Determine the heat input magnitude and distribution along the
tool/workpiece interface through a detailed analysis of FSW tools.
Friction Stir Welding (FSW) is a solid-state joining
process by which an interface-free union of two workpieces
is formed. It has increased in popularity over the last
decade and numerous researchers have attempted to model
the process different ways. However, there has not been
a study specifically dedicated to determining the heat
input at the tool/workpiece interface. Using mathematical
relations, heat conduction modeling of FSW tools, and
actual experimental temperature measurements of FSW tools,
an in-depth study of the heat input at the interface will
be performed. It is felt that a greater understanding
of the heat input at the interface, specifically its quantity
and distribution, can thus be obtained.
In general, the approach will be to produce a numerical model of an FSW tool, apply a heat input along the tool/workpiece interface, and then analyze the temperature contours within the tool. FLUENT commercial software will be used to perform the numerical heat conduction analysis. Experiments will be run concurrently where thermocouples, mounted within the tool, will record temperatures during the weld. Tool temperatures are transmitted to the data acquisition system using and RF telemetry system. Infrared Imaging will also be used throughout the weld to determine the surface temperature of the tool (Figure 1). The numerical and experimental results will be compared and the heat input in the numerical model will then be adjusted until an agreement is reached, thus characterizing the heat input at the interface.
A typical plot showing tool temperatures and tool depth
throughout a weld is shown in Figure 2 above. The inset
picture shows the locations of three thermocouples, each
of which is within 0.050 in. (1.27 mm.) of the tool/workpiece
interface. Tool temperature (left) and approximate tool
depth (right) are plotted versus weld time. As can be
seen in the plot, as the tool is plunges into the workpiece
(decreasing tool depth) the tool temperatures raise over
time and then level out, reaching a final plateau near
the end of the weld. Average values for the temperatures
and z-force were taken for this last plateau and are displayed
on the plot. It can be noted that small fluctuations in
tool depth affect tool temperature. It is interesting
that the temperature at pin center is considerably higher
than the temperatures at the other two locations. This
result is consistent between welds. Current efforts are
focusing on obtaining temperature data and analyzing the
infrared images to confirm tool surface temperatures at
a number of different weld parameters.
Although much work is still to be done by way of numerical
modeling, initial results show the method to be very promising.
One numerical prediction, which correlates with the parameters
used to obtain the plot shown in Figure 2, gave the following
temperatures: Pin Center: 521 C; Root:
457 C; Shoulder: 479 C. It is
interesting here that although the temperatures are higher
than those shown in Figure 2, the trend showing the Pin
Center as the hottest, followed by the Shoulder and then
the Root as the coldest is exhibited in both the numerical
and experimental results. Further work will need to be
done to adjust the numerical heat input until the numerical
and experimental results agree.