Maurizio Ferrante

Diffusion bonding of aluminium oxide to stainless steel using stress relief interlayers

Abstract Aluminium oxide was diffusion bonded to AISI 304 steel using Ti, Cu or Mo as interlayer materials. It was observed that Ti joins easily to both ceramic and steel parts, giving an average shear strength equal to 20 MPa. However, within the experimental conditions applied, the adhesion between Al 2 O 3 and Cu or Mo was unsuccessful. A finite element modelling (FEM) was employed to evaluate the residual stresses in the joints and showed that the use of Mo and Cu reduced the stress level, when comparing to Ti. Results from the FEM suggests that similar behaviour is expected when an interlayer comprising a thin Ti foil in contact with the alumina and a thick Cu or Mo foil is used. Diffusion bonding experiments using such combinations substantiated the FEM results and the average strength of the joints reached 27 MPa with Ti/Mo and 65 MPa with Ti/Cu. Ti 3 Al formation was found to be responsible for the Al 2 O 3 /Ti adhesion whilst β-Ti phase stabilisation was detected at the Ti/steel interface and attributed to the intense diffusion of Fe into the Ti.

Al2O3/Ti interlayer/AISI 304 diffusion bonded joint : Microstructural characterization of the two interfaces

Abstract Sintered alumina and AISI 304 stainless-steel discs were joined at 900°C by solid state diffusion bonding, making use of a Ti foil acting as thermal stress relief interlayer. The microstructure of the two interfaces thus formed, that is, Al2O3/Ti and Ti/AISI 304 was investigated by a variety of characterization techniques such as scanning and transmission electron microscopy (TEM), energy-dispersive X-ray (EDX) microanalysis, X-ray diffraction and Vickers microhardness, with the following results: (i) precipitation of Ti3Al particles plus Al and oxygen diffusion were detected within the Ti and close to the ceramic; (ii) within the Ti foil, coexistence of α-Ti and β-Ti was observed and ascribed to Fe and Cr diffusion and to β-Ti decomposition during slow cooling from the bonding temperature; and (iii) adjacent to the Ti/steel interface a 5-μm-thick layer was found to contain TiFe and Fe2Ti, as well as sigma phase, M23C6 precipitates and TiC. An interpretation of these results in terms of element partition during the bonding process was attempted.

Rheology and microstructural development of a Al–4wt%Cu alloy in the semi-solid state

Abstract Rheological and microstructural studies were performed on a Al–4wt%Cu alloy in the semi-solid state employing two different experimental techniques: the indentation test and the simple compression test. Results showed that recrystallized microstructures exhibit lower viscosities than dendritic microstructures and that the rheological behavior can be described by the Power Law. The viscosity values were analyzed in terms of solid volume fraction and deformation rate and the pseudoplasticity of the Al–4wt%Cu alloy was thus ascertained. As for the solid particle size and shape evolution within the semi-solid temperature range, the experimental growth kinetics obeyed the classic D 3 = K c t relation, which indicates a Ostwald Ripening type of growth, but the particles size distribution was wider than the one predicted by the LSW theory. Also, comparison of the experimental and the theoretical growth rate constants suggested that coarsening due to hard impingement of the solid particles was the main growth mechanism.

The role of oxygen rich inclusions in determining the microstructure of weld metal deposits

The influence of the deoxidation product size distribution in determining the prior austenite grain and the subsequent balance between inter- and intragranular nucleation has been studied in C-Mn-Nb weld metals. The growth of austenite appears to be controlled by pinning of the grain boundaries by the deoxidation inclusions, and follows the classical Zener precipitate-boundary interaction equations. Some evidence of the direct nucleation by inclusions of the acicular ferrite phase has been obtained.

Rheological behaviour and deformation characteristics of a commercial and a laboratory-cast Al-4%Cu alloy in the semi-solid state

Abstract A commercial AA2024 and a Al-4%Cu alloy in the semi-solid state (high solid volume fraction) were studied with regard to both rheological and mechanical behaviour during simple compression. From stress–strain curves and power law exponents analysis two deformation regimes were identified. The deformation response was adequately described by the Ngyuen, Favier and Suery theoretical model. Comparison of stress–strain curves of the two materials showed that the 2024 alloy has higher intrinsic resistance. Interrupted and jump compression tests, plus different dwelling time experiments gave information on both thixotropy and microstructural evolution of the alloys. Microstructural observation showed that deformation is highly inhomogeneous and takes place by the formation of liquid-filled channels surrounding particle aggregates. Correlations between liquid phase segregation, particle size distribution and process parameters are discussed. Kurzfassung— Eine Handels-AA2024- und eine Al-4%Cu-Legierung wurden im Fest-Flussig-Zustand (hoher Feststoffanteil) in Bezug ihres rheologischen und mechanischen Verhaltens durch einfache Zusammenpressung untersucht. Aus den Spannungs-Verformung-Kurven und der Analyse des Potenzgesetz-Exponentes wurden zwei Verformungsarten festgestellt. Der Verformungsanspruch wurde in angemessener Weise durch das theoretische Modell von Ngyuen, Favier und Suery beschrieben. Der Vergleich zwischen den Spannungs-Verformung-Kurven beider Werkstoffe zeigte, daβ die 2024-Legierung eine hohere Eigenfestigeit besitzt. Unterbrochener- und Sprung-Zusammenpressungstest bzw. Verweilzeit-Experimente haben Informationen uber die Tyxotropie und die mikrostrukturelle Entwicklung der Legierungen ergeben. Die mikrostrukturelle Beobachtung zeigte, daβ die Verformung sehr ungleichmassig ist und, daβ sie durch die Entstehung von mit Schmelze gefullten Kanalen, die von Feststoff-Aggregaten umgeben werden, vorgeht. Der Zusammenhang zwischen Seigerung der flussigen Phase, Partikelgroβenverteilung und Prozessparametern wurde diskutiert.

Interface compounds formed during the diffusion bonding of Al2O3 to Ti

The interfacial reaction products of Ti/Al2O3 joints obtained in the context of real diffusion bonding technology were investigated by means of X-ray diffraction analysis, X-ray photoelectron spectroscopy, and transmission electron microscopy. Some Ti reacted with Al2O3 giving titanium oxides, but the main mass transport occurred into the bulk Ti due to Al2O3 dissolution. The formation of a Ti[Al, O] solid solution followed by a order/disorder reaction yielded Ti3Al. Further Al enrichment at the interface could lead to the formation of TiAl, which was not observed in the present work due to either the short residence time at the bonding temperatures or to its lower oxygen solubility. For joints obtained at 800°C and shear test fractured it was ascertained that the crack always propagated within the Ti3Al layer.

Materials selection as an interdisciplinary technical activity: basic methodology and case studies

The technical activity known as Materials Selection is reviewed in its concepts and methodologies. Objectives and strategies are briefly presented and two important features are introduced and discussed; (i) Merit Indices: a combination of materials properties, which maximises the objectives chosen by the designer and (ii) Materials Properties Maps: a bi-dimensional space whose coordinates are pairs of properties in which materials can be plotted and compared directly in terms of their merit indices. A general strategy for the deduction of these indices is explained and a formal methodology to establish a ranking of candidate materials when multiple constraints intervene is presented. Finally, two case studies are discussed in depth, one related to materials substitution in the context of mechanical design and a less conventional case linking material selection to physical comfort in the home furniture industry.

Microstructure, material flow and tensile properties of A356 alloy thixoformed parts

The present work examines microstructure, tensile properties and quality (porosity level) of two thixoformed products, namely an assembly of three cylindrical rods from which tensile samples were machined out, and a cup-shaped part. The material is an A356 aluminium alloy and the microstructural conditioning was carried out by the deformation - recrystallization technique. Additionally, the microstructures of an A356 ingot produced by the magnetic stirring process was also investigated. Results showed that solid fraction and soaking time in the semi-solid state determine the thixoforming forces, that very low porosity levels can be achieved and minimized by the maintenance of high pressures during solidification, and that the material flow during injection in the mould is relatively homogeneous, except for the material conditioned by magnetic stirring. Physical simulation experiments of the thixoforming process (rods) showed a strong correlation between liquid segregation and porosity level. Finally, the tensile properties were shown to be much higher that those of permanent cast samples.

A general study of commercially pure Ti subjected to severe plastic deformation: microstructure, strength and corrosion resistance

Bars of Titanium Grade 2 were subjected to deformation by Equal Channel Angular Pressing, both at room temperature and at 300°C. Additionally, some specimens were cold rolled up to 70% reduction. From tensile tests, data such as yield, maximum strength, elongation and area reduction were obtained. Results show that the best strength - ductility combination is produced by four passes followed by cold rolling. Finally, the corrosion behavior was assessed following ASTM F2129 and no noticeable difference between the starting material and the ECAP-deformed was detected.

The effect of Sn additions on the semi-solid microstructure of an Al-7Si-0.3Mg alloy

Abstract The influence of Sn additions on both the binary Al–Si equilibrium phase diagram and the microstructural evolution of an Al7Si0.3Mg alloy (A356) having a solid/liquid ratio close to 0.5 were studied. Application of the THERMOCALC method showed that in the range 0.2–10.0 wt.%, Sn decreases all the L→S transformation temperatures. The microstructural evolution was studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and by quantitative optical microscopy. Results showed that Sn additions decreased the rate of attainment of the equilibrium liquid fraction besides reducing the kinetics of particle spheroidisation and growth. This behaviour was attributed to a decrease of solid–liquid surface energy brought about by the presence of Sn as small globules within the Si or Al phases and at the Al–Si interface. This interpretation was supported by experimental measurements of particle contiguity and by dihedral angle calculations. The less accentuated effect observed on sample 2.0Sn, was explained by a different spatial distribution of the Sn globules.