ON THE RESIDUAL STRESS FIELD IN THE ALUMINIUM ALLOY FSW
JOINTS
V. Dattoma, M. De Giorgi, R. Nobile
Dipartimento di Ingegneria dell’Innovazione
Università degli Studi di Lecce
Via Arnesano – 73100 Lecce
[email protected], [email protected], [email protected]
ABSTRACT
In this work, we evaluate the residual stress field originated by a new welding process named Friction Stir Welding. We
analysed aluminium alloy butt-welded joints. Both similar and dissimilar joints have been considered in 2024-T3 and 6082T6 aluminium alloy 0.8 and 3 mm thick. For each joint, the longitudinal and transversal residual stress distribution have
been obtained in a direction normal to the weld cord. In the dissimilar joints, the longitudinal residual stress distribution is
very similar to the distribution present in the traditional welded joints. It presents, in fact, a tensile region near the weld
cord, which is balanced by compressive regions away. On the contrary, other joints present a low compressive stress at
the weld toe, which began tensile away.
Introduction
Friction Stir Welding (FSW) is an innovative joining process, patented at The Welding Institute (TWI) in the 1991 [1]. The
aerospace and automotive industries have devoted large attention to this welding technology since it is based on a solidstate process thus is able to weld the traditionally unweldable materials. FSW uses a rotating (non consumable) cylindrical
tool that consists of a shoulder and a probe (Figure 1).
Figure 1. A schematic illustration of FSW butt-joint, the two sheets are transparently represented to show the probe.
The shoulder is pressed against the surface of the materials being welded, while the probe is forced between the two
components by a downward force. The rotation of the tool under this force generates a frictional heat that decreases the
resistance to plastic deformation of the material. The softened material then easily moves behind the tool and forms a
solid state weld as the stirred material is consolidated [2-5]. This process is efficient and fast (about 4 mm/s for aluminium
alloys with high thickness), and produces a high quality weld. In particular, the joint presents low distortion, excellent
mechanical properties, fine microstructure, and absence of cracking. The possibility to control the welding parameters
allows obtaining a high repeatability, no shielding gas neither surface cleaning is required.
Therefore, FSW appears as an excellent technique, but, as the traditional welding process, also FSW introduces residual
stress field potentially compromising mechanical strength. Even if the FSW is a solid-state process where the maximum
temperature is lower than the melting point, the rigid clamping used impedes the contraction of the weld nugget and heataffected zone during cooling in both longitudinal and transverse directions and originates residual stresses [6].
The existence of high value of residual stress exerts a significant effect on the postweld mechanical properties, particularly
the fatigue properties. Therefore, it is of practical importance to investigate the residual stress distribution in the FSW
welds, considering also that the relevant literature is not at all exhaustive [3, 7-11]. In the following, we report a short
review on the argument.
James and Mahoney [7] measured the residual stress in Al7050-T7451, C458 Al–Li e Al2219 FSW joints by means X-ray
diffraction and they observe that:
the residual stress in the FSW joints was generally lower than that originates by fusion welding;
the residual stress was higher at the transition between the fully recrystallized and partially recrystallized regions
than that observed in other regions of the weld;
- generally, longitudinal residual stresses were tensile and transverse residual stresses were compressive.
Recently, Donne et al. [8] measured the residual stress by means different measurement techniques in Al2024-T3 e
Al6013-T6 FSW joints, concluding that:
the longitudinal residual stresses were always higher than the transverse ones, independently of the pin
diameter, tool rotation rate and traverse speed;
longitudinal and transversal residual stresses exhibited an ‘‘M’’-like distribution across the weld with the maximum
longitudinal stress localised at about 10 mm away from the weld axis in the HAZ, on the contrary low
compressive residual stress was present in the region adjacent to the HAZ;
across the weld, residual stress distribution was similar in the top and root side;
the width of M-distribution was proportional to the diameter of the tool and the residual stress value was
proportional to welding speed and tool rotation rate;
in small specimen, the maximum longitudinal residual stress was about 30–60% of the yield stress of the welded
material and 20–50% of the yield stress of the base material, higher residual stresses were found in bigger
specimens [9].
Peel et al. [3] analysed the aluminium alloy Al5083 concluding that:
longitudinal residual stress showed an ‘‘M’’-like distribution according to Donne [8], the transversal residual stress
showed a peak value at the weld center;
both longitudinal and transversal residual stress were tensile in the nugget zone;
peak tensile residual stress was localised at 10 mm from the weld centerline, a distance corresponding to the
edge of the tool shoulder;
longitudinal residual stress was proportional to welding speed, transversal residual stresses did not exhibit this
dependence;
longitudinal distribution showed a mild asymmetry within the nugget zone, the stresses was 10% higher on the
advancing side.
The common result of cited studies [3, 7–9] and others [12-14] was that, in the aluminium alloys, the maximum longitudinal
residual stress was lower than 100 MPa, much lower than yield stress of the base material and the ones observed in the
traditional joints.
Reynolds et al. [10] studied the 304L stainless steel FSW joints and they obtained the typical distribution of residual stress
originates by the fusion welding: high tensile longitudinal residual (near the base material yield stress) and a negligible
transversal residual stress [15]. The tool rotation rate did not influence the peak residual stress but only slightly the tensile
zone width. Longitudinal residual stress was only slightly variable with depth, whereas the transversal stress varied
significantly through the thickness. Therefore, residual stress distributions differ if we consider aluminium alloys or steel
joints, mainly because of the different dependence of the material properties on the temperature.
The aim of this work is to evaluate the residual stress field in the aluminium alloys FSW butt-joints. In particular the
Al2024-T3 e Al6082-T6 FSW joints were analysed.
The peculiarity of this study is the use of thin FSW sheets, having a thickness of 0.8 and 3mm. As consequence, the
residual stress field resulted limited in the peak value; on the contrary, the sheets distortion resulted more evident.
The FSW process allows to join dissimilar materials without difficulty [16]. Therefore, we have analysed also Al2024Al6082 dissimilar joints with thickness of 0.8 and 3 mm. We can evaluate the influence of the thickness and of the
mechanical properties on the residual stress field magnitude.
Geometry and methods
The residual stress field has been evaluated on different aluminium alloy FSW joints. The analysed joints are 0.8 mm thick
butt-welded joints in Al2024-T3 and Al6082-T6, in both similar and dissimilar configuration. In the dissimilar configuration,
also the 3 mm thick joint has been considered. The welded plate geometry is represented in Figure 2. The optimum
welding parameters for each type of joint are presented in Table 1. In this table, the measurements plan is also
summarised.
Measurement points
ondithe
lower
linea
misura
sullaface
faccia
inferiore
Measurement
linea
di misura sulla line
facciaon the upper face
superiore
6082-T6
2024-T3
Figure 2: Analysed plate geometry and measurement points.
Table 1: Experimental plan of the residual stress evaluation
Joint
Welding parameters
Distance of the weld toe
[mm]
6082-2024, t=3mm,
1
11
21
41
61
6082 side
6082-2024, t=3mm,
1
11
27
40
59
2024 side
6082-2024, t=0.8mm,
n=2085 rpm, v=762
1
11
21
41
6082 side
mm/min
6082-2024, t=0.8mm,
n=2085 rpm, v=762
1
11
21
41
2024 side
mm/min
6082-6082, t=0.8mm
n=2085 rpm, v=762
1
11
21
41
mm/min
2024-2024, t=0.8mm
n=1809 rpm, v=460
1
11
21
41
mm/min
The residual stress measurements have been performed by means of the hole-drilling method on FSW joint 0.8 mm thick
in the similar and dissimilar configuration, according to ASTM E837-01 standard. RESTAN automatic system and
rectangular rosettes CEA-062UM-120 were used in the half-bridge configuration to compensate the possible thermal
strain. An incremental through hole was drilled using an advancing speed of 0.08 mm/min with parabolic steps distribution.
For each joint, longitudinal and transversal residual stress distribution were evaluated in a transversal section to the weld.
To obtain correct measurements, the joints, significantly deformed by FSW process, were clamped to the worktable. This
operation originated secondary bending strains, which were measured before the hole execution and then elaborated
according to the thin shell theory, in order to calculate bending spurious stress and to correct the real stress relaxed by
drilling. In the case of 3 mm thick joints, a blind hole 2 mm depth has been executed and two measurements have been
carried out also on the 2024 and 6082 root side at 1 mm away from the weld toe.
Results and discussions
Figures 3-6 show the longitudinal and transversal residual stress distribution in each joint versus the distance of the weld
axis. The graphs have been reconstructed at the half depth hole, i.e. 0.4 mm for the 0.8 mm thick joints and 1 mm for the 3
mm thick joint.
Longitudinal residual stress
2024
80
Transversal residual stress
60
2
Residual stress [N/mm ]
6082
40
20
0
-65 -55 -45 -35 -25 -15 -5
-20
5
15 25 35 45 55 65
-40
-60
Distance from the weld axis [mm]
Figure 3. Longitudinal and transversal residual stress distribution for 3 mm thick joint.
Residual stress [N/mm2]
6082
2024
80
Longitudinal residual stress
60
Transversal residual stress
40
20
-45
-35
-25
-15
0
-5
-20
5
15
25
35
45
-40
-60
Distance from the weld axis [mm]
Figure 4. Longitudinal and transversal residual stress distribution for 0.8 mm thick joint.
Longitudinal residual stress
2024
80
Residual stress [N/mm2]
Transversal residual stress
60
40
20
0
-20
0
5
10
15
20
25
30
35
40
45
-40
-60
Distance from the weld axis [mm]
Figure 5. Longitudinal and transversal residual stress distribution for 0.8 mm thick joint, 2024-2024 configuration.
Residual stress [N/mm2]
Longitudinal residual stress
6082
80
Transversal residual stress
60
40
20
0
-20
0
5
10
15
20
25
30
35
40
45
-40
-60
Distance from the weld axis [mm]
Figure 6. Longitudinal and transversal residual stress distribution for 0.8 mm thick joint, 6082-6082 configuration.
In the dissimilar joints, the longitudinal residual stress distribution is very similar to those originated by the traditional
welding process. The thicker joint presents a higher residual stress field with respect to the thinner joint and the maximum
stress is localised on the Al2024 side which is characterised by a higher yield strength than the Al6082. According to [8, 9],
the peak stress is about 20% of the yield strength of the base material (Table 2) in the thicker joint, but it is lower in the
thinner joint, in all cases it is localised at the weld toe on the advancing side.
Table 2. Mechanical properties of the base materials Al2024-T3 and Al6082-T6 [17-20].
σy [N/mm2]
345
250
E [N/mm2]
73000
73000
Aluminium alloy
Al 2024
Al 6082
σu [N/mm2]
483
290
A % at break
18
10
The transversal residual stress is generally lower than longitudinal residual stress.
The similar joints present a low compressive residual stress field near to the weld toe, which become tensile away.
Considering the in-depth distribution (Figure 7-8), we observe that in the thicker joint the longitudinal residual stress is
significantly higher than the transversal residual stress, while in the thinner joint we have an hydrostatic residual stress
field.
Longitudinal residual stress
Transversal residual stress
40
2
Residual stress [N/mm ]
60
20
0
0
0,2
0,4
0,6
0,8
1
1,2
1,4
1,6
1,8
2
-20
-40
-60
Depth [mm]
Figure 7. The in-depth residual stress distribution for 3 mm thick dissimilar joint, on 6082 side
Longitudinal residual stress
60
2
Residual stress [N/mm ]
Transversal residual stress
40
20
0
0
0,2
0,4
0,6
0,8
-20
-40
-60
Depth [mm]
Figure 8. The in-depth residual stress distribution for 0.8 mm thick dissimilar joint, on 6082 side
The measurements on the root side show that the residual stress field is not significantly different on the top and bottom
surfaces, neither on Al2024 neither on the Al6082 side.
Conclusions
In this work, the residual stress field has been evaluated in FSW butt-joints. The analysed joints were in two different
aluminium alloys: Al2024-T3 e Al6082-T6, obtained by welding sheets with thickness of 0.8 e 3 mm in the similar and
dissimilar configuration. The longitudinal and transversal residual stress distributions were obtained in a transversal path
of the weld cord by the hole-drilling method.
The first striking result, in the case of the dissimilar joints, was that the longitudinal residual stress distribution was very
similar to that originated by the fusion welding process, according to Donne [8] and Peel [3] relatively to the measurements
performed at the weld toe and away from it. We cannot say whether we are in presence of an M-distribution because we
did not perform the measurements on the weld cord but we can certainly state that the longitudinal peak value was found
at a distance corresponding to the edge of the tool shoulder [3].
The transversal residual stress was in general compressive and lower than the longitudinal residual stress [7]. Both
longitudinal and transversal residual stress were lower than that observed in the traditional joints. The longitudinal residual
stress was about 20% of the base material yield stress, at least in the thicker joints.
The longitudinal peak values on the top and the root surface, near the weld cord, were similar both on the Al2024 both on
the Al6082 side.
In the dissimilar joints, the residual stress distribution was asymmetric with respect to the weld axis. In the thicker joint, the
longitudinal peak value was in the Al2024 side (with highest yield strength), it was the opposite in the thinner joint. This
arises from the fact that in the first case the advancing side was the Al2024 side, while in the second case the advancing
side was the Al6082 side [3]. Considering the in-depth distribution, it can be observed a similar behaviour but also a
significant difference in the values, since in the thicker joint very high longitudinal residual stresses were present.
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