Research On The Superplasticity And Advanced Superplastic Forming Of Light Alloy

Superplastic forming (SPF) is a low investment process that takes advantage of certain material's ability to undergo large strains to failure when deformed under the right conditions, which usually involve elevated temperature and low strain rates. Product development and manufacturing benefits associated with SPF include low capital investment, part consolidation and increased design freedom with materials that have limited room temperature ductility. Although these attributes have led to significant interest in the SPF of aluminum automotive panels, with long forming times often exceeding 20 minutes and the high cost of aluminum sheet specially processed for SPF, conventional superplastic forming has found limited applicability in automotive manufacturing. Advanced SPF die technology need to be developed to reduce forming time and enable the forming of low cost aluminum sheet; accurate forming simulation is needed for the successful design of tooling for complex SPF processes.The superplastic forming potential of two fine-grained 5083 aluminum alloys were studied under various stress states with the use of both high temperature tensile testing and pneumatic bulge testing. The failure modes of superplastic materials were studied and the metrics necessary to establish practical superplastic forming limits for aluminum sheet were considered. Experiments were performed at temperatures ranging from 475°C to 525°C with tensile test and under three different strain paths ranging from equi-biaxial to approaching plane strain using a pneumatic bulge test. The effects of temperature on final thickness distribution, dome height and cavitation were investigated for the case of equi-biaxial stretching. Although results indicate that the tensile and bulge tests both have significant limitations in terms of accurately quantifying a materials practical forming limit in SPF, when used in conjunction, they offer valuable insight into a material's superplastic formability.Although superplastic deformation leads to large"neckfree"elongation, most superplastic materials develop internal cavitation during deformation. It has been recognized that for such materials, cavitation often precedes failure and excessive cavitation can impose significant limitations on the industrial usage of superplastically formed components. To assess the extent of cavitation damage in formed parts, a method for accurate prediction of cavitation is desirable. Through microstructural examination of the evolution of population density and volume of cavitation with strain, the effect of stress state and strain on cavitation has been evaluated. Based on experimental observations, a stress state dependent cavity nuclation model was proposed and combined with the Gurson model to predict the cavitation behavior of SPF in the present work. Correlation with experiments shows that this model is capable of adequately predicting cavitation behavior during bulge superplastic forming of AA5083.A novel SPF process that utilizes a mechanical pre-forming process to enhance formability was also developed in this present work, which can reduce costs and improve production efficiency. A simulation capability has been established and experimentally verified for modeling superplastic forming with finite element analysis and is the result of fundamental research into experimental forming, simulation and constitutive modeling. Contributions include simulation parameters, die design and finite element model guidelines as well as novel approaches to material model refinement and pressure cycle optimization. Experimental forming trials verified that this novel die technology can deliver a superior thickness profile and significantly decrease forming time as compared to conventional superplastic forming for high aspect-ratio, deep-draw panels. Microstructure evolution during superplastic forming was also studied on an AA5083 superplastic alloy. The electron backscattered diffraction (EBSD) technique was applied in this present work to investigating pole figures, orientation map, misorientation map, grain size structure and cavitation behavior. Analysis indicate that there existed texture in the as-received AA5083 plate and the texture evolved but still keep weak texture during deformation, grain boundary sliding (GBS) and potential dynamic recrystallization were believed to be occurring as well.