镁合金表面防腐层的研究现状

本文综述了镁合金防腐层的研究现状,介绍了包括金属及其合金涂层、陶瓷涂层、化学转化膜以及有机膜在内的四种典型表面防护层特点和研究应用现状,并展望镁合金表面防腐层的发展前景。     

Modification of Roll Pass Design of Low Pearlite,Fine-Grained Sheet Steel Using Numerical Modeling and Processing Maps

Abstract

The main objective of this pass design is to achieve fine-grained structure with high strength properties and good ductility for 2.0 mm thickness steel sheet on a six stand finishing mill. Numerical modeling is used to evaluate the different rolling parameters while processing maps are used to determine a workability “window” in which dynamic recrystallization must occur.

The existing rolling parameters would not lead to a dynamically recrystallized structure. The proposed technique retains the sequence of reduction in thickness at the working temperatures, while it diminishes the rotational speed of the first stand and consequently all the other stands. The first stand is proposed to rotate with 25 rpm at 7.2 s-1 rate of deformation. The final rotational roll speed would be 177 rpm at stand no. 6 with 83.4 s-1 rate of deformation at 850 °C. Rolling force and torque needed in the proposed technique are lower than those in the existing ones at most of the passes. The proposed technique saves energy as it reduces the power consumption.

Le principal objectif de cette conception de passe de laminoir est d'obtenir une structure à grain fin avec des propriétés de résistance élevée et une bonne ductilité pour une tôle d'acier d'une épaisseur de 2.0 mm, sur un laminoir finisseur à six cages. On utilise la modélisation numérique pour évaluer les différents paramètres de laminage alors qu'on utilise les plans de formage pour déterminer une "fenêtre” de fabricabilité dans laquelle doit se produire une recristallisation dynamique.Les paramètres actuels de laminage ne peuvent conduire à une structure dynamiquement recristallisée. La technique proposée retient la séquence de réduction en épaisseur aux températures d'opération, tout en diminuant la vitesse de rotation de la première colonne et, par conséquent, de toutes les autres colonnes. On propose que la rotation de la première colonne soit de 25 rpm, avec un taux de déformation de 7.2 s-1. La vitesse du dernier rouleau serait de 177 rpm à la colonne no. 6, avec un taux de déformation de 83.4 s-1, à 850 °C. La force de laminage et le moment de torsion nécessaires pour la technique proposée sont plus bas que ceux qui existent déjà pour la plupart des passes. La technique proposée conserve l'énergie puisqu'elle réduit la consommation de pouvoir.     

Protective Oxide Film on Alloy X750 Formed in Air at 973 K

An effective pre-oxidation method for Alloy X750 was developed to reduce general corrosion in an oxygenated aqueous environment such as in BWR core water. The optimum condition of preoxidation in air at elevated temperatures was found to be 5–20 h at 973 K by considering the allowance condition of heat treatment for age-hardening.

Some characteristics of the corroded oxide film have been clarified by surface analyses with XMA, SIMS, AES, XPS etc. The film was composed of double oxide layers, namely a highly crystallized NiFe₂O₄ outer layer and a high Cr₂O₃ content inner layer. The passive property of the film has been recognized to be due to the nature of the oxides whereby NiFe₂O₄ restricts the dissolution of metals because of its low solubility and Cr₂O₃ restricts the diffusion of metal ions because of its high binding energy and low diffusion coefficient.     

Corrosion Behavior of Extruded near Eutectic Al—Si—Mg and 6063 Alloys

In this work,a comparison study on corrosion behavior of extruded near eutectic Al—12.3%Si—0.26%Mg and 6063 alloys has been carried out by mass loss test in 4%H₂SO₄ aqueous solution in the open air and potentiodynamic polarization test in 3.5 wt.%NaCI aqueous solution.Results indicate that the corrosion resistance of the near eutectic Al—Si—Mg alloy is less than that of 6063 alloy.Macro/microscopy and scanning electron microscopy results clearly show the difference of the corrosion progress of these two alloys in 4%H₂SO₄ aqueous solution.The corrosion type of 6063 alloy is pitting corrosion.The Mg₂Si and AlFeSi particles and surface defects act as nucleation sites for pitting,and the amount and distribution of them have a significant effect on the pitting behavior.For the near eutectic alloy,there are two types of corrosion cells. One is between the extruded primaryα-AI and the eutectic,the other is between the eutectic Al and eutectic Si particles.Combination of these two types of corrosion cells leads to a lower corrosion resistance,a higher mass loss of the near eutectic alloy compared with 6063 alloy,and the formation of the paralleling corroded grooves.     

Effect of Yttrium Addition on Microstructural Characteristics and Superplastic Behavior of Friction Stir Processed ZK60 Alloy

As-extruded ZK60 and ZK60-Y magnesium alloy plates were successfully processed via friction stir processing （FSP） at a tool rotation rate of 1600 r/rain and a traverse speed of 200 mm/min. FSP resulted in the formation of equiaxed recrystallized microstructures with the average grain sizes of ,-8.5 and -4.7 μm in the ZK60 and ZK60-Y alloys, respectively. Moreover, FSP broke and dispersed the MgZn2 and W-phase （Mg3Zn3Y2） particles and dissolved MgZn2 phase in the FSP ZK60 alloy. With the addition of rare earth element yttrium （Y） into the ZK60 alloy, the ratio of the high angle grain boundaries （HAGBs） in the FSP alloys increased from 64% to 90%, and a certain amount of twins appeared in the FSP ZK60-Y alloy. The maximum elongation of 1200% and optimum strain rate of 3 X 10-3 s-1 achieved at 450 °C in the FSP ZK60-Y alloy were substantially higher than those of the FSP ZK60 alloy. This is attributed to the fine grains with high ratio of HAGBs and the distribution of a large number of dispersed second phase particles with high thermal stability in the FSP ZK60-Y alloy. Grain boundary sliding was identified as the primary deformation mechanism in the FSP ZK60 and ZK60-Y alloys from the superplastic data analyses and surficial morphology observations.     

Microstructure and mechanical behaviour of asymmetric extruded Mg–3Al–1Zn alloy sheets

Abstract

Microstructural evolution and mechanical responses of Mg–3Al–1Zn (AZ31) sheet processed by the asymmetric extrusion (ASE) and conventional extrusion (CE) are examined. Mechanical properties of ASE sheets were remarkably enhanced compared with CE samples. This is attributed to the subdivision of the asymmetric extrusion die along the flow passage equipped with a chamfer on one side, which would trigger the angular spread of the basal texture by introducing an asymmetry shear deformation. Moreover, subsequent annealed ASE specimens show a significant weakening of the basal texture and a combination of the superior stretch formability.     

Synthesis and properties of light weight magnesium–cenosphere composite

Significantly light weight magnesium composite foams are synthesised by addition of fly ash cenosphere particles (waste from coal-fired power plants) in biocompatible pure magnesium using solidification-based disintegrated melt deposition technique. The density of the composite foams synthesised in this study approaches that of plastics- and polymer-based composites. Microstructure development of Mg/cenosphere composite foams was favourable as they exhibited better dimensional stability (reduced coefficient of thermal expansion) and remarkable improvements in tensile strengths, compressive strengths, compressive total strain and microhardness. The present study highlights the processing, microstructure and mechanical properties of Mg/cenosphere composite foams which hold great potential as light weight metal-based green materials for diverse weight critical applications spanning from engineering to biomedical sector.     

Heat transfer during solidification of chemically modified Al–Si alloys around a copper chill

Abstract

The solidifying metal/chill contour will significantly affect the boundary heat transfer coefficients, and solidification modellers should be aware of the casting conditions for which the heat transfer coefficients are determined. The previous work carried out on solidification of Al–Si alloys in a metallic mould and solidification against bottom/top chills has shown that modification and chilling have synergetic effect resulting in a significant increase in the heat flux transients at the casting/chill interface. In the present work, the heat transfer during solidification of unmodified and chemically modified Al–Si alloys around a cylindrical copper chill was investigated. Heat flux transients were estimated using lumped heat capacitance method. Lower peak heat flux was obtained with chemically modified alloy. This is in contrast to the results reported for alloys solidifying against chills and in metallic moulds. The chill thermal behaviour and heat transfer to the chill material when surrounded by modified and unmodified alloys were explained on the basis of the decrease in the degree of undercooling in the case of modified alloy as compared to unmodified alloy and the change in contact condition and shrinkage characteristics of the alloy due to the addition of chemical modifiers.     

Microstructural features of spray-formed AZ31 magnesium alloy

Abstract

Mg alloy AZ31 was spray-formed using an indigenously developed spray atomisation and deposition unit under protective atmosphere and various processing parameters were optimised. The microstructural features of the bell shaped AZ31 spray-formed deposit were characterised using optical microscope, scanning electron microscope/energy dispersive spectrometer, X-ray diffraction and high resolution transmission electron microscope. It was observed that the microstructural features are critically dependent on location in the spray-formed deposits. Under optimised processing conditions, the central region of the bell shaped deposit exhibited minimal porosity and a uniform fine grained equiaxed microstructure with fine Mg₁₇Al₁₂ intermetallics preferably located at the grain boundaries. However, the peripheral regions of the spray-formed deposit indicate higher porosity with distinct microstructural characteristics different from those in the central region. These microstructural features, observed at different locations in the spray-formed deposit, have been analysed and their evolution is discussed in the light of variations in thermal and solidification conditions of the droplets in flight, during impingement as well as those of the deposition surface.