Metal-based additive manufacturing, or three-dimensional (3D) printing, is a potentially disruptive technology across multiple industries, including the aerospace, biomedical and automotive industries. Building up metal components layer by layer increases design freedom and manufacturing flexibility, thereby enabling complex geometries, increased product customization and shorter time to market, while eliminating traditional economy-of-scale constraints. However, currently only a few alloys, the most relevant being AlSi10Mg, TiAl6V4, CoCr and Inconel 718, can be reliably printed; the vast majority of the more than 5,500 alloys in use today cannot be additively manufactured because the melting and solidification dynamics during the printing process lead to intolerable microstructures with large columnar grains and periodic cracks. Here we demonstrate that these issues can be resolved by introducing nanoparticles of nucleants that control solidification during additive manufacturing. We selected the nucleants on the basis of crystallographic information and assembled them onto 7075 and 6061 series aluminium alloy powders. After functionalization with the nucleants, we found that these high-strength aluminium alloys, which were previously incompatible with additive manufacturing, could be processed successfully using selective laser melting. Crack-free, equiaxed (that is, with grains roughly equal in length, width and height), fine-grained microstructures were achieved, resulting in material strengths comparable to that of wrought material. Our approach to metal-based additive manufacturing is applicable to a wide range of alloys and can be implemented using a range of additive machines. It thus provides a foundation for broad industrial applicability, including where electron-beam melting or directed-energy-deposition techniques are used instead of selective laser melting, and will enable additive manufacturing of other alloy systems, such as non-weldable nickel superalloys and intermetallics. Furthermore, this technology could be used in conventional processing such as in joining, casting and injection moulding, in which solidification cracking and hot tearing are also common issues.

3D printing of high-strength aluminium alloys

Mechanical properties in the semi-solid state and hot tearing of aluminium alloys

Solidification microstructures and solid-state parallels: Recent developments, future directions

A criterion for cracking during solidification

Using deep neural network with small dataset to predict material defects

A new criterion for the appearance of hot tears in metallic alloys is proposed. Based upon a mass balance performed over the liquid and solid phases, it accounts for the tensile deformation of the solid skeleton perpendicular to the growing dendrites and for the induced interdendritic liquid feeding. This model introduces a critical deformation rate (
 
$$\dot \varepsilon _{p,\max } $$

 ) beyond which cavitation, i.e., nucleation of a first void, occurs. As should be expected, this critical value is an increasing function of the thermal gradient and permeability and a decreasing function of the viscosity. The shrinkage contribution, which is also included in the model, is shown to be of the same order of magnitude as that associated with the tensile deformation of the solid skeleton. A hot-cracking sensitivity (HCS) index is then defined as 
$$\dot \varepsilon _{_{p,\max } }^{ - 1} $$

 . When applied to a variable-concentration aluminum-copper alloy, this HCS criterion can reproduce the typical “Λ curves” previously deduced by Clyne and Davies on a phenomenological basis. The calculated values are in fairly good agreement with those obtained experimentally by Spittle and Cushway for a non-grain-refined alloy. A comparison of this criterion to hot cracks observed in ring-mold solidification tests indicates cavitation depression of a few kilo Pascal and tensile stresses in the coherent mushy zone of a few mega Pascal. These values are discussed in terms of those obtained by other means (coherency measurement, microporosity observation, and simulation). Even though this HCS criterion is based only upon the appearance of a first void and not on its propagation, it sets up for the first time a physically sound basis for the study of hot-crack formation.

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A New Hot-Tearing Criterion

In order to quantify the cooling conditions undergone by an ingot during direct-chill (DC) casting, thermocouples were immersed in the liquid pool and consequently entrapped in the solid, thus monitoring the temperature of the metal during its descent. Assuming steady-state thermal conditions, the time-dependent temperatures measured by these thermocouples were then converted into spacedependent temperature profiles. These values were the input of a Maximum A Posteriori (MAP) inverse method described by Rappaz et al.,[1] which has been adapted in this case to steady-state thermal conditions. This MAP method permits the deduction of the temperature-dependent thermal conductivity of the alloy, initially, and then of the highly nonuniform heat-flux distribution along the ingot rolling faces, in a second step. The obtained values are in good agreement with literature and clearly reflect the widely different boundary conditions associated with primary cooling (contact with the mold) and secondary cooling (water jet).

Determination of thermophysical properties and boundary conditions of direct chill-cast aluminum alloys using inverse methods

Laser metal forming is an attractive process for rapid prototyping or the rebuilding of worn parts. However, large tensile stress may arise in layers deposited by laser melting of powder. A potential solution is to preheat the substrate before and during deposition of layers to introduce sufficient contraction during cooling in the substrate to modify the residual stress distribution in the deposited layers. To demonstrate the value of this approach, specimens were prepared by depositing stellite F on a stainless steel substrate with and without preheating. Residual stresses were computed by numerical simulation and measured using the crack compliance method. For non-preheated specimens simulation and experiment agreed well and showed that extremely high residual tensile stresses were present in the laser melted material. By contrast, pre-heated specimens show high compressive stresses in the clad material. However, in this case the numerical simulation and experimental measurement showed very different stress distribution. This is attributed to out of plane deformation due to the high compressive stresses which are not permitted in the numerical simulation. A ‘‘strength of materials’’ analysis of the effect of out of plane deformation was used to correct the simulation, Agreement with experimental results was then satisfactory.

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The Computation and Measurement of Residual Stresses in Laser Deposited Layers

A numerical program has been developed in order to calculate the time-dependent thermal gradient and isotherm velocity within a solidifying pool obtained by stationary laser treatment. The heat diffusion equation is solved for an axisymmetric geometry with an enthalpy formulation using a standard FDM implicit scheme. The program has been used to investigate to which extent the cellular dendritic growth orientation is affected by the direction and amplitude of the thermal gradient.Experiments were carried out on Al-1 mass%Si dendritic single crystals oriented by Laue back reflection technique and cut along precise crystallographic orientations prior to laser irradiation and on Al-3 mass%Cu eutectic polycrystalline alloys. Deviation of cellular dendritic growth direction from crystallographic and heat flux directions are reported. In addition, the comparison of the microstructural spacings with theoretical predictions outlines the influence of crystallography on the microstructural development during stationary laser treatment.

M. Gremaud

Papers

A New Hot-Tearing Criterion

Determination of thermophysical properties and boundary conditions of direct chill-cast aluminum alloys using inverse methods

The Computation and Measurement of Residual Stresses in Laser Deposited Layers

Influence of Crystallographic and Heat Flow Orientations on Growth of Cellular Dendrites.

State‐of‐the‐Art in the Modelling of Aluminium and Copper Continuous Casting Processes