Metallurgy

No more tears for metal 3D printing

3D printing could revolutionize manufacturing processes involving metals, but few industrially useful alloys are compatible with the technique. A method has been developed that might open up the 3D printing of all metals. See Letter p.365

Additive manufacturing of metals, also known as metal 3D printing, was not long ago seen only as a means of making prototypes of mechanical components for industry. But it is now considered to be a potentially transformative manufacturing process for many sectors. This change has been driven by diverse factors1, such as the complex geometries that can be achieved, the relatively low number of parts required for components made in this way, and the fact that manufacturing timescales are much shorter than those of conventional manufacturing methods. However, the limited range of industrially useful alloys that can be processed in the additive manufacturing of metals is a barrier to its wide adoption. On page 365, Martin et al.2 report an approach that expands the range of metallic materials that can be used, taking lessons from a much older manufacturing process — casting.

Metal additive manufacturing (AM) often involves the deposition of layers of an alloy feedstock in the form of powders or wires, which are melted together by a rapidly moving heat source to form a solid mass. Successive layers are built up to produce a 3D component. The rate of solidification is often an order of magnitude higher than that seen during conventional casting techniques, and the process of building up layers causes non-uniform cooling, which, in turn, leads to large temperature gradients or thermal stresses in the alloy.

The picture is further complicated by the solidification processes that occur in alloys used for engineering applications, such as high-strength aluminium and nickel superalloys. These materials tend to have relatively wide temperature ranges over which liquid and solid phases can coexist, and often solidify as either columnar grains or dendrites (multi-branched crystal formations) under the conditions imposed by AM. As solidification proceeds, the liquid content of the alloy decreases, and the residual liquid becomes confined to channels between the cells or dendrites, where it forms a film. Localized contraction of the solid can then cause cavities to form in the liquid film; if these cavities propagate, they generate cracks known as hot tears3. The resulting material can therefore contain both a columnar microstructure and numerous cracks — neither of which would be acceptable for engineering applications.

Martin et al. now suggest that a possible way of suppressing hot tearing in AM would be to change the dominant solidification mode from directional columnar growth to non-directional growth that produces 'equiaxed' grains — which have roughly equal width, length and height. However, once again, the local solidification conditions that occur during AM work against this: steep thermal gradients are generated, which suppress the repeated formation (nucleation) of the tiny crystal seeds that would allow equiaxed grains to form. The challenge is to find a way of allowing this nucleation process to occur under AM conditions.

Previous attempts to solve this problem have generally focused on changing the processing parameters of AM, such as the speed, the power of the laser or electron beam used to heat the alloy feedstock, or the pattern in which the printer moves to build up an object, to disrupt the conditions that promote columnar growth (see refs 4 and 5, for example). Unfortunately, it has proved extremely difficult to exert sufficient control over such process variables to promote nucleation and hence develop the desired microstructure.

Luckily, a potential solution to this problem can be found from casting — in which additives called inoculants are commonly mixed into a liquid metal to 'seed' nuclei on which new crystals can grow, even in the presence of steep thermal gradients and high solidification velocities. The first reported instance of an addition being made to deliberately manipulate microstructure was in 1906, when ferro-silicon was added to a ladle of cast iron6. Since then, developments in casting have enabled the production of strong materials that lack holes or tears, and which contain equiaxed microstructures, using high-performance engineering alloys7.

Inoculants are normally added to an alloy in its molten state. This poses a problem in AM, because the melt pool is only tens of micrometres long, and exists at any given point for just tens of microseconds8. Martin and colleagues' solution allows a precise quantity of inoculant to be delivered to such melt pools on this timescale.

The authors demonstrate the potential of their approach using two aluminium alloys that are well characterized and widely used: Al7075, a wrought (mechanically worked) material used in aerospace applications and which is not well suited to melt processing, and Al6061, a high-strength alloy used for casting. Crucially, both are difficult to process by AM. Martin et al. first modified the surface of the feedstock alloy powders by decorating them with nanoparticulate inoculants, which were tailored to the composition and crystal lattices of each alloy. These 'functionalized' powders were then used in a standard AM machine, following manufacturer-recommended processing conditions. For comparison, the authors also tested alloys that had not been surface-modified using the same processing conditions.

The difference in the microstructures obtained for the two types of sample was dramatic. The samples made using unmodified alloys contained large columnar grains and a high number density of cracks, as might be expected (Fig. 1a). By contrast, the functionalized powders produced fine, equiaxed microstructures that were free of cracks (Fig. 1b). The mechanical properties of the inoculated Al7075 were also markedly better than when it was made from the unmodified powder, and approached those of the same alloy in the wrought condition.

Figure 1: Improving the properties of alloys produced using additive manufacturing.
figure1

a, When many industrially useful alloys are processed using additive manufacturing (3D printing), the resulting materials contain large cracks and columnar grains (as shown here for the aluminium alloy Al7075). This makes them unsuitable for engineering applications. b, Martin et al.2 attached nanoparticles to the surfaces of the granules in Al7075 powder. No cracking was observed when this powder was used as a feedstock for additive manufacturing, and the microstructure consisted of equiaxed particles (which have roughly equal widths, lengths and heights). Scale bars, 20 micrometres. (Images from ref. 2.)

There is still some way to go, however, before this becomes the 'go-to' manufacturing technology for aerospace applications. In this context, the resistance of materials to fatigue — weakening caused by repeatedly applied loads — is of equal, if not greater, importance to their strength9. More work is needed to better understand and control the fatigue resistance of materials produced using AM. Another barrier to uptake by industry is the slow speed of current metal AM processes. Methods are emerging that deliver a step change in the speed of 3D printing of polymers10, and the race is on to achieve the same for metals, but this presents a major technological challenge.

In the meantime, however, Martin and colleagues have identified an approach that allows alloys to be made more suitable for AM. Although they used aluminium alloys, they note that the method could be readily extended to other industrially useful alloy classes, such as non-weldable nickel alloys, superalloys and intermetallics. This might take some time to achieve, however, because inoculants for these materials remain elusive. But if inoculants can be found to functionalize the surfaces of powders of these alloys, then we really would be moving towards the 3D printing of any metal. Footnote 1

Notes

  1. 1.

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Todd, I. No more tears for metal 3D printing. Nature 549, 342–343 (2017). https://doi.org/10.1038/549342a

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