Effect of Micro-arc Oxidation on the Mechanical Properties of Hierarchical Porous Ti6Al4V Prepared by Selective Laser Melting
The work aims to investigate the change rule of mechanical properties of porous Ti6Al4V after surface modification through micro-arc oxidation (MAO). In this research, porous Ti6Al4V lattice materials with relative densities of 0.30, 0.38, and 0.47 were prepared by selective laser melting (SLM). The surface was pre-treated by chemical polishing and then the MAO film layer was formed on the surface by the MAO process, and then the microscopic morphology and mechanical properties were analyzed by microscopic observation and uniaxial compression test.The surface chemical polishing pretreatment of porous Ti6Al4V was carried out by Kroll's reagent for 50 minutes to remove the residual unfused powder particles on the surface of porous Ti6Al4V, not causing acid etching of porous Ti6Al4V pillars, and the pore size of the MAO film layer on the surface of hierarchical porous Ti6Al4V after MAO was positively correlated with the pulse voltage and oxidation time, and the thickness of the MAO film layer was positively correlated with the oxidation time. The growth rate of the film layer thickness increased rapidly at first and then slowed down.With the prolongation of the micro-arc oxidation time, the ratio of calcium and phosphorus atoms in the MAO film layer started to increase, and then tended to be stable, as the deposition and transformation of Ca, P compounds basically reached equilibrium, while the atomic ratio of calcium to phosphorus continued to rise, with the rate of increase gradually slowing down. The film layer prepared under the conditions of 350 V pulse voltage and 10 minutes of oxidation time was the most homogeneous. The compressive stress-strain curves of porous Ti6Al4V before and after MAO were basically the same, and the elastic modulus and the yield strength of the two both increased with the increase of the relative density. Compared to the theoretical values calculated by the G-A equation, the measured elastic modulus decreased slightly but not significantly, which might be attributed to the reduction of the elastic modulus of porous Ti6Al4V due to the generation of residual stresses caused by rapid heating and cooling during the SLM forming process, and also due to the fact that the pore wall in the formed porous Ti6Al4V lattice material might be lower than the value assumed in the theoretical predictions. This would cause the pore walls to deform or break down during the loading process, leading to a decrease in the overall modulus of elasticity of the material. The measured yield strength was on the higher side than the theoretical value calculated by the G-A equation, which might be due to the more regular pore structure of the SLM formed porous Ti6Al4V lattice materials compared to the theoretical model of the G-A equation. In addition, in logarithmic coordinates, the yield strength and elastic modulus before and after MAO showed a strong positive relationship, with slopes of 1.10 and 1.18, respectively, which were very close to the theoretical values of G-A equation. This also indicated that the overall effect of MAO on the mechanical properties of porous Ti6Al4V was limited.Under the conditions of 350 V pulse voltage and 10 minutes of oxidation time, the MAO process contributes the most homogeneous MAO film layer. Moreover, the impact of MAO on the mechanical properties of SLM formed porous Ti6Al4V lattice materials is found to be minimal.
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