The relaxation phenomenon during melt superheating treatment of DZ125 alloy

As a precipitation hardened unidirectionally solidified Ni-based superalloy, DZ125 has been extensively applied as structural materials in advanced aeroengine for gas turbine blades and vanes operating at elevated temperatures. The solidification microstructure of alloy plays an important role in determining the alloy's mechanical properties. For a given alloy, the thermal history of alloy melt has an obvious influence on the final solidification microstructure. However, previous researches mainly focused on melt superheating temperature. There are few investigations about the influence of the relaxation process of alloy melt on the solidification structure. In this paper, under the same conditions of processing parameters, e.g., solid/liquid interface temperature gradient, withdrawing velocity, by changing the melt holding time, the relaxation phenomenon during melt superheating treatment of DZ125 was investigated. The melt was superheated to 1650°C and kept for 30 min, and then it was rapidly cooled down to 1500°C. In order to study the relaxation phenomenon during melt superheating treatment, the melt was held for 15, 30 and 60 min, respectively, and then was withdrawn into the Ga-In-Sn liquid metal bath at a rate of 50 μm/s. The results show that dendrite arm spacing, segregation ratios, size and morphology of γ' phase and γ+γ' eutectic did not change with the increase of the holding time when melt holding time was less than 30 min. But, when melt holding time was prolonged to 60 min, the dendrite arm spacing, γ+ γ' eutectic and the size of γ' phase in interdendrite area increased, and the segregation becomes serious, but the γ' phase on dendrite trunk was lightly refined. The morphology of MC carbide mainly showed disconnected blocky and nodule, and un- changed with the prolongation of holding time. These changes in solidification structure should be primarily attributed to the influence of relaxation process on melt structure. When the melt holding time was less than 30 min, there was no evidently change in met structure. But, further prolonging melt holing time to 60 min, the melt structure had obvious variation. This variation affected the solidification processing and finally solidification structure. Therefore, the variation of melt holding time resulted in the change of solidification structure.