Influence of withdrawing rate transition on the primary dendrite arm spacing and microsegregation of directionally solidified single crystal superalloy DD3

The microstructure of single crystal Ni-based superalloys is virtually determined by both processing parameters of growth rate v and thermal gradient G ahead of the solidification front. Previous researches have established the relationship between dendrite spacings and G or v under supposing those parameters to be constant from beginning to end of solidification, resulting in the disaccord between the predicted and experimental results for actual blade productions. Furthermore, previous experimental and theoretical works seldom involve the influence of processing parameter change on dendrite growth for multi-component industrial alloys. Therefore, investigation of directional solidified microstructure varying with processing parameter is the focus of present study. The influence of the withdrawing rate transition during directional solidification on the primary dendrite arm spacing (PDAS) and microsegregation for single crystal superalloy DD3 has been studied by liquid metal cooled (LMC) directional solidification method. A wide range of withdrawing rate variations from 50 to 600 μm/s is allowable under G high to 250 K/cm. The results indicate that the average PDAS is remarkably dependent on the history of growth rate variation. The smaller PDASs of 56.5 and 86 μm can be obtained under withdrawing rate transition from higher values of 600 and 300 μm/s into 100 μm/s, while they are kept being 111.5 μm if withdrawing rate is 100 μm/s all the time. In contrast, PDASs increase to 109 and 93 μm under withdrawing rate transition from lower values of 50 and 100 μm/s into 300 μm/s, while they are kept being a small value of 70 μm if growth rate is 300 μm/s at beginning. These experimental results agree approximately with those from Hunt-Lu model, but the maximum to minimum ratio of PDASs at a given solidification parameter, i.e., λ₁^max/ λ₁^min, is proved to be more than 2. It is also shown that the dendrite microsegregation will be lessened with the decrease of PDASs at the same current solidification parameters.