Maria Elisa Tata

A Weldability Study of Al–Cu–Li 2198 Alloy

Al–Cu–Li alloys, conceived for automotive and aeronautic applications thanks to the high mechanical resistance/density ratio, exhibit weldability issues common to all light alloys. In this paper, the weldability of Al–Cu–Li 2198 alloy was studied by comparing features of welds carried out by two processes, the traditional arc welding and the friction stir welding (FSW). Welded joints were submitted to optical and SEM metallographic examinations with EDS microanalysis measurements. Mechanical characteristics were evaluated through microhardness tests and the instrumented indentation test FIMEC (Flat-top cylinder Indenter for MEchanical Characterization).

Pressure effect on Al alloy cast behaviour: microstructures and mechanical properties

Dealing with pressure-assisted fabrication processes of aluminium alloys, the applied pressure has a double effect, a direct and an indirect one. Except for porosity, the direct effect is connected to the thermodynamic equilibrium modification and the indirect effect is related to the cooling rate variation. A suitable apparatus was built and an experimental procedure was carried out in order to investigate both pressure effects, showing the predominance of the indirect one. Microstructural analyses and mechanical tests were performed for this aim. An FE numerical model was implemented in order to correctly assist the experimental activity too. Starting from the evidence of the greater importance of the pressure indirect effect, a model was proposed so as to predict the alloy mechanical properties as a function of the cooling rate. The proposed model allows to realise important process considerations and is the first step in order to predict the final performances of a given aluminium alloy component when the geometry and the process parameters employed during fabrication are known.

IR Thermography and Resistivity Investigations on Ni-Ti Shape Memory Alloy

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.

CHACTERIZATION OF CU TUBE FILLED WITH AL ALLOY FOAM BY MEANS OF X-RAY COMPUTER TOMOGRAPHY

Copper tubes filled with aluminium foams were prepared by directly foaming metal powder compacts inside them. Compressive behaviour and foam-shell interface, that characterizes mechanical properties of reinforced tubes, were investigated by means of variable focus X-ray computer tomography. Compression tests were performed on empty and filled samples at increasing deformation steps: at each stage the samples were observed by tomography. A geometric evaluation of porosity on 2D sections was performed by calculating, for each pore, its area, equivalent diameter and circularity.

Effects of thermal treatments on the ductile to brittle transition of MANET steel

The effect of the cooling rate T from austenitic field on the microstructural evolution and the mechanical stability of MANET steel for increasing times of tempering at 700 degrees C has been studied by means of Charpy tests, SEM observations with EDS microanalysis and X-ray diffraction analysis. The investigated rates T range from 150 to 3600 degrees C/min. The ductile to brittle transition temperature, the upper shelf energy and the slope a Delta E/Delta T in the transition region depend on the time of treatment with variations more pronounced in samples slowly cooled. After tempering times of about 20 h at 700 degrees C the samples cooled with T = 150 degrees C/min show a mixed mode of brittle fracture (quasi-cleavage and intercrystalline) with good correspondence between Cr-enriched zones and surfaces fractured in intercrystalline mode. Only quasi-cleavage mechanism is operative in samples quenched with T = 3600 degrees C/min, which present fracture surfaces with uniform Cr distribution. The results are discussed on the basis of C-Cr associate distributions determined by internal friction tests.

Mechanical and metallurgical characterization of 8090 Al–Li alloy welded joints

The properties of Al–Li alloys have been extensively studied recently for structural applications in automotive and aerospace industry, thanks to their lower density and enhanced modulus of elasticity. In common with many Al alloys, one of the main issues is the loss of toughness and soundness of welded joints consequent upon welding operations, in particular, possible cracking in the weld metal and metallurgical modifications induced in the heat-affected zone. In this work, welding trials were carried out on 8090 Al–Li plates welded by electric arc, using a 5% Mg filler metal. Joints were mechanically and metallurgically characterized through tensile and microhardness tests, optical and scanning electron microscopy, and fractography on both welded and unwelded tensile specimens. Energy dispersive spectroscopy (EDS) measurements were also performed to evaluate chemical composition locally and determine the nature of precipitates.

Effect of Temperature on the Mechanical Behaviour of Ni-Ti Shape Memory Sheets

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.

Recycling of exhaust batteries in lead-foam electrodes

Lead and lead-alloy foams have been investigated in this research. In particular low-cost techniques for the direct production of lead-based electrodes have been analyzed and discussed in this work. The relevance of the main process parameters (powder compacting pressure, granulometry, base metal composition, sintering temperature and time) have been focused and the effect on foam morphology has been discussed too. In particular “Sintering and Dissolution Process” (SDP) and “Replication Process” (RP) have been employed and suitable modified. Both spherical urea and NaCl have been adopted in the SDP method. In the replication process it has been evidenced that the viscosity of the melt is fundamental. Furthermore the research examines lead recovery and recycling of exhaust batteries into foam-based electrodes. A novel method for the direct conversion of Pb scrap into lead foam is discussed too.

New capabilities in the numerical simulation of aluminium alloy casting processes

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.

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.