Papers by Author: Alessandro Morri

Abstract: The high temperature tensile behaviour of the A354 casting aluminum alloy was investigated also evaluating the influence of secondary dendrite arm spacing (SDAS). Cast specimens were produced through a gradient solidification equipment, obtaining two different classes of SDAS, namely 20-25 µm (fine microstructure) and 40-50 µm (coarse microstructure). After hot isostatic pressing and T6 heat treatment, the samples underwent mechanical characterization both at room and high temperature (200 °C). Results of tensile tests and hardness measurements were related to the microstructural features and fractographic characterization, in order to investigate the effect of microstructure and high temperature exposure on the mechanical behaviour of the alloy.

Abstract: The present study aims at evaluating the gas bubbling method, based on the use of dry air as a gaseous phase, for the production of Al based metal matrix nanocomposites through a proper gas-liquid reaction. In particular, Al₂O₃ reinforcement particles were in-situ synthesized in molten commercially pure Al through a gas bubbling oxidation technique. Dry air was injected in the melt in order to induce a controlled oxidation of the molten matrix. SEM-EDS analysis on the produced samples revealed the presence of alumina particles, ranging from the nanoto the micrometric size, demonstrating the feasibility of the process. A hardness increase on the produced samples confirmed the strengthening effect of the in-situ produced ceramic particles.

Abstract: The widespread use of metal matrix composites (MMC) is often limited due to the difficulties related to their joining by means of traditional fusion welding processes. The aim of this work was to evaluate the effect on microstructure and mechanical properties (hardness and tensile strength) of two different friction welding techniques used for joining two Al-based metal matrix composites. In particular, Friction Stir Welding was applied to a 6061 (Al-Mg-Si) alloy matrix, reinforced with 20vol.% of Al₂O₃ particles (W6A20A), while Linear Friction Welding was applied to a 2124 (Al-Cu-Mg) alloy matrix reinforced with 25vol.% of SiC particles (AMC225xe). Both the welding processes permitted to obtain substantially defect-free joints, whose microstructures was found to be dependent on both the initial microstructure of the composites and the welding processes. Hardness decrease was in the order of 40% for the FSW joint and of 10% for the LFW joint, mainly due to overaging of the matrix induced by the frictional heating, while the joint efficiency in respect to the ultimate tensile strength was 72% and 82%, respectively. Elongation to failure increased in the FSW joint due to coarsening of precipitates, whereas it decreased in the LFW joints due to the fibrosity in the thermomechanically altered zone. Fracture surface analysis showed good matrix/reinforcement interface for both composites.

Abstract: This paper presents the results of microstructural and mechanical characterization of Friction Stir Welding joints of two aluminum-based particles reinforced composites. The composites were FSW in the extruded and T6 heat treated condition. No post-weld heat treatment was carried out on the FSW joints. Hardness, tensile, low-cycle fatigue and impact tests were carried out. Microstructural and fractographic characterization were performed both on the base and FSW material, in order to investigate the effect of the solid-state welding process on the reinforcement particles and aluminum matrix. The FSW produced high quality joints with good microstructural characteristics: the welded zone displayed a refinement of the Al matrix grain size and reinforcement particles, and a better particle distribution. The FSW specimens showed high efficiency, both in the tensile, impact and fatigue tests.

Abstract: The aim of the present work is to evaluate the possibility of using the Linear Friction Welding (LFW) technique to produce similar and dissimilar joints between a 2024 Al alloy and a 2124Al/25vol.%SiCP composite. In this solid state joining process the bonding of two flat edged components is achieved through frictional heating induced by their relative reciprocating motion, under an axial compressive force. Microstructural characterization of the welds was carried out by optical and scanning electron microscopy, to investigate the effect of LFW both on the aluminium alloy matrix and the reinforcement particles. The mechanical behaviour of the welded specimens was studied by means of hardness and tensile tests. The mechanisms of failure were investigated by SEM analyses of the fracture surfaces. LFW joints in MMCs resulted substantially defect free, with a uniform particle distribution, while a partial lack of bonding at the corners was observed in the others welds. The hardness decreased by approximately 10% in the welded zone, with some data fluctuations due to the complex microstructural modifications introduced by the LFW process. The joint efficiency, evaluated in respect to the UTS, was 90% for the Al alloy joints and 80% for the MMC joints. A decrease in the elongation to failure was measured in all the LFW specimens, probably related to the orientation of the plastic flow in the TMAZ, where the fracture generally occurred.

Abstract: In the present study, the process modelling of AMCs linear friction weldment was carried out. Four major stages of the process (Part 1: Warm-Up; Part 2: Osci-Forging; Part 3: Forging; Part 4: Cool-Down) were identified and simulated consecutively to generate the temperature field and residual strains distribution within the model. An eigenstrain model calibrated by the neutron diffraction results was also employed to capture the permanent deformation distribution. Good agreement between the process modelling and the experimental measurements was found.

Abstract: The aim of the present work was to evaluate the potential for superplastic deformation of the AZ31 magnesium alloy produced by Twin Roll Casting (TRC), a continuous casting technology able to convert molten metals directly into a coiled strip. In order to develop a superplastic microstructure, the TRC sheets were heated at 400 °C for 2 h, then rolled by multiple passes with re-heating between them, with a total thickness reduction of about 60%. The superplastic behaviour of the alloy was studied by tensile tests, carried out at in the temperature range from 400 °C to 500 °C and with initial strain rates of 1•10-3 s-1 and 5•10-4 s-1. The microstructural and fractographic characterization of the alloy was carried out by means of optical (OM) and scanning electron microscopy (SEM). The tensile tests evidenced a superplastic behaviour of the processed AZ31 Mg alloy, with a maximum elongation to failure of about 500% at 460 °C, with a strain rate of 5•10-4 s-1. The microstructure of the alloy after superplastic deformation showed fine and equiaxed grains, with a large fraction of high angle boundaries. Analyses of the fracture surfaces evidenced flow localization around the grains, suggesting that grain boundary sliding (GBS) was the main deformation mechanism. Failure occurred by cavitation, mainly at the higher testing temperature, due to the prevailing effect of grain growth.

Abstract: This work presents the results of a set of endurance tests performed on steel to steel shaft/bushing tribosystems, without re-lubrication after start-up. The experimental procedure was designed in order to provide simulation for the operative conditions of earthmoving machinery. For this purpose, a new testing machine was specifically manufactured so as to accommodate shaft/bushing assemblies up to a nominal coupling diameter of 90 mm, with a maximum radial load capability of 300 kN. The machine was operated in alternating rotation, with a span angle of 180 degrees, at an angular speed of 10 rpm. The effects of contact pressure intensity were investigated, as well as the effects of grease lubrication type and of the geometry of grease breeding grooves that were located on the bushings inner surface. The tribology behaviour of the tested assemblies was quantified by means of the transmitted torque, grease temperature at the surface of contact and the metallographic analysis of wear out surfaces.