Mechanism Study Of Solidification Microstructure Evolution And Morphology Formation Of Hypoeutectic Aluminum-copper Alloy With Synchrotron Radiation

Controlling microstructure is critical point to obtain high quality castings, as well as demishing defects, especially for large complex thin walled parts, which have complicated structures, such as, variable cross-section, T-type structure and thin-walled structure. It is well known that the distribution of local thermal field, solute field and flow field is the key point to the formation of solidification microstructure.In this work, stood on the fundamentals of thermal and solute effect on metallic crystallization, used synchrotron radiation methods, to study the relationship between solidification microstructure and multi-field distribution. The results present a new doublon growth mode in Al-Cu alloy, a mechanism of Ti B2 paricle refinement, and features of microstructure formation during solidifcation.The dendrite pattern and growth orientation of the dendritic doublon in hypoeutectic Al-Cu alloy studied by synchrotron radiation imaging and traditional electron backscatter diffraction(EBSD). The experimental results show that this kind of doublon has two-dimensional growth pattern, and the secondary dendrite arms growing perpendicularly to the primary stem. This doublon pattern can appear as an equiaxed dendrite, columnar dendrite, or coexist with a regular dendrite. EBSD date show the dendritic doublon tips grow along <110>, and the secondary arms are towards <001> direction, respectively. The probability of dendritic doublon strongly depends on Cu concentration. It gradually increass as increasing the alloy concentration, and reach the highest point at the Al-20 wt% Cu. According to the experimental data and theoretical calculations, the dynamics crystal morphology diagram of hypoeutectic Al-Cu alloy was modified, and defined the dendritic doublon is a special transitional morphology pattern between fractal dendrite(FD) and compact dendrite(CD).Microstructure evolution of hypoeutectic Al-Cu alloy at each characteristic structure mentioned above was studied by synchrotron radiation imaging. According to the absorption contrast differences between different substances, and the time-related properties of imaging, the effects of thermal field, solute field, and flow field on the solidification microstructure evolution were investigated. It is found that for the variable cross-section structure and T-type structure, which solidification cavities are subjected to the spatial structure changes, the thermal distribution in front of the growth interface presents a significant relevance with structural features. Different structure leads to different enrichment region and melt flow status, resulting in a different freezing sequence and microstructure characteristics. For the thin-walled structure, the thickness is the vital factor to solidification microstructure. It is inessential to improve the filling and feeding ability of the thin-walled structure through increasing pouring temperature and mold temperature. The cooling rate increases with decreasing wall thickness, and the homogeneity of equiaxed grains reduces as reducing wall thickness.Further, in order to analyze the effect factors on refinement of solidification microstructure to regulate the solidification microstructure, the effect of addition of Ti B2 particles on the melt structure of Al-15 wt% Cu was studied by Small angle X-ray scattering(SAXS). The results indicate that the aggregates in Al-Cu alloy melt present mass fractal characteristics and incompact 3D flocs structure. Doping Ti B2 particles can destroy the chain network structure of the melt aggregates, dramatically reduce its size at temperatures ranging from melting point to 800°C. The results of synchrotron radiation imaging show that addition of Ti B2 particles can refine the microstructure effectively, eliminate the large bulk eutectic zone, significantly change the solute and flow field at variable cross-section region, and improve the uniformity of solidification microstructure.In summary, the study of the mechanism of hypoeutectic Al-Cu alloy dendrite morphology transition, the solidification microstructure evolution of the characteristic structure and the mechanism of refiners inhibiting the structural effects can provide a scientific basis for controling the solidification microstructure. Research results will act as a useful complement to the dendritic morphology study.