# Mechanical Behaviour and Microstructure of  Aluminum-Steel Sheets Joined by FSSW

Authors: S. Bozzi; A. L. Etter; T. Baudin; A. Robineau; J. C. Goussain
Journal: Texture, Stress, and Microstructure
 (2008)

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## Abstract

At the prospect of a lightening of the automobile
structures, welded spots have been realized on a stacking of
two sheets (a 6008 aluminum alloy on steel) Friction Stir
Spot welding (FSSW). Different process parameters have been
tested, but only the influence of the dwell time will be
described in the present paper. The dwell time corresponds to
the time during which the probe stays in rotation at its
bottom location before extracting. A study of the
microstructures and textures associated to mechanical tests
(tensile shear tests) allowed determining the best set of
welding parameters. The recrystallized area around the welding
spot has been characterized by electron back-scattered
diffraction (EBSD). A mechanism of continuous dynamic
recrystallization has been identified since the misorientation
of subboundaries increases close to the weld,  and this is for all
the dwell times tested. Elsewhere, the increase of the dwell
time induced a larger recrystallized zone. It has also been
found that a long dwell time induced a larger welded area but
also a higher quantity of intermetallic compounds (especially
FeAl,Fe2Al7, and FeAl2) with high-microhardness values (up to 800 Hv). Thus, the dwell time must not exceed a certain value, otherwise it can weaken the weld.


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## Body

## 1. Introduction

Introducing the aluminum on the automotive structures would permit to reduce their weight [1, 2] and so to
reduce the fuel consumption.  However,  the aluminum presents a lower mechanical
resistance comparing to steel.  So assembling both aluminum and steel would
permit to solve this problem and then to reduce the weight of the automotive
bodies. The friction stir spot welding (FSSW) is expected to provide a practical
solution for joining aluminum and steel [3, 4].This kind of bimetallic welding is
still little known,   and many tests and studies must be performed in order to
industrialize the process. The purpose of this work consists in studying the
microstructures and the recrystallization mechanisms of the aluminum steel
welding according to the dwell time, in order to optimize their mechanical
resistance. Some authors dedicated their works for visualizing the
microstructure obtained during the linear FSW of aluminum alloys [5, 6] and the
FSSW [7, 8]. Concerning the microstructural evolution on FSSW, the studies permitted
to identify the different areas inside the joint [8]: the BM (base metal), the
HAZ (heat affected zone), the TMAZ (thermomechanically affected zone), and the
SZ (stir zone); but the recrystallization mechanisms are less touched on, and so
this present work will be useful to better understand the metallurgical
phenomena occuring during this process.Moreover, the formation of brittle
intermetallic compounds varies according to the welding conditions and has to
be avoided as much as possible. Indeed, it has been shown that those
intermetallic compounds can cause a weakening of the spot welds because of high
microhardness [9, 10].


## 2. The Friction Stir Spot Welding (fssw)

The technique FSSW is purely
mechanical. It permits an assembly without fusion and presents several
advantages, especially for the materials such as aluminum, magnesium, and
copper alloys, which are not easily joined with conventional fusion welding
method.The process is composed of three different steps
(Figure1). At first, the tool
is formed by a probe and a shoulder is put in fast rotation (Phase 1).  Under a defined effort,
the shoulder gets in contact with the surface of sheets, which must be welded (Phase 2). The heat generated by
friction involves a local softening of the material, and so it permits the
mixing of both parts; a new metallurgical structure common to the two materials
is formed. In this manner, the metallic link is insured without reaching the
fusion temperature, which permits to avoid the problems which generally occur
for the classical ways of welding (porosities, blisters, and cracking, etc.).
Lastly, the tool in rotation is removed from the material (Phase 3), and it remains a print at
the surface.Figure 1
The different steps of FSSW (Friction Stir Link).Therefore, the principal welding
parameters are the rotation speed of the tool, the effort of welding, and the
dwell time, which corresponds to the time during which the tool stays in rotation
at the bottom of the weld and the geometry and the material used to machine the
FSW tool. The quality of the spot welds depends directly on the different
parameters.


## 3. Material and Experiments

### 3.1. Configuration and Materials

The studied spot welds were realized
at the FSW Center of the Institut de
Soudure at Goin, in France. The FSW probe is machined into rhenium tungsten alloy (W25Re), and the shoulder
is made of steel Z38CDV5 treated at 50 HRC.A 6008 aluminum alloy plate of 2.5 mm thickness is superimposed
to a galvanized steel plate of 2 mm thickness. The nature of the steel used here is specific
to the automotive industry. The tool has a diameter of 6 mm. As the tool is 4 mm length, it penetrates
completely into the aluminum sheet and partially into the steel sheet.Each sample was cut out (Figure2) and mechanically polished.
Lastly, the samples underwent an electrolytic polishing with a solution Struers
A2 adapted to steel and aluminum alloys (made up of 60% of perchloric acid), at
20°C, with a tension of 45 V during 12 seconds.Figure 2
Photography of a cut sample, which shows the studied plan of the welds.Three samples with different values
of dwell time were studied (the other parameters remained constant, with a
rotation speed of 2000 rpm). From sample 1 to sample 3, the dwell time, Dt,
increases from 0 second to 2 seconds.


### 3.2. Characterization and Analysis Methods

To make first observations on the welding,
optical microscopes, SEM, and FEG/SEM were used. The scanning electron
microscope was also used for the chemical analysis by EDS. Then, orientation
imaging microscopy (OIM) attached to a Zeiss DSM 940 SEM was employed to
evaluate grain-size distribution and grain-boundary misorientation.To assess the fragility of the
intermetallic compounds and for more complete information, Vickers
microhardness tests were realized with a 100 g loaded tester, using a LECO
M400H.Analyses by microprobe were also
performed to obtain the distribution of the chemical elements across the
welding. Those experiments were realized at the LCMTR (Laboratoire de Chimie Métallurgique
des Terres Rares) at Thiais.Lastly, concerning the mechanical
properties, some tensile shear tests were carried out.


## 3.1. Configuration and Materials

The studied spot welds were realized
at the FSW Center of the Institut de
Soudure at Goin, in France. The FSW probe is machined into rhenium tungsten alloy (W25Re), and the shoulder
is made of steel Z38CDV5 treated at 50 HRC.A 6008 aluminum alloy plate of 2.5 mm thickness is superimposed
to a galvanized steel plate of 2 mm thickness. The nature of the steel used here is specific
to the automotive industry. The tool has a diameter of 6 mm. As the tool is 4 mm length, it penetrates
completely into the aluminum sheet and partially into the steel sheet.Each sample was cut out (Figure2) and mechanically polished.
Lastly, the samples underwent an electrolytic polishing with a solution Struers
A2 adapted to steel and aluminum alloys (made up of 60% of perchloric acid), at
20°C, with a tension of 45 V during 12 seconds.Figure 2
Photography of a cut sample, which shows the studied plan of the welds.Three samples with different values
of dwell time were studied (the other parameters remained constant, with a
rotation speed of 2000 rpm). From sample 1 to sample 3, the dwell time, Dt,
increases from 0 second to 2 seconds.


## 3.2. Characterization and Analysis Methods

To make first observations on the welding,
optical microscopes, SEM, and FEG/SEM were used. The scanning electron
microscope was also used for the chemical analysis by EDS. Then, orientation
imaging microscopy (OIM) attached to a Zeiss DSM 940 SEM was employed to
evaluate grain-size distribution and grain-boundary misorientation.To assess the fragility of the
intermetallic compounds and for more complete information, Vickers
microhardness tests were realized with a 100 g loaded tester, using a LECO
M400H.Analyses by microprobe were also
performed to obtain the distribution of the chemical elements across the
welding. Those experiments were realized at the LCMTR (Laboratoire de Chimie Métallurgique
des Terres Rares) at Thiais.Lastly, concerning the mechanical
properties, some tensile shear tests were carried out.


## 4. Results and Discussion

### 4.1. Mechanical Resistance

The influence of welding parameters
on the mechanical resistance of the spot weld was analyzed. With the better
offset of parameters, tensile shear resistance of about 4 kN has been reached. A lack
of time and a lack of samples have limited the number of possible tensile shear
tests. However, even if three points are not enough to reach a conclusion, the
average of mechanical strength tends to show that the more the dwell time
increases, the more the welding is resistant. More experiments will be realized
to confirm this trend, but it seems that over a threshold the mechanical
resistance stays equivalent (Figure3).
It can also be observed that there seems to be more reproductibility problems
for the dwell times of 1 second and of 2 seconds, than without dwell time (0
second).Figure 3
Evolution
of tensile shear strength versus the dwell time.


### 4.2. Material Flow

#### 4.2.1. General View

A general view of the sample was
obtained using the scanning electron microscope (Figure4). The welding holds thanks to the area called the “hanging.”
It corresponds to the entrance of the steel into the Al alloy, possibly because
of the stirring of the material during the welding.Figure 4
General view of a welding obtained by the scanning electron microscope.The comparison of the hangings shows
geometrical differences depending on the dwell time. To understand its influence,
different distances were measured: the size of the base (b), the height (h),
and the width (w) of the “hanging” (Figures5, 6). It shows that while the dwell time increases, the size of the
base increases too, which can explain the better mechanical resistance obtained
with a high dwell time. It also shows that when the dwell time is low, the
hanging stretches mostly in height. Conversely, when the dwell time is high,
the hanging stretches laterally, without gaining height.Measurements of the dimensions of the hangings SEM micrographes of two hangings (dwell time of 0 second (a) and 2 seconds (b)).
(a)(b)Figure 6
Curves of h and b versus the dwell time.


#### 4.2.2. Chemical Distribution

Then, by observing the cartographies
obtained by microprobe (Figure7),
it can be noted that there is migration of steel into the Al alloy, Moreover,
the areas with intermetallic compounds can be located. An important chemical
heterogeneity exists. The maps of the chemical elements highlight also that the
increase of the dwell time induces an increase of the heterogeneity of the
intermetallic areas.Microprobe maps of the Fe and Al distributions around a hanging (weight percentage, 3μm step, LCMTR Thiais in France) for the samples with Dt = 0 second (a) and with Dt = 2 seconds (b).
(a)
Dt = 0 s(b)
Dt = 2 sA chemical analysis was performed by
EDS, and it permitted to conclude that, according to the dwell time, there are
different imtermetallic compounds, which are formed during welding. Indeed, for
the lower value (Figure7(a)),
there is formation of FeAl, Fe3Al. On the other hand, for the higher
dwell time (Figure 7(b)), there
is formation of FeAl, Fe3Al, Fe2Al7, and FeAl3.
The microhardness of the intermetallic compounds which appear at the higher
dwell time is much more important, and so much more weakening than for the
lower (Table 1).Table 1
Description of the different intermetallic compounds.
Bimetallic compoundsDwell time (s)Al content (atomic percentage)Structure [10]Microhardness (HV) [10]Fe3Al0 and 225Ordered body-centered cubic structure260–370FeAl0 and 250Ordered body-centered cubic structure420–520Fe2Al7263Complex body-centered cubic structure620–730FeAl3274–76Highly complex monoclinic body-centered cubic structure820Then, the intermetallic areas have
been quantified (Table2), and it has been found that increasing the dwell time results
in creating more intermetallic compounds.Table 2
Intermetallic compounds surface fraction according to the dwell time (obtained by weighing).
Dwell time (second)012Intermetallic compounds fraction0.170.180.20It can be noted that there are important movements of zinc inside the aluminum
part (Figure8), which comes from
the galvanized layer of the steel sheet. The cartography of zinc distribution
permits to observe the expanse of the area affected by zinc, with a high
concentration under the shoulder. As the melting temperature of zinc is worth 420°C, it can be assumed that
this element becomes liquid during welding.Figure 8
Microprobe cartographies of Zn distribution around the “hanging” (weight percentage, 3μm step, LCMTR Thiais in France) for the sample with Dt = 2 seconds.


#### 4.2.3. The Cracked Areas

Many cracks appeared in the
intermetallic areas around the hanging (Figure9), but there are also frequent cracks at the interfaces between steel
and the intermetallic compounds. These cracks may come from the passage to a
molten state of the material induced by high temperatures. Then, during the
cooling, the difference between the expansion coefficient of the steel and that
of the intermetallic compounds may have resulted in the cracking at the
interfaces.Figure 9
SEM micrograph of the right hanging of the
sample with Dt = 2 seconds (back-scattered electrons).To conclude, the weld holds
thanks to the area called “the hanging.” The more “the hanging” is wide, the
more the welding is resistant. As the welding is bimetallic, there is formation
of intermetallic compounds at the aluminum steel interface around the hanging.
It has been found that the more the dwell time is high, the more “the hanging”
is wide, but also the more there is presence of intermetallic compounds, which weaken
the weld because of their high microhardness [9, 10], and so which could
explain the mechanical properties evolution.


#### 4.2.4. The Microstructure of Alloys Before Welding

The Al alloy sheet was hot rolled.
Therefore, original microstructure is composed of medium-sized grains (with a
diameter between 60 and 80μm) (Figure 10(a)). There are almost no subboundaries (defined as grain boundary which
misorientation is lower than 15°). And from the obtained microstructure, it can
also be noticed that most grains have a {001} plan parallel to the studied plan.(a) The original microstructure in the studied plan under a step size of 1μm. (b) The texture in the rolling plan of the Al alloy.
(a)(b)Then, the calculation of the orientation
distribution function (ODF) corresponding to the aluminum before welding (Figure10(b)) presents a classical
texture composed of the cube orientation {100}〈010〉. The presence of a
secondary component close to Goss orientation can be observed.Concerning the steel microstructure,
it is constituted of small grains (around 8μm) (Figure 11). There are many grains whose plan is {101}. The
obtained texture, represented in the Euler space in Figure 11(b), corresponds
to a classical texture of rolling, with the presence of α and γ fibres visible
on the section at φ2 = 45°. It can also be noted that the sharpness stays relatively
weak because the maximum reached is only 6.4.(a) The original microstructure in the studied plan under a step size of 0.2μm. (b) The texture in the rolling plan of the steel.
(a)(b)


#### 4.2.5. Comparison of the Microstructures Obtained  with Different Dwell Times

Different orientation maps were
measured by EBSD in the aluminum part, all of dimension 200μm × 400 μm with a
1 μm step. To observe the evolution of the microstructure closer and closer to
the welding spot, horizontal displacements were performed from the original
microstructure to “the hanging” (Figure 12) for 2 values of dwell time (0 and 2 seconds).Figure 12
Comparison of the microstructure evolution between Dt = 0 second and Dt = 2 seconds (under a step size of 1μm).The most important difference
between both dwell times concerns the extent of the recrystallized area.
Indeed, for Dt = 0 second, at 9000μm far from the weld center, the microstructure
corresponds to the original one. But, on the contrary, for Dt = 2 seconds, at
9000 μm, the microstructure is still far from the original one; the grains are
elongated, and it remains subboundaries. This difference is highlighted by the
curve in Figure 13, which shows the evolution of the percentage of subboundaries
depending on the dwell time.Figure 13
Percentage of low-angle boundaries versus the
distancex from the welding center.The passage from a microstructure composed of subboundaries to a microstructure
without subboundaries, when approaching the hanging, corresponds to the maximum
of the curves in Figure13. Indeed, getting closer to the welding center, the
mixing tends to increase the boundary misorientation. Then, when the
misorientation reaches 15°, the low-angle boundaries are replaced by the high-angle
boundaries, which correspond to the maximum of the curves. There seems to be a
shift of this maximum depending on the dwell time because of the difference in the temperature
and the deformation reached in the welds. Indeed, if we assume that the passage
to a misorientation higher than 15° is associated to a critical couple (ε,T)c, it is obvious that this couple
will be reached farther from the welding center for the high dwell time, as
illustrated in Figure 14.Figure 14
Hypothetical
curves of the couple (ε,T) as a function of the distance from the
welding center.After those maxima, the decrease in percentage
of low-angle boundaries is due to the transformation of low-angle to high-angle
boundaries. Then, the change of slope in the curve (ε, T) at around 1 mm from the weld center can
be due to the friction effect of the tool on the material. This change of slope
induces the drastic drop of low-angle boundaries observed very close to the nugget
(Figures 13, 14), resulting in
the formation of dynamically recrystallized microstructure with small grains
highly misoriented. Obviously, it remains to check this hypothesis of (ε, T) evolution by a simulation of temperature
and deformation, for instance, [11].


#### 4.2.6. Evolution of the Microstructure of the Aluminum  Part After Welding

Far from the welding nugget, the
microstructure is described above, that is, the microstructure of the original aluminum
sheet. Then, getting closer to the welding center, there is an apparition of
dislocation substructure, characterized by low-angle (misorientation <15°)
boundaries inside grains. Indeed, when the temperature and the deformation
increase, the phenomenon of recrystallization is more activated and so the subboundaries
misorientation increases [12, 13].  Consequently, near the welding center, a
recrystallized microstructure of small grains is obtained. This metallurgical
phenomenon corresponds to the continuous
dynamic recrystallization (Figure 15).
The texture obtained
close to the welding center seems to be identical with the two values of the
dwell time; it is composed of the orientation {012}〈221〉, but these
results must be taken with precaution because the studied areas are small and
contain large grains (less than 100 grains per map), and so the statistic is
poor. In order to refine the analysis of the texture, it would be better to
increase the size of the analyzed areas.Figure 15
The mechanism of continuous dynamic recrystallization.


## 4.1. Mechanical Resistance

The influence of welding parameters
on the mechanical resistance of the spot weld was analyzed. With the better
offset of parameters, tensile shear resistance of about 4 kN has been reached. A lack
of time and a lack of samples have limited the number of possible tensile shear
tests. However, even if three points are not enough to reach a conclusion, the
average of mechanical strength tends to show that the more the dwell time
increases, the more the welding is resistant. More experiments will be realized
to confirm this trend, but it seems that over a threshold the mechanical
resistance stays equivalent (Figure3).
It can also be observed that there seems to be more reproductibility problems
for the dwell times of 1 second and of 2 seconds, than without dwell time (0
second).Figure 3
Evolution
of tensile shear strength versus the dwell time.


## 4.2. Material Flow

### 4.2.1. General View

A general view of the sample was
obtained using the scanning electron microscope (Figure4). The welding holds thanks to the area called the “hanging.”
It corresponds to the entrance of the steel into the Al alloy, possibly because
of the stirring of the material during the welding.Figure 4
General view of a welding obtained by the scanning electron microscope.The comparison of the hangings shows
geometrical differences depending on the dwell time. To understand its influence,
different distances were measured: the size of the base (b), the height (h),
and the width (w) of the “hanging” (Figures5, 6). It shows that while the dwell time increases, the size of the
base increases too, which can explain the better mechanical resistance obtained
with a high dwell time. It also shows that when the dwell time is low, the
hanging stretches mostly in height. Conversely, when the dwell time is high,
the hanging stretches laterally, without gaining height.Measurements of the dimensions of the hangings SEM micrographes of two hangings (dwell time of 0 second (a) and 2 seconds (b)).
(a)(b)Figure 6
Curves of h and b versus the dwell time.


### 4.2.2. Chemical Distribution

Then, by observing the cartographies
obtained by microprobe (Figure7),
it can be noted that there is migration of steel into the Al alloy, Moreover,
the areas with intermetallic compounds can be located. An important chemical
heterogeneity exists. The maps of the chemical elements highlight also that the
increase of the dwell time induces an increase of the heterogeneity of the
intermetallic areas.Microprobe maps of the Fe and Al distributions around a hanging (weight percentage, 3μm step, LCMTR Thiais in France) for the samples with Dt = 0 second (a) and with Dt = 2 seconds (b).
(a)
Dt = 0 s(b)
Dt = 2 sA chemical analysis was performed by
EDS, and it permitted to conclude that, according to the dwell time, there are
different imtermetallic compounds, which are formed during welding. Indeed, for
the lower value (Figure7(a)),
there is formation of FeAl, Fe3Al. On the other hand, for the higher
dwell time (Figure 7(b)), there
is formation of FeAl, Fe3Al, Fe2Al7, and FeAl3.
The microhardness of the intermetallic compounds which appear at the higher
dwell time is much more important, and so much more weakening than for the
lower (Table 1).Table 1
Description of the different intermetallic compounds.
Bimetallic compoundsDwell time (s)Al content (atomic percentage)Structure [10]Microhardness (HV) [10]Fe3Al0 and 225Ordered body-centered cubic structure260–370FeAl0 and 250Ordered body-centered cubic structure420–520Fe2Al7263Complex body-centered cubic structure620–730FeAl3274–76Highly complex monoclinic body-centered cubic structure820Then, the intermetallic areas have
been quantified (Table2), and it has been found that increasing the dwell time results
in creating more intermetallic compounds.Table 2
Intermetallic compounds surface fraction according to the dwell time (obtained by weighing).
Dwell time (second)012Intermetallic compounds fraction0.170.180.20It can be noted that there are important movements of zinc inside the aluminum
part (Figure8), which comes from
the galvanized layer of the steel sheet. The cartography of zinc distribution
permits to observe the expanse of the area affected by zinc, with a high
concentration under the shoulder. As the melting temperature of zinc is worth 420°C, it can be assumed that
this element becomes liquid during welding.Figure 8
Microprobe cartographies of Zn distribution around the “hanging” (weight percentage, 3μm step, LCMTR Thiais in France) for the sample with Dt = 2 seconds.


### 4.2.3. The Cracked Areas

Many cracks appeared in the
intermetallic areas around the hanging (Figure9), but there are also frequent cracks at the interfaces between steel
and the intermetallic compounds. These cracks may come from the passage to a
molten state of the material induced by high temperatures. Then, during the
cooling, the difference between the expansion coefficient of the steel and that
of the intermetallic compounds may have resulted in the cracking at the
interfaces.Figure 9
SEM micrograph of the right hanging of the
sample with Dt = 2 seconds (back-scattered electrons).To conclude, the weld holds
thanks to the area called “the hanging.” The more “the hanging” is wide, the
more the welding is resistant. As the welding is bimetallic, there is formation
of intermetallic compounds at the aluminum steel interface around the hanging.
It has been found that the more the dwell time is high, the more “the hanging”
is wide, but also the more there is presence of intermetallic compounds, which weaken
the weld because of their high microhardness [9, 10], and so which could
explain the mechanical properties evolution.


### 4.2.4. The Microstructure of Alloys Before Welding

The Al alloy sheet was hot rolled.
Therefore, original microstructure is composed of medium-sized grains (with a
diameter between 60 and 80μm) (Figure 10(a)). There are almost no subboundaries (defined as grain boundary which
misorientation is lower than 15°). And from the obtained microstructure, it can
also be noticed that most grains have a {001} plan parallel to the studied plan.(a) The original microstructure in the studied plan under a step size of 1μm. (b) The texture in the rolling plan of the Al alloy.
(a)(b)Then, the calculation of the orientation
distribution function (ODF) corresponding to the aluminum before welding (Figure10(b)) presents a classical
texture composed of the cube orientation {100}〈010〉. The presence of a
secondary component close to Goss orientation can be observed.Concerning the steel microstructure,
it is constituted of small grains (around 8μm) (Figure 11). There are many grains whose plan is {101}. The
obtained texture, represented in the Euler space in Figure 11(b), corresponds
to a classical texture of rolling, with the presence of α and γ fibres visible
on the section at φ2 = 45°. It can also be noted that the sharpness stays relatively
weak because the maximum reached is only 6.4.(a) The original microstructure in the studied plan under a step size of 0.2μm. (b) The texture in the rolling plan of the steel.
(a)(b)


### 4.2.5. Comparison of the Microstructures Obtained  with Different Dwell Times

Different orientation maps were
measured by EBSD in the aluminum part, all of dimension 200μm × 400 μm with a
1 μm step. To observe the evolution of the microstructure closer and closer to
the welding spot, horizontal displacements were performed from the original
microstructure to “the hanging” (Figure 12) for 2 values of dwell time (0 and 2 seconds).Figure 12
Comparison of the microstructure evolution between Dt = 0 second and Dt = 2 seconds (under a step size of 1μm).The most important difference
between both dwell times concerns the extent of the recrystallized area.
Indeed, for Dt = 0 second, at 9000μm far from the weld center, the microstructure
corresponds to the original one. But, on the contrary, for Dt = 2 seconds, at
9000 μm, the microstructure is still far from the original one; the grains are
elongated, and it remains subboundaries. This difference is highlighted by the
curve in Figure 13, which shows the evolution of the percentage of subboundaries
depending on the dwell time.Figure 13
Percentage of low-angle boundaries versus the
distancex from the welding center.The passage from a microstructure composed of subboundaries to a microstructure
without subboundaries, when approaching the hanging, corresponds to the maximum
of the curves in Figure13. Indeed, getting closer to the welding center, the
mixing tends to increase the boundary misorientation. Then, when the
misorientation reaches 15°, the low-angle boundaries are replaced by the high-angle
boundaries, which correspond to the maximum of the curves. There seems to be a
shift of this maximum depending on the dwell time because of the difference in the temperature
and the deformation reached in the welds. Indeed, if we assume that the passage
to a misorientation higher than 15° is associated to a critical couple (ε,T)c, it is obvious that this couple
will be reached farther from the welding center for the high dwell time, as
illustrated in Figure 14.Figure 14
Hypothetical
curves of the couple (ε,T) as a function of the distance from the
welding center.After those maxima, the decrease in percentage
of low-angle boundaries is due to the transformation of low-angle to high-angle
boundaries. Then, the change of slope in the curve (ε, T) at around 1 mm from the weld center can
be due to the friction effect of the tool on the material. This change of slope
induces the drastic drop of low-angle boundaries observed very close to the nugget
(Figures 13, 14), resulting in
the formation of dynamically recrystallized microstructure with small grains
highly misoriented. Obviously, it remains to check this hypothesis of (ε, T) evolution by a simulation of temperature
and deformation, for instance, [11].


### 4.2.6. Evolution of the Microstructure of the Aluminum  Part After Welding

Far from the welding nugget, the
microstructure is described above, that is, the microstructure of the original aluminum
sheet. Then, getting closer to the welding center, there is an apparition of
dislocation substructure, characterized by low-angle (misorientation <15°)
boundaries inside grains. Indeed, when the temperature and the deformation
increase, the phenomenon of recrystallization is more activated and so the subboundaries
misorientation increases [12, 13].  Consequently, near the welding center, a
recrystallized microstructure of small grains is obtained. This metallurgical
phenomenon corresponds to the continuous
dynamic recrystallization (Figure 15).
The texture obtained
close to the welding center seems to be identical with the two values of the
dwell time; it is composed of the orientation {012}〈221〉, but these
results must be taken with precaution because the studied areas are small and
contain large grains (less than 100 grains per map), and so the statistic is
poor. In order to refine the analysis of the texture, it would be better to
increase the size of the analyzed areas.Figure 15
The mechanism of continuous dynamic recrystallization.


## 4.2.1. General View

A general view of the sample was
obtained using the scanning electron microscope (Figure4). The welding holds thanks to the area called the “hanging.”
It corresponds to the entrance of the steel into the Al alloy, possibly because
of the stirring of the material during the welding.Figure 4
General view of a welding obtained by the scanning electron microscope.The comparison of the hangings shows
geometrical differences depending on the dwell time. To understand its influence,
different distances were measured: the size of the base (b), the height (h),
and the width (w) of the “hanging” (Figures5, 6). It shows that while the dwell time increases, the size of the
base increases too, which can explain the better mechanical resistance obtained
with a high dwell time. It also shows that when the dwell time is low, the
hanging stretches mostly in height. Conversely, when the dwell time is high,
the hanging stretches laterally, without gaining height.Measurements of the dimensions of the hangings SEM micrographes of two hangings (dwell time of 0 second (a) and 2 seconds (b)).
(a)(b)Figure 6
Curves of h and b versus the dwell time.


## 4.2.2. Chemical Distribution

Then, by observing the cartographies
obtained by microprobe (Figure7),
it can be noted that there is migration of steel into the Al alloy, Moreover,
the areas with intermetallic compounds can be located. An important chemical
heterogeneity exists. The maps of the chemical elements highlight also that the
increase of the dwell time induces an increase of the heterogeneity of the
intermetallic areas.Microprobe maps of the Fe and Al distributions around a hanging (weight percentage, 3μm step, LCMTR Thiais in France) for the samples with Dt = 0 second (a) and with Dt = 2 seconds (b).
(a)
Dt = 0 s(b)
Dt = 2 sA chemical analysis was performed by
EDS, and it permitted to conclude that, according to the dwell time, there are
different imtermetallic compounds, which are formed during welding. Indeed, for
the lower value (Figure7(a)),
there is formation of FeAl, Fe3Al. On the other hand, for the higher
dwell time (Figure 7(b)), there
is formation of FeAl, Fe3Al, Fe2Al7, and FeAl3.
The microhardness of the intermetallic compounds which appear at the higher
dwell time is much more important, and so much more weakening than for the
lower (Table 1).Table 1
Description of the different intermetallic compounds.
Bimetallic compoundsDwell time (s)Al content (atomic percentage)Structure [10]Microhardness (HV) [10]Fe3Al0 and 225Ordered body-centered cubic structure260–370FeAl0 and 250Ordered body-centered cubic structure420–520Fe2Al7263Complex body-centered cubic structure620–730FeAl3274–76Highly complex monoclinic body-centered cubic structure820Then, the intermetallic areas have
been quantified (Table2), and it has been found that increasing the dwell time results
in creating more intermetallic compounds.Table 2
Intermetallic compounds surface fraction according to the dwell time (obtained by weighing).
Dwell time (second)012Intermetallic compounds fraction0.170.180.20It can be noted that there are important movements of zinc inside the aluminum
part (Figure8), which comes from
the galvanized layer of the steel sheet. The cartography of zinc distribution
permits to observe the expanse of the area affected by zinc, with a high
concentration under the shoulder. As the melting temperature of zinc is worth 420°C, it can be assumed that
this element becomes liquid during welding.Figure 8
Microprobe cartographies of Zn distribution around the “hanging” (weight percentage, 3μm step, LCMTR Thiais in France) for the sample with Dt = 2 seconds.


## 4.2.3. The Cracked Areas

Many cracks appeared in the
intermetallic areas around the hanging (Figure9), but there are also frequent cracks at the interfaces between steel
and the intermetallic compounds. These cracks may come from the passage to a
molten state of the material induced by high temperatures. Then, during the
cooling, the difference between the expansion coefficient of the steel and that
of the intermetallic compounds may have resulted in the cracking at the
interfaces.Figure 9
SEM micrograph of the right hanging of the
sample with Dt = 2 seconds (back-scattered electrons).To conclude, the weld holds
thanks to the area called “the hanging.” The more “the hanging” is wide, the
more the welding is resistant. As the welding is bimetallic, there is formation
of intermetallic compounds at the aluminum steel interface around the hanging.
It has been found that the more the dwell time is high, the more “the hanging”
is wide, but also the more there is presence of intermetallic compounds, which weaken
the weld because of their high microhardness [9, 10], and so which could
explain the mechanical properties evolution.


## 4.2.4. The Microstructure of Alloys Before Welding

The Al alloy sheet was hot rolled.
Therefore, original microstructure is composed of medium-sized grains (with a
diameter between 60 and 80μm) (Figure 10(a)). There are almost no subboundaries (defined as grain boundary which
misorientation is lower than 15°). And from the obtained microstructure, it can
also be noticed that most grains have a {001} plan parallel to the studied plan.(a) The original microstructure in the studied plan under a step size of 1μm. (b) The texture in the rolling plan of the Al alloy.
(a)(b)Then, the calculation of the orientation
distribution function (ODF) corresponding to the aluminum before welding (Figure10(b)) presents a classical
texture composed of the cube orientation {100}〈010〉. The presence of a
secondary component close to Goss orientation can be observed.Concerning the steel microstructure,
it is constituted of small grains (around 8μm) (Figure 11). There are many grains whose plan is {101}. The
obtained texture, represented in the Euler space in Figure 11(b), corresponds
to a classical texture of rolling, with the presence of α and γ fibres visible
on the section at φ2 = 45°. It can also be noted that the sharpness stays relatively
weak because the maximum reached is only 6.4.(a) The original microstructure in the studied plan under a step size of 0.2μm. (b) The texture in the rolling plan of the steel.
(a)(b)


## 4.2.5. Comparison of the Microstructures Obtained  with Different Dwell Times

Different orientation maps were
measured by EBSD in the aluminum part, all of dimension 200μm × 400 μm with a
1 μm step. To observe the evolution of the microstructure closer and closer to
the welding spot, horizontal displacements were performed from the original
microstructure to “the hanging” (Figure 12) for 2 values of dwell time (0 and 2 seconds).Figure 12
Comparison of the microstructure evolution between Dt = 0 second and Dt = 2 seconds (under a step size of 1μm).The most important difference
between both dwell times concerns the extent of the recrystallized area.
Indeed, for Dt = 0 second, at 9000μm far from the weld center, the microstructure
corresponds to the original one. But, on the contrary, for Dt = 2 seconds, at
9000 μm, the microstructure is still far from the original one; the grains are
elongated, and it remains subboundaries. This difference is highlighted by the
curve in Figure 13, which shows the evolution of the percentage of subboundaries
depending on the dwell time.Figure 13
Percentage of low-angle boundaries versus the
distancex from the welding center.The passage from a microstructure composed of subboundaries to a microstructure
without subboundaries, when approaching the hanging, corresponds to the maximum
of the curves in Figure13. Indeed, getting closer to the welding center, the
mixing tends to increase the boundary misorientation. Then, when the
misorientation reaches 15°, the low-angle boundaries are replaced by the high-angle
boundaries, which correspond to the maximum of the curves. There seems to be a
shift of this maximum depending on the dwell time because of the difference in the temperature
and the deformation reached in the welds. Indeed, if we assume that the passage
to a misorientation higher than 15° is associated to a critical couple (ε,T)c, it is obvious that this couple
will be reached farther from the welding center for the high dwell time, as
illustrated in Figure 14.Figure 14
Hypothetical
curves of the couple (ε,T) as a function of the distance from the
welding center.After those maxima, the decrease in percentage
of low-angle boundaries is due to the transformation of low-angle to high-angle
boundaries. Then, the change of slope in the curve (ε, T) at around 1 mm from the weld center can
be due to the friction effect of the tool on the material. This change of slope
induces the drastic drop of low-angle boundaries observed very close to the nugget
(Figures 13, 14), resulting in
the formation of dynamically recrystallized microstructure with small grains
highly misoriented. Obviously, it remains to check this hypothesis of (ε, T) evolution by a simulation of temperature
and deformation, for instance, [11].


## 4.2.6. Evolution of the Microstructure of the Aluminum  Part After Welding

Far from the welding nugget, the
microstructure is described above, that is, the microstructure of the original aluminum
sheet. Then, getting closer to the welding center, there is an apparition of
dislocation substructure, characterized by low-angle (misorientation <15°)
boundaries inside grains. Indeed, when the temperature and the deformation
increase, the phenomenon of recrystallization is more activated and so the subboundaries
misorientation increases [12, 13].  Consequently, near the welding center, a
recrystallized microstructure of small grains is obtained. This metallurgical
phenomenon corresponds to the continuous
dynamic recrystallization (Figure 15).
The texture obtained
close to the welding center seems to be identical with the two values of the
dwell time; it is composed of the orientation {012}〈221〉, but these
results must be taken with precaution because the studied areas are small and
contain large grains (less than 100 grains per map), and so the statistic is
poor. In order to refine the analysis of the texture, it would be better to
increase the size of the analyzed areas.Figure 15
The mechanism of continuous dynamic recrystallization.


## 5. Conclusion

This study had the aim to understand
the influence of the dwell time on the microstructure around the weld. At
first, it has been found that when
increasing the dwell time, there seems to be an improvement of the tensile
shear strength. Indeed, a long dwell time induced a larger welded area, but
simultaneously a long dwell time induced a higher quantity of intermetallic
compounds (especially FeAl, Fe2Al5, and FeAl2)
with high-microhardness values [10] (Table 1). Thus, the dwell time
must not exceed a certain value; otherwise it can weaken the weld.Moreover, a continuous dynamic
recrystallization has been evidenced whatever the dwell time is. Near the spot
welding, the microstructure is composed of small equiaxed and misoriented recrystallized
grains. At last, a higher dwell time tends to widen the recrystallized area.So, to conclude, an intermediate dwell time
value gives a good compromise between a sufficient width of the hanging and limited
intermetallic compounds formation, in order to optimize the mechanical
resistance. However, more experiments are needed to confirm the mechanical
trend. Consequently, this study on the influence of the dwell time should be
helpful for further works on weld parameters optimization. It is also necessary
to study the other parameters that can increase temperature and strain such as
the rotation speed and the welding effort. Moreover, getting information on the deformation and the
temperature fields is needed.



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*Source: 360617-2008-06-02.xml*