关键词： grain refinement   magnesium alloy   microsegregation   quasi-crystalline phase   subrapid solidification  

摘要： The evolution of the microstructure and microsegregation in a new type of Mg-6Al-4Zn-1.2Sn (wt%) cast magnesium alloy solidified at cooling rates from 4.5 x 10(1) degrees C s(-1)to 2.3 x 10(3) degrees C s(-1)is investigated. The results indicate that the secondary dendrite arm spacing and the average grain sizes decrease from 14.3 to 2.9 mu m and from 187 to 78 mu m, respectively, as the cooling rate increases from 4.5 x 10(1) degrees C s(-1)to 2.3 x 10(3) degrees C s(-1). The relationship between the secondary dendrite arm spacing (lambda(2)) and cooling rate (R) is fitted as lambda(2) = 69.7 x R-0.42. When the alloy solidifies in the range of cooling rates, a new quasi-crystalline I phase is formed due to faster solidification. The microsegregation ratios of Al, Zn, and Sn elements decrease significantly with the increase in cooling rate. Compared with the specimen solidified at 4.5 x 10(1) degrees C s(-1), the microsegregation ratios of Al, Zn, and Sn of that solidified at a cooling rate of 2.3 x 10(3) degrees C s(-1)decrease by 37.4%, 45.7%, and 31.6%, respectively.

关键词： HPDC parameters   Defect band   Porosity   3D   Tensile fracture  

摘要： For understanding the processing-microstructure-properties correlation of high pressure die casting (HPDC) AZ91D alloy, 3D reconstruction was carried out to characterize the microstructural morphologies of castings produced by four designed HPDC processes. The influence of different parameters (vacuum, slow-shot speed, fast-shot speed) on porosity volume and morphology, defect band width and average externally solidified crystals (ESCs) size of castings were systematically studied. Microstructural characterization revealed that the defect band width was strongly related with the size and quantity of ESCs, rather than the porosity volume. Meanwhile, ESCs were mainly influenced by the slow-shot speed in shot sleeve (ESCs growth time) and fast-shot speed into the die cavity (shear stress of melt flow). Tensile fracture revealed that cracks easily propagated within the defect bands due to the accumulated ESCs and connected irregular pores, while the cracks were restrained in center zone. The mechanical properties of HPDC castings was inversely proportional to the defect band width, and is not dependent on the average volume fraction porosity.

关键词： AZ31 magnesium alloy   Cross variable thickness rolling   Edge crack   Edge curve   Microstructure  

摘要： According to the MAS rolling and cross rolling theory, the hot rolling experiments were carried out on the AZ31 magnesium alloy specimens at the initial rolling temperature of 350 degrees C, and the grain distribution and edge damage of the strip after rolling by a controlled rolling reduction in different regions were analyzed. Meanwhile, the edge curves were designed to improve the edge crack and grain refinement of magnesium alloy during cross variable thickness rolling. The results indicated that the variable thickness rolling with the edge curve could effectively reduce the edge crack of magnesium alloy. The results also indicated that different edge curves could produce varying degrees of grain refinement at the strip edge and realize the ideal grain refinement in different regions during the rolling process;meanwhile, the optimization of the edge curve during cross variable thickness rolling could provide a theoretical basis for improving the magnesium alloy edge crack.

关键词： Wire-arc additive manufacturing   AZ31 magnesium alloy   Cold metal transfer   Processing parameter   Anisotropy  

摘要： The modified cold metal transfer (CMT) heat sources by adjusting the characteristic processing parameters were conducted in wire-arc additive manufacturing (WAAM) of AZ31 magnesium alloy for thin-wall component. The processing, microstructure, and mechanical behaviour were investigated. The CMT characteristic deposited a wide and shallow weld bead coupled with good wettability and low equivalent heat input was selected for CMT-WAAM process. The microstructure of thin-wall component showed obvious layer characteristics, which consisted of the deposited layers with fine columnar dendrite grains and the HAZ interlayers with coarse equiaxed grains and some pores. The thin-wall component exhibited anisotropic tensile properties with lower yield strength, higher elongation, and higher ultimate tensile strength in the deposition direction than in the build direction. CMT-WAAM component of AZ31 magnesium alloy has an accepted mechanical property compared with the as-forged and as-cast components. To improve the component strength, minimizing the softening effect of the HAZ interlayers by preventing grain coarsening and suppressing porosity is essential.

关键词： casting and solidification   magnesium   optical microscopy   scanning electron microscopy   ultrasonic processing  

摘要： The reduction of harmful greenhouse gas emissions can be realized by utilizing lightweight structural metals, such as magnesium (Mg). Magnesium alloys have the potential to replace higher-density aluminum and ferrous components in automotive and aerospace industries, thereby decreasing vehicle weight and the associated fuel requirements. However, their strength and ductility must be improved to ensure widespread application. This goal can be achieved through ultrasonic processing, a technique that is gaining popularity in the manufacturing of light alloys. In this study, the effects of high-intensity ultrasonic vibration on the microstructure and mechanical properties of AZ91E were investigated. The molten alloys were subjected to 60, 120, 180 and 240 s of sonication at a frequency and amplitude of 20 kHz and 30 mu m, respectively. The resultant castings were characterized using optical microscopy, scanning electron microscopy and tensile testing. Sonication was found to improve the mechanical properties of the alloy relative to the base condition. The improvement was attributed to the refinement of the Mg grain structure, decreased area fraction and average size of the brittle Mg17Al12 eutectic phase, in addition to increased area fraction, homogenous distribution and spheroidization of the Mn-Al intermetallic phase. The refinement in microstructure was determined to be a result of improved nucleation and cavitation-induced undercooling of the melt as well as a finer grain size. Therefore, ultrasonic processing can enhance the viability of Mg alloys as engineering materials, offering important opportunities for the production of greener, light-metal components.

关键词： detonation sprayed   dry sliding wear   Fe-based amorphous coating   magnesium alloy   wear mechanism  

摘要： A Fe-based amorphous coating was successfully deposited on an AZ31B magnesium alloy substrate by detonation spraying. The microstructure and sliding wear behavior of the coating were investigated in detail under dry conditions in ball-on disk mode by using a Si3N4 ball as the counterpart. It was found that the Fe-based amorphous coating presents an almost perfect amorphous structure, and the thickness, microhardness, porosity and amorphous content are 200 +/- 30 mu m, 825 +/- 15 Hv(0.1), 1.0% and 89.3 +/- 2%, respectively. The dry sliding wear tests indicate that the Fe-based amorphous coating presents a fairly low dry sliding wear rate of 3.14-11.5 x 10(-6) mm(3) N-1 m(-1), which is two orders of magnitude lower than that of the AZ31B substrate under a normal load of 5 N and at a sliding speed of 0.25 m/s. This distinguished wear resistance is mainly attributed to the fact that the Fe-based amorphous coating prepared by detonation spraying has an extremely low porosity and high amorphous phase content. Moreover, as the increase in load and sliding speed, the dry sliding wear mechanism of the Fe-based amorphous coating changes from fatigue wear and oxidative wear to delamination wear and oxidative wear.

关键词： Nano-composited film   ALD   In-situ oxidation   Microstructure   Corrosion behavior  

摘要： The high corrosion rate of Mg alloy restricts its application. In this paper, a nano-composite film as TiN/TiO2 was introduced to Mg alloy to improve the corrosion resistance. The nano-composited film was fabricated by atomic layer deposition (ALD) and in-situ oxidation (20 SCCM oxygen flow, at 250 degrees C, duration >= 20 min). When oxygen is introduced, the initial ALD-ed crystalline TiN layer is rapidly oxidized till the amorphous TiO2 layer is saturated. A stable TiN/TiO2 nano-composited film would be obtained then. X-ray photoelectron spectroscopy depth (XPS-depth) and transmission electron microscopy image (TEM) show that the nano-composite film has an interlayer structure. The nano-composite film shows good adhesion to the Mg alloy substrate in the nano-indentation analysis. Moreover, the nano-indentation scratches heal to a certain degree after 48 h, which means the film could self-heal. With the film, the surface potential mapping is not only overall higher but also less fluctuating by scanning Kelvin probe (SKP) analysis. There is a higher E-corr (increasing similar to 0.42 V) and a much lower i(corr) (decreasing similar to 1/200) for potentiodynamic polarization (PDP) measurement. Equivalent circuit fitting for electrochemical impedance spectroscopy (EIS) profile changes from R-s(CPE1R(ct)) to R-s(CPE2(R-f(CPE1R(ct)))). The film makes less corroded withing a more uniformed corroded mode during neutral salt spray test (NSS). All these indicate that Mg alloy corrosion resistance is greatly improved. The work not only provides a flexible method for the interlayer composite film fabrication but also make an effective surface modification for Mg alloy corrosion resistance.

关键词： Mg-Al-Zn alloy   Al-Ti-B master Alloy   Grain refinement   Mechanical properties  

摘要： Enhancement of mechanical properties of AZ91 magnesium alloy based on the structural refinement induced by a newly developed Al-5Ti-2B master alloy and subsequent hot working through the extrusion process was studied. The addition of Al-5Ti-2B master alloy resulted in the grain refinement of alpha-Mg grains mainly due to the inoculating effect of TiB2 particles, where the optimal amount of the grain refiner was found to be similar to 0.3 wt%. The hot extrusion process led to a remarkable grain refinement and the fracturing and distribution of intermetallics along the extrusion direction. The enhancement of mechanical properties was successfully rationalized based on the grain size, and as a result, Hall-Petch relationships were developed for the yield stress, tensile strength, and hardness. However, the ductility of the extruded alloys was larger than those predicted by the grain size effects alone, which resulted in a superior strength-ductility trade-off compared to the as-cast counterparts. This was related to the disappearance of the grain boundary beta-Mg17Al12 phase, closure of casting defects, and formation of a recrystallized and homogeneous microstructure in the extruded alloys.

关键词： Metal matrix composite   Equal channel angular pressing   Microstructure   Hydrogen storage  

摘要： In this study, AZ61 alloy processed by equal channel angular pressing (ECAP) method, is presented. This research seeks to find out the relationship that exists between the microstructure of the AZ61-magnesium alloy doped with SiC and Ni and processed by ECAP in terms of hydrogen storage properties. The results indicate that grain size reduces nearly 10 times after 4 passes of ECAP. Homogeneous distribution of the SiC in the metal matrix composite (MMC) and dynamic recrystallization are also observed. There is a gradual reduction in the grain size when the number of ECAP passes are increased. This microstructural refinement induced by ECAP improves the storage capacity and reaction kinetics of the alloy. Besides, the addition of Ni catalyst achieves faster absorption and desorption kinetics. However, it does not change its capacity. Ni reacts with Mg to form an intermetallic, which is hydrogen-absorbing, which in turn, catalyzes the reaction. Apart from introducing defects, the ECAP process also produces Mg/Al intermetallic, which acts catalytically. It is shown that SiC-doped AZ61-magnesium alloy can absorb and desorb 6.8 wt% H-2 at 648 K and 50 atm. (C) 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

关键词： AZ31B alloy   Superhydrophobic   Self-healing   Magnesium silicate nanotubes   2-Mercaptobenzimidazole  

摘要： In this work, clinochrysotile-like magnesium silicate nanotubes (MS-TNs) were fabricated by a hydrothermal method and then modified with dodecyltrimethoxysilane (DTMS) for the first time to act as a superhydrophobic material (MS-TNs/DTMS), whilst the MS-TNs was used as a nanocontainer to load a corrosion inhibitor 2-mercaptabenzimidazole (2-MBI). Taking the advantage of epoxy resin as a coating matrix, the AZ31B Mg alloy substrate was coated successfully by MS-TNs(2-MBI) as the first layer (for self-healing anti-corrosion) and superhydrophobic MS-TNs/DTMS as the second layer (for superhydrophobicity and anti-corrosion). The wettability, functionality and physicochemical stability of the superhydrophobic surface as well as the anticorrosion performances of the dual-function coatings were systematically investigated by characterization, testing and electrochemical measurements. The as-prepared dual function coating with water contact angle at about 155 degrees and sliding angle at circa 5 degrees not only showed robust mechanical stability and chemical durability, but also exhibited substantial improvement of anti-corrosion property for protection of AZ31B Mg in a long run. The corrosion current density decreased from 8.33 x 10(-4) ***(2) of bare Mg to 8.12 x 10(-8) ***(2) of the coated sample. Furthermore, MS-TNs/DTMS was applicable to various kinds of substrates (e.g., copper mesh, sponge, glass slide, cloth, A4 typing paper) and showed an excellent water repellency, demonstrating its potential usage in fabrication of superhydrophobic surface as a low-cost and environmentally-benign material.