Boosted by the success of high-entropy alloys (HEAs) manufactured by conventional processes in various applications, the development of HEAs for 3D printing has been advancing rapidly in recent years. 3D printing of HEAs gives rise to a great potential for manufacturing geometrically complex HEA products with desirable performances, thereby inspiring their increased appearance in industrial applications. Herein, a comprehensive review of the recent achievements of 3D printing of HEAs is provided, in the aspects of their powder development, printing processes, microstructures, properties, and potential applications. It begins with the introduction of the fundamentals of 3D printing and HEAs, as well as the unique properties of 3D-printed HEA products. The processes for the development of HEA powders, including atomization and mechanical alloying, and the powder properties, are then presented. Thereafter, typical processes for printing HEA products from powders, namely, directed energy deposition, selective laser melting, and electron beam melting, are discussed with regard to the phases, crystal features, mechanical properties, functionalities, and potential applications of these products (particularly in the aerospace, energy, molding, and tooling industries). Finally, perspectives are outlined to provide guidance for future research.

Recent Advances on High‐Entropy Alloys for 3D Printing

Additive manufacturing of high entropy alloys: A practical review

The quick-emerging paradigm of additive manufacturing technology has revealed salient advantages in enabling the tailored-design of structural components with more exceptional performances over ordinary subtractive processing routines. As a peculiar feature, sub-micro cellular structures widely exist in additively manufactured (AM) metallic materials. This phenomenon primarily appears with high-density dislocations and segregated elements or precipitates at the cellular boundaries. The discovery of novel metastable substructures in various alloys through numerous investigations has proven their substantial effects on the engineering properties of AM components. This paper reviews the most recent research momentum regarding the formation mechanisms (elemental segregation, dislocation cell and oxide inclusion), the kinetics of the size and morphology, the growth orientation and the thermodynamic stability of these cellular structures by taking AM austenitic stainless steel as an exemplary material. Another topic of concern here is the inherent correlation between the unique cellular microstructure and the corresponding mechanical properties (strength, ductility, fatigue, etc.) and corrosion responses (passivity, irradiation damage, hydrogen embrittlement, etc.) for this category of AM materials. The design, control, and optimization of cellular structures for additive manufacturing techniques are expected to inspire new strategies for advancing high-performance structural alloy development.

About metastable cellular structure in additively manufactured austenitic stainless steels

Additive manufacturing of metals: Microstructure evolution and multistage control

As a revolutionary industrial technology, additive manufacturing creates objects by adding materials layer by layer and hence can fabricate customized components with an unprecedented degree of freedom. For metallic materials, unique hierarchical microstructures are constructed during additive manufacturing, which endow them with numerous excellent properties. To take full advantage of additive manufacturing, an in-depth understanding of the microstructure evolution mechanism is required. To this end, this review explores the fundamental procedures of additive manufacturing, that is, the formation and binding of melt pools. A comprehensive processing map is proposed that integrates melt pool energy- and geometry-related process parameters together. Based on it, additively manufactured microstructures are developed during and after the solidification of constituent melt pool. The solidification structures are composed of primary columnar grains and fine secondary phases that form along the grain boundaries. The post-solidification structures include submicron scale dislocation cells stemming from internal residual stress and nanoscale precipitates induced by intrinsic heat treatment during cyclic heating of adjacent melt pool. Based on solidification and dislocation theories, the formation mechanisms of the multistage microstructures are thoroughly analyzed, and accordingly, multistage control methods are proposed. In addition, the underlying atomic scale structural features are briefly discussed. Furthermore, microstructure design for additive manufacturing through adjustment of process parameters and alloy composition is addressed to fulfill the great potential of the technique. This review not only builds a solid microstructural framework for metallic materials produced by additive manufacturing but also provides a promising guideline to adjust their mechanical properties. 

Additively manufactured CoCrFeNiMn high-entropy alloy via pre-alloyed powder

An interstitial solute strengthened high entropy alloy (iHEA), Fe49.5Mn30Co10Cr10C0.5 (at.%), was successfully additively manufactured via selective laser melting. The as-built iHEA exhibit...

https://www.tandfonline.com/doi/pdf/10.1080/21663831.2019.1650131

Selective laser melting enabling the hierarchically heterogeneous microstructure and excellent mechanical properties in an interstitial solute strengthened high entropy alloy

High mechanical strengths and ductility of stainless steel 304L fabricated using selective laser melting

The microstructure, mechanical properties and deformation mechanisms of the 304L stainless steel (SS) additively manufactured by selective laser melting (SLM) were systematically investigated. The SLM fabricated 304L SS contains two phases (face-centered-cubic γ-austenite and body-centered-cubic δ-ferrite) and exhibits a hierarchical microstructure with length scales spanning several orders of magnitude. The hierarchical microstructure includes the melt pools and slightly elongated columnar grains at the micron scale, cellular structures decorated with a high density of dislocations at the sub-micron scale and oxides at the nanoscale. Stacking faults formed due to the residual stress in addition to the low stacking fault energy of the 304L SS (19.2 mJ/m2) while massive annealing twins were generated arising from the combined effects of residual stress and intrinsic heat treatment. The as built 304L SS exhibits a significantly enhanced strength–ductility synergy compared to that of wrought and annealed counterparts. The enhanced yield strength stems from the hierarchically heterogeneous microstructure, while the outstanding tensile elongation is ascribed to the activation of multiple deformation mechanisms, involving the dislocation activities, the formation of stacking faults and mechanical twins, and the transformation-induced plasticity.

Fern Lan Ng

Papers

Additively manufactured CoCrFeNiMn high-entropy alloy via pre-alloyed powder

Selective laser melting enabling the hierarchically heterogeneous microstructure and excellent mechanical properties in an interstitial solute strengthened high entropy alloy

High mechanical strengths and ductility of stainless steel 304L fabricated using selective laser melting

Enhanced strength-ductility synergy and transformation induced plasticity of the selective laser melting fabricated 304L stainless steel

Additive manufacturing of high-entropy alloys by thermophysical calculations and in situ alloying