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It is well established in literature that, when processing intermetallic γ-TiAl components by electron beam powder bed fusion, a banded microstructure is frequently formed because of an inhomogeneous Al distribution since more pronounced evaporation of Al occurs at the top of the melt pool. This feature is particularly promoted when highly energetic process parameters (high beam currents, slow beam speeds, narrow line offsets) are used. Therefore, an approach already suggested in the literature to reduce the Al loss is to minimize the energy level of the process parameter during production. However, there is a limit to such kind of approach: minimizing the beam current or increasing the beam speed, or increasing the line offset will, at a certain point, results in not being able to achieve a completely dense material and thus some process induced porosity, the so-called lack-of-fusion defects, starts to occur in the produced parts. In this study, the effect of an additional parameter of the electron beam powder bed fusion process is taken under consideration: the focus offset (FO), i.e. the distance between the focusing plane of the electron beam with respect to the powder bed. The effect of the FO on the residual porosity, microstructure, phase composition, hardness as well as chemical composition is investigated, thus having the possibility to demonstrate that also the FO can affect the Al loss and play a fundamental role in the generation of a homogenous microstructure, contributing to mitigate the appearance of a banded microstructure.

The number of materials processed via additive manufacturing (AM) technologies has rapidly increased over the past decade. As of these emerging technologies, electron beam powder bed fusion (EB-PBF) process is becoming an enabling technology to manufacture complex-shaped components made of thermal-cracking sensitive materials, such as AISI H13 hot-work tool steel. In this process, a proper combination of process parameters should be employed to produce dense parts. Therefore, one of the first steps in the EB-PBF part production is to perform the process parameter optimization procedure. However, the conventional procedure that includes the image analysis of the cross-section of several as-built samples is time-consuming and costly. Hence, a new model is introduced in this work to find the best combination of EB-PBF process parameters concisely and cost-effectively. A correlation between the surface topography, the internal porosity, and the process parameters is established. The correlation between the internal porosity and the melting process parameters has been described by a high robust model (R2adj = 0.91) as well as the correlation of topography parameters and melting process parameters (R2adj = 0.77–0.96). Finally, a robust and information-rich prediction model for evaluating the internal porosity is proposed (R2adj = 0.95) based on in situ surface topography characterization and process parameters. The information-rich prediction model allows obtaining more robust and representative model, yielding an improvement of about 4% with respect to the process parameter-based model. The model is experimentally validated showing adequate performances, with a RMSE of 2% on the predicted porosity. This result can support process and quality control designers in optimizing resource usage towards zero-defect manufacturing by reducing scraps and waste from destructive quality controls and reworks.

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.

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.

The automotive field is continuously researching safer, high-strength, ductile materials. Nowadays, dual-phase (DP) steels are gaining importance, since they meet all these requirements. Dual-phase steel made of ferrite and bainite is the object of a complete microstructural and mechanical characterization, which includes tensile and bending tests. This specific steel contains ferrite and bainite in equal parts, ferrite is the soft phase while bainite acts as a dispersed reinforcing system. This peculiar microstructure, together with fine dispersed carbides, an extremely low carbon content (0.09 wt %), and a minimal degree of strain hardening (less than 10%) allow this steel to compete with traditional medium-carbon single-phase steels. In this work, a full pearlitic C67 steel containing 0.67% carbon was used as a benchmark to build a comparative study between the DP and SP steels. Moreover, the Crussard–Jaoul (C-J) and Voce analysis were adopted to describe the hardening behavior of the two materials. Using the C-J analysis, it is possible to separately analyze the ferrite and bainite strain hardening and understand which alterations occur to DP steel after being cold rolled. On the other hand, the Voce equation was used to evaluate the dislocation density evolution as a function of the material state.

Hot Isostatic Pressing (HIP) is a net-shape powder metallurgy technique where powders densification is achieved through the application of high temperature and pressure at the same time. Powders are allocated into a hollow steel mold called capsule or canister which gives the final shape to the particles. This technique is particularly useful for manufacturing complex components made of materials which are extremely difficult to process via forging or casting. Thus, HIP is particularly indicated to handle superalloy powders such as Astroloy, which is the object of the following study. One of the most attractive peculiarities of HIP is the low material waste obtained since the overstock is limited to the layers immediately beneath the steel capsule. At the end of the HIP cycle, the canister is typically removed using an acid leaching bath which is responsible for the alteration of the outermost layers of the final product. Only a little number of research papers deal with the optimization of the removal of these layers; Consequently, manufacturers often apply a very conservative approach by eliminating more material than is actually needed with a final machining procedure. This paper aims to optimize this procedure by systematically assessing the total thickness of the altered layer of material deriving from the HIPping and leaching process together. To achieve this goal, a set of samples were prepared by removing progressively thicker layers of material and then they were bend tested. Finally, the recorded mechanical properties were compared with those obtained with the samples machined from the core material. One of the main findings is that the removal of 500 μm of material is enough to recover mechanical properties which are comparable with those observed in samples coming from the core. More specifically, by eliminating the first 100 μm material, all the corroded layer is removed, which results in an overall increase of all the mechanical properties except for ductility. This property strongly depends on the number of prior particle boundaries arising from the HIPping process itself. Thus, the correct amount of overstock material must include both these layers.

This work aims to investigate the effect of the process parameters on the densification and microstructure of Inconel 939 (IN939) alloy processed by laser powder bed fusion (LPBF). IN939 is a Ni-based superalloy with high creep and corrosion resistance that can be used up to around 850 &deg, C under load, resulting in higher operative temperatures than the ones commonly allowed for Inconel 718 and Inconel 625 alloys (around 650 &deg, C). However, this alloy can suffer from poor weldability involving possible crack formation. In order to minimize the residual porosity and the cracking density, specific process parameters were investigated. The parameters to generate IN939 samples almost pores-free (porosity &le, 0.22%) with a cracking density &le, 1.36 mm/mm2 as well as samples almost crack-free (&le, 0.10 mm/mm2) with limited residual porosity (&le, 0.89%) were determined. The microstructure revealed fine dendritic/cellular structures with the formation of sub-micrometric phases. A high concentration of these phases was also found along the intergranular cracks, suggesting that their presence, coupled to the high thermal stresses, can be the primary reason for crack formation during the LPBF process.

The Al-alloy die casting die is a sector where the operating environment imposes a very severe aggression to hot working tools. Steel grades for such application, techniques for their surface modification and specifically conceived lubricants are continuously improved so as to limit Al soldering. Within this scenario the interaction between lubricant and die surface and the effect of finishing levels of such surface is poorly studied. This paper deals with a study of the influence of dies surface roughness on the working behavior of a die casting lubricant and on surface damages of a tool steel grade. Tool steel samples were prepared for the research and two different levels of surface roughness (as polished and as finely sand blasted) were investigated. Apart from the base characterization of steels and surface, two specific test rigs were used to study the lubricant-tool steel surface-Al alloy interactions. One of test rig was devoted to study the coupling principles of tool die surface-lubricant, while the other test rig was used to perform a cyclic immersion test in molten Al-alloy. The derived data were compared to the experimental investigation of cracks and craters as provided by cycling with a correlation with the surface finishing level of samples.

The effect of aging treatment on Astroloy fabricated via hot isostatic pressing and subjected to super-, and sub- solvus solutioning has been investigated. The evolution of hardness and microstructural features were followed after each step of the treatment. Since this alloy is commonly subjected to a double aging treatment at two different temperatures, particular attention was given to the effectiveness of the first aging treatment compared to the second one. Coarsening and modification of the γ' reinforcing system together with carbides formation were made the object of research. The cooling rate used after solutioning treatment was also kept into account. Finally, a model to describe secondary and ternary gamma prime coarsening upon aging treatments is presented.

This study investigated the mechanical behaviour and microstructure of as-built and heat-treated Inconel 625 (IN625) samples processed by laser powder bed fusion (LPBF). This process offers freedom in design to build complex IN625 components in order to overcome extensive machining. However, post heat treatments must be performed to obtain specific mechanical properties to match industrial requirements. For this purpose, different heat treatments were performed on IN625 samples, and through hardness measurements, three different heat treatments were selected, as optimised conditions. A direct ageing, a solutioning and a solutioning followed by ageing treatments were chosen to study the effects of these specific heat treatments on the microstructure and tensile properties, comparing them to those of as-built condition. The tensile properties of as-built and selected heat-treated IN625 samples showed superior values to minimum requirements for wrought IN625 alloys, whereas the investigation on the microstructures and fracture surfaces of as-built and heat-treated IN625 contributed to an understanding of the tensile properties evolution. The high tensile strength of as-built samples essentially derived from very fine dendritic structures mainly below 1 µm with high dislocation density and nanometric MC carbides. The high tensile properties of ageing treatments performed at 700 °C for 24 h, whether directly aged or post-solutioning, were found to be primarily dependent on γ" phases (10–30 nm) and M23C6 carbides formation. By contrast, the tensile properties of solution-treated IN625 samples at 1150 °C for 2 h showed higher ductility coupled to lower strength than other conditions, due to the grain growth.

WC cemented carbides are used extensively as cutting and drilling tools in the cutter heads of the full face Tunneling Boring Machines (TBMs). This paper describes the effects of microstructural properties of selected cemented carbide grades, operating as cutting tools in dry and foam conditioned soil, towards abrasive wear resistance. The work was carried out with the purpose of identifying the most promising cemented carbide microstructural formulation against abrasive wear and of defining the possible changes induced on the wear mechanisms by conditioning agents. Microstructures of six commercial grades of cemented carbides with different Co content and WC grain size were characterized. The wear tests were carried out using a testing device simulating the working environment of TBM cutter heads both in dry and conditioned soil. According to all experimental findings, wear resistance of the investigated cemented carbides strongly decreases as the grain size and mean binder free path increase, regardless of the test conditions. Furthermore, the carbides distribution, hereinafter called Delaunay distance, plays a crucial role on wear behavior, having a linear relation with the specific volume loss. Moreover, the optimal microstructural features to promote wear resistance are here described. Finally, foam conditioning of soils was found to strongly influence the wear resistance, by reducing binder ploughing and WC microcracking.

The influence of solutioning treatment on Astroloy fabricated via Hot Isostatic Pressing has been investigated. Both sub- and super-solvus solutioning treatments were performed, gradually increasing the test temperature from 1115 to 1200 °C in order to cross the ɣ′ solvus. The effects of the solutioning temperature and of the following cooling step were studied in terms of amount of reinforcing precipitate ɣ′, of γ-grain coarsening and of alloy hardness. Grain boundaries morphology was observed to be clearly modified by the solutioning conditions moving from the “smooth” to the “serrated” type. It was also possible to draw a clear correlation between cooling rates and Astroloy hardness.

Nowadays, an increasing number of components are manufactured using Hot Isostatic Pressing (HIP), which is considered a near net or net shape forming technology, depending on the overstock thickness. One of the most challenging tasks with regard to improving HIP is limiting the interaction between the capsule and the consolidating powders, aiming to reduce material waste and costs. The capsule is typically made of mild carbon steel and it is used to correctly transfer the forming pressure to the loose powders at the HIP temperature. The great inhomogeneity between the two materials at such a high pressure and temperature lead to a strong elemental diffusion between the powders and the capsule, which leads to the formation of an interface. This study investigates in depth, the effects of this diffusion during the consolidation of Astroloy powders.

Nickel-based Inconel 718 is a very good candidate for selective laser melting (SLM). During the SLM process, Inconel 718 develops a complex and heterogeneous microstructure. A deep understanding of the microstructural features of the as-built SLM material is essential for the design of a proper post-process heat treatment. In this study, the microstructure of as-built SLM Inconel 718 was investigated at different length scales using optical microscopy (OM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Electron backscatter diffraction (EBSD) was also used to analyze the grain morphology and crystallographic texture. Grains elongated in the build direction and crossing several deposited layers were observed. The grains are not constrained by the laser tracks or by the melt pools, which indicates epitaxial growth controls the solidification. Each grain is composed of fine columnar dendrites that develop along one of their &lt, 100&gt, axes oriented in the direction of the local thermal gradient. Consequently, prominent &lt, crystallographic texture was observed and the dendrites tend to grow to the build direction or with occasional change of 90&deg, at the edge of the melt pools. At the dendrite length scale, the microsegregation of the alloying elements, interdendritic precipitates, and dislocations was also detected.

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.

In this work, carried out within the European project TIALCHARGER, TiAl-based powders of Ti-48Al-2Nb-0.7Cr-0.3Si, were used to fabricate specimens and prototypes of hollow turbocharger turbines by Electron Beam Melting (EBM). This additive manufacturing technology uses an electron beam to generate parts by selectively melting the powders layer by layer according to CAD data. Components produced with these powders were characterized in terms of residual defects and resulted appropriate both in terms of overall residual porosity and maximum defect size. Furthermore the microstructure of the as produced material was investigated and heat treatments were set up in order to obtain the desired microstructure. This is an essential point for these materials because their mechanical properties are extremely sensitive to microstructure. Chemical composition of powders and massive materials were analysed. Loss of Al was observed, while the levels of impurities were very low and similar for both the powder and the part. Mechanical tests were performed on the heat treated samples and the results are in well agreement with literature data for this TiAl alloy for the desired automotive application.

This paper reports the activities carried out at the Department of Applied Science and Technology of Politecnico di Torino about Titanium Aluminide alloys for aerospace and automotive applications processed by the Electron Beam Melting (EBM) Additive Manufacturing technique. In particular, it describes the material characterization and the heat-treatments performed on different γ-TiAl alloys in order to improve their properties in view of their final applications.

Ni-based superalloy components for high-temperature applications rely on the long term stability of the microstructure and mechanical properties at service temperatures. Nowadays, the production of such types of components is frequently performed via Additive Manufacturing (AM) technologies. Nevertheless, few studies are dedicated to understanding the behavior of AM Ni-based superalloys upon prolonged exposure to high temperatures. This work aims at studying the effect of prolonged thermal exposures on the microstructure and mechanical properties of Inconel 625 processed by laser powder bed fusion. Thermal exposures within the range of 600 °C and 900 °C for 200 h were performed on this material. The as-built and solution annealed Inconel 625 conditions were selected. The solution annealed state implies a complete chemical homogenization, typically recommended for working at high temperatures, whereas the initial as-built state is characterized by segregations and fine dendritic structures. Upon the studied prolonged thermal exposures, the peculiar as-built microstructure formed a higher quantity of phases with smaller dimensions with respect to the solution annealed condition under thermal exposures. The smaller phases of the as-built state resulted in similar mechanical properties evolution under different temperatures. Differently, the prolonged heat-treated solution annealed conditions exhibited more marked mechanical properties variations due to coarser phases.

Instrumented indentation test is a non-conventional hardness test, relevant for performing both hardness measurement and estimating the elastic modulus. The indentation modulus, which depends on the contact stiffness, estimates the elastic modulus. To evaluate contact stiffness, the literature requires fitting force–displacement curve according to mathematical models, which are hardly representative of the curve. This research aims at proposing alternatives, which estimate contact stiffness directly from experimental data. Proposed methods are applied to nano-indentations performed on a reference material and a high-speed bearing steel; indentation modulus is evaluated along with uncertainty contribution of contact stiffness. Comparison with standard methods is provided.

The effect of aging treatment on Astroloy fabricated via hot isostatic pressing and subjected to super-, and sub- solvus solutioning has been investigated. The evolution of hardness and microstructural features were followed after each step of the treatment. Since this alloy is commonly subjected to a double aging treatment at two different temperatures, particular attention was given to the effectiveness of the first aging treatment compared to the second one. Coarsening and modification of the &gamma, &prime, reinforcing system together with carbides formation were made the object of research. The cooling rate used after solutioning treatment was also kept into account. Finally, a model to describe secondary and ternary gamma prime coarsening upon aging treatments is presented.

Titanium aluminide alloys are good candidates for structural applications thanks to their low density and good balance of properties up to relatively high temperatures. However, their application is still limited by significant oxidation. Four γ-TiAl alloys with different content of aluminum and niobium were produced by electron beam melting: Ti-45Al-2Cr-2Nb, Ti-48Al-2Cr-2Nb, Ti-45Al-2Cr-8Nb, and Ti-46Al-2Cr-8Nb. The behavior of these alloys in response to oxidation in air during constant heating up to 1000 °C and isothermal oxidation for 10 h at 850 and 950 °C were studied by thermogravimetric analysis. The mass gain due to oxidation of the low Nb-containing alloys was always at least twice that of the high Nb-containing alloys. Both low and high Nb-containing alloys exhibited on their surface oxidation products of the same nature: oxides TiO2 and Al2O3, and nitrides TiN and Ti2AlN. Niobium addition up to 8 at.% did not suppress the growth of rutile and promote the formation of a protective alumina layer. However, it efficiently reduced the formation of rutile, mainly responsible for the mass gain due to oxidation of γ-TiAl alloys and with tendency to spallation.

The wear phenomenon evaluation in EPB shield tunneling machines is not a simple issue, as a large number of parameters are involved, such as soil and tool material properties, soil conditioning and pressure in the bulk chamber. The evaluation of the influence of these parameters and predicting this influence is a complex task and the research has proposed different test procedures and approaches. In this paper a new procedure for testing wear of tools with an innovative concept and design is presented. The experimental results obtained using conventional steel and hard material tools, tested with natural and conditioned soils, are discussed. The outcomes show the feasibility of the proposed procedure and the quality of the measurements that can be obtained using the proposed wear tool shape.

Die-casting processes are commonly employed in the automotive area to cast aluminum alloys. These processes involve the use of steel dies, typically realized in AISI H11 or H13, with or without a coated surface. Mechanical and chemical stresses and thermal cycles act producing stresses, plastic deformation, corrosion and metallization on the mold surfaces. Particularly, the interaction between the die and the molten aluminum causes the nucleation of intermetallic compounds at the interface. In this work the interaction between the hot work steel AISI H13 and the aluminum alloy EN–AB 46000 were evaluated in terms of adhesion strength for different roughness of the die surface. Two of the tested roughness were finally applied on the surface of die inserts; the use of injection pressure in a high pressure die casting (HPDC) machine, led to observe the possible variations in terms of soldering severity on the surface of the die in real operating conditions. The comparison between casting tests carried out in laboratory conditions (aluminum alloy casted on the steel surface) and in real-work conditions (aluminum alloy injected with a certain pressure on the steel surface) allowed to evaluate the effect of surface-roughness on soldering mechanisms.

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.

In this work, the characterization of the assembly of a steel shaft into a γ-TiAl part for turbocharger application, obtained using Electron Beam Welding (EBW) technology with a Ni-based filler, was carried out. The Ti-48Al-2Nb-0.7Cr-0.3Si (at %) alloy part was produced by Electron Beam Melting (EBM). This additive manufacturing technology allows the production of a lightweight part with complex shapes. The replacement of Nickel-based superalloys with TiAl alloys in turbocharger automotive applications will lead to an improvement of the engine performance and a substantial reduction in fuel consumption and emission. The welding process allows a promising joint to be obtained, not affecting the TiAl microstructure. Nevertheless, it causes the formation of diffusive layers between the Ni-based filler and both steel and TiAl, with the latter side being characterized by a very complex microstructure, which was fully characterized in this paper by means of Scanning Electron Microscopy, Energy Dispersive X-ray Spectroscopy, and nanoindentation. The diffusive interface has a thickness of about 6 µm, and it is composed of several layers. Specifically, from the TiAl alloy side, we find a layer of Ti₃Al followed by Al₃NiTi₂ and AlNi₂Ti. Subsequently Ni becomes more predominant, with a first layer characterized by abundant carbide/boride precipitation, and a second layer characterized by Si-enrichment. Then, the chemical composition of the Ni-based filler is gradually reached.

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.

Hot Isostatic Pressing (HIP) is a powder metallurgy technique which densifies metallic powders via the contemporary application of high temperature and pressure. Such manufacturing route demonstrated highly efficient to produce components made of material with low forgeability or castability such as the Ni-based superalloys. Such materials are designed to withstand very high temperatures, which typically are met in aeronautical engines. Despite Astroloy, the Ni-based superalloy studied in the following work, is designed to work in the temperature range of 650–760 °C, only few research papers discussed its thermal stability in depth. In this work, the thermal stability of the alloy was studied in a more demanding condition, i.e., at 820 °C for 200 h. This experimental condition was chosen to assess the effects of repeated exposures of a component above the standard operating temperature. The over-ageing treatment applied to the alloy severely altered its microstructure, causing γ’ coarsening and second phases precipitation. More precisely, σ phase and M23C6 carbides precipitated inside and at the grain boundaries respectively. Nano-indentation trials and tensile tests at high temperature were used to assess how these microstructural alterations impacted on mechanical properties of the material. The main conclusions are: only a slight lowering of tensile properties and a strong increase of ductility was measured after tensile test up to 760 °C, as well as an higher reduction of both tensile properties and ductility when sample were tested at 820 °C.

Inconel 625 (IN625) alloy has high-temperature strength coupled with high oxidation and corrosion resistance. Additionally, due to its excellent weldability, IN625 can be processed by laser powder bed fusion (LPBF) additive manufacturing (AM) process allowing the production of complex shapes. However, post-AM heat treatment is necessary to develop the desired microstructure and mechanical properties to meet industrial needs. This work is focused on the influence of different heat treatment processes, namely stress relieving, recrystallization annealing and solution annealing on the microstructure and tensile properties of LPBF IN625 alloy. Investigation of the crystallographic texture by electron backscattered diffraction indicated that heat treatments at 1080 °C and 1150 °C tend to eliminate anisotropy in the material by the recrystallization and grain growth resulting in the formation of equiaxed grains. Tensile properties of heat-treated LPBF IN625 alloy built along different orientations revealed higher tensile properties than the minimum recommended values of wrought IN625 alloy in the annealed and solution annealed states.

The adhesive bonding technology can be used in combination with other traditional joining methods, such as mechanical fastening techniques (e.g. rivets or bolts) or welding techniques, generating a hybrid joint. Hybrid adhesive joints are designed to exploit the advantages of the different techniques and, if possible, overcome their drawbacks. This study focuses on the interference fitted/adhesive bonded joining technique. This method consists in two cylindrical components coupled together by inserting one into the other, after having placed an adhesive on the mating surfaces. This hybrid joint, generally realized by exploiting acrylic anaerobic systems, has been studied to evaluate the interaction between the tensile field of the components (at the interference level) and the adhesive strength under both static and dynamic loading conditions. Notwithstanding this, the contributions of the adhesive and the interference on the performance of the final joint are still not completely clear. The aim of this research is then to further study the factors affecting the resistance of this hybrid joint. Hub-shaft samples, joined by means of a press fit, were tested under an axial push out load. Hybrid joints were compared to both adhesive joints in clearance conditions and interference joints. In particular, different levels of interference were analyzed in order to clarify the role played by the tensile field between the hub and the shaft, and the influence of the friction effects between them. Numerical simulations were also employed to support the experimental data in the evaluation of the behavior of the adhesive and its relation with all the phenomena influencing the hybrid system. It was found that tribological phenomena played an important role in governing the mechanical behavior of the unbonded samples, while they can be considered negligible in presence of the adhesive. The correlation between bonded and unbonded press-fitted joints was investigated pointing out that the maximum strength of the hybrid joint is mainly related to the resistance of the epoxy adhesive.

Directed energy deposition (DED) has been employed to produce AISI 316L samples. Microstructure and primary cellular arm spacing (PCAS) are studied analysing the relationship with the cooling rate at the different heights of DED processed 316L stainless steel sample. It is found that, by increasing the deposition distance from the substrate, the PCAS of the sample increases from 2.9 to 4.5 μm, as a consequence of the decreased cooling rate and thermal gradient. On the other hand, in the last deposited layers, the PCAS of the sample decreases from 4.5 to 3.3 μm, because of the changes in cooling mechanisms. The phase composition of samples after deposition is revealed and compared with the predictions based on the Schaeffler and Pseudo-binary diagrams. It is revealed that the final microstructure is characterized by austenitic dendrites together with some residual delta ferrite located at dendritic arms location. Lastly, the effect of using fresh or recycled powders, on the microstructure and mechanical properties of DED 316L stainless steel parts is investigated. It is found that the samples fabricated using recycled powders have rather similar tensile strength levels, but much lower elongation than those produced using fresh powder due to a lower inclusions content and of their average lower size. The nature of these inclusions is discussed as well as the reason for their increase both in numbers and size.

Thermal fatigue (TF) is a common problem in many tool steel components. It is caused by thermal cycling in presence of internal constraints. The resulting thermomechanical stresses induce thermal cracking (heat checking). A laboratory test was developed to reproduce TF damage on a laboratory scale, under oxidizing conditions. Two different test configurations were used to induce unidirectional and bidirectional cracking. Nitriding impairs TF resistance of plain steel due to the easier propagation of cracks through the diffusion layer. The efficacy of PVD coatings is dependent on their oxidation resistance and microstructure. The higher oxidation resistance of AlCrN/AlTiN than CrN results in delayed crack initiation. TF resistance is increased if the coating is free of defects and the interface adhesion is good.

The paper describes used a beam of negative ions Au- and performed high-temperature annealing for recrystallization structure of X-ray amorphous AlN-TiB 2 -TiSi 2 , having typical dimensions of the regions near the ordering 0, 8–1 nm. Direct methods of measurement of high-resolution transmission electron microscopy, X-ray diffraction and scanning electron microscopy and X-ray microanalysis demonstrated, that thermal annealing at 1300 °C in air results to the formation of nanoscale phase of 10–15 nm AlN, AlB 2 , Al 2 O 3 and TiO 2 , and ion implantation of negative ions Au-leads to fragmentation (decrease) in sizes up to 2–5 nm nanograin with formation of nanocrystals from Au-“ball” shape in a few nm in size and formation of an amorphous oxide film in the depths (surface layer) cover by ballistic ion mixing and collision cascades. The work performed in collaboration with the National Institute for Materials Science, Tsukuba, Japan and Politecnico di Torino, Italy.

In heavy loaded mating components, such as sliders and sliding bearings, guaranteeing the efficiency of lubricant films for long times during severe service conditions is very complicated. In this work, the benefits deriving from the use of fiber laser sources for surface texturing of very thin TiN coatings in severe wear working conditions were demonstrated. Evaluations of the laser textured dimples shape, geometry and density are given. Wear performance of the fiber laser textured surfaces was evaluated in discontinuous oil lubricated conditions with a flat contact. High normal load and low sliding speed were applied. Comparison tests were also performed on commercial TiN and WC/C coatings. In terms of average wear volume and maximum wear depth, Laser Surface Texturing of TiN provided respectively a 70% and a 45% reduction if compared to plain TiN. If compared to WC/C the wear resistance gains were lower but LST TiN maintained such benefits for longer wear runs. SEM analysis also revealed that the laser interaction provided a localized thermal cracking to the TiN coating. However, the sliding action caused very limited and localized coating fragmentation or delamination.

Hastelloy X (HX) is a Ni-based superalloy which is employed to produce gas turbine and gas-cooled reactor sectors due to its outstanding oxidation resistance and high tensile strength at high temperatures. This alloy can be processed by laser powder bed fusion (LPBF) fabricating complex geometries in a single step. However, post-processing thermal treatments must be applied to generate a suitable microstructure for high-temperature applications. The investigation reports the microstructure evolution of LPBF HX samples under specific post-processing treatments. A hot isostatic pressing (HIP) treatment can close the internal cracks and reduce the residual porosity (less than 0.1%). Moreover, the HIP-triggered recrystallization generated equiaxed grains, while the slow cooling rate generated a film of intergranular carbides (Mo-rich M6C and Cr-rich M23C6) and intragranular carbides (Mo-rich M6C carbides). Therefore, a solution annealing was performed to dissolve the film of carbides which may reduce the ductility. The post solution annealed material consisted of equiaxed grains with ASTM grain size number mainly 4.5-5.5 and inter/intragranular Mo-rich M6C carbides. The microstructure is highly comparable with solution annealed wrought HX alloy. Finally, after simulating short thermal exposure at 745 &deg, C for 6 h, a significant formation of Cr-rich M23C6 carbides was observed strengthening the LPBF HX alloy.

The main focus of this paper is on materials for radiant burners application. Two advanced metallic alloys, a Ni and Fe-based alloy are studied and compared to a reference ferritic stainless steel. Oxidation kinetics of such alloys at different temperatures are reported. Oxide formation mechanisms are discussed. Furthermore, thermo-mechanical resistance and eventual strengthening mechanism in temperature are studied. Finally, technical and brief tentative economical analysis of different alloys as potential candidates for the fabrication of radiant burners are given.

Wear and failure of die casting dies involve a complex interaction between various mechanisms. The most important wear and failure modes are summarized as follows: (i) the so-called washout damages on working die surfaces are attributed to erosion, corrosion and soldering; (ii) thermal fatigue is the most important failure mode in die casting. The sector of surface thin PVD coatings is constantly enhancing in order to meet the increasing demand for improved performances of tooling. Advanced PVD coatings are designed to withstand severe mechanical and thermal stress conditions. A CrAlSiN nanostructured coating system was deposited on the base material, modulating the chemical composition so as to increase either the chromium or the aluminium-silicon content. Then, another set of specimens was subjected to a cyclic immersion program in a molten aluminium bath. The washout signs were detected and monitored in function of the increasing number of cycles.

Direct metal laser sintering (DMLS) is an additive manufacturing technique for the production of parts with complex geometry and it is especially appropriate for structural applications in aircraft and automotive industries. Aluminum-based metal matrix composites (MMCs) are promising materials for these applications because they are lightweight, ductile, and have a good strength-to-weight ratio This paper presents an investigation of microstructure, hardness, and tribological properties of AlSi10Mg alloy and AlSi10Mg alloy/TiB2 composites prepared by DMLS. MMCs were realized with two different compositions: 10% wt. of microsize TiB2, 1% wt. of nanosize TiB2. Wear tests were performed using a pin-on-disk apparatus on the prepared samples. Performances of AlSi10Mg samples manufactured by DMLS were also compared with the results obtained on AlSi10Mg alloy samples made by casting. It was found that the composites displayed a lower coefficient of friction (COF), but in the case of microsize TiB2 reinforcement the wear rate was higher than with nanosize reinforcements and aluminum alloy without reinforcement. AlSi10Mg obtained by DMLS showed a higher COF than AlSi10Mg obtained by casting, but the wear rate was higher in the latter case.

A combined experimental and numerical study on the variation of the elastic properties of defective single-crystal diamond is presented for the first time, by comparing nano-indentation measurements on MeV-ion-implanted samples with multi-scale modeling consisting of both ab initio atomistic calculations and meso-scale Finite Element Method (FEM) simulations. It is found that by locally introducing defects in the 2 × 10 18 –5 × 10 21 cm −3 density range, a significant reduction of Young’s modulus, as well as of density, can be induced in the diamond crystal structure without incurring in the graphitization of the material. Ab initio atomistic simulations confirm the experimental findings with a good degree of confidence. FEM simulations are further employed to verify the consistency of measured deformations with a stiffness reduction, and to derive strain and stress levels in the implanted region. Combining these experimental and numerical results, we also provide insight into the mechanism responsible for the depth dependence of the graphitization threshold in diamond. This work prospects the possibility of achieving accurate tunability of the mechanical properties of single-crystal diamond through defect engineering, with significant technological applications, e.g. the fabrication and control of the resonant frequency of diamond-based micromechanical resonators.