SOLID-STATE FLOW VISUALIZATION IN THE FRICTION-STIR WELDING OF 2024 Al TO 6061 Al
Ying Li, L.E. Murr and J.C. McClureDepartment of Metallurgical and Materials Engineering, The University of Texas at El Paso,
El Paso, TX 79968
(Received November 16, 1998)(Accepted in revised form February 15, 1999)
Introduction
It is now well established that friction-stir welding (FSW) is a solid-state, extreme plastic deformationprocess which culminates in dynamic recrystallization to provide a mechanism for the superplastic flowwhich accommodates the stirring of one metal workpiece into another (1–4). In the welding of 6061aluminum to itself, maximum, measured centerline temperatures have been observed not to exceedabout 0.8 TM (where TM is the absolute melting temperature, K); there is no melting (5).
In the case of fluid flow, and especially complex turbulence, it is often very difficult to examine andcharacterize the flow because of an inability to visualize it. Flow visualization has therefore become animportant area of fluid and related flow research (6).
This study examines the complex flow patterns developed in the FSW of 2024 aluminum to 6061aluminum. These flow patterns are visualized by the differential etching of the two aluminum alloys.
Experimental Details
Plates of 2024 aluminum (4.5% Cu, 1.5% Mg, 0.6% Mn, balance Al in weight percent) and 6061aluminum (0.25% Cu, 1% Mg, 0.6% Si, 0.15% Cr, balance Al in weight percent), nominally 0.65 cmthick, were friction-stir welded as shown schematically in Fig. 1(a) and (b). The rotating tool orhead-pin (HP in Fig. 1(a)) was a standard 1/4–20, carbon-steel screw (nominally 0.65 cm diameter)inserted into a 1.9 cm diameter milling chuck. The clockwise rotation (R) of the tool draws theworkpiece against the milling chuck and expands the FSW zone at the upper surface of the specimenas shown in Fig. 1(b). The tool (HP) rotation speeds (R in Fig. 1(a)) were varied between 400 and 1200rpm, and the actual traverse (T) or linear welding speed was constant at 1 mm/s. In addition, the tool(HP) axis was adjusted to be a few degrees from the perpendicular z-axis illustrated schematically inFig. 1(c). Following FSW, sections were cut from the weld zone to expose the flow geometries asillustrated ideally in Fig. 1(c) and polished and etched using a Keller’s reagent (nominally 150 mLwater, 3 mL nitric acid, 6 mL hydrochloric acid, and 6 mL hydrofluoric acid; at 0°C). The 2024aluminum was normally most responsive to this etchant, but by making slight adjustments in thecomposition, especially the hydrofluoric acid concentration, and extending the etching time, the etchingsensitivity could be shifted to the 6061 aluminum. In this way, the residual, FSW mixing and flowpatterns could be visualized by metallographic contrast in light microscopy. This feature is illustrated
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for a typical weld zone as shown in Fig. 1(d) which shows contrast variation with deformation andrecrystallization associated with the FSW process. The arrow denotes the dynamic recrystallizationzone boundary (extended by dashed line into the 6061 Al side). In addition, Fig. 1(e) and (f) compare
Figure 1. Friction-stir welding of 2024 Al to 6061 Al. (a), (b), and (c) show schematic sequence. In (a) the rotating (R) tool orhead pin (HP) advances into contiguous workpieces to form a weld as in (b). (c) shows the cutting of sections from the weld zone.(d) shows an actual FSW example viewed along x after welding at 400 rpm. Note section reference shown by vertical dashed line.(e) and (f) show corresponding 2024 Al and 6061 Al workpiece grain structures respectively; at same magnification shown in (e).Black dots represent precipitates.
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the workpiece grain structures through the thickness sections indicated 2024 and 6061 respectively inFig. 1(d). It can be noted that the grains in either case are elongated. In the 2024 aluminum workpiecethe mean aspect ratio for the grains is 4:1 (length:width) and average actual grain sizes range from about8 to 150 mm; with a mean actual grain size of 60mm. In the 6061 aluminum workpiece thecorresponding aspect ratio averages 3:1 and the grain dimensions vary from about 8 to 200mm; witha mean actual grain size of 90mm.
Results and Discussion
Figure 2 shows reconstructed flow patterns for the FSW section shown in Fig. 1(d) corresponding toa rotational speed of 400 rpm. The linear striations through the weld zone (perpendicular to -y andrunning in the x-direction) correspond to the spacing of the HP screw threads, and the complex patternsillustrate solid-state flow in intercalated vortices both parallel and perpendicular to the tool axis. Thedetails of these features are shown in the enlarged views in Fig. 3(a) and (b) which also show thecorrespondence of flow pattern features at 90° (-y'x respectively). Note also the contrast reversal oncomparing Fig. 2 and Fig. 3(a) and (b) as a result of the optical feature differences in imaging. Figure3(c) and (d) show successive enlargements of Fig. 3(a); with Fig. 3(d) illustrating the very small,dynamically recrystallized and equiaxed grain structure (with nominal grain sizes ranging from 1 to 2mm; compared with Fig. 1(e)). The grain structure view in Fig. 3(d) in fact is characteristic of theidealized fluid particle concept which accommodates shear flow in liquid and liquid-like regimes(extremely fine powder flow for example).
Figure 4 illustrates for comparison equivalent weld center zone flow patterns at 800 and 1200 rpmtool (HP) rotation speeds. Figure 4(a) shows essentially the same zone area as Fig. 3(a) but at 800 rpm.Enlargements of one flow region in Fig. 4(a) (marked by an arrow) are shown in Fig. 4(b) and (c)respectively; with Fig. 4(c) showing the residual, recrystallized grain size in this zone to be about 15
Figure 2. Section views through a FSW weld at 400 rpm rotation speed. Note the cut through the weld shown by dashed linein Fig. 1(d) is from the opposite side shown in Fig. 1(c) (-y or left).
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mm, and noticeably larger than that shown in Fig. 3(d). This is due in part to a slightly elevatedtemperature difference (;40°C (5)) at 800 rpm in contrast to 400 rpm tool rotation speed whichpromotes grain growth. Figure 4(d) shows for comparison with Figs. 3(a) and 4(a) the same weld zone
Figure 3. Enlarged views of flow patterns. (a) Enlarged view of area in dashed line window (-y) in Fig. 2. (b) Enlarged view ofweld cross-section along (x) in Fig. 2. (c) Enlarged view (arrow) in (a). (d) Enlarged view showing grain structure in dashed linewindow in (c).
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region at 1200 rpm tool rotation speed where the apparent increase in FSW turbulence has effectivelydestroyed the regular flow zone features at lower rotation speeds shown in Figs. 2 and 3(a). Of coursesome of the variance in flow patterns shown in Figs. 3(a), 4(a), and 4(d) may be due to the positions
Figure 4. Flow patterns at 800 and 1200 rpm. (a) Region (-y) corresponding to Fig. 3(a) but at 800 rpm. (b) Enlarged view of(a) (at arrow). (c) Enlarged view of (b) showing equiaxed, recrystallized grain structure. (d) Region (-y) corresponding to (a) butat 1200 rpm.
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of the reference cut (shown as a dashed line in Fig. 1(d) and the movement of this cut along -y or y),and more systematic and additional comparisons will be required to confirm these implications.
Conclusions
Differential etch contrast has allowed complex, residual solid-state flow patterns to be visualized inFSW of dissimilar aluminum alloys: 2024/6061. Intercalating lamellae of these two dynamicallyrecrystallized alloys creates complex vortex, whorl, and swirl features characteristic of chaotic-dynamicmixing (7–9), and more detailed and systematic visualization of these patterns may provide moreinsightful views of this complex process.
Acknowledgments
This research was supported by a NASA Marshal Space Flight Center Cooperative AgreementNCC8–137.
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