This study investigates the influence of hydrogen charging on the mechanical behaviour and deformation mechanisms of a nanoparticle-reinforced CoCrFeNi medium-entropy alloys fabricated via laser powder bed fusion (LPBF). Changing the precursor nanoparticles from TiN to TiO2, resulted in a change in the microstructure from columnar to equiaxed, while all the printing parameters remained the same. Nanoindentation mapping was conducted on both the microstructures before and after electrochemical hydrogen charging to quantify any variation in mechanical behaviour. The columnar matrix grain morphology reveals progressive matrix softening with hydrogen exposure, with hardness decreasing from 3.53 ± 0.16 GPa to 2.84 ± 0.19 GPa after 24 h charging, attributed to hydrogen-enhanced localized plasticity (HELP). In contrast, the same alloy with an equiaxed microstructure exhibited minimal softening, demonstrating that grain morphology governs hydrogen susceptibility. Scanning transmission electron microscopy analysis from beneath the nanoindents revealed that while the uncharged state exhibited predominant deformation twinning, the hydrogen-charged specimens displayed spatially heterogeneous deformation features characterized by well-defined subgrains and deformation twins, indicating a net softening effect where HELP is partially offset by twin-mediated hardening. Thermal desorption spectroscopy yielded a binding energy of 24 kJ/mol, which is indicative of dislocations acting as the dominant hydrogen traps given the extremely high defect density inherent to LPBF processing. The dispersed nanoparticles maintain their strengthening effect while showing no evidence of preferential hydrogen accumulation at particle-matrix interfaces. These findings establish the critical role of grain boundary tortuosity and interfacial trap density in controlling embrittlement resistance in additively manufactured nanocomposite alloys.

Most of the high-strength aluminium alloys for Laser Powder Bed Fusion (L-PBF) in research are based on conventional hardenable Al-Cu, Al-Mg-Si and Al-Zn-Mg alloys that have issues with hot-cracking during the L-PBF process. This study investigates the effect of alloying elements on two novel Al-Cr-Mo-Sc-Zr alloys (Scancromal (R) variants) with a good, crack-free processability and strength-ductility combination. A layer thickness of 100 mu m was used, highlighting the high productivity and good processability of Al-Cr alloys. Heat treatments and hardness measurements were conducted to investigate peak age heat treatment and revealed a relation between Sc/Zr-ratio and peak aged temperature. SEM observations of small precipitates at grain boundaries and fracture surfaces contributed to understanding the changing fracture mechanism from less ductile (2.4-10.2% elongation) to very ductile (14.1-19% elongation). Tensile tests at elevated temperatures and creep tests showed a positive effect of Cr and Mo on high temperature strength. High-temperature properties, especially creep results of Al-Cr alloys and Scalmalloy (R) (Al-Mg-Mn-Sc-Zr) are presented for the first time and emphasize the increased heat-resistance by Cr and Mo. The threshold stress, below which theoretically no creep should occur, of the presented Scancromal (R) variants is at least 5 times higher (142-189 MPa) compared to Scalmalloy (R) (28 MPa). A comprehensive comparison of the results to literature values of AlSi10Mg, other Al-Cr alloys and high temperature Al alloys gives valuable insight on how the aging response, microstructure, ductility and high-temperature strength of this type of alloy is influenced by its alloying elements.

Improving the sustainability of metals and alloys is essential for slowing down global warming. The reason is that their extraction and production stand for about 40 % of all greenhouse gas emissions in the industrial sector. This motivates new alloy design and processing criteria such as the (1) preferred use of abundant and sustainable alloying elements and (2) improved material tolerance against impurity intrusion from recycling. In this context, additive manufacturing (AM) is attractive, through rapid solidification, capable of quenching impurities into a solid solution state, avoiding formation of large intermetallics and introduction of metastable phases. Here, by using this approach we show how iron, an important scrap-related contaminant in aluminum alloys, can be turned from a harmful into a valuable ingredient. Specifically, the addition of Mo and Si facilitates the formation of beneficial metastable body-centred cubic (BCC) Al12(Fe, Mo)3Si phase, instead of more stable but detrimental intermetallic variants commonly observed in Al-Fe alloys. The as-built microstructures have excellent thermal stability, tested up to 200 hours at 300 degrees C, because of low diffusivity of Fe and the formation of Zr shell. We find that for such supersaturated alloys, two issues are important, namely (a) the heterogeneous microstructures in the as-built condition, (b) the evolution of metastable precipitates during heating. We suggest that this type of approach help to guide sustainable alloy design via AM and other rapid solidification processes.

The present study investigates the effect of coupled thermo-mechanical phenomena, referred to thermal evolution and plastic deformation, during hot stamping of precipitation hardenable aluminum alloy AA7075 by using an in-situ approach of differential cooling in combination with electron microscopy. To explore the intrinsic geometry-dependent local strain distribution of the tailored microstructure, digital image correlation coupled with tensile tests is used. The precipitates size distribution, morphology and types are investigated using electron channeling contrast imaging and differential scanning calorimetry. Results indicate that the forming tool temperature and contact time within closed forming tools are influential parameters for tailoring the microstructure distribution. A sigmoidal curve-like hardness distribution is found for the forming tool temperature difference of 24 degrees C-300 degrees C and 24 degrees C-350 degrees C, showing a steep decrease from 180 HV5 to 123 HV5. This is explained by the detected coarse and lath-shaped quench-induced precipitates in the soft zone and formation of fine strengthening eta '-phase type in the hard zone in areas with higher supersaturation. Transition zones contain a mixture of precipitates structures of varying morphology and types, with a lower aspect ratio on the heated section. This fact leads to an inhomogeneous plastic deformation mainly located in the heated zones and consequently a high strain hardening rate due to the increased inter-particle-spacing and higher dislocation mean free path. Post-cooling after differential cooling generates higher mechanical properties for water-quench compared to air-cooling, as it prevents the nucleation of quench-induced precipitates and segregation of solute atoms to grain boundaries.

Nanoparticle reinforced metallic composites manufactured using laser powder bed fusion (LPBF) provide an economically viable avenue to obtain high strength near-net shaped critical components in automotive and aviation industry. In this study, the equiatomic compositionally complex alloy (CCA) CoCrFeNi is manufactured by LPBF with two types of reinforcing particles, titanium nitride (TiN) and titanium oxide (TiO2). The reinforcing particles are introduced with varying size and volume concentration to the CCA powder fulfilling two purposes – improving the flowability of the feedstock and to cause nanoparticle strengthening, as demonstrated here using nanoindentation. We focus on the microstructure and texture evolution of all the alloys, and the phase transitioning of the particles. TiN nanoparticles of all sizes dissolve in the melt pool and uniformly precipitate as TiO2. We observed distinctive core-shell nanoparticle formation with higher TiN content that also resulted in 45 % higher tensile strength. The successful integration of homogenously distributed nanoparticles without inducing cracks or defects and with superior mechanical properties signifies a leap forward in the fabrication of high-performance metal matrix composites like the commercial oxide dispersion strengthened (ODS) alloys.

Market availability of aluminum alloys for laser powder bed fusion (L-PBF) is still highly limited in comparison to conventional manufacturing processes. The demand for high-strength but inexpensive alloys specifically designed for L-PBF is high. This demand has led to research on a variety of adapted conventional alloys which are still limited to utilize the full potential of L-PBF. Scalmalloy® (Al–Mg–Sc–Zr) satisfies the demand for high-strength L-PBF-alloys but needs a high energy input and has troubles with evaporation of Mg. Scancromal® (Al–Cr–Sc–Zr) is a novel alloying system for L-PBF and was first introduced in 2019 with the possibility of higher build rates and comparable strengths to Scalmalloy®. In this paper, a more economic low Sc-containing version of Scancromal® is presented. A parameter study was performed for 100 m layer thickness reaching high build rates of about 47 cm^3 h^-1 . Hardness tests for different parameters were carried out and showed a stable process window with a hardness comparable to AlSi10Mg. Additionally, two-dimensional multilayer process simulations showed a potential for increasing the layer thickness to 150 m and therefore a significant increase in build rate of up to 70 cm^3 h^-1 highlighting the high productivity potential of Al–Cr alloys for L-PBF.

Particle-reinforced aluminum-matrix-composites (AMCs) promise enhanced mechanical properties while maintaining the lightweight potential of aluminum. Fabricating AMCs near-net-shape is desirable because it avoids the drawback of the difficult machining. Laser powder bed fusion (PBF-LB) has the potential to achieve this goal. An economically attractive powder blending approach was used to modify AlSi7Mg0.6 powder with TiC nanoparticles. The resulting blend was processed in a PBF-LB machine to fabricate AMCs. This paper shows how to maximize the grain refinement potential of TiC nanoparticles by using a high mixing time of 1215 min to break up agglomerates. For a better understanding of the process related microstructure, investigations of the TiC distribution in blend and composites have been conducted by SEM and EDS. The formation of TiC microparticles formed by sintering of agglomerates are an integral part of the final microstructure. Measuring the overall carbon content of the specimens reveals a loss of TiC during PBF-LB. The root causes are discussed in terms of agglomeration and smoke generation and countermeasures are proposed. The presented approach is an effective way to introduce nanoparticles and to refine the microstructure. It can also be adapted to other material systems to modify the grain size and reduce the texture.

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The hydrogen embrittlement (HE) is investigated for the titanium alloyTi-6Al-4V, processed by different manufacturing methods. The mechanical characteristics of conventionally cold rolled, as-built laser powder bed fused (L-PBF) and heat treated L-PBF Ti-6Al-4V are compared under in-situ electrochemical hydrogen charging at room temperature. The microstructure determines the susceptibility to HE, which is examined by a combination of in-situ constant extension rate tensile tests, X-ray tomography, X-ray diffraction, hydrogen melt extraction, scanning electron microscopy, and electron backscatter diffraction. The presence of the beta phase facilitates an increased hydrogen uptake capacity, resulting in an enhanced HE-susceptibility. In-situ hydrogen assisted failure of a duplex alpha-beta microstructure in conventionally cold rolled Ti-6Al-4V occurs via a strain rate dependent mechanism, i.e. hydrogen enhanced localised plasticity (HELP) at fast strain rates and a combination of HELP and stress induced hydride formation (SIHF) at slow strain rates. Meanwhile, the martensitic alpha' microstructure of as-built L-PBF Ti-6Al-4V shows clear evidence of prevalent hydrogen degradation via SIHF after slow straining. However, hydrogen significantly deteriorates the mechanical behaviour in L-PBF material due to a fatal synergetic interaction with porosity related stress fields, dominating failure characteristics after hydrogenation. The same effect applies for the inherent porosities in heat treated L-PBF Ti-6Al-4V with a fine alpha-beta microstructure. Therefore, knowledge on these microstructural interactions with hydrogen will allow the development of better HE resistant Ti-6Al-4V.

Metal powders in laser-powder-bed-fusion (L-PBF) often exhibit cohesive flow resulting from interparticle adhesion. Nanoparticle dry-coating can improve powder flowability and promote powder layer densification. A Co25Cr25Fe25Ni25 metal powder (20–90 µm) is dry-coated with TiN particles with a diameter of 16 nm at low concentrations of up to 69 ppm. The dynamic angle of repose decreased and bulk powder density increased compared to the uncoated state from 49 ° and 4.67 g/cm3 to 29 ° and 4.81 g/cm3 with dry-coating of TiN, respectively. UV/Vis spectroscopy showed negligible alterations by TiN additions on the powder light absorption. The powder modifications strongly affected their corresponding processability in L-PBF and reduced the melt pool signatures of the in situ detected confocal single-color pyrometer signal as well as ex situ measured melt pool depth and width. With increasing flowability, a significant decrease in thermal emission and melt pool size was observed. The results demonstrate the impact of powder flowability and bulk powder density on the quality of L-PBF parts when particle interactions are actively modified.

Laser additive manufacturing (LAM) offers high flexibility in the production of customized and geometrically complex parts. The technique receives great interest from industry and academia but faces substantial challenges regarding processability and insufficient mechanical properties of LAM-produced material. One reason is that currently mainly conventional alloys are being used in LAM, which were developed for different processes such as casting. Since these alloys are not optimized for the specific process conditions encountered in LAM such as fast cooling and cyclic re-heating, they cannot be expected to perform ideally in such processes regarding processability and resulting mechanical properties. Here we present the development of a new, simple ternary Fe-NiTi maraging-type alloy tailor-made for LAM. We used compositionally graded samples to screen Ti compositions from 0 to 21 at. % and efficiently identify promising microstructures and mechanical properties. Under LAM solidification conditions the desired mainly martensitic microstructure needed for a maraging steel formed at Ti compositions ranging from 0 to 7 at. %. Within this composition range, the intended microstructure is formed and additionally some unique process conditions of LAM such as cyclic re-heating can be exploited. Specifically, in-situ phase transformations can be controlled during LAM, via the thermal history. At higher Ti compositions two different eutectic microstructures with different primary phases were found that show a high hardness of up to 700 HV.

The interaction between hydrogen and laser powder bed fused (L-PBF) Ti-6Al-4V is investigated, in combination with the influence of (post-)processing parameters, i.e. the building orientation (horizontal vs. vertical) and the surface finish (as-built vs. polished). After an extensive characterisation of the intrinsic properties of the additive manufactured material (including porosity and surface roughness), its interaction with hydrogen is thoroughly assessed via electrochemical hydrogen charging at room temperature. The stressed martensitic α’ microstructure shows beneficial properties against hydrogen assisted degradation. The higher effective surface area of the as-built material compared to the polished condition offers more opportunities for hydrogen absorption. Hydrogen uptake is also influenced by the orientation of the prior β grains, depending on the building orientation. Furthermore, porosities tend to accumulate hydrogen. Therefore, a flawless (sub)surface layer and a maximal density are essential for optimal safe use of L-PBF Ti-6Al-4V in hydrogen-rich environments.

Titanium alloys are particularly sensitive to temperature during additive manufacturing processes, due to their dual phase microstructure and sensitivity to oxygen uptake. In this paper, laser powder bed fusion (LPBF) was used in conjunction with a heated substrate bed at 100 °C, 570 °C and 770 °C to produce specimens of Ti–6Al–4V, to investigate the change in mechanical properties and segregation of alloying elements. An initial increase in ductility was observed when increasing the temperature from 100 °C to 570 °C, followed by a significant loss in ductility when samples were produced at 770 °C. A suite of multi-scale characterisation techniques revealed that the as-printed microstructure was drastically different across the range of temperatures. At 100 °C, α + α ′ phases were identified. Deformation twinning was extensively observed in the a phase, with Al and V segregating at the twin interfaces. At 570 °C (the most ductile sample), α ′, α and nano-particles of β were observed, with networks of entangled dislocations showing V segregation. At 770 °C, no martensitic α ′ was identified. The microstructure was an α + β microstructure and an increased volume fraction of tangled dislocations with localised V segregation. Thermodynamic modelling based on the Gibbs-free energy of formation showed that the increased V concentration at dislocations was insufficient to locally nucleate β phase. However, b-phase nucleation at grain boundaries (not dislocations) caused pinning of grain boundaries, impeding slip and leading to a reduction in ductility. It is likely that the increased O-content within specimens printed at increased temperatures also played a key role in high-temperature embrittlement. Building operations are therefore best performed below sub-transus temperatures, to encourage the growth of strengthening phases via solute segregation, and the build atmosphere must be tightly controlled to reduce oxygen uptake within the samples.

Designing a metastable microstructure with a coherent nano-sized precipitation phase in the matrix is an effective strategy in improving the strength of materials. Recently, the rapid fusion and solidification cycle associated with laser-based additive manufacturing (AM) has emerged as a promising strategy to design unique microstructures with lattice distortion, solute segregation, and nano-sized precipitations. In this study, the evolution of nano-sized Cu-rich clusters in an AM-processed Fe-15Cu-15Ni alloy (wt.%) was investigated by conducting multiscale microstructural characterization. The results reveal that nano-sized Cu-rich clusters were generated inside the matrix due to a phase decomposition induced by the intrinsic heat treatment during the AM process. The heat energy generated by the laser beam not only initiated Cu-rich cluster formation, but also induced precipitation growth. Therefore, the average Cu-rich cluster size increased with an increase in the volumetric energy density. The hardness of the AM-processed Fe-15Cu-15Ni alloy at first increased with an increase in the energy density until a medium energy density level (140 J/mm(3)), due to formation of Cu-rich clusters. The hardness decreased with further increase in energy density (185 J/mm(3)), due to the Cu-rich cluster growth and retained austenite. The results reveal that laser-based AM successfully induces nano-cluster without the need for post-treatment and that the mechanical properties of materials can be optimized by adjusting the processing parameters in a way to enable nano-sized cluster and phase formation.

The temperature distribution, geometry and size of the melt pool, and solidification parameters were computed using the heat transfer and material flow model for the directed energy deposition process. The thermal cycle and melt pool size were computed for different process parameters such as laser power, deposition speed, and laser beam radius across the multiple layers for deposition of Al-0.5Sc-0.5Si alloy. The computed thermal cycles at various locations from the centerline and melt pool size showed fair agreement with the experimentally measured result. The computed thermal gradient and solidification rate were mapped over the solidification map, which was in agreement with the experimentally observed microstructure. The transition in solidification morphology with varying melt pool depth and process parameters were fairly observed inside the melt pool. The fraction of equiaxed solidification morphology increases with the reduction in a thermal gradient.

Disadvantageous complex residual stress distributions are common in parts manufactured by laser-based powder bed fusion metals (PBF-LB/M). Thus, the residual stress state determination is essential for understanding the part's limitations. One residual stress measurement technique applicable to PBF-LB/M parts is the slitting method. This destructive technique allows through-thickness measurements and copes well with discontinuities in the material, yet is limited to one-dimensional stress profiles. Conversely, other stress measurement techniques, e.g. X-ray diffraction, are restricted to the part's surface. This article presents an inexpensive and straightforward stress determination approach that combines an implementation of the slitting method with X-ray diffraction measurements to create a continuous two-dimensional residual stress map along a part's cross-section. The approach was numerically validated using finite element models that simulate, on the one hand, the PBF-LB/M process and, on the other hand, the measuring process; further, it was experimentally tested in PBF-LB/M AlSi10Mg samples. The use of linear elastic fracture mechanics allowed a straightforward formulation of the approach, which enabled a high degree of automation. The accurate stress distribution results and the correlation with the simulations and previous studies demonstrate the approach's robustness and effectiveness for complex residual stress states determination in two dimensions.

The influence of hydrogen on the mechanical behavior of Inconel 718 fabricated by laser powder-bed-fusion was investigated through a series of tensile experiments. Samples subjected to two different post-fabrication heat treatments, viz. direct aging (DA) and homogenization plus aging (HA), were tested. Detailed microstructural characterization showed that a solidification substructure including a high density of dislocations and precipitates prevails in the DA sample while the HA sample is free from such a substructure. The DA sample exhibited a comparatively higher strength, but a lower resistance to hydrogen embrittlement. By recourse to a statistical analysis of the hydrogen-assisted cracks, the severe hydrogen embrittlement of the DA sample was proven to be due to the significant portion of hydrogen-assisted intergranular cracks that occurred without the aid of slip localization. These results are discussed in terms of the changes in microstructure upon heat treatments, and their influences on the hydrogen trapping sites.

The significant strengthening of selectively laser melted (SLM) 304L stainless steel that occurs due to high-pressure torsion (HPT) is examined by recourse to detailed nanomechanical and microstructural characterization. In the as-built alloy, dislocation hardening is the main strengthening mechanism. After HPT, however, the synergistic combination grain refinement and martensitic transformation dominate the strength of the alloy. In the nanocrystalline regime, the dominance of the grain boundary-mediated plasticity retards the martensite-induced hardening.