Metal additive manufacturing (MAM) has been called the third wave of manufacturing and is currently revolutionizing the production across multiple industries, in particular for aerospace, automotive and biomedical applications. A major barrier to the widespread adoption of MAM, however, is the control of the grain structure which can affect factors such as susceptibility to hot cracking and lead to anisotropic mechanical behavior, particularly in high performance alloys. Alloys currently employed in industry have been originally designed for conventional manufacturing routes and not optimized for MAM. New alloys with high strength and optimum solidification behavior are required to maximize uptake of MAM as a competitive manufacturing route for high performance components.
It has been recognized for decades that fine and equiaxed grains can decrease tendency to hot cracking and improve properties, e.g. strengthening through the Hall–Petch relationship. However, in MAM, the grains are predominantly characterized by columnar and textured microstructures due to the non-equilibrium solidification of extremely high cooling rate and thermal gradient. As a result, it is a big challenge to form equiaxed grains in MAM. Whilst there has been reasonable progress is achieving fine equiaxed grains in MAM of aluminium alloys by adding grain refiner, there is no commercial grain refiner for titanium that is able to effectively refine the microstructure.
In the RMIT Centre for Additive Manufacturing, Prof Mark Easton is leading a team including Dr Dong Qiu and Dr Duyao Zhang working on designing tuneable microstructures for MAM components, especially for light alloys. The theory behind this project is based on the Interdependence Theory proposed by Prof David StJohn et al. (Acta Mater. 2011, 59, 4907). We also worked with some of the world’s experts on Ti-alloys including Dr Mark Gibson from CSIRO and Prof Hamish Fraser and his team (Dr Yufeng Zheng) at the Ohio State University in the USA. The idea was to exploit the constitutional supercooling capacity, due to partitioning of the alloying elements, to override the negative effects of solidification conditions during MAM. In addition, in order to take the full advantage of the MAM process, such as the thermal cycling during the layering process, it appeared that eutectoid systems and their pearlite microstructures could benefit. We envisioned that the unique thermal cycles and high cooling rate of MAM will help to refine the severe microsegregation that occurs in pearlite-Ti in conventional processing, and thus to achieve desired mechanical properties.
As demonstrated in our paper "Additive manufacturing of ultrafine-grained high-strength titanium alloys" that published in Nature (https://www.nature.com/articles/s41586-019-1783-1), Ti-Cu alloys manufactured by MAM that have fully fine equiaxed primary grains and eutectoid lamellae, as well as excellent mechanical properties. It has shown that tuneable microstructures can be achieved by MAM across multiple microstructural length scales. The new alloy design strategy proposed focuses on smart manipulation of the thermodynamics of alloying elements and the solidification conditions of MAM synergistically. It is also expected that our alloy design concept can be applied to other alloy systems and lead to the development of more high-performance engineering alloys for MAM in the future.
a, Optical micrograph of an as-printed Ti–6Al–4V alloy showing coarse columnar grains. b, By contrast, optical microstructures of an as-printed Ti–8.5Cu alloy show fine, fully equiaxed grains along the building direction under the same manufacturing conditions.