While conventional forming processes (e.g. stamping) of wrought Mg alloys are problematic at room temperature, high-temperature forming facilitates the manufacture of near-net shapes from Mg alloy sheets. To reduce energy consumption and manufacturing costs, it is desirable to reduce the processing temperatures of Mg sheet forming. Hence, understanding the deformation mechanisms which drive microstructural and texture evolution of Mg alloys during forming rather than post-forming is crucial to achieve optimized processing and end products. In this task, we designed novel experimental techniques to investigate the deformation behavior during sheet bending of AZ31 Mg alloys. One of the techniques is in-situ EBSD (electron backscatter diffraction) analysis during sheet bending. This technique allows examinations of microstructure and texture evolution during sheet bending without unloading the specimen. Very interesting results were obtained. In another technique, we designed and fabricated fixtures to perform three-point bending with Instron machine such that the strain rate can be controlled. A ground-breaking discovery was made. Meanwhile, continuum modeling of sheet bending was performed.
Fig. 1. Multiple twin bands were observed that span both the compression and tension zones near the center line of the specimen (indicated by a dash-dotted line in the middle). The twin bands grow accross the center line, indicative of a shift of neutral axis.
(1) In-situ EBSD on sheet bending. A special fixture was fabricated to conduct in-situ EBSD on a sheet sample that is being loaded. Fig. 1 shows that during bending, multiple twin bands were produced in the compression zone of the sample. But the twins are localized in bands. In between the bands, no twins can be seen. As the bending angle increases, the twin bands continue to grow across the center line. The pole figure is typical of extension twinning. Details
The highly localized twin bands were not observed before. The mechanism responsible for these twin bands and how they influence sheet forming require further studies. This critical information can serve the community of sheet metal forming by improving the understanding of damage in textured magnesium during bending.