Zhen-hua Chen

Phosphating process of AZ31 magnesium alloy and corrosion resistance of coatings

Abstract Zinc phosphate films were formed on AZ31 magnesium alloy by immersing into a phosphatation bath to enhance the corrosion resistance of AZ31. Different films were prepared by changing the processing parameters such as immersing time and temperature. The corrosion protection of the coatings was studied by electrochemical measurements such as electrochemical impedance spectroscopy, potentiodynamic polarization curves, and the structure of the films were studied by metalloscopy and X-ray diffraction (XRD). The results show that, the film formed at 80 °C, 10 min has the highest corrosion resistance. The XRD patterns show that the film consists of hopeite (Zn 3 (PO 4 ) 2 · x H 2 O).

Microstructures and properties of rapidly solidified Mg-Zn-Ca alloys

Ternary alloys based on the Mg-Zn-Ca system were produced by twin-roll rapid solidification. The alloys were characterized by OM, SEM, HRTEM, XRD, EDS and Micro-hardness. The results show that the rapidly solidified flakes are of fine dendritic cell structures with the cell size ranging from 1 to 5 μm. The Mg-6Zn-5Ca alloy in RS and annealing (200℃ for 1 h) states are mainly composed of α-Mg, Mg2Ca, Ca2Mg6Zn3 and a small quantity of Mg51Zn20, MgZn2 and Mg2Zn3. Micro-hardness increases with the increment of Ca content and age hardening occurs after aging at 200℃ in the flakes probably due to the precipitation strengthening of the fine precipitates Mg2Ca and Ca2Mg6Zn3. Some phases at the grain boundary in Mg-6Zn-5Ca alloy are identified by means of FIRTEM, which may be beneficial to the improvement in thermal stability of the alloy.

Corrosion properties of AZ31 magnesium alloy and protective effects of chemical conversion layers and anodized coatings

Abstract The corrosion properties of AZ31 magnesium alloys were studied by potentiodynamic polarization curves and electrochemical impedance spectroscopy(EIS) techniques, meanwhile, the protective properties of two environmentally protective types of chemical conversion layers and anodized coatings of AZ31 magnesium alloys were also discussed. The component of chemical conversion bath is NaH 2 O 4 12H 2 O 20 g/L, H 3 PO 4 7.4 mL/L, NaNO 2 3 g/L, Zn(NO 3 ) 2 ·6H 2 O 5 g/L and NaF 1g L, and components of the anodization bath is Na 2 SiO 3 60 g/L, C 6 H 5 Na 3 O 7 2H 2 O 50 g/L, KOH 100 g/L and Na 2 B 4 O 7 ·2H 2 O 20 g L. The results show that the corrosion resistance of AZ3 1 magnesium increases with the increase of pH value of the corrosive medium. For the chemical conversion layer acquired at 80 °C, 10 min is the best processing time and the charge transfer resistance of the chemical conversion layer is enhanced nearly by 10 times. The optimum processing time for the anodization of AZ31 is 60 min, the charge transfer resistance value of the anodized sample at the early immersion stage is nearly 26 times of that of the blank sample and the corrosion type of the anodized samples is pitting.

Fracture behaviors of AZ80 magnesium alloy during multiple forging processes

The initiation, propagation and the accompanied dislocation structures of the cracks in AZ80 magnesium alloy during multiple forging processes were investigated. The results show that the cracks firstly initiate at the Mg/Mg17Al12 interface under the hoop tensile stress on equatorial free surface. On further deformation, the cracks in the Mg17Al12 particles tend to propagate along the grain boundaries(GBs) in a zigzag pattern and link with adjacent cracks in other Mg17Al12 particles to form one whole crack, leading to the fracture surface. Low deformation temperature and too many forging passes during the deformation will promote the nucleation of interfacial microcrack inside the specimens due to the strong plastic strain incompatibility and the high internal stresses near the GBs. The loading axis rotating during the process can change the stress field at the tip of cracks, leading to the change of the crack propagating path and assisting in inhabiting microcracking development.

Effect of electropolymer sizing of carbon fiber on mechanical properties of phenolic resin composites

Abstract Carbon fiber/phenolic resin composites were reinforced by the carbon fiber sized with the polymer films of phenol, m -phenylenediamine or acrylic acid, which was electropolymerized by cyclic voltammetry or chronopotentiometry. The contact angles of the sized carbon fibers with deionized water and diiodomethane were measured by the wicking method based on the modified Washburn equation, to show the effects of the different electropolymer film on the surface free energy of the carbon fiber after sizing by the electropolymerization. Compared with the unsized carbon fiber, which has 85.6° of contact angle of water, 52.2° of contact angle of diiodomethane, and 33.1 mJ/m 2 of surface free energy with 29.3 mJ/m 2 of dispersive components (γ L ) and 3.8 mJ/m 2 of polar components (γ sp ), respectively. It is found that the electropolymer sized carbon fiber tends to reduce the surface energy due to the decrease of dispersive γ L with the increase of the polymer film on the surface of the carbon fiber that plays an important role in improving the mechanical properties of carbon/phenolic resin composites. Compared with the phenolic resin composites reinforced by the unsized carbon fiber, the impact, flexural and interlaminar shear strength of the phenolic resin composites were improved by 44 %, 68% and 87% when reinforced with the carbon fiber sized by the electropolymer of m -phenylenediamine, 66%, 100%, and 112% by the electropolymer of phenol, and 20%, 80 %, 100% by the electropolymer of acrylic acid. The results indicate the skills of electropolymerization may provide a feasible method for the sizing of carbon fiber in a composite system, so as to improve the interfacial performance between the reinforce materials and the matrix and to increase the mechanical properties of the composites.

Wear property of high-resistivity carbon brushes made with and without MoS2 in variable humidity

Four kinds of high-resistivity carbon brushes with MoS2 contents of 0%, 2%, 4% and 6% (mass fraction) were prepared, respectively. Wear tests of brushes were conducted on an alternate current(AC) motor. Scanning electron microscopy(SEM) and energy dispersive X-ray analysis(EDS) were used to analyze the worn surface of brushes, and a thermocouple was used to measure the bulk temperature of the brush. The results show that wear rate of brush made without MoS2 in 10% relative humidity(RH) is two times larger than that in 50% RH, whereas the wear rates change little for the brushes made with MoS2. The wear of brushes has much to do with the surface film. In low humidity, the surface film can not be formed for the former brush while a sulfur layer can be formed for the latter brushes, which can reduce sparks, frictional heat and wear rate, and play a role like the water film in high humidity.

Preparation of carbon brushes with thermosetting resin binder

Carbon brushes with a resin binder were prepared according to an industrial process and the effects of the molding pressure, grains size and cure temperature on the properties of brush samples were discussed. The results show that the bulk density, bending strength and Rockwell hardness increase, while resistivity decreases with increasing molding pressure. Cure temperature has much more influence on the properties of brushes than molding pressure and grains size. Isothermal differential scanning calorimetry(DSC) was used to estimate the degree of cure of resin binder and a novel method of using the true density to measure the degree of cure of resin binder was presented and discussed briefly. Based on optimal process parameters carbon brushes were manufactured, durability tests for brushes were carried out on an alternate current motor and scanning electron microscope(SEM) was adopted to observe the morphology of worn surface of brushes. The results show that a luster oxide film can be formed on the surface of brushes and their service life reaches 380 h.

Warm Deformation Mechanism of hot-rolled Mg Alloy

Tension attachment of high temperature microscopy was proposed to research the microstructure evolution and plastic behavior of AZ31 magnesium, alloy in a temperature range of 473-523K and a load range of 80-160N. Transmission electron microscopy (TEM) was utilized to observe the morphology of twins after deformation process. The results show that as Zener-Hollomon parameter Z increases (temperature falls, strain rate rises), the peak stress obviously increases, while the ductility tends to become worse. A great amount of twins can be found at moderate temperatures. Therefore, basal slip, a+c non-basal slipping and twinning are considered the dominant mechanisms at moderate temperatures. Some DRXed grains can be observed in the twinned regions and grain boundaries, suggesting both twinning-induced DRX and continuous DRX occurs in the deformation process.

Twinning in weld HAZ of ZK21 commercial magnesium alloy

The microstructure and properties of Mg ZK21 laser beam weld without filler were researched using optical microscopy (OM), electron microscopy and mechanical test. The results show that the fracture strain of the joints after laser beam welding reduces by about 10.7% at room temperature. By means of laser beam welding, the fusion zones contain tensile RS, while the base material far away from the fusion line is under balancing compressive RS. The microstructures of the weld were characterized by a narrow heat affected zone and twins. Significant {10 1(average) 2} tension twins occur in the weld HAZ during laser welding processing. Due to the influence of temperature field and stress on morphologies, most of twins form twinning bands, which are nearly parallel to the welding direction.