Finite Element Modeling of Burnishing and the Effects of Process Parameters on Surface Integrity of Orthopedic Implants

Hydrostatic burnishing is mainly a ceramic ball supported by a pressurized cushion of hydraulic oil and pushed against the workpiece surface. As the ball rolls along the surface it produces a unique combination of three physical effects in the surface layer: i) work hardening and increased hardness, ii) burnishing and decreased roughness, and iii) increased compressive residual stresses. This process has gained an increasingly great attention in automotive, aerospace, and especially medical device manufacturing industries. However, most of the research in hydrostatic burnishing has been performed experimentally and there is still lack of numerical studies providing fundamental understanding of the mechanics and the way process parameters interact with surface integrity characteristics particularly surface roughness and residual stresses. Understanding the correlation between process parameters and surface integrity is critical in efficiently adjusting the surface integrity in order to achieve proper biodegradation rate in human body after implantation. In this study, the dynamic mechanical behavior of the material is simulated using internal state variable (ISV) plasticity model. A semi-infinite, two-dimensional, plane strain FE model is developed and the ISV material model is incorporated into it using a user defined material subroutine. The effects of burnishing pressure and feed on surface roughness and residual stresses are investigated. The simulation results are validated with experimental measurements of residual stresses and surface roughness.