Y.-H. Wei

Microporosity formation in Mg alloys and its effect on protective coatings

Abstract A notebook computer component with a complex geometry was manufactured with a die cast process (DCP), using an AZ91D alloy. Chemical conversion and organic coatings were sequentially applied to provide protection against physical and chemical damage. Air content in the component, which gives rise to microporosity, was determined with a DCP computer simulation using MAGMA software. The surface layer characteristics of the component were also investigated using scanning electron microscopy coupled with energy dispersive X-ray spectroscopy. The microporosity content was higher at the end of the filling process compared with the regions that filled first. Corrosion resistance was poorer for discontinuous conversion coatings that resulted from surface microporosity. Moreover, adhesion of an organic coating was degraded at areas with higher microporosity.

Protective compound coating on AZ91D Mg alloy fabricated by combination of cold spraying with die casting

A novel processing that leads to the formation of a protective compound coating upon the AZ91D Mg alloy by the combination of cold spraying of Al–Mg alloy on a die cavity surface with subsequent die casting was discussed. The microstructure and phase composition of the coating were investigated using X-ray diffraction and scanning electron microscope coupled with energy dispersive spectroscopy. Results revealed that the coating consisted mainly of β phase (Mg17Al12) plus some Al and integrated with the substrate by mechanical interlocking and slender metallurgical bonding formed during the die casting process. A high bond strength of 82±5 MPa for the coating was achieved. The hardness and wear resistance of the AZ91D Mg alloy were greatly improved due to the compound coating. Simultaneously, the results of immersion tests and electrochemical corrosion tests indicated that the coated AZ91D Mg alloy specimens had better corrosion resistance compared with the bare AZ91D alloy specimen.

High-frequency corrosion fatigue behavior of AZ31 magnesium alloy in different environments

The high-frequency corrosion fatigue behavior of extruded AZ31 magnesium alloy was investigated in different environments: air, gear oil and 3.5% NaCl solution. Compared with that in air, the corrosion fatigue limits were degraded by approximately 3.52% and 58.91%, respectively, and the corrosion fatigue lives were shortened by about 13.43% and 89.36%, respectively. In the same environment, the high-frequency fatigue limits are all higher than those tested at low frequency. The specimen geometrical shape plays a certain factor on stage characteristics of S–N curves. Compared with that of arc transition specimens, the stress sensitivity of line transition specimens is only reflected in a relatively high cycle fatigue life region. Different environments influence the corrosion failure kinetics processes (the crack initiation mechanism), but do not change the fatigue fracture mechanism of the alloy, and the higher loading frequency only accelerate the crack nucleation processes.

Calculation of Second Phase Particle-Grain Boundary Interaction Range

On the basis of the grain boundary equation by Hellman and corresponding analysis of Worner, this article deals with the interaction range between the second-phase particle (SPP) and grain boundary (GB) as viewed from the applicability of grain boundary equation. Also, a new expression describing the interaction range has been derived, which solves the problem in theory that the interaction range between SPP and GB can only be qualitatively analyzed previously. It is shown that given the interaction position between SPP and GB, the interaction range can be quantitatively determined by use of this expression.

Wear and electrochemical behaviour of Mg–Al intermetallic coating

Abstract An erbium modified Mg17Al12 based coating was fabricated by hot press sintering on as extruded AZ61 Mg alloy. The Er modified coating was composed of Mg17Al12 and Al3Er phases. As a result of the presence of the dispersed Al3Er phase, the coating had a greater microhardness than Mg17Al12. Thermal effects of sintering at 400°C for 1 h caused no obvious deterioration in the wear resistance of the AZ61 Mg matrix. Electrochemical and wear resistance tests showed that the Er modified Mg17Al12 based coating had a lower corrosion current density and a lower friction coefficient than Mg17Al12, the AZ61 Mg matrix and a thermal diffusion coating (TDC). The superior wear resistance of the sintered coating resulted from the hard Al3Er phase. The corrosion resistance of the sintered coating was better than that of the TDC as a result of suppression of hydrogen evolution by the rare earth metal Er.

Evolution of intergranular corrosion resistance for HR3C heat-resistant austenitic stainless steel at elevated temperature

ABSTRACT An attempt has been made to investigate the evolution of intergranular corrosion (IGC) resistance of HR3C heat-resistant austenitic stainless steel. The double-loop electrochemical potentiokinetic reactivation (DL-EPR) tests were conducted to evaluate the IGC resistance of HR3C steel. The results show that the side peaks can be observed in active–passive region of DL-EPR curves due to inhomogeneous distribution of the chromium dissolved in matrix. The variation of the degree of sensitisation (DOS) during sensitising at 800°C can be divided into three regions (sensitisation dominant region, desensitisation dominant region and equilibrium region). When HR3C steel is sensitised at 800°C for over 24 h, the equilibrium state in DOS can be reached. Simultaneously, the IGC morphologies after DL-EPR tests can correlate well with the DOS. On the basis of the equilibrium state in DOS, proper heat treatments and corrosion protection measures can be taken to avoid IGC of HR3C steel.

Research on Corrosion Resistance of AZ91D Magnesium Alloy Coated with Epoxy Resin

The corrosion resistance of a 1~2mm thick AZ91D magnesium alloy die-casting coated with epoxy varnish after phosphatizing was tested. Zinc phosphating solution was used. In the experiment, uniform paint was obtained by dipping method. Scratch test showed that the adhesion force between the coating and matrix is excellent. Within the test range the optimal phosphating temperature and time are 50°C and 1 min tested by dynamic potential scanning method. With the organic coating the corrosion current density decreased 3 orders of magnitude, the polarization resistance increased 3 orders of magnitude. The coated specimens were immersed in10% H2SO4, 10% NaOH and acetone for 10 days, respectively. The results showed that blistering did not occur on the paint films. This work illustrated that the simple zinc phosphating process combined with simple organic coating can meet the corrosion resistance requirements of thin-wall die casting magnesium components that require higher quality appearance.

Cu-Ni-Ti Composite Diffusion Coating on Si3N4 Ceramic Surface

Si3N4 ceramic surface was metallized using multi-ionic composite coating technology. The obtained Cu-Ni-Ti composite coating was examined and evaluated by energy dispersive spectroscopy (EDS), scanning electron microscopy (SEM), X-ray diffusion (XRD) and sound emissive scratch test (SEST). The composite coating composition, the element distribution, the phase structure of the deposited coating, and the jointing mechanism of steel/ceramics were analyzed. It was found that the obtained alloy coating layer contained elements such as Cu, Ti, Ni and Fe, etc, and no element segregation was observed. XRD result showed that the diffusion coating alloy layer was composed of Cu2Ti4O, Cu, NiTi and CuTi2. Sound emissive scratch test (SEST) results revealed a good adhesion between the diffusion coating alloy layer and ceramic, and no spallation occurred under the maximum load of 100N.