Abstract

Recently, Magnesium based alloys have been identified as a potential bio-degradable material for implants. While the biggest advantage of magnesium based implants is that it eliminates the need for additional surgery for removal, magnesium corrodes within human body much faster than the cure of broken bones. Hence, reduction of corrosion rate in magnesium surface is important to successful implementation. Laser Shock Peening (LSP) as mechanical treatment has been used successfully on implant surfaces to reduce the corrosion rate. Having reviewed the literature in LSP, it can be seen that the lasers used are low repetition rate high power lasers. Owing to cost of low repetition lasers, high repetition laser shock peening (HRLSP) is introduced in this work.

The general objective of this thesis dissertation is to develop HRLSP for low mechanical strength metals/alloys like Magnesium. To this end, a feasibility study of HRLSP was performed and pertinent laser parameters for successful peening of magnesium were evaluated. The finite element analysis using Abaqus Dynamic/Static was performed in both single shot mode as well as in multi shot mode. The simulation was used to predict the surface deformation on the peened area, magnitude of Compressive Residual Stress (CRS), and the propagation of CRS along the depth from the surface. The effect of laser parameters and scanning parameters on these values have been analyzed by Finite Element Analysis (FEA).

Based on the results from the feasibility study, an experimental setup involving both optical as well as scanning arrangement was performed and design of experiment was done for peening. From the experiments, with two laser intensities at 0.91GW/cm2 and 2GW/cm2, peening was possible only at 2GW/cm2. The magnitude of peening was varied by changing the %overlap between subsequent spots, and the total number scans. Increasing the number of peened shots within the specimen surface area of 10 X 10 mm, increases the magnitude of CRS that was shown by Finite Element Analysis. Comparison of peened samples to unpeened samples showed significant improvements in the mechanical attributes, comparable to that seen in the literature. The hardness increased from 45 HV to 103 HV; Surface roughness (Ra) increased from 0.35 m to 3.3 m; surface wettability measured as function of contact angle reduced from 68.5° to 44.4°; and wear resistance improved from 5.5E-4 gr/s to 1.8E-4 gr/s. The results mentioned above clearly indicate the relationship between improvements in mechanical attributes to the magnitude of peening.