Materials Science Forum Vols. 488-489

Abstract: A model was developed for the analysis and prediction of correlation between composition and mechanical properties of Mg-Al-Zn (AZ) magnesium alloys by applying artificial neural network (ANN). The input parameters of the neural network (NN) are alloy composition. The outputs of the NN model are important mechanical properties, including ultimate tensile strength, tensile yield strength and elongation. The model is based on multilayer feedforward neural network. The NN was trained with comprehensive data set collected from domestic and foreign literature. A very good performance of the neural network was achieved. The model can be used for the simulation and prediction of mechanical properties of AZ system magnesium alloys as functions of composition.

Abstract: The use of magnesium and its alloys is limited by their high susceptibility to corrosion, that may be attributed to the presence of critical impurities and the high chemical reactivity of the magnesium itself. Such characteristic justifies the great interest of performing coating technologies for the protection of magnesium parts by using environmentally friendly and cheap processes, with particular relevance in the framework of the European context. While magnesium technologies are rapidly taking advantages of the research results obtained during the last years, some more efforts are necessary to evaluate the impacts of its production on the environment and on the use of existing energy resources. Following this main lines, the paper presents some results obtained in NANOMAG (“Development of Innovative Nanocomposite Coatings for Magnesium Casting Protection” – Growth Programme n. 40548), a research project sponsored by the European Commission in which magnesium and its coating processes are evaluated taking into account also their environmental performances with a life-cycle approach.

Abstract: The high-purity magnesium alloy specimens prepared by applying die-cast [1] paint and various types of surface treatments, and then the corrosion resistance of practical coating materials after ten-year outdoor exposure tests was evaluated. These tests were performed at the Choshi (long. 140°45' E, lat. 35°43' N), the Asahikawa (long. 142°18' E, lat. 43°47' N), and the Miyakojima (long. 125°20' E, lat. 24°44' N) Exposure Laboratories of the Japan Weathering Test Center. The environmental factors generally believed to be associated with corrosion mechanisms (temperature, humidity, precipitation, wetness time, solar and UV radiation, and concentration of chloride and sulfur oxide) were measured. Evaluation of the adhesion of paint and corrosion resistance, chromate and anodic oxide films were superior as pretreatment paint films on AZ91D magnesium alloys; these films were comparable to paint-coated ADC12 aluminum alloys.

Abstract: Mg-0.5%Zn-4%Al-0.5~2.0%Ca-0.5~2.0%La alloys are investigated in order to determine the optimum composition with superior casting properties and excellent creep resistance for automatic transmission case applications. The results show that regardless of the La content, the hot-tearing susceptibility of alloys containing 0.8%Ca or more is low. However, at 0.5%Ca content, hot-tearing occurs in alloys containing more than 1.2%La. The creep resistance of the alloys increases as the Ca content increases and Mg-0.5%Zn-4%Al-0.9%Ca-0.8%La alloy is found to be the optimum alloy exhibiting good casting properties and adequate heat resistance for automatic transmission cases.

Abstract: The electrodeposition of aluminium and magnesium in lithium chloride-sodium chloride (mass 5:1) melt at 700°C has been studied using cyclic voltammetry, square wave voltammetry and chronopotentiometry. The reduction of aluminium and magnesium ions on tungsten electrode are both reversible processes, with three and two electrons exchanged: Al3++3e → Al, Mg2++2e →Mg. The diffusion coefficients of Al3+ and Mg2+ were determined as 2.72×10-5 cm2 s-1 and 1.58×10-4cm2 s-1 respectively. A laboratory test of the direct production for the Mg-Al alloy was performed to show the feasibility of electrochemical co-deposition of magnesium base alloys.

Abstract: Flow stress and microstructures evolution of AM50 alloy during upsetting forging tests were investigated in this paper. In upsetting forging deformation, cylindrical billets of AM50 alloy as-cast were compressed in the temperature range of 250°C to 350°C and strain rate range of 1.1×10-3s-1 to 2.8×10-2s-1. Under the deformed conditions, the true stress-true strain curves indicated that the true stress increased quickly at the initial time, then stayed at certain value for a short duration and increased continuously at lower temperature and high strain rate. It should be noted that the true stress kept the dynamic balance with the true strain at higher temperature and lower strain rate that the true stress obeyed the regulation as the same as the ordinary materials. The peak stress at all conditions increased with the strain rate increasing and the forging temperature decreasing. A great many recrystallized grains and dislocations were observed in the deformed specimens of AM50 alloy. The grain size of the deformed specimens decreased with decreasing forging temperature and increasing the deformation.

Abstract: A phosphate conversion coating was deposited on diecast AZ91D magnesium alloys, the film was a complex phosphate containing Mg and Al,which showed amorphous structure. Corrosion resistance of conversion coating in 3.5%NaCl was investigated by polarization curve. It was showed that the anodic branch of polarization curve for the phosphate conversion coating presented typical passivation characteristic. Immersion test results showed that the corrosion rate of phosphate conversion coating was less than that of chromate Dow7 film.

Abstract: The wear and corrosion protective coatings were synthesized on AZ91 Mg alloy and Mg matrix SiCw/AZ91 composite by a microarc oxidation (MAO) technique in an alkali-silicate electrolyte solution. The phase structure of the coatings was analyzed using X-ray diffraction (XRD). The tribological properties of the coatings sliding against GCr15 steel were investigated on a pin-on-disc friction and wear tester. The morphology and elemental composition of the worn coating surface were determined by scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS). The potentiodynamic polarizing curves were measured to evaluate the corrosion resistance of the coatings. The results showed that the application of the coatings changed evolution tendency of friction coefficients and wear mechanism. For coated Mg alloy and Mg matrix composite, both the wear resistance and corrosion resistance were improved greatly.

Abstract: Compressive deformation behaviors of extruded SiCw/AZ91 were investigated in Gleeble-1500 thermal simulator at temperatures from 743 K to 783 K and strain rates from 6.4×10-2 s-1 to 1.0×101s-1. Results showed that high strain rate sensitivity (~0.5) occurred during compression; deformation activation energy normalized by threshold stress was higher than the lattice self-diffusion activation energy of magnesium. Dynamic recovery (DRV) and dynamic recrystallization (DRX) took place during compression, which refined the grains. The increase of deformation energy was attributed to non-basal planes slip and climbing of dislocations and also the presence of liquid phase.

Abstract: In the present paper, phosphoric and chromic conversion coatings were prepared for the die-cast magnesium alloy ZM5. Both two kinds of chemical converting coatings are of evident corrosion resistance. Their compositions, morphologies and crystal characteristics were discussed.