Application of ultrasonic vibration to shape-casting based on resonance vibration analysis

The application of ultrasonic vibration during the casting process has been proven to refine the microstructure and enhance the properties of the casting.By using the direct inserting method,wherein the ultrasonic horn is inserted directly into the melt,ultrasonic treatment can be utilized in the semi-continuous casting process to produce aluminum ingots with simple shapes.However,due to the attenuation of ultrasound,it is challenging to apply the direct inserting method in the die casting process to produce complex castings.Thus,in this study,the impact of ultrasonic vibration on the microstructure of a gravity die-cast AlSi9Cu3end cap was investigated by applying ultrasonic vibration on the core(indirect method).It is found that the effect of ultrasonic vibration relies greatly on the resonance mode of the core.Selection of ultrasonic vibration schemes mainly depends on the core structure,and only a strong vibration can significantly refine the microstructure of the casting.For castings with complex structures,an elaborated ultrasonic vibration design is necessary to refine the microstructure of the specified casting.In addition,strong vibration applied on the feeding channel can promote the feeding ability of casting by breaking the dendrites during solidification,and consequently reduce the shrinkage porosity.

Influence of non-uniform ultrasonic vibration on casting fluidity of liquid aluminum alloy

The application of ultrasonic vibration to the casting process can be realized through mould(die)vibration.However,the resonant vibration of the mould is always accompanied by a non-uniform vibration distribution at different parts,which may induce a complex liquid flow and affect the casting fluidity during the mould filling process.The influence of non-uniform ultrasonic vibration on the fluidity of liquid AlSi9Cu3 alloy was studied by mould vibration with different vibration gradients.It is found that ultrasonic mould vibration can generate two opposite effects on the casting fluidity:the first,ultrasonic cavitation in melt induced by mould vibration promotes the casting fluidity;the second,the non-uniform mould vibration can induce a melt flow toward the weak vibration areas and turbulence there,consequently decreasing the casting fluidity.When the melt flow and turbulence are violent enough to offset the promoting effect of cavitation on fluidity,the ultrasonic vibration will finally induce a resultant decrease of casting fluidity.The decreasing effect is proportional to the vibration gradient.

Computer simulation of super-magnetoelastic behavior near critical region of magnetic materials based on phase-field method

Since the discovery of ferromagnetic morphotropic phase boundary(MPB)in 2010,the connotation and extension of MPB have been becoming more and more abundant.Over the last dozen years,much experimental work has been done to design magnetostrictive materials based on the MPB principle.However,due to the difficulty in direct experimental observations and the complexity of theoretical treatments,the insight into the microstructure property relationships and underlying mechanisms near the ferromagnetic MPB has not been fully revealed.Here,we have reviewed our recent computer simulation work about the super-magnetoelastic behavior near the critical region of several typical materials.Phase-field modeling and simulation are employed to explore the domain configuration and engineering in single crystals as well as the grain size effect in polycrystals.Besides,a general nano-embryonic mechanism for superelasticity is also introduced.Finally,some future perspectives and challenges are presented to stimulate a deeper consideration of the research paradigm between multiscale modeling and material development.

Nanoembryonic thermoelastic equilibrium and enhanced properties of defected pretransitional materials

Behaviors of displacive phase-transforming materials above the temperature of transformation,where abnormal thermal,elastic,magnetic properties are often observed,are mostly explained by intrinsic peculiarities in electronic/atomic structure.Here,we show these properties may also be attributed to extrinsic effects caused by a thermoelastic equilibrium in highly defected pretransitional materials.We demonstrate that the stress concentration near stress-generating defects such as dislocations and coherent precipitates could result in the stress-induced transformation within nanoscale regions,producing equilibrium embryos of the product phase.These nanoembryos in thermoelastic equilibrium could anhysteretically change their equilibrium size in response to changes in applied stress or magnetic fields leading to superelasticity or supermagnetostriction.Similar response to cooling may explain the observed diffuse phase transformation,changes in the coefficient of thermal expansion and effective elastic modulus,which,in turn,may explain the invar and elinvar behaviors.

Recent progress of vanadium-based alloys for fusion application

Low-activation vanadium alloys,with the reference composition of V-4Cr-4Ti have been considered as one of the most promising candidate materials for structural components such as the blanket in future fusion reactors,thanks to their excellent neutron irradiation resistance,superior high-temperature mechanical properties,and high compatibility with liquid lithium blankets.The self-cooled liquid lithium blanket using structural materials of vanadium alloys is an attractive concept because of the high heat transfer and high tritium breeding capability.After more than 2 decades of research,technological progress has been made in reducing the number of critical issues for application of vanadium alloys to fusion reactors.In this paper,the recent research and development activities of vanadium alloys are summarized,including significant progress achieved on fabrication technology and composition optimization,coating and corrosion,improved understanding of irradiation effects upon microstructure and material properties,retention of hydrogen isotopes,as well as advancements in joining and weld-ing.In particular,the fact that recent products from China,Japan,US and France showed similar properties which meant the fabrication technology has been almost standardized.