| Improving the strength and room temperature toughness of magnesium alloys is the key to broaden the applications of magnesium alloys.According to the Hall-petch formula,the grain size of magnesium alloys has a great influence on the room temperature properties of magnesium alloys,and the reduction of the grain size of magnesium alloys to improve their mechanical properties is a hot topic of research.Ultrasonic vibration is of great importance in refining the grain size of magnesium alloys through its unique cavitation and acoustic flow effects.In this paper,direct ultrasonic vibration is used to prepare AZ91 D magnesium alloy semi-solid slurry,and the process of preparing slurry by ultrasonic vibration is systematically studied.The main conclusions obtained are as follows:(1)The ultrasonic vibration power,starting vibration temperature and termination temperature have a large influence on the morphology of the primary phase,where the optimum ultrasonic vibration power is 1500 W.The optimum ultrasonic vibration temperature range is 615°C-590°C when the ultrasonic vibration tool head is at the same temperature as the melt,and 635°C-590°C when the tool head is preheated at 400°C.(2)The mechanism of nucleation promoted by ultrasonic vibrations is the cavitation effect promoting heterogeneous nucleation,which occurs above the liquid phase line,where nuclei above the critical nucleus size are stable and grow as dendrites.(3)Grain growth always tends towards dendritization,but the formation of spherical crystals does not result from dendrite fracture.During grain growth,the redistribution of the solute layer at the solid-liquid front by acoustic flow shows inhibition of the tips of the dendrites,which are fast in heat and mass transfer,while the slower roots and the sides of the dendrites show promotion.When the number of grains is sufficiently large,the overlap of the solute field causes the nucleus growth drive to be insufficient,the grains stop growing and spherical crystals are formed,and the growth drive disappears when the solid-phase ratio exceeds 45% in this paper.(4)If the size of the dendrite is large enough at the time of application of the ultrasonic vibration,the energy exerted by the ultrasonic vibration on the melt can break the dendrite from the lower melting point or the lower strength of the root necking,not due to the high pressure generated by cavitation,but due to the perturbation of the whole dendrite from the root by the action of the acoustic current. |
