| Author: | Xie, Weijie |
| Title: | Laser additive manufacturing of magnesium alloys for biomedical applications |
| Advisors: | Man, H. C. (ISE) |
| Degree: | Ph.D. |
| Year: | 2024 |
| Department: | Department of Industrial and Systems Engineering |
| Pages: | xxi, 257 pages : color illustrations |
| Language: | English |
| Abstract: | There is an increasing demand for medical implants due to factors such as obesity, sedentary lifestyles, and an aging population. While stainless steel 316L and Ti6Al4V are common metallic materials for permanent implants, their use as temporary implants requires a second surgery for removal, together with other limitations. Biodegradable magnesium (Mg) alloys are emerging as a promising alternative for temporary applications since they do not require a second surgery for removal. In addition, they offer mechanical properties comparable to natural bone. Selective laser melting (SLM) offers the capability to create tailor-made implants but is complicated by its numerous interrelated process parameters, and the occurrence of defects such as porosity. The high degradation rate of Mg implants, which depends on the alloy composition and microstructure, highlights the need for a thorough understanding of the SLM process and its parameters. The effect of composition and process parameters on porosity in SLM and the resulting material properties were investigated in this project. Characterization techniques such as Scanning-Electron Microscopy (SEM), Energy Dispersive X-ray (EDX), X-ray Diffraction (XRD), Optical Microscopy (OM), and X-ray Computed Tomography (XCT) were utilized. A real-time monitoring system was developed to observe the degradation behavior of Mg alloys. The results are presented in three work packages: (i) Preliminary SLM Experiment and Cell Culture Analysis; (ii) Influence of Compositional Content on Corrosion Properties; and (iii) Influence of Laser Power on Defects and Degradation Behaviors. SLM experiment and cell culture analysis were performed using stainless steel (SS) 316L as a preliminary study to understand the SLM process, since SS is easier to handle than Mg alloys in SLM. The effect of Argon flow velocity inside the build chamber and the biocompatibility of the printed surfaces through in-vitro culturing of Mesenchymal Stem Cells (MSCs) for 96 hours were investigated. The study achieved highly-dense parts (>99.8% density) and linked the micro-sized surface defects (i.e., balling) to the gas flow velocity. The resulting printed surfaces were hospitable for MSCs, and healthy cell response was recorded throughout the 96-h culture periods. To study the influence of compositional content on corrosion properties, the effects of Zinc (Zn) content on the powder characteristics, porosity, microstructure, and corrosion properties, Mg-xZn-0.2Mn (x=1, 4, 7 wt.%) alloys were produced via SLM. The corrosion resistance and degradation behavior were assessed through electrochemical corrosion tests and immersion tests in Hanks’ solution at 37.5 °C. It was observed that both the powder and porosity characteristics were influenced by Zn content, as evidenced by OM and SEM results. As the Zn content increased, the pore fraction rose from 1.0% to 5.3%, and the pore size grew from 2.2 μm to 3.0 μm. All printed samples consisted of an α-Mg matrix, confirmed by XRD. Additionally, a higher Zn content resulted in more distinct grain boundaries. The corrosion resistance decreased with Zn, leading to more pronounced localized corrosion after immersion in Hanks’ solution. Ca-P was found as white corrosion products on all samples, as observed by OM and SEM-EDX. To elucidate the influence of laser power on defects and the degradation behaviors, the multi-physics phenomena of the SLM process and the subsequent porosity characteristics of ZK60 Mg alloys were studied. High-speed camera was employed to monitor process signals in real-time, revealing the dynamics of melt pools and vapor plumes under varying laser power conditions between 40 W and 60 W. In addition, porosity was holistically investigated using both destructive (OM and SEM) and non-destructive (XCT) methods, with OM capturing detailed and full surface images and XCT providing comprehensive internal views. An increase in laser power was found to lead to a denser structure with reduced porosity. Moreover, lower laser power favored the formation of interconnected pores, while a reduction in interconnected pores and an increase in isolated pores were observed at higher power. The interplay between melt pool size, vapor plume effects, and laser power was found to significantly influence the resulting porosity, indicating a need for effective management of these factors to optimize the SLM process of Mg alloys. The corrosion tests showed a correlation between porosity and degradation rate, and the self-developed real-time monitoring system revealed various types of corrosion in 60-W samples. |
| Rights: | All rights reserved |
| Access: | open access |
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