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
A magnesium alloy AZ31 sheet was processed by ultrasonic shot peening treatment to fabricate a surface nanocrystalline,and a ball-on-disk dry sliding wear test was performed to evaluate the tribological behavior after...A magnesium alloy AZ31 sheet was processed by ultrasonic shot peening treatment to fabricate a surface nanocrystalline,and a ball-on-disk dry sliding wear test was performed to evaluate the tribological behavior after treatment.The microstructure observation indicated a gradient nanocrystalline structure was formed after USSP treatment.The microhardness at the top surface was improved from 60 HV to 145 HV after treatment.The formed nanocrystalline resulted in an easy formation of MgO patches on the surface and reduced the coefficient of friction.Moreover,the formed nanocrystalline leaded to a retard of delamination with increasing the sliding speed and applied load,which was due to its stronger sub-surface.Under high sliding speed(0.5 m/s)and high applied load(50 N),it was firstly found that the formed nanocrystalline prevented the happening of thermal softening and melting.The possible reasons accounting for the prevention of thennal softening and melting were discussed accordingly.展开更多
The modification of eutectic silicon is of general interest since fine eutectic silicon along with fine primary aluminum grains improves mechanical properties and ductilities. In this study, high intensity ultrasonic ...The modification of eutectic silicon is of general interest since fine eutectic silicon along with fine primary aluminum grains improves mechanical properties and ductilities. In this study, high intensity ultrasonic vibration was used to modify the complex microstructure of aluminum hypoeutectic alloys. The ultrasonic vibrator was placed at the bottom of a copper mold with molten aluminum. Hypoeutectic AI-Si alloy specimens with a unique in-depth profile of microstructure distribution were obtained. Polyhedral silicon particles, which should form in a hypereutectic alloy, were obtained in a hypoeutectic AI-Si alloy near the ultrasonic radiator where the silicon concentration was higher than the eutectic composition. The formation of hypereutectic silicon near the radiator surface indicates that high-intensity ultrasonic vibration can be used to influence the phase transformation process of metals and alloys. The size and morphology of both the silicon phase and the aluminum phase varies with increasing distance from the ultrasonic probe/radiator. Silicon morphology develops into three zones. Polyhedral primary silicon particles present in zone I, within 15 mm from the ultrasonic probe/radiator. Transition from hypereutectic silicon to eutectic silicon occurs in zone II about 15 to 20 pm from the ultrasonic probe/radiator. The bulk of the ingot is in zone III and is hypoeutectic AI-Si alloy containing fine lamellar and fibrous eutectic silicon. The grain size is about 15 to 25 IJm in zone I, 25 to 35 IJm in zone II, and 25 to 55 pm in zone II1. The morphology of the primary a-AI phase is also changed from dendritic （in untreated samples） to globular. Phase evolution during the solidification process of the alloy subjected to ultrasonic vibration is described.展开更多
The effect of solutes on grain formation has been studied over a century but is still under debating,simply because it is a very complex topic.This article focuses on the effect of dissolved solute on the growth,fragm...The effect of solutes on grain formation has been studied over a century but is still under debating,simply because it is a very complex topic.This article focuses on the effect of dissolved solute on the growth,fragmentation of a dendrite and the resultant grain size.Experimental data on grain size in magnesium and aluminum alloys with various solute concentrations are collected and analyzed using phase diagram variables including Q,P,andΔT.The physical meaning of each phase diagram variable is discussed.Curve fitting of the recently proposed two-parameter models with experimental data suggests that there is a clear correlation between the measured grain size and the solidification range of the alloy over the entire range of the hypoeutectic composition.Such a trend of grain size vs.ΔT is closely related to the grain refining mechanisms operating under relevant experimental conditions.A critical review of the grain refining mechanisms indicates that the mechanisms associated with the dendrite fragmentation are operating under the conditions where convection is substantial in the melt during its solidification.展开更多
A surface nanocrystalline was fabricated by ultrasonic shot peening(USSP)treatment at AZ31 Mg alloy.The effect of nanocrystalline thickness and its placed side(external or internal)on the bendability was studied by a ...A surface nanocrystalline was fabricated by ultrasonic shot peening(USSP)treatment at AZ31 Mg alloy.The effect of nanocrystalline thickness and its placed side(external or internal)on the bendability was studied by a V-bending test.Three durations,5,10,and 15 min,were applied to form the surface nanocrystalline with thicknesses of 51,79,and 145μm,respectively.Two-side treatment led to a similar bendability as that of as-received.One-side internal treatment for 5 min resulted in an improved bendability while the improvement was limited and degenerated for longer treatment.The improvement was related to the drawing back of the neutral axis.The one-side external treatment also improved the bendability,and the improvement was due to the redistribution of strain and stress during bending.With nanocrystalline at external side,it resulted in a larger stress but a smaller strain at the convex,which prevented the happening of crack during bending.展开更多
Walnuts are one of nature’s more waste-heavy products. 67% of the nut is shell and husk, low value by-products that are rich in phenolic compounds. The phenolic compounds extracted from walnut shells are potentially ...Walnuts are one of nature’s more waste-heavy products. 67% of the nut is shell and husk, low value by-products that are rich in phenolic compounds. The phenolic compounds extracted from walnut shells are potentially good natural sources of antioxidants for the food and pharmaceutical industries. In this study, phenolic compounds were extracted using an ultrasonic bath, an ultrasonic probe and a standard shaking method. The extraction yield achieved with an ultrasonic probe was 51.2 mg GAE/g DW, two times higher than both the shaking method and the ultrasonic bath method which were 20.6 mg GAE/g DW and 25.8 mg GAE/g DW, respectively. Phenolic extraction was further improved by a size reduction of the walnut shells. The best extraction yield of 52.8 mg GAE/g DW was attained when the particle size was between 45 - 100 mesh. The ultrasonic probe treatment is the best method for extraction of phenolic compounds from walnut shells. Scanning electron microscopy (SEM) imaging indicated that the ultrasonic probe treatment could better rupture the hard structure of the cells, increasing the penetration of solvents and thus the extraction yield.展开更多
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.
文摘A magnesium alloy AZ31 sheet was processed by ultrasonic shot peening treatment to fabricate a surface nanocrystalline,and a ball-on-disk dry sliding wear test was performed to evaluate the tribological behavior after treatment.The microstructure observation indicated a gradient nanocrystalline structure was formed after USSP treatment.The microhardness at the top surface was improved from 60 HV to 145 HV after treatment.The formed nanocrystalline resulted in an easy formation of MgO patches on the surface and reduced the coefficient of friction.Moreover,the formed nanocrystalline leaded to a retard of delamination with increasing the sliding speed and applied load,which was due to its stronger sub-surface.Under high sliding speed(0.5 m/s)and high applied load(50 N),it was firstly found that the formed nanocrystalline prevented the happening of thermal softening and melting.The possible reasons accounting for the prevention of thennal softening and melting were discussed accordingly.
基金supported by the US Department of Energy,Office of Energy Efficiency and Renewable Energy,Industrial Technologies Program,Industrial Materials for the Future(IMF),under Contractor No.DE-PS07-02ID14270 with UT-Battelle,LLC
文摘The modification of eutectic silicon is of general interest since fine eutectic silicon along with fine primary aluminum grains improves mechanical properties and ductilities. In this study, high intensity ultrasonic vibration was used to modify the complex microstructure of aluminum hypoeutectic alloys. The ultrasonic vibrator was placed at the bottom of a copper mold with molten aluminum. Hypoeutectic AI-Si alloy specimens with a unique in-depth profile of microstructure distribution were obtained. Polyhedral silicon particles, which should form in a hypereutectic alloy, were obtained in a hypoeutectic AI-Si alloy near the ultrasonic radiator where the silicon concentration was higher than the eutectic composition. The formation of hypereutectic silicon near the radiator surface indicates that high-intensity ultrasonic vibration can be used to influence the phase transformation process of metals and alloys. The size and morphology of both the silicon phase and the aluminum phase varies with increasing distance from the ultrasonic probe/radiator. Silicon morphology develops into three zones. Polyhedral primary silicon particles present in zone I, within 15 mm from the ultrasonic probe/radiator. Transition from hypereutectic silicon to eutectic silicon occurs in zone II about 15 to 20 pm from the ultrasonic probe/radiator. The bulk of the ingot is in zone III and is hypoeutectic AI-Si alloy containing fine lamellar and fibrous eutectic silicon. The grain size is about 15 to 25 IJm in zone I, 25 to 35 IJm in zone II, and 25 to 55 pm in zone II1. The morphology of the primary a-AI phase is also changed from dendritic （in untreated samples） to globular. Phase evolution during the solidification process of the alloy subjected to ultrasonic vibration is described.
文摘The effect of solutes on grain formation has been studied over a century but is still under debating,simply because it is a very complex topic.This article focuses on the effect of dissolved solute on the growth,fragmentation of a dendrite and the resultant grain size.Experimental data on grain size in magnesium and aluminum alloys with various solute concentrations are collected and analyzed using phase diagram variables including Q,P,andΔT.The physical meaning of each phase diagram variable is discussed.Curve fitting of the recently proposed two-parameter models with experimental data suggests that there is a clear correlation between the measured grain size and the solidification range of the alloy over the entire range of the hypoeutectic composition.Such a trend of grain size vs.ΔT is closely related to the grain refining mechanisms operating under relevant experimental conditions.A critical review of the grain refining mechanisms indicates that the mechanisms associated with the dendrite fragmentation are operating under the conditions where convection is substantial in the melt during its solidification.
基金financially supported by the Natural Science Basic Research Program of Shaanxi,China(No.2021JQ-250)the Fundamental Research Funds for the Central Universities(No.300102220301)。
文摘A surface nanocrystalline was fabricated by ultrasonic shot peening(USSP)treatment at AZ31 Mg alloy.The effect of nanocrystalline thickness and its placed side(external or internal)on the bendability was studied by a V-bending test.Three durations,5,10,and 15 min,were applied to form the surface nanocrystalline with thicknesses of 51,79,and 145μm,respectively.Two-side treatment led to a similar bendability as that of as-received.One-side internal treatment for 5 min resulted in an improved bendability while the improvement was limited and degenerated for longer treatment.The improvement was related to the drawing back of the neutral axis.The one-side external treatment also improved the bendability,and the improvement was due to the redistribution of strain and stress during bending.With nanocrystalline at external side,it resulted in a larger stress but a smaller strain at the convex,which prevented the happening of crack during bending.
文摘Walnuts are one of nature’s more waste-heavy products. 67% of the nut is shell and husk, low value by-products that are rich in phenolic compounds. The phenolic compounds extracted from walnut shells are potentially good natural sources of antioxidants for the food and pharmaceutical industries. In this study, phenolic compounds were extracted using an ultrasonic bath, an ultrasonic probe and a standard shaking method. The extraction yield achieved with an ultrasonic probe was 51.2 mg GAE/g DW, two times higher than both the shaking method and the ultrasonic bath method which were 20.6 mg GAE/g DW and 25.8 mg GAE/g DW, respectively. Phenolic extraction was further improved by a size reduction of the walnut shells. The best extraction yield of 52.8 mg GAE/g DW was attained when the particle size was between 45 - 100 mesh. The ultrasonic probe treatment is the best method for extraction of phenolic compounds from walnut shells. Scanning electron microscopy (SEM) imaging indicated that the ultrasonic probe treatment could better rupture the hard structure of the cells, increasing the penetration of solvents and thus the extraction yield.
