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Over the last decade, the enormous potential of metal additive manufacturing (AM) processes has led these technologies to establish their position in many industries. Much effort is being made toward their widespread application; however, much remains to be done to achieve full industrialization of these processes. Therefore, many companies, research centers and universities are investing in comprehensive research and development activities in order to further promote the industrialization of metal AM. This review traces the progress of metal AM technologies through an investigation of patents. In the present study, beam-based metal AM patents were searched through the Orbit Intelligence database. First, the number of patents per year was studied, indicating that, as expected, there is strong growth in AM patenting activities. The patents were afterward examined in order to highlight the key players in the field, and it was found that the main players investing in this market are: multidisciplinary companies, AM machine producers, end users working, especially in the aerospace sector, universities and research centers. The patents were then analyzed to understand the technology domains covered by each key player and their trend of investments. Finally, the patents in the field of Materials and Metallurgy were studied individually to identify the main topics faced by the most used alloy classes: Al-, Ni- and Ti-based alloys and steels. The extensive study of these patents clearly indicated that the main gaps to fill in metal AM are strongly material dependent and that it is possible to find correlations between the alloy classes, their main industrial applications and their specific AM processability issues. The current study provides insights into global trends that can help industrial markets to identify the right investment direction and research to identify topics for future investigation.

Conformal cooling coils have been developed during the last decades through the use of additive manufacturing (AM) technologies. The main goal of this study was to analyze how the presence of an internal channel that could act as a conformal cooling coil could affect compressive strength and quasi-elastic gradient of AlSi10Mg lattice structures produced by laser powder bed fusion (LPBF). Three different configurations of samples were tested in compression at 25 °C and 200 °C. The reference structures were body centered cubic (BBC) in the core of the samples with vertical struts alongZ(BCCZ) lattices in the outer perimeter, labelled as NC samples. The main novelty consisted in inserting a straight elliptical channel and a 45° elliptical channel inside the BCCZ lattice structures, labelled as SC and 45C samples respectively. All the samples were then tested in as-built (AB) condition, and after two post process heat treatments, commonly used for AlSi10Mg LPBF industrial components, a stress relieving (SR) and a T6 treatment. NC lattice structures AB exhibited an overall fragile fracture and therefore the SC and 45C configuration samples were tested only after thermal treatments. The test at 25 °C showed that all types of samples were characterized by negligible variations in their quasi-elastic gradients and yield strength. On the contrary, the general trend of stress-strain curves was influenced by the presence of the channel and its position. The test at 200 °C showed that NC, SC and 45C samples after SR and T6 treatments exhibited a metal-foam like deformation.

Among recently developed high-strength and lightweight alloys, the high-performance Scalmalloy® certainly stands out for laser powder bed fusion (LPBF) production. The primary goal of this study was to optimize the Scalmalloy® LPBF process parameters by setting power values suitable for the use of lab-scale machines. Despite that these LPBF machines are commonly characterized by considerably lower maximum power values (around 100 W) compared to industrial-scale machines (up to 480 W), they are widely used when quick setup and short processing time are needed and a limited amount of powder is available. In order to obtain the optimal process parameters, the influence of volumetric energy density (VED) on the sample porosity, microstructure and mechanical properties was accurately studied. The obtained results reveal the stability of the microstructural and mechanical behaviour of the alloy for VEDs higher than 175 Jmm−3. In this way, an energy-and-time-saving choice at low VEDs can be taken for the LPBF production of Scalmalloy®. After identifying the low-power optimized process parameters, the effects of the heat treatment on the microstructural and mechanical properties were investigated. The results prove that low-VED heat-treated samples produced with an LPBF lab-scale machine can achieve outstanding mechanical performance compared with the results of energy-intensive industrial production.

Directed Energy Deposition (DED) is an additive manufacturing process that enables the production of large metal components by melting the feedstock material while being deposited. An improvement of the production speed of this process would further increase its applicability in many industrial fields. The DED building rate is strictly related to the building parameters adopted, in particular to the laser spot diameter, which also affects the build accuracy and the surface quality of the components. The possibility of using a variable laser spot would result in a significant increase in the production rate in bulky zones, while also providing a good surface quality where needed. In the present work, an oscillating scanning strategy was used to create a large apparent laser spot (+ 170% of the nominal value) to produce 316L stainless steel samples via DED. The optimisation of the DED parameters with the oscillating strategy was performed using the single scan tracks (SSTs) approach. The morphologies of the SSTs obtained with different process parameters were assessed and the geometrical features related to the melt pools were analysed in order to select the most suitable X and Z displacements for the production of the cubic samples. The analyses of the cubes revealed that, if the correct overlap among nearby scans is selected, it is possible to obtain dense samples with all the oscillating diameters tested. Finally, comparing the building rate and powder efficiency values confirmed that this method can accelerate the building process and improve its overall performance. Graphical Abstract

This work focuses on the effect of different heat treatments on the Ti-6Al-4V alloy processed by means of electron beam melting (EBM). Super β-transus annealing was conducted at 1050 °C for 1 h on Ti-6Al-4V samples, considering two different cooling paths (furnace cooling and water quenching). This heat treatment induces microstructural recrystallization, thus reducing the anisotropy generated by the EBM process (columnar prior-β grains). Subsequently, the annealed furnace-cooled and water-quenched samples were aged at 540 °C for 4 h. The results showed the influence of the aging treatment on the microstructure and the mechanical properties of the annealed EBM-produced Ti-6Al-4V. A comparison with the traditional processed heat-treated material was also conducted. In the furnace-cooled specimens consisting of lamellar α+β, the aging treatment improved ductility and strength by inducing microstructural thickening of the α laths and reducing the β fraction. The effect of the aging treatment was also more marked in the water-quenched samples, characterized by high tensile strengths but limited ductility due to the presence of martensite. In fact, the aging treatment was effective in the recovery of the ductility loss, maintaining high tensile strength properties due to the variation in the relative number of α/α’ interfaces resulting from α’ decomposition. This study, therefore, offers an in-depth investigation of the potential beneficial effects of the aging treatment on the microstructure and mechanical properties of the EBM-processed super β-transus heat-treated Ti-6Al-4V alloy under different cooling conditions.

Their high strength-to-weight ratio, good corrosion resistance and excellent thermal and electrical conductivity have exponentially increased the interest in aluminium alloys in the context of laser-based powder bed fusion (PBF-LB/M) production. Although Al-based alloys are the third most investigated category of alloys in the literature and the second most used in industry, their processing by PBF-LB/M is often hampered by their considerable solidification shrinkage, tendency to oxidation, high laser reflectivity and poor powder flowability. For these reasons, high-strength Al-based alloys traditionally processed by conventional procedures have often proved to be unprintable with additive technology, so the design and development of new tailored Al-based alloys for PBF-LB/M production is necessary. The aim of the present work is to explore all the challenges encountered before, during and after the PBF-LB/M processing of Al-based alloys, in order to critically analyse the solutions proposed in the literature and suggest new approaches for addressing unsolved problems. The analysis covers the critical aspects in the literature as well as industrial needs, industrial patents published to date and possible future developments in the additive market.

Directed energy deposition (DED) process is recognized as an alternative technology to produce the complex-shape AISI 316L components. The critical production step in this technology is the optimization of process parameters that can directly affect the final properties of the components. To optimize the process parameters, the residual defects of specimens produced with different combinations of process parameters are evaluated, and the optimum condition is chosen. Therefore, the residual defects assessment is a vital step in finding the optimum process parameters; therefore, this evaluation should be carried out carefully. One of the main issues in the production of AISI 316L by DED process is oxidation during the process that should be considered besides the other defects such as porosity and cracks. However, the identification between the oxides and porosities is not an easy task, and so this study aims to provide more clear insight into the evaluation of pores and oxides in DED 316L samples. The outcomes of this work show that at the best process parameters suitable for a porosity-free sample, there are some oxides that can be misinterpreted as porosity and consequently deteriorate the mechanical properties of the dense sample.

Laser powder bed fusion (LPBF) is an additive manufacturing technology that implies using metal powder as a raw material. The powders suitable for this kind of technology must respect some specific characteristics. Controlled gas atomization and post-processing operations can strongly affect the final properties of the powders, and, as a consequence, the characteristics of the bulk components. In fact, a complete characterization of the powders is mandatory to fully determine their properties. Beyond the most used tests, such as the volume particle size distribution (PSD) and flowability, the PSD number, the Hausner ratio and the oxidation level can give additional information otherwise not detectable. The present work concerns the complete characterization of two AlSi10Mg powders: a commercial-grade gas atomized powder and a laboratory-scale gas atomized counterpart. The laboratory-scale gas atomization allows to better manage the amount of the fine particles and the oxidation level. As a consequence, a higher particle packing can be reached with an increase in the final density and tensile strength of the LPBF bulk samples.

During Laser Powder-Directed Energy Deposition (LP-DED), many complex phenomena occur. These phenomena, which are strictly related to the conditions used during the building process, can affect the quality of the parts in terms of microstructural features and mechanical behavior. This paper investigates the effect of building parameters on the microstructure and the tensile properties of AISI 316L stainless-steel samples produced via LP-DED. Firstly, the building parameters were selected starting from single scan tracks by studying their morphology and geometrical features. Next, 316L LP-DED bulk samples built with two sets of parameters were characterized in terms of porosity, geometrical accuracy, microstructure, and mechanical properties. The tensile tests data were analyzed using the Voce model and a correlation between the tensile properties and the dislocation free path was found. Overall, the data indicate that porosity should not be considered the unique indicator of the quality of an LP-DED part and that a mechanical characterization should also be performed.

This study deals with the Inconel 625 (IN625) alloy reinforced with micro-TiC particles processed by laser powder bed fusion. The microstructure and hardness in the as-built and solution-annealed states were investigated. The microstructures of the as-built IN625 and IN625/TiC states were primarily made up of columnar grains along the building direction. After the solution annealing at 1150 °C for 2 h, the IN625 alloy consisted of equiaxed grains due to recrystallization and grain growth. On the contrary, the solution-annealed IN625/TiC composite still presented columnar grains. Therefore, the TiC particles hinder the recrystallization, indicating higher microstructure stability for the composite. For the IN625/TiC composite, both the reduced alteration of the grains and the more intensive formation of carbides prevent a remarkable hardness reduction in the solution-annealed state.

Directed energy deposition is an additive manufacturing technology which usually relies on prototype machines or hybrid systems, assembled with parts from different producers. Because of this lack of standardization, the optimization of the process parameters is often a mandatory step in order to develop an efficient building process. Although, this preliminary phase is usually expensive both in terms of time and cost. The single scan approach allows to drastically reduce deposition time and material usage, as in fact only a stripe per parameters combination is deposited. These specimens can then be investigated, for example in terms of geometrical features (e.g. growth, width) and microstructure to assess the most suitable process window. In this work, Ti-6Al-4V single scans, produced by means of directed energy deposition, corresponding to a total of 50 different parameters combinations, were analyzed, focusing on several geometrical features and relative parameters correlations. Moreover, considering the susceptibility of the material to oxygen pick-up, the necessity of an additional shielding gas system was also evaluated, by comparing the specimens obtained with and without using a supplementary argon flow. A process window, which varies according to the user needs, was found together with a relationship between microstructure and process parameters, in both shielding scenarios.Graphic Abstract

Laser Powder Bed Fusion (LPBF) is an additive manufacturing technology which has been the subject of thorough research and successfully adopted in several industrial sectors. Among all the processable classes of materials, titanium alloys are especially interesting due to their favourable combination of mechanical properties and corrosion resistance. Most of the literature focuses on Ti-6Al-4V, although there are other alloys which are widely applied in fields that can benefit from the advantages of LPBF techniques, such as Ti-6Al-2Sn-4Zr-6Mo, thus far not investigated for this technology. This alloy is generally preferred to Ti-6Al-4V for the production of some components in the aerospace industry, mostly due to its superior strength. In this work, the most suitable process window for this alloy was investigated. Samples produced with two different combinations of process parameters, located in the selected process window, were then thoroughly studied in order to assess the effect of building conditions on the microstructure, phases and mechanical properties of the as-built and heat-treated material. To do so, an X-ray diffraction analysis was conducted with the aim of determining the phase composition and lattice parameters. Moreover, microstructural features, such as α” needles and α lath widths, were analysed in order to correlate the thermal history of the process to the final microstructure of the specimens. Furthermore, the hardness and the tensile properties of the alloy processed by LPBF were quantified and compared with the data available in the literature relative to conventional manufacturing technologies.

Blown powder additive manufacturing technologies are not restricted to the use of a process chamber. This feature allows to build larger components with respect to conventional powder bed processes. This peculiarity is mostly promising for manufacturing large components or repairing/rebuilding parts of large systems. The main downside of using an open environment, even if a protective shielding gas system is adopted, is the lack of control of process atmosphere. This is particularly critical for titanium alloys which are very sensitive to oxygen/nitrogen pick-up, they have a detrimental effect on ductility, by causing embrittlement and possibly leading to the formation of cracks. It is then important to address how environmental factors, such as process atmosphere and platform temperature, impact not only on the processability but also on the final component properties, both from a compositional and mechanical point of view. The correlations between these environmental factors and microstructure, interstitials content, grain size, and hardness were investigated. Moreover, the Hall&ndash, Petch equation was then adopted to additive manufacturing microstructures, characterized by a columnar grain morphology, and used to further investigate the relationship intercurring between grains and hardness and how different microstructures might influence this correlation.

The growing need to develop cost-effective and efficient alloys requires enhancing the flexibility of the current production methods. Additive manufacturing (AM), as an emerging technology, has re-shaped the manufacturing strategies and largely influenced the industrial production lines. The feedstock, the main concern with regard to the powder-based AM methods, are mostly in the form of pre-alloyed powders. Pre-alloyed powders have a narrow composition range, suffer from limited availability and are expensive, making additive manufacturing of new alloys inflexible and costly. A growing number of works in the literature are dedicated to AM in-situ alloying, i.e. employing pure elemental blends as opposed to pre-alloyed ones for producing samples using additive manufacturing. This strategy gives added flexibility to the AM methods by benefiting from the laser local energy and paves the way towards an on-demand alloy design framework that is proportionate to the growing needs of the industry. This review is intended to shed light on the key components of AM in-situ alloying process, from initial blend preparation to requirements for the final composition homogeneity.

Background:Silicon carbide ceramics obtained by reactive infiltration of silicon (SRI) have many industrial applications especially involving severe and high temperature conditions. In this study, the oxidation behavior in air of Si-SiC-ZrB2systems at a high temperature (1500°C) for dwelling times of up to 48 hours was examined.Methods:The oxidation process was analyzed on the basis of elemental maps and X-ray diffraction patterns taken, respectively, on the core and on the surface of the specimens, together with weight gains and the average thicknesses of the resulting scale. Further, flexural strength at room temperature was examined as a function of different oxidation times.Results:The main chemical reactions and phase transformations involved in the oxidation process are reported. Several oxides were detected on the surface: zirconia, silica, zircon and 3-zirconium monoxide. All of the samples showed a parabolic oxidation kinetics, suggesting that the controlling mechanism was the diffusion; however, even after 48 hours, the oxidation process was not finished – indeed, all of the samples continued to gain weight.Conclusions:The oxidation of Si-SiC-ZrB2material produced via SRI was slower compared with previously investigated ZrB2-SiC composites processed with a different techniques and tested in similar conditions. The oxidation mechanism was found to be consistent with the convection cells model.

Banded and discontinuous eutectic microstructures were found in melt quenched ribbons and laser melted single tracks of an Al-3%wt Er alloy, respectively. The bands occurring in ribbons are made of alternate primary α-Al phase and eutectic. Their formation is well described by the Han-Trivedi model for banding in alloy solidification. Microstructure fluctuations between fibrous and discontinuous eutectic are seen in laser melted single tracks on the same length scale of bands due to change in local temperature gradient. Overall, various transitions in phase morphology are found in thin zones of material constituting a valuable guideline for interpretation of the solidification mechanism. Nano-indentation tests show that the fine microstructures induce substantial hardening.

AISI 316L stainless steel samples were produced by means of laser powder-directed energy deposition using optimized building conditions with two different deposition strategies. The samples, in the as-built state, were at first characterized in terms of microstructure, hardness and residual stresses by the incremental hole-drilling strain-gauge method. The results highlighted that the deposition strategy mainly affects the residual stress values, while the material hardness value is not strongly varied. The distribution of the residual stresses along the sample height was also evaluated by measuring the stress at different distances from the building platform. Furthermore, some samples underwent a homogenizing heat treatment for 2 h at 600 and 800 °C and were characterized and compared with the as-built ones. The results showed that the suggested heat treatments allow not only a reduction in residual stresses but also the homogenization of the microstructure confirmed by comparing the variations of the Vickers hardness values along the building direction. Tensile tests were also performed on the as-built and heat-treated samples in order to investigate the effect of the heat treatment on the tensile properties of the material.

Aluminum alloys are key materials in additive manufacturing (AM) technologies thanks to their low density that, coupled with the possibility to create complex geometries of these innovative processes, can be exploited for several applications in aerospace and automotive fields. The AM process of these alloys had to face many challenges because, due to their low laser absorption, high thermal conductivity and reduced powder flowability, they are characterized by poor processability. Nowadays mainly Al-Si alloys are processed, however, in recent years many efforts have been carried out in developing new compositions specifically designed for laser based powder bed AM processes. This paper reviews the state of the art of the aluminum alloys used in the laser powder bed fusion process, together with the microstructural and mechanical characterizations.

Additive manufacturing has emerged as a promising method for developing new Ti-based alloys. However, very few works have been dedicated to the in-situ synthesis of Ti-Cu alloys using Laser powder bed fusion (LPBF) method. The current work investigates the correlations between the volumetric energy density (VED) and some structural features of the in-situ Ti-5 wt.% Cu alloys produced by LPBF of the elemental powders. The results highlighted the importance of selecting the size of constituents in accordance with their thermophysical properties. It was also shown that crystallite size and lattice distortion of the α phase increased with increasing the VED. These were attributed to the higher temperature and lower cooling rate of the melt pools as well as improved particles melting and copper dissolution in the titanium lattice at higher VEDs. Isotropy of the microstructure was also improved at higher VEDs.

Laser-Directed Energy Deposition was used to produce AISI 316L stainless steel samples. The effect of the protective atmosphere on the microstructure and mechanical performance of AISI 316L deposited parts was investigated by building samples using a simple nitrogen shielding gas or using a nitrogen-filled build chamber. The effect of the different processing conditions on the microstructure was evaluated by X-ray analysis, optical and scanning electron microscopy. Only slight differences in the cellular dendrites morphology of samples built under different protective atmosphere conditions were observed. However, the presence of oxides was monitored too: the oxides composition and area fraction were analysed and compared by image analyses, and it was demonstrated that the protective atmosphere mainly affects the oxides dimensions. The effect of the oxides and nitrogen pick-up on the mechanical performance of the samples was evaluated by tensile tests. The results revealed that the nitrogen-filled build chamber allowed the achievement of slightly higher tensile strength and elongation with respect to the other processing conditions as a consequence of the reduced size of the oxide inclusions.

The material palette available for Laser Powder Bed Fusion (LPBF) process is still limited. The mixing of different powders, here AlSi10Mg + Cu, and alloy synthesis by Single Scan Tracks (SSTs) and Melt Spinning (MS) were performed in this work for fast testing the new composition and reducing the building time and amount of powder employed. Then LPBF massive samples were manufactured to verify the outcome of the procedure. In all three cases an AlSi10Mg + Cu alloy was obtained, some residual inhomogeneity in Cu distribution being found in the SST sample. Microstructural features of the LPBF sample are intermediate with respect to SST and MS ones in terms of cell size of primary Al, while the eutectic morphology of Si is close to that of the ribbons. Nanoindentation tests showed an increase in hardness of the Cu-added samples with respect to that of the base AlSi10Mg alloy. XRD results indicated the occurrence of supersaturation of both Si and Cu in the as quenched samples and DSC analyses provided information on temperature and enthalpy of exothermal signals due to precipitation.

Al-Si-Zn-Mg-Cu samples were produced using Laser Powder Bed Fusion from mixed AlSi10Mg and 7075 powders. It was observed that the introduction of silicon to an Al-Zn-Mg-Cu alloy strongly reduced the crack density, probably because of the reduction of the solidification range, the improved fluidity of the molten phase and the reduction of the coefficient of thermal expansion. The density measurements showed that crack-free samples can be successfully produced with this powder mixture. The obtained Al-Si-Zn-Mg-Cu samples were characterized in terms of microstructure, hardness and tensile properties showing that this composition is very promising for future powder bed additive manufacturing processes.

Recently, additive manufacturing techniques have been gaining attention for the fabrication of parts from aluminium alloys to composites. In this work, the processing of an AlSi10Mg based composite reinforced with 0.5% in weight of MgAl2O4 nanoparticles through laser powder bed fusion (LPBF) process is presented. After an initial investigation about the effect of process parameters on the densification levels, the LPBF materials were analysed in terms of microstructure, thermo-mechanical and mechanical properties. The presence of MgAl2O4 nanoparticles involves an increment of the volumetric energy density delivered to the materials, in order to fabricate samples with high densification levels similar to the AlSi10Mg samples. However, the application of different building parameters results in modifying the size of the cellular structures influencing the mechanical properties and therefore, limiting the strengthening effect of the reinforcement.

Powder bed additive manufacturing technologies gained much attention in past years not only thanks to design freedom but also because of the peculiar microstructures and mechanical properties that can be obtained thanks to the extremely high cooling rate. However the phenomena that arise during the laser scanning are not yet deeply understood. In this work the effect of the main building parameters and of powder properties on the shape, the microstructure and the properties of Al-based LPBF single scan tracks was evaluated. The experiments were carried out with three different powders in order to understand of the effect of the material thermo-physical properties on the consolidation phenomena.

Hastelloy X (HX) is a Ni-based superalloy which suffers from high crack susceptibility during the laser powder bed fusion (LPBF) process. In this work, the microstructure of as-built HX samples was rigorously investigated to understand the main mechanisms leading to crack formation. The microstructural features of as-built HX samples consisted of very fine dendrite architectures with dimensions typically less than 1 µm, coupled with the formation of sub-micrometric carbides, the largest ones were mainly distributed along the interdendritic regions and grain boundaries. From the microstructural analyses, it appeared that the formation of intergranular carbides provided weaker zones, which combined with high thermal residual stresses resulted in hot cracks formation along the grain boundaries. The carbides were extracted from the austenitic matrix and characterized by combining different techniques, showing the formation of various types of Mo-rich carbides, classified as M6C, M12C and MnCm type. The first two types of carbides are typically found in HX alloy, whereas the last one is a metastable carbide probably generated by the very high cooling rates of the process.

Silicon carbide ceramics obtained by reactive infiltration of silicon (SRI) have many industrial applications especially involving severe and high temperature conditions. In this study, the oxidation behavior in air of Si-SiC-ZrBThe oxidation process was analyzed on the basis of elemental maps and X-ray diffraction patterns taken, respectively, on the core and on the surface of the specimens, together with weight gains and the average thicknesses of the resulting scale. Further, flexural strength at room temperature was examined as a function of different oxidation times.The main chemical reactions and phase transformations involved in the oxidation process are reported. Several oxides were detected on the surface: zirconia, silica, zircon and 3-zirconium monoxide. All of the samples showed a parabolic oxidation kinetics, suggesting that the controlling mechanism was the diffusion; however, even after 48 hours, the oxidation process was not finished - indeed, all of the samples continued to gain weight.The oxidation of Si-SiC-ZrB

The mechanical properties and biocompatibility of titanium alloy medical devices and implants produced by additive manufacturing (AM) technologies – in particular, selective laser melting (SLM), electron beam melting (EBM) and laser metal deposition (LMD) – have been investigated by several researchers demonstrating how these innovative processes are able to fulfil medical requirements for clinical applications. This work reviews the advantages given by these technologies, which include the possibility to create porous complex structures to improve osseointegration and mechanical properties (best match with the modulus of elasticity of local bone), to lower processing costs, to produce custom-made implants according to the data for the patient acquired via computed tomography and to reduce waste.