[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 ...
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).
