Franco Bonollo

Morphology evolution of gold nanoparticles as function of time, temperature, and Au(III)/sodium ascorbate molar ratio

In this work the morphology evolution of Au nanoparticles (AuNPs), obtained by direct reduction, was studied as a function of time, temperature, and Au(III)/sodium ascorbate molar ratio. The NPs morphology was examined by transmission electron microscope with image analysis, while time evolution was investigated by visible and near-infrared absorption spectroscopy and dynamic light scattering. It is found that initially formed star-like NPs transform in more spheroidal particles and the evolution appears more rapid by increasing the temperature while a large amount of reducing agent prevents the remodeling of AuNPs. An explication of morphology evolution is proposed.Graphical Abstract

Influence of matrix hardness on the dry sliding behaviour of 20 vol.% Al2O3-particulate-reinforced 6061 Al metal matrix composite

Abstract In the present investigation the wear behaviour of the 6061 Al alloy reinforced with 20 vol. % Al 2 O 3 particles dry sliding against a tool steel counterface was studied as a function of load and with reference to different values of the matrix hardness, obtained by submitting the extruded composite to thermal and forging treatments. The obtained wear rates were interpreted on the basis of the analysis of the surface and subsurface damage to the composite due to sliding. If the hardness of the materials is increased by a treatment, their wear rate also increases, because of the occurrence of subsurface softening and the formation of surface mixed scales prone to leaving the tribological system. On the other hand, the as-extruded composite is characterized by a lower matrix hardness (and thus a higher ductility) which prevents the conditions for subsurface softening and formation of the mixed scale from being reached. In this case a transfer layer mainly consisting of iron oxides then forms and protects the composite, thus reducing its wear rate.

The influence of phase transformations on residual stresses induced by the welding process—3D and 2D numerical models

In this work, a numerical study of laser beam welding of steel was performed. In particular, phase transformation effects were considered, which consist mainly of volume change and transformation plasticity. Thanks to the possibilities of numerical modelling, additional analyses were performed (a) without taking into account phase transformations and (b) considering only the transformation plasticity phenomenon.The aim of this study was to examine the influence of phase transformation on the residual stress induced by the welding process, by comparing the results obtained with the described differences in the analyses. Finally, the residual stress field computed by the three-dimensional (3D) model was compared with the one computed by a two-dimensional (2D) model in order to estimate the grade of reliability of the more efficient 2D analyses, also in the presence of phase transformations. It was found that both volume changes due to phase transformations and transformation plasticity have a great influence on the residual stress induced by the welding process. 2D numerical models can be used with good accuracy instead of 3D models, if the in-plane stresses are of primary interest. All analyses in this investigation were performed with the finite element code SYSWELD®.

Microstructure, texture evolution, mechanical properties and corrosion behavior of ECAP processed ZK60 magnesium alloy for biodegradable applications

Ultra-fine grained ZK60 Mg alloy was obtained by multi-pass equal-channel angular pressing at different temperatures of 250°C, 200°C and 150°C. Microstructural observations showed a significant grain refinement after ECAP, leading to an equiaxed and ultrafine grain (UFG) structure with average size of 600nm. The original extrusion fiber texture with planes oriented parallel to extrusion direction was gradually undermined during ECAP process and eventually it was substituted by a newly stronger texture component with considerably higher intensity, coinciding with ECAP shear plane. A combination of texture modification and grain refinement in UFG samples led to a marked reduction in mechanical asymmetric behavior compared to the as-received alloy, as well as adequate mechanical properties with about 100% improvement in elongation to failure while keeping relatively high tensile strength. Open circuit potential, potentiodynamic and weight loss measurements in a phosphate buffer solution electrolyte revealed an improved corrosion resistance of UFG alloy compared to the extruded one, stemming from a shift of corrosion regime from localized pitting in the as-received sample to a more uniform corrosion mode with reduced localized attack in ECAP processed alloy. Compression tests on immersed samples showed that the rate of loss of mechanical integrity in the UFG sample was lower than that in the as-received sample.

Rapid solidification in laser welding of stainless steels

Abstract The microstructural characterization of both weld beads and heat affected zones (HAZ) was carried out on austenitic (AISI 304, 316) and duplex (UNS 31803) stainless steels, laser welded under various working parameters (power, traverse speed, shielding gas), by means of light microscopy, SEM, TEM, and image analysis, with the aim of pointing out changes in the amounts of the present phases, with respect to those predicted by equilibrium diagrams. Moreover, an analytical thermal model of laser beam welding was employed in order to evaluate the cooling rates involved in the process. The thermal field analysis, checked by comparing the calculated and the actual weld beads, has been used as a tool aimed at correlating cooling rates and microstructural characteristics.

New Classification of Defects and Imperfections for Aluminum Alloy Castings

In recent years, aluminum alloys have become more and more relevant because of their low density, coupled with good mechanical and corrosion properties. Different processes are available for the production of aluminum alloy components, such as rolling, extrusion, and powder metallurgy, but a significant role is played by foundry processes. Defects and imperfections are physiologically generated by different casting techniques as a result of the process stages, alloy properties and die or mold design.In the present work, a multi-level classification of structural defects and imperfections in Al alloy castings is proposed. The first level distinguishes defects on the basis of their location (internal, external, or geometrical), the second level distinguishes on the basis of their metallurgical origin, while the third level refers to the specific type of defect, because the same metallurgical phenomenon may generate various defects.

Influence of Injection Parameters on the Porosity and Tensile Properties of High-Pressure Die Cast Al-Si Alloys: A Review

Aluminum-Silicon alloys are the most extensively used Al foundry alloys and are widely used in high-pressure die casting (HPDC) of automotive components. Several process parameters need to be controlled during HPDC in order to obtain sound and reliable castings. Among the different process variables, the determination and control of the injection parameters, such as the gate velocity and intensification pressure (IP), remain a key requirement throughout the HPDC process. This work critically reviews the effects of the injection parameters on the porosity and tensile properties of the die castings. The results of the literature review are summarized and optimal values for the gate velocity and IP are suggested.

A Semiempirical Model for Sigma-Phase Precipitation in Duplex and Superduplex Stainless Steels

Sigma phase is known to reduce the mechanical properties and corrosion resistance of duplex and superduplex stainless steels. Therefore, heat treatments and welding must be carefully performed so as to avoid the appearance of such a detrimental phase, and clearly, models suitable to faithfully predict σ-phase precipitation are very useful tools. Most fully analytical models are based on thermodynamic calculations whose agreement with experimental results is not always good, so that such models should be used for qualitative purposes only. Alternatively, it is possible to exploit semiempirical models, where time-temperature-transformation (TTT) diagrams are empirically determined for a given alloy and the continuous-cooling-transformation (CCT) diagram is calculated from the TTT diagram. In this work, a semiempirical model for σ-phase precipitation in duplex and superduplex stainless steels, under both isothermal and unisothermal conditions, is proposed. Model parameters are calculated from empirical data and CCT diagrams are obtained by means of the additivity rule, whereas experimental measurements for model validation are taken from the literature. This model gives a satisfactory estimation of σ-phase precipitates during both isothermal aging and the continuous cooling process.

Fluidity of aluminium die castings alloy

Abstract The aim of the present work was to investigate the fluidity of four different high pressure die cast Al–Si alloys at different pouring temperatures. A vacuum fluidity test apparatus was employed to measure fluidity. The analysed alloys showed different flow sensitivities to casting temperatures. Furthermore, it is showed that among the considered alloying elements, magnesium and silicon affected the fluidity of the melt. One alloy was then contaminated with 50% scrap addition, increasing the amount of oxide inclusions. The fluidity of the contaminated melt has then been measured and compared with the fluidity of the clean melt. The results show that the fluidity of the alloy with scrap addition is lower than that of the clean melt. Further the fluidity linearly increases at increasing temperatures within the range between 580 and 680°C until it reaches a plateau at the highest pouring temperatures.

Mechanical and impact behaviour of (Al2O3)p/2014 and (Al2O3)p/6061 Al metal matrix composites in the 25–200°C range

The present work is aimed at studying the impact behaviour of commercially available Aluminium matrix composites, in a temperature interval ranging from 25°C to 200°C. The results of instrumented impact tests and of microstructural and fractographic observations are correlated with the tensile properties of these materials. A description of the phenomena involved (particles cracking, interfacial failure associated to matrix-reinforcement reaction layers, ductile behaviour of the matrix) is given. The effect of testing temperature as well as that of the matrix characteristics are presented and discussed.