Research Of Ultrasonic Treatment On Solidification Characteristics And Structure Of Metals

It is well known that ultrasonic treatment can refine solidification structure effectively and strengthen mechanical properties of metals. However, due to some disadvantages of the traditional ultrasonic treatment process, such as inclusion immixture, difficult manipulation to high-melting metals and so on, the application of ultrasonic treatment in the industry field has great difficulties.A novel ultrasonic treatment process, i.e. ultrasonic side-introduced technology with "self-attraction amplitude rod" is developed to solve the problems of traditional ultrasonic treatment and extend its applications. The aim of this work is to systemically investigate its application to metal solidification.In order to investigate the feasibility of side-introduced ultrasonic treatment, we examined the effects of ultrasonic treatment on solidification characteristics and structure of low-melting A1-7.3%Si alloy. Differential heat analysis method is selected for measuring solidification initial temperature, eutectic reaction temperature and time of A1-7.3%Si alloy during solidification. The experimental results show that solidification initial temperature of the melt has few changes until the inputting ultrasonic power reaches 600W. With the further increase of ultrasonic power, solidification initial temperature is decreased, causing grain refinement and morphology development of primaryαphase. From 0W to 700W, average grain size can be expressed as the negative exponential of ultrasonic power. Mechanical tests show that both tensile strength and elongation percentage are enhanced.Next, we evaluate the contribution of ultrasonic treatment on solidification characteristics and structure of high-melting T10 steel. Results of differential heat analysis show that once the ultrasonic power is beyond 300W, solidification initial temperature decreases with the increment of ultrasonic power. Microstructure study of T10 steel reveals that ultrasonic treatment results in drastic refinement of grain with more uniform distribution. The significant structure variation obviously has an effect on mechanical properties. Mechanical test demonstrates that ultrasonic treatment can improve both tensile strength and ductility remarkably. Furthermore, fracture characteristics of samples are transformed from brittle fracture to quasi-cleavage fracture.Based on the above investigation, we investigate the effect of ultrasonic treatment on solidification characteristics, structure and properties of 1Crl 8Ni9Ti austenite stainless steel so as to meet the demand of the industry field. The differential heat analysis suggests that the solidification initial temperature decreases and solidification time shortens with the application of ultrasonic during solidification. The application of ultrasonic treatment on 1Crl 8Ni9Ti also makes the peritectic reaction temperature decrease. The relation between solidification initial temperature and ultrasonic power is quantitatively determinate. Analsis results of EBSD show that with the increase of ultrasonic power, important structure changes occur with the equiaxed grain being substituted for column one. Moreover, not only the area of grain and subgrain boundary, but also the dislocation density increases. From 0W to 700W, austenite content is ratio to linear increase of ultrasonic power. When the power is 700W, the structure is almost austenite phase. Meanwhile, the investigation of the effect of ultrasonic treatment on rnicrostructure of 1Crl 8Ni9Ti is carried out. Ultrasonic treatment plays the role in the columnequiaxed dendrite transformation. According to EDS results, we exhibit the distribution of main elements in different positions. It is further revealed that ultrasonic substantially suppressed the zonal segregation of Cr and Ni due to the acceleration of solute diffusion in the front of solid/liquid surface, which leads to the decrease of solute boundary layer thickness. The experimental results of mechanical properties indicate that the ultrasonic treatment improves tensile strength remarkably and slightly increases elongation percentage of 1CrlSNi9Ti austenite stainless steel. Tensile strength can be expressed as the negative exponential of propagation distance of ultrasonic. In addition, the stainless steel has better corrosion resistance after ultrasonic treatments.On the basis of the solidification and acoustic theory, the refinement mechanism of ultrasonic treatment during solidification is discussed. It is indicated that micro-area supercooling during cavitation bubble expansion and high pressure induced by bubble close give rise to the increase of nucleation rate of melts, causing the refinement of structure. In addition, shock force generated by cavitation bubble and heat pulse caused by ultrasonic also make great contributions to improve dendrite morphology and refinement structure. The effect of ultrasonic treatment on theory crystallization temperature of the metal is studied. Calculation shows that theory crystallization temperature rises after the application of ultrasonic treatment. According to the experimental results of solidification initial temperature, it can be concluded that ultrasonic treatment causes the increase of degree of supercooling. By means of Newton cooling model and fluidics, we analyze the relation between cooling rate and ultrasonic power.Attenuation model of ultrasonic propagated into the liquid metal is built to reveal the mathematic relation among ultrasonic attenuation, propagation distance and melt temperature. Meanwhile, in virtue of self-developed attenuation-testing device, we collect ultrasonic attenuation of Al-7.3%Si alloy with different propagation distances during solidification and fit the function of attenuation distribution of the alloy. Verification between the experiment results and the theory model shows that they are in good agrerment with each other.