Additive manufacturing of metals: a brief review of the characteristic microstructures and properties of steels,Ti-6Al-4V and high-entropy alloys

Abstract

We present a brief review of the microstructures and mechanical properties of selected metallic alloys processed by additive manufacturing (AM). Three different alloys, covering a large range of technology readiness levels, are selected to illustrate particular microstructural features developed by AM and clarify the engineering paradigm relating process–microstructure–property. With Ti-6Al-4V the emphasis is placed on the formation of metallurgical defects and microstructures induced by AM and their role on mechanical properties. The effects of the large in-built dislocation density, surface roughness and build atmosphere on mechanical and damage properties are discussed using steels. The impact of rapid solidification inherent to AM on phase selection is highlighted for high-entropy alloys. Using property maps, published mechanical properties of additive manufactured alloys are graphically summarized and compared to conventionally processed counterparts.     

Microstructures and liquid phase separation in multicomponent CoCrCuFeNi high entropy alloys

The microstructures of high entropy alloys of the system CoCrCuFe_xNi and CoCrCuFeNi_x (where x indicates the molar ratio, which, where not specified, is 1) have been investigated. Many Cu rich spheres were evident in the microstructure of CoCrCuFe_0.5Ni and CoCrCuFeNi_0.5 alloys, which indicates that liquid phase separation had occurred before solidification. During liquid phase separation, the original liquids separated into two liquids: Cu rich and Cu depleted. In contrast, in other alloys (x?=?1.0, 1.5 and 2.0), typical dendritic and interdendritic structures are obtained. Cu and/or Cr rich precipitates, with various morphologies, can be seen in the interdendritic region. Additionally, Cu rich nanoparticles and Cr rich bird shaped structures can be observed in the Cu rich spheres. Sluggish cooperative diffusion causes the element segregation and formation of nanoprecipitates in the microstructures. The calculated positive mixing enthalpies of CoCrCuFe_0.5Ni and CoCrCuFeNi_0.5 alloys are likely reasons for their liquid phase separation.     

Effect of Mo and Ti on the microstructure and microhardness in AlCoFeNiMoTi high entropy alloys prepared by mechanical alloying and conventional sintering

In this investigation, the synthesis of equiatomic AlCoFeNi, AlCoFeNiMo, AlCoFeNiTi, and AlCoFeNiMoTi high entropy alloys, fabricated by mechanical alloying and conventional sintering processes is presented aiming to elucidate the effect of Mo and Ti additions on the properties of the AlCoFeNi base system. X-ray diffraction studies revealed that after 15 h of milling, only BCC and FCC structures were formed. It was also found that by increasing the crystallite size after sintering, phase transformations and composition variations were observed for all the systems studied but BCC and FCC structures prevailed. Further, the addition of the different alloying elements had a significant effect on the microhardness of the HEAs and particularly, the addition of Mo and Ti to form the AlCoFeNiMoTi system presented the highest value of 894 HV0.2. Finally, it was also found that Mo- containing alloys presented considerable porosity.     

Microstructure and mechanical properties of CoCrNi-Mo medium entropy alloys:Experiments and first-principle calculations

The effect of Mo additions on the microstructures and mechanical properties of CoCrNi alloys was investigated,meanwhile,ab initio calculations are performed to quantitatively evaluate the lattice distortion and stacking fault energy(SFE).The yield strength,ultimate tensile strength,and elongation of(CoCrNi)₉₇Mo₃alloy are 475 MPa,983 MPa and 69%,respectively.The yield strength is increased by~30%and high ductility is maintained,in comparison with CoCrNi alloy.Besides the nano-twins and dislocations,the higher density of stacking faults is induced during the tensile deformation for(CoCrNi)₉₇Mo₃alloy.Ab initio calculation results indicate the mean square atomic displacement(MSAD)and SFE value of(CoCrNi)₉₇Mo₃alloy is 42.6 pm²and-40.4 mJ/m²at 0 K,respectively.The relationship between mechanical properties and MSAD,SFE for various multiple principal element alloys is discussed.     

Microstructure and corrosion behaviour of FeCoNiCuSnx high entropy alloys

Abstract

The FeCoNiCuSn_x alloys with different Sn contents are prepared, the microstructure and the corrosion behaviour of the alloys are investigated. When Sn content is lower than 0.09, FeCoNiCuSn_x alloys consist of a single FCC phase. While Sn content of the alloy is 0.09, a small quantity of BCC structure is present. The FeCoNiCuSn_x alloys have a wider passive region in the NaOH solution. FeCoNiCuSn_x alloys exhibit a better corrosion resistance in NaCl solution than 304 stainless steel, the corrosion resistance of FeCoNiCuSn_0.04 alloy is best among all the alloys. The corrosion resistance of FeCoNiCuSn_x alloys in NaOH solution is lower than that of 304 stainless steel, the corrosion resistance of FeCoNiCuSn_0.03 alloy is best among all FeCoNiCuSn_x alloys.     

Metal Alloys for Fusion‐Based Additive Manufacturing

Metal additive manufacturing (AM) is an innovative manufacturing technique, which builds parts incrementally layer by layer. Thus, metal AM has inherent advantages in part complexity, time, and waste saving. However, due to its complex thermal cycle and rapid solidification during processing, the alloys well suit and commercially used for metal AM today are limited. Therefore, it is important to understand the alloying strategy and current progress with materials performance to consider alloy development for metal AM. This review presents the current range of alloys available for metal AM, including titanium, steel, nickel, aluminum, less common alloys (including Mg alloys, metal matrix composites alloys, and low melting point alloys), and compositionally complex alloys (including bulk metallic glasses and high entropy alloys) with a focus on the relationship between compositions, processing, microstructures, and properties of each alloy system. In addition, some promising alloy systems for metal AM are highlighted. Approaches for designing and optimizing new materials for metal AM have been summarized.

     

Influence of Al content on thermal stability of nanocrystalline AlxCoCrFeNi high entropy alloys at low and intermediate temperatures

Thermal stability of mechanically alloyed nanocrystalline Al_xCoCrFeNi (x = 0, 0.3, 0.6, 1 mol) high entropy alloys (HEAs) has been investigated for the low and intermediate temperature range of 673–1073 K. Single phase FCC structure is observed in the as milled CoCrFeNi. A mixture of FCC and BCC phases is exhibited by × = 0.3, 0.6 and 1, alloys where the volume fraction of BCC increases with increasing Al content. Phase evolution in heat-treated Al_xCoCrFeNi HEAs proceeds via increasing BCC fraction at 673 K, followed by subsequent reduction at elevated temperatures. For each alloy, the major phase observed in as milled condition and it is retained even after prolonged exposure at the 1073 K. Al favors the formation of the BCC phase due to its high affinity to form ordered B2 structures with constituent elements Co, Fe and Ni. Thermal exposure of Al_xCoCrFeNi HEAs also leads to the formation of Cr₇C₃, owing to the higher negative free energy of carbide formation for Cr among other constituents. Transmission electron microscopy (TEM) investigations substantiated that nanostructure of milled powder is maintained even after the heat treatment. Grain growth factor for quinary HEAs is relatively lower than quaternary CoCrFeNi owing to their slower rates of diffusion.     

Alloying behaviour,thermal stability and phase evolution in quinary AlCoCrFeNi high entropy alloy

An equiatomic quinary AlCoCrFeNi high entropy alloy (HEA) has been synthesized by mechanical alloying. Milled powder after 30?h shows good chemical homogeneity and refined morphology with a mean particle size of ～4?μm. Solid solution phase with BCC crystal structure (a?=?2.89?±?0.02?Å) has been confirmed from XRD and transmission electron microscopy in the as-synthesized high entropy alloy. The milled alloy powder is not thermally stable. Differential scanning calorimetric (DSC) thermogram of 30?h milled powder exhibits the presence of a small peak at ～600?°C (873?K) with a thermal shift near the peak. This thermal shift indicates the diffusive type of phase transformation in this alloy while heating. The analysis of the in-situ heating X-ray diffraction patterns at various temperatures extends support to the diffusive nature of the phase transformation. Upon heat treatment, the disordered BCC solid solution phase partially transforms to Ni₃Al prototype L1₂ phase which precipitates at a lower temperature (350?°C (623?K)) as observed by in-situ XRD experiments. However, at high temperature annealing (575–800?°C (848–1073?K)) the evolution of a partially ordered BCC phase (B2) with lattice parameter (a?=?2.87?±?0.02?Å), and L1₂ phase (a?=?3.58?±?0.05?Å), along with tetragonal σ phase (a?=?8.8?Å and c?=?4.53?Å) are observed. Similar types of phases have also been identified after annealing and microwave sintering at 800?°C (1073?K) & 900?°C (1173?K) respectively. The transformation of ordered BCC phases along with two intermetallics such as L1₂ phase and σ phase suggests that the evolution of the high entropy phase in the milled condition leads to a combination of high entropy and medium entropy phases in the annealed condition.     

Strengthening the mechanical properties and wear resistance of CoCrFeMnNi high entropy alloy fabricated by powder metallurgy

In this study, an equiatomic CoCrFeMnNi high entropy alloy (HEA) was fabricated by a rapid solidified gas atomization process. Subsequently, the high-energy mechanical milling was carried out to further refine the microstructure of pre-alloyed powder to improve the sintering ability and strengthening of HEAs. The microscopic results show that the powder morphology significantly changed from spherical to flatten, flake, irregular, and partially spherical shape with increasing milling time. The XRD results exhibited HEA bulks consisting of major FCC and minor Cr₇C₃ phases. The hardness of HEA bulks increased from 270±10 Hv to 450±10 Hv with increasing milling time, while the compressive yield strength increased from 370 MPa to 1050 MPa due to grain boundary strengthening and dislocation strengthening. Meanwhile, the lowest coefficient of friction ～0.283 and specific wear rate ～1.03×10^-5 mm³/Nm were obtained for the 60 min milled HEA due to increased surface hardness and oxidation behavior. The developed powder metallurgy approach could be considered as a promising way to improve the strength and wear resistance when compared to the conventional processed CoCrFeMnNi HEAs.     

Nickel-titanium shape memory alloys made by selective laser melting:a review on process optimisation

Selective laser melting (SLM) is a mainstream powder-bed fusion additive manufacturing (AM) process that creates a three-dimensional (3D) object using a high power laser to fuse fine particles of various metallic powders such as copper, tool steel, cobalt chrome, titanium, tungsten, aluminium and stainless steel. Over the past decade, SLM has received significant attention due to its capability in producing dense parts with superior mechanical properties. As a premier shape memory alloy, the nickel-titanium (NiTi) shape memory alloy is attractive for a variety of biomedical applications due to its superior mechanical properties, superelasticity, corrosion resistance and biocompatibility. This paper presents a comprehensive review of the recent progress in NiTi alloys produced by the SLM process, with a particular focus on the relationship between processing parameters, resultant microstructures and properties. Current research gaps, challenges and suggestions for future research are also addressed.The full text can be downloaded at https://link.springer.com/article/10.1007/s40436-021-00376-9     

Formation rules of single phase solid solution in high entropy alloys

A new empirical rule to form single phase solid solution structured high entropy alloys was proposed by integrating three dependent parameters, i.e. atomic size difference δ, mixing enthalpy ΔH_mix and valence electron concentration VEC. It was found that the single phase face centred cubic solid solution will form if δ?<?4.27%, ??7.27?kJ?mol^??1?<?ΔH_mix?<?4?kJ?mol^??1 and VEC>8, while the single phase body centred cubic solid solution will form in the case of δ?<?4.27%, ??7.27?kJ?mol^??1?<?ΔH_mix?<?4?kJ?mol^??1 and VEC?<?6.87. This empirical rule was verified by typical high entropy alloys that have been reported.     

NbMoTiVSi_x refractory high entropy alloys strengthened by forming BCC phase and silicide eutectic structure

In order to improve mechanical properties of refractory high entropy alloys,silicide was introduced and NbMoTiVSi_x(x=0,0.1,0.2,0.3,and 0.4,molar ratio) refractory high entropy alloys are prepared by vacuum arc melting.Phase composition,micro structure evolution and mechanical properties were systematically studied.Results show that the silicide phase is formed in the alloys with addition of silicon,and the volume fraction of silicide increases from 0 to 8.3 % with increasing of silicon.Microstructure observation shows that the morphology of dendrite changes from columnar to near equiaxed,eutectic structure is formed at grain boundaries and composed of secondary BCC phase and silicide phase.The average length of the primary and second dendrites decreases with the increasing of silicon.Whereas,the ratio of eutectic structure increases from 0 to 19.8 % with the increment of silicon.The refinement of microstructure is caused by heterogeneous nucleation from the silicide.Compressive tests show that the yield and ultimate strength of the alloys increases from 1141.5 MPa to 2093.1 MPa and from 1700.1 MPa to 2374.7 MPa with increasing silicon content.The fracture strain decreases from 24.7 %-11.0 %.Fracture mechanism is changed from ductile fracture to ductile and brittle mixed fracture.The improvement of the strength is caused by grain bounda ry strengthening,which includes more boundaries around primary BCC phase and eutectic structure in grain boundary,both of them is resulted from the formation of silicide.     

Strengthening FCC-CoCrFeMnNi high entropy alloys by Mo addition

In order to strengthen the face-centered-cubic(FCC) type CoCrFeMnNi high entropy alloys(HEAs), different contents of Mo(0–16 at.%, similarly hereinafter) were alloyed. Phase evolution, microstructure,mechanical properties and related mechanism of these HEAs were systematically studied. The results show that sigma phase is appeared with addition of Mo, and the volume fraction of it increases gradually from 0 to 66% with increasing Mo content. It is found that Mo is enriched in sigma phase, which indicates that Mo element is beneficial to form sigma phase. Compressive testing shows that the yield strength of the alloys increases gradually from 216 to 765 MPa, while the fracture strain decreases from 50%(no fracture) to 19% with increasing of Mo. The alloy exhibits the best compressive performance when Mo content reaches 11%, the yield strength, fracture strength and fracture strain are 547 MPa, 2672 MPa and44% respectively. The increased volume fraction of sigma phase plays an important role in improving the compressive strength of(CoCrFeMnNi)_(100-x)Mo_xHEAs.     

Effect of titanium on the ductilization of Fe-B alloys with high boron content

Binary Fe-B alloys with a high boron concentration are hard but brittle. The brittleness is caused by the formation of a cast microstructure which consists of a continuous network of brittle Fe₂B. In the present investigation, we demonstrated that, by alloying with titanium, and particularly with a titanium-to-boron atom ratio of 0.5, a fine and uniform microstructure can be formed. Also, as a result of the microstructural modification, the alloy is improved from being completely brittle (∼ zero elongation) in the Ti-free alloy to a 4% tensile elongation. The preliminary result indicates that additional rolling at 1223 K further enhances the ductility to over 16%.     

Wire + Arc Additive Manufacturing

Depositing large components (>10?kg) in titanium, aluminium, steel and other metals is possible using Wire + Arc Additive Manufacturing. This technology adopts arc welding tools and wire as feedstock for additive manufacturing purposes. High deposition rates, low material and equipment costs, and good structural integrity make Wire+Arc Additive Manufacturing a suitable candidate for replacing the current method of manufacturing from solid billets or large forgings, especially with regards to low and medium complexity parts. A variety of components have been successfully manufactured with this process, including Ti–6Al–4V spars and landing gear assemblies, aluminium wing ribs, steel wind tunnel models and cones. Strategies on how to manage residual stress, improve mechanical properties and eliminate defects such as porosity are suggested. Finally, the benefits of non-destructive testing, online monitoring and in situ machining are discussed.     

A novel high-entropy alloy with multi-scale precipitates and excellent mechanical properties fabricated by spark plasma sintering

Body-centered-cubic (BCC) high entropy alloys (HEAs) usually exhibit high strength but poor ductility. To overcome such strength-ductility trade-off, a novel (FeCr)₄₅(AlNi)₅₀Co₅ HEA was presented in this paper, which was designed and fabricated with mechanical alloying (MA) followed by spark plasma sintering (SPS), and has a heterogeneous microstructure with multi-scale precipitates. Electron microscopy characterization revealed that the sizes of the precipitates range from nano (<300 nm), sub-micron (300～800 nm) to micron (>1 μm). The bulk HEA exhibits excellent mechanical properties, of which the compressive yield strength, fracture strength, and plasticity at room temperature can reach 1508 MPa, 3106 MPa and 30.4 %, respectively, which are much higher than that of most HEAs prepared by Powder Metallurgy reported in the literatures, suggesting that the HEA developed is highly promising for engineering applications. The excellent mechanical properties of the bulk HEA can be attributed to that the multi-scale precipitates are fully coherent with the matrix, which could reduce the misfit strain at the interface, and relieve the stress concentration during deformation.     

Elemental partitioning as a route to design precipitation-hardened high entropy alloys

Precipitation-hardened high entropy alloys (HEAs) with carefully tuned compositions have shown excellent mechanical properties,demonstrating great potential for engineering applications.However,due to the lack of precise multiple phase diagrams,the composition design of multi-principal-component HEAs still inevitably relies on the extremely time-consuming trial-and-error approach.The present study,on the basis of powerful composition quantification ability of atom probe tomography (APT) technology,proposed a framework to guide the quantitative design of precipitation-hardened HEAs.In this framework,the elemental partitioning was used as a crucial route to avoid the thermodynamic challenge of designing precipitation-hardened HEAs.As a case study,the role of Ti/Al ratio in the design of γ-γ'HEAs was predicted through the proposed framework and then validated by experimental studies.The framework predicted that when the total content of Ti and Al is fixed,a higher Ti/Al ratio makes γ-γ'HEA stronger.APT and mechanical results agreed well with these predictions and validated the feasibility of the framework.These findings provided a new route to design the precipitation-hardened alloys and a deeper insight into the design of γ-γ'HEA.     

Progress in additive manufacturing on new materials: A review

Recent efforts and advances in additive manufacturing (AM) on different types of new materials are presented and reviewed. Special attention is paid to the material design of cladding layers, the choice of feedstock materials, the metallurgical behavior and synthesis principle during the AM process, and the resulted microstructures and properties, as well as the relationship between these factors. Thereafter, the trend of development in the future is forecasted, including: Effects of the particles size and size distribution of powders; Approaches for producing fine microstructures; Opportunities for creating new materials by AM; Wide applications in reconditioning of damaged components; Challenges for deep understanding and applications of the AMed new materials. The idea of “Develop Materials” or “Create Materials” by AM is highlighted, but a series of scientific, technological and engineering problems remain to be solved in future.     

Additive manufacturing of copper alloys: influence of process parameters and alloying elements

Owing to the physical properties of copper and its alloys it is challenging to achieve good surface quality and low porosity by the widely used laser-based additive manufacturing processes. This paper deals with the role of alloy composition, powder size and process parameters in additive manufacturing with laser beam melting machine (with power up to 100 W). Test parts were produced in pure copper and CuNiSi(Cr) alloys. The porosity was investigated as a function of different process parameters and powder size ranges. The effects of the alloy physical properties (reflectivity, thermal conductivity, melting range and surface tension) are discussed. Moreover, the effect of thermal treatment on the properties of CuNiSi parts was assessed in conventional two-step heat treatments.     

A novel Cu-doped high entropy alloy with excellent comprehensive performances for marine application

High entropy alloy(HEA) has attracted great interests as one of the promising multifunctional materials in marine applications. However, Cu as an effective biocide tends to form segregation in HEA, which could deteriorate corrosion and induce brittle fracture. Herein, we report a strategy to tailor the existing form of Cu in HEA from undesired large-scale segregation to uniform distribution with dispersed nanoscale precipitation, while retaining the unique structure characteristics of HEA. Eliminating Cu segregation improves toughness and avoids serious corrosion in the grain boundary. Uniform distribution with dispersed nanoscale precipitation of Cu further enhances the antifouling and lubricating abilities of Cu-doped HEA. Tailored AlCoCrFeNiCu_(0.5) HEA in this work has excellent comprehensive properties combining good mechanical properties, outstanding antifouling abilities, superior resistance to corrosion and wear. Furthermore, the corresponded mechanisms are discussed in terms of Cu-segregation-eliminated,nanoscale-Cu-precipitate-forming and comprehensive properties.