Friction Stir Processing of Aluminum Alloy A206: Part I—Microstructure Evolution
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Friction stir processing (FSP) was applied to plates of cast aluminum A206 alloy. The focus of the work was on the evolution of microstructure during processing, which yielded dramatic decrease in the local grain size of the processed material. Reduction in pore morphology features, as well as intermetallic particle size, was also observed. Friction-stir-processed metal is characterized by a high ratio of high-angle grain boundaries to low-angle boundaries compared to what is observed in the as-cast alloy. Consistent with findings of other investigators, FSP generates a product nugget which has a portion of its structure having an onion skin-type structure that is characterized by a preferred crystallographic texture within individual bands in the nugget.
Keywordsfriction stir processing microstructure evolution casting defect elimination shape casting components
Friction stir processing (FSP) is an emerging solid-state metal processing technique. Through FSP, localized and significant microstructure modification, including grain refinement, is attained with concomitant mechanical property improvements in the processed region. One of its unique and attractive features as it relates to cast components is that it can be incorporated in the overall manufacturing cycle (post casting); it can be used to modify the properties locally, and to render tailored properties to different regions within a single component. The application of FSP has particular appeal in locally refining or alleviating casting heterogeneities.1, 2, 3, 4, 5
FSP has emerged as an important processing technique for refining the grains of metallic components demonstrating the capability to generate ultra-fine grains down to sizes below 200 nm.13, 14, 15 Abundant studies have been carried out on wrought Al alloys such as 2XXX and 7XXX series alloys, which are widely applied in the aerospace industry,16, 17, 18 however, not much work has been done with cast Al alloys.
This study was performed on alloy A206, which is a commonly used cast aluminum alloy with high-strength and widespread applications. The microstructure of the A206 alloy via FSP was locally manipulated to refine the grain structure and to eliminate heterogeneities such as large porosity as well as coarse intermetallic phases within the castings. Detailed microstructural analyses were carried out to study the effects of FSP on the resultant microstructures, and to establish the operating mechanism.
The apparatus for FSP consists of the main FSP platform (machine table), the fixture, and instruments (thermocouple and dynamometer) used to detect and record the temperature distribution during processing. For the current study, the main platform for FSP is a HAAS CNC machine to which a specifically designed tool has been attached. The diameter of the tool shoulder is 16 mm, and at the end of the shoulder there is a tapered probe of length 3.2 mm. A tilt angle of 3° (angle between machine spindle and workpiece normal) has been applied to induce forging action at the trailing edge of the shoulder. This is achieved by inserting a back plate whose surface is machined into a slant surface. Proper tool penetration depth is very important in generating enough friction heat between the tool and the work piece to produce a good FSP finish on the workpiece. The minimum tool penetration depth requires the shoulder of the tool to have enough contact with the work piece, and this number is calculated based on the specific contact condition between the tool and the material.
Samples for metallographic analysis were sectioned perpendicular to the FSP traverse direction and prepared for microstructural analysis. Barker’s etchant and polarized light were used to reveal grain morphology. Specimen preparation for the electron backscattering diffraction (EBSD) analysis was carefully performed in order to properly image the structure and obtain high-quality diffraction patterns. A Carl Zeiss SUPRA-55 SEM equipped with the EBSD detector was used for local grain orientation data acquisition. The step size chosen for the FSPed specimen was 0.5 μm, and for the as-cast specimen was 5 μm. Thin films for transmission electron microscopy (TEM) analysis were prepared via focus ion beam (FIB) technique at selected locations in the processed region. TEM work was carried out with a JOEL 2000 microscope operated at 200 kV.
Results and Discussion
Grain boundaries and grain size in the FSP zone are clearly depicted in Figure 5c. After FSP, grains are equiaxed and their size decreased to ~ 10 μm, a reduction of more than one order of magnitude compared with the original grain size. Figure 5d and e shows grain structures of the transition zones (retreating and advancing sides) between the FSP region and the matrix material. There is no distinct boundary between the FSP region and the dendritic structure in the unworked plate. Rather, there is a zone where the grains have been sheared to an ever increasing extent at locations close to the FSP nugget. In transition areas, grains are elongated and bent and seem to deform along the shape of the nugget.
The FSP region contains a special microstructure feature, “the onion ring” structure, which is also called the banded structure.20, 21, 22 The onion ring structure is marked with a white rectangular shape in Figure 5a. From the transverse cross-sectional view (Figure 5f), we can see that the onion rings consist of concentric, elliptical rings located near the middle of the FSP nugget. It has been reported that the space between each ring corresponds to the advance per revolution of the tool.22 Previous studies reported that the appearance of onion rings was due to a periodic particle density variation or grain size variation.23 However, as shown in Figure 5f and g, one cannot observe variations of grain size or particle density in the onion ring region. Polarized light was used to reveal grain morphologies; different colors indicate different grain orientations. As shown in Figure 5g, within each ring, there are many grains that show a similar orientation (shown in light blue color). This result is supported by a number of researchers,24, 25, 26, 27, 28 indicating that there exists a texture or grain orientation component to the onion ring structure.
Grain refinement during FSP is generally acknowledged to be attributed to dynamic recrystallization; however, there is still some debate among researchers as to whether the dynamic recrystallization is continuous (CDRX) or discontinuous (DDRX) during FSP. Discontinuous recrystallization is the most commonly recognized form of recrystallization and involves the generation of high-angle boundaries and the subsequent migration of those boundaries into the deformed matrix under the driving force of the deformation strain. Continuous recrystallization is less commonly recognized and is perhaps more appropriately considered a form of extended recovery as dislocations and dislocation networks rearrange via recovery-type processes (climb, glide) into high-angle boundary networks. High-angle boundary migration is not involved, and the interior of the recrystallized grains would be populated with residual dislocation substructures. Rhodes et al.10 and Su et al.11 have proposed the DDRX mechanism is the active mode in FSP 7075. Several others have concluded that grain refinement mechanism is by CDRX.7,9,24,29
It is difficult to be conclusive as to whether the recrystallization is continuous or discontinuous in nature. While the high processing temperature of FSP and high stacking fault energy of aluminum can favor recovery-type mechanisms, the extensive shear deformation imparted by the FSP process is expected to generate high misorientation gradients throughout the processed nugget to support the high-angle boundary formation required by the CDRX process. The misorientation distributions (shown in Figure 7b, d) indicate that although the number of high-angle boundaries increased after FSP, there is still a large frequency of low-angle boundaries which might support an argument for CDRX. But low-angle boundaries can also result from preferred crystallographic texture that develops as a result of the shear-type deformation imparted to the FSP nugget.
The temperature and strain rate of the deformed material are two factors that affect dynamic recrystallization during FSP. Chang et al.30 have used the Zener–Hollomon parameter which combines these two factors into a temperature compensated strain rate to help explain the grain refinement of AZ31 Mg alloy during FSP. They developed a relationship which expresses the natural log of the grain size as being proportional to negative of the natural log of Z. The latter implies that grain size will decrease with increasing strain rate and that grain size will increase with increasing temperature.
In brief, the recrystallized grain size will be influenced by the total amount of local strain imposed in the material and the thermal history through the stirring process and subsequent cooling stage. As reported in Figure 6, there was a gradient in grain size through the FSP nugget. The small grains were found at the top of the nugget. The metal at top of the nugget experiences the highest heat from friction at the shoulder of the tool, but it will also undergo the highest strain from the shoulder as that part of the tool continues to shape the nugget after the probe has passed through. The high level of strain experienced by the near surface is expected to be the dominant influence refining the grains. Near the bottom of the nugget, the grains are also quite small. The material at the bottom of the nugget undergoes a substantial amount of strain, though less strain than the rest of the nugget, and it is also be the coolest portion of the nugget. As a result, the time for recovery, recrystallization, and coarsening at the bottom of the nugget is curtailed. In the middle of the nugget, the strains are quite high and the structure will have additional time for thermally activated processes to occur, explaining the slightly larger grain size.
Friction stir processing of an A206 aluminum alloy cast plate has been shown to be capable of producing significant local changes to the microstructure of the casting. The most dramatic effect is a refinement of the grain size from about 400 microns as cast to 10 microns and finer within the FSP nugget. The size of porosity features and intermetallic particles was also seen to be greatly reduced. A significant increase in the ratio of high-angle grain boundaries to low-angle boundaries is another result of FSP of the cast alloy. The grain structure after FSP develops by a dynamic recrystallization process. As observed by previous investigators, FSP generates an onion skin-type structure visible in the transverse nugget cross section. Consistent with previous work, the grain structure within the onion skin structure was found to contain aspects of preferred texture as part of its structural makeup.
Our intention has been to illustrate the value of FSP in modifying the structure and properties of cast parts. We recognize that this work employed a cast flat workpiece, while most shape cast parts have more complicated geometries, which will certainly present a challenge for applying FSP. It is envisioned that FSP would only be employed in accessible locations on a cast part to resolve some specific issue, such as to heal porosity. If porosity can be controlled to form only near flat or convex surfaces, then FSP conceptually can be used to heal it. Clearly, there will be challenges in designing the proper fixtures for the workpiece and in controlling the tool path and load. The exit hole will also need to be addressed. Strategies to deal with exit holes could include designing the path of the tool to leave the hole at a non-critical portion of the part or employing a welding operation as a post-FSP process to seal the hole. Obviously, each cast geometry would present its own challenges; FSP is a novel enabling technology to locally engineer the microstructure.
The authors gratefully acknowledge the member companies of the Advanced Casting Research Center (ACRC) for their support of this work, and for their continued support of research focused on the science and technology of metal casting at Worcester Polytechnic Institute.
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