关键词： Magnesium   Microstructure   Micro hardness   Friction Stir Processing   Spiral tool path  

摘要： Friction stir processing is an advance technique to modify the microstructure of metals for enhanced mechanical properties. Previous works have demonstrated that the friction stir processing of Mg alloys is feasible. These works are restricted to small areas or sample sizes. The objective of this work is to adopt spiral tool path strategy to process a complete blank of Mg alloy and study the effect of process parameters on the microstructure and hardness. The friction stir processed samples are prepared with different combinations of tool shoulder overlaps and tool rotation directions. Experimental results show fully consolidated processed Mg blanks are achieved with the tool rotation direction, which forces the advancing side towards the 'yet to be processed' side of the tool. Further, tool rotation direction plays a crucial role towards defect formation and retention upon friction stir processing of Mg alloy blanks with spiral tool path. The microstructural studies and hardness test are performed for the fully consolidated processed blanks. The microstructure of the processed region consists of refined grains, with banded regions comprising even finer grains. The friction stir processing of the as-cast Mg alloy also lead to the dissolution of second phase and more uniform distribution of the alloying elements in the Mg matrix. The grain refinement and second phase dissolution lead to enhanced hardness upon friction stir processing. Higher tool shoulder overlap promotes higher temperatures and plastic deformation, which leads to more grain refinement and second phase dissolution with corresponding increase in hardness levels. The local variation in average hardness within a processed sample corresponds well to the local variation of the average grain size. This work shows that large samples of Mg alloy can be friction stir processed towards microstructure modification and mechanical property enhancement, with appropriate combination of tool rotation direction and t

关键词： Laser shock peening   MB8 magnesium alloy   Microhardness   Friction coefficient   Wear rate   Anti-wear mechanism  

摘要： Due to low hardness of magnesium alloy, surface wearing often causes fatigue fracture of magnesium alloy component, which seriously affects flight safety of aircraft. Laser shock peening (LSP) is a new surface plasticity strengthening technology, which can increase the hardness and strength of metal materials by work hardening, thereby significantly improving friction and wear performance. In this work, MB8 magnesium alloy samples were treated by LSP with a group of optimized parameters. The laser-peened samples were tested on friction-wear testing machine under different loads of 50 gf, 100 gf, 150 gf, and 200 gf;rotate speeds of 100 r/min, 200 r/min, 300 r/min, and 400 r/min;and friction pairs of Si3N4 ball and 416L ball. Microhardness, abrasion morphology, and elemental contents were measured by a microhardness tester, scanning electron microscope (SEM), and energy dispersive spectroscopy (EDS), respectively. The experimental results show that LSP could effectively enhance the anti-wear performance of the magnesium alloys, but the amplitude for the enhancement depends on the working conditions. To be more specific, LSP has a better effect on anti-wear property under low load, low rotate speed, and ceramic friction pair conditions. Moreover, the underlying anti-wear mechanisms of the LSPed samples under different working conditions were revealed.

关键词： dynamic precipitation   high strain rate rolling   mechanical properties   Mg-Al-Mn alloys   shear bands   textures  

摘要： AM60B magnesium alloy sheets with excellent mechanical properties are prepared by a double-pass high strain rate rolling (HSRR) with gradient cooling from 370 to 340 degrees C. A bimodal lamellar structure, consisting of fine dynamically recrystallized grains, shear bands and fine precipitates, is obtained after the second-pass HSRR. Formation mechanisms of shear bands are studied in depth. The results show that the formation of shear bands are closely related to the high-density intersected twin lamellae, boundaries of initial coarse grains and the extensive dynamic recrystallization (DRX), respectively. It is also clarified that the HSRR process of the coarse grained AM60B alloy is dominated in succession by the twinning deformation stage and the following shear bands flow stage. Dynamic precipitation behavior is induced in the double-pass HSRR process. The DRX is promoted by fine precipitates, resulting in local refinement of the HSRR'ed microstructure. Moreover, the weak basal texture is obtained with peak intensity less than 10. The high-density twins, DRX and shear bands are suggested as main causes for the basal texture weakening. An outstanding matching between tensile strength and ductility at room temperature are obtained owing to the bimodal lamellar structure and the weakened basal texture.

关键词： biodegradable magnesium   corrosion behavior   microgalvanic corrosion   precipitation   beta-compounds  

摘要： Microstructure analyses, immersion tests, and electrochemical measurements are performed to investigate the corrosion behavior of Mg-1.59Nd-2.91-Zn-0.05Zr-0.35Mn (NZKM) alloy under three different conditions (as-cast, heat-treated, and as-extruded) in Hanks' solution. The results indicate that the corrosion rates of the three types of NZKM alloys are all lower than 0.5 mm year(-1). The primary driving force of the NZKM alloys' corrosion is the microgalvanic coupling between alpha-Mg and beta-compounds/precipitates in Hanks' solution during the corrosion process. The lowest corrosion rates can be attributed to a low content of beta-compounds and less precipitation, a necklace-type appearance, and discontinuous distribution of the beta-compounds in the heat-treated NZKM alloy. The clustered precipitation that occurs within the as-extruded NZKM exacerbates the corrosion process due to the formation of clustered microgalvanic couple groups. These results indicate that the content of beta-compounds and precipitation of the NZKM alloys, as well as their appearances and distributions, should be controlled to allow for further application as biodegradable materials.

关键词： Conversion coating   Mg alloy   Corrosion resistance   Hydrogen induced cracking  

摘要： Cracks formed on chromate-free conversion coating (CFCC) have long been regarded as the result of dehydration. In this work, multiple forms of defects inside the phosphate-vanadium (P-V) coating and phosphate-calcium-vanadium (P-Ca-V) coating were observed and analyzed by TEM. Results show that the conversion coatings are rich with defects near the metal/coating interface. The "vena contracta" phenomenon of cracks, the trench found at the root of crack, and the isolated pores found inside the coating indicate that the cracks originate from the metal/coating interface. An entrapped hydrogen-induced coating cracking mechanism was proposed. Solutions towards governing the hydrogen-induced cracking were proposed.

关键词： AZ31B   deformation behavior   magnesium alloy sheet   pulsating loading condition  

摘要： Magnesium alloys have been known as the next generation material for lightweight body structures. Pulsating hydroforming is an effective method to improve magnesium alloy sheet forming performance, and the formed parts are characterized by lightweight, high-specific strength and stiffness. The deformation performance of magnesium alloy sheet AZ31B with a thickness of 0.6 mm under pulsating hydroforming has been investigated by means of experimental study, numerical simulation and theoretical analysis. The results show that under the same maximum hydraulic pressure, compared with simple linear loading, the magnesium alloy forming parts with pulsating hydraulic loading not only have better wall thickness uniformity and larger bulging height but also can delay the occurrence of fracture, improve the forming performance and ultimate the forming ability of magnesium alloy sheet. A new evaluation index is proposed to simplify the comprehensive forming performance of magnesium alloy parts with different amplitudes and frequencies more accurately, which can also be applied to determine the optimal forming parameters of magnesium alloy sheet AZ31B in the pulsating loading condition.

关键词： Epoxy coating   Silane   Inhibitors   EIS   EN   Pull-off adhesion  

摘要： PEO/Silane/Epoxy coating system with inorganic inhibitor (Ce(NO3)(3)) in the plasma electrolytic oxidation (PEO) layer as well as organic inhibitors in the silane layer was investigated in terms of active corrosion protection properties and adhesion strength. The results of electrochemical impedance spectroscopy (EIS), electrochemical noise (EN) and pull-off tests indicated that the presence of the 8-Hydroxyquinoline (8-HQ) organic inhibitor provides the optimal performance for coating systems. The formation of Ce(OH)(3) in the PEO coating structure and the complex chelate of Mg(HQ)(2) in the silane layer lead to the creation of active corrosion protection properties for the coating systems.

关键词： magnesium alloys   rare-earth metals   post-aging retrogression   decomposition of solid solution  

摘要： The phenomenon of post-aging retrogression in magnesium alloys containing two rare-earth metals belonging to different groups, i.e., yttrium Y (the yttrium group) and samarium Sm (the cerium group), is investigated. It is shown that retrogression may occur in alloys with different proportions of yttrium and samarium aged at 200 degrees C for maximum hardness and then annealed at somewhat higher temperatures (250 and 300 degrees C). The process is possible due to decomposition of the supersaturated magnesium solid solution. The degree of the retrogression is shown to depend on the temperature and duration of the post-aging annealing and on the yttrium- to-samarium proportion in the alloys.

关键词： Aluminum   Coating   Cold spray   Corrosion   Hardness   Mg alloy  

摘要： Magnesium (Mg) alloys have a high strength/weight ratio, high dimensional stability, good machinability, and the ability to be recycled. However, their poor corrosion resistance in humid environments limits their usage for exterior aerospace components. This study aims to improve the corrosion resistance of two Mg alloys (AZ31B and AZ91) by using aluminum coatings. The latter have been deposited by a low pressure and temperature cold spray process. An aluminum powder (60 wt%) with a particle size ranging between 1 and 8 mu m and nickel powder (40 wt%) with a particle size of about 70 mu m were blended and used as feedstock powder. The coating thickness was about 240 mu m. Its densification was achieved by the in-situ hammering effect of the nickel particles. The shot-peening effect also resulted in an enhanced coating hardness. The microstructure, mechanical properties, and corrosion resistance of the coatings have been investigated. They showed that the aluminum had a face centered cubic structure. Potentiodynamic polarization tests were performed along with a combination of materials characterization techniques to assess the corrosion resistance of the coatings when immersed in a 3.5 wt% NaCl solution for long durations. The results revealed that the corrosion resistance increased with the immersion time because of the formation of a protective oxide layer on the surface. These results were supported by elemental and structural analyses. This study shows that cold-sprayed aluminum coatings are a promising candidate for enhancing the corrosion resistance of AZ31B and AZ91Mg alloys compared to other thermal spray processes.

关键词： fine structure of magnesium alloy   self-organizing LPSO-phases   deformation process parameters   redistribution of Zn and Y hardening elements  

摘要： Results of a study of the fine structure of an alloy of the Mg - Zn - Zr - REE system proving the presence of nanosize plates of self-organizing LPSO-phases after deformation are presented. The Zn and Y hardening elements are redistributed in the periodic rows of the LPSO phases under the effect of the deformation process parameters. The optimum deformation mode of the new alloy provides formation of periodic rows in plates of LPSO phases bearing (3.2 - 4.3) at.% Y and (3.8 - 4.2) at.% Zn.