By outreach of Friction Stir Welding (FSW), the solid-state joining method
may possibly present excessive potential for aluminum alloys since it can perform
without any toxic fumes and ease or remove some welding defects that related
to solidification. Among these alloys, AA6061-T6 is progressively used in many
industries (Liu et al., 1997; Mishra
and Ma, 2005). In addition to welding in a butt shape, friction stir welding
is also, widely used in lap joints forms in some industries such as automotive,
aerospace, shipbuilding and so on. Figure 1 shows a lap joint
as illustrated, two metal plates are overlapped by a certain width (upper plate
and bottom plate). A rotating tool, plunged into the material for prearranged
depth with the tool shoulder in adjacent interaction with the upper plate, is
traversed along the centerline of the overlap.
More than a few efforts were carried out to completely recognize FSW process
mechanics and to develop joint performances in butt joints but in fact, the
very restricted effort has been achieved for the friction stir welding of the
lap joints (Mishra and Ma, 2005; Cederqvist
and Reynolds, 2001).
Material flow in FSW is asymmetrical due to the fact that tool rotation direction
and welding direction are different in advancing and retreating sides. In advancing
side these two are in the same direction whereas in retreating side are in opposite
(Arbegast et al., 2003). Joining two plates together
in a lap joint configuration needs the welding tool to penetrate to the lower
plate in a certain depth. Inserting the pin causes the interface to bend upward
or downward depending on the pin shape and welding parameters. The curvatures
(bends) were known as hooking effect (Cantin et al.,
2005). This wavy flaw in advancing side called hooking whiles in retreating
side called thinning.
|| Schematic friction stir lap welding (joint design was used
in this research) and hook and plate thinning geometries
Moreover, the hook and plate thinning geometries on these two sides display
heterogeneity (Fig. 1). Hooking height and size alteration
will change the Effective Plate Thickness (EPT) which has a greater effect on
welding strength than microstructure (Babu et al.,
2012; Yazdanian et al., 2012). The EPT is
known as a minimum plate thickness determined by measuring the smallest distance
between hooking flaw (hooking or thinning) and the top of the upper plate if
the flaw located in the upper plate (and/or, the bottom of the lower plate if
the flaw located in lower plate (Cederqvist and Reynolds,
2001). Moreover, decreasing in EPT lowers the load carrying capacity.
In the current research, the effects of welding speed on the microstructure
and mechanical properties of FSW AA6061-T6 aluminum alloy lap joints were studied.
Four different welding speed (30, 40, 50 and 60 mm min-1) was used.
Regarding to above subject, microstructure, hardness and tensile shear properties
were investigated and, also, hooking effect and its effect on tensile with Effective
Plate Thickness (EPT) were illustrated.
MATERIALS AND METHODS
Base metal: Plates of Al-alloys AA6061-T6, 5 mm thick, 220 mm long and
140 mm wide were selected for both the top and bottom plates of lap joints.
The chemical compositions in mass% of the parent metals AA6061-T6 is 1.0 Mg,
0.66 Si, 0.27 Cu, 0.30 Fe, 0.03 Mn, 0.02 Ti, 0.05 Zn, 0.18 Cr and Al balance
(UNS A96061). The mechanical properties of this alloy are shown in Table
Tool and joint design: The longitudinal direction of the plates was
parallel to the plate rolling direction. The overlap along the linear direction
of the plates was 50 mm wide.
||Mechanical properties of the 6061-T6 aluminum alloy used in
The designed tool for Friction Stir Lap Welding was cylindrical (Ø 8x5
mm) with a sub-conical headpin (Ø 8x3 mm) having standard left-hand threads
and a cylindrical shoulder (Ø 20 mm), which was made from hardened high-speed
tool steel (AISI H13). This twofold pin was modeled from multistage pins (for
top plate was cylindrical and for bottom plate was conical). The reason for
selecting this design for the pin was focusing on interface behavior for hooking
Welding procedure: FSLW was performed at a constant rotation speed (ω)
of 1000 rpm together with different welding speeds in 20, 40, 50 and 60 mm min-1.
To improve weld joining, the tool was tilted at 3 degrees from the normal direction
of the plate in the direction of the behind the tool during welding and the
rotation direction was selected in clockwise. In addition, the tool shoulder
was plunged into the upper workpiece by a depth of around 0.15 mm. Dwell time
at the start point of welding was 20 sec.
Lap Shear and hardness test: Strength of welds loaded technically in
lap shear test was inspected. Dimensions of the samples that used for overlap
shear testing shows in Fig. 2. Due to the asymmetric feature
of Friction Stir Lap Welding, two welding locations were considered, thus resulting
in two different loading forms. In type (a) The advancing side of a lap weld
on the upper workpiece is loaded, while in type, (b) The retreating side of
a lap joint on the upper workpiece is loaded, as schematically explained in
Fig. 3 (Babu et al., 2012).
|| Overlap shear test sample (thickness is 5 mm and all dimensions
are in mm)
||Overlap shear testing methods, (a) Type A, (b) Type B
To balance the offset axes of the lab samples and reduce the bending effects,
two compensation pieces with the plate thickness were applied for each specimen,
during the tensile shear testing as indicated in Fig. 3. Room
temperature lap shear tests were done by using a 100 KN Instron mechanical testing
machine with the cross head speed fixed at 2.0 mm min-1. No fewer
than three specimens were examined to find average weld strength for a process
condition. For each tensile shear-testing specimen, fracture locations were
recorded. Vickers hardness (HV 0.5) of the FSW region was measured on cross-section
perpendicular to weld direction of the mid-thickness of the plate using a load
of 100 g and a dwell time of 10 sec.
Metallography and SEM: Macro structural and microstructural examinations
were performed on as-welded specimens. The metallographies samples were exposed
to mechanical grinding and then polished with1-micron diamond paste. The specimens
were then etched with Kellers reagent. Microstructure, hook and plate
thinning were characterized using Lecia light microscope joined with a Qwin
image analysis. Hooking and plate thinning geometry was measured on planar macrostructures.
The fracture surfaces were investigated using Field-Emission Scanning Electron
Microscopy (FEI, Nova NanoSEM 230) by selecting the condition of 20 KV and 15
mm work-distance and also Energy-Dispersive X-ray Spectroscopy (EDX).
RESULTS AND DISCUSSION
Visual inspection and microstructural study of weld zones: All of the
samples that cut in perpendicular to welding direction, indicates the free defects
regions of welds. Figure 4 displays a macrostructure and related
microstructures of each zone of the weldment. That will be demonstrated later.
Nugget zone (NZ, region d, Fig. 4d) represented a wider range
near the top of the upper plate than lower plate like a semi-taper shape because
the upper plate undergo the maximum deformation and frictional heat began with
directly contacting welded plates with the shoulder. Macroscopic inspection
of the weld zone revealed a comparative non-symmetric nugget zone that was mainly
related to the tilt angle of the tool and the relation between the direction
of tool rotation and welding direction (Mishra and Ma, 2005;
Liu et al., 2004). It was obvious that the weld
zone exhibited any discontinuity and porosity. In spite of the base metal (BM,
Fig. 4a), NZ, in upper plate (Fig. 4d) and
in bottom plate (Fig. 4d), has fine and equiaxed grains and
the grain size was much smaller than that of the BM. This structure (NZ) was
produced by the dynamic recrystallization and static grain growth after welding,
which was caused by the frictional heat and plastic deformation. It was evident
that the original grain structure was microscopically disappeared in the thermo-mechanically
affected zone (TMAZ, Fig. 4c) and the transient microstructure
between NZ and heat affected zone (HAZ, Fig. 4b) was achieved.
The elongated grains (in the same direction) and dynamic recovered grain structure
are regarded as TMAZ because the thermal and deformation condition was not enough
to create the recrystallized grain structure.
||Cross-section macrostructure of lap joint section AA6061-T6
in 60 mm min-1 and related microstructures indicated in the macrograph,
(a) BM (base metal), (b) HAZ (heat-affected zone), (c) TMAZ (thermo-mechanically
affected zone), (d) NZ (nugget zone) in upper plate, (d') NZ (nugget zone)
in bottom plate (all images are in 75X)
The HAZ has a coarser grain size than the unaffected Base Metal (BM).
Due to higher pressure on the upper plate than the bottom plate, the grain
size in upper nugget was smaller than in lower nugget. The actual results that
were measured by Hean method is in full agreement with Table 2.
It also shows that although increasing welding speed from 20-60 mm min-1
decreases grain size 9.8 μm in upper nugget and 9.7 μm in the lower
one but the grains at lower nugget remain always bigger than that the grains
in upper one. These, also, have been reported by Rajakumar
et al. (2011). Changes in grain size in different zones of weld can
clearly be seen in hardness of weldments.
|| Grain sizes of nugget zones in upper plate and bottom plate
Investigation of hardness in different areas of weldment: Figure
5 indicates the hardness distributions in the middle of the upper and bottom
plates in the lowest welding speed (20 mm min-1) and the highest
welding speed (60 mm in-1). That is almost symmetric around the weld
centerline. It can be seen that, by applying the higher welding speed, the hardness
in NZ, HAZ and TMAZ increased meanwhile the difference between the top and the
bottom plate hardness, in NZ decreased. Microstructure variation is the
main reason for the above mentioned phenomenon.
||Hardness distribution in the middle of upper and bottom plate
(a) 20 mm min-1 welding speed and (b) 60 mm min-1
It is justified by grain size changes in different area of weld (NZ, HAZ and
TMAZ) and also the contrast between up and bottom NZs microstructure (Liu
et al., 2004). The HAZ areas at the both AS and RS of the joints
(almost 10 mm distance from weld center) were found to have the minimum hardness
among the other regions of the weld reported because of the grain coarsening.
Weld flaws (hooking and thinning): Due to hooking effect that explained
in introduction part and illustrated in Fig. 1, strength of
friction stir welded samples in lap joint condition is affected by hooking and
thinning. Variations in height and size of hooking and thinning can change the
Effective Plate Thickness (EPT) that's known as a minimum plate thickness determined
by measuring the smallest distance between hooking flaw (hooking or thinning)
and the top of the top plate if the flaw located in top plate and the bottom
of the bottom plate if the flaw located in there (Babu
et al., 2012).
|| Effective plate thickness-EPT
EPT of all four samples in different welding speeds are indicated in Table
3. According to data from this table, increasing welding speed from 20-60
mm min-1, causes EPT to rise 0.79 mm. Yazdanian
et al. (2012) and Buffa et al. (2009)
have shown hooking and plate thinning are looming to the interface of two plates
with increasing of welding speed. It means that the height of hooking and plate
thinning (compared with interface line) will be decreased when the welding speed
is increasing. In this research, due to the shape of pin, that changes from
cylindrical to conical at interface, the position of hooking and thinning firstly
was placed in the lower plate (Fig. 4 macrostructure) but,
based on increasing welding speed; variation in these positions can be noticed,
even Fig. 6 indicates that in 60 mm min-1 welding
speed direction of hooking tip is altering toward the top plate.
||Changing in hooking shape and direction, (a) 40 mm min-1
welding speed (hooking tip is moving down), (b) 50 mm min-1 welding
speed (tip is almost parallel to interface) and (c) 60 mm min-1
welding speed (tip is folding up) (all images are in 150X)
|| Overlap shear test results
The importance of hooking and thinning tip direction identified in selective
path for fracture in shear overlap testing.
Mechanical properties of lap joints by using overlapping shear method:
Shear Tensile test specimens were prepared perpendicular to welding line. The
interval between welding and shear testing was five days. In consequence of
the asymmetric nature of the weld and related zone of FSW a lap joint shall
be tested with two conditions as mentioned in Material and Method part, which
results were illustrated in Fig. 7. It appears that increasing
welding speed from 20-40 mm min-1, causes shear strength to soar
up to 12% for type A followed by a steady increase in shear strength for both
types in higher welding speeds. Moreover, the test results also show that shear
strength in type A is always higher than type B. In these tensile shear tests,
fracture path occurred at two locations. In samples with 20 and 40 mm min-1
welding speed, as it can be seen in Fig. 8a (upper right corner),
the direction of the tip is toward the bottom plate, fracture taken place from
thinning to the bottom plate.However, in specimen with 50 mm min-1,
due to higher welding speed, hook tip is an almost parallel with interface of
two plates and even in specimen with 60 mm min-1 is changing up (Fig.
6), route of fracture, in these samples, was started from hook or advancing
side and then, thinning in retreating side also started to break (both through
nugget zone) and connected to each other in lower NZ (Fig. 8b-upper
Surfaces of fractures were investigated by FE-SEM and EDX. Figure
8a shows the fracture surface of the sample with 40 mm min-1
welding speed that failed due to break from retreating flaw (thinning) toward
bottom plate. The fracture surface is composed of areas which clearly exhibit
the features of ductile fracture such as relatively deep dimples.
But in Fig. 8b that shows fracture located through the welds
nugget zone (sample with 50 mm min-1 welding speed). Its illustrated
shallow dimples and sheared cleavage facets these introduce mixture fracture
(Zadpoor et al., 2009). The dimples relate to
the onset of the fracture. After beginning of breaking, consequently the specimen
experiences overloading and the dimples are sheared (cleavage). EDX analyze
of initial points (dimples) emphasizes existing intermetallic compounds of Fe/Al
as it can be seen in Fig. 8 (point A). Niranjani
et al. (2009) has shown that majority of precipitation components
of AA6061 can be solved about 600°C and, also, perceptions dissolution could
have occurred due to the large plastic deformation imposed on the alloy, whereas
Al/Fe compounds are the most stable among these intermetallic compounds and
they can be created above 500°C and remain stable till 600°C and more.
In FSW of AA6061-T6, temperature under tool shoulder rises from 550°C up
to 650°C (Nandan et al., 2008) and because
of high plastic deformation in NZ (Mishra and Ma, 2005),
the majority of precipitations can be dissolved. Due to mentioned reasons, Al/Fe
compounds, due to high stability, are expected as an effective point in fractures
that can be seen in Fig. 8.
||Fracture surface of the overlap shear test samples, (a) 40
mm min-1 welding speed-fracture from thinning to the bottom plate
and (b) 50 mm min-1 welding speed-fracture through nugget zone,
(c) EDX spectrum of point A, (d) EDX spectrum of point B, (e) Quantitative
results of point A and (f) Quantitative results of point B
The gained outcomes, in this research, permission to appeal the following
conclusions: With increasing welding speed, microstructure of NZ gets finer
which is a reason for increasing the hardness and Due to shoulder pressuring,
grains in the top plate are smaller than the bottom plate whereas. This alteration
is less at higher welding speed. The minimum hardness is in the HAZ and NZ belongs
to the lower welding speed.
The result of overlap shear testing shows that the motivation of hooking and
thinning in fracture is related to the size and orientation of them. In this
way, the maximum EPT-effective plate thickness formed in maximum welding speed
(60 mm min-1) that shows the highest amount of shear tensile strength
among samples. The height of hooking and plate thinning (in analogy with interface
line) will be decreased when the welding speed is increasing.
FE-SEM study of fracture surface indicates that initial point of fracture can
be caused by Fe/Al intermetallic compounds through NZ.
The authors wish to place sincere special thanks to Mr. Tajul Ariffin b. Md.
Tajuddin Senior Technician of Engineering Faculty, UPM University and Mr. Shaiful
Hisham B Samsudin Senior Technician of Engineering Faculty, UTP. Also, authors
appreciate Dr. Amir Abbas Nourbakhsh for his help and guidance to do this research.