Materials Science Forum Vols. 546-549

Abstract: In the present study, the conventional process of Compo-casting was carried and the microstructural and mechanical propertied were then investigated. The matrix alloy used was Mg- 6Al, and B2O3 was added into Mg-Al alloys was investigated at two levels of 3 and 6wt%. The other experimental materials were NaCl and CaCl2. The microstructures were of the samples analyzed with Optical Microscopy, SEM and XRD, and the mechanical properties were determined by micro-hardness and tensile test. The results showed that the mechanical properties of Mg- 6Al- 3B2O3- 1NaCl- 1CaCl2 increased and the microstructure was satisfactory, for a cast alloy, i.e.the tensile strength was 175MPa.

Abstract: Nano-indentation technique was used to survey creep stress exponent of ZM6 (Mg-2.8Nd-0.7Zn-0.6Zr) alloy at room temperature. The results showed that average press creep stress exponent of ZM6 alloy was about 89.75, and independent of strain rate and hardness, which has been verified by linear relationship of the double logarithmic plots between strain rate ( ε& ) and hardness (H ) measured by a nano-indentation equipment with constant load of 500mN.

Abstract: Mg is the lightest structural metal with the highest specific strength. It is therefore considered as a solution for vehicular mass reduction so as to reduce the fuel consumption and greenhouse gas emission. Unfortunately, Mg and its alloys are highly susceptible to corrosion, particularly in salt-spray conditions. This has limited its use in the automotive and aerospace industries, where exposure to harsh service conditions is unavoidable. To improve its corrosion resistance it is viable to apply a desirable engineering coating on the Mg surface.This communication discusses the practical aspects of magnesium surface modification based on our review. A novel immersion coating process is also discussed along with the corrosion behavior of coated and uncoated AZ91 magnesium alloy.

Abstract: A hybrid biomaterial was produced by dressing titanium with magnesium in order to utilize the most advantageous properties of both materials. Titanium was coated with magnesium via arc physical vapor deposition (PVD) method, and a third agent between titanium and magnesium, as a bonding medium was also used. The microstructures and chemical compositions of magnesium arc-PVD coated Ti-6Al-4V alloy and silicon wafer were investigated. The experimental results showed that Mg can be deposited onto Ti-6Al-4V alloy and on silicon wafer by using arc-PVD method.

Abstract: The common joining techniques for Mg sheet and die cast alloys such as riveting, friction-stir welding and adhesive bonding all introduce additional challenges for protection in the joining areas. First of all, the sheet products are prone to high rate of corrosion due to surface contamination. Introduction of iron-rich contaminants can be encountered from the friction-stir welding process. Although powder coating on top of conversion pre-treatments is practical for mitigating corrosion, the lap-shear adhesion of such surfaces can be negatively impacted when the adhesively bonded joints are exposed to a corrosion environment. Anodized surfaces are better in terms of their ability to retain adhesion strength, but their resistance to galvanic corrosion is an issue. Development of galvanically compatible coatings for steel rivets will benefit the adaptation of riveting as a joining technology for magnesium.

Abstract: Aluminum spray coating was applied onto AZ91D magnesium alloy. The effects of post heat treatment on the interfacial characteristics, especially the diffusion kinetics of magnesium and aluminum atoms across the interface were studied in this paper. It was observed that there was an obvious interface between the aluminum coating and the magnesium alloy substrate. It was found that post heat treatment could reduce the number of pores existed in the aluminum coating. The post heat treatment resulted in not only the change in the composition and microstructure of the aluminum coating, but also the improved metallurgical bonding between the coating and the substrate.

Abstract: Microstructural examinations of T6-treated WE54 magnesium alloy were conducted after different aging conditions, and the influence of aging on electrochemical behavior of this alloy was investigated. For three batches of samples, with increasing aging time, the amount of precipitate phases was greatly promoted, and they formed in a strengthening continuous way. Electrochemical study showed that the value of corrosion potential followed the tendency to decrease when the aging condition transformed from under-aged to peak-aged. However, after peak-aging, the corrosion potential was raised to higher values. Explanations for the observed phenomena were based on the relationship between microstructure, especially precipitate phases, and electrochemical behavior. At under-aged condition, matrix served as galvanic cathode and precipitates acted as galvanic anode. Because the amount of precipitate would rise with prolonging aging time, it is natural that corrosion potential decreased. For over-aged alloys, metastable precipitate phases might in situ change to equilibrium phases, rendering increase of corrosion potential possible.

Abstract: The corrosion of AZ91 magnesium alloy with residual stress was observed in-situ in an atmosphere of water vapor with cyclic wet-dry conditions using environmental scanning electron microscope (ESEM). The results showed that the oxygen concentration on the surface generally increased with the wetting time. And the average corrosion rate of AZ91 alloy with residual stress (RS) was much larger than that without RS, indicating RS accelerated corrosion. The corrosion localized at primary α-Mg area at short wetting time, but with the increase of wetting time spread to the eutectic α-Mg region. Small-size as-cast defects (<50μm) contributed little to the average corrosion rate while large-size as-cast defects (>100μm) influenced the distribution of water film and corrosion on the surface.

Abstract: Effect of pulsed high-energy electron beam on the surface modification and the state of surface layer and wear resistance of AZ91 magnesium alloy have been investigated in this study. Optical microscope (OM) and X-ray diffraction (XRD) were employed to characterize the microstructure and phase composition of the modified surface layer. It was found that the thickness of melted layer on the surface varied with electron beam current and the numbers of pulses, the treated surface layer exhibited higher hardness than AZ91 alloy. The friction coefficient and the wear volume of AZ91 alloy after electron beam treatment decrease markedly. The wear resistance of treated samples were significantly improved, which may be attributed to high hardness as a result of grain refinement.