RD&D-Programme 2004 - SKB

6.3.3 The welding process
The parameters that are controlled are:
• The downward force of the tool (welding force).
• Spindle speed.
• Welding speed.
In addition to the process parameters, the important resultants are also measured. These process
indicators are:
• The temperature of the tool probe.
• The torque of the rotating spindle.
• The force on the tool in the travel direction.
• The position (depth) of the tool shoulder in relation to the canister surface.
One goal in the development work has been to keep the parameters constant throughout the
welding process. Important points in the development work have been to:
• Reduce the risk that the FSW probe is exposed to high temperatures that could reduce its life.
• Achieve even weld quality and grain size in the weld metal.
• Develop a simple and robust method for monitoring the process.
Following is a description of the different sequences in the welding process and how they are
controlled:
• Positioning and heating. A pilot hole is drilled above the joint line to facilitate the
positioning of the FSW tool and reduce the risk that the probe will be damaged. The tool
is rotated and pushed into the pilot hole until the welding force is reached. The welding
force is maintained throughout the heating sequence.
• Start and acceleration. When a given welding temperature has been reached the
acceleration sequence starts. The welding speed is a function of the welding temperature.
• Steady state. The welding temperature is controlled by maintaining a constant heat input
(spindle torque x spindle speed), which is achieved by small variations in the welding force.
• Termination. When welding is terminated and the tool is removed from the canister, an
exit hole is formed. Since the exit hole may not remain in the joint, the tool is parked above
the joint line, see Figure 6-18. This area is machined away, along with the start area, after
welding.
Figure 6-18. The start area (at right) and the exit hole (at left) above the joint line.
78 RD&D-Programme 2004