Analysis of the Increased Formability of Aluminum Alloy Sheet Formed Using Electromagnetic Forming

One of the main challenges associated with the use of aluminum alloys in the automotive industry is increasing their limited formability. Electromagnetic forming has been considered recently as a way of addressing this issue. Increases in formability for several commercial aluminum alloys have been reported in electromagnetic (EM) and other high speed forming processes. These increases are typically attributed to high strain rate and inertial effects; however, these effects alone cannot account for the increases in formability observed. The present authors have previously reported that the increased formability is likely due to damage suppression caused by the tool/sheet interaction. This paper presents an analysis of this interaction and how it affects the formability of the sheet. Experimental and numerical work was carried out to determine the details of the forming process and its effects on formability, damage evolution and failure. It has been determined that when the sheet makes contact with the tool, it is subject to forces generated due to the impact, and very rapid bending and straightening. These combine to produce complex non-linear stress and strain histories. The predictions indicate that relatively little damage is generated in the process except in specific areas of the parts. Damage measurements agree with the predicted trends and fractographic analysis shows that parts formed with the EM process do not fail in pure ductile failure, but rather in a combination of plastic collapse, shear fracture and ductile failure. The majority of the plastic deformation occurs at impact, leading to strain rates in the order of 10,000 s -1 . It is concluded that the rapid impact, bending and straightening that results from the tool/sheet interaction is the main cause of the increased formability observed in EM forming. The tool/sheet interaction produces a non-plane stress condition, very high strain rates and highly non-linear strain paths.

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