The Ultrasonic Treatment Of Al Alloy Melts And The Preparation Of Surface Composites By Ultrasonic Method

In the production of industry and agriculture, a great deal of parts bearing friction and wear need not only better surface performance but also better overall strength and toughness. In addition, with the aggravation of the global energy crisis year after year, the decrease of the weight and consumption of vehicle has been mentioned on the agenda. It is an important means for lightening own weight of the machine, has become a hot area of the material research at home and abroad, and is also the trend of the future development of the transport industry that light metals such as aluminum alloys replace the steel and iron materials with high density. However, some performances of light metals such as Al alloys etc, such as hardness and wear resistance, can not often satisfy the operating requirement. Therefore, it has important practical significance to improve the microstructure and properties of aluminum alloys in order to solve the contradiction between the wear resistance and decreasing weight of the mechanical parts. Recently, it is also one of the hotspots in material research that power ultrasonic is used to treat the melts of Al alloys, refine the microstructures, and improve the properties of Al alloys. Especially that power ultrasonic was used to treat the melts of Al alloys and prepare the surface composites in order to obtain both the good surface properties and preferable overall strength and toughness was not reported till now. Therefore, it has important theoretical and practical significance to research deeply and systematically in these fields.In this paper, power ultrasonic was adopted to treat the melts of Al and Al alloys. The effects and their mechanisms of power ultrasonic on the microstructures and properties of Al and Al alloys were discussed. A novel method was developed to prepare in-situ particles reinforced surface composites.Firstly, the system that the ultrasonic treated the melts was improved. Horn crucible was designed and invented. It was employed as a part of horn to treat the melts. The results were as fellows:1. A novel horn crucible has been designed and invented. The horn crucible is a part of the horn. The cooling system was set in the node of the horn to decrease the influence of the temperature on the ultrasonic transducer during ultrasonic treatment, which exhibited sufficiently the ability of the transducer.2. The melts as a part of the horn could be coupled very well with ultrasonic in the course of the ultrasonic treatment. As a result, the melts were treated intensively near ultrasonic zone. During the ultrasonic treatment and solidification, the ultrasonic was introduced upward so that the surface of melts could not be destroyed when the surface composites were fabricated. This provides a guarantee for the heat preservation of the melt at high temperature and the investigations of the movement of particles in the sound field during the ultrasonic treatment and the change of alloy microstructure.Secondly, the effects of ultrasonic on the microstructure and properties of the pure Al and hypereutectic Al-Si alloy were researched. The commercial pure Al was refined by the effects of the cavitation and acoustic flowing of ultrasonic treatment. After ultrasonic treatment, the microstructures of hypereutectic Al-Si alloy were homogenized and fined well, and this alloy had better friction and wear properties. The main results were as fellows:1. The microstructure of commercial pure Al treated by ultrasonic at 720℃was refined in comparison with that untreated by ultrasonic whether it was the permanent mold pouring or rapid water cooling. The dendriticα-Al was changed to equiaxial crystal. The commercial pure Al could be refined well by ultrasonic with as-ultrasonic treatment Al-5Ti-0.25C alloy grain refiner. The minimal grain size was 62μm when the contact interval was 30 s, and the anti-decay ability of the grain refiner was enhanced.2. The macrostructures of hypereutectic Al-23%Si alloy treated by ultrasonic (150 W, 10 min) were smaller than those untreated by ultrasonic. With increasing ultrasonic power, the microstructure of primary Si changed from five-petaled star flowers and flat bar shape to octahedron (75 W, 10 min). On the condition of high power (150W, 10min) the microstructure of primary Si was changed to fine irregular cluster primary Si. The distribution of the primary Si was uniform, and the mean size of primary Si decreased from 500μm to 180μm. The grain size ofα-Al increased firstly and then decreased, and the morphology was changed from dendrite to equiaxed crystal. The size of eutectic Si decreased from 40μm to 20μm, while the space length increased from 2.2μm to 2.8μm. With the increase of ultrasonic treating time, the microstructure of the primary Si changed from five-petaled star flowers and flat bar shape to fine irregular cluster primary Si.3. The hardness of hypereutectic Al-Si alloy treated by ultrasonic was analyzed with the mathematical statistics for the first time, and the distribution of the hardness was much uniform. The tensile strength of hypereutectic Al-Si alloy treated by ultrasonic was improved, but the ductility was reduced. The wear resistance of the alloy was much improved and the wear mechanism was transformed from spalling to microcutting.Thirdly, the effects of the ultrasonic treatment on eutectic Al-Si alloy were investigated, and Sip/Al-Si alloy surface composites were prepared. The microstructures and properties of the surface composites, such as hardness and wear resistance, were investigated. The main results were as fellows:1. In the eutectic Al-Si alloy treated by ultrasonic, the primary Si particles were concentrated in the surface layer due to the ultrasonic radiation force, weight and buoyance in the ultrasonic field, thus the surface composites of Sip/Al-Si alloy were fabricated. The microstructure distribution with the primary Si in the surface layer and the dendriteα-Al and eutectic in others was obtained creatively.2. With the increase of the ultrasonic power, the thickness of rich Si surface layer reduced and the unit area fraction of the primary Si increased. The eutectic Si was modified from needle and/or flake shape to fiber shape in middle of samples. Theα-Al dendritic was precipitated increasedly in the underside of samples. The primary dendritic arm was fined and the space of secondary dendritic arm was decreased.3. The hardness of the surface layer of Sip/Al-Si surface composites was higher than that of other position. The wear resistance of surface composite layer was good and the block primary Si was distributed uniformly in surface layer in contrast to that untreated by ultrasonic.Finally, in-situ Al₃Ti/Al surface composites were prepared by ultrasonic method. The microstructure and hardness were studied. The results were as fellows:1. As the melt of Al₃Ti/Al composites was treated by ultrasonic, the Al₃Ti particles were concentrated in the surface layer and formed the surface composites under the ultrasonic radiation force, weight and buoyance in the ultrasonic standing wave field. With the increase of the ultrasonic power and treating time, the thickness of the particle reinforced surface layer reduced.2. The sizes of Al₃Ti particles were decreased because of the cavitation and sound flowing effect of ultrasonic treatment. Meanwhile, the relative content of Ti in Al₃Ti phase increased. There were the homologous crystal lattice between Al₃Ti andα-Al, andα-Al phase solidified on the nucleus of Al₃Ti.3. By ultrasonic treatment, the dislocation density in the interface between the reinforce phase and matrix was increased. In the interior and grain boundary ofα-Al a great deal of dense dislocation was generated by the cavitation and sound pressure of ultrasonic. The hardness of the surface composites was increased distinctly after ultrasonic treatment.To sum up, the microstructures were refined and the properties were improved when the melts of Al and Al alloys were treated by ultrasonic. By the ultrasonic treatment of certain Al alloys, the surface composites could be fabricated. The results have great importance in enriching the theory of ultrasonic treating melts and broadening the application field of ultrasonic and fill up the theoretical blank of the surface composites prepared by ultrasonic method.