Enhancement of Formability of AA5052 Alloy Sheets by Electrohydraulic Forming Process

Formability of lightweight materials like Al and Mg alloys is a major concern for their application in automobiles. Forming limit diagram (FLD) and strain distribution are extremely useful in the assessment of overall formability of sheet metals. At very high strain rates, the deformation behavior of Al alloys and the safe forming window could be different from quasi-static conventional forming. In this paper, formability of Al 5052 alloy sheets of 0.5 mm thickness has been assessed in electrohydraulic forming (EHF) in terms of FLD and strain distribution and compared with formability in conventional forming by punch-stretching experiments. EHF is a high strain rate forming process which utilizes energy released from a capacitor bank to generate shockwaves in a fluid medium. Experiments have been conducted at different energy levels to identify the highest safe strains in different modes of deformation. From the experimental results, it has been observed that the limit strains increased by nearly 45-50% in all the three regions of the FLD (tension-tension, plane strain and tension compression). Unlike in the case of conventional forming, no clear necking due to strain localization has been observed prior to failure due to very high strain rates of the order of 103/s. The strain distribution has been found to be more uniform in the case of EHF with a single strain peak at the pole. Absence of friction in EHF also leads to higher degree of biaxiality leading to higher limit strains in biaxial tension. In the case of EHF, the effective strain and hardness are maximum at the pole and their variation correlated well with the findings from the strain distribution analysis. In all modes of deformation, the features of fractured surface in EHF appeared different from a normal ductile failure.