Research On Temperature Field And Control Of Stray Grains In Directional Solidification Of DD5 Single Crystal Alloys

Many of problems occur during preparation process of Ni-base single crystal superalloy, such as stray grains, misorientation, freckle, coarse microstructure. The formation of stray grains destroys the integration of single crystal and greatly reduces mechanical property and service life of the superalloy blades. Control of stray grains formation has great guidance on the performance improvement of single crystal blade.The formation of stray grains was simulated by using computer simulation and experiment during directional solidification. The main results are summarized as follows:(1) Utilizing an improved Monte Carlo method a calculation model of temperature field during the dynamical withdrawal process has been built. A simplified key parameters--view factor has been obtained. The effect of temperature change on process parameters such as withdrawal rate, positions of ceramic shell and module quantity has been researched. Above insulating baffle, the temperature is lower at the far end of furnace wall than near-end; below insulating baffle, temperature status is just the opposite. Radiation angle increases more quickly in unit time with faster withdrawal rate so that heat dissipation potential of ceramic shell increases; larger radiation angle enhances cooling capacity of shell; radiation decreases because radiation space at the far end of furnace wall becomes smaller in the case of more bars on shell.(2) A specimen of variable cross-section was designed to study temperature field, mushy zone and stray grains in the platform area by numerical simulation. There are three kinds of stray grains formation: longitudinal growth, lateral growth, undercooling nucleation in variable section platform of tangential direction. An improved time criterion was presented for decision of stray grain formation.(3)The influence of withdrawal rate on stray grains formation was studied. With faster withdrawal rate, super-cooling degree of platform increases. It makes ΔL(critical dimension of platform) decrease. At the withdrawal rate of 3mm/min, ΔL is 20cm; At the withdrawal rate of 7mm/min, ΔL is 4cm. At the same withdrawal rate, probability of stray grain formation increases for super-cooling degree of platform increasing with larger size of platform.(4) Directional solidification parameters were optimized and the position of stray grains formation in specimen was forecasted by numerical simulation. Through the comparison for simulation results and experimental results, accuracy of simulation was proved.(5) Growth morphology of stray grains in variable section platform was studied. Simulation results show that: rule-oriented stray grains formed in platform at lowest position of specimen; stray grains did not form in platform at top position; disordered stray grains formed in platform at middle position. The reason for difference growth morphology of stray grains is that cooling velocity and undercooling degree are different for platform with different height. With the height increase of platform, heat conduction between casting and coolship weakens. Radiation heat transfer also recedes with the solidification process. Lower heat dissipation rate makes undercooling degree in platform decline.