Jixue Zhou

Overall micro-arc oxidation treatment for AZ31B–6061 magnesium–aluminium dissimilar metal connecting parts

ABSTRACT The magnesium–aluminium connecting parts are important application form for lightweight structural materials. But the conductive connect of magnesium and aluminium will cause serious galvanic corrosion problems. Therefore, the overall corrosion protective treatment is necessary. A ceramic coating was prepared via overall micro-arc oxidation to wrap the magnesium–aluminium connecting part integrally. The surface morphologies and compositions of the coatings were analysed by scanning electron microscopy and X-ray energy dispersive spectroscopy. The corrosion behaviour of the coatings was investigated with potentiodynamic polarisation tests in 3.5 wt-% NaCl solution. The growth process of ceramic coating on aluminium and magnesium surface was investigated, which showed the micro-arc oxidation reaction priority and the balanced growth process of ceramic coating under unbalanced micro-arc distribution. The results demonstrated that the overall micro-arc oxidation treatment improved the corrosion resistance and reduced the corrosion potential difference of each metal of magnesium–aluminium connecting part.

Characteristics of AZ31 Mg alloy joint using automatic TIG welding

The automatic tungsten-inert gas welding (ATIGW) of AZ31 Mg alloys was performed using a six-axis robot. The evolution of the microstructure and texture of the AZ31 auto-welded joints was studied by optical microscopy, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and electron backscatter diffraction. The ATIGW process resulted in coarse recrystallized grains in the heat affected zone (HAZ) and epitaxial growth of columnar grains in the fusion zone (FZ). Substantial changes of texture between the base material (BM) and the FZ were detected. The {0002} basal plane in the BM was largely parallel to the sheet rolling plane, whereas the c-axis of the crystal lattice in the FZ inclined approximately 25° with respect to the welding direction. The maximum pole density increased from 9.45 in the BM to 12.9 in the FZ. The microhardness distribution, tensile properties, and fracture features of the AZ31 auto-welded joints were also investigated.

The Study on the Overall Plasma Electrolytic Oxidation for 6061–7075 Dissimilar Aluminum Alloy Welded Parts Based on the Dielectric Breakdown Theory

Electrical connection of dissimilar metals will lead to galvanic corrosion. Therefore, overall surface treatment is necessary for the protection of dissimilar metal welded parts. However, serious unbalanced reactions may occur during overall surface treatment, which makes it difficult to prepare integral coating. In this paper, an overall ceramic coating was fabricated by plasma electrolytic oxidation to wrap the 6061–7075 welded part integrally. Moreover, the growth mechanism of the coating on different areas of the welded part was studied based on the dielectric breakdown theory. The reaction sequence of each area during the treatment was verified through specially designed dielectric breakdown tests. The results showed that the high impedance overall of ceramic coating can inhibit the galvanic corrosion of the 6061–7075 welded part effectively.

Effects of scandium addition on the in vitro degradation behavior of biodegradable Mg–1.5Zn–0.6Zr alloy

Heretofore, recognitions of the systematic effects of scandium addition on corrosion behavior of biodegradable magnesium alloys are not yet clear. In the present study, a series of Mg–1.5Zn–0.6Zr–xSc (ZK21–xSc, x = 0, 0.2, 0.5, 1.0 wt.%) alloys were casted and investigated with respect to the immersion and electrochemical degradation behavior. The hydrogen evolution, pH monitoring, ion release and mass loss results demonstrated that ZK21–0.2Sc alloy exhibited the lowest corrosion rate. The surface morphology analyses displayed that an obvious uniform corrosion occurred in ZK21–xSc alloys with Sc content below 0.5, while localized corrosion occurred in ZK21–1.0Sc alloy. Corrosion potentials of ZK21–xSc alloys shifted toward more positive with the increasing Sc content. But ZK21–0.2Sc alloy exhibited the lowest corrosion current density and the largest corrosion film resistance. Compared with other developed Mg alloys, the ZK21–0.2Sc alloy demonstrated a superior degradation behavior.

FE Analysis of Dynamical Recrystallization during the Seamless Tube Extrusion of Semicontinuous Casting Magnesium Alloy and Experimental Verification

Abstract In this paper, empirical dynamic recrystallization (DRX) models for the semicontinuous AZ31 magnesium alloy were established based on the stress–strain curves and microstructure observations generated from hot compression tests. The DRX evolution during the seamless tube extrusion of the alloy was studied by numerical and experimental methods. The extruding temperature and the ram speed were two important parameters during the extrusion. With the increase of the two parameters, the volume fraction of DRX and the recrystallized grain size were observably increased. The predicted DRX fraction and grain size were in an excellent agreement with the experimental results.

Influence of Initial Microstructure on the Hot Working Flow Stress of ZK60 Mg Alloy

The hot working flow stress of as-cast and two as-extruded magnesium alloys ZK60 was investigated through uniaxial compression tests. It was found that the hot deformation behavior was sensitive to the deformation conditions and initial microstructure. The peak flow stress was affected by the deformation temperature and the strain rate. Its value was decreased with increasing the deformation temperature or decreasing the deformation rate. The peak flow stresses for the as-cast and as-extruded samples showed a different Z dependency. However, the correlations between Z parameters and as-extruded materials with different initial microstructures were almost the same. These key features of deformation behavior can be explained by twinning and dynamic recrystallization.

Enhanced Corrosion Resistance of 6063 Aluminum in Surface Modification Using γ-Aminopropyl Triethoxysilane

The corrosion protection of 6063 aluminum with film of 3-aminopropyl triethoxysilane was studied in this paper. Electrochemical tests in neutral 3.5% (wt%) NaCl solution, scanning electronic microscopy (SEM), energy dispersive spectroscopy (EDS), and confocal laser scanning microscope (CLSM) were performed. The result showed that the silane films could offer unexceptionable corrosion resistance to metal. A highly crosslinked and dense interfacial layer formed on the surface of the metal, which makes the substrate and the coating firmly combined to reduce the corrosion current density and inhibit the occurrence of corrosion. The film with bridging group had excellent hydrophobicity which can heavily restrict corrosion product proceeds at the interface.

Effects of Post-welded Heat Treated on the AZ31B Magnesium Alloy Joint Welded by Automatic TIG Welding

The automatic TIG welding on AZ31B alloys was carried out by the six-axis robot in the present work. After post-welded heat treatment (PWHT), the microstructure, precipitates, and mechanical properties of the AZ31B joints were investigated by using the optical microscope (OM), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD). The experimental results reveal that after PWHT, the microstructure and mechanical properties of the AZ31B joints were both significantly improved. After PWHT, the grains of the AZ31B joint were still fine, whereas the number of the precipitates along with the grain boundaries was dramatically decreased. The ultimate tensile strength (UTS) and the elongation (EL) of the PWHT AZ31B joints were up to the 100 and 96.9% of the base metal (BM), respectively.