There are many potential benefits to using additive manufacturing — also known as 3D printing — for making metal parts, rather than conventional manufacturing processes. For example, additive manufacturing is highly customizable, it can produce complex structures and it can be used for the economical production of low numbers of metal components. But to achieve the strict specifications needed for some applications, the microscopic structure of printed metal objects must be controlled. Writing in Nature, Zhang et al.1 describe titanium–copper alloys that produce practically useful microscopic structures during additive manufacturing, removing the need for subsequent treatment. The resulting materials exhibit promising combinations of mechanical properties, comparable to those of the ubiquitous structural alloy Ti-6Al-4V, produced using conventional and additive manufacturing processes.
In metal additive manufacturing, an alloy (in the form of powders or wires) is deposited in a layer and then melted by a rapidly moving heat source to form a solid mass; successive layers are built up to produce a 3D part. The process typically produces large temperature gradients, high solidification rates and repeated cycles of heating and cooling. A common characteristic of 3D-printed metals is coarse columnar grains that grow along specific directions of the crystal lattice that are favourably oriented with the heat flow (Fig. 1a).
Coarse columnar grains are usually undesirable because they can cause the printed material to have direction-dependent (anisotropic) mechanical properties and make it susceptible to tearing or cracking during solidification2–4. However, columnar solidification can undergo a transition to equiaxed solidification — in which the grains produced have similar dimensions in all directions — by changing the processing conditions used for additive manufacturing2. Alloys with equiaxed grains have desirably uniform properties, and so methods for producing them are of great technological value4.
Models and experiments have been used to study the columnar-to-equiaxed transition (CET) in nickel-based alloys that have been melted using an electron beam2,3. The number of nuclei (tiny crystals that ‘seed’ the growth of the solid phase) in the liquid metal, and the processing conditions used during electron-beam additive manufacturing, were found to have a larger influence on grain structure than did the composition of the alloy3. This suggests that the CET can be controlled through process design and by promoting nucleus formation in alloy melts. Additives called inoculants, which cause nuclei to form in the melt, have been incorporated into metal-alloy powders used in additive manufacturing, to increase the density of nuclei and thereby promote the formation of equiaxed grains4. However, suitable inoculants for titanium alloys remain elusive.
Zhang et al. now show that fine equiaxed grains, on average less than 10 micrometres in diameter, can be produced in titanium–copper alloys during additive manufacturing, without adding inoculants (Fig. 1b). The authors propose that nucleation and CET are promoted in these alloys by the formation of a large zone of supercooled liquid — melted alloy that is fully liquid, despite its being below the temperature at which the alloy should start to solidify. The final product consists of two solid phases that contain different amounts of titanium and copper, forming a microstructure that includes nanoscale plates (lamellae). The mechanical properties of the printed material compare favourably with those of Ti-6Al-4V, and of cast (and heat-treated) titanium–copper alloys.
The authors suggest that equiaxed grains are produced during solidification of the melt, and that further microstructural refinement might then occur during the cyclical temperature changes associated with the 3D-printing process. However, it is difficult to tell unambiguously whether the solidification step is the genesis of the fine grains, because the microstructures produced at high temperatures during solidification will be replaced by features that develop during subsequent solid-state phase transitions. Another plausible scenario is that columnar grains form during solidification, and that equiaxed grains are produced and refined during solid-state thermal cycling. Such grain refinement has been reported in steels5.
When steels that have a two-phase lamellar microstructure at low temperatures are heated above a critical temperature, new grains of a third phase (austenite) nucleate and grow. The two low-temperature phases then re-form on cooling5. Repeated nucleation and growth of the various phases can therefore occur under suitable conditions during thermal cycling, leading to significant grain refinement.
Alloys such as Ti-6Al-4V typically do not undergo grain refinement during thermal cycling6, because no new grains of the high-temperature phase nucleate. However, it is unclear whether new grains of high-temperature phase can nucleate and grow in Ti-6Al-4V during thermal cycling typical of additive manufacturing7, which might conceivably refine grains. Zhang and colleagues’ titanium–copper alloys have high- and low-temperature phases analogous to those of steels. Clarifying the role of nucleation and growth of these phases in grain refinement during thermal cycling should be a topic of future research.
A deeper understanding of solidification and solid-state phase transitions is clearly needed to guide the design of future alloys for additive manufacturing and to control their microstructures — although the nucleation stage is hard to study experimentally. It is also imperative that we have a better understanding of how the rapidly changing conditions during additive manufacturing influence microstructure development. In situ characterization of phase transitions and dynamic phenomena, for example using imaging and diffraction techniques in experiments that simulate the conditions of additive manufacturing8,9, might help to unveil some of the complexity of the processes involved. Such efforts are timely, and are necessary to produce optimized alloys that will lead to the widespread adoption of additive manufacturing for the production of high-performance structural parts, for which reliably high-quality microstructures and mechanical properties are of the utmost importance.
Nature 576, 41-42 (2019)