DYNAMIC BLANKHOLDER CONTROL FOR THE ENHANCED FORMING LIMIT OF MAGNESIUM SHEETS

The deep drawability of magnesium is known to be affected significantly not only by forming temperature and speed, but by blankhold force. Since magnesium alloys typically exhibit very weak strain hardening at the elevated temperature, thinning (instability) can be triggered by small amount of excessive blankhold force. In this study, a servo-controlled blankholder mechanism was implemented to the dies/tools for magnesium warm drawing. As reported in the previous studies of high strength steels and aluminum alloys, improvement of forming limit coupled by lower forming load was reproduced for magnesium warm forming. Cyclic 'clamp-and-release' blankholder motion eased off the resistance of metalflow from flanges to walls, which resulted in smaller stretcher strain and more uniform thickness distribution. By optimizing the control function of blankholder action, the forming limit can be increased by 20% and more.