Research On Laser Peening And Tribological Properties Of Biomedical Magnesium Alloys

The corrosion and friction and wear properties of magnesium alloy internal fixation bone plates in implantable medical devices are worthy of attention.In this paper,different laser power densities were used to study the effects of laser peening on the surface parameter characteristics,corrosion properties,tribological properties,microstructure and mechanical properties of magnesium alloy ZK60.The three-dimensional surface morphology,roughness,microhardness,metallographic structure,residual stress and phase transformation were characterized.The effects of different laser power densities on the corrosion and friction and wear properties of magnesium alloys are explored through simulated body fluid immersion corrosion test,electrochemical test and friction and wear test.A scanning electron microscope(SEM)equipped with an energy spectrometer(EDS)was used to observe and analyze the corrosion morphology,corrosion products,wear morphology,and elemental composition of wear debris.The microstructure was observed and analyzed by transmission electron microscope(TEM),the tensile properties were tested and the fracture morphology was observed,and the fracture mechanism was determined.The following results can be obtained within the scope of this thesis.The surface parameter characteristic results showed that the surface roughness,microhardness value,number of crystal grains,and residual compressive stress increase with the increase of laser power density.The roughness increased from the initial 0.32?m to 3.6?m,6.273?m and 9.3?m.The surface microhardness increased from the initial HV76.9 to HV90.6,HV99.2 and HV106.9,and the hardness values increased by 17.8%,29%,and 39%,respectively.Laser peening refined the surface metallographic structure,and the number of crystal grains increases from 1045 to 1280,1250,and 1461.The surface residual stress can reach-82MPa,-95MPa,and-106MPa,respectively.Laser peening does not induce phase transformation in magnesium alloys.The simulated body fluid immersion corrosion test showed that the corrosion rate of the laser peened samples was reduced.In terms of the amount of corrosion,the corrosion resistance of the laser shot peened samples increased by 52.1%,45.1%and 49%,respectively.Laser peening can improve the pitting resistance of magnesium alloys and inhibit the initiation and propagation of cracks.The increase in the deposition of calcium and phosphorus on the surface of the laser peened sample is beneficial to further improve the biocompatibility.The corrosion potential increased from the initial-1.3884V to-1.2256V,-1.1707V and-1.1094V,respectively,and the corrosion current density decreased from 1.378×10-5A/cm~2 to 1.267×10-5A/cm~2,1.23×10-5A/cm~2 and 1.196×10-5A/cm~2,the corrosion tendency is reduced by 20.1%.Variable load dry sliding wear test results showed that laser peening can reduce the friction coefficient,wear volume and wear rate of magnesium alloys,and laser peening can improve the wear resistance under higher loads.The fretting wear test results showed that the friction coefficient of magnesium alloy was not significantly reduced under dry friction conditions and SBF lubrication conditions,but the maximum wear volume is reduced by 67.4%and 73.4%,respectively.laser peening can significantly reduce the adhesive wear,oxidative wear and fretting corrosion of magnesium alloys in SBF.Adhesive wear and oxidative wear are the main wear mechanisms of the original magnesium alloy,while the wear mechanism of laser peened samples is mainly abrasive wear.TEM microstructure results and tensile test results showed that laser peening can induce the surface grains of magnesium alloys to be nanosized,and the fine grain strengthening and the pinning strengthening effect of the second phase particles on the dislocations together enhance the mechanics of the magnesium alloy.The yield strength and tensile strength were increased by 15.02%and 10.6%.After laser peening,the number of equiaxed dimples increased and the size of dimples decreased.The fracture mechanisms before and after laser peening were ductile fracture and brittle fracture.The Abaqus software was used to simulate the residual stress field generated by the laser parameters of 1.19GW/cm~2,1.99GW/cm~2 and 2.79GW/cm~2 laser parameters shock peening magnesium alloy.The results show that the average surface residual stress values are about-90MPa,-100MPa and-120MPa,and the depth of the residual stress layer generated is about500?m,600?m and 700?m respectively,which is in good agreement with the actual measured residual compressive stress value.