Perspectives on Titanium Science and Technology

Additive manufacturing, often known as three-dimensional (3D) printing, is a process in which a part is built layer-by-layer and is a promising approach for creating components close to their final (net) shape. This process is challenging the dominance of conventional manufacturing processes for products with high complexity and low material waste1. Titanium alloys made by additive manufacturing have been used in applications in various industries. However, the intrinsic high cooling rates and high thermal gradient of the fusion-based metal additive manufacturing process often leads to a very fine microstructure and a tendency towards almost exclusively columnar grains, particularly in titanium-based alloys1. (Columnar grains in additively manufactured titanium components can result in anisotropic mechanical properties and are therefore undesirable2.) Attempts to optimize the processing parameters of additive manufacturing have shown that it is difficult to alter the conditions to promote equiaxed growth of titanium grains3. In contrast with other common engineering alloys such as aluminium, there is no commercial grain refiner for titanium that is able to effectively refine the microstructure. To address this challenge, here we report on the development of titanium-copper alloys that have a high constitutional supercooling capacity as a result of partitioning of the alloying element during solidification, which can override the negative effect of a high thermal gradient in the laser-melted region during additive manufacturing. Without any special process control or additional treatment, our as-printed titanium-copper alloy specimens have a fully equiaxed fine-grained microstructure. They also display promising mechanical properties, such as high yield strength and uniform elongation, compared to conventional alloys under similar processing conditions, owing to the formation of an ultrafine eutectoid microstructure that appears as a result of exploiting the high cooling rates and multiple thermal cycles of the manufacturing process. We anticipate that this approach will be applicable to other eutectoid-forming alloy systems, and that it will have applications in the aerospace and biomedical industries.

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Additive manufacturing of ultrafine-grained high-strength titanium alloys

High-density 316 L specimens were fabricated by selective laser melting (SLM). Different processing parameters, including laser power (100, 200 W) and scanning strategies (alternating stripes without and with re-melting after every layer) were employed to evaluate their impact on microstructure and texture of the specimens. Microstructures of the specimens in as-built condition were characterised by columnar grains of austenite with intercellular segregation of Mo, Cr and Si, resulting in creation of non-equilibrium eutectic ferrite. It was found that laser energy density and scanning strategy strongly affect cellular substructure of austenite and amount of ferrite, as well as kind and degree of texture. Specific microstructure of austenite in as-built condition is the cause of almost double increase of yield strength accompanied by much smaller improvement of ultimate tensile strength and 1.4 times reduction of elongation at fracture in comparison of properties of hot-rolled SS316L sheet. Moreover, features of this substructure determine kind of the changes occurring during stress relieving at 800 °C for 5 h (among others, precipitation of sigma-phase strongly activated by presence of ferrite and residual stresses), demonstrated by decreased yield strength value with no significant changes of ultimate tensile strength and elongation. At the same time, an attempt was made to explain some unclearly interpreted observations in the literature related to a correlation between process parameters, microstructure and properties of SLM-processed steel 316 L.

Correlation between process parameters, microstructure and properties of 316 L stainless steel processed by selective laser melting

Phase stability change with Zr content in β-type Ti–Nb alloys

Effect of Cu content on the antibacterial activity of titanium–copper sintered alloys

Aging response of the Ti–35Nb–7Zr–5Ta and Ti–35Nb–7Ta alloys

The correlation between the composition, aging heat treatments, microstructural features and mechanical properties of β Ti alloys is of primary significance because it is the foundation for developing and improving new Ti alloys for orthopedic biomaterials. However, in the case of Ti–Mo alloys, this correlation is not fully described in the literature. Therefore, the purpose of this study was to experimentally investigate the effect of composition and aging heat treatments on the microstructure, Vickers hardness and elastic modulus of Ti–Mo alloys. These alloys were solution heat-treated and water-quenched, after which their response to aging heat treatments was investigated. Their microstructure, Vickers hardness and elastic modulus were evaluated, and the results allow us to conclude that stabilization of the β phase is achieved with nearly 10% Mo when a very high cooling rate is applied. Young's modulus was found to be more sensitive to phase variations than hardness. In all of the compositions, the highest hardness values were achieved by aging at 723 K, which was attributed to the precipitation of α and ω phases. All of the compositions aged at 573 K, 623 K and 723 K showed overaging within 80 h.

Ti-Mo alloys employed as biomaterials: effects of composition and aging heat treatment on microstructure and mechanical behavior.

http://www.fem.unicamp.br/~giorgia/Afonso%202010.pdf

High resolution transmission electron microscopy study of the hardening mechanism through phase separation in a β-Ti–35Nb–7Zr–5Ta alloy for implant applications

Effect of cooling rate on Ti-Cu eutectoid alloy microstructure

An evaluation was made of liquid to solid transformation during the solidification of an austenitic stainless steel based on an investigation of the material's solidification mode and microstructure. The solidification mode was studied using directional solidification at low growth rates and differential thermal analysis. The alloy analyzed here showed solidification, with the liquid transforming completely into austenite; however, depending on the solute segregation level, it could present the formation of austenite and ferrite. In addition, most of the alloying elements in this steel showed a partition coefficient of less than 1.

P.L. Ferrandini

Papers

Aging response of the Ti–35Nb–7Zr–5Ta and Ti–35Nb–7Ta alloys

Ti-Mo alloys employed as biomaterials: effects of composition and aging heat treatment on microstructure and mechanical behavior.

High resolution transmission electron microscopy study of the hardening mechanism through phase separation in a β-Ti–35Nb–7Zr–5Ta alloy for implant applications

Effect of cooling rate on Ti-Cu eutectoid alloy microstructure

Solute segregation and microstructure of directionally solidified austenitic stainless steel