Micro-arc oxidation (MAO) coatings were synthesized on MB3 magnesium alloy substrate. Two different electrolyte (Ⅰ) and electrolyte (Ⅱ) were adopted, and the performance of ceramic coatings was compared. The morphology feature, phase composition, and chemical composition of the formed coatings were studied by metallographic microscope, scanning electron microscopy (SEM), energy disperse spectroscopy (EDS), XRD, respectively. The corrosion resistance of the coatings was evaluated by salt spray test. The research results show that, the starting and terminal voltage are different during the oxidation process, they are 220V and 375V in the electrolyte (Ⅰ), and 150V and 270 V in electrolyte (Ⅱ). After MAO, The sample surface in electrolyte (Ⅰ) is light gray, and looks tender and smooth. The ceramic coatings obtained in electrolyte (Ⅱ) are relative rough and white in colour. But MAO coatings obtained in electrolyte (Ⅱ) has higher deposition rate. Salt spray test show the corrosion resistance of samples obtained in electrolyte (Ⅱ) are better than those in electrolyte (Ⅰ), which is related to the thickness of the MAO coatings.
γ-TiAl alloys are emerging as potential light-weight, high-temperature structural materials and possess wide capacities of engineering applications in aeronautics, space and automobile industries because of their low density, high specific strength and specific modulus, good oxidation-resistance and creep-resistance. Investment casting is introduced to complex TiAl net-shape or near net-shape components. In this research, ZrO2 (CaO stabilized) was chosen as the face coat materials for the investment casting of TiAl alloys. The present study mainly focuses on the fabrication of ceramic shell mould for TiAl investment casting. Optimisation of reducing the stress in cast-mould system was carried out. The processing technology of the invented ceramic shell moulds was successfully verified in the investment casting of prototype TiAl parts. The interfacial reaction between TiAl alloys and ZrO2 ceramic mould was analyzed using OM, SEM, EDS and XRD. The experimental results showed that, when the rotation speed is 200 rpm and 400 rpm, the thickness of reaction layer is about 5μm and 20μm, respectively.
The true stress-strain curves of AZ91 magnesium alloy as-homogenized were studied with compression tests on Gleeble 3500 thermal simulated test machine at 200°C—400°C and strain rate range of 10-3S-1 — 5S-1. Critical recrystallization behavior of AZ91 magnesium alloy was investigated by using strain hardening rate. The relationship between critical stress of dynamic revrystallization and temperatures and strain rates is determined by the curves between strain hardening rate and flow stress. According to the irreversible principles of thermo dynamics, the critical stresses at different temperatures and strain rates are calculated by two derivative of θ—σ curves. The map of the critical stress-temperatures-strain rates is established.
Micro-arc oxidation (MAO) method was used for the surface modification of MB3 magnesium alloy. The morphology feature, phase composition, and chemical composition of the formed ceramic coatings were studied by metallographic microscope, scanning electron microscopy (SEM), XRD, respectively. Drop test has been applied to study the corrosion resistance of MB3 Mg alloy with and without micro-arc oxidation treatment. The present result indicates that, through MAO, ceramic coatings were in-situ fabricated on the surface of MB3 Mg alloy, the micro-arc oxidation coating is relatively dense and uniform, maximum thickness is more than 97μm. The dominant phase of the coating is spinal Mg2SiO4 (Forsterite) and dissociative MgO (Periclase). Drop test shows that after oxidation the corrosion resistance of MB3 Mg alloy is greatly improved.
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