Influence of silane on corrosion resistance of magnesium alloy AZ31 with thermally sprayed aluminum coatings

Aluminum coatings on Mg alloy AZ31 were fabricated using the thermal spraying technique, and then sealed with silane.The surface morphology and chemical groups were discerned using scanning electron microscopy and examined using Fourier transformation infrared spectroscopy, respectively.The salt fog tests and the potentiodynamic electrochemical technique were applied to evaluate the influence of silane on corrosion of the AZ31 alloy with aluminum coatings.The results showed that the corrosion resistance of the aluminum-coated AZ31 alloy was superior to that of the substrate.The aluminum coating sealed with various silane layers led to a further increase in the corrosion resistance of the alloy.Double silane layers were more corrosion-resistant than the single one.Also, it was no longer significant for more than two silane layers to improve the corrosion resistance.It implied that the optimum choice for silane treatment on the aluminum coatings was two layers.

Galvanic Corrosion of Magnesium Alloy and Aluminum Alloy by Kelvin Probe

Galvanic corrosion on samples of AZ91D magnesium alloy coupled with 2A12 aluminum alloy during neutral salt spray test was investigated.The variations of the surface potential were measured using scanning kelvin probe（SKP）.The results showed that galvanic effect on the corrosion of AZ91D magnesium alloy is closely related to the potential difference between the anodic and cathodic materials.In the initial period,corrosion only occurred in a narrow area at the coupling interface because of the limited distance galvanic current.Then,the corrosion rate of 2A12 aluminum alloy was accelerated due to its poor stability in strong alkali environment,which was attributed to the strong alkalization caused by the corrosion of AZ91D magnesium alloy.With the increase of the potential of 2A12 aluminum alloy as a result of the continuous covering of corrosion products,the potential difference between the two materials was enlarged,which enhanced the galvanic corrosion.

Tsinghua University Tsinghua-TOYO Research and Development Center of Magnesium and Aluminum Alloys Processing Technology

Tsinghua-TOYO Research and Development Center of Magnesium and Aluminum Alloys Processing Technology was officially established between Tsinghua University and TOYO Machinery & Metal Co. on Feb. 26, 2002, which is a non-independent legal research organization located in Department of Mechanical Engineering of Tsinghua University. The center was equipped with one set of 650t automatic magnesium and aluminum alloys diecasting machine and necessary accessories including the melting furnaces for magnesium alloys and aluminum alloys, an accurate magnesium pump to transfer the magnesium alloys to the shot sleeve of the die casting machine, a die temperature controller, and data logging systems for cavity pressure and die temperature distributions, etc. The center is aimed to the research and development of magnesium and aluminum die casting process and their related technologies, and the main research contents include:

Effect of magnesium on the aluminothermic reduction rate of zinc oxide obtained from spent alkaline battery anodes for the preparation of Al–Zn–Mg alloys

The aluminothermic reduction of zinc oxide（ZnO） from alkaline battery anodes using molten Al may be a good option for the elaboration of secondary 7000-series alloys. This process is affected by the initial content of Mg within molten Al, which decreases the surface tension of the molten metal and conversely increases the wettability of ZnO particles. The effect of initial Mg concentration on the aluminothermic reduction rate of ZnO was analyzed at the following values： 0.90wt%, 1.20wt%, 4.00t%, 4.25wt%, and 4.40wt%. The ZnO particles were incorporated by mechanical agitation using a graphite paddle inside a bath of molten Al maintained at a constant temperature of 1123 K and at a constant agitation speed of 250 r/min, the treatment time was 240 min and the ZnO particle size was 450？500 mesh. The results show an increase in Zn concentration in the prepared alloys up to 5.43wt% for the highest initial concentration of Mg. The reaction products obtained were characterized by scanning electron microscopy and X-ray diffraction, and the efficiency of the reaction was measured on the basis of the different concentrations of Mg studied.

Homogenization and Recrystallization of Al-6Mg Alloys with and without Sc and Zr

Plates of Al-6Mg alloys with and without scandium were prepared by semi-continuous cast, homogenization, hot-rolled, cold-rolled and annealing. Based on studying of effects of temperature and time on hardness and conductivity during homogenization, it was found that homogenization of ingot with Sc and Zr can result in dispersion strengthening. Recrystallization temperature of alloys was studied by hardness method and optical metallographic method. Staring recrystallization temperature of Al-6Mg alloys with Sc and Zr is 375 ℃, increasing by 150 ℃ than Al-6Mg alloys without Sc and Zr. Mechanical properties (σ_b, σ_(0.2), δ) of hot-rolled alloy with Sc and Zr are 400 MPa, 280 MPa and 18%; cold-rolled and anneal alloy with Sc and Zr are 420 MPa, 310 MPa and 12% respectively.

Finite element analysis of temperature and stress fields during selective laser melting process of Al−Mg−Sc−Zr alloy

A 3D finite element model was established to investigate the temperature and stress fields during the selective laser melting process of Al−Mg−Sc−Zr alloy.By considering the powder−solid transformation,temperaturedependent thermal properties,latent heat of phase transformations and molten pool convection,the effects of laser power,point distance and hatch spacing on the temperature distribution,molten pool dimensions and residual stress distribution were investigated.Then,the effects of laser power,point distance and hatch spacing on the microstructure,density and hardness of the alloy were studied by the experimental method.The results show that the molten pool size gradually increases as the laser power increases and the point distance and hatch spacing decrease.The residual stress mainly concentrates in the middle of the first scanning track and the beginning and end of each scanning track.Experimental results demonstrate the accuracy of the model.The density of the samples tends to increase and then decrease with increasing laser power and decreasing point distance and hatch spacing.The optimum process parameters are laser power of 325−375 W,point distance of 80−100μm and hatch spacing of 80μm.

Improved strength and ductility of high alloy containing Al–12Zn–3Mg–2.5Cu alloy by combining non-isothermal step rolling and cold rolling

Al-12Zn-3Mg-2.5Cu alloy was prepared using a liquid metallurgy route under the optimized conditions. A sample cut from the ingot was rolled non-isothermally from 400℃ to 100℃ in 100℃ steps, with 15% reduction in thickness; it was then cold rolled isothermally at room temperature for 85% reduction. The cold-rolled alloys were characterized by electron microscopy, hardness test, and tensile test to elucidate their structural evolution and evaluate their mechanical behavior. In the results, the cast alloy consists of a-aluminum and various intermetallic compounds. These compounds are segregated along the grain boundaries, which makes the alloy difficult to roll at room tem- perature. The combined effect of non-isothermal step rolling and cold rolling results in the nano/microsized compounds distributed uniformly in the matrix. The hardness is substantially increased after rolling. This increase in hardness is attributed to the ultra-fine grain size, fine-scale intermetallic compounds, and structural defects （e.g., dislocations, stacking faults, and sub-grains）. The ultimate tensile strength of the rolled alloy is approximately 628 MPa with 7% ductility.

Mechanical properties and friction behaviors of CNT/AlSi_(10)Mg composites produced by spark plasma sintering

The mechanical properties and friction behaviors of CNT/AlSi10Mg composites produced by spark plasma sintering (SPS) were investigated. The results showed that the densities of the sintered composites gradually increased with increasing sintering temperature and that the highest microhardness and compressive strength were achieved in the specimen sintered at 450A degrees C. CNTs dispersed uniformly in the Al-Si10Mg matrix when the addition of CNTs was less than 1.5wt%. However, when the addition of CNTs exceeded 1.5wt%, the aggregation of CNTs was clearly observed. Moreover, the mechanical properties (including the densities, compressive strength, and microhardness) of the composites changed with CNT content and reached a maximum value when the CNT content was 1.5wt%. Meanwhile, the minimum average friction coefficient and wear rate of the CNT/AlSi10Mg composites were obtained with 1.0wt% CNTs.

Improving friction stir weldability of Al/Mg alloys via ultrasonically diminishing pin adhesion

Formation of intermetallic compounds （IMCs） during friction stir welding （FSW） of alu- minum/magnesium （AI/Mg） alloys easily results in the pin adhesion and then deteriorates joint formation. The severe pin adhesion transformed the tapered-and-screwed pin into a tapered pin at a low welding speed of 30 mm/min. The pin adhesion problem was solved with the help of ultrasonic. The weldability of Al/Mg alloys was significantly improved due to the good material flow induced by mechanical vibration and the fragments of the IMCs on the surface of a rotating pin caused by acoustic streaming, respectively. A sound joint with ultrasonic contained long Al/Mg interface joining length and complex mixture of AI/Mg alloys in the stir zone, thereby achieving perfect metallurgical bonding and mechanical interlocking. The ultrasonic could broaden process window and then improve tensile properties. The tensile strength of the Al/Mg joint with ultrasonic reached 115 MPa.