Grain refinement of aluminum alloys by friction stir processing

Philosophical Magazine, Vol. 86, No. 1, 1 January 2006, 1–24

Grain refinement of aluminum alloys by friction stir processing J.-Q. SU*y, T. W. NELSONy and C. J. STERLINGz yDepartment of Mechanical Engineering, Brigham Young University, 435 CTB, Provo, UT 84602, USA zLockheed Martin Missiles and Fire Control, PO Box 650003 M/S L10-03, Dallas, Texas 75265, USA (Received 3 June 2005; in final form 20 July 2005) Combining friction stir processing (FSP) with rapid cooling, samples with grain sizes of 100, 180, 300 and 500 nm have been produced in commercial 7075 Al by controlling the cooling rate. Microstructure characteristics of the processed materials were investigated. The nanocrystalline structures formed in the sample processed with the highest cooling rate consist of high-angle grain boundaries, and are free of dislocation cell structures. High temperature discontinuous dynamic recrystallization is the mechanism responsible for the nanocrystalline creation. Dislocations and recovery structures were observed in the large grains of samples with slower cooling rates. The developed grains are a result of the evolution of the initial nanocrystals formed around pin tool during the FSP.

1. Introduction Nanocrystalline (NC) and ultrafine crystalline (UFC) metals and alloys exhibit high strength, superior wear resistance, and excellent superplasticity at higher strain rates and lower temperature compared to their conventional coarse-grained polycrystalline counterparts [1–5]. These attributes have generated considerable interest in the use of these materials for structural applications. The development of NC and UFC metals and alloys for structural applications must address issues related to the fabrication of bulk samples. For this task, many methods have been developed to produce NC and UFC materials. The currently available techniques can be generally classified into the following broad categories: (a) powder metallurgy methods which include inert gas condensation and consolidation of nanopowder [6, 7] and mechanical alloying [8–11], (b) crystallization of an initially amorphous material [12, 13], and (c) deposition methods such as electrodeposition [14–16], or physical vapor deposition [17]. These techniques are attractive for producing materials with grain sizes below 100 nm, but there are disadvantages with them that are hard to overcome.

*Corresponding author. Email: [email protected] Philosophical Magazine ISSN 1478–6435 print/ISSN 1478–6443 online # 2006 Taylor & Francis http://www.tandf.co.uk/journals DOI: 10.1080/14786430500267745

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Powder metallurgy methods typically suffer from the limitation that the consolidated nanostructured alloy is not fully dense, and that processing-induced porosity can curtail ductility and fracture resistance. Crystallization of an initially amorphous structure does not provide the flexibility to produce nanocrystalline alloys of broad composition ranges and properties. Deposition methods offer the possibility to produce clean, full dense nanocrystalline metallic materials with a narrow range of grain size and large in-plane dimensions, but the thickness of the specimens produced is no more than a few millimeters; in addition, methods such as electrodeposition can introduce contamination from hydrogen, sulfur or other impurities. As a consequence of these difficulties, much attention has been paid to alternative procedures of introducing ultra-fine grains in materials by severe plastic deformation (SPD) [18–20]. The general experimental approach of SPD involves large-scale deformation using processes such as rolling, equal-channel angular pressing or extrusion (ECAP/E), or high-pressure torsional straining. Using this approach, nanostructured bulk materials have been produced from ductile metals and alloys of initial low to moderate strength [19]. However, high-strength metals and alloys are difficult to process by SPD methods. Furthermore, these processing techniques produce relatively small quantities of material, are very difficult to scale up, and are unlikely to be able to produce materials at low cost. Recently, a new processing technique, friction stir processing (FSP), has been developed by adapting the concepts of friction stir welding (FSW) [21]. The basic concept of FSP is remarkably simple [22]. A cylindrical rotating tool with pin and shoulder is plugged in the material to be processed, and traversed along the line of interest. Localised heating is produced by severe plastic deformation of the material, and by friction between the rotating tool shoulder and the top surface of the base metal. This new thermo-mechanical processing technique has been found to be an effective grain refinement technique for aluminum alloys [22–26]. Mishira et al. [22] have reported that fine grains of 3–4 mm were obtained in 7075 Al alloy by using FSP. The finest grain size achieved in a previous FSP study has been reported to be 0.5–0.8 mm [26]. Most recently, combining FSP technique with rapid cooling, the authors successfully refined grain size to a nanoscale level in 7075 Al [27]. It is also realized that the resulting microstructures might be controlled by changing the processing parameters and cooling rate. In this work, we report fabrication of nanocrystalline and ultrafine-grained structures by FSP technique. Also, we present a detailed investigation on the microstructure characteristics of the processed samples. 2. Experimental The basic principle of FSP in the present research is schematically illustrated in figure 1. A tool with shoulder diameter, pin tool diameter and length of 9 mm, 3 mm and 1.9 mm, respectively, was used to perform the friction stir process. The material for this study was commercial 7075-T6 Al with nominal composition of Al-5.6Zn-2.5Mg-1.6Cu-0.23Cr. A single pass friction stir processed zone 30 cm

Grain refinement of aluminum alloys

Figure 1.

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Schematic illustration of friction stir processing.

long was produced at a travel speed of 12 cm/min and a rotational speed of 1000 rpm. A mixture of water, methanol, and dry ice, was used to quench the plate immediately behind the FSP tool. The cooling rate was controlled by adjusting the volumetric flow rate of cooling liquid. In this investigation, four cooling rates were chosen to generate the various grain sizes within the resulting processed microstructure. The exact cooling rate was unknown. But a greater flow of cooling fluid was assumed to cause faster cooling that is also supported by the microstructures observed. Microstructures were investigated by optical microscopy (OM), transmission electron microscopy (TEM) and orientation imaging microscopy (OIM). Thin foil TEM samples were prepared by cutting the processed materials parallel to the processed surface at the mid-plane of the processed regions into discs of 3 mm in diameter using an electrical-discharge machine (EDM). The discs were ground to a thickness of about 80 mm, then subject to twin-jet electropolishing. The OIM samples were polished using progressively finer grits of SiC papers to a 4000 grit finish and then polished in 0.05 mm colloidal silica using a vibratory polisher for 6 hours. The TEM observations were carried out on a JEOL 2000FX instrument with a tungsten filament operated at 200 kV. For OIM, the observations were recorded using a Philips XL-30S FEG scanning electron microscope with a TSL orientation imaging system.

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3. Results 3.1. Overview of as processed microstructures The samples produced under different cooling rates will be denoted as samples A, B, C and D with decreasing cooling rate from A to D. The surface of the processed samples is rather rough, but no cracks were observed. Figure 2 shows macroscopic views of the transverse cross sections of friction stir regions (nugget zones) after single processing at the various cooling rates. The stir zone widens near the upper surface. This is because the upper surface experiences extreme deformation and frictional heating due to contact with the shoulder of the tool during FSP. With decreasing cooling rate, it can be seen that there is an increase (from sample A to sample D) in the size of the plasticized zone, suggesting an increase of heat input during FSP. Annular rings can also be seen within the nugget zones, the spacing of which have been previously related to the incremental advance of the tool each revolution [28]. It is noted that in sample A, which experienced the highest cooling rate, processing defects were present. These defects were detected in the processed zone near the bottom, as indicated by the arrows in figure 2a. This suggests that under these processing conditions the heat input or time at elevated temperature is insufficient to locally raise the temperature of the material to the range where it is easily plastically deformed generating the necessary flow around the pin tool to completely

Figure 2. Macro images of cross section perpendicular to the processing direction; from (a) specimen A, (b) specimen B, (c) specimen C, and (d) specimen D.


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Figure 3. TEM micrographs of friction stir processed 7075 Al alloy; (a) in specimen A with the corresponding SAD pattern, (b) in specimen B, (c) in specimen C and (d) in specimen D.

consolidate the stir zone. For samples B-D, it is apparent that the nugget zones exhibit a high degree of continuity and no defects (figure 2b, c, and d), indicating an excellent processing quality under the processing conditions imposed. A TEM overview of grain structures in various specimens is shown in figure 3. In specimen A, very small equiaxed grain structure with an estimated average

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Figure 3.

Continued.

grain sizes 100 nm was obtained (figure 3a). Corresponding select area diffraction (SAD) pattern in figure 3a, taken from a 1.7 mm diameter region, exhibits a distinct diffraction ring pattern that is indicative of a high population of grain boundaries separated by large angles of misorientations, with no obvious signs of preferential orientations in the region of analysis. With a decrease in cooling rate, the post processed grain size increased. As shown in figure 3b, c and d, the average grain


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sizes in specimens B, C and D are 180, 300 and 500 nm, respectively. The resulting microstructures show that a nanocrystalline structure was produced during FSP and the resulting grain size can be controlled during FSP by controlling the cooling rate. 3.2. OIM observation Quantitative measurements of grain sizes and grain boundary misorientations were performed on sample A using OIM. The scanning step size corresponding to a pixel size, which was 10 nm 10 nm. The error in the determination of the crystallographic orientation of each grain was less than 2 . For orientation imaging analysis, the as-obtained OIM maps were subjected to a ‘‘cleaning’’ procedures as: (1) unsolved pixels were filled with the value from its nearest neighbours; (2) one grain contains at least two scan points. All the OIM results are from the ‘‘cleaned’’ grain maps. Figure 4a shows OIM image and the relevant (111) pole figure recorded from sample A at the center point of cross section perpendicular to the processing direction. The unit triangle on the left represents the inverse pole figure color-coded as the reference of the crystallographic orientations, parallel to the normal direction. The red, green, and blue colours represent the h001i, h101i, and h111i crystal directions normal to the specimen surface, respectively. The microstructure appears that the grains are mostly equiaxed and many grains were refined into the nanometer regime (