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A new method for the manufacturing of pre-determined open porosity Al foams is presented in this work. Starting from a 3D foam model designed with CAD and printed with fused deposition modeling (FDM), a porous structure of poly lactic acid (PLA) has been replicated. The choice of this material has been driven by the properties in terms of recyclability and low cost. The truncated octahedron for the porosity shape has been selected due to benefit and properties ascribable to its morphology. After creating the 3D PLA model, it has been filled with liquid plaster. After its solidification, PLA is removed in oven at 600 °C so that a negative-shaped plaster mold is obtained. Successively liquid Al at 750 °C is poured in the mold inside the oven. After Al solidification, plaster can be easily removed in ultrasonic bath thus obtaining Al foam with the same morphology of the starting PLA model. This process shows great flexibility allowing to manufacture different kind of elementary cell types. Tailoring of the selected properties of the manufactured foams, e.g. the morphology, the density or the surface/volume ratio may be obtained. With this technique many other metals and alloys can be manufactured too. The potential of this production technique can be successfully employed in many application fields of metal foam. Light-weight 3D lattice structures are widely employed for multifunctional applications such as loading bearing, negative and zero thermal-expansion structures, vibration attenuation, impact and blast proof structures.

Due to the manifold properties of porous materials in general, and metal foams in particular, they are increasingly used in industrial, structural and functional applications. The properties of metal foams depend on: chemical composition, metallurgical aspects, processing morphology (the type of porosity: open or closed), amount of porosity, cells size and shape. There are challenges and difficulties encountered in welding and bonding metal foams from wider use in the production of complex-shaped components. Some of the problems that associate with welding processes in metal foams are reduction in mechanical properties, intermetallic compound formation, microstructural inhomogeneity and low fatigue strength. More complicated challenges are envisaged during welding metal foams with other dissimilar materials. Laser welding technologies seem to be the most promising joining techniques. This paper presents an overview on laser welding and joining techniques involving metal foams based on laser technology in similar and dissimilar combinations, with and without filler metal. The discussion on laser welding of metal foams is presented keeping the focus on CuZn open-cell foams, lotus type porous iron, foam-filled steel sandwich beams, aluminum foam cores inside a hollow profile and sandwich panels.

In this paper Powder Compact Melting Technique (PCMT) and Sintering and Dissolution Process (SDP) are compared and discussed. In the first process melting of the powders is required for the foam production. Mechanical characterization of Al foams produced with the two techniques has been performed by means of static compressive tests in order to make a direct comparison. The PCMT foams show higher mechanical properties in compressive tests if compared to the SDP foams and are ideal for structural applications in which energy absorption is the main task. When the control of the morphology as well functional properties (e.g. noise and vibration absorption) related to the interconnected porosity are fundamental, SDP foams are to be preferred.

In the last decades, great attention has been focused on the characterization of cellular foams, because of their morphological peculiarities that allow for obtaining effective combinations of structural properties. A predictive analytical model for the compressive behavior of closed-cell Al foams, based on the correlation between the morphology of the cellular structure and its mechanical response, was developed. The cells’ morphology of cylindrical specimens was investigated at different steps of compression by X-ray computed tomography, in order to detect the collapse evolution. The structure, typically inhomogeneous at local level, was represented by developing a global virtual model consisting of homogeneous cells ordered in space, that was fitted on the experimentally detected structure at each deformation step. As a result, the main parameters characterizing the two-dimensional cells morphology (equivalent diameter, circularity), processed by the model, allowed to simulate the whole compression stress–strain curve by enveloping those obtained for each step. The model, fitted on the previous foam, was validated by comparing the simulated stress–strain curve and the corresponding experimental one, detected for similar foams obtained by different powder compositions. The effectiveness in terms of an accurate prediction of the compression response up to the final densification regime has been confirmed.

Recent research findings on the mechanical behavior of metal foams are summarized in this work. Thanks to their properties in compressive tests, a wide range of foamed materials has been considered for energy-absorption applications such as Al, Fe, Ti, Ni and its alloys. The main parameters affecting energy absorption are focused and presented: cell size, relative density, strain rate, hybrid foam (Al-Cu, Al-Ni), base metal, and composites structures (Al-foam filled tube and sandwich). Metal foam response, impact resistance and failure are discussed in many configurations and test conditions. The results of finite elements modelling and its validation by means of mechanical tests are discussed too.

The present research investigates the application of a cardan suspension making use of permanent magnet (PM) bearings employed to obtain high reliable/low-cost solutions for the permanent alignment of directional payloads such as laser reflectors for the Next Generation Lunar Retroreflector (NGLR) experiment or antennas to be deployed on the moon’s surface. According to Earnshaw’s Theorem, it is not possible to fully stabilize an object using only a stationary magnetic field. It is also necessary to provide axial control of the shaft since the PM bearings support the radial load but, they produce an unstable axial force when losing alignment between the stator and rotor magnets stack. In this work, the use of commercial off-the-shelf (COTS) sapphire as axial bearings in the cardan suspension has been investigated by testing their behavior in response to some of the dynamic loads experienced during the qualification tests for space missions. The work is innovative in the sense that COTS sapphire assembly has never been investigated for space mission qualification. As Artemis mission loads have not been yet provided for NGLR, test loads for this study are those used for the proto-qualification of the INFN INRRI payload for the ESA ExoMars EDM mission. Tests showed that, along the x and y directions, no damages were produced on the sapphire, while, unfortunately, on the z direction both sapphires were badly damaged at nominal loads.

Space vehicles may be propelled by solar sails exploiting the radiation pressure coming from the sun and applied on their surfaces. This work deals with the adoption of Nickel-Titanium Shape Memory Alloy (SMA) elements in the sail deployment mechanism activated by the Joule Effect, i.e., using the same SMA elements as a resistance within suitable designed electrical circuits. Mathematical models were analyzed for the thermal analysis by implementing algorithms for the evaluation of the temperature trend depending on the design parameters. Several solar sail prototypes were built up and tested with different number, size, and arrangement of the SMA elements, as well as the type of the selected electrical circuit. The main parameters were discussed in the tested configurations and advantages discussed as well.

Al alloys, conceived for automotive and aeronautic applications thanks to the high strength/density ratio, exhibit weldability issues common to all light alloys. In particular loss of toughness and soundness of welded joints consequent upon welding operations, possible cracking in the weld metal and metallurgical modifications induced in the heat affected zone. In this paper the weldability of AA2139, AA6110 and AA6156 with the same filler of AA4047 was investigated by comparing features of welds carried out by Nd:Yag laser process. Some samples have been welded of different Al alloys. Welded joints were submitted to optical and SEM metallographic examinations with EDS. Microanalysis measurements were performed to evaluate locally chemical composition and to investigate the nature of the precipitates. Mechanical properties were evaluated through tensile test (T-pull and Hoop stress) and fatigue test (T-pull and Hoop stress). One of the main results is the goodness of this kind of welding between the different alloys for mechanical properties and metallographic features. In particular the configuration AA6156-AA2139-AA4047 is typical for resistance structure in aircraft applications, consisting in extruded AA2139 stringer, responsible to absorbing structural loads and AA6156 skin with high corrosion and crack propagation resistance.

The actuation capability and the reliability of hybrid composites activated by shape memory alloys are discussed in this work. The manufacturing of compact and safe actuators has been possible employing thermo-activated SMA which allows the polymer-based matrix to undertake many geometries. Different technological procedures are proposed for the manufacturing of this type of composites and the production problems are discussed too. The adoption of non-metallic materials as deformation recovery elements, even if at present they do not allow a complete reversibility of the imposed deformation, represent an interesting research field, due to their properties of lightness, flexibility and low cost.

The research is focused on the design of a magnetic suspension employable to achieve Earth-pointing for a space reflector or other devices integrated on a lunar lander or rover deck just utilizing the local gravity. The classical solution is based on heavy and expensive actuators, still representing critical subsystems as their failure might often lead to severe or catastrophic effects on the mission. This proposal is based on a magnetic suspension realized with two small magnetic bearings. They can be contained in a cylinder smaller than 15 mm diameter and 20 mm height and capable of supporting 20 N. After designing, a prototype has been built employing commercially available neodymium permanent magnets. Preliminary results of carrying capacity have been correlated in good agreement both with analytical calculation and finite elements analysis. The study has showed that the goal of 20 N can be attainable. The structure stiffness and carrying load for the accurate alignment of each stator/rotor pack is crucial for the performance of the suspension. The next step is the design and build-up of an upgraded model to be qualified for the mission dynamic loads.

This work deals with the feasibility and reliability about the use of shape memory alloys (SMAs) as mechanical actuators for solar sail self-deployment instead of heavy and bulky mechanical booms. Solar sails exploit radiation pressure a as propulsion system for the exploration of the solar system. Sunlight is used to propel space vehicles by reflecting solar photons from a large and light-weight material, so that no propellant is required for primary propulsion. In this work, different small-scale solar sail prototypes (SSP) were studied, manufactured, and tested for bending and in three different environmental conditions to simulate as much as possible the real operating conditions where the solar sails work. Kapton is the most suitable material for sail production and, in the space missions till now, activated booms as deployment systems have always been used. In the present work for the activation of the SMA elements some visible lamps have been employed to simulate the solar radiation and time-temperature diagrams have been acquired for different sail geometries and environmental conditions. Heat transfer mechanisms have been discussed and the minimum distance from the sun allowing the full self-deployment of the sail have also been calculated.

In this work a novel actuation system based on NiTi shape memory springs has been designed and characterized in a small-scale prototype to permit setting of the right aperture and its modulation when necessary. The manufactured prototype exploits an actuator in the antagonist configuration. In particular, the aim has been focused on the modulation of the stroke by the activation of Shape Memory Alloy (SMA) actuators. SMA springs of 15 turns and 7.5 mm average diameter are the optimal solution relatively to force and stroke required from the system. One of the possible applications of such actuator is in the field of solar reactors in which one of the main problem is to keep constant temperature into the reactor in order to allow a continuous and efficient transformation, from sunrise to sunset.

During fusion welding the molten metal is shielded from contact with the atmospheric gas by means of a gaseous flux. The shielding gas prevents weld embrittlement, affects welding quality, because of its influence on filler metal transfer, and has a direct impact on welding costs as well. Argon is the most common shielding gas, often used with some adds of other gases that can be inert, as helium, or active, as CO 2 , O 2 or H 2 . In this work the effects of mixtures with different composition have been considered for the arc welding of austenitic steels. Metallographic samples of welded sections have been undergone to visual and optical microscopy observations, microhardness, indentations and tensile tests.

Shape memory alloys (SMAs) show a particular behavior that is the ability to recuperate the original shape while heating above specific critical temperatures (shape memory effect) or to withstand high deformations recoverable while unloading (pseudoelasticity). In many cases the SMAs play the actuator’s role. Starting from the origin of the shape memory effect, the mechanical properties of these alloys are illustrated. This paper presents a review of SMAs applications in the aerospace field with particular emphasis on morphing wings (experimental and modeling), tailoring of the orientation and inlet geometry of many propulsion system, variable geometry chevron for thrust and noise optimization, and more in general reduction of power consumption. Space applications are described too: to isolate the micro-vibrations, for low-shock release devices and self-deployable solar sails. Novel configurations and devices are highlighted too.

Solar sails exploit the radiation pressure as propulsion system. Sunlight is used to propel space vehicles by reflecting solar photons from a large and lightweight material, so that no propellant is required for primary propulsion. Kapton seems to be the most suitable material for the sail production and in the space missions till now activated booms as deployment systems have always been used. In this work, an innovative self-deploying system based on NiTi shape memory wires has been designed and manufactured in a small-scale prototype. As kapton has always been employed with a thin Al coating on the surface of the sail, commercial pure Al thin sheets with thin adhesive kapton have been used in order to simulate the sail. The attention has been focused, in the deployment experiments performed in the laboratory, on the effect of different heating methods and different pressure conditions on the activation times. The folded configuration chosen has been deployed in atmospheric condition and in low pressure condition (0.05 bar) inside a oven connected to a rotary pump. For what concerns the heating methods, the attention has been focused on low-pressure oven ISCO NSV 9035 (1.3 kW) and on halogen lamp (1 kW) in order to obtain the self-deployment of the sail. Some comparisons between the two configurations in the different environmental conditions have been performed. In all cases, the full self-activation of the sail has been achieved.

Solar sails are propellantless systems where the propulsive force is given by the momentum exchange of reflecting photons. In this study, a self-deploying system based on NiTi shape memory wires and sheets has been designed and manufactured. A small-scale prototype of solar sail with carbon fibre loom has been developed. Different configurations have been tested to optimize material and structure design of the small-scale solar sail. In particular the attention has been focused on the surface/weight ratio and the deployment of the solar sail. By reducing weight and enlarging the surface, it is possible to obtain high values of characteristic acceleration that is one of the main parameters for a successful use of the solar sail as propulsion system. Thanks to the use of shape memory alloys for self-actuation of the system, complexity of the structure itself decreases. Moreover, sail deployment is simpler.

The evolution of the mold temperature during squeeze casting of EN-AB46000 aluminum alloy has been correlated with the final mechanical performances of cast ingots. Starting from a material model which expresses hardness and yielding stress of cast aluminum alloys as a function of the cooling rate during the melt solidification, an experimental approach has been used to provide a useful tool for process monitoring. As a result, the mold temperature increase during the melt squeezing phase is directly correlated with the main mechanical and microstructural parameters. Experiments were made by squeeze cast small cylinders (14 mm in diameter and 18 mm height) at different values of squeezing pressure, mold pre-heating temperature, and melt temperature. Microscopic observations of the sample sections were made as well as hardness measurements and indentation tests. In conclusion, because of the material solidification, a temperature gradient has been observed in the sample which can be directly related with the aluminum alloy dendrite size and, in turn, with microhardness and yielding stress.

The shape recovery efficiency of Ni-Ti shape memory springs has been investigated upon the application up to 6 X 105 thermo-activation cycles. The hysteretic behaviour of the Martensitic-Austenitic phase transition has been characterized by resistivity measurements and infrared thermography. A loss in the recovery efficiency of the original shape has been observed and has been ascribed to functional fatigue leading to the formation of the R phase upon sample heating. Nevertheless, one way shape memory effect was found to exhibit an asymptotic stable behaviour which makes possible the realization of Ni-Ti actuators able to operate for a relative large number of activation cycles.

Microstructural stability is one of the utmost important requirements for metallic materials in engineering applications, particularly at high temperatures. The paper shows how Mechanical Spectroscopy (MS) (i.e., damping and dynamic modulus measurements) permits the monitoring of the evolution of lattice defects, porosity, and cracks which strongly affect the mechanical behavior of metals and sometimes lead to permanent damage. For this purpose, some applications of the technique to different metals and alloys (AISI 304 stainless steel, PWA 1483 single crystal superalloy, nanostructured FeMo prepared via SPS sintering and tungsten) of engineering interest are presented. These experiments have been carried out in lab conditions using bar-shaped samples at constant or increasing temperatures. The results can be used to orient the interpretation of frequency and damping changes observed through other instruments in components of complex shape during their in-service life.

Solar sails exploit the radiation pressure as propulsion system for the exploration of the solar system. Sunlight is used to propel space vehicles by reflecting solar photons from a large and light-weight material, so that no propellant is required for primary propulsion. Kapton seems to be the most suitable material for the sail production and in the space missions till now activated booms as deployment systems have always been used. In this work a self deploying system based on NiTi Shape Memory wires has been designed and manufactured in a small-scale prototype. As kapton has always been employed with a thin Al coating on one or both surfaces of the sail, for the first experiments commercial pure Al thin sheets have been used in order to simulate the sail. In the small-scale prototype manufactured, three different configurations have been studied for bending the sail while two different Nitinol wires have been used as active materials for the self-deployment of the sail. Infrared lamps have been employed in order to warm the solar sail and obtain the activation of the shape memory active elements.

In this research the shape memory effect of commercial NiTi sheets has been investigated. Four commercial SMA elements have been characterized. Tensile tests (load-unload cycles) have been performed at various constant temperatures (from room temperature up to 60 °C). Stress-strain curves have been analyzed and discussed in order to quantify the shape-recovery which has been found higher at increasing testing temperature. A study on the energy dissipation on each cycle has been carried out too and the energy absorption on the whole load-unload cycle has been found higher as the temperature increases.

In tubular elements filled with metal foams the structural collapse is delayed in comparison with the empty tubes,consequently compression strength and absorbed energy increase. Production methods of foams are crucial, asthey determine cellular morphology and bonds formation with tube wall. In this work AISI 316 steel tubes filledwith foam of commercially pure Al and Al-Si alloys with hypoeutectic compositions were produced. The processparameters were optimized to obtain closed cells foams with an acceptable morphology of porosity and goodmechanical properties. Foams were characterized by optical and scanning electron microscopy and by computertomography; mechanical properties were investigated by axial compression tests (performed on foam samplesand on Cu tubes, empty or filled with foams) and radial compression “Brazilian test” (carried out on AISI 316tubes, empty or filled with foams).

AISI 316 steel samples have been investigated by infrared thermography (IRT) after introducing controlled degrees of plastic deformation by means of tensile tests. Microstructural examinations of the same samples showed that the progressive decrease of thermal diffusivity observed for successive steps of plastic deformation is due to the increase of dislocation density. Dislocations affect thermal diffusivity in two ways, by acting as centres of scattering and by inducing elastic stress fields. To evaluate the two contributions, IRT measurements have been also carried outin-situ,during tensile tests at progressively increasing strain. For relevant plastic deformation, the dislocations mainly affect the thermal diffusivity by scattering mechanism. On the basis of these results, it can be concluded that thermal diffusivity is significantly affected by the increase of dislocations density therefore locally resolved IRT measurements seem promising to detect states of local plasticization in mechanical components.

This paper proposes a new technique based on artificial neural network useful for the characterization of superplastic behaviour, in particular for PbSn60 alloy. A three-layer neural network with back propagation (BP) algorithm is employed to train the network. The network input parameters are: alloy grain size, strain and strain rate. Just one is the output: the flow stress. Experiments are performed to evaluate the behaviour of PbSn60 alloy, subject to uniaxial tensile test, when the cross speed is kept constant. The strain rate sensitivity value (m) has been estimated analyzing the slope of the log σ – log e ˙ curve. It is shown that BP artificial neural network can predict the flow stress and, consequently, the m index during superplastic deformation with considerable efficiency and accuracy.

Evidence of exposure of a metal component to a small charge explosion can be detected by observing microstructural modifications; they may be present even if the piece does not show noticeable overall plastic deformations. Particularly, if an austenitic stainless steel (or another metal having a face-centered cubic structure and a low stacking fault energy) is exposed to an explosive shock wave, high-speed deformation induces primarily mechanical twinning, whereas, in nonexplosive events, a lower velocity plastic deformation first induces slip. The occurrence of mechanical twins can be detected even if the surface is damaged or oxidized in successive events. In the present research, optical metallography (OM) and scanning electron microscopy (SEM), and scanning tunneling microscopy (STM) were used to detect microstructural modifications caused on AISI 304Cu steel disks by small-charge explosions. Spherical charges of 54.5 or 109 g TNT equivalent mass were used at explosive-to-target distances from 6.5 to 81.5 cm, achieving peak pressures from 160 to 0.5 MPa. Explosions induced limited or no macro-deformation. Two alloy grain sizes were tested. Surface OM and SEM evidenced partial surface melting, zones with recrystallization phenomena, and intense mechanical twinning, which was also detected by STM and X-ray diffraction. In the samples' interior, only twins were seen, up to some distance from the explosion impinged surface and again, at the shortest charge-to-sample distances, in a thin layer around the reflecting surface. For forensic science locating purposes after explosions, the maximum charge-to-target distance at which the phenomena disappear was singled out for each charge or grain size and related to the critical resolved shear stress for twinning.

X-ray diffraction (XRD) and Mossbauer spectroscopy (MS) measurements were used to evaluate the content of α′ ferromagnetic phase in AISI 304 steel cold rolled and then heat treated for increasing times (up to 7.2×104 s) at 673 K. Both analytical techniques show the same trend of α′ content versus annealing time, characterized by two maxima. The first maximum has been attributed to the growth of preexisting martensitic zones due a different microstress relaxation in the two phases, which induces a stress gradient through the α′/γ interface forcing it to move. The second maximum was explained assuming that local composition variations were induced by carbide precipitation and that a new martensitic phase may form during cooling to room temperature. MS data, i.e. the increase of average hyperfine field of magnetic fraction and the constancy of austenite isomer shift, and the analysis of precision XRD peak profiles support this assumption. Furthermore, MS measurements evidenced in cold rolled samples the presence of layers with decreasing martensite content towards the inside.

A numerical procedure for the prediction of the mechanical performances of squeeze cast ingots of an aluminium alloy is presented. A 2D finite element (FE) model was used to simulate the ingot cooling under squeezing. Extracting the ingot cooling rate across the aluminium alloy freezing range, it was possible to predict the average microhardness by using a material equation which takes into account the dependence of the mechanical properties on the cooling rate. The model was experimentally calibrated by means of laboratory tests. Some cylindrical ingots were direct squeeze cast at different mould temperatures and squeezing pressures. The ingots were characterised by surface analysis, optical microscopy, instrumented macro-indentation and Vickers microhardness. The agreement between the experimental average microhardness and the predicted one was optimal for each ingot.

In this paper, the application of a new technique based on artificial neural network is used for the characterisation of mechanical properties. Three applications are shown in this article: the room temperature superplastic behaviour in PbSn60 alloy, the compressive behaviour of Al foam with different starting powder composition (different pores amount) and the fatigue behaviour in rotating bending tests of Al 6082T6 (as-received and after the fluidised bed treatment). Three neural networks with back-propagation (BP) algorithm have been implemented. After validation, the artificial neural networks are able to predict the output values with considerable efficiency and accuracy.