Extension of formability of the magnesium wrought alloy AZ31B-O at room temperature by pulse magnetic forming

Abstract

For having the lowest density of all metal construction materials of 1.75 kg/dm3, magnesium wrought alloys are outstanding lightweight materials. The low formability at room temperature limits the industrial use of magnesium AZ31B-O. In this paper the influence of high strain rates was investigated with the aim to improve the formability of the alloy AZ31B-O at room temperature. The negative strain rate sensitivity of quasi-static strain rates causes an early loss of material stability due to formation of local deformation zones on the work piece surface. This leads to a low formability in the forming state of plane strain, in which the forming limit (FLC) of magnesium alloy AZ31B-O has a critical minimum. For process illustration of multi-axial stress states - which appear in conventional forming processes - the pulse magnetic forming process is used. To create plane strain formability a flat coil is used. The applied die is used to control the free formability. Hereby, a change of the maximum loads on the power transfer zone to areas of plane strain formability occurs. The results that have been achieved show that high strain rates at room temperature increase the permitted loads of the material with plane strain formability significantly. High speed forming is linked to a rising strain rate sensitivity which increases the flow resistance in critical forming areas, in favor of a rising material stability. This fact is represented by equally reduction of the sheet thickness on the power transfer area. The homogeneous work piece stress clearly increases the formability of AZ31B-O at room temperature compared to quasi-static forming process.