Predictions of stress distribution and material flow in coining process for bi-material commemorative coin

Abstract

Bi-material commemorative coins have the characteristics of weight reduction, beauty and corrosion resistance. However, due to different characteristics of bi-materials, defects such as stress concentration, drop of inner core and micro-cracks often occur at interface of materials in usage. Engineers usually employ multiple mold trials to check the bonding state of two material regions, judge the stress concentration area and modify the process to avoid micro-crack sprout. Based on the professional metal forming software Deform 3D, stress distribution and material flow in coining processes for single and bi- material commemorative coins are studied. Numerical examples demonstrate that there are three typical stages over the whole process whether the coin is single or bi- material. And stress concentrations appear at the corners of upper/lower dies and the material interface in case of bi-material. The former concentration lead to sprout of micro-cracks along the shear bands and the latter triggers cracks at the interface. In addition, two materials with different hardness are adopted in the core and ring, respectively, to investigate the mutual interaction behavior at interface. Numerical findings indicate that large strains occur at the edge and the interface in case of bi-material with soft core. These imply that deep adhesion happens at the interface in case of that soft material is in the core. It suggests that hard material in the inner core and soft one in the outer ring may cause falling of the core.