FSW as riveting replacement on a fuselage panel

FSW replaced traditional riveting on an example of an outer fuselage panel. Usual design of fuselage outer panels consists of stringers riveted onto the panel surface. Riveting is a well known and widely used technology however its usage on the outer panels brings problems with impermeability.

Expected improvements lie mainly in productivity and speed of production, better tensile properties of the joint, better fatigue properties and corrosion properties.

Panel design

Theoretical mechanical properties of the panels

New panel design

The new panel keeps all the outer dimensions of the riveted panel, only the construction of the stringer reinforcement has been changed. Details of the new panel design are in Attachment 3.

Comparison of mechanical properties and weight of the riveted and FS welded panel

6.5      Production process

The stringers are held in the gripping device and the panel plate will be put on them. Positioning parts of the gripping device will assure correct position of the panel plate. This all will be gripped down to the workbench by 3 rods using clamps a shown on the scheme below.

Due to deviations from the steady welding state in the first and last 20 mm of the weld [22] the work piece has 20 mm allowance on the both sides as the approach and drifting area. After the welding has been performed the allowance will be cut off.

The movement of the welding tool was designed in order to assure highest possible productivity as shown on Figure 43.

Figure 43: Welding process scheme

6.6      Used tool

As the known tools produced by TWI are suitable for welding of aluminium plates thicker than 6 mm, the custom made tool was chosen to be used. The basic parameters based on the tools used on similar jobs in Attachment 2 are following:

-          Tool type                       Conical threaded

-          Pin diameter 1               5 mm

-          Pin diameter 2               3 mm

-          Pin length                      4 mm

-          Shoulder diameter         15 mm

-          Tool material                  W-Co tool steel

Detailed design of the tool is not within the scope of this thesis and would require more detailed analysis.

6.7      Welding parameters

Determination and optimization of the welding parameters is a process requiring testing on the actual samples of the material, machine and the tool used. The parameters used as the starting point in the welding parameters optimization process is based on the welding parameters of similar welds as in Attachment 2.

-          Rotational speed            1400 min-1

-                     Welding speed               110 mm.min-1

-          Tool tilt                          2.5°

-          Vertical down force      3.5 kN

Before introducing a serial production, the welding parameters have to be optimized. The proposed optimization would consist of a series of experiments for welding speeds in range of 100 to 150 mm.min-1 by step of 10 mm.min-1 and rotational speed in range of 1200 to 1600 by step of 200 min-1. For each of the combinations 5 samples should be done and their tensile strength in the axis perpendicular to the stringer plane would be evaluated. Also the joint would be examined visually for presence of cracks and examination of overall weld quality.

These optimization steps are vital for successful and efficient application of the technology on the panel and further investigation like fatigue tests might be needed to comply with certification requirements.

6.8      Machine used

The welding machine chosen is ESAB FSW SuperStir, which allows welding of large work pieces. Parameters of the machine are following [82].

-          Maximum work piece dimensions         2500 x 4000 mm

-          Maximum welding speed                      2000 mm.min-1

-          Maximum vertical down force              12.5 kN

-          Maximum revolutions of the tool          2500 min-1

-          Maximum welding depth                      6 mm

The machine parameters are sufficient for the expected range of welding parameters.

6.9      Gripping equipment

Gripping equipment was designed to assure the correct positioning of panel plate and the stringers as well as stiffness of the whole work piece needed for the welding process involving high welding forces.

Figure 44 shows the scheme of the welding device. Stringer holders together with the stringer stop assure the appropriate position of the stringers. Corner and side stops guarantee the right position of the panel plate with the 20 mm allowance on the both sides towards the stringers. The panel plate together with the stringers are clamped down using the beams.

The low distance between the stringer holders together with the fact that the welded stringer always carries the vertical down force initiated by the tool, should assure low deformation of the work pieces during the welding process. Further examination of the deformation of the work pieces under effect of welding process should be done before introducing the gripping device as a part of serial production.

The full welding force should not be applied in the first and the last 20 mm of the weld, as stringer do not support the welded panel plate in that area. The time behaviour of the vertical down force, induced by the welding tool, during the first and last 20 mm of the weld should be a part of the further optimization of the welding process.

Attachment 4 contains the system drawing of the gripping device.

Figure 44: Griping device scheme: 1. Clamped beams holding the weld pieces,
2. Stringer holders attached to the main desk, 3. Corner stop, 4. Side stops,
5 Stringer stop

6.10 Manufacturing steps

The process of manufacturing the reinforced panel has following steps:

-          Cutting of panel plate work piece

-          Cutting stringer work pieces

-          Clamping down the gripping device to the welding machine workbench

-          Positioning the stringers into the device

-          Positioning the panel plate into the device

-          Clamping the work pieces down to the machine work bench using the beams

-          Defining the work piece position towards the tool in the known base position for given CNC welding program (in serial production for the first piece only)

-          Automated welding program execution

-          Removing the clamping beams

-          Removing the weldment from the griping device

-          Basic visual control of the weld

-          Cutting the 20 mm material allowance off

-          Cutting the side U shaped notches into the panel

Time consumption