Microstructure Strain Localization during Sheet Metal Hydroforming Processes

Abstract

This investigation deals with the interdependence between microstructure constituents and local material flow under stress in regard to deep drawing of cold rolled steel sheets. The studies of the local flow and phase deformation of DP, TRIP and ULC steels included hydraulic bulge testing, stretch-forming with a semicircular punch and tensile testing. Changes in the microstructure distribution and orientation of the constituents were determined metallographically by optical and scanning electron microscopes (SEM), by in situ tensile tests as well as by determining texture by electron back-scattered diffraction (EBSD) and X-ray diffraction methods. A quantitative microstructure examination of phase deformation was carried out and used for the analysis of strain localization under uniaxial and biaxial tension. In the examined forming processes the different microstructure constituents showed different local deformations; this phenomenon can be described by the coefficient of heterogeneity deformation (CHD). As expected, there is a correlation between the CHD, texture and the disappearance of the material's plastic reserve during deformation. Variations in the material flow are due to changes in microstructure constituents during the forming processes. These variations and changes can be expressed by the coefficient of macro heterogeneity. For the examined cold deformation processes three characteristic ranges of inhomogeneity development were found in each case, which are based on the corresponding changes in microstructure. The CHD-formation can be used in combination with a forming limit diagram to evaluate the suitability of a material to a forming process.