| Aluminum alloys have been the material of choice for aircraft construction since the 1930's. Currently, the automotive industry is also showing an increasing interest in aluminum alloys as structural materials. 6xxx aluminum alloys possess a combination of strength and formability which makes them attractive to both industries. In addition, 6xxx alloys are highly weldable, corrosion resistant, and low in cost as compared with the 2xxx and 7xxx aluminum alloys.; Superplastic forming (SPF) is a manufacturing process which exploits the phenomenon of superplasticity in which gas pressure is used to form complex-shaped parts in a single forming operation. This reduces part counts and the need for fasteners and connectors, resulting in reduced product weight. Reduced product/vehicle weight improves fuel economy.; Most alloys must be specially processed for superplasticity. Much research effort has been directed at the development of thermomechanical processes for the grain refinement of aluminum alloys by static or dynamic recrystallization. to induce superplasticity. While large numbers of studies have been conducted on 2xxx, 5xxx, 7xxx, and 8xxx aluminum alloys, very few studies have been focused on the grain refinement of 6xxx aluminum alloys for superplasticity.; The current research describes a new thermomechanical process for application to 6xxx aluminum alloys for grain refinement and superplasticity. The process is shown to successfully refine and induce superplasticity in an Al-Mg-Si-Cu alloy which falls within the compositional limits of both 6013 and 6111. The grain refinement is by particle-stimulated nucleation of recrystallization.; The microstructural evolution during the thermomechanical processing is characterized in terms of precipitate size, shape, distribution and composition; texture; recrystallization; and grain size, shape, and thermal stability. The new process produces a statically-stable, weakly-textured, equiaxed grain structure with an average grain diameter of ∼10 μm.; The refined microstructure exhibits superplasticity above 500°C, where the strain rate sensitivity reaches a maximum of 0.5 (at 540°C for strain rates between 2 × 10−4 s−1 and 5 × 10−4 s−1). The maximum uniaxial elongation (375%) occurred in the regime of the maximum strain rate sensitivity. The corresponding flow stress was 680 psi (4.7 Mpa).; Biaxial cone tests were performed in order to better evaluate the high-temperature forming characteristics of the material. During tests with back pressure, cone height-to-radius ratios near 1.2 were obtained with maximum strain approaching 2.0 for strain rates near 1 × 10−3 s−1 . The effect of superplastic deformation on the microstructure is described in terms of the effect of strain on grain size and porosity for a cone sample.; The ultimate goal of the project is to advance the fundamental understanding of the complex interrelationships between processing, microstructure, and superplastic performance. |
