Superplastic forming (SPF) is an important technology for the fabrication of complex shapes using aluminum alloys. However, the process has some drawbacks: it is limited to thin sheet, forming rates are slow, the microstructure of the entire sheet is altered, and the cost of the specially processed fine grain material is relatively high.
For economical, efficient manufacturing, SPF is applied either at high strain rate to increase throughput or at as low a temperature as possible to minimize die wear and reduce energy consumption. Conventional thermomechanical processing creates a relatively equiaxed grain morphology with a minimum grain size typically 8 to 10 micron. SPF response in 7075 aluminum having this conventionally processed SPF microstructure is optimized at a processing temperature of approximately 516C (960F) with typical strain rates of 2 to 6 x 10(-4)s(-1).
Now, a unique thermomechanical metalworking process called friction stir processing (FSP) can locally tailor a material's properties to achieve maximum superplasticity performance. FSP is an expansion of the friction stir welding (FSW) process, a solid state joining process invented at The Welding Institute (Cambridge, UK) in 1991. FSP can be used to create superplastic properties never before achievable. Very fine equiaxed FSP grain size (<5 micron) yields high strain rate superplasticity at lower temperatures. In addition, superplasticity can be created in thick section aluminum alloys or in a controlled area for subsequent local forming. FSP can be applied selectively to a location in a material or structure to tailor specific properties at specific locations without altering the performance characteristics of other structural parts. The long-term goal is to use friction stir processing to control local properties in structural metals including aluminum and other nonferrous and ferrous alloys.
Friction stir processing uses the same methodology as friction stir welding. Figure 1 shows a schematic illustration of FSP. To "friction process" a location within a plate or sheet, a cylindrical tool consisting of a small diameter pin with a concentric larger diameter shoulder is rotated and plunged into the selected area. Friction and deformation between the rotating pin and the surface rapidly heats and softens a small column of metal. Depth of penetration is controlled by the tool shoulder and length of entry pin.
Contact of the rotating shoulder with the surface creates additional frictional heat and plasticizes a larger cylindrical metal column around the inserted pin. The shoulder contains the upward metal flow caused by the tool pin. The area to be processed and the tool are moved relative to each other with overlapping passes during FSP. Hot working by the rotating tool plasticizes metal within a narrow zone and transports it from the leading face of the pin to its trailing edge. A defect-free recrystallized, fine grain microstructure forms as the processed zone cools.
Process-zone SPF characteristics
FSP generates a fine, equiaxed grain morphology having a banded, bimodal grain size of 1 to 5 micron. The microstructure of friction stir processed aluminum alloy 7075 is stable under superplastic conditions of high temperature and dynamic strain, and high-angle grain boundaries can enhance grain boundary sliding and related superplasticity. However, optimum superplasticity requires a homogeneous distribution of equiaxed grains of minimum grain size. Microstructures resulting from FSP do not have a uniform grain size distribution for any one set of process parameters. Grain size varies from the top to the bottom as well as from the advancing to the retreating side. The differences in grain size likely are associated with differences in both peak temperature and times at temperature.
SPF properties were evaluated from tensile tests on ground and polished ~0.5-mm thick minisamples having a 1-mm gage length centered within the fine-grain FSP nugget (Fig. 2a and b) and on bulk 5-mm thick through-thickness samples (Fig. 2c). For optimum test conditions of 1 x 10(-3)s(-1) strain rate and a temperature of 460C for single-pass FSP minisamples, strain to failure approached 600%. Compared with FSP 7075, both the total SPF strain for 7050 (600% vs. 1000%) and the optimum strain rate are lower (1 x 10(-3)s(-1) vs. 1 x 10(-2)s(-1).
Multipass FSP samples (Fig. 3) experience different pre- and post-FSP thermal history at different locations within the FSP zone. Prior thermal and/or deformation history influences the total SPF elongation. Total elongation is lower for multiprocessed locations versus a single pass. Also, there are consistent differences in superplastic behavior among the three multipass locations (1-2, 2-3 and 3) with very limited superplasticity at location 3.
In full thickness specimens, a moderately high strain rate of 5 x 10(-3)s(-1) yields an optimum SPF temperature of 450C (840F) and a true strain of 1.2 (engineering strain >230%). In most forming applications, a true strain of 0.7 is more than sufficient to form a complex shape. An even greater elongation (a true strain of 1.55, engineering strain >370%) can be achieved at the lower strain rate of 1 x 10(-3)s(-1). A strain rate of 2 x 10(-4)s(-1) (a strain rate typical for SPF), produces a true strain to failure of 2.2 (>800% engineering strain), and also illustrates strain uniformity (Fig. 4) where an SPF test was stopped prior to failure at a true strain of 1.9 (>550% engineering strain).
At a temperature of 450C, a strain rate of 1 x 10(-3)s(-1), and 900 psi hydrostatic pressure, strain rate sensitivity (m-value) as a function of true strain remains relatively high for true strains >1.5 (engineering strain >350%).
Aluminum alloys can develop cavities at grain boundaries and triple points during superplastic extension if the strain exceeds the initial incubation strain for cavitation. A well-known method to extend the strain limit for onset of cavitation is to apply a hydrostatic pressure during superplastic extension. Cavitation volume is high for a sample tested under atmospheric pressure compared with the lower levels of cavitation for a sample tested under a hydrostatic pressure of 600 psi.
In most cases, cavitation is unacceptable because of the detrimental influence on SPF ductility and post-forming properties. Using in-situ hydrostatic pressure during SPF extension effectively limits cavitation.
The application of this initial work demonstrates the ability of friction stir processing to change the local microstructure via thermomechanical working. In this case, we demonstrated the ability to use FSP to create high strain rate superplasticity in thick section aluminum alloys at moderate temperatures. This is the first time true superplasticity has been demonstrated in a 5-mm thick 7000 series aluminum alloy. Because FSP can create a relatively fine grain, homogeneous microstructure in a section thickness greater than 25 mm, the thickness limit for superplasticity has yet to be reached.