Additive manufacturing of ultrafine-grained high-strength titanium alloys

Abstract

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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Fig. 1: Additive manufacturing of Ti–6Al–4V and Ti–8.5Cu alloys.
Fig. 2: Scanning electron microscopy (SEM) characterization of Ti–8.5Cu alloy.
Fig. 3: Transmission electron microscopy characterization of as-printed Ti–8.5Cu alloy.
Fig. 4: Mechanical properties of as-printed Ti–Cu alloys.

Data availability

The datasets generated or analysed during the current study are available from the corresponding author on reasonable request.

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Acknowledgements

We acknowledge the Australian Research Council (ARC) for financial support (grant number DP160100560). D.Q. would like to thank the RMIT Vice-Chancellor’s Senior Research Fellowship Fund for support. We thank M. Brandt and A. Jones for their support during laser metal deposition manufacturing, K. Yang for her support in etching additively manufactured titanium samples and E. Lui for his support in tensile testing. We acknowledge the facilities, and the scientific and technical assistance, of the RMIT Microscopy and Microanalysis Facility (RMMF). We also acknowledge the Center for Electron Microscopy and Analysis (CEMAS) at the Ohio State University for providing access to research facilities.

Author information

M.A.E., M.A.G., D.H.StJ. and H.L.F. conceived the idea. D.Q. and D.Z. designed the experiments. D.Z. and D.Q. helped with processing parameter development and sample manufacturing, and performed the microstructure characterization. Y.Z. and D.Z. conducted the transmission electron microscopy and analysed the results. D.Z. performed mechanical testing, simulations and X-ray computed tomography. M.A.E. supervised the project. D.Z. and D.Q. drafted the manuscript. All authors discussed the results and edited the manuscript at all stages.

Correspondence to Hamish L. Fraser or Mark A. Easton.

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Competing interests

The authors declare no competing interests.

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Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Peer review information Nature thanks Amy Clarke, David Dye and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

Extended data figures and tables

Extended Data Fig. 1 Ti–Cu phase diagram.

Portion of the Ti–Cu phase diagram indicating the compositions selected for laser metal deposition. We selected 3.5, 6.5 and 8.5 wt% copper to explore the behaviour of hypo-eutectoid, eutectoid and hyper-eutectoid compositions under additive manufacturing. This figure is adapted from ref. 30, with the permission of ASM International.

Extended Data Fig. 2 3D visualization of the porosity of the manufactured specimens in the xyz coordinate system.

a, Ti–3.5Cu. b, Ti–6.5Cu. c, Ti–8.5Cu. d, Calculated relative density of the as-printed specimens. Error bars represent one standard deviation.

Extended Data Fig. 3 XEDS results of the copper content along the building direction for Ti–8.5Cu alloy.

The base point is 0 mm and the chemical composition is homogeneous. Error bars represent one standard deviation.

Extended Data Fig. 4 Polarized optical microstructures.

a, b, The equiaxed grains of as-printed Ti–3.5Cu (a) and Ti–6.5Cu (b). The average grain size is 69.8 μm for Ti–3.5Cu and 16.3 μm for Ti–6.5Cu.

Extended Data Fig. 5 Solidification curves.

The data are shown for different copper compositions under equilibrium and Scheil conditions. The Scheil curves show a substantially enlarged temperature interval between liquidus and solidus temperatures compared with the equilibrium condition.

Extended Data Fig. 6 XRD spectra.

Experimental XRD spectra collected from the as-printed Ti–8.5Cu alloy indicates that only two phases are present in the specimen: α-phase titanium and Ti2Cu.

Extended Data Fig. 7 BSE images.

ad, BSE images of as-printed specimens showing the fine α phases when multiple layers were deposited, for Ti–3.5Cu (a) and Ti–6.5Cu (b); and the martensite phase when only a single layer was deposited for Ti–3.5Cu (c) and Ti–6.5Cu (d). Images were taken at the first layer of build specimens, indicated by the red spots.

Extended Data Fig. 8 SEM images of the cross-section of raw powders.

a, b, SEM images of the titanium powder (a) and copper powder (b) cross-sections. The powders are spherical in shape with a diameter between 50 µm and 100 µm, and porosity can be observed within some powder particles. The yellow arrows indicate examples where powder particles fell out of the resin during the polishing process.

Extended Data Fig. 9 Engineering stress–strain curves.

The data for the additively manufactured materials tested in this study indicate good repeatability.

Extended Data Table 1 Measured chemical compositions (wt%) and volume fraction of eutectoid lamellae in the as-printed alloys

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Zhang, D., Qiu, D., Gibson, M.A. et al. Additive manufacturing of ultrafine-grained high-strength titanium alloys. Nature 576, 91–95 (2019) doi:10.1038/s41586-019-1783-1

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