Process Mechanics in Deep Rolling of Magnesium-Calcium (MgCa) Biomaterial

Magnesium-Calcium (MgCa) alloys have received considerable attention recently in medical device manufacturing industry specially in making biodegradable bone implants. Deep rolling (DR) is as a promising manufacturing technique to adjust surface characteristics of implants with the ultimate goal of being able to adjust corrosion rates of MgCa implants. Contact mechanics between rolling ball and the workpiece is essential to understand the DR process. Contact mechanics is further complicated by the normal force reduction due to hydraulic pressure loss at the tip of DR tool, and the penetration depth reduction due to elastic recovery. The measured normal force, in this study, shows maximum 23% reduction compared to theoretical value. The normal force drop is not uniform and increases with increasing applied pressure. A 2D axisymmetric, semi-infinite FEM model is developed and validated to predict the amount of elastic recovery after deep rolling. The dynamic mechanical behavior of the material is simulated using the internal state variable (ISV) plasticity model and implemented in FEM code using a user material subroutine. The simulated dent geometry agrees with the measured ones in terms of profile and depth. Simulation results suggest 8% elastic recovery on average.