Materials Science Forum Vols. 546-549

Abstract: Recently, a new super-high-strength Al-Zn-Mg-Cu alloy with Zr bearing was developed in BIAM. In this paper, the effect of microalloying element Zr on the microstructure and mechanical properties of the alloy was investigated. It was found that the influence of Zr on the microstructure and age-hardening behaviors was remarkable. The strength of the alloy increases with increasing the content of zirconium, and the peak value of the elongation appeared when the content of Zr was 0.06%(mass fraction) and then decreases. The fracture behavior was also studied. The result shows that the fracture mechanism of the alloy was converted from intergranular brittle to transgranular ductile model with increasing the content of zirconium. Finally, the optimum contents of Zr in the alloy were proposed as 0.10%~0.14%.

Abstract: Large extrusion and forgings of Al-9Fe-1.9Mo-1.7Si (wt.%, FMS0918) aluminum alloy for elevated temperature applications were produced by rapidly solidified powder metallurgy process. Powders of FMS0918 alloy were produced by inert gas atomization, and then screened, canned, degassed, extruded and forged. The extrusion exhibited good strengths and elongation in longitudinal orientation, but low elongation in long- and short-transverse orientations. After forged, the tensile strengths of the forgings showed little change, but the long- and short-transverse elongation was improved.

Abstract: The flow field and gas-bubble size during the process of aluminum melt degassing were investigated in water model. A Φ400mm×400mm transparent water model and an impellor degassing device were used in this study. The instantaneous velocity fields of water and bubbles under the mixture of rotary injector were measured with PIV velocity field measurement technique. Then the pictures of bubbles gained from PIV were analyzed with software to get the bubble size distribution. The results showed that bubble flow field and bubble size were influenced by rotary speed of rotor and gas flow-rate. With the increase of rotary speed of rotor, the horizontal velocity components of bubbles became larger, the stagnant time of bubbles in water increaseed correspondingly and bubble size became smaller. With the increase of gas flow-rate, the longitudinal velocity components of bubbles became larger, the stagnant time of bubbles in water decreased correspondingly, and the bubble size became larger also.

Abstract: Corrosion performances of several metallic materials (Al6061 and Al319 alloys, 304 stainless steel and grey cast iron) in the ethanol-gasoline alternative fuels were investigated. Cyclic potentiodynamic polarization tests were used to study their corrosion behavior. Anodizing and plasma electrolytic oxidation (PEO) techniques were used to produce oxide coatings on the Al6061 and Al319 alloys, and the corrosion properties of these coatings in the alternative fuel environments were also evaluated. The results showed that, the 304 stainless steel, Al6061 and the coating materials are compatible with the alternative fuels. The oxide coatings on both Al alloys provided effective corrosion protection in the alternative fuel environments.

Abstract: 2024-T6 Al alloy sheet s were modified by bis-[triethoxysilylpropyl] tetrasulfide (BTESPT) silane film to improve the corrosion resistance. Fourier-Transform Reflection Absorption (FTIR-RA) spectroscopy was used for structural characterization of BTESPT silane film formed on surface of the sheet. Potentiodynamic polarization and immersion test in 3.5% NaCl solution were used for evaluating the corrosion performances of the silane film. The results showed that the film formed after curing at 120 °C for 40 min was cross-linked through Si-O-Si and that it was covered on the entire surface of the sheet. The content of elements S and Si on the Al2CuMg particles is a little higher that of on the matrix. The strong peak at 1032 cm-1 indicated that the film was linked to the sheet by Si-O-Al. Compared to the untreated case, the corrosion current density of the sheet treated with the silane film was reduced by close to 2 orders. Treatment of BTESPT silane can provide about 670 h protection of corrosion for the sheet in 3.5% NaCl water solution.

Abstract: The effect of pre-rolling reduction prior to ageing on the size and distribution of the precipitates, the width of precipitation free zones (PFZ) along grain boundaries and intergranular corrosion (IGC) of aluminum alloy 2519A were investigated by hardness tests, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The experimental results showed that the time for peak-age shortened when the reduction increased, which resulted in refining the precipitatates and distributing homogeneously within the grains and hence the IGC decreased. With increasing pre-rolling reduction, the PFZ along grain boundaries became narrower and precipitates changed from continuous chains to discrete distribution, which resulted in high IGC resistance.

Abstract: The corrosion behavior of cold worked Al-Mg-Sc-Zr-Ni alloys prepared by vacuum induction melting in acidic chloride solution was studied. The morphological characteristics of the corroded specimens were examined by OM(optical microscopy), SEM
and EDX techniques. The results indicated that the intergranular and exfoliation corrosion susceptibility dramatically depended on the Ni content. The Al-Mg-Sc-Zr alloy with 0 and 0.5 wt.% Ni were lightly susceptible to intergranular corrosion as the precipitation of Mg2Al3 phases presented at grain boundaries and the Al3Ni particles were finely dispersive. The intergranular corrosion was enhanced by the Al3Ni particles enrichment and became pitting corrosion with increasing Ni to 1.0 wt.%. Finally, the exfoliation corrosion happened to the alloy with 2.0wt.% Ni. This trend correlated well with the electrochemical property and distribution of Al3Ni phases. The corrosion potential of Al3Ni intermetallic phase is nobler than the β phase and the matrix, which result in an appearance of galvanic coupling. In addition, the increase of Al3Ni particles enlarged the attack area and the inhomogeneous segments of Al3Ni and Al3Mg2 phases accelerated the localized corrosion.

Abstract: After surface mechanical attrition treatment (SMAT) for Al-Zn-Mg alloy, a gradient structure with average grain size increased from 20nm in surface layer to about 100nm at a depth of 20μm was formed. The thermal stability of surface nanostructured layer in Al-Zn-Mg alloy samples was investigated by vacuum annealing at 100°C, 150°C, 200°C and 250°C for 1h, respectively. The microstructural evolution as well as the microhardness along the depth from top surface layer to matrix of SMATed samples was analyzed. Experimental results showed that the grain size of surface nanocrystallites remains in submicro-scale, ranging from 300nm to 400nm, when annealed at a temperature of 250°C, and the microhardness of surface nanostructured layer was still high compared with that of matrix, indicating satisfying thermal stability of nanocrystallized layer. This might be attributed to the presence of substantive trident grain boundaries and pinning effect of dispersive precipitated phases in nanocrystalline materials, which hindered the grain boundary migration that leading to grain growth.

Abstract: The surface roughness and morphology of the ceramic coatings fabricated on industrially pure aluminum by the plasma electrolytic oxidation (PEO) technique with a hetero-polar pulsed current ceramic synthesizing system were investigated. A model for the process was established. Based on this a formula was deduced to estimate the temperature rise rate at the corresponding locations before the plasma discharge channels have formed during the PEO process, and the channels’ temperature when the plasma discharges occur. Results show that the PEO ceramic coatings grow by an alternating coating melting and solidifying way; The coating roughness becomes greater and the plasma discharge channel population in the ceramic coatings decreases while the pores enlarge with PEO treatment time and current. With the deduced formula, the PEO alumina coating on industrially pure aluminum was taken as an example for validation of the model.