Papers by Author: E. Evangelista

Abstract: TMP of Al alloys includes hot working with dynamic substructures and deformation bands for texture components combined with static recovery or recrystallization as well as cold working altered by annealing. The above processes are separately tailored for solute (Al, Mg), dispersoid (Al-0.7Fe) and precipitation hardening alloys; aging combined with deformation can raise strength or improve fatigue or corrosion resistance. Hot and cold rolling with suitable holding intervals are managed to combine deformation and annealing textures for planar anisotropy or for producing less fibrous grains to avoid delamination corrosion; grains may be severely refined by discontinuous or continuous recrystallization for superplastic sheet. In hot-billet and impact extrusion as an addition to substructure and texture strengthening, the intense heating near the die may be employed for precipitate solution with exit quenching for press heat treatment to T5 temper. Similarly, friction stir surface treatment and welding provide intense hot straining with additional softening as metal is swept behind the pin. In combination with some of the above, forging provides grain and dispersoid fibering oriented for crack retardation; semi solid forming competes with this.

Abstract: The modeling of extrusion of various Al alloys and their particulate metal matrix composites was conducted by DEFORM™ finite element analysis to develop strain rate, stress and temperature distributions through the peak load and into steady state following development of the hot zone. The hot strength and ductility, constitutive constants and microstructural evolution had been determined by hot torsion. The relative load-stroke curves were determined for several billet temperatures, extrusion ratios and ram speeds. The grid distortion and distributions of important internal parameters define the evolution of microstructure. The extrudability was estimated on the basis of load, ductility and the potential for modeling the microstructure developed.

Abstract: One of the most challenging activities for magnesium industry is to increase productivity by introducing cost-effective processes. While die-cast magnesium alloys are in worldwide use, even in the demanding sector of automotive industry, where cost is the major asset, wrought magnesium alloys still not widely available. The aim of the present study was to evaluate the high temperature response by torsion testing of a ZM21 alloy, to calculate the constitutive equations to be used in FEM simulation of the extrusion process. The ZM21 alloy that was considered in the present study was Direct-Chill casted by Alubin Ltd., Israel. Torsion tests were carried out in air on a computer-controlled torsion machine, under strain rates ranging from 10-2 to 5 s-1 and temperatures from 200 to 400°C. The equivalent peak flow stress was related to temperature and strain rate by means of the conventional power-law and by the sinh equation. The microstructure of the alloy, even at 300°C, appeared largely unrecrystallized, with elongated grains; also at 400°C, the structure is more equiaxed, but elongated structures still appears.

Abstract: The influence of initial texture on the formation of primary twin system of AZ31 Magnesium rolled plate was investigated in this work. Uniaxial compression tests were carried out on samples cut along the rolling direction (RD) and normal direction (ND) of rolled AZ31 Mg plate at various temperatures (RT, 150, 200, 250, 300, 350, 400, 450°C) with the fixed strain rate (0.01s-1). The results showed that the primary twinning system of AZ31 Mg alloy (c-axis extension twin) occurred actively in the RD compression specimens, which promoted homogeneous deformation as compared to the ND compression specimens. The effect of temperature on the formation of deformation twins was also investigated, and slip/twin transition temperature was found to be 250°C.

Abstract: The tensile properties and the microstructure of an Al-7%Si-0.6%Mg-0.5%Cu rheo-cast component were investigated. The material underwent a T5 treatment, consisting in ageing at 160, 175 and 190°C for durations ranging from 0.5 to 48h. Tensile testing indicated that the T5 treatment resulted in a relatively good level of strength and in a comparatively low ductility. In order to improve ductility, maintaining as low as possible the cost of the final component, a single solution treatment at 500°C for 4h was subsequently applied. The tensile strength and ductility of the solution treated and aged material were higher than in the T5 condition. These differences were attributed to the microstructural evolution occurring during exposure at 500°C, in particular to the spheroidization of eutectic-Si and to a more homogeneous distribution of the precipitates.

Abstract: The microstructural evolution with strain was investigated either in a Zr-modified 6082 Al-Mg-Si alloy and in the same alloy added with 0.117wt.% Sc, subjected to severe plastic deformations. Materials were deformed by equal-channel angular pressing using route BC, up to a true strain of ∼12. A strain of ~4 produced a sub-micrometer scale microstructure with very fine cells (nanometer scale) in the grain interior. The role of fine dispersoids (Al3(Sc1-x,Zrx)) was investigated by transmission electron microscopy techniques and discussed. Dispersoids were responsible for a more complex dislocation substructure with strain. Compared to the commercial parent alloy, block wall formation and propagation were favored by the presence of Sc-Zr containing dispersoids, while cell boundary evolution was less affected, compared to the commercial parent alloy. Mean misorientation across block walls increased with strain much more in the Sc-Zr containing alloy, reaching a plateau, starting from a true strain of ∼8. Misorientation across cell boundaries continuously increased to ∼8° and ∼5° for the Sc-Zr and Zr containing alloy, respectively.

Abstract: Mechanical spectroscopy was employed to investigate the microstructure evolution of a Zr-modified 6082 Al-Mg-Si alloy and the same alloy with Sc addition after ageing and following severe plastic deformation through equal channel angular pressing. Measurements of the internal friction and dynamic young modulus have been performed in isothermal and isochronal runs in the frequency range 0.1 - 104 Hz. The anelasticity spectra reveal in the temperature range 470-870 K both structural and anelastic relaxation processes. Two structural damping maxima connected with inverse temperature trend of the modulus occur in the alloys submitted to equal channel angular pressing, the first one is strongly suppresed by Sc and Zr addition. An anelastic relaxation peak whose strength depends on the nature and morphology of precipitates and dispersoids and on the deformation and ageing condition was observed in all samples investigated. The high background damping occurring before the first structural damping maximum is analyzed with reference to a superplastic behavior of the equal channel angular pressing processed alloys.

Abstract: The micron-size grain refinement of pure a-zirconium obtained with elevated temperature tensile deformation was investigated. The development of low-misorientation subboundaries caused the serration of the original grain boundaries at low strains. The final microstructure (e.g. strains > 3) was predominantly composed of fine, equiaxed “crystallites” with ⅔ of the boundaries being of very low misorientations (< 3°) and the remaining ⅓ being high angle boundaries (θ > 8°, and typically 25-35°). Discontinuous dynamic recrystallization was excluded as a possible mechanism due to the absence of newly formed grain nuclei. The bimodal distribution of the crystallite or (sub)grain boundary misorientations is inconsistent with the occurrence of continuous dynamic recrystallization and rotational recrystallization. The continual thinning of the original grains, the serration of the high angle boundaries, the bimodal misorientation distribution of misorientations, ⅔ of boundaries of very low misorientations at high strains all strongly suggest geometric dynamic recrystallization and dynamic recovery as the grain refinement and restoration mechanisms.