Materials Science Forum Vols. 488-489

Abstract: Microstructure and mechanical properties of AM50+xTi (x=0,0.01,0.1wt%) magnesium alloys extruded from as-cast and solution treated conditions have been studied. Results show that Ti element obviously refines the microstructure of AM50 magnesium alloy and Mg17Al12 phase. Only 0.01 wt% Ti addition can make the Mg17Al12 phase turn into particles and small rod-like shape. Ti addition improves tensile strength at room temperature, and obviously improves elongation at elevated temperatures up to 200°C. The AM50+xTi alloys extruded from as-cast have better tensile strength at room temperature and better elongation at 100°C, 150°C and 200°C than that of AM50+xTi alloys extruded from solution treatment; The plasticity of AM50 magnesium alloys increases with Ti content increasing and temperature increasing for the tensile fractograph.

Abstract: Orientation mapping based on EBSD technique was applied to analyze the rules of orientation evolution of grains in AZ31 magnesium alloy. Results show that not only under deformation strain rate of 1×10-2s-1, but under 4×10-4s-1(the superplastic deformation condition), grains in all samples with initial textures rotate gradually to near basal orientation ({0002} || compression plane) at different ways, and basal texture becomes stronger with increasing strain, which indicates plastic slip plays an important role during hot deformation. Otherwise, no evident non-basal pyramidal slip of as some studies mentioned was observed in the sample with the initial basal texture, and the basal orientation is kept unchanged during the deformation process, which suggests that basal slip is the uppermost plastic slip mechanism in this sample. In addition, the phenomenon of viscous laminar flow was observed in the sample with initial basal texture.

Abstract: The paper researched formability ability of AZ31 Mg-alloy tube by hot pneumatic bulging. FEM simulations were carried out to reveal the effect of internal pressure changing on bulging process and forming limit. Several loading paths with different pressure changing were used in the simulations. From the research, the original expansion ratio of diameter using a bilinear loading path, namely the internal pressure was increased linearly, is only 22%, and bursting occurs quickly. With a step-like loading path, namely the internal pressure is increased step by step, among the steps, the pressure is kept constant, and the strain rate of bulging can be kept into a small range. Thus the deformation around the hoop direction is more even than the linear loading path, and than bursting can be postponed. During bulging, the inner pressure should be lowered with the increase of diameter to keep the strain rate in a small range around a constant value. Through optimization of loading path, the forming limit can be enhanced obviously so that the expansion ratio of diameter can be increased to 25.1%.

Abstract: To examine the formability of a magnesium-based alloy AZ31 sheet, a temperature and strain rate related constitutive model for AZ31 sheet was developed based on tensile experiments. The relative parameters were obtained by fitting the equation to the experimental data. The comparison between the fitted and the experimental data proved the effectiveness of the model. Based on this model, the deep drawing process has been simulated with the finite element method and the limit drawing ratio (LDR) of AZ31 sheet was numerically studied. The study result was helpful to the application of the stamping technology for the magnesium alloy sheet.

Abstract: There is growing interest in magnesium alloys as structural materials for the automotive, aerospace and electronic industries. However, the corrosion performance of most magnesium alloys is not good enough for the increasingly diverse practical applications. The Cooperative Research Centre for Cast Metals Manufacturing (CAST) is an Australian research organisation established to cope with the problems associated with development and application of light metals. Corrosion and prevention of magnesium and its alloys has been an important part of CAST’s research program since 1995. The research effort in this area is focused on solving corrosion problems relative to the application of magnesium alloys in the automotive industries. Nevertheless, encompassed by the requirements of the applied research, some fundamental studies have also been conducted. This paper presents a brief summary of some of the research achievements in this area recently made by CAST. They include studies of corrosion behaviour, alloying effects, corrosion inhibition and surface treatment of magnesium alloys.

Abstract: Among four AZ91 samples, two were once anodized and the others were twice anodized in two electrolytic baths. After twice anodization, two coatings coexist on the base metal. Surface morphology showed that for one of twice anodic coatings (sample 2), the second coating only existed on some places and sealed some pores of the first coating after short time on the second anodizing. However, for another twice anodic coating (namely sample 4), the second anodization spent very long time and the coating was thick and loose. Salt spray testing showed that sample 2 had the most excellent corrosion resistance and the reason is discussed.

Abstract: Formation behavior and performances of anodic film grown on AZ31B alloys were systematically studied in Na2SiO3 system electrolyte with varying triethanolamine (TEA) content. The growth behavior and appearance of anodic film were significantly affected by the TEA content. The film growth speed increased because of the addition of TEA. Whitish gray lubricous barrier-type film were obtained in the electrolyte with TEA. The film density was obviously increased by the addition of TEA. Moreover, the film produced in electrolyte with TEA possessed a high degree of corrosion resistance and obviously better than that formed in electrolyte without TEA. EDX analysis indicates that, as the concentration of TEA increased, the contribution of the elements from the base alloy increased.

Abstract: An organic compound of dihexyl-contained triazine dithiol was specially synthesized for surface modification of magnesium alloy AZ91 in order to improve its corrosion resistance. The nano-scale polymer film on the surface of AZ91 was created with the synthesized compound by means of electrochemical measuring system called as polymer plating in the electrolytic solution. The modified surface of AZ91 had the peculiar functional characteristic of water repellency to inhibit corrosion. Corrosion tests were carried out with methods of polarization curve and electrochemical impedance. The corrosion resistance was evaluated from corrosive current density and reactive resistance. When concentration of the compound was set on 8 mol/m3, the good corrosion resistance was obtained for low corrosive current density and high reactive resistance in NaCl aqueous solution at 303K.

Abstract: A chromium-free conversion coating for AZ91D magnesium alloys has been obtained by using a phosphate-permanganate solution. Examinations have been carried out on the conversion coating for morphology, composition, adhesion force and corrosion resistance. Results show that the conversion coatings are relatively uniform and continuous, with thickness from 7µm to 10µm. They exhibit good adhesion to matrix and have some non-penetrate tiny holes on the surface. The main elements of the conversion coating of AZ91D alloy are Mg、O、P、K、Al、Mn. Results of corrosion resistance test indicate that the corrosion resistance of the conversion coating by phosphate-permanganate solution is in match to that of the conversion coating formed in a chromate solution, but for the corrosion resistance after painting, the former is better than the later.