Abstract
The investigation of a columnar to equiaxed transition (CET) and grain refinement is of high commercial importance for the improvement of the solidification structure of metal castings. The crystal rain from the free surface is frequently generated to produce grain refinement and promote a CET in alloys under the application of electromagnetic fields. However, the mechanism underlying the CET influenced by the generated crystal rain is not clear because the employed metallic alloys are opaque. In the present paper, the crystal rain in a transparent NH4Cl–H2O solution is produced by blowing a cooled nitrogen gas on the free surface to observe in situ its impact on the occurrence of a CET. The results show that the crystal rain can significantly promote a CET even in a high temperature gradient and that a CET only can occur when the temperature gradient is almost close to zero in the reference experiment. Finally, the most likely mechanism is discussed and clarified.
Highlights
Solidified structures and segregation of castings are of great importance for its mechanical, chemical, and processing properties
In order to generate crystal rain in the bulk solution, nitrogen gas was cooled in a spiral copper tube, which was immersed in the ice water mixture, and directly blown onto the free surface of the solution
It should be noted here that indicates that the crystal rain can trigger the columnar to equiaxed transition (CET) at one elevated temperature gradient ahead of the interface grows over the lowest thermocouple
Summary
Solidified structures and segregation of castings are of great importance for its mechanical, chemical, and processing properties. The solidified structure in ingots and billets consists of columnar and equiaxed grains. The most effective way to improve the homogeneity of ingots and billets is to generate large amounts of equiaxed grains and promote a columnar to equiaxed transition (CET). More equiaxed grains can be obtained by adding nucleating agents [4] This chemical method has limitations in many cases. The crystal rain can block the growth of columnar dendrites and refine the solidification structure significantly. In situ observation of solidifying metals by a synchrotron X-ray technique [15,16] was successfully performed to study the solidification behavior of alloys. The sedimentation of free crystals from the melt surface (crystal rain) was generated and observed in situ in a NH4 Cl–H2 O solution. The influence of the generated crystal rain on the occurrence of a CET was considered, and the related mechanism is discussed
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