The friction stir welding process was developed by TWI (Welding Institute in
Cambridge, UK in 1991) for joining aluminum alloys. During friction stir
welding, a joint is made between contacting sheets by traversing a rotating tool
comprising a pin and shoulder between the mating surfaces. Dynamically
recrystallized material formed adjacent to the periphery of the rotating tool
creates a weld between the contacting work pieces. Friction stir welding has
also been used to weld magnesium, titanium, and copper alloys, stainless steel,
and thermoplastics in a wide range of thicknesses.
Currently, FSW is used particularly for joining aluminum alloys in shipbuilding and marine industries,
aerospace, automotive and rail industry. Furthermore, the technology provides
significant advantage to the aluminum extrusion industry. Automotive suppliers
are already using the technique for wheel rims and suspensions arms. Fuel tanks
joined by FSW have already has been launched in spacecraft, and many other space
advances are under development. Aluminum panels for high speed ferries and
panels for rail vehicles are also produced. Moreover, the friction stir welding
of 50 mm thick copper material has provided a potential solution for nuclear
encapsulation of radioactive waste. Friction stir welding is making an impact as
a material processing technique and the prognosis for the successful welding of
steel products by FSW looks promising.
Friction stir spot welding is a process variant where the rotating tool is plunged into and out of two overlapping
sheets at a single location. When the pin penetrates into the contacting sheets,
a stir zone is formed comprising dynamically recrystallized material and a
keyhole region is left in the welded component when the rotating tool is
retracted. The keyhole can be filled via the use of a specially-designed tool
fixture if desired. The tool penetration process and the factors determining
weld mechanical properties have been investigated during spot welding of
AI-alloy and Mg-alloy sheet materials. Although the microstructural features and
mechanical properties of Al 5754 and Al 6061 friction stir seam welds have been
studied extensively, this is not the case in Al 5754 and Al 6061 friction stir
spots welds made using a range of tool rotational speed settings.
North et al. proposed that local melting and tool slippage could account for the low travel
speeds that are obtainable during friction stir seam welding of Al 2024 and Al
7075 sheets. Gerlich et al. provided support for this proposal in detailed
investigations of Al 7075 and Al 2024 friction stir spot welds. The remarkable
decrease in estimated strain rate values in spot welds made using high tool
rotational speed settings was associated with spontaneous melting of
second-phase particles (η, S, and T phases), which promoted
tool slippage at the contact interface between the periphery of the rotating
tool and adjacent material in the stir zone.
High tool rotational speed settings produced a combination of high heating rate and high stir zone temperature,
which facilitated local melting and tool slippage during friction stir spot
welding.
The possibility that local melting and tool slippage are characteristic
features when AI-alloy sections are joined using friction stir spot welding is
examined in the present article. Because energy generation resulting from
viscous dissipation during friction stir spot welding is ultimately limited by
the solidus temperature, local melting and tool slippage may be precluded when
the AI-alloy base materials being fabricated do not contain second phase
particles, which have melting points less than their solidus temperature.
This is the primary reason why the present article reports the results of a detailed
investigation of Al 5754 and Al 6061 friction stir spot welding. The possibility
that local melting and tool slippage occur during friction stir spot welding of
Al 5754 and Al 6061 base materials was examined and the strain rate in the
contact region between the periphery of the rotating tool and adjacent material
in the stir zone is estimated by incorporating the average grain dimensions and
stir zone temperatures in friction stir spot welds into the Zener-Hollomon
relation.
Reliable measurements of the average subgrain dimensions and stir zone
temperature are critical inputs when the strain rate in the contact region is
estimated. Although aluminum alloys have high stacking fault energy and are not
significantly strengthened by grain refinement, the subgrain dimensions of the
deformed microstructure are related to the Zener-Hollomon parameter.
The influence of tool rotational speed variations from 750 to 3000 RPM on the
average sub grain dimensions and temperatures in the stir zone was investigated.
Because almost all of the energy generated during friction stir spot welding is
due to the torque resulting from tool rotation, stir zones made using rotational
speeds ≤750 RPM did not have bonded regions having acce8table dimensions and
joint mechanical properties. In the present investigation, the welding
parameters used are those that produced stir zones having widths > 100 µm.
Optical micrograph of a typical spot welded cross section is shown at Figure1.
Region I, which is located 100 µm from the keyhole periphery halfway up the
length of the rotating pin, was examined using EBSD. The average EBSD grain
dimensions were calculated from misorientation maps using the linear intercept
method, with only grain boundaries having > 2 deg misorientations considered. At
least two locations in two repeat test welds were examined, which provided a
minimum of 400 intercept measurements for each welding condition. It has already
been shown that the average subgrain dimensions found using TEM are similar to
those estimated using EBSD when a 2 deg misorientation criterion is applied
during the examination of the stir zones produced in Al 2024, AI 7075, and Al
5754 friction stir spot welds.
The temperature gradually increased during the
dwell period prior following complete penetration by the rotating pin. In
addition, no temperature fluctuations were observed during the 4-s-long dwell
period in Al 5754 and Al 6061 spot welding.
The heating rates increased from
150°C/s to 354°C/s in Al 5754 and Al 6061 friction stir spot welds produced
using different tool rotational speeds. Also, higher peak temperatures were
measured close to the tip of the rotating pin when tool rotational speed
increased from 750 to 3000 RPM. The highest temperature measured in Al 5754 spot
welds made using a tool rotational speed of 3000 RPM (565°C) is lower than the
reported solidus temperature of this alloy (590°C). The highest temperature in
Al 6061 spot welds (541°C) was also less than the solidus temperature of the
alloy (582°C).
The temperatures measured by the thermo-couple embedded within
the tool shoulder were consistently lower than those found close to the tip of
the rotating pin at all tool rotation speed settings during Al 5754 and Al 6061
spot welding.
Figure 1: Polarized light optical micrograph of an Al 6061 friction stir
spot weld made using a tool rotational speed of 1500 RPM.
The stir zone boundary in indicated by the dashed line.
The highest temperature in Al 5754 spot welds made using a tool rotational speed
of 3000 RPM was 565°C. Based on the chemical composition the Al 5754 alloy can
be approximately represented as an AI-3 wt pct Mg alloy. The AI-Mg binary phase
diagram indicates that the solidus temperature of this binary alloy is 613°C. As
a result, the highest temperature found in the stir zone of Al 5754 spot welds
corresponds with a homologous temperature of 0.97.
In a similar manner, the highest temperature in Al 6061 friction stir spot welds (541°C) corresponds with
a homologous temperature of 0.95. These homologous temperature values correspond
well with those reported during friction stir spot welding of different Al alloy
and Mg alloys. For example, the highest temperatures found in the stir zones of
Al 7075, Al 2024, AI 6111, AZ31, AZ91, and AM50 friction stir spot welds
corresponded with homologous temperatures from 0.94 to 0.99.
It has been recently concluded that the stir zone temperature in friction stir spot welds
cannot be determined by embedding thermocouples in drilled holes adjacent to the
periphery of the rotating tool, because the helical vertical rotational flow of
material setup in the stir zone formed adjacent to the periphery of the rotating
pin displaces thermocouples from their original locations. Also, because a steep
temperature gradient exists immediately adjacent to the tool periphery and the
temperature decreases markedly at small distances from the contact surface, it
is particularly difficult to obtain consistent temperature output when the
thermal cycles in the TMAZ region of friction stir spot welds are measured.
As mentioned earlier, tool slippage during AI 7075 and Al 2024 friction stir spot
welding operations has been associated with spontaneous melting of second-phase
η (MgZn2) and S (AI2CuMg) particles during Al 7075-T6 and
Al 2024-T3 spot welding. In as received Al 5754 base material, the second-phase
particles comprised mainly Al6(Fe,Mn). Al6Fe is a
metastable phase, while Al6Mn intermetallic has a melting temperature
of 705°C that is much higher than the solidus temperature of Al 5754. Also,
although it has been reported that AlMg2 particles have a melting
temperature of 450°C detailed TEM examination confirmed that they were not
present in the as-received Al 5754 base material.
The results of the present investigation suggest that the assumption of a no-slip condition may be
appropriate during numerical modeling of Al 5754 and Al 6061 friction stir spot
welding. However, this is not the case when Al 7075, Al 2024, and Mg-alloy AZ91
are spot welded.
There was no evidence of grain growth in the stir zones of Al
5754 friction stir spot welds. However, the average grain sizes were higher in
the stir zones of air-cooled Al 6061 spot welds, indicating that grain growth
occurred when spot welds cooled in air to room temperature. The final stir zone
grain size in air-cooled spot welds could be represented using an Arrhenius
growth equation. Also, there was no evidence of abnormal grain growth in the
stir zones of Al 6061 friction stir spot welds.