SSFSW Butt joint of a 0.5 mm thick aluminium alloy
Conventional FSW of lap and butt joints in aluminium alloys is well established. However when welding components below 2mm thickness, it is challenging to produce high quality joints as the rotating shoulder tends to damage the thin aluminium sheet surrounding the weld area and the weld can be undercut, reducing the strength. It is therefore proposed to use the SSFSW technique to produce butt welds. There are a number of reasons to use SSFSW over conventional FSW, especially for thin section joints, including:
- Lower distortion
- Improved surface finish
- Increased robustness when performed by a robot
- Lower undercut
- Improved mechanical properties and fatigue resistance
Recent developments in SS-FSW tool design and process control have resulted in joint strengths in excess of 90% of the parent material, including aerospace grade aluminium alloys such as AA2024-T3.
The conventional approach to producing T-joints is by tool penetration from the skin side into the rib or frame. This approach, a so-called “stake joint”, leads to undercutting of the top plate surface, difficult to achieve full joint coverage and the sharp corners produced can act as stress raisers. To eliminate these issues, a new FSW technique called Corner SS-FSW was developed. Like SSFSW for butt and lap joints, the probe rotates and protrudes through a hole in a stationary shoulder/slide component. For Corner SSFSW, however, the shoulder is V-shaped with a 90º angle to fit the internal corner of a T-joint. Using this technique full joint coverage is a guaranteed.
REFILL FRICTION STIR SPOT WELDING
Of all the FSSW process variants, only the RFSSW process does not leave an exit hole after welding, which would be very beneficial for aerostructure applications. The main advantage of the RFSSW over the basic FSSW technique is the ability to provide a larger weld area which provides improved weld strength. Although the process cycle may be slightly longer than the “Basic” FSSW process, it is substantially quicker than riveting.
For aerostructures, cost savings that can be achieved by using RFSSW due light weighting by removing the weight of the rivets from the assembly and also that this can be done without changing the design of assembly, which is very advantageous. The average tensile shear strength of a RFSSW is much higher than that of a riveted joint and the thickness of the flange could be significantly lower than with riveting.
LASER BEAM WELDING
A number of different welding approaches for laser beam welding cargo door demonstrator are examined in this project.
In addition the state of the art such as fibre-delivered single-sided stake (through flange), single-sided T-butt welding and simultaneous double-sided T-butt welding), more novel approaches are also being explored, recognising that their respective possibilities will be determined by factors including parent material, joint design and ease of access to the joints to be welded.
NUMERICAL MODELLING AND RESIDUAL STRESS SIMULATION
A key to successful numerical modelling is the accuracy of the input data. In addition to the mechanical testing required for qualification of the welds, additional material testing is needed as input for the numerical modelling. Dynamic material testing will be performed using a Gleeble system, to generate stress-strain curves of aerospace aluminium alloys, at various temperatures, strain rates and thermal expansion coefficients, needed for input and verification of the numerical models. The Gleeble system uses resistance heating for the heating of a small test specimen according to a pre-programmed temperature and mechanical load cycle and is monitored through percussion welded thermocouples in a closed circuit loop. This enables fundamental investigations and possibilities to simulate numerous welding processes and abilities to generate input data to simulation models as well as validation.
The numerical model, inclusive of weld distortion and residual stress model developed will then be used to make predictions regarding the optimum welding approach, materials, profile and joint designs and a proposed weld sequence to minimise distortion and residual stress in the LBW demonstrator. An example of the residual stress and displacements results on a T-joint is shown below.