Shape memory alloys for aerospace, recent developments, and new applications: a short review

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

HYBRID SOLUTION FOR TWO-WAY INDUCED SHAPE MEMORY ACTUATOR

The actuation capability and the reliability of hybrid composites activated by shape memory alloys (SMA) 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 problems related to the production of such elements 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

Mechanical behavior of Nd:YAG laser welded aluminum alloys

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

Mechanical behavior of PCMT and SDP Al foams: A comparison

n 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

Interfacial reactions between alsi10 foam core and aisi 316l steel sheets manufactured by in-situ bonding process

Aluminum foam sandwiches (AFS) with AlSi10 foam cores and AISI 316L steel skins are manufactured by an in-situ bonding process. The precursor of the core foam was made with the powder compacted method. The precursor and skins, coupled together, were then heated up to the melting point of the Al alloy. The gas released by the blowing agent formed hydrogen bubbles in the melt. producing the foam. Such a porous structure was kept frozen at room temperature via cooling in cold water. To optimize the process conditions, some foaming experiments have been conducted with different holding times and temperatures. Such manufactured AFS were cut, chemically etched and studied with an optical microscope associated with image analysis software to get information about pores morphology in terms of circularity and equivalent diameter. The interface AlSi10-AISI316L has been characterized by SEM and EDX to investigate the bonding conditions between cores and skins. Finally, the AFS have been polished and etched to analyze the microstructure. Quasi-static compressive tests have been performed on the AFS. Obtained results showed that the interface formed during the foaming can be characterized by the inter-diffusion of alloying elements, as confirmed by the good quality of metallurgical joints

Manufacturing and Characterization of AlSi Foams as Core Materials

AbstractAlSi alloys and their foaming properties have been studied in this paper. For adequate comparison it has been necessary to define process parameters and optimal chemical composition of the Al alloys. Such foams have been evaluated in terms of structure and mechanical properties, in particular in the use of foams as cores materials of cylindrical massive skins

Al foams manufactured by PLA replication and sacrifice

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. Lightweight 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

Experimental Set-Up of the Production Process and Mechanical Characterization of Metal Foams Manufactured by Lost-PLA Technique with Different Cell Morphology

A flexible and versatile method for manufacturing open-cell metal foams, called lost- PLA, is presented in this work. With a double extruder 3D printer (FDM, Ultimaker S3, Utrecht, The Netherlands), it is possible to make polymer-based samples of the lost model. Through CAD modeling, different geometries were replicated so as to get black PLA samples. This method combines the advantages of rapid prototyping with the possibility of manufacturing Al-alloy specimens with low time to market. The production process is articulated in many steps: PLA foams are inserted into an ultra-resistant plaster mix, after which the polymer is thermally degraded. The next step consists of the gravity casting of the EN-6082 alloy in the plaster form, obtaining metal foams that are interesting from a technological point of view as well as with respect to their mechanical properties. These foam prototypes can find application in the automotive, civil and aeronautical fields due to their high surface/weight ratio, making them optimal for heat exchange and for the ability to absorb energy during compression. The main aspects on which we focus are the set-up of the process parameters and the characterization of the mechanical properties of the manufactured samples. The main production steps are examined at first. After that, the results obtained for mechanical performance during static compression tests with different geometry porosities are compared and discussed. The foam with truncated octahedron cells was found to show the highest absorbed energy/relative density ratio

Correlation modeling between morphology and compression behavior of closed-cell al foams based on x-ray computed tomography observations

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

Design and Characterization of a Small-Scale Solar Sail Prototype by Integrating NiTi SMA and Carbon Fibre Composite

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