Study On Method And Mechanism Of Ultrasonic Vibration Aided Rheological Forming Of Amorphous Alloy

Amorphous alloy is an ideal material for micro-forming,superplastic rheological deformation in supercooled liquid region is the main means to realize the formation of amorphous alloy.In order to improve the superplastic rheological forming ability of the amorphous alloy,two process parameters,deformation temperature and time are optimized.However,these two factors are mutually constrained,a higher forming temperature can improve the rheological forming ability,but it can also shorten the crystallization time and increase the risk of crystallization,so it is necessary to find out new ways to improve the rheological forming ability of amorphous alloys.Vibration aided plastic forming?VPF?have has been studied at home and abroad for a long time,vibration aided metal plastic forming technology can reduce deformation resistance and improve surface quality,vibration aided polymer rheological forming can reduce apparent viscosity of the melt and increase filling capacity.In this paper,the ultrasonic vibration is introduced into the rheological forming of Zr₅₅Cu₃₀Al₁₀Ni₅ bulk amorphous alloy,the method and mechanism of ultrasonic vibration aided rheological forming of amorphous alloy were studied.In this paper,an ultrasonic aided thermoplastic forming experiment is designed,the ultrasonic vibration device was developed by the combination of analytic method and finite element simulation.The main work includes:analyzing the demand of the device,selecting the appropriate ultrasonic generator,designing the sandwich piezoelectric ceramic transducer,calculating the material,quantity and size of the piezoelectric ceramic sheet,the front and rear cover plates,designing the ultrasonic horn,and the modal of the horn is analyzed by ANSYS,the magnification factor of the horn is calculated.The bearing is designed to connect the vibration device and the testing machine,and the actual output frequency and amplitude of the vibration device are tested.The Zr55 bulk amorphous alloy billet was prepared by copper mould casting.Compression deformation experiments at different frequencies?19kHz,20kHz?and vibration power?45%to 90%?were performed by using the designed ultrasonic vibration apparatus at a compression rate of 10?m/s and a temperature of 452?.The rheological forming of amorphous alloy is characterized by the deformation under the same limit load,the results show that vibration can improve the rheological forming ability of amorphous alloy,the diameter of deformed specimen increases with the increasing of the vibration power,and when the vibration power is 1800W,the deformed diameter increases by 11%,and it does not change with frequency.The rheological behavior of bulk amorphous alloys in supercooled liquid region under the vibration field is analyzed,it is found that the flow stress decreases with the increase of the vibration power,which can be reduced by up to30%.The apparent viscosity of the supercooled liquid is calculated,it is found that the apparent viscosity also decreases with the increase of vibration power,and the empirical equation of viscosity change with power is fitted,the flow stress and viscosity are not affected by frequency.It is found that vibration caused surface damage of specimen,the mechanism of load reduction by vibration is that the billet separated from the platen cyclically.The amorphous structure of the deformed specimen was confirmed by X-ray diffraction?XRD?analysis,and the mechanical properties of amorphous alloy after deformation were analyzed by Vickers hardness measurement and nanoindentation,it is found that the hardness and elastic modulus of the deformed specimen are increased compared with the as-cast specimen,while the hardness and elastic modulus of the vibration aided deformed specimen are between the constant load specimen and the as-cast specimen which indicates that the ultrasonic vibration causes the softening of the deformed specimen.According to the relationship between hardness and free volume,it is considered that the reason for the softening of vibration aided deformed specimen is that the compression under high strain rate introduces too much free volume,The results show that the vibration caused the change of the structure and properties of amorphous alloys.Finally,it is proposed to introduce the ultrasonic vibration into the micro-extrusion process of the amorphous alloy,and design the micro-extrusion molding die to explore the ultrasonic vibration aided micro-reverse extrusion process.