G Costanza

Effect of powder mix composition on Al foam morphology

The effect of mix composition on foam morphology has been examined by image analysis carried out on metallographic sections of Al foams prepared by powder metallurgy. Two sets of samples have been prepared by using SiC particles with mean sizes of 37 and 60 µm. Each set consists of 16 groups of samples with different amounts of TiH2 (0.1, 0.2, 0.4, and 0.6 wt%) and SiC (0.8, 2.8, 6, and 9 wt%). The distribution of SiC particles on the internal walls of the bubbles has been investigated by scanning electron microscopy observations, which evidenced also the presence of particles of another phase, identified as Ti3Al by energy dispersion spectroscopy. Some tests, performed without SiC particles, showed that the Al foaming occurs also under these conditions, however foams exhibit few bubbles of very large size and irregular shape. Experimental data have been used for training two multi-layer feedforward artificial neural networks. The models represent useful tools for predicting morphologic features of foams for any given mix composition in the training range.

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).

Auxetic epoxy foams produced by solid state foaming

Auxetic epoxy resin foams were produced by solid-state foaming thanks to the use of properly shaped precursors. In fact, a re-entrant hexagonal shape of the precursors is preserved during foaming and results in a foam with a complex structure: a thin macro-structure with the re-entrant geometry filled with foam. The auxetic behavior was observed by using tensile tests at different temperatures (room temperature, 80℃, and 100℃). Indentation tests were also carried out to evaluate the gradient properties across the lines of the thin re-entrant macro-structure. In order to show that the auxetic behavior depended on the internal macro-structure, tests were also performed on foam panels obtained by cylindrical tablets and, therefore, with a standard-hexagonal macro-structure. In conclusion, the auxetic behavior was observed only for the foam panels with re-entrant hexagonal structure at 80℃. In this case, a negative Poisson’s ratio is immediately achieved at small strains and tends to a zero plateau value for longitudinal strains up to 1%.

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.

Fatigue behaviour of Ti sputtered Al composites

Remarkable enhancement of fatigue life and endurance limit of Al matrix composites has been achieved by thin coatings of Ti sputtered at room temperature. Three materials have been investigated. They differ for matrix (6061, A359, 2618), reinforcement particles (SiC and Al2O3) and preparation process (casting and powder metallurgy). Fatigue tests employing a rotating bending machine showed that the method works on all of them, higher effectiveness has been observed in the composite (6061/ 20%SiCp) which has more homogeneous reinforcement distribution. An improvement of fatigue behaviour has been observed also in tests performed on the composite A359/20%SiCp at 200°C, typical working temperature for some industrial applications of Al matrix composites. Coatings seal the defects present on the external surface of the composites, retard crack growth thus lead to an improvement of fatigue behaviour. The effect of coating on fatigue behaviour is discussed with reference to the surface state, which has been investigated by scanning electron microscopy (SEM), scanning tunnelling microscopy (STM), X-ray diffraction (XRD) and microhardness 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.