BYU - Brigham Young UniversityFSRL - Friction Stir Research Laboratory
By Jefferson Pew

Effects of Spindle Speed on Welding Forces


Investigate the various parameter interactions of the FSW process in hopes to build a model of the evolution of heat generation (heat input) via FSW.

Experimental approach:

Continuously vary the spindle RPM at different feed rates in Aluminum 7075-T7. All welds were performed by maintaining a constant depth (instead of a constant z-force). Parameters will be varied from 200-800RPM and 2-10 ipm.

In order to minimize variation in the data, four welds were performed at each feed rate and then averaged together. Two of the four welds were executed increasing the RPM from low to high. The other two were executed decreasing the RPM from high to low.


Some interesting trends were discovered to depend heavily on RPM. These are described below.


In the x-direction (travel direction), forces are initially high but decrease to a local minimum at about 200-300 RPM for all travel speeds investigated. As RPM increases beyond this minimum, the X-force continuously rises to a local maximum. The RPM at which this maximum occurs are travel rate dependant. Beyond this maximum, x-forces at all travel speeds decrease and become increasing sporadic.

Figure 1: X-force as a function of RPM and Feed Rate


In the y-direction (perpendicular to the travel direction), forces are initially high but decrease to zero around 200RPM. As RPM increases, the y-force returns to its original magnitude, but in the opposite direction. This maximum depends on the feed rate, but falls between 250-300RPM. After reaching this maximum, the y-forces switch back to their original direction, crossing the axis between 300 and 400 RPM. Beyond this point, the y-forces tend to increase at low feed rates and become increasingly sporadic at high feed rates.

Figure 2: Y-Force as a function of RPM and Feed Rate


In the z-direction, forces are initially high at low RPM. The force decreases nearly linearly with increasing spindle speed. The slope of decrease is not the same for all feed rates.

Figure 3: Z-Force as a function of RPM and Feed Rate.

Future Work


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.

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