is used for optimizing the process parameters, including active in-process cooling, in FSP. This model is simulated using a solid mechanics FEM solver capable of analyzing the three dimensional flow and of estimating the state variables associated with materials processing. Such processing (input) parameters of the FSP as spindle rotational speed, travel speed, and cooling rate are optimized to minimize the heat affected zone, while maintaining reasonable travel speeds and producing uniformity of the desired grain size distribution of the microstructure in the stirred zone. The simulation results predict that such optimized parameters will result in submicron grain sized structure in the stirred zone and at the corresponding stirred surface. These simulation predictions were verified using published experimental data.
Article
Proper numerical modeling of the Friction Stir Processes (FSPs) requires the identification of a suitable constitutive equation which accurately describes the stress-strain material behavior under an applicable range of strains, strain rates, and temperatures. While some such equations may be perfectly suitable to simulate processes characterized by low (or high) strains and temperatures, FSPs are widely recognized for their relatively moderate ranges of such state variables. In this work, a number of constitutive equations for describing flow stress in metals were screened for their suitability for modeling Friction Stir Processes of twin roll cast (TRC) wrought magnesium Mg–AL–Zn (AZ31B) alloy. Considered were 4 different reported variations of the popular Johnson–Cook equation and one Sellars–Tegart equation along with their literature–reported coefficients for fitting AZ31B stress–strain behavior. In addition, 6 variations of the (rarely used in FSPs simulations) Zerilli–Armstrong equation were also considered along with their literature–reported coefficients. The screening assessment was based on how well the considered constitutive equations fit experimental tensile stress–strain data of twin roll cast wrought AZ31B. Goodness of fit and residual sum of squares were the two statistical criteria utilized in the quantitative assessment whereas a ‘visual ’ measure was used as a qualitative measure. Initial screening resulted in the selection of one best fitting constitutive equation representing one of each of the Johnson–Cook, Sellars–Tegart, and Zerilli–Armstrong equations. An HCP–specific Zerilli–Armstrong constitutive equation (dubbed here as ZA6 ) was found to have the best quantitative and qualitative fit results with an R2 value of 0.967 compared to values of 0.934 and 0.826 for the Johnson–Cook and Sellars–Tegart constitutive equations, respectively. Additionally, a 3D thermo–mechanically coupled FEM model was built in DEFORM 3D to simulate the experimental tensile test from which the experimental load–deflection data was obtained. The three ‘finalist ’ equations were fed into the FEM simulations and were compared based on the 1) simulations’ running times and 2) goodness of fit of the simulation results to the experimental load–deflection data. It was found that the ZA6 constitutive equation exhibited favorable run times even when contrasted against the simpler mathematical form of the Sellars–Tegart equation. On average, the ZA6 equation showed improvements in solution time by 5.4% as compared with the Johnson–Cook equation and almost identical solution time (0.9% increase) with that of the ST equation. This result indicates that the proposed equation is not numerically expensive and can be safely adopted in such FEM simulations. Based on the favorable running times and goodness of fit, it was concluded that the HCP–specific Zerilli–Armstrong constitutive equation ZA6 holds an advantage over all other considered equations and was, therefore, selected as most suitable for the numerical modeling of FSP of twin roll cast AZ31B.
Article
This article investigates the effects of the strain rate and temperature on the microstructural evolution of twin-rolled cast wrought AZ31B sheets. This was achieved through static heating and through tensile test performed at strain rates from 10−4 to 10−1 s−1 and temperatures between room temperature (RT) and 300 °C. While brittle fracture with high stresses and limited elongation was observed at the RT, ductile behavior was obtained at higher temperatures with low strain rates. The strain rate sensitivity and activation energy calculations indicate that grain boundary diffusion and lattice diffusion are the two rate-controlling mechanisms at warm and high temperatures, respectively. An analysis of the evolution of the microstructure provided some indications of the most probable deformation mechanisms in the material: twinning operates at lower temperatures, and dynamic recrystallization dominates at higher temperatures. The static evolution of the microstructure was also studied, proving a gradual static grain growth of the AZ31B with annealing temperature and time.
Article
In the present study, an investigation has been carried out on the friction stir processing (FSP) of an AlSi9Mg cast aluminium alloy. The relationship between the FSP parameters and the torque action on the tool, temperature of the modified surface and microstructure was investigated. It was found that an increase in rotational speed of the tool causes a decrease in the torque and an increase in temperature of the processed material. Simultaneously, the results showed that an increase in travelling speed of the tool increases the torque and decreases the temperature. The metallographic examination of the processed surface layer of the material has shown that the microstructure of stir zone can be refined. The penetration depth in which the microstructure was modified by the shoulder action mainly depended on the rotation speed and to a lesser extent on the travelling speed.
Article
Microstructure and tensile behaviors of AZ31 magnesium alloy prepared by friction stir processing (FSP) were investigated. The results show that microstructure of the AZ31 hot-rolled plate with an average grain size of 92.0 μm is refined to 11.4 μm after FSP. The FSP AZ31 alloy exhibits excellent plasticity at elevated temperature, with an elongation to failure of 1050% at 723 K and a strain rate of 5×10−4 s−1. The elongation of the FSP material is 268% at 723 K and 1×10−2 s−1, indicating that high strain rate superplasticity could be achieved. On the other hand, the hot-rolled base material, which has a coarse grain structure, possesses no superplasticity under the experimental conditions.
Article
The extrusion process of AZ31 magnesium alloy has been simulated. First, a series of tests were designed to obtain the simulation parameters: stress–strain curves, friction factors and heat transfer coefficient. Then, a special extrusion process was carried out and simulated by DEFORM-2D simultaneously. The results reveal that the simulated temperature–time curve agrees well with the measured one, indicating the good accuracy of the simulation parameters. Accordingly, the extrusion of AZ31 at different conditions was analyzed in detail. The extrusion loads and temperature distribution of billets were obtained.