Investigation Of Casting Defects Formation Mechanism Of Directionally Superalloy DZ417G

As a key component of aircraft engines and industrial gas turbines, the performance of superalloy turbine blades is closely related to solidification microstructure. However, there are some casting defects exist in directionally solidified superalloy blade, such as:porosity, orientation deviation, coarse dendrite in the cross-section transition blade, which seriously affects the performance of superalloy blade. Therefore, it has a very important significance for the manufacture of superalloy blade to control the casting defects in directional solidification process, and improve its microstructure and performance. The paper focus on the formation of casting defects during the process of manufacturing superalloy blades. By using nickel-based superalloy DZ417G dummy blades with cross-section transition was investigation, with fluorescence detection, optical microscopy (OM), scanning electron microscopy (SEM), electron probe microanalysis (EPMA) and other analytical methods to analyze the process parameters and assemble mode and other factors on the effect of casting defects and microstructure in cross-section transition DZ417G directional superalloy blades. The main results are as follows:It is found through quantitative statistics casting defects and microstructure which located in cross-section transition of dummy blade within the different process parameters that:the content of porosity near the central runner cross-section transition is an order of magnitude higher than near the heater cross-section side. In the range of 1480～1580℃, with the casting temperature increase, the content of porosity near the central runner increase at first then decrease, meanwhile the content of porosity near the heater cross-section transition decrease slightly, the degree of elements segregation decline at first then increase, the primary and second dendrite arm spacing in cross-section place decrease gradually, while the content of y/y’eutectic increase at first then decrease, carbide morphology change from fine needle and small block to a massive big block with sharp chamfered, the size of carbide increase at first then decrease; the maximum content of porosity and y/y’eutectic is present at 1550℃, and the maximum size of carbide is in 1550℃, besides, the blade have the slightest degree of segregation at 1520℃.In the range of 1～10mm/min withdrawal rate, with the increase of withdrawal rates, the content of porosity near the central runner increase gradually, however, the content of porosity near the heater decrease gradually. Besides, the degree of elements segregation increase at first then decline, the primary and second dendrite arm spacing decrease both, the content of y/y’eutectic gradually decrease, the morphology of carbide transition from block to acicular gradually, the size and content of carbide both gradually decrease. The content of porosity reach the peak at the velocity of 10mm/min, the blade in dendrite region have the slightest degree of segregation at the velocity of lOmm/min.Growth direction of the blade has a certain effect on the microstructure, when growth direction changed from airfoil solidification after the root blade into inverse direction, the content of porosity near the central runner cross-section transition reduce, the content of porosity at the other part do not changed significantly, besides, change the growth pattern would result in the formation of stray grain in cross-section transition. When assemble mode changed from single layer blade growth into double layer of blades stacking, the content of porosity for double layer blades at the cross-section transition is higher than single layer blade, besides. When arranged in the way of the double blades turn 90°, the distribution of porosity at cross-section transition changed from wide cross-section into narrow side, furthermore, the area and content of porosity reduce both.By quantitating statistics the casting defects within different process parameters and assemble mode, it found that increasing the casting temperature appropriate(1480-1520℃), decline withdrawal rates(1-6mm/min), and resonable assemble mode(the display location turn 90°) in the blade could decline the casting defects both in the cross-section, ultimately improve the metallurgical quality of the blade.