An inherent feature of friction stir welding (FSW) is the presence of an exit hole, due to the retraction of the tool at the end of the weld.
Many components cannot tolerate this exit hole, and this has led to the development of a number of techniques to eliminate the feature. Run-off tabs can be used successfully provided that the tool can be traversed off the edge of the component onto the tab. This often cannot be achieved due to the geometry of the component as in the case of circumferential welds, lid sealing or crack repair for example.
A more complicated solution is the use of a two piece tool allowing the probe to be retracted near the end of the weld while the shoulder remains in contact. The equipment necessary to produce this motion is expensive, not always easy to integrate onto existing FSW machinery and not compatible with some probe designs. Therefore TWI has developed a simple ramp as an effective method of eliminating the exit hole from the workpiece material.
There are a variety of control methods available when friction stir welding. Force control uses a feedback sensor to monitor and record the welding force and this information is fed back to a control system that adjusts the plunge depth of the tool to maintain the programmed force value. This allows the tool shoulder to stay in contact with the workpiece surface and responds to variations in material thickness.
Using force control in conjunction with a ramp, which is firmly clamped to the workpiece at the end of the weld, allows the tool to follow the angle of the ramp until the full depth of the probe is clear of the workpiece. This is achieved without the need to programme any positional information into the machine. The ramp is then machined away from the workpiece leaving no evidence of an exit hole.
Preliminary trials, performed on an aluminium alloy using a selection of ramp angles, indicated that the technique was very effective. Photographs are shown in the figure of the ramp before and after welding. The ramp angle was5° and the tip of the 4mm length tool probe was raised 2mm above the upper surface of the workpiece at the end of the ramp.
Work is ongoing to understand the extraction rate limitations with trials on patch welding and crack repair in stainless steel looking to be promising first applications.
Initial trials have proven the concept in aluminium and steel materials with work now continuing in other high temperature materials such as nickel and titanium.
For more information see Friction Processes or please contact us.