Download Validation of Marker Material Flow in 4mm Thick Friction Stir

Validation of Marker Material Flow in 4mm Thick Friction Stir
Welded Al 2024-T351 as reported by Computed
Microtomography using Standard Metallographic Techniques
1
R. Zettler, J.F. dos Santos, T. Donath, F. Beckmann, T. Lippmann, D. Lohwasser and A. Schreyer
Institute for Materials Research, GKSS Research Centre, Max-Planck-Str. 1, 21502 Geesthacht, Germany
1
Airbus Deutschland GmbH
Friction Stir Welding (FSW) was invented and patented by The Welding Institute (TWI) of
Cambridge, in 1991 [1]. The process is referred to as a solid state joining process where no bulk
melting of the base material occurs. In comparison to fusion welds there is no evidence of an as
cast structure in the weld nugget. The process essentially relies on frictional heating and plastic
deformation brought about by a rotating and non-consumable tool that is plunged into and then
traversed along the join line between typically two abutting workpieces. A schematic of the process
can be found in Figure 1.
Figure 1: Schematic of FSW process: Left to right: tool rotation, tool plunge, tool traverse, tool exit.
The FSW tool in conjunction with the processing parameters; axial load or downforce, rotation and
travel speed provide the necessary energy input required to thermally soften the workpiece
material. The joining tool comprises essentially of two parts: a shoulder and a pin. The tool
shoulder not only helps to produce heat by means of friction induced rubbing with the surface of
the workpiece but also acts as a barrier preventing expulsion of locally plasticised material from the
immediate weld zone. The FSW pin then forces this thermally softened material, contained at the
underside of the workpieces by a backing bar or anvil, to flow in the direction of rotation where it is
transferred from in front of and then to the back of the pin where it cools and consolidates. The
residue of this interaction between the welding tool, the workpieces and the clamping system can
be evidenced in the microstructure and flow induced patterns of the weld nugget. These flow
patterns appear like the layers of an onion prompting the term onion ring structure for the weld
nugget, Figure 2.
Figure 2: Macrographs of two friction stir welds as seen transverse to weld travel direction produced in
4mm thick Al 2024 T351 using identical tool shoulders but dissimilar tool pins, demonstrating very different
flow patterns.
Although many aluminium alloys have proven to be capable of being joined using the FSW
process, much conjecture still exists concerning the nature of the deformation process, the bonding
mechanisms involved and their influence on subsequent weld properties.
The use of minute embedded marker materials strategically placed in the path of the FSW tool have
allowed for much greater insight into material flow resulting from the interaction of the FSW tool
with the workpieces. The visualisation and displacement of a Ti powder marker material has here
been investigated for friction stir welds produced in a 4 mm thick Al 2024 T351 alloy using X-ray
computed microtomography (µCT) [2]. The results from this investigation have demonstrated that
marker flow originating from two different locations and for two different tool geometries
generates significant differences in the observed flow patterns of the marker material. The accuracy
of the µCT technique in validating marker material flow has been assessed using standard
metallographic techniques. The results confirm the accuracy of the CT measurements but also
highlight advantages and disadvantages associated with each procedure.
Figure 3: Comparison of metallographic images (top) and renderings from µCT-data (bottom). From left to
right: Tool 1B marker placed l.h.s top., Tool 1B marker placed r.h.s top, Tool 1C marker placed l.h.s top,
Tool 1C marker placed r.h.s top. Note tool rotation is clockwise and traveling up the page:
Samples were generated by using a stop action technique where the welding tool pin is abruptly
halted as it enters the marker embedded region of the workpieces. Figure 3 compares the
information obtained by the standard metallographic technique and the µCT data obtained at
beamline W2 at a photon energy of 60 keV.
The metallographic technique is capable of defining the location of the marker material in relation
to the onion ring or banded structures within the weld nugget. This is important if one wishes to
correlate the relationship between material flow or banding with that of crack growth within the
weld nugget under cyclic e.g. fatigue loading. Obtaining a 3D marker distribution for flow studies
is not possible using standard metallographic techniques where only one surface can be ground,
polished and etched at any one time. The metallographic technique thereby requires the ultimate
destruction of the specimen. By use of the non-destructive µCT technique the 3D distribution of
marker material around the FSW pin could be obtained at 10 µm spatial resolution. From this data
set information on the material flow is accessible. The combination of the two techniques will give
new insights in the understanding of the welding process.
References
[1] W.M. Thomas, E.D. Nicholas, J.C. Needham, M.G. Church, P. Templesmith, C.J. Dawes:
International Patent Application No. PCT/GB92/02203 and GB Patent Application No. 9125978.9,
(1991).
[2] T. Donath, F. Beckmann, R. Zettler, J. dos Santos, D. Lohwasser, T. Lippman, H. Clemens, and
A. Schreyer, AIP Conf. Proc. 705, 1312, (2004).