Study On Preparation And Rheocasting Of Semisolid Al Alloy Slurry With Ultrasonic Vibration

The semisolid metal processing, including thixoforming and rheocasting, offers the opportunity to manufacture net-shaped components with complicated shape and good mechanical properties. More and more researches have been focused on rheocasting in recent years because of its low cost and high productivity. The most critical task in rheocasting is to develop efficient processes for making of semisolid slurry, and the ultrasonic vibration process is one of these processes. The 5000 series Al-Mg alloys have been found a large variety of applications in aircraft, automobiles, ships and home appliances. Nevertheless, it is hard for traditional plastic forming technologies to produce components with great difference in thickness. Rheocasting process is expected to solve this problem and expand the application fields of these alloys. In this paper, semisolid slurry of 5052 Al alloy was prepared by direct ultrasonic vibration (DUV). A new type of indirect ultrasonic vibration (IUV) process was developed based on DUV. The semisolid slurry was formed by rheo-diecasting or rheo-squeeze casting.The feasibility of preparing 5052 Al alloy semisolid slurry and its filling capability were firstly explored. The effects of vibration starting-temperature, treatment time and isothermal holding time on the microstructure of semisolid slurry produced by DUV were systematically investigated. The results indicate that non-dendritic 5052 Al alloy semisolid slurry with average particle diameters about 110μm and average shape coefficients above 0.4 could be obtained when the melt is treated by DUV for 90s with starting-temperatures between 665℃and 675℃. The slurry has the best filling ability when the pouring temperature, specific pressure of injection and injection speed are 637℃,80MPa and 3.5m/s, respectively.A new IUV apparatus, in which the horn was vibrated under the bottom of the metallic cup containing alloy melt, has been developed in this research. Statistical energy analysis was employed to calculate the acoustic pressure distribution of the melt and the mean-square vibration velocity of the cup. The results indicate that the ultrasound is mainly introduced into the melt through the resonance of the cup bottom, and an acoustic pressure gradient is induced by IUV. The acoustic pressure of the melt has a maximum value of 4.86MPa near the bottom and decreases from the bottom to the melt surface. The mean-square vibration velocity of the cup bottom reaches 5.93m/s. Physical simulation experiments confirm that acoustic stream, cavitation and thermal effect can be induced by IUV.The IUV process can produce excellent Al alloy semisolid slurry. IUV process not only has a higher efficiency and better effect for preparing slurry but also avoid the erosion problem of titanium alloy horn by the melt, compared to DUV process. The effects of processing parameters such as IUV starting-temperature and treatment time on the microstructures of semisolid slurry of ZL101,5052 and 5083 Al alloys were investigated. In order to eliminate rosettes in the slurry, the optimum vibration starting-temperature is the liquidus temperature of an alloy in a specified time. When the alloys are treated by IUV for more than 30s, the average diameters and shape coefficients of primary a-Al particles increase with the time. Semisolid slurry with good combination of grain size and shape coefficient could be obtained by IUV treatment for 50s. In this case, the average diameters of the ZL101,5052 and 5083 Al alloys are less than 75μm, and shape coefficients above 0.52. Apart from preparing semisolid slurry with fine microstructure, IUV had degassing effect on semisolid slurry.The mechanism of preparing semisolid slurry with IUV was discussed by analyzing the evolution of non-dendritic grain in the semisolid slurry with high solid fraction. Cavitation generated by IUV enhances heterogeneous nucleation in the melt, and the cup bottom plays an important role in producing nuclei because of its cooling effect for the melt. High frequency vibration causes micro-cracks on the interface between the cup and grains, and then cavitation and acoustic streaming infiltrate liquid into the cracks and lead to the separation of the grains which act as heterogeneous nuclei. The nuclei directly grow into spherical particles under stirring by cavitation and acoustic streaming.The effects of IUV treatment time on the microstructures and mechanical properties of rheo-diecasting samples were studied. The rheo-diecasting samples of ZL101 and 5083 Al alloys treated by IUV for 50s have the maximum mechanical properties which are superior to liquid diecasting samples.The rheo-squeeze casting of 5052,5083 and ZL101 alloys was studied, and semisolid components with good mechanical properties were obtained. The emphasis was to study the effects of processing parameters such as IUV treatment time, squeeze pressure and mould preheating temperature on the micro structure and mechanical properties of 5052 Al alloy. The best mechanical properties of rheo-squeeze casting 5052 Al alloy were achieved under a squeeze pressure of 100MPa, IUV treatment time of 50s and mould preheated temperature of 300℃. The tensile strength and elongation were 225MPa and 17.8% respectively, which were improved by 9.7%and 42.4%respectively compared to conventional squeeze casting sample. Under this processing conditions, the tensile strength and elongation of 5083 Al alloy were 274MPa and 13.5%respectively, and the T6 heat treated ZL101 Al alloy with a tensile strength of 338MPa and an elongation of 8%.