This paper presents a new phenomenological model for describing the main features of the viscoplastic behavior of superplastic sheet metals, namely, strain hardening, softening, and damage. The proposed model is based on a variable strain rate sensitivity index (m-value) measured from uniaxial tensile tests at different strain rates under constant temperature. In this study, the uniaxial tensile tests were carried out at three strain rates (i.e., 10 -3 , 10 -2 , and 10 -1 s -1 ) on a superplastic grade AA5083 aluminum sheet alloy. In addition, the volume fractions of cavities at different plastic strain levels were assessed using X-ray microtomography. The performance of the model was investigated by comparing its predictions with the experimental data. In addition, the model was validated with two sets of reference data for AA5083 aluminum alloy and AZ31 magnesium alloy. In particular, it was observed that the new model could predict the flow behavior of these metals more successfully compared with two reference models; nevertheless, it requires minimal experimentation and calculation efforts.

Abstract. The strain rate sensitivity index, m-value, is being applied as a common tool to evaluate the impact of the strain rate on the viscoplastic behaviour of materials. The m-value, as a constant number, has been frequently taken into consideration for modeling material behaviour in the numerical simulation of superplastic forming processes. However, the impact of the testing variables on the measured m-values has not been investigated comprehensively. In this study, the m-value for a superplastic grade of an aluminum alloy (i.e., AA5083) has been investigated. The conditions and the parameters that influence the strain rate sensitivity for the material are compared with three different testing methods, i.e., monotonic uniaxial tension test, strain rate jump test and stress relaxation test. All tests were conducted at elevated temperature (470ºC) and at strain rates up to 0.1 s -1. The results show that the m-value is not constant and is highly dependent on the applied strain rate, strain level and testing method.

High-speed blow forming (HSBF) is a new technology for producing components with complex geometries made of high strength aluminum alloy sheets. HSBF is considered a hybrid-superplastic forming method, which combines crash forming and gas blow forming. Due to its novelty, optimization of the deformation process parameters is essential. In this study, using the finite element (FE) code ABAQUS, thinning of an aluminum component produced by HSBF under different strain rates was investigated. The impact of element size, variation of friction coefficient, and material constitutive model on thinning predictions were determined and quantified. The performance of the FE simulations was validated through forming of industrial size parts with a complex geometry for the three investigated strain rates. The results indicated that the predictions are sensitive to the element size and the coefficient of friction. Remarkably, compared to a conventional power law model, the variable m-value viscoplastic (VmV) model could precisely predict the thickness variation of the industrial size component.

Cavitation is the main defect encountered during superplastic forming (SPF) of thin sheet aluminum alloys. In the present paper, the influence of preforming operation (cold or hot) on the superplastic forming ability and quality of 1.6-mm-thick sheet of 5083 SPF aluminum alloy is investigated. Specifically, grain size evolution and the characteristics of the cavitation process are discussed as a function of prior deformation and the preforming temperature. Optical and field emission gun scanning electron microscopy (FEG-SEM) were used to study the characteristics of the cavities and microstructure evolution. Image processing was used to measure the surface and volume fractions of the cavities. The results indicate that hot preforming leads to a lower number of cavities per unit surface compared to cold preforming prior to the SPF operation. However, the average cavity sizes and the average grain size are higher in the case of hot preforming compared to cold preforming, which lead to higher susceptibility to crack formation and reduced SPF ability of the alloy.

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