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Metal additive manufacturing is a recent breakthrough technology that promises automated production of complex geometric shapes at low operating costs. However, its potential is not yet fully exploited due to the low reproducibility of quality in mass production. The monitoring of parts quality directly during manufacturing promises to solve this problem, while machine learning showed efficient performance correlating versatile manufacturing measurements with different quality grades. Today, most monitoring algorithms are based on semi- or supervised learning, thus, requiring a collection and ground-truth validation of training sets. This is costly and time consuming in real-life conditions. Our work is a feasibility study of the application of unsupervised machine learning to monitor different manufacturing regimes and quality in metal additive manufacturing. The algorithm combines the kernel Bayes rule for inference and Bayesian adaptive resonance for structuring the incoming data. Airborne acoustic emission from laser powder bed fusion is used as an algorithm input. The recognition of the main manufacturing regimes (conduction mode, stable, and unstable keyholes) are shown on real-life data, while the self-learning accuracy of developed algorithm exceeds 88 %. Our approach promises future development of plug-and-play quality monitoring systems for laser processing technology, requiring minimum modifications of the existing machines, reducing time/cost for algorithm preparation and providing continuous data driven adaptation of the algorithm to changes in manufacturing conditions.

Laser powder bed fusion (L-PBF) allows for the fabrication of samples with complex geometries based on thin struts or walls. However, only few studies have focused on the effect of these geometries on the properties of the material fabricated using this technology. In this work, we studied the impact of wall thicknesses below 1 mm on microstructure formation and mechanical properties in 316L parts fabricated by L-PBF. The size and geometry of melt pools varied significantly between different wall thicknesses due to powder denudation and local preheating, resulting in non-symmetrical melt pools for thicker samples. Furthermore, in the sub-grain microstructure, the thinnest samples consisted of solidification cells oriented almost parallel to the building direction. In the thicker walls, side branching and slender columnar grains were observed in the center lines of the melt pools. On the grain size scale, the thinnest samples consisted of finer grains with a more pronounced texture 〈100〉, while large grains growing parallel to the build direction and texture 〈101〉 were found for the thicker samples. Mechanical tests showed that the strength and ductility were higher in thicker samples, which was attributed to finer solidification cells.

Laser powder bed fusion (L-PBF) has emerged as an additive manufacturing technique that offers unprecedented design freedom. Besides being capable of producing complex and near net shape objects, L-PBF can impact tremendously the engineering materials community due to the possibility of locally manipulating metallic microstructures. Here we exploit the latter potentiality of L-PBF, to produce site-specifically tailored stainless steel components, in terms of their crystallographic texture. The tailored materials are tested and exhibit superior energy dissipation capabilities under bending deformation compared to uniformly textured materials. This is enabled by the strong dependence of the secondary hardening mechanisms, namely the deformation twinning and/or martensite formation, of these materials on the locally tuned microstructures. With the aid of finite element simulations, it is possible to identify the stress state and hence, the crystallographic orientations that facilitate twinning or martensite formation. Then, by engineering favorable crystallographic textures, matched to the complex stress state during bending, enhanced work hardening behavior is obtained. This site-specific microstructure design enabled by L-PBF provides a new pathway for the design of “smart” components that exhibit superior mechanical response under complex stress states.

Architected materials exhibit extraordinary properties in comparison with conventional materials and structures, resulting in additional functionality and efficiency by engineering the geometry in harmony with the base material. Buckling-induced architected materials (BIAMs) are a class of architected materials that exhibit a significant potential to absorb and dissipate energy owing to their local instabilities. Previous studies have shown a trade-off between energy dissipation and geometrical recoverability in metallic BIAM, which limits their use in applications that require both of these features. This study, for the first time, presents 4D printing of buckling-induced architected iron-based shape memory alloys (BIA Fe-SMAs) using laser powder bed fusion (LPBF). The results show that 4D printing of BIA Fe-SMAs can offer both energy dissipation and geometrical recoverability (i.e., recentring). The study was conducted on two different alloy compositions of Fe-17Mn-5Si-10Cr-4Ni. Quasi-static cyclic tests were performed on the two BIA Fe-SMAs, and the samples were subsequently heated to 200 °C to activate the shape memory effect (SME) of the base material. The samples could recover the residual deformations accumulated during the cyclic load owing to the SME of the base material, which led to shape-recovery ratios of 96.8 and 98.7% for the studied BIA Fe-SMAs. The results of this study demonstrate that 4D printing of BIA Fe-SMAs can yield an enhanced multi-functional behavior by combining the material's inherent functional behavior with the functionalities of the architected structure. Notably, BIA Fe-SMA samples could reconfigure their initial shape without damage after densification, which sets them apart from conventional crushable lattices.

In the present work, a Fe–Mn–Si shape memory alloy containing V and C has been fabricated by laser powder bed fusion for the first time. A pronounced pseudo-elasticity compared to a similar alloy but conventionally manufactured is achieved after aging treatment, as a result of the fine microstructure developed during the process and of the evaporation of Mn. The formation of high precipitate densities is also responsible for the improved thermo-mechanical properties. The size and density of precipitates significantly vary with aging conditions and account for important variation in material's performance. A maximum pseudo-elasticity is observed in samples aged at 750 °C for 6 h. The recovery strain upon unloading exceeds the values achieved in the conventionally manufactured alloy by more than 50%.

The fcc-γ → hcp-ε → bcc-α' martensitic transformation of a Fe-Mn-Si-based shape memory alloy fabricated by laser powder bed fusion (LPBF) is studied for the first time by in-situ neutron diffraction, in-situ high-resolution electron backscatter diffraction and digital image correlation during uniaxial tensile testing. The comparison between horizontal specimens (fabricated with no inclination on the build platform) and vertical specimens (fabricated with 90° inclination on the build platform) shows that sample orientation affects the deformation and transformation behavior of the material, considerably influencing alloy's shape memory and mechanical properties. The reason for the variation in the alloy's behavior is found in the distinct crystallographic texture generated during the LPBF process. In the vertical samples, the preferential 〈110〉 orientation of grains along the loading direction facilitates the martensitic transformation, resulting in enhanced shape memory properties and pronounced work hardening.

Tensile and creep rupture properties of crack-free CM247LC alloy, processed via laser powder bed fusion, have been characterised in this work at temperatures up to 1000°C. The subject alloy matches or even exceeds the tensile performance of its directionally solidified counterpart up to 700°C, beyond which both the strength and ductility drop off relatively rapidly with increasing temperature. It was found that the agglomeration of discrete carbides – that essentially pin the grain boundaries – facilitate the nucleation, linking and propagation of microcracks along the adjacent grain boundaries at elevated temperatures. Relatively short-term creep-rupture tests conducted at 800°C show the need for greatly improving the microstructure of this alloy to compete with its directionally solidified or conventionally cast counterparts, especially considering that this material finds extensive application up to ∼950°C in turbomachinery components. Increasing the grain size, incorporating grain boundary engineering concepts and a better understanding of carbide/γ' precipitate evolutionary characteristics can greatly help to improve the material's suboptimal creep response.

Fe-Mn-Si shape memory alloys are materials, whose functional properties strongly depend on microstructural factors such as grain size and orientation, phase fraction and chemical composition. The present study demonstrates the possibility of microstructure modification via process parameter variation on a Fe-Mn-Si based shape memory alloy fabricated by laser powder bed fusion. By varying the scan speed, samples characterized by coarse elongated grains with strong orientation along the build direction or by finer equiaxed grains without preferential orientation can be fabricated. Changes in the volume phase fraction of bcc-δ ferrite and fcc-γ austenite are also introduced by selective Mn evaporation. A direct correlation between the generated microstructure and achieved mechanical and shape memory properties is found.

Non-destructive characterization methods to observe phase transformations and thus, gain insights to transformation mechanisms in representative volumes are key for the development of advanced materials and manufacturing. Conventional methods are constrained to the surface and small sizes, thus, access to the bulk often implies tedious destructive approaches and hinders in-situ observations of phase evolution. In this work, we introduce a non-destructive technique that overcomes key limitations prevailing today for mapping the spatial distribution of magnetic phases in bulk materials. The use of polarized neutrons, being sensitive to sub-percent fractions of ferromagnetic phases and able to penetrate centimeter sized samples, enables micrometer-scale spatial and second-scale time resolutions. We demonstrate ex-situ and in-situ quantitative mapping of magnetic phases, in particular the evolution of martensite induced by uniaxial- and biaxial deformation in metastable 304 steel. The quantitative results obtained during in-situ deformation testing prove polarization contrast neutron imaging to be particularly effective in detecting small fractions of martensite. Especially during early stages of deformation where neutron diffraction, which in contrast does not provide full field spatial resolution, fails. The short exposure times and high sensitivity renders the method well suited for rapid 3D tomographic mapping and/or operando investigations of phase distributions.

Laser-powder bed fusion (L-PBF) has gained significant research interest, not only for its profound advantage of producing near-net shape complex geometries of metallic parts, but also for the possibility of producing tailored microstructures. Here we exploit the capability of manipulating the crystallographic texture by L-PBF to tailor the deformation behavior of austenitic stainless steels. In specific, by adjusting the laser power and the laser scanning speed, tailored crystallographic textures can be obtained, along the uniaxial loading direction in 304L stainless steel samples produced by L-PBF. In situ neutron diffraction and uniaxial tension and compression tests are undertaken to investigate the extent of the transformation induced plasticity effect and to correlate it with the tailored macrostructures. The influence of the initial and the evolving crystallographic texture on the deformation behavior is demonstrated and elaborated accordingly. The observed asymmetry in the deformation behavior between tension and compression is also discussed in detail.

Neutron dark-field imaging (DFI) was used to investigate the microstructure of additive manufactured steels. Several DFI methods were combined to assess the microstructure over more than two orders of magnitude in size. Different degrees of porosity and other building features were found depending on the parameters of the selective laser melting additive manufacturing process. A sample built with processing parameters yielding the lowest porosity was deformed which induced a phase transformation of the austenitic phase (fcc) into the martensitic phase (bcc). In the deformed sample an increased dark-field contrast was observed which can only be explained by accounting for the fcc-bcc phase distribution and the magnetic properties of the martensitic phase. We demonstrate that neutron dark-field imaging is well suited to not only detect build flaws like cracks but quantitatively characterize the microstructure in additive manufactured steels.

The stress recovery and cyclic deformation behaviour of Fe–17Mn–5Si–10Cr–4Ni–1(V,C) shape memory alloy (Fe-SMA) strips, which are often used for pre-stressed strengthening of structural members, were studied. The evolution of recovery stress under different constraint conditions was studied. The results showed that the magnitude of the tensile stress in the Fe-SMA member during thermal activation can have a signification effect on the final recovery stress. The higher the tensile load in the Fe-SMA (e.g., caused by dead load or thermal expansion of parent structure during heating phase), the lower the final recovery stress. Furthermore, this study investigated the cyclic behaviour of the activated SMA followed by a second thermal activation. Although the magnitude of the recovery stress decreased during the cyclic loading, the second thermal activation could retrieve a significant part of the relaxed recovery stress. This observation suggests that the relaxation of recovery stress during cyclic loading is due to a reversible phase transformation-induced deformation (i.e., forward austenite-to-martensite transformation) rather than an irreversible dislocation-induced plasticity. Retrieval of the relaxed recovery stress by the reactivation process has important practical implications as the prestressing loss in pre-stressed civil structures can be simply recovered by reheating of the Fe-SMA elements. [ABSTRACT FROM AUTHOR]

Laser powder bed fusion is an additive manufacturing technique extensively used for the production of metallic components. Despite this process has reached a status at which parts are produced with mechanical properties comparable to those from conventional production, it is still prone to introduce detrimental tensile residual stresses towards the surfaces along the building direction, implying negative consequences on fatigue life and resistance to crack formations. Laser shock peening (LSP) is a promising method adopted to compensate tensile residual stresses and to introduce beneficial compressive residual stress on the treated surfaces. Using neutron Bragg edge imaging, we perform a parametric study of LSP applied to 316L steel samples produced by laser powder bed fusion additive manufacturing. We include in the study the novel 3D-LSP technique, where samples are LSP treated also during the building process, at intermediate build layers. The LSP energy and spot overlap were set to either 1.0 or 1.5 J and 40[Formula: see text] or 80[Formula: see text] respectively. The results support the use of 3D-LSP treatment with the higher LSP laser energy and overlap applied, which showed a relative increase of surface compressive residual stress (CRS) and CRS depth by 54[Formula: see text] and 104[Formula: see text] respectively, compared to the conventional LSP treatment., (© 2021. The Author(s).)

Laser powder bed fusion (L-PBF) of copper alloys with high copper content is difficult due to the high infrared reflectivity and thermal conductivity of these alloys. In this study a simple and scalable method for coating copper powder with tin and nickel is presented, and suggested as an alloying strategy for such alloys. The coated powders were processed in a commercial L-PBF-machine at various scanning speeds. The samples made from coated powders show a lower amount of porosity compared to samples made from in-situ alloyed powders of similar composition.

Laser beam-based deposition methods such as laser cladding or additive manufacturing of metals promises improved properties, performance, and reliability of the materials and therefore rely heavily on understanding the relationship between chemical composition, rapid solidification processing conditions, and resulting microstructural features. In this work, the phase formation of four Ni-Cr-Si alloys was studied as a function of cooling rate and chemical composition using a liquid droplet rapid solidification technique. Post mortem x-ray diffraction, scanning electron microscopy, and in situ synchrotron microbeam X-ray diffraction shows the present and evolution of the rapidly solidified microstructures. Furthermore, the obtained results were compared to standard laser deposition tests. In situ microbeam diffraction revealed that due to rapid cooling and an increasing amount of Cr and Si, metastable high-temperature silicides remain in the final microstructure. Due to more sluggish interface kinetics of intermetallic compounds than that of disorder solid solution, an anomalous eutectic structure becomes dominant over the regular lamellar microstructure at high cooling rates. The rapid solidification experiments produced a microstructure similar to the one generated in laser coating thus confirming that this rapid solidification test allows a rapid pre-screening of alloys suitable for laser beam-based processing techniques.

Laser welding is a key technology for many industrial applications. However, its online quality monitoring is an open issue due to the highly complex nature of the process. This work aims at enriching existing approaches in this field. We propose a method for real-time detection of process instabilities that can lead to defects. Hard X-ray radiography is used for the ground truth observations of the sub-surface events that are critical for the quality. A deep artificial neural network is applied to reveal the unique signatures of those events in wavelet spectrograms from the laser back-reflection and acoustic emission signals. The autonomous classification of the revealed signatures is tested on real-life data, while the real-time performance is reached by means of parallel computing. The confidence of the quality classification ranges between 71% and 99%, with a temporal resolution down to 2 ms and a computation time per classification task as low as 2 ms. This approach is a new paradigm in the digitization of industrial processes and can be exploited to provide feedbacks in a closed-loop quality control system.

We present combined in situ X-ray diffraction and high-speed imaging to monitor the phase evolution upon cyclic rapid laser heating and cooling mimicking the direct energy deposition of Ti-6Al-4V in real time. Additive manufacturing of the industrially relevant alloy Ti-6Al-4V is known to create a multitude of phases and microstructures depending on processing technology and parameters. Current setups are limited by an averaged measurement through the solid and liquid parts. In this work the combination of a micro-focused intense X-ray beam, a fast detector and unidirectional cooling provide the spatial and temporal resolution to separate contributions from solid and liquid phases in limited volumes. Upon rapid heating and cooling, the β ↔ α' phase transformation is observed repeatedly. At room temperature, single phase α' is observed. Secondary β-formation upon formation of α' is attributed to V partitioning to the β-phase leading to temporary stabilization. Lattice strains in the α'-phase are found to be sensitive to the α' → β phase transformation. Based on lattice strain of the β-phase, the martensite start temperature is estimated at 923 K in these experiments. Off-axis high speed imaging confirms a technically relevant solidification front velocity and cooling rate of 10.3 mm/s and 4500 K/s, respectively.

The joining of macroscopic films of vertically aligned multiwalled carbon nanotubes (CNTs) to titanium substrates is demonstrated by active vacuum brazing at 820 °C with a Ag-Cu-Ti alloy and at 880 °C with a Cu-Sn-Ti-Zr alloy. The brazing methodology was elaborated in order to enable the production of highly electrically and thermally conductive CNT/metal substrate contacts. The interfacial electrical resistances of the joints were measured to be as low as 0.35 Ω . The improved interfacial transport properties in the brazed films lead to superior electron field-emission properties when compared to the as-grown films. An emission current of 150 μ A was drawn from the brazed nanotubes at an applied electric field of 0.6 V μ m -1 . The improvement in electron field-emission is mainly attributed to the reduction of the contact resistance between the nanotubes and the substrate. The joints have high re-melting temperatures up to the solidus temperatures of the alloys; far greater than what is achievable with standard solders, thus expanding the application potential of CNT films to high-current and high-power applications where substantial frictional or resistive heating is expected.

The quaternary system Al-Fe-Si-Ti was studied in the iron-rich corner for sections at 50, 60 and 70 at.% Fe at 900 °C. Isothermal phase equilibria were investigated by a combination of optical microscopy, X-ray powder diffraction (XRD) followed by Rietveld refinement and Electron Probe Microanalysis (EPMA). Phase boundaries of the phases, in particular of the Laves phase (Fe 2 Ti) and of the extended phase field of A2/B2/D0 3 , were investigated. Selected samples containing the Laves phase and the B2 phase were characterized by microhardness measurements at different compositions throughout the quaternary homogeneity range of the phases.

The ternary phase diagram Al-Ge-Ni was investigated between 0 and 50 at.% Ni by a combination of differential thermal analysis (DTA), powder- and single-crystal X-ray diffraction (XRD), metallography and electron probe microanalysis (EPMA). Ternary phase equilibria and accurate phase compositions of the equilibrium phases were determined within two partial isothermal sections at 400 and 700 °C, respectively. The two binary intermediate phases AlNi and Al 3 Ni 2 were found to form extended solid solutions with Ge in the ternary. Three new ternary phases were found to exist in the Ni-poor part of the phase diagram which were designated as τ 1 (oC24, CoGe 2 -type), τ 2 (at approximately Al 67.5 Ge 18.0 Ni 14.5 ) and τ 3 (cF12, CaF 2 -type). The ternary phases show only small homogeneity ranges. While τ 1 was investigated by single crystal X-ray diffraction, τ 2 and τ 3 were identified from their powder diffraction pattern. Ternary phase reactions and melting behaviour were studied by means of DTA. A total number of eleven invariant reactions could be derived from these data, which are one ternary eutectic reaction, six transition reactions, three ternary peritectic reactions and one maximum. Based on the measured DTA values three vertical sections at 10, 20 and 35 at.% Ni were constructed. Additionally, all experimental results were combined to a ternary reaction scheme (Scheil diagram) and a liquidus surface projection.

In this investigation, the cyclic deformation behaviour of commercially pure titanium was characterized in axial stress controlled constant amplitude and load increase tests, as well as in rotating bending tests. The influence of different clinically relevant surface treatments (polishing, thermal and anodic oxidizing) on the fatigue behaviour was investigated. All tests were realized in oxygen-saturated Ringer's solution. The cyclic deformation behaviour was characterized by mechanical hysteresis measurements. In addition, the change of the free corrosion potential and the corrosion current during the fatigue tests in simulated physiological media indicated such types of surface damage as slip bands, microcracks and oxide film ablation. Microstructural changes on the specimen surfaces were examined by scanning electron microscopy.

In this investigation, the cyclic deformation behaviour of the binary titanium alloys Ti-6Al-4V and Ti-6Al-7Nb was characterized in axial stress-controlled constant amplitude and load increase tests as well as in rotating bending tests. The influence of different clinically relevant surface treatments (polishing, corundum grit blasting, thermal and anodic oxidizing) on the fatigue behaviour was investigated. All tests were realized in oxygen-saturated Ringer's solution. The cyclic deformation behaviour was characterized by mechanical hysteresis measurements. In addition, the change of the free corrosion potential and the corrosion current during testing in simulated physiological media indicated surface damages such as slip bands, intrusions and extrusions or finally microcracks. Microstructural changes on the specimen surfaces were examined by scanning electron microscopy (SEM).