Author Identifiers

Peng Qin
ORCID: 0000-0003-2557-197X

Date of Award

2018

Degree Type

Thesis

Degree Name

Master of Engineering Science

School

School of Engineering

First Advisor

Associate Professor Lai-Chang Zhang

Second Advisor

Professor Hongqi Sun

Field of Research Code

091207

Abstract

Recently, the additive manufacturing techniques (e.g., selective laser melting and electron beam melting) have attracted great attention in manufacturing titanium and titanium alloys because the additive manufacturing techniques could produce parts with almost no geometric constraints. Many researchers have studied the mechanical properties of additive manufactured titanium and titanium alloys, and the results indicated that the additive manufactured titanium and titanium alloys have different degrees on enhanced mechanical properties compared with the alloy manufactured by traditional methods. However, the titanium alloys in applications usually encounter the environment of corrosion, such as biomedical, aerospace, chemical implants, etc. Unfortunately, rare studies have addressed the corrosion behavior of the additive manufactured titanium products. The present corrosion studies on additive manufactured titanium alloys show that the α’ phase presented in the selective laser melted samples has a detrimental effect on the corrosion resistance in simulated seawater and simulated human body fluids. Even the α’ phase in the selective laser melted titanium alloys was eliminated by heat treatment, the selective laser melted titanium alloys still show inferior corrosion resistance in simulated seawater and human body fluids compared to the counterparts produced by traditional methods. By contrast, the electron beam melted titanium alloy have a slightly better corrosion resistance than the wrought sample as the microstructure of electron beam melted titanium alloy only contain α phase and β phase. Therefore, the current issue is arising and to whether the unique phase produced by selective laser melted titanium alloys affect the corrosion resistance? By overviewing the current literature, the corrosion study of the additive manufactured titanium alloys are mainly focused on the Ti-6Al- 4V, CP-Ti, and Ti-Al alloys but rare research on the other titanium alloys.

In this thesis, the corrosion behavior of Ti-5Cu and Ti-24Nb-4Zr-8Sn produced by the selective laser melting has been systematically investigated and discussed. The electrochemical testing methods, such as open circulate potential, electrochemical impedance spectroscopy, and potentiodynamic polarization curve has been employed. Transmission electron microscopy, scanning electron microscopy, optical microscopy, and X-ray diffraction has been jointly used to characterize the microstructure and surface morphology of the samples. The inductively coupled plasma has been applied to examine the released ions in the electrolyte after electrochemical corrosion tests. The selective laser melted Ti-5Cu has a severe pitting phenomenon which can be alleviated by heat treatment. The heat-treated Ti-5Cu samples produced by selective laser melting exhibit a significant passivation behavior instead of pitting phenomenon in potentiodynamic polarization test. Such a phenomenon is attributed to that the present α’ phase in melting pool boundary forms a galvanic couple, causing severe pitting corrosion. In another study, the selective laser melted Ti-24Nb-4Zr-8Sn and wrought Ti-24Nb-4Zr-8Sn only contain single β phase and they display similar corrosion results. The above results suggest that the selective laser melted titanium alloys have a great potential to replace the conventionally manufactured titanium alloys in the simulated seawater environment and simulated body fluid environment.

Available for download on Wednesday, September 06, 2023

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