Zongyi Ma

Superplastic Deformation Mechanism

The steady-state deformation of polycrystalline materials at elevated temperatures is usually analyzed through the equation.

Friction Stir Welding and Superplastic Forming for Multisheet Structures

One of the key benefits of superplastic forming (SPF) is the ability to form multisheet components. These integrally stiffened structures or sandwich structures are used in many aerospace applications. Most of the initial examples were developed for titanium alloys as they can be diffusion bonded easily. The simple approach of diffusion bonding/superplastic forming (DB/SPF) involved DB of sheets with masked areas and subsequent gas forming by applying pressure in between the sheets. Very complex structures could be obtained by using three, four, or five sheets. Aluminum alloys on the other hand have surface oxide film that hindered easy DB. This can be overcome with friction stir welding (FSW) of sheets in lap joint configuration with patterns. FSW retains the fine-grained microstructure of superplastic sheet. The work of Grant et al. has demonstrated the feasibility of making multisheet structures by combining FSW and FSSW with SPF.

Low-Temperature Superplasticity

Conventional superplasticity is typically achieved at slow strain rates (10−4–10−3xa0s−1) and higher homologous temperatures (0.80Tm, where Tm is the melting point of the matrix alloy expressed in K). It is well established that a decrease in grain size enhances the optimum superplastic response by lowering temperature and/or increasing strain rate. Therefore, if the grain size could be reduced to ultrafine-grained (UFG) range, low-temperature superplasticity (LTSP) could be achieved. LTSP is attractive for commercial superplastic forming, for lowering energy requirement, increasing life for conventional or cheaper forming dies, improving surface quality of the formed component, preventing severe grain growth and solute-loss from the surface layers, thus, resulting in better post-forming mechanical properties.

Chapter 13 – Summary

Unitized structures are very attractive for metallic structures as they reduce component count and reduce cost. High strain rate superplasticity is critical to reduction of forming time during superplasticity while low temperature superplasticity is very promising for reduction of energy consumption during manufacturing.

Friction Stir Microstructure for Superplasticity

The microstructural prerequisites for a superplastic material are well established in metallic alloys. The first prerequisite is a fine-grain size, typically less than 15xa0μm. The optimum strain rate for superplasticity increases with decreasing grain size when grain boundary sliding (GBS) is the dominant process. For a given strain rate, the finer grain sizes lead to lower flow stresses, which is beneficial for practical forming operation. The second prerequisite is thermal stability of the fine-grained microstructure at high temperatures. Single-phase materials generally do not show superplasticity because grain growth occurs rapidly at elevated temperatures. Hence, presence of second phases at grain boundaries is required to resist excessive grain growth. An appropriate amount of fine, uniformly distributed, and thermally stable second-phase particles is necessary to keep stable microstructure during superplastic deformation. The third prerequisite is high grain boundary misorientation. High-angle grain boundaries (HAGBs, angle ≥15°), particularly random ones promote GBS, whereas low-angle grain boundaries are generally believed to be not suitable for GBS. The fourth prerequisite is equiaxed grain shape. With equiaxed grains, grain boundaries can experience shear stress easily promoting GBS. The fifth prerequisite is mobility of grain boundaries. During GBS, stress concentration could be produced at various grain boundary discontinuities such as triple points. The migration of grain boundaries could lead to reduction in stress concentration. Thus, GBS can continue as a major deformation mechanism.

Superplastic Punch Forming and Superplastic Forging

Superplastic punch forming and closed die forging are other possibilities because of the ability to convert thick sheets and plates into superplastic microstructure. Dutta et al. reported on punch forming of 9-pass FSP 7075Al with a thickness of 5xa0mm. They used the approach of limiting dome height to establish formability. Punch forming tests were carried out at 400°C and 450°C. While punch forming tests at 400°C were performed at only one strain rate, i.e., at 10−2xa0s−1, three different strain rates were employed at 450°C, as higher elongations and larger cup depths could be obtained at this temperature. The maximum cup depth of 52xa0mm was achieved at a slow strain rate of 10−3xa0s−1 at a temperature of 450°C consistent with the high tensile elongation under these conditions. They also conducted stage-wise measurements of thickness strain and compared with FEM simulation at 450°C and 10−2xa0s−1. The progress of deformation and the change of shape of the blank at three stages of forming under the above parameters have been demonstrated.

Potential of Superplastic Forming of Low-Cost Cast Plate

One of the most cost-effective ways of producing sheets is twin-roll casting. Twin-roll cast sheets can be used as a starting material. Friction stir processing can create selective regions of fine grain size which can then be gas formed.

Chapter 7 – Cavitation During Superplasticity

It is well established that cavitation can occur in a wide range of materials during tensile superplastic flow. In recent years, extensive attention has been paid to the cavitation behavior of superplastic alloys because cavitation leads to degradation of the overall properties of the post-SPF materials. It has been demonstrated that the post-SPF mechanical properties of the materials are significantly reduced when the cavity volume fraction exceeds approximately 1%.

Chapter 3 – High-Strain-Rate Superplasticity

High-strain-rate superplasticity (HSRS) refers to the superplasticity achieved at an optimum strain rate of ≥1×10−2xa0s−1. According to the prediction by the constitutive relationship for superplasticity, the optimum strain rate increased with decreasing the grain size. Based on the fine-grain size of 0.7–10xa0μm produced in aluminum alloys via friction stir processing (FSP), it is expected that the FSP fine-grained aluminum alloys would exhibit high-strain-rate superplastic behavior with the decrease in the grain size. Table 3.1 summarizes the superplastic data of a number of aluminum alloys prepared by FSP. It is indicated that HSRS has been achieved in several FSP aluminum alloys, such as 7075Al, 2024Al, Al–Mg–Zr, Al–Zn–Mg–Sc, and Al–Mg–Sc.

Superplastic Forming of Friction Stir Processed Plates

Conventional superplasticity is based on starting sheet material of <3xa0mm thickness. This limitation mainly comes from the fact that the processing of superplastic sheets requires thermomechanical processing in the form of controlled hot rolling to obtain fine-grained microstructure with grain size <15xa0μm. So applications of friction stir processing can be visualized at several different levels.