Effect of Cooling Rate on Microstructure and Element Segregation Behavior of Cu-9Ni-6Sn alloy

With copper blocks, nickel plates, and tin ingots as raw materials, Cu-9Ni-6Sn alloy was prepared by melting and subsequently casting into square iron molds, cylindrical water-cooled copper molds, and twin-roll thin strip casting-rolling machines, with corresponding cooling rates of 2.3, 18.7, and 138.6 ℃·s⁻¹, respectively. In-situ observation of the solidification process was conducted by high-temperature laser confocal microscopy at cooling rates from 0.1 to 100 ℃·s⁻¹. The effect of cooling rate on the microstructure and elemental segregation behavior of the test alloy was studied. The results show that when the cooling rate was 0.1 ℃·s⁻¹, the dendritic crystals of the alloy during solidification were relatively coarse, with a maximum diameter of 83 μm. When the cooling rate increased to 0.5 ℃·s⁻¹, the dendrite size decreased, and the maximum diameter reduced to 65 μm. When the cooling rate continued to increase to 10, 100 ℃·s⁻¹, the dendrite size decreased by several orders of magnitude. The solidification microstructure of the alloy prepared by iron mold casting was coarse dendrites, with a primary dendrite spacing of 180 μm, and Sn elements were periodically enriched and depleted. When the cooling rate was increased to 18.7 ℃·s⁻¹, which was the same as that used for the water-cooled copper mold preparation, the primary dendrite spacing decreased to approximately 36 μm, and the Sn element segregation was effectively alleviated. When the cooling rate continued to increase to 138.6 ℃·s⁻¹, which was the same as that used for the twin-roll strip casting, the solidification microstructure changed from dendrites to equiaxed crystals, with an average grain size of 8 μm in the center, and the Sn element segregation was significantly suppressed at the grain interiors and grain boundaries.