Data Overviews

The data overviews are a series of tables which contain all the results from the welding trials and the testing carried out post welding. Table 7.1 shows the welding trials carried out on the Parkson milling machine. Table 7.2.a) and b shows the results from welding trials carried out on FW22. The last overview table 7.3.a) and b) contains the trials performed on the ESAB SuperStir machine, this is by far the

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largest of the overview tables as it contains the results from the entire bead on plate trial.

Table 7.1, Data Overview Table: Friction Stir Welds Created using the Parkson Milling Machine.

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Table 7.2.a) Data Overview Table: Bead on Plate Welds in AA5083 completed onFW22.

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Table 7.2. b) Data Overview Table: Bead on Plate Welds in AA2004 Carried out on FW22.

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Table 7.3.a) Data Overview Table: Welds in AA5083 Created using the ESAB Machine.

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Table 7.3.b Data Overview Table: Welds in AA2004 Created using the ESAB Machine.

82 7.3 Real Time Force Monitoring

The real time force monitoring only applies to the welds carried out on FW22 and the ESAB SuperStir machines. All welds carried out of FW22 were collected using the LoStir device [32]. The ESAB machine utilises built in systems. Values are taken at 10ms intervals creating a force/torque versus time plot. Figure 7.1 shows an example of the plots derived from the different force monitoring systems.

Figure 7.1, Real Time Force Data. a) FW22: Weld Forces versus Time. b) FW22:

Spindle Torque versus Time. c) ESAB SuperStir: Weld Forces versus Time. d) ESAB SuperStir: Spindle Torque and Rotation Speed versus Time.

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From the plots produced the most significant is the torque versus time plot shown in Figure 7.1 c) and d). This plot shows the initial tool touchdowns for both the probe and shoulder these are annotated on the plots as 1 and 2. It also shows start of the tool traverse, annotated as 3 and the onset of the steady state regime. During FSW it will take some time for the process to become steady state hence the removal of material at the start of the weld when testing is carried out. This steady state region is present for the weld force versus time plots but is a little noisy and less distinct.

Using the torque plots the steady state regions were defined and averages for the forces and torque have been taken from this time span to be included in the data overviews.

7.3.1 Weld-Force

The weld-force is strongly linked to the amount of heat supplied by the FSW tooling and process variables. It equates to the amount of force the machine requires to traverse the tooling through the plasticised material. More heat implies softer material beneath the tool making it easier to traverse. This is what is expected, as the weld pitch increases the force required to traverse the tooling increases; this trend is shown in Figure 7.2, for the welds created using FW22. The difference between the hot welding conditions (low weld pitch) and the cold welding conditions (high weld pitch) ranges between ~50% for the small diameter tooling and ~232% for the largest tooling.

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Figure 7.2, Weld-Force Versus Weld Pitch for Bead-On-Plate Welds Created on FW22.

7.3.2 Down-Force

The downwards force acting through the tooling is much larger in magnitude than the welding force. This is the force required to maintain constant contact between the FSW tools and weld material. The down-force through the tool is relatively constant;

with only very small changes in force for changes in weld pitch, this suggests that the major process variables (spindle speed and feed speed) have less affect on the down-force than the tooling used to create the welds and the material being welded. The same trend as the weld force was expected for the down-force, hot welding conditions implying softer material beneath the tool requiring less force through the tooling. This however is not the case. For all the tooling, with the exception of the 7.5mm tool welding the AA2004, the down-force decreased slightly. This difference is much less in magnitude than the weld force, only ~10-20%. It can be seen in Figure 7.3, that the forces experienced when welding AA5083 is higher than that for the AA2004, this can be attributed to the differences in the alloy systems, implying that it is harder to plasticise the AA5083 due to its strain hardening effects of this alloy.

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Figure 7.3, Down-Force Versus Weld Pitch for Bead-On-Plate Welds Created on FW22.

7.3.3 Side-Force

The side force measurements only apply to the welds carried out on the ESAB machine. Only a small number of welds have been created and so a complete analysis of the side force cannot be made, therefore the side force Fy will not be discussed further.

7.3.4 Spindle Torque

The spindle torque produced during the process gives a good initial estimate of the amount of heat generated during the FSW process. A high torque value yields a low heat input and vice versa. Spindle torque (Nm) is the tendency of a force vector to rotate an object about an axis. For a high value of spindle torque more force is required to rotate the spindle, this in turns means more power is required to rotate the tooling; indicating that the material is not fully plasticized; further indicating a low heat input to the weld. As the weld pitch falls, moving from cold to hot welding conditions, the material is more plasticised resulting in a reduction in the spindle torque and therefore the power required to rotate the tool. This torque/heat input relationship behaves in the same way as in conventional rotational friction welding.

As the heat input rises, the torque decreases, up until a critical point. After this point

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no further increase in heat input is possible due to extreme softening and localised melting of the material directly beneath the shoulder, resulting in a loss of traction between the tool and the weld material [30, 31]. Figure 7.4 shows the torque versus increasing weld pitch. As the welding pitch is increased the torque rises; indicating that the tool requires more power to rotate the tools as the weld material is harder under cold welding conditions. When the weld pitch is low the material becomes soft and in some extreme cases can cause localized melting of material in direct contact with the tool shoulder. This thin layer of melted material acts as a lubricant and so decreases the amount of torque; this makes FSW a self regulating process unable to attain fusion of the bulk material.

Figure 7.4, Spindle Torque Versus Weld Pitch for Bead-On-Plate Welds Created on FW22.

7.3.5 Heat Input

The heat input for a friction stir weld is best estimated using the spindle torque. It has been suggested that the rotation of the tool in contact with the weld material accounts for as much as 99% of the total heat generated, with the remaining heat derived from the traverse of the tool. The efficiency factor represents the amount of heat which remains in the work piece and has been estimated to be ~0.85 [14, 30, 31].

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(2.1) (KJ/mm)

Where: Q is Heat Input, E is Efficiency factor (0.85), r is Spindle rotation speed (rpm), T is Spindle torque (Nm) and v is Traverse speed (mm/min) [31, 32].

Figure 7.5 shows the calculated heat input from equation (2.1) versus the welding pitch. It is clearly visible that as the welding pitch increases the amount of heat supplied falls. This is expected as the higher the welding pitch, the more material is processed for each rotation of the tooling. This supports the notion of using the welding pitch as a preliminary evaluation of the heat supplied to the weld.

Figure 7.5, Calculated Heat Input Versus Welding Pitch for Bead-On-Plate Welds Created on FW22.