FSW is a solid-state joining process invented by Wayne Thomas and his colleagues
at The Welding Institute (TWI) UK in December 1991 (Thomas
et al., 1991). It has been widely used in joining aluminum alloys
especially for production of fuel tank, engine cradles, and deck panels in automotive
industry. In FSW, a cylindrical, shouldered tool with a profiled probe is rotated
and slowly plunged into the joint line between two pieces of sheet or plate
material, which are butted together. The parts have to be clamped onto a backing
bar in a manner that prevents the abutting joint faces from being forced apart.
Frictional heat is generated between the wear resistant welding tool and the
material of the workpieces. This heat causes the latter to soften without reaching
the melting point and allows traversing of the tool along the weld line. The
plasticized material is transferred from the leading edge of the tool to the
trailing edge of the tool probe and is forged by the intimate contact of the
tool shoulder and the pin profile. It leaves a solid phase bond between the
two pieces. The process can be regarded as a solid phase keyhole welding technique
since a hole to accommodate the probe is generated, then filled during the welding
sequence (Yuh and Xinhai, 1999). Figure
1 depicts schematically the friction stir welding process.
FSSW is a derivative of the FSW without lateral movement of the tool during
welding process. It has been used in the production of aluminum doors, engine
hoods, and deck lids in automotive industry. The FSSW are created by the combined
action of frictional heating and mechanical deformation due to a rotating tool.
The probe penetrates the work piece whereas the shoulder rubs with the top surface
(Badeshia, 2003). The FSSW process consists of three
phases; plunging, stirring and retraction as shown in Fig. 2.
It starts with tool spinning and slowly plunge the tool into a weld spot until
the shoulder contacts the top surface of the work piece. The stirring phase
enable the materials of two-work pieces mix together. Once a predetermined penetration
is reached, the process stops and the tool retract from the work piece (Badeshia,
As the rotating tool is plunged into the upper and lower sheets, it extrudes
material sideways. It is logically hypothesized that analogously to the FSW
process; a Thermo-mechanically Affected Zone (TMAZ) and a heat affected zone
are created around the pin. However, the micro structural details of both zones
are yet to be elucidated in open literature. There has been a recent series
of conference proceedings concerning the mechanics or heat transfer in FSSW.
There are also current works regarding pin insertion depth on failure mode of
the lap shear specimens with its microstructure but not all process parameters
has been studied their effect on weld joint strength (Mitlin
et al., 2006).
FSW has made a significant impact on the welding industries in a short time
and will continue to grow in both research and commercial application. Early
years of FSW, many researchers believed it is a relatively simple process. However,
the metallurgical fundamentals that results in this remarkable post weld properties
have found to be quite complex. Most data in many research and literatures provide
empirical observation but not an understanding of the process itself. A concerted
effort is needed to address the basic components of the process to integrate
them into a complete process description (Mishra and Mahoney,
2007). Therefore, in the first part of this research, using fabricated FSW
tool, which operated, produced FSW welds of aluminum alloy 2011 by CNC milling
The region occurred on weld was characterized based on the microstructure examination
by using optical microscope and Scanning Electron Microscopy (SEM) whereas the
strength of the FSW welds was tested using Universal Tensile Machine to compare
with the strength of the parent metal. For the second part of this research,
spot welds of FSSW on 1100 Aluminum alloy was produced by using fabricated FSSW
tool which operated by CNC milling machine. The region occurred on weld was
characterized based on the microstructure examination whereas the strength of
the FSW welds was tested to compare with the parent metal.
MATERIALS AND METHODS
A tool for FSW was fabricated by Mazak Integrex 200-III CNC Turning Machine from 50 mm diameter and 200 mm long AISI H13 tool steel to a length of 8 mm with a diameter of shaft around 62 mm. The design of the tool is attached in the Appendix. A tube furnace to prevent tool wear then heated the tool. Two plates of 100x100 mm of aluminum alloy 2011 with thickness of 10 mm was butted joint together and clamped on the backing support (127 mmx280 mmx25 mm of aluminum block) by vise jaw of CNC Milling Machine.
The two workpieces were friction stir welded by using Mazak Variaxis 630 5-X CNC milling machine. The rotating tool speed was in the range of 550 to 3500 rpm, the transverse speed was 100 mm/min and the depth of penetration was 8 mm. The FSW welds were then prepared by metallographic techniques to characterize by optical microscopy. Transverse tensile tests were performed according to the ASTM E8-00b Standard Test Methods for Tension Testing of Metallic Materials. A Universal Testing Machine was used to conduct the pull-to-break test method at 1.27 mm min-1 to determine the tensile strength of the weld.
A 25 mm diameter and 305 mm long AISI H13 tool steel was fabricated by using
CNC Turning Machine to 100 mm length and 25 diameter of FSSW tool steel. The
design of FSSW tool steel is attached in the Appendix . A tube furnace to prevent
tool wear then heated the tool. Two sheets of 200x30 mm of 1100 Aluminum with
thickness of 10 mm was butted joint together and clamped on the backing support
(127 mmx280 mmx25 mm of aluminum block) by vise jaw of CNC Milling Machine.
A 200 mmx30 mm sheet 1100 Aluminum with the same thickness was put under on
the sheet to be joined in order to make a spot weld.
Like FSW, the 1100 Aluminum sheets was friction stir spot welded by using Mazak Variaxis 630 5-X CNC Milling machine. The rotating tool speed was varied from 4000 to 6000 rpm, the plunge rate of 45 to 75 mm min-1 and the plunge depth of 1.7 mm. The FSSW spot welds were then prepared by metallographic techniques to characterize by optical microscopy. Transverse tensile tests were performed according to the ASTM E8-00b Standard Test Methods for Tension Testing of Metallic Materials. Universal Testing Machine was used to conduct the pull-to-break test method at 1.27 mm min-1 to determine the tensile strength of the weld.
RESULTS AND DISCUSSION
Figure 3 shows the top surface of the welds joined by FSW at 2500 rpm with a 100 mm min-1 and plunge depth of 8 mm. Even though the top surface of the welds has good surface finish, the radiography examination shows that there are defect and hole along the weld line.
Figure 4 shows the hole produced by FSW along the weld line. Data in Table 1 and 2 shows the parameters of FSW that joined the two plates of aluminum alloy 2011 but all of them produce defect along the weld line.
Colligan et al. (2001) reported that the hole
along the weld line FSW can be characterized as common FSW defect which is called
wormhole or tunnel defect. This defect is a result of insufficient material
transport around the tool pin to the advancing side. It occurs when the tool
advance per revolution is too high. The defect in this research possibly occurs
because of insufficient dwell time during the plunging of FSW tool in order
to give steady state heat input to the material before it travels along the
Figure 5 shows the different welded region generated from
FSW process. The finest grain structure in the welded region is called the nugget
|| FSW parameters
This zone-undergone recrystallization by direct rubbing of the tool pin and
the tool shoulder without exceed the solidus temperature, Ts. This
region undergoes extreme levels of plastic deformation that leads to a very
fine recrystallized grain structure being formed in the center of the weld.
The finer grain structure with little porosity in the welded region is called
the Thermos-Mechanically-Affected Zone (TMAZ). In this region, the FSW tool
has plastically deformed the material and the heat from the process will also
exerted some influence on the material. In aluminum, there is generally a distinct
boundary between recrystallized zone and the deformed zones of TMAZ.
|| FSW of Aluminum Alloy 2011
||Wormhole defects along weld line of Aluminum Alloy 2011
|| FSW Parameters and their Tensile Strength
||Cross section area indicating different welded region generated
from FSW process. (a) Friction stir processed zone or Nugget. (b) Thermo-mechanically
affected zone (TMAZ). (c) Heat affected zone (HAZ). (d) Base metal. All
optical microscopic images are taken at 20X magnification. Etching reagent:
25mL NH4OH, 20mL distilled water and 8-25mL of H2O2
The region, which has fine grain structure with porosity, is called Heat-Affected
Zone (HAZ). It is a zone that clearly lies closer to the weld centre. The material
has experienced a thermal cycle, which has modified the microstructure and the
mechanical properties. However, there is no plastic deformation occurred in
this area. Whereas the region with least fine grain structures with large porosity
is the base metal. This part is remote from the weld and not been deformed.
It might have experience a thermal cycle from the weld but not affected the
microstructure or mechanical properties.
Besides having microstructure evolution, Table 2 also illustrates the value of tensile strength of FSW welds is near to the tensile strength of the parent metal. The most optimum set of parameters from the experiment was at 3000 rpm, travelling speed of 40 mm min-1 and penetration depth of 8 mm. The lower tensile strength was expected because of the material did not has high composition of alloy and not being tempered to give more influences to alloy response. The high thickness of the material also results in low cooling rate, which affect the properties of the material.
Figure 6 shows the base metal which is not affected by the
FSSW. The microstructure has very coarse grains with lots of small pores. The
porosity might be because of the inclusions of Copper in the 1100 Aluminum alloy
or oxide stringers that interrupted during the manufacturing process of 1100
Aluminum alloy. Whereas the TMAZ shows the microstructure at this region has
coarse grains with small pores but quite less than the base metal. These might
happened because of the force from the stirring tool gives more compaction to
the grains. For the finest grain structure in the spot welded region is called
the stir zone. Even though it has large size porosity, the porosity formed is
quite less than base metal region and TMAZ zone.
||(a) Micrograph view of base metal region (b) region affected
by the tool shoulder (c) region affected by the tool pin. All optical microscopic
images are taken at 10X magnification
The large size of porosity might happen because of the higher force from the
stirring tool pin that gives more compaction to the grains of inclusions of
copper or oxide stringers in the metal.
The tensile test in Table 3 illustrates the maximum load
withstand by FSSW spot welds at different parameters. The optimum parameter
for FSSW is 2000 rpm rotational speed with penetration rate of 75 mm/min which
results maximum load which is 2182.8 N. The area proportional to the force is
0.00006 m2. Thus, the maximum tensile strength of FSSW spot weld
in this experiment is 36.38 MPa. The tensile strength of 1100 Aluminum alloy
is 110 MPa (Davis, 1993). Thus, the friction stir spot
welded tensile strength is closed to 1100 Aluminum alloy by 33.1%. The lower
tensile strength was expected because of the material has no high alloy composition
and not being tempered to give more influences to alloy response.
For FSW, there are four different regions appeared from the welds, which are friction stir nugget zone, TMAZ, HAZ and the base metal or unaffected material. The tensile strength of friction stir welds is closer to the tensile strength of the aluminum alloy 2011. Heat treatment to the material could increase the tensile strength of FSW welds. Heat treatable aluminum alloy such as 2, 6 or 7 xxx series are preferable to give closer tensile strength of welds to the parent metal tensile strength. Whereas the tunnel defect occurred in the weld can be avoided by choosing suitable welding process parameters for aluminum alloy such as rotational speed, transverse speed or stirring time during the plunging.
In the FSSW, there are three different regions formed by the spot welds which are base metal or non-affected region, stir zone and TMAZ. The tensile strength of the spot welds is closer to the tensile strength of the 1100 Aluminum Alloy. The porosity of the material could be avoided by using high strength aluminum alloy and by increasing the dwell time of the welding process.
For future works, two plates of Aluminum Metal Matrix Composite with composition of 242 Aluminum alloy and 30% volume percent of reinforced particle of Alumina, Al2O3 will be jointed using FSW and will be compared with conventional welding, Gas Tungsten Arc Welding (GTAW) on the same material in the scope of microstructure evolution and tensile strength of both welds.
||Detailed design of FSW tool
|| Detailed design of FSSW