STRUCTURE-PROPERTY RELATIONSHIPS OF CORROSION AND MECHANICAL PROPERTIES IN POLYMER-COATED ADDITIVELY MANUFACTURED STAINLESS STEEL AND TITANIUM ALLOYS

 

Alloys manufactured using additive manufacturing (AM) techniques have been increasingly used for medical and structural applications. However, their corrosion properties are not sufficiently studied which hinders their use in applications, such as orthopedic implants and ocean structures. The objective of this thesis is to systematically investigate the corrosion and mechanical properties of polymer-coated additively manufactured (AM’ed) 316L stainless steel (SS) and Ti-6Al-4V(Ti64) titanium alloys, thus understanding their structure-property relationships, which will provide the guidelines for the use of these alloys in corrosive environments.

To achieve the objective, the following research tasks have been identified: (1) characterization of the corrosion properties of AM’ed and conventional wrought 316L SS; (2) study of the mechanical and corrosion properties of epoxy coated AM’ed and conventional wrought 316L SS; (3) investigation of biocompatible polymer coated 316L SS and Ti64 alloy; (4) machine learning-based lattice structure design of AM’ed alloys; and (5) development of computational models to simulate the mechanical and corrosion properties of the AM’ed alloys.

The results show that the corrosion potential of AM’ed 316L SS is lower, but the corrosion rate is higher than that of conventional 316L SS. From the phase-field model of corrosion, the corrosion area increases with the corrosion time, the number of holes, and the size of holes.

The epoxy-coated 316L SS shows a bond strength of 1.68 MPa and 1.81MPa for the conventional and AM’ed 316L SS, respectively. The molecular dynamics (MD) model illustrates that the interface bond is broken in the epoxy layers, and the mechanical strength increases with strain rate and decreases with temperature. The electrochemical tests show that the coating layer improves the anti-corrosion properties compared to the uncoated specimens. The Monte Carlo (MC) model illustrates that the polymer has good adhesion properties on the substrate with anti-corrosion behavior, indicated by low adsorption energy in NaCl solution.

For the AM’ed 316L SS and Ti64, Young’s modulus and yield strength are similar to the conventional counterparts. The Tafel polarization results show that both uncoated AM’ed 316LSS and Ti64 have higher corrosion potential and current density than the uncoated conventional ones. The biocompatible polymer coated AM’ed 316L and Ti64 samples demonstrate relatively low cytotoxicity.

The developed neural network (NN) model is capable to predict reasonably well about Young’s modulus and yield strength of AM’ed alloys. An example of lattice structure is designed with matching mechanical properties with natural bones.