Title

Selective laser melting of Ti-35NB alloy: Processing, microstructure and properties

Author Identifiers

Jincheng Wang

https://orcid.org/0000-0003-2476-928X

Date of Award

2021

Degree Type

Thesis

Degree Name

Doctor of Philosophy

School

School of Engineering

First Advisor

Laichang Zhang

Second Advisor

Timothy Barry Sercombe

Abstract

The initiative of a sustainable material system needs to lower the environmental and economic impact of production processes and adopt new ways of synthesizing and re-using materials. Even though the current conventional manufacturing processes, such as powder metallurgy, casting, forging, and rolling, have already shown their excellent ability to manufacture a large variety of parts and efficiently yield high volume products. Nevertheless, there are still many obstacles in manufacturing metallic components, such as complicated process procedures, high-energy consumption, large material waste, and high machinery cost for reasons that the excess materials need to be removed and extra post-processing time needs to be taken to acquire desired shapes during the machining stage. Thus, finding innovative solutions for producing complex structures is becoming increasingly desirable in the industry. Innovative additive manufacturing (AM, also known as 3D printing) techniques have proved their capacity to manufacture metallic materials with designed complex shapes and tailored properties. The selective laser melting (SLM) is one of the most popular AM techniques, which has the ability to manufacture a wide range of metallic powders in a layer-wise manner and fabricate complex shapes without compromising dimensional accuracy.

The toxicity, biocompatibility, corrosion resistance, and stress shielding effect are the key challenges for developing titanium biomaterials for orthopedic applications. Adding nontoxic alloying elements into titanium can solve the issues of toxicity and biocompatibility. One of the best solutions for minimizing the stress-shielding effect and prolonging implant lifetime is to tailor the modulus of implant materials closer to that of bones. Nb is a nontoxic alloying element and an excellent β phase stabilizer, which plays a significant role in reducing the elastic modulus and in improving the corrosion resistance of Ti-based alloys. Accordingly, obtaining a highperformance simple alloy by reducing the alloying elements and substituting toxic elements can facilitate the improvement of sustainability. Thus, the β-metastable Ti-Nb alloys with relatively low elastic modulus have been studied for orthopedic implants due to their high strength to weight ratio, excellent corrosion resistance, and high biocompatibility in the human body.

In addition, the high reactivity of titanium with hydrogen and oxygen as well as the high melting points of titanium alloys make conventional manufacturing difficult and cost intensive. As such, the SLM provides an innovative solution to manufacture shape-complicated products in a building chamber under the flow of high purity argon gas to minimize oxidation. However, the availability, printability, and high cost of high-quality raw metallic alloy powder are the limits for the SLM process. The individual elemental powder is relatively cheap and easy to manufacture. Thus, the use of elemental powder mixture results in greater alloy choices as well as lower cost and wider commercial availability. The issues of resultant microstructural and chemical inhomogeneity of the produced parts using the powder mixture have been the major concerns and challenges in the field. Since the mechanical behaviors and chemical properties directly depend on the microstructural homogeneity and phase composition, an in-depth understanding of the effect of inhomogeneity is required. It is necessary to have further advances in manufacturing optimization to extend the benefit of low production costs. In particular, in-situ alloying prospects make SLM a potential route to use a powder mixture with near infinite chemical compositions to synthesize desired titanium alloys for broad applications. As such, synthesizing the proper titanium alloys using the SLM technique, minimizing defect formation, controlling phase composition, evaluating their properties, and investigating the performances of SLM-processed products could significantly advance the applications in various industries and academia. The aim is to apply the SLM technique to process titanium alloys for biomedical and industrial applications. The results help to improve the scientific understandings of the interrelation among alloy compositions, processes, microstructures, defects, properties, and deformation behaviors of 3D-printed parts.

Chapter 1 introduces additive manufacturing (AM) has huge potential to realize new alloys with flexible design and easy manufacturing. Especially for the customized healthcare products and services, such as biomedical implants, prosthetics, and hip replacement. Titanium alloys have desirable properties for various applications. Combining additive manufacturing with affordable and biocompatible titanium alloys can further advance and benefit the healthcare industry. Accordingly, the objectives are to fabricate titanium alloys by SLM and to investigate the microstructure, mechanical performance, and corrosion properties.

Chapter 2 overviews the type, utilization, and advantage of AM techniques, biomaterials, and titanium alloys. The SLM process can manufacture parts with high precision and superb asbuilt surface quality but relatively high residual stress due to the rapid cooling rate. The raw powder properties and processing parameters play important roles in the densification and mechanical property of built products. The physical factors in the melting process and simulation are shown to understand the melt pool characteristics and stability, which is the critical factor to a successful and desired part. The microstructure, mechanical properties, and corrosion performance of different titanium alloys are also reviewed in order to design the powder, understand the mechanism, and improve the properties.

Chapter 3 shows insight into the manufacturing of a Ti-35Nb composite using SLM and post heat treatment. The results emphasize the capability of SLM to fabricate alloys from elemental powder mixtures, even suitable for those with a significant difference in melting point. It provides a significant advance in the understanding of the effect of microstructural inhomogeneity on the resultant mechanical and chemical properties. Heat treatment can further enhance the corrosion resistance of SLM-produced Ti-35Nb samples because the improved chemical homogeneity can facilitate the homogeneous formation of titanium oxides and niobium oxides. It presents a different method of synthesizing novel β-type composites at a relatively lower cost and in easy manufacture.

Chapter 4 shows the microstructure, phase response, and mechanical properties of the SLM-fabricated Ti-35Nb using an elemental powder mixture with reduced Nb particle size and its heat-treated counterpart. The results provide significant advances in the understanding of the role of undissolved Nb particles, Nb-rich interfaces, and Ti-Nb-based β phases on the mechanical performance. The nanoindentation mappings provide direct evidence of the contribution of the different phase responses to overall mechanical properties. The Nb particle segregation zones have lower hardness and higher deformation compared to the Ti-Nb matrix. The as-SLMed Ti- 35Nb exhibits relatively high tensile yield strength (648 ± 13 MPa) due to the formation of dendritic β grains. However, the ductility is relatively low (3.9 ± 1.1%) as a result of the weak bonding of undissolved Nb particles within the matrix. The heat-treated counterpart shows a slightly lower yield strength (602 ± 14 MPa) but a nearly 43% increase in ductility (5.6 ± 1.9 %) due to the improved homogeneous Ti-Nb β phase.

Chapter 5 shows the microstructure, phase composition, melt pool morphology, and mechanical properties of a prealloyed Ti-35Nb alloy manufactured using SLM and compares it to one produced using an elemental powder mixture. The SLM-processed Ti-35Nb from both feedstocks retained a high volume fraction of β phase due to adequate β stabilization by the Nb and the fast cooling of the SLM process; however, other phase compositions were quite different. The chemical heterogeneity and inhomogeneous microstructure of the SLM-produced sample from powder mixture are results of the fast cooling rate of the melt pool and the high difference of melting temperature and density between elemental powders. However, a uniform microstructure and chemical composition can be achieved in the SLMed prealloyed Ti-35Nb. The variances of powder morphology, density, and melting point between mixed powder and prealloyed powder induce different melt pool status, where the stability of the melt pool plays a critical role in the homogeneity and microstructure. The SLMed Ti-35Nb prealloyed powder samples present a slightly lower yield strength (485 ± 28 MPa) but higher plastic strain (23.5 ± 2.2 %). The excellent ductility has been attributed to the high homogeneity, strong interface bonding, and the existence of a large amount of β phase.

Chapter 6 shows the understanding of the homogeneity effect on the coexistence of the acicular α″, β grains, and melt pool boundary for a homogeneous microstructure. It provides some new insight into the phase response and the effect of homogeneity on the SLMed Ti-35Nb alloy using prealloyed powder. The reduced elastic modulus of β phase (89.6 ± 2.1 GPa) is close to that of α″ phase (86.3 ± 2.0 GPa) from the indentation measurement, which is in favor of orthopedic implants application. It also reveals that the nanoindentation test can provide a fast mapping and considerable potential to evaluate the homogeneity, microstructural features, individual phase strength, and deformation behavior in a fine microstructure of SLM-fabricated metallic alloys.

Chapter 7 shows the preliminary design in porous structures and compressive behavior of different prealloyed Ti-35Nb sandwich composite porous structures manufactured using SLM. The simulation results were in good agreement with the compression tests. The compression tests show that the sandwich composites with different layers have different deformation behavior and mechanical properties. The rhombic dodecahedron porous structure with added layers could achieve balanced compressive strength and ductility. The preliminary sandwich design with the verified finite element method (FEM) models can be employed in other metallic porous structures to improve the strength and ductility without affecting the porosity.

Chapter 8 concludes the present findings in this thesis and suggests the future challenges and development using SLM to tailor titanium alloys for specific applications.

As such, the SLM technique is a promising route to develop titanium alloys from powder mixture with wider alloy choices at a cheaper cost and in easier availability. Even though a uniform microstructure and chemical composition can be achieved in the SLM-produced Ti-35Nb using prealloyed powder, there are still challenges on how to achieve full melting of elemental powder particles and obtain a homogeneous β phase microstructure. With the investigation of β- type Ti-Nb alloys, this thesis aims to further understand the effect of the unmelted Nb particles in the synthesized Ti-Nb alloys and melt pool stability as well as improve the Nb melting, microstructure, and mechanical properties for industrial and biomedical applications. Understanding the effect of powder feedstock type and phase features of the SLM-produced Ti- 35Nb using prealloyed powder further provides insights into the homogeneity, microstructure, and resultant properties. The novel design in Ti-35Nb sandwich composite cellular structures can benefit biomedical and industrial applications. By taking advantage of the commercial availability and lower cost of elemental powder, finding solutions to achieve full melting and homogeneous microstructure for nontoxic and biocompatible β-type Ti-Nb alloys with promising mechanical and corrosion properties is significant in future research and development.

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