This study emphasizes on the evaluation and comparison of the anticorrosive properties of sol-gel coatings with and without inhibitor loaded nanocontainers.In this case,naturally available clay nanotubes(halloysite)we...This study emphasizes on the evaluation and comparison of the anticorrosive properties of sol-gel coatings with and without inhibitor loaded nanocontainers.In this case,naturally available clay nanotubes(halloysite)were loaded with cationic corrosion inhibitors Ce 3+/Zr 4+.These nanocontainers were dispersed in hybrid organic-inorganic sol-gel matrix sol.Coating was applied on magnesium alloy AZ91D using the sols containing modified and unmodified nanocontainers employing the dip coating method and cured at 130℃for 1 h in air.Corrosion resistance of coated/uncoated substrates were analyzed using electrochemical impedance spectroscopy,potentiodynamic polarization and weight loss measurements after exposure to 3.5 wt%NaCl solution for varying time durations between 24 h to 120 h.Self-healing ability of coatings was evaluated by micro-Raman spectroscopy after 120 h exposure to 3.5 wt%NaCl solution.Coatings generated after dispersion of corrosion inhibitor loaded clay in hybrid sol-gel matrix have shown more promising corrosion resistance when compared to just the sol-gel matrix coatings,after prolonged exposure to corrosive environment.展开更多
The effects of rare earth elements on the microstructure andproperties of magnesium alloy AZ91D alloy were studied. The differentproportion of rare earth elements was added to the AZ91D and thetensile tests were carri...The effects of rare earth elements on the microstructure andproperties of magnesium alloy AZ91D alloy were studied. The differentproportion of rare earth elements was added to the AZ91D and thetensile tests were carried out at different temperatures. Theexperimental results show that at room temperature or at 120 deg. Cthe AZ91D's strength decrease with the increasing amount of the rareearth elements. However, the ductility is improved. The influence of0.14/100Sb(mass fraction)on the AZ91D's strength is like that of rareearth elements(0.2/100-0.4/100)(mass fraction). Microstructure graphsdemonstrate that appropriate amount of rare earth elements(0.1/100-0.2/100)can fine AZ91D's grain and improve its ductility.展开更多
Halloysite nanotubes were used as nanocontainers to hold corrosion inhibitors such as Ce^(3+)-Zr^(4+),2-mercaptobenzothiazole and 8-hydroxyquinoline in their lumen.An acid assisted etching of the nanotubes was carried...Halloysite nanotubes were used as nanocontainers to hold corrosion inhibitors such as Ce^(3+)-Zr^(4+),2-mercaptobenzothiazole and 8-hydroxyquinoline in their lumen.An acid assisted etching of the nanotubes was carried out with a view to increase the lumen diameter and thereby,increase the amount of loading of the corrosion inhibitor.The morphology of as-received and etched halloysite nanotubes was ob-served using TEM analysis.The loading of corrosion inhibitors was confirmed using SEM-EDS and BET analysis.Polymeric microcapsules were used as capping agents for the ends of the loaded HNTs following which,they were dispersed into a hybrid sol-gel silica matrix.Dip coating method was used to generate coatings on AZ91D substrates followed by heat treatment at 130℃ for 1 h.The release rate kinetics of corrosion inhibitors from as-received and etched nanotubes was investigated in buffer solutions of 3.5 wt%NaCl at different pH.The release mechanism of corrosion inhibitors from the HNT lumen was validated using various semi-empirical models.Coatings were also evaluated for their corrosion protection ability using electrochemical techniques after exposure to 3.5 wt%NaCl solution for 120 h.Coatings generated using Ce^(3+)-Zr^(4+)loaded into as-received halloysite nanotubes have shown more effective corrosion protection when compared to other corrosion inhibitors after 120 h exposure to the corrosive medium.展开更多
In this research we presented a non-cyanide plating process of Ni-P alloy coating on Mg alloy AZ91D. By applying a new process flow of electroless nickel plating in which zinc coating is used as transition of Ni-P coa...In this research we presented a non-cyanide plating process of Ni-P alloy coating on Mg alloy AZ91D. By applying a new process flow of electroless nickel plating in which zinc coating is used as transition of Ni-P coating on Mg alloy AZ91D, the process of copper transition coating plated in the cyanides bath can be replaced. A new bath composed of NiSO4 was established by orthogonal test. The results show that zinc transition coating can increase the adhesion and protect the Mg alloy substrate from the bath corrosion. The optimal plating bath composition is NiSO4·6H2O 20 g/L, NaH2PO2·H2O20g/L and C6H8O7·H2O 2.5 g/L, and optimal bath acidity and optimal plating temperature are pH 4.0 and 95℃, respectively. The present process flow is composed of ultrasonic cleaning→alkaline cleaning→acid pickling→activation→double immersing zinc→electroplating zinc→electroless nickel plating→passivation treatment. The present non-cyanide process of electroless nickel plating is harmless to our surroundings and Ni-P coating on Mg alloy AZ91D produced by present process possesses good adhesion and corrosion resistance.展开更多
The preparation techniques of Mg-mischmetal intermediate alloy and the effects of the mischmetal addition ranging from 0.45% to 1.04% on the microstructure and properties of AZ91D alloy prepared by die casting were in...The preparation techniques of Mg-mischmetal intermediate alloy and the effects of the mischmetal addition ranging from 0.45% to 1.04% on the microstructure and properties of AZ91D alloy prepared by die casting were investigated. The Mg-MM intermediate alloy was prepared by permanent mold casting and then was extruded into the bars. The microstructure and analytical studies were carried out using optical microscopy and differential scanning calorimetry (DSC). Testing results shows the Mg-MM intermediate alloy could melt easily down at die casting temperature of 680 ℃ that was lower than the melting point of lanthanum (918 ℃) and that of cerium (798 ℃). This was propitious to protection the alloy from the oxidation at high temperatures. Then magnesium alloy test bars were produced under conventional cold chamber die casting condition with addition of different weight of the Mg-MM intermediate alloy. Observation and analysis indicated that the microstructures of the alloy were refined and RE containing Al phase was formed with increasing RE addition. The data obtained by tensile tests showed that alloying with mischmetal improved the tensile property of the AZ91D magnesium die casting alloy at ambient temperature.展开更多
This work proposed a strategy of indirectly inducing uniform microarc discharge by controlling the content and distribution ofβ-Mg_(17)Al_(12)phase in AZ91D Mg alloy.Two kinds of nano-particles(ZrO_(2)and TiO_(2))wer...This work proposed a strategy of indirectly inducing uniform microarc discharge by controlling the content and distribution ofβ-Mg_(17)Al_(12)phase in AZ91D Mg alloy.Two kinds of nano-particles(ZrO_(2)and TiO_(2))were designed to be added into the substrate of Mg alloy by friction stir processing(FSP).Then,Mg alloy sample designed with different precipitated morphology ofβ-Mg_(17)Al_(12)phase was treated by microarc oxidation(MAO)in Na_(3)PO_(4)/Na2SiO3electrolyte.The characteristics and performance of the MAO coating was analyzed using scanning electron microscopy(SEM),energy dispersive spectrometer(EDS),X-ray diffraction(XRD),X-ray photoelectron spectroscopy(XPS),contact angle meter,and potentiodynamic polarization.It was found that the coarseα-Mg grains in extruded AZ91D Mg alloy were refined by FSP,and theβ-Mg_(17)Al_(12)phase with reticular structure was broken and dispersed.The nano-ZrO_(2)particles were pinned at the grain boundary by FSP,which refined theα-Mg grain and promoted the precipitation ofβ-Mg_(17)Al_(12)phase in grains.It effectively inhibited the“cascade”phenomenon of microarcs,which induced the uniform distribution of discharge pores.The MAO coating on Zr-FSP sample had good wettability and corrosion resistance.However,TiO_(2)particles were hardly detected in the coating on TiFSP sample.展开更多
Wire arc additive manufacturing offers advantages in producing large metal structures.The current research on GTA-based wire arc additive manufacturing(GTA-WAAM)of magnesium alloys is focused on deformed magnesium all...Wire arc additive manufacturing offers advantages in producing large metal structures.The current research on GTA-based wire arc additive manufacturing(GTA-WAAM)of magnesium alloys is focused on deformed magnesium alloys,mainly on the Mg-Al alloy system.However,there is little research on GTA-WAAM for casting magnesium alloy.This study investigates the microstructural characteristics and mechanical properties of AZ91D magnesium alloy(AZ91D-Mg)deposited by GTA-WAAM.Single-pass multilayer thin-walled components were successfully fabricated.The results show that equiaxed grains dominate the microstructure of the deposited samples.During the remelting process,the precipitated phases dissolve into the matrix,and they precipitate and grow from the matrix under the thermal effect of the subsequent thermal cycle.The mechanical properties in the vertical and horizontal directions are similar,showing higher overall mechanical properties than the casting parts.The average yield strength is 110.5 MPa,the ultimate tensile strength is 243.6 MPa,and the elongation is 11.7%.The overall hardness distribution in the deposited sample is relatively uniform,and the average microhardness is 59.6 HV_(0.2).展开更多
Hot tearing is a serious destructive solidification defect of magnesium alloys and other casting metals.Quantitative and controllable measurements on the thermal and the mechanical behavior of an alloy during its soli...Hot tearing is a serious destructive solidification defect of magnesium alloys and other casting metals.Quantitative and controllable measurements on the thermal and the mechanical behavior of an alloy during its solidification process are crucial for the understanding of hot tearing formation.We developed a new experimental method and setup to characterize hot tearing behavior via controlled cooling and active loading to force hot hearing formation on cooling at selected fractions of solid.The experimental setup was fully instrumented so that stress,strain,strain rate,and temperature can be measured in-situ while hot tearing was developing.An AZ91D magnesium alloy,which is prone to hot tearing,was used in this study.Results indicate that when hot hearing occurred,the local temperature,critical stress,and cumulative strain were directly affected by strain rate.Depending on the applied strain rate,hot tearing of the AZ91D magnesium alloy could occur in two solidification stages:one in the dendrite solidification stage(fS∼0.81-0.82)and the other in the eutectic solidification stage(fS∼0.99).AZ91D alloy exhibited distinct mechanical behaviors in these two ranges of fraction solid.展开更多
Ceramic oxide coatings were prepared on AZ91D magnesium alloys in alkaline silicate solution using micro-arc oxidation（MAO） technique.The corrosion behavior of MAO coating on AZ91D magnesium alloys in NaCl solutions...Ceramic oxide coatings were prepared on AZ91D magnesium alloys in alkaline silicate solution using micro-arc oxidation（MAO） technique.The corrosion behavior of MAO coating on AZ91D magnesium alloys in NaCl solutions with different concentrations（0.1%,0.5%,1.0%,3.5% and 5.0% in mass fraction） was evaluated by electrochemical measurements and immersion tests.The results showed that the corrosion rate of the MAO coated AZ91D increased with increasing chloride ion concentration.The main form of corrosion failure was localized corrosion for the MAO coated AZ91D immersed in higher concentration NaCl solutions（1.0%,3.5% and 5.0%）,while it was general corrosion in dilute NaCl solutions（0.1% and 0.5%）.Two different stages of the failure process of the MAO coated AZ91D could be identified：1） occurrence of the metastable pits and 2） growth of the pits.Different equivalent circuits were also proposed based on the results of electrochemical impedance spectroscopy（EIS） for the MAO coated AZ91D immersed in different concentrations of NaCl solutions for 120 h.展开更多
The microarc oxidation（MAO） coatings produced at different current frequencies on AZ91 D magnesium alloys were studied systematically. The morphologies, thickness, corrosion performances, and tribological properties...The microarc oxidation（MAO） coatings produced at different current frequencies on AZ91 D magnesium alloys were studied systematically. The morphologies, thickness, corrosion performances, and tribological properties of the coatings were investigated by the scanning electron microscopy, the electrochemical measurement system, and MS-T3000 friction test rig, respectively. The results show that the structure of the coatings becomes denser, and thickness becomes thinner with the increase of the current frequency. It is also found that the corrosion resistance of the coatings produced at higher frequency is improved greatly and the difference of the corrosion current density becomes small with increasing current frequency, which is similar to that of the coating thickness. The tribological test shows that the friction coefficient decreases with increasing the current frequency and the wear resistance of the coatings is influenced by both the thickness and structures. All these results were explained by analyzing the growing process of the MAO coating.展开更多
Pouring and mold temperatures are two important parameters during casting magnesium components. The present study examined their influence on hot tearing susceptibility （HTS） of commercial AZ91D and newly developed ...Pouring and mold temperatures are two important parameters during casting magnesium components. The present study examined their influence on hot tearing susceptibility （HTS） of commercial AZ91D and newly developed Mg-3Nd-0.2Zn-Zr （mass fraction, %; NZ30K） magnesium alloys in gravity permanent mold casting condition. The results indicate that mold temperature shows much more significant influence on the HTS of both alloys than pouring temperature whose influence only can be distinguished at low mold temperature （341 K for AZ91D alloy and 423 K for NZ30K alloy）. Hot tearing susceptibility prediction model concerning feeding parameters, grain size and solidification range, is more suitable to estimate the HTS of different magnesium alloys than the model only concerning feeding parameters. In order to achieve better hot tearing resistance, the ranges of pouring and mold temperatures are suggested to be 961-991 K and≥641 K for AZ91D alloy, 1003-1033 K and≥623 K for NZ30K alloy, respectively.展开更多
The flow behavior and dynamic recrystallization（DRX） behavior of an as-cast AZ91 D alloy were investigated systematically by applying the isothermal compression tests in temperature range of 220-380 ℃ and strain ra...The flow behavior and dynamic recrystallization（DRX） behavior of an as-cast AZ91 D alloy were investigated systematically by applying the isothermal compression tests in temperature range of 220-380 ℃ and strain rate range of 0.001-1 s^-1.The effect of temperature and strain rate on the DRX behavior was discussed.The results indicate that the nucleation and growth of dynamic recrystallized grains easily occur at higher temperatures and lower strain rates.To evaluate the evolution of dynamic recrystallization,the DRX kinetics model was proposed based on the experimental data of true stress-true strain curves.It was revealed that the volume fraction of dynamic recrystallized grains increased with increasing strain in terms of S-curves.A good agreement between the proposed DRX kinetics model and microstructure observation results validates the accuracy of DRX kinetics model for AZ91 D alloy.展开更多
Microstructural evolution of AZ91 magnesium alloy was investigated during homogenizing annealing treatment, hot extrusion and ageing treatment, respectively. The results exhibited that both the divorced eutectic β-Mg...Microstructural evolution of AZ91 magnesium alloy was investigated during homogenizing annealing treatment, hot extrusion and ageing treatment, respectively. The results exhibited that both the divorced eutectic β-Mg17Al12 and the precipitated β-Mg17Al12 phases appeared in the as-cast alloy. The β-Mg17Al12 phase dissolved into α-Mg matrix mostly and the structure kept fine after the optimized homogenization treatment at 380 °C for 15 h. Dynamic recrystallization and consequent grain refinement occurred during extrusion. The banded β-Mg17Al12 precipitates paralleled to the extrusion direction were observed after ageing treatment. The banded precipitation should be attributed to the solidification segregation which was elongated during the subsequent extrusion. Furthermore, the effects of temperature, holding time of homogenization and ageing treatment, and extrusion processing parameters on the microstructural evolution of AZ91 alloy were also discussed in details according to the experimental results.展开更多
Effects of ultrasonic treatment on microstructures were investigated by introducing the ultrasonic oscillation into AZ91 alloy melts. The results show that the ultrasonic treatment does not change the phase compositio...Effects of ultrasonic treatment on microstructures were investigated by introducing the ultrasonic oscillation into AZ91 alloy melts. The results show that the ultrasonic treatment does not change the phase composition but has great influence on solidification microstructures. The area percentage of lamellar eutectic phase increases to the maximum when the applied ultrasonic power is increased to 600 W and then decreases gradually with the further increase of power;meanwhile, the average area of Al8Mn5 phase owns a completely opposing variation trend. The area percentage of Mg17Al12 decreases gradually with increasing the applied ultrasonic power. Mechanisms accounted for the refinement and fraction of different intermetallic phases were also discussed.展开更多
Micro-arc oxidation （MAO） process was carried out on AZ91D alloy in alkaline borate solution using an alternative square-wave power source with different parameters. The effects of voltage, frequency and duty cycle ...Micro-arc oxidation （MAO） process was carried out on AZ91D alloy in alkaline borate solution using an alternative square-wave power source with different parameters. The effects of voltage, frequency and duty cycle on the coatings were investigated by orthogonal experiment. It is found that the thickness of coatings increases with the increase of voltage and duty cycle, but decreases with the increase of frequency. The structure and morphology of the coatings also depend on voltage, frequency and duty cycle. The coatings become more porous and crack with increasing voltage and duty cycle. The coating is thin and transparent when the voltage is lower than 120 V. The corrosion resistances of different coatings were evaluated by polarization curves and electrochemical impedance spectroscopy （EIS） in 3.5% NaCl （mass fraction） solution. When the optimized values of voltage, frequency and duty cycle are 140 V, 2 000 Hz and 0.4, respectively, the anodic coating shows the best corrosion resistance.展开更多
The semi-solid slurry of AZ91 D magnesium alloy was prepared by gas bubbling process.The effect of processing parameters,including gas flow rate,cooling rate and stirring end temperature,on microstructure of AZ91 D se...The semi-solid slurry of AZ91 D magnesium alloy was prepared by gas bubbling process.The effect of processing parameters,including gas flow rate,cooling rate and stirring end temperature,on microstructure of AZ91 D semi-solid slurry was investigated.With increasing the gas flow rate from 0 to 5 L/min,the average size of primary α-Mg particles decreases from 119.1 to77.2μm and the average shape factor increases continuously from 0.1 to 0.596.The formation of non-dendritic primary α-Mg particles during gas bubbling is the result of combined effects of dendrite fragmentation and copious nucleation.With increasing the cooling rate from 3.6 to 14.6℃/min,the average particle size of primary α-Mg phase decreases from 105.0 to 68.1μm while the average shape factor peaks at 9.1℃/min.Both high and low cooling rates can induce dendritic growth of primary α-Mg particles.Changing the stirring end temperature from 590 to 595℃ has little effect on the average size and shape factor of primary α-Mg particles in AZ91 D semi-solid slurry.The insensitivity of semi-solid microstructures to the stirring end temperature is attributed to the sufficient quantity of primary particles formed in the melt.展开更多
The microstructural evolution of AZ91D magnesium alloy prepared by means of the cyclic upsetting-extrusion and partial remelting was investigated. The effects of remelting temperature and holding time on microstructur...The microstructural evolution of AZ91D magnesium alloy prepared by means of the cyclic upsetting-extrusion and partial remelting was investigated. The effects of remelting temperature and holding time on microstructure of semi-solid AZ91D magnesium alloy were studied. Furthermore, tensile properties of thixoextruded AZ91D magnesium alloy components were determined. The results show that the cyclic upsetting-extrusion followed by partial remelting is effective in producing semi-solid AZ91D magnesium alloy for thixofonning. During the partial remelting, with the increase of remelting temperature and holding time, the solid grain size increases and the degree of spheroidization tends to be improved. The tensile mechanical properties of thixoextruded AZ91D magnesium alloy components produced by cyclic upsetting-extrusion and partial remelting are better than those of the same alloy produced by casting.展开更多
To develop AZ91D alloys with fine microstructure, effects of the addition of rare earth (RE), Sr and RE + Sr on the dendrite growth and phase precipitation in AZ91D magnesium alloy were studied, respectively. The resu...To develop AZ91D alloys with fine microstructure, effects of the addition of rare earth (RE), Sr and RE + Sr on the dendrite growth and phase precipitation in AZ91D magnesium alloy were studied, respectively. The results show that the microstructure is refined and the morphology of β-Mg17A112 phase is modified with RE or Sr addition, especially with the RE+Sr composite addition which can reduce the average grain size of AZ91D alloy obviously to 141 μm. The needle-like or block-like new phases adhering to β-Mg17A112 phase form at interdendrites during solidification. The enrichment of RE or/and Sr elements in front of the solidification interface, especially at the tips of α-Mg dendrite, which restricts the growth of α-Mg dendrite, changes the preferential growth of α-Mg and finally results in the grain refinement and the blunting of α-Mg dendrite.展开更多
Low temperature mechanical properties of AZ91D magnesium alloy fabricated by solid recycling process from recycled scraps were studied. Various microstructural analyses were performed using optical microscopy （OM） a...Low temperature mechanical properties of AZ91D magnesium alloy fabricated by solid recycling process from recycled scraps were studied. Various microstructural analyses were performed using optical microscopy （OM） and scanning electron microscopy （SEM）. The recycled specimens consist of fine grains due to dynamic recrystallization and the interfaces of original individual scraps are not identified. Tensile tests were performed at a strain rate of 5 x 10 3 s 1 at room temperature （27 ~C）, -70, -100 and 130 ~C, respectively. Ultimate tensile strength of the specimens increases slightly with decreasing the tensile temperature, and elongation to failure decreases with decreasing the tensile temperature. The tensile specimens at -130 ~C show the highest ultimate tensile strength of 360.65 MPa and the lowest elongation to failure of 5.46%. Impact tests were performed at room temperature （27 ~C）, -70 and -130 ~C, respectively. Impact toughness decreases with decreasing the impact temperature. The impact specimens at -130 ~C show the lowest impact toughness of 3.06 J/cm2.展开更多
基金financial sup-port from SERB,DST for the funding provided through grant number SB/S3/ME/007/2014.
文摘This study emphasizes on the evaluation and comparison of the anticorrosive properties of sol-gel coatings with and without inhibitor loaded nanocontainers.In this case,naturally available clay nanotubes(halloysite)were loaded with cationic corrosion inhibitors Ce 3+/Zr 4+.These nanocontainers were dispersed in hybrid organic-inorganic sol-gel matrix sol.Coating was applied on magnesium alloy AZ91D using the sols containing modified and unmodified nanocontainers employing the dip coating method and cured at 130℃for 1 h in air.Corrosion resistance of coated/uncoated substrates were analyzed using electrochemical impedance spectroscopy,potentiodynamic polarization and weight loss measurements after exposure to 3.5 wt%NaCl solution for varying time durations between 24 h to 120 h.Self-healing ability of coatings was evaluated by micro-Raman spectroscopy after 120 h exposure to 3.5 wt%NaCl solution.Coatings generated after dispersion of corrosion inhibitor loaded clay in hybrid sol-gel matrix have shown more promising corrosion resistance when compared to just the sol-gel matrix coatings,after prolonged exposure to corrosive environment.
基金[The work is financially supported by "973" key foundation of China (No. 2000067202).]
文摘The effects of rare earth elements on the microstructure andproperties of magnesium alloy AZ91D alloy were studied. The differentproportion of rare earth elements was added to the AZ91D and thetensile tests were carried out at different temperatures. Theexperimental results show that at room temperature or at 120 deg. Cthe AZ91D's strength decrease with the increasing amount of the rareearth elements. However, the ductility is improved. The influence of0.14/100Sb(mass fraction)on the AZ91D's strength is like that of rareearth elements(0.2/100-0.4/100)(mass fraction). Microstructure graphsdemonstrate that appropriate amount of rare earth elements(0.1/100-0.2/100)can fine AZ91D's grain and improve its ductility.
文摘Halloysite nanotubes were used as nanocontainers to hold corrosion inhibitors such as Ce^(3+)-Zr^(4+),2-mercaptobenzothiazole and 8-hydroxyquinoline in their lumen.An acid assisted etching of the nanotubes was carried out with a view to increase the lumen diameter and thereby,increase the amount of loading of the corrosion inhibitor.The morphology of as-received and etched halloysite nanotubes was ob-served using TEM analysis.The loading of corrosion inhibitors was confirmed using SEM-EDS and BET analysis.Polymeric microcapsules were used as capping agents for the ends of the loaded HNTs following which,they were dispersed into a hybrid sol-gel silica matrix.Dip coating method was used to generate coatings on AZ91D substrates followed by heat treatment at 130℃ for 1 h.The release rate kinetics of corrosion inhibitors from as-received and etched nanotubes was investigated in buffer solutions of 3.5 wt%NaCl at different pH.The release mechanism of corrosion inhibitors from the HNT lumen was validated using various semi-empirical models.Coatings were also evaluated for their corrosion protection ability using electrochemical techniques after exposure to 3.5 wt%NaCl solution for 120 h.Coatings generated using Ce^(3+)-Zr^(4+)loaded into as-received halloysite nanotubes have shown more effective corrosion protection when compared to other corrosion inhibitors after 120 h exposure to the corrosive medium.
基金This work is partially supported from the National Sciences and Engineering Research Council(NSERC)of Canada Discovery Grant RGPIN48158 awarded to M.Hasan of McGill University,Montreal,for which the authors are grateful.
文摘In this research we presented a non-cyanide plating process of Ni-P alloy coating on Mg alloy AZ91D. By applying a new process flow of electroless nickel plating in which zinc coating is used as transition of Ni-P coating on Mg alloy AZ91D, the process of copper transition coating plated in the cyanides bath can be replaced. A new bath composed of NiSO4 was established by orthogonal test. The results show that zinc transition coating can increase the adhesion and protect the Mg alloy substrate from the bath corrosion. The optimal plating bath composition is NiSO4·6H2O 20 g/L, NaH2PO2·H2O20g/L and C6H8O7·H2O 2.5 g/L, and optimal bath acidity and optimal plating temperature are pH 4.0 and 95℃, respectively. The present process flow is composed of ultrasonic cleaning→alkaline cleaning→acid pickling→activation→double immersing zinc→electroplating zinc→electroless nickel plating→passivation treatment. The present non-cyanide process of electroless nickel plating is harmless to our surroundings and Ni-P coating on Mg alloy AZ91D produced by present process possesses good adhesion and corrosion resistance.
基金the National Key Technology R&D Program for the 11th Five-Year Plan(2006BAE04B01 ,2006BAE04B04) the National Basic Research Program ("973") of China (2007CB613705)
文摘The preparation techniques of Mg-mischmetal intermediate alloy and the effects of the mischmetal addition ranging from 0.45% to 1.04% on the microstructure and properties of AZ91D alloy prepared by die casting were investigated. The Mg-MM intermediate alloy was prepared by permanent mold casting and then was extruded into the bars. The microstructure and analytical studies were carried out using optical microscopy and differential scanning calorimetry (DSC). Testing results shows the Mg-MM intermediate alloy could melt easily down at die casting temperature of 680 ℃ that was lower than the melting point of lanthanum (918 ℃) and that of cerium (798 ℃). This was propitious to protection the alloy from the oxidation at high temperatures. Then magnesium alloy test bars were produced under conventional cold chamber die casting condition with addition of different weight of the Mg-MM intermediate alloy. Observation and analysis indicated that the microstructures of the alloy were refined and RE containing Al phase was formed with increasing RE addition. The data obtained by tensile tests showed that alloying with mischmetal improved the tensile property of the AZ91D magnesium die casting alloy at ambient temperature.
基金funded by China Postdoctoral Science Foundation(No.2021M700569)Chongqing Postdoctoral Science Foundation(No.7 cstc2021jcyj-bshX0087)。
文摘This work proposed a strategy of indirectly inducing uniform microarc discharge by controlling the content and distribution ofβ-Mg_(17)Al_(12)phase in AZ91D Mg alloy.Two kinds of nano-particles(ZrO_(2)and TiO_(2))were designed to be added into the substrate of Mg alloy by friction stir processing(FSP).Then,Mg alloy sample designed with different precipitated morphology ofβ-Mg_(17)Al_(12)phase was treated by microarc oxidation(MAO)in Na_(3)PO_(4)/Na2SiO3electrolyte.The characteristics and performance of the MAO coating was analyzed using scanning electron microscopy(SEM),energy dispersive spectrometer(EDS),X-ray diffraction(XRD),X-ray photoelectron spectroscopy(XPS),contact angle meter,and potentiodynamic polarization.It was found that the coarseα-Mg grains in extruded AZ91D Mg alloy were refined by FSP,and theβ-Mg_(17)Al_(12)phase with reticular structure was broken and dispersed.The nano-ZrO_(2)particles were pinned at the grain boundary by FSP,which refined theα-Mg grain and promoted the precipitation ofβ-Mg_(17)Al_(12)phase in grains.It effectively inhibited the“cascade”phenomenon of microarcs,which induced the uniform distribution of discharge pores.The MAO coating on Zr-FSP sample had good wettability and corrosion resistance.However,TiO_(2)particles were hardly detected in the coating on TiFSP sample.
基金supported by the National Natural Science Foundation of China[Grant Nos.52275324 and 51975148]the Fundamental Research Funds for the Central Universities[Grant No.FRFCU5710051321].
文摘Wire arc additive manufacturing offers advantages in producing large metal structures.The current research on GTA-based wire arc additive manufacturing(GTA-WAAM)of magnesium alloys is focused on deformed magnesium alloys,mainly on the Mg-Al alloy system.However,there is little research on GTA-WAAM for casting magnesium alloy.This study investigates the microstructural characteristics and mechanical properties of AZ91D magnesium alloy(AZ91D-Mg)deposited by GTA-WAAM.Single-pass multilayer thin-walled components were successfully fabricated.The results show that equiaxed grains dominate the microstructure of the deposited samples.During the remelting process,the precipitated phases dissolve into the matrix,and they precipitate and grow from the matrix under the thermal effect of the subsequent thermal cycle.The mechanical properties in the vertical and horizontal directions are similar,showing higher overall mechanical properties than the casting parts.The average yield strength is 110.5 MPa,the ultimate tensile strength is 243.6 MPa,and the elongation is 11.7%.The overall hardness distribution in the deposited sample is relatively uniform,and the average microhardness is 59.6 HV_(0.2).
基金supported by the NSFC(grant nos.52171039&52130109)the National Key Research and Development Program of China(grant no.2020YFB2008400).
文摘Hot tearing is a serious destructive solidification defect of magnesium alloys and other casting metals.Quantitative and controllable measurements on the thermal and the mechanical behavior of an alloy during its solidification process are crucial for the understanding of hot tearing formation.We developed a new experimental method and setup to characterize hot tearing behavior via controlled cooling and active loading to force hot hearing formation on cooling at selected fractions of solid.The experimental setup was fully instrumented so that stress,strain,strain rate,and temperature can be measured in-situ while hot tearing was developing.An AZ91D magnesium alloy,which is prone to hot tearing,was used in this study.Results indicate that when hot hearing occurred,the local temperature,critical stress,and cumulative strain were directly affected by strain rate.Depending on the applied strain rate,hot tearing of the AZ91D magnesium alloy could occur in two solidification stages:one in the dendrite solidification stage(fS∼0.81-0.82)and the other in the eutectic solidification stage(fS∼0.99).AZ91D alloy exhibited distinct mechanical behaviors in these two ranges of fraction solid.
基金Project (2007CB613700) supported by the National Basic Research Program of ChinaProject supported by Research Program of Excellent Scholars Studying Abroad of Ministry of Human Resources and Social Security,China
文摘Ceramic oxide coatings were prepared on AZ91D magnesium alloys in alkaline silicate solution using micro-arc oxidation（MAO） technique.The corrosion behavior of MAO coating on AZ91D magnesium alloys in NaCl solutions with different concentrations（0.1%,0.5%,1.0%,3.5% and 5.0% in mass fraction） was evaluated by electrochemical measurements and immersion tests.The results showed that the corrosion rate of the MAO coated AZ91D increased with increasing chloride ion concentration.The main form of corrosion failure was localized corrosion for the MAO coated AZ91D immersed in higher concentration NaCl solutions（1.0%,3.5% and 5.0%）,while it was general corrosion in dilute NaCl solutions（0.1% and 0.5%）.Two different stages of the failure process of the MAO coated AZ91D could be identified：1） occurrence of the metastable pits and 2） growth of the pits.Different equivalent circuits were also proposed based on the results of electrochemical impedance spectroscopy（EIS） for the MAO coated AZ91D immersed in different concentrations of NaCl solutions for 120 h.
基金Project(11005151)supported by the Young Scientists Fund of the National Natural Science Foundation of ChinaProject(YETP1297)supported by the Beijing Higher Education Young Elite Teacher Project,ChinaProject(BEIJ2014110003)supported by the Undergraduate Research and Innovative Undertaking Program of Beijing,China
文摘The microarc oxidation（MAO） coatings produced at different current frequencies on AZ91 D magnesium alloys were studied systematically. The morphologies, thickness, corrosion performances, and tribological properties of the coatings were investigated by the scanning electron microscopy, the electrochemical measurement system, and MS-T3000 friction test rig, respectively. The results show that the structure of the coatings becomes denser, and thickness becomes thinner with the increase of the current frequency. It is also found that the corrosion resistance of the coatings produced at higher frequency is improved greatly and the difference of the corrosion current density becomes small with increasing current frequency, which is similar to that of the coating thickness. The tribological test shows that the friction coefficient decreases with increasing the current frequency and the wear resistance of the coatings is influenced by both the thickness and structures. All these results were explained by analyzing the growing process of the MAO coating.
基金Project (2009AA033501) supported by the High-tech Research and Development Program of China
文摘Pouring and mold temperatures are two important parameters during casting magnesium components. The present study examined their influence on hot tearing susceptibility （HTS） of commercial AZ91D and newly developed Mg-3Nd-0.2Zn-Zr （mass fraction, %; NZ30K） magnesium alloys in gravity permanent mold casting condition. The results indicate that mold temperature shows much more significant influence on the HTS of both alloys than pouring temperature whose influence only can be distinguished at low mold temperature （341 K for AZ91D alloy and 423 K for NZ30K alloy）. Hot tearing susceptibility prediction model concerning feeding parameters, grain size and solidification range, is more suitable to estimate the HTS of different magnesium alloys than the model only concerning feeding parameters. In order to achieve better hot tearing resistance, the ranges of pouring and mold temperatures are suggested to be 961-991 K and≥641 K for AZ91D alloy, 1003-1033 K and≥623 K for NZ30K alloy, respectively.
基金Project (51075098) supported by the National Natural Science Foundation of ChinaProject (HIT.NSRIF.2014006) supported by the Fundamental Research Funds for the Central Universities,China
文摘The flow behavior and dynamic recrystallization（DRX） behavior of an as-cast AZ91 D alloy were investigated systematically by applying the isothermal compression tests in temperature range of 220-380 ℃ and strain rate range of 0.001-1 s^-1.The effect of temperature and strain rate on the DRX behavior was discussed.The results indicate that the nucleation and growth of dynamic recrystallized grains easily occur at higher temperatures and lower strain rates.To evaluate the evolution of dynamic recrystallization,the DRX kinetics model was proposed based on the experimental data of true stress-true strain curves.It was revealed that the volume fraction of dynamic recrystallized grains increased with increasing strain in terms of S-curves.A good agreement between the proposed DRX kinetics model and microstructure observation results validates the accuracy of DRX kinetics model for AZ91 D alloy.
基金Project (2011A090200104) supported by Guangdong Science and Technology Special Projects, China
文摘Microstructural evolution of AZ91 magnesium alloy was investigated during homogenizing annealing treatment, hot extrusion and ageing treatment, respectively. The results exhibited that both the divorced eutectic β-Mg17Al12 and the precipitated β-Mg17Al12 phases appeared in the as-cast alloy. The β-Mg17Al12 phase dissolved into α-Mg matrix mostly and the structure kept fine after the optimized homogenization treatment at 380 °C for 15 h. Dynamic recrystallization and consequent grain refinement occurred during extrusion. The banded β-Mg17Al12 precipitates paralleled to the extrusion direction were observed after ageing treatment. The banded precipitation should be attributed to the solidification segregation which was elongated during the subsequent extrusion. Furthermore, the effects of temperature, holding time of homogenization and ageing treatment, and extrusion processing parameters on the microstructural evolution of AZ91 alloy were also discussed in details according to the experimental results.
基金Project(51071128)supported by the National Natural Science Foundation of ChinaProject(2011CB610401)supported by the National Basic Research Program of ChinaProject(SKLSP201002)supported by Free Research Fund of State Key Laboratory of Solidification Processing,China
文摘Effects of ultrasonic treatment on microstructures were investigated by introducing the ultrasonic oscillation into AZ91 alloy melts. The results show that the ultrasonic treatment does not change the phase composition but has great influence on solidification microstructures. The area percentage of lamellar eutectic phase increases to the maximum when the applied ultrasonic power is increased to 600 W and then decreases gradually with the further increase of power;meanwhile, the average area of Al8Mn5 phase owns a completely opposing variation trend. The area percentage of Mg17Al12 decreases gradually with increasing the applied ultrasonic power. Mechanisms accounted for the refinement and fraction of different intermetallic phases were also discussed.
基金Project (50801056) supported by the National Natural Science Foundation of ChinaProject (2005DKA10400-Z20) supported by the National R&D Infrastructure and Facility Development Program of ChinaProject supported by the Zijin Project of Zhejiang University, China
文摘Micro-arc oxidation （MAO） process was carried out on AZ91D alloy in alkaline borate solution using an alternative square-wave power source with different parameters. The effects of voltage, frequency and duty cycle on the coatings were investigated by orthogonal experiment. It is found that the thickness of coatings increases with the increase of voltage and duty cycle, but decreases with the increase of frequency. The structure and morphology of the coatings also depend on voltage, frequency and duty cycle. The coatings become more porous and crack with increasing voltage and duty cycle. The coating is thin and transparent when the voltage is lower than 120 V. The corrosion resistances of different coatings were evaluated by polarization curves and electrochemical impedance spectroscopy （EIS） in 3.5% NaCl （mass fraction） solution. When the optimized values of voltage, frequency and duty cycle are 140 V, 2 000 Hz and 0.4, respectively, the anodic coating shows the best corrosion resistance.
基金Project(51275295)supported by the National Natural Science Foundation of ChinaProjects(20120073120011,20130073110052)supported by the Research Fund for the Doctoral Program of Higher Education of China
文摘The semi-solid slurry of AZ91 D magnesium alloy was prepared by gas bubbling process.The effect of processing parameters,including gas flow rate,cooling rate and stirring end temperature,on microstructure of AZ91 D semi-solid slurry was investigated.With increasing the gas flow rate from 0 to 5 L/min,the average size of primary α-Mg particles decreases from 119.1 to77.2μm and the average shape factor increases continuously from 0.1 to 0.596.The formation of non-dendritic primary α-Mg particles during gas bubbling is the result of combined effects of dendrite fragmentation and copious nucleation.With increasing the cooling rate from 3.6 to 14.6℃/min,the average particle size of primary α-Mg phase decreases from 105.0 to 68.1μm while the average shape factor peaks at 9.1℃/min.Both high and low cooling rates can induce dendritic growth of primary α-Mg particles.Changing the stirring end temperature from 590 to 595℃ has little effect on the average size and shape factor of primary α-Mg particles in AZ91 D semi-solid slurry.The insensitivity of semi-solid microstructures to the stirring end temperature is attributed to the sufficient quantity of primary particles formed in the melt.
文摘The microstructural evolution of AZ91D magnesium alloy prepared by means of the cyclic upsetting-extrusion and partial remelting was investigated. The effects of remelting temperature and holding time on microstructure of semi-solid AZ91D magnesium alloy were studied. Furthermore, tensile properties of thixoextruded AZ91D magnesium alloy components were determined. The results show that the cyclic upsetting-extrusion followed by partial remelting is effective in producing semi-solid AZ91D magnesium alloy for thixofonning. During the partial remelting, with the increase of remelting temperature and holding time, the solid grain size increases and the degree of spheroidization tends to be improved. The tensile mechanical properties of thixoextruded AZ91D magnesium alloy components produced by cyclic upsetting-extrusion and partial remelting are better than those of the same alloy produced by casting.
基金Project(50774075) supported by the National Natural Science Foundation of ChinaProject(2007CB613705) supported by the National Basic Research Program of China
文摘To develop AZ91D alloys with fine microstructure, effects of the addition of rare earth (RE), Sr and RE + Sr on the dendrite growth and phase precipitation in AZ91D magnesium alloy were studied, respectively. The results show that the microstructure is refined and the morphology of β-Mg17A112 phase is modified with RE or Sr addition, especially with the RE+Sr composite addition which can reduce the average grain size of AZ91D alloy obviously to 141 μm. The needle-like or block-like new phases adhering to β-Mg17A112 phase form at interdendrites during solidification. The enrichment of RE or/and Sr elements in front of the solidification interface, especially at the tips of α-Mg dendrite, which restricts the growth of α-Mg dendrite, changes the preferential growth of α-Mg and finally results in the grain refinement and the blunting of α-Mg dendrite.
基金Projects(50674038,50974048)supported by the National Natural Science Foundation of ChinaProject(200802140004)supported by Doctoral Fund of Ministry of Education of China
文摘Low temperature mechanical properties of AZ91D magnesium alloy fabricated by solid recycling process from recycled scraps were studied. Various microstructural analyses were performed using optical microscopy （OM） and scanning electron microscopy （SEM）. The recycled specimens consist of fine grains due to dynamic recrystallization and the interfaces of original individual scraps are not identified. Tensile tests were performed at a strain rate of 5 x 10 3 s 1 at room temperature （27 ~C）, -70, -100 and 130 ~C, respectively. Ultimate tensile strength of the specimens increases slightly with decreasing the tensile temperature, and elongation to failure decreases with decreasing the tensile temperature. The tensile specimens at -130 ~C show the highest ultimate tensile strength of 360.65 MPa and the lowest elongation to failure of 5.46%. Impact tests were performed at room temperature （27 ~C）, -70 and -130 ~C, respectively. Impact toughness decreases with decreasing the impact temperature. The impact specimens at -130 ~C show the lowest impact toughness of 3.06 J/cm2.
