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2.4 Design and performance of FSW tool
Chapter 2
Figure 2.10. Hot gas stream assisted FSW setup using a nitrogen stream to preheat the workpieces [20].
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Figure 2.11. Different types of FSW tools (a) fixed, (b) adjustable and (c) self-reacting [12].
Shoulder is important part of FSW tool which generate 70-80% of the total heat and produce the downward forging action necessary for welding consolidation to yield proper material flow during welding. The tool shoulder can also provide confinement for the heated volume of the material [13]. Therefore, tool shoulder end surface inclination with respect to workpiece surface (such as flat, concave and convex) and feature on end surface play vital role to improve performance of tool. Reynolds et al. [171] and Nelson et al. [172] used concave shoulder end surface and restricted material extrusion from the sides of the shoulder. The concave shoulder inclines only a small angle from the flat shoulder end surface. During tool plunging, the material displaced by the probe is fed into tool shoulder cavity. Hence the concave surface profile of the tool shoulder serves as an escape volume or reservoir for the displaced material from the probe. Another possible end shape of the shoulder is a convex profile and it pushes the material away from the probe due to that it is unsuccessful [173].
However, Nishihara and Nagasaka [174] reported that a smooth convex end surface shoulder with a 5 mm diameter was successful to weld 0.4 mm thick AZ31 Mg alloy sheets.
Whereas, some researchers [175-176] utilized the shoulder end surfaces with some features to increase material friction, shear and deformation for increased workpiece mixing and higher weld quality. The typical shoulder end styles include flat (smooth or featureless), scrolls, ridges, knurling, grooves and concentric circles, as revealed in Fig. 2.12. These features can be applied to concave, flat or convex shoulder ends. Scrolls are the most commonly used shoulder feature. Dawes et al. [177] described the outlined of the tool design aspects of the scroll shoulder concept.
Chapter 2
Figure 2.12. Summarizes the typical shoulder outer surfaces, the bottom end surfaces and the end features [176].
An important parameter in FSW is the ratio of dynamic volume (volume swept by the pin during rotation) to static volume (volume of the pin itself). Increasing this ratio results in a reduction in the formation of voids in the welds and allows the surface oxide to be more effectively disrupted and dispersed within the microstructure. In conventional FSW, the dynamic/static ratio can be increased via the use of re-entrant features, flutes, threads and or flats machined into the pin as shown in Fig. 2.13 [176]. Dawes and Thomas [177]
found that the addition of flat features can change material movement around a probe. This is due to the increased local deformation and turbulent flow of the plasticised material by the flats acting as paddles. Elangovan et al. [178] investigated the effect of axial force and tool pin profiles on FSP zone formation in AA6061 aluminium alloy. Five different tool pin profiles (straight cylindrical, tapered, cylindrical, threaded cylindrical, triangular and square) were used to fabricate the joints at three different axial force levels. They found that the square tool pin profile produces mechanically sound and metallurgically defect free welds compared to other tool pin profiles. Chowdhurya et al. [179] studied the effect of pin tool thread orientation right-hand thread (RHT) and left-hand thread (LHT)) in the clockwise rotation on the fatigue resistance of friction stir welded (FSWed) AZ31B-H24 Mg alloy butt joints. They found that the fatigue strength was higher in the joints made with the LHT pin tool than with the RHT pin tool due to the elimination of the welding defects near the bottom surface via a downward material flow. Similarly, Zhao et al. [180] and Schnider and Nunes [121] made FSW joints using right hand thread (counterclockwise) tool profile and
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left hand thread (clockwise) tool profile respectively to observed occurrence of welding defects. The combination of the thread orientation with the rotation of the pin tool increased an extra downward force and heat generation that would be beneficial to accelerate the flow of plasticized material to ensure good mechanical bonding.
On the other hand, Cao and Jahazi [181] recently observed some noticeable defects at the bottom or root of the welded joints with a right hand screw threaded pin in the clockwise rotation. These observations indicated that the material flow pattern during FSW is a function of pin tool thread and direction of rotation. But little is known of the effect of the left hand thread (LHT) and right hand thread (RHT) pins on the microstructure and mechanical properties of the FSWed. Zettler et al. [182] welded 4 mm thick 2024-T351 and 6056-T4 Al alloys using three different tapered probe designs: non-threaded, threaded and threaded with flats. It was found that the non-threaded probe produced voids, while the two threaded probes produced fully consolidated welds. The flats on the probe act as the cutting edge of a cutter. The material is trapped in the flats and then released behind the tool, promoting more effective mixing. The addition of the flats was also shown to increase the temperature and nugget area. Mishra and Mahoney [183] conducted experimental studies FSW of aluminum alloy using tool which contains three flutes cut in to the helical ridge. The flutes reduce the displaced volume of a cylindrical pin by approximately 70% and supply additional deformation at the weld line in addition it increases the tool travel speed and it can be used advantageously to welding thick-section aluminum alloys.
The fundamental requirement of the FSW tool is high resistance to wear, high strength and hardness at both room and elevated temperatures, and effective heat-dissipation ability with respect to welding material. However, due to extreme load condition tool experience excessive wear which changes the tool shape, thereby increasing the probability of defect generation, and possibly degrading the weld quality. The exact wear mechanism depends on the interaction between the workpiece and the tool materials, the selected tool geometry and the welding parameters. Liu et al. [184] analyzed the effect of welding speed which has a decisive effect on radial wear rate of the pin. The FSW tool was made of a WC- Co hard alloy and the tool pin possessed right handed threads. A series of tool photographs
Chapter 2 was obtained and the variations in tool geometry were accurately calculated in a computer system. Thompson and Babu [185] used three tungsten-based tool materials namely material A (99% W-1% La2O3), material B (75% W-25% Re), and material C (70% W-20% Re-10%
HfC) to join high strength steel. They identified the tool degradation mechanisms by studying the pre- and post-weld microstructures of the tool. They found that the grain deformation was the most significant source of tool degradation. The primary degradation mechanism of material A was deformation, for material B it was twinning and for C it was inter-granular failure. The shoulder size and pin length are changed slightly and the radial wear of the pin is most severe for the whole tool. The radial wear of tool is different at different locations of the tool and maximum wear is produced at a location of about one- third pin length from the pin root. The welding speed also effect on radial wear rate of the pin that is lower the welding speed, higher the wear rate and the maximum wear rate is produced in the initial welding.
Figure 2.13. Different re-entrant features, flutes, threads and or flats on pin surface [184- 185].
Prado et al. [186] studied the tool wear and the rate of wear in the FSW of Al 6061 aluminum alloy with respect to different welding speed. They observed that the tool shape is
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changed with increase of welding speed and suggested that the tool wear and shape change are due to counter motion of solid-state flow regimes which depend upon both tool rotation speed and actual weld traverse speed. Plane tool shape was more sustainable than threaded tool which continues to produce excellent welds but without any additional tool wear. The self-optimized shape changed with increase in weld speed and at a constant tool rotational speed of 1000 rev/min. Lienert et al. [187] studied tool deformation and wear by comparing critical tool dimensions for a given tool before and after each weld using an optical comparator. Metallographic and metrology techniques suggested changes in tool dimensions resulted from both rubbing wear and deformation of the tool. The greatest changes in dimensions occurred during the initial plunging stage. Wu et al. [188] investigate tool wear at different travel distances of polycrystalline cubic boron nitride (pcBN) tool at rotation rates of 400, 800 and 1200 rpm during FSW of Ti–6Al–4V alloy. They found that at high rotation rates of 800 and 1200 rpm, the greatest tool wear, including mechanical and chemical wear, occurred at the initial tool plunge point.
Park et al. [189] conducted a detailed study on the tool wear in FSW of stainless steel using pcBN tool. They reported that apart from mechanical wear, chemical wear is also occurred and Cr-rich borides formed through the reaction between the workpiece and the pcBN tool. These Cr-rich borides have adverse effect on the tool strength during welding.
Zhang et al. [190] succinctly reported tool wear of pcBN during FSW of pure titanium using electron microprobe analysis (EMPA) and energy dispersive spectrometer (EDS) and they inferred that tool might have reacted with titanium. They had not identified the FSW parameters which influenced tool wear and the nature of the wear products. However, Pilchak et al. [191] showed that millimeters of bands consisted of sub-micro-sized tungsten- rich particles as the result of tool wear were deposited in the stir zone. Tool wear not only reduces the lifetime of the tool but also probably affect the material flow and the properties of the welds. Agrawal et al. [192] studied tool wear during FSW of AA6063 aluminum alloy and pure copper. The rubbing action with high strength alloys such as brass and AA6063 materials at a higher rotational speed may result in wear of the tool. Additionally, the sticking of Cu–Al mixed material on the surface of the tool after every welding run is a big issue which causes defects and tool wear. However, this problem can be avoided by
Chapter 2 inserting the FSW tool into the fresh Al material after every experiment. Insertion of tool in fresh material helps to react Cu–Al mixed material with fresh Al material which in turn clean the tool pin and prevent the defects.