A Study On Micro-forming Ability Of Zr-based Bulk Metallic Glass Under Low Frequency Vibrating Field

Bulk metallic glasses(BMGs) exhibit great application potential in modern industry because of their attractive mechanical properties such as high strength, high elasticity and so on. BMGs have good superplastic formability when reheated into the supercooled liquid region(SCLR). It is noted that most of the present studies are focusing on choosing optimum processing parameters such as temperature and processing time to get the highest formability of BMGs in their SCLR. However, these two processing parameters are interactional. At higher temperature, the processing time is shortened because of a risk of possible crystallization may occur in such forming conditions, which distinctly degrades the mechanical performance of the materials. In this study, a vibration loading was introduced into the micro-forming of Zr55Cu30Al10Ni5 BMG and lateral extrusion experiment under vibration field was established to investigate the micro-forming ability of BMG in the SCLR.Zr55Cu30Al10Ni5 BMG was prepared by copper mould casting in vacuum. Differential scanning calorimeter(DSC) study of the as-received BMG revealed that the temperature of the SCLR ranges from 420 to 490?. The H13 dies of lateral extrusion were manufactured, which designed according to self-positioning and overall mold handling principles.The micro-extrusion deformation of Zr55Cu30Al10Ni5 under low frequency vibrating field was simulated by DEFORM-3D software. The average force and processing time were determined according to the simulation results. The experiment of micro-lateral extrusion was carried out on a Zwick Z020 under different amplitude(38~760N) and different loading frequency(0.1~2.0Hz) at a fixed temperature of 450? and the processing time was set to 5min. The extrude lengths, which can be applied to characterize the micro-forming ability of BMG in the SCLR under vibration loading, were measured after vibration forming. Experimental results reveal that vibration can effectively improve the micro-forming ability of BMG in the SCLR. The extrude lengths significantly increase with increasing frequencies, but as the amplitudes increase, the extrude lengths increase and then come to a flat, which is consistent with the results of finite element analysis.According to the experimental results and rheological theory, the influence of vibrating parameters on flow viscosity of Zr55 BMG in the SCLR was discussed. At a certain loading amplitude, the flow viscosity declines with increasing frequency, and the viscosity decreases more quickly at lower amplitude than that at higher amplitude. The viscosity declines up to 29.7% with increasing amplitude, but it is not obvious as that with increasing frequency.Finally, the physical mechanism of the experimental phenomenon is studied and rationalized in terms of the material loss modulus, free volume theory and surface effect. The loss modulus caused by vibration stress on viscoelastic material like BMG decreases with the increasing frequency, thus, the dynamic viscosity declines with that. Vibration as energy can improve the concentration of free volume on the BMG in the SCLR, which increases the flowing rate and promotes the micro-forming ability. On the other hand, the friction of the interface between dies and specimens is reduced by vibration, which is also an important factor for improving the micro-forming efficiency.