A Study On Preparation, Microstructure And Properties Of Semi-solid Slurry Of Al-Si Alloy By Vibration

Semi-solid metal forming is considered as one of the most promising technologies intwenty-first century. Recently, rheoforming, which die-casts or extrudes semi-solid slurrydirectly and characterizes as energy-saving, low costs, has become the hot research topicsin semi-solid metal forming field. Consequently, preparation of semi-solid slurry, themost critical step of rheoforming, has drawn great attention and extensive research. Inthis research, based on the review of the research and development of the semi-solidslurry preparation method around the world, the author systematically discussed thepreparation of Al-Si alloy semi-solid slurry by vibration.The technique of semi-solid metal slurry preparation by low-frequency mechanicalvibration and the relevant equipment were developed, and the process and mechanism ofslurry preparation by mechanical vibration were studied. The effects of processingparameters such as mechanical vibration frequency, vibration time and pouringtemperature on the primary solid phase of ZL101 hypoeutectic Al-Si alloy wereinvestigated. The results show that the ZL101 alloy semi-solid slurry with average graindiameter of 85μm, average shape coefficient of 0.61 could be prepared by low-frequencymechanical vibration; the microstructures of semi-solid rheo-diecasting and liquiddiecasting of ZL101 hypoeutectic Al-Si alloy are very different, as the former exhibitsthe primary non-dendriticα-Al phases and secondary solidifiedα-Al phases. The slurrypreparaton process which can gain the best mechanical properties of rheo-diecasting ofZL101 aluminum alloy is: vibration frequency about 41.7Hz, preparation temperaturearound 605℃, vibration time for 3-5min. Compared to liquid diecasting, the tensilestrength and elongation of rheo-diecasting of ZL101 aluminum alloy were increased by6% and 11%, respectively.The technique of semi-solid metal slurry preparation by high-energy ultrasonicvibration and the relevant equipment were developed, and the process and mechanism ofslurry preparation by high-energy ultrasonic vibration were studied. The role ofprocessing parameters such as ultrasonic input temperature, treatment time and rest-work ratio in semi-solid slurry preparation of ZL101 hypoeutectic Al-Si alloy by high-energyultrasonic vibration were explored. The microstructural evolution of semi-solid ZL101hypoeutectic Al-Si alloy solidified in ultrasound field was investigated. It was found thatthe morphology of primary particles is dominated by the temperature of the ultrasonicvibration applied: the primary grains grow into globular shape when ultrasound isapplied from 640℃; after the ultrasound is applied at 610℃(solid fraction 0.1) for 120 s,the originally formed dendrites are granulated; when the ultrasound is applied at 600℃(solid fraction 0.25), a mixture structure consisted of dendrites and granulated grains areformed. The primaryα-Al crystals could be prepared with average grain diameter ofabout 90μm and average shape coefficient of above 0.5 by 144s ultrasonic vibration.Finer and rounder grains could be fabricated with decrease of the rest-work ratio ofultrasonic vibration under the same other conditions.The impact of parameters such as the pouring temperature, preheating temperature ofsample cup and ultrasonic vibration power on primary Si of A390 hypereutectic Al-Sialloy were systematically studied. After ultrasonic vibration, primary Si of A390hypereutectic Al-Si alloy could be refined to about 20μm. When isothermal holding afterthe slurry treated by ultrasonic vibration, the primary Si grains grow up fast with a speedof 8μm/min while slow with a rate of less than 2.5μm/min under ultrasonic vibration andkeeping the average shape factor of primary Si at 0.5-0.6. Suitable intermittent vibrationhas better grain refinement effect of Si than the continous vibration. P modification haslittle impact on the primary Si under ultrasonic vibration. The optimal process parametersof slurry preparation of A390 alloy by ultrasonic vibration are as follows: pouringtemperature around 685℃, ultrasonic time for 60-120s. Ultrasound has an indirect roleon refinement of eutectic Si. Although ultrasounic vibration does not change thecomposition phases of A390 alloy, it does influence their relative contents.The effect of A390 hypereutectic Al-Si alloy slurry-making conditions on thecorresponding properties of rheo-diecasting were examined. Suitable rheo-diecastingtemperature of A390 slurry is in the range of 600-620℃. Rheo-diecasting temperature ofslurry has a significant influence on the tensile strength of the correspondingrheo-diecasting as well as its density. The tensile strength of rheo-diecasting samplesreduces with increase of primary Si size linearly. Compared with the liquid diecasting, the tensile strength and hardness of rheo-diecasting specimen increased by 25.2%, 48.9%respectively. Meanwhile, the intermetallic compound in rheo-diecasted alloys reducedand not connected. Rheo-diecasting samples after heat treatment present a significantincrease in performance, and P modification has little effect on the A390 properties ofrhe-diecasting samples. Toughness of the rheo-diecasting specimen increased, and thefriction coefficient and wear rate decreased.The formation, size and scope of ultrasonic cavitation in the bulk melt were analysed,and the roles of cavitation and ultrasonic streaming in the melt on nucleation and graingrowth were investigated, the mechanism of non-dendrite formation of primary crystalswere put forward, and a dendrite infiltration model with low-solid fraction in ultrasonicfield were established. Non-wettable particles in aluminum melt provide the nuclei ofultrasonic cavitation, and ultrasonic cavitation threshold increases with reduced nucleussize. Pressure up to 10~9pa is generated when the cavitation bubbles breakdown. Thenumerical simulation indicates that the shape of the ultrasonic cavitation region beneaththe end of vibrator was semi-sphericals. The intensity of ultrasonic streaming is related tothe melt viscosity and ultrasonic power, and the ultrasonic streaming speed could reach1.37m/s in aluminum alloy melt near liquidus temperature. The intensive pressuregenerated by ultrasonic cavitation increases the solidification temperature ofmicro-region melt, which promotes nucleation. On the other hand, surface coolingresulted from the expansion of cavitation bubbles is very small, thus it is unlikely topromote the nucleation in the melt. The jet and pressure wave generated by ultrasoniccavitation could cause dendritic arms bending and capillary infiltration of roots, whichfinally results in dendrites fragmentation. Furthermore, ultrasonic cavitation and acousticstreaming could impede grain growth and homogenize microstructure.