Title

Corrosion behavior and mechanism of metallic biomaterials produced by laser powder bed fusion

Author Identifiers

Peng QIN

https://orcid.org/0000-0003-2557-197X

Date of Award

2022

Degree Type

Thesis

Degree Name

Doctor of Philosophy

School

School of Engineering

First Advisor

Laichang Zhang

Second Advisor

Hongqi Sun

Abstract

For biomedical applications, the implants always require specific shapes to fulfill the requirements of the different bone tissues. Recently, laser powder bed fusion (L-PBF) in additive manufacturing techniques (e.g., selective laser melting) has attracted extensive attention in manufacturing complex and near net-shaped parts with almost no geometric constraints on the final product. It also has the advantage of less production cycle with high material utilization rate when compared with the conventional manufacturing methods (materials removal from ingot).

The L-PBF-produced alloys are proved to have enhanced or comparable mechanical properties that benefit from the rapid cooling rate in the manufacturing process. However, the corrosion resistance of some produced alloys has been proved to have inferior corrosion resistance in some harsh environments. Although the corrosion mechanism of the L-PBFproduced alloys has been studied gradually in recent, and the results are still not enough to support the L-PBF-produced alloys for commercial use. Therefore, some of the corrosionrelated aspects are investigated in this thesis, which include feedstock types for the L-PBF process, electrolyte types and conditions, plastic deformation, and long-term immersions.

In the aspect of feedstock types, the corrosion behavior and mechanism of L-PBFproduced Ti35Nb are separately produced by using mixed powder (Ti35Nb-M) and pre-alloyed powder (Ti35Nb-P) were investigated (in Hank’s solution). Both produced alloys demonstrate similar corrosion behavior, but the oxide film formed on Ti35Nb-M shows inferior stability with more defects. Dual-layered films are found on the TiNb regions both in Ti35Nb-M and Ti35Nb-P, while the outer porous layer on Ti35Nb-M exhibits more defects due to the microgalvanic effect.

In the aspect of electrolyte condition, the corrosion behavior and passivation behavior of L-PBF-produced Ti-6Al-4V were investigated in different concentrations of NaCl solutions under static and dynamic conditions. The results indicate the higher concentration of Cl- could promote the reactions between metal matrix and electrolyte, and therefore, increase the thickness of the produced oxide film. The effect of Cl- on corrosion is mainly attributed to the dissolution of the TiO2 where located in the outermost oxide film.

In addition, the corrosion behavior and passivation behavior of L-PBF-produced Ti-6Al- 4V were also investigated under various compressive strains (from initial until failure). A decreased corrosion resistance of the alloy was found with an increase in the compressive strain. The reason could be attributed to the deformation and breakages of the acicular α'-Ti phase, resulting in the enhancement of the oxide film growth with an inferior corrosion resistance (oxide film containing more point defects).

In the aspect of long-term immersion, the L-PBF-produced CoCrW alloys also used the electrochemical methods and surface characterization method to investigate their corrosion behavior and mechanisms in 0.9 wt% NaCl solution with a pH of 2. The corrosion resistance of the produced alloys decreased during the 28 days of immersion due to the diffusion of element W in the oxide film, which results in the decreased stability of the formed oxide film (Cr2O3). In addition, the corrosion of the L-PBF-produced CoCrW alloy on a macro-scale is related to the dissolution of the melt pool that results from the scan strategy in the PBF process.

The results suggest that the L-PBF-produced Ti-35Nb, Ti-6Al-4V, and CoCrW are highly corrosion-resistant, although there is a small difference in the electrochemical behavior and the formed oxide film. Therefore, this research demonstrates that the alloy produced through LPBF- produced alloy has more desirable properties in corrosion performance, and it also laid a foundation for the application of L-PBF-produced metallic biomaterials in biomedical applications.

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