Fabrication and finite element simulation of sequential laser shock peening of biodegradable Mg-Ca implants

Current biocompatible metals such as steel and titanium alloys have excellent corrosive properties and superior strengths. However, their strengths are often too high and as a result have a negative effect on the body. Therefore, Magnesium (Mg) alloys with relatively low strengths are ideal biocompatible metallic materials. The problem with Mg implants is how to control corrosion rates so that the degradation of Mg implants may match with bone growth. The high compressive residual stress induced by laser shock peening (LSP) has a great potential to slow down the corrosion rate. LSP is a known surface treatment method to impart compressive residual stress in subsurface of a metal. Therefore, LSP was initiated in this study to investigate surface topography and integrity produced by peening a Mg alloy. A 3D semi-infinite simulation has also been developed to predict the topography and residual stress fields produced by sequential peening. The dynamic mechanical behavior was modeled using a user material subroutine of the internal state variable plasticity model. The temporal and spatial peening pressure was modeled using a user load subroutine. The simulated dent agrees with the measured dent topography in terms of profile and depth. Sequential peening was found to increase the tensile pile up region which is critical to tribological applications. The predicted residual stress profiles are also presented.