Dexin Ma

History effects during the selection of primary dendrite spacing. Comparison of phase-field simulations with experimental observations

The paper describes numerical experiments to investigate the selection of primary dendritic spacing λ under transient conditions. The phase field method coupled to solutal diffusion is combined with a moving frame algorithm to enlarge the total calculation domain. For constant growth conditions of an Al–7%Si alloy, a range of λ has been achieved depending on initial conditions. Quasistationary decrease of the solidification velocity leads to extinction of redundant trunks. Increasing the velocity leads to the formation of new trunks from ternary arms, but a striking incubation delay suggests a history-dependent spacing assignment. The results are compared to experimental and theoretical results.

Some New Observations on Freckle Formation in Directionally Solidified Superalloy Components

To improve the understanding of freckle formation in superalloys, several directional solidification experiments were carried out in a production-scale Bridgman furnace. During the careful inspections of the many cast specimens, several interesting phenomena were observed, indicating new aspects of freckle formation. Accordingly, several new factors influencing the freckle formation, such as the shadow effect, the edge effect, the step effect, and the crystal orientation effect, are proposed. This will contribute to our current understanding of freckle formation and the future development of new measures to minimize freckle defects in directionally solidified components of superalloys.

The Geometrical Effect on Freckle Formation in the Directionally Solidified Superalloy CMSX-4

In order to better understand the geometrical effect on freckle formation in superalloys, samples with uniform increases and decreases in their cross sections were directionally solidified in a Bridgman furnace. In comparison to our conventional knowledge, some new features of freckle appearance have been observed. Freckles could occur at sloped surfaces where the freckle pattern is no longer roughly parallel to the direction of gravity but has the same slope as the surface. At significantly angled surfaces, the freckles appear as discrete flakes, having the shape of tree roots, instead of the long and narrow chains which are usually observed. The component portions having inward sloping surfaces are very freckle prone while those with outward sloping surface are mostly freckle free, although the completely opposite freckling tendency is indicated by the simulated solidification condition. Therefore, as an independent factor the geometrical feature of the components can more effectively affect the freckle formation than the local thermal conditions.

Comparative Investigation of the Downward and Upward Directionally Solidified Single-Crystal Blades of Superalloy CMSX-4

Single-crystal blades of Ni-base superalloys CMSX-4 have been directionally solidified using the downward directional solidification (DWDS) process. The possible benefits of the process were comparatively evaluated with respect to the Bridgman process’ results. The DWDS process exhibits good capabilities for casting the single-crystal components. The thermal gradients of this process are approximately seven times higher than those of the Bridgman process. It provides more advantages for solidifying the single-crystal superalloy blades by reducing the casting defects, refining the microstructure, decreasing the size of the γ/γ′ eutectic pools, refining the γ′ precipitates, alleviating the degree of the microsegregation, and minimizing the size and volume fraction of the micropores.

The Influence of Surface Roughness on Freckle Formation in Directionally Solidified Superalloy Samples

A series of directional solidification experiments with superalloy CMSX-4 was carried out with various surface profiles. The results obtained clearly demonstrate the influence of surface features on the occurrence of freckles. The rough surfaces exhibit less freckling tendencies than smooth surfaces. For a notch depth larger than the critical layer beneath the surface, the freckle formation can be completely suppressed. The freckle formation is not only a surface defect but can initiate the occurrence of internal equiaxed and stray grains. The results of the present study can be applied to completely prevent freckle formation and the accompanying internal grain defects by using oversize with an appropriately roughened surface.

Freckle Formation in Directionally Solidified Superalloy Components with Expanding Cross-Section

In the present work the influence of a cross-section transition on freckle formation in the directionally solidified superalloy CMSX-4 was investigated. It was found that in stepwise expanding specimens the new freckle chains were not formed immediately at the bottom edge of the steps, but after an incubation height of about 10 mm. The flow behaviour through a mushy zone was simulated in a similar system, in which the cross section was changed stepwise. After an expansion of the cross section, the flow velocity slowed down significantly. This explained the suppression effect of the expanding step on the interdendritic convection and furthermore on the freckle formation. The step effect was successfully applied to avoid the freckle formation in expanding specimens by applying a step length smaller than the critical incubation length.

Avoiding grain defects in single crystal components by application of a heat conductor technique

Abstract During single crystal solidification in turbine blades, grain defects are often formed in the platform region with abrupt variation in cross-section. In order to reduce grain defects arising from component geometries an HC (Heat Conductor) technique was developed. In the production process for ceramic shell moulds, a graphite heat conductor of excellent heat conductivity is inserted close to the inner corner of the platform. This effectively extracts local heat from this critical region during directional solidification. Numerical simulations show that by using HC the thermal condition in the platform region can be significantly improved. The single crystal growth in the blade body can spread into the platform more quickly before the melt at the platform extremity becomes deeply undercooled. Structural investigations reveal a remarkable reduction in grain defect formation, providing confirmation of the effectiveness of the HC technique in improving the casting quality of single crystal components.

Preferred growth orientation and microsegregation behaviors of eutectic in a nickel-based single-crystal superalloy

Abstract A nickel-based single-crystal superalloy was employed to investigate the preferred growth orientation behavior of the (γ + γ′) eutectic and the effect of these orientations on the segregation behavior. A novel solidification model for the eutectic island was proposed. At the beginning of the eutectic island’s crystallization, the core directly formed from the liquid by the eutectic reaction, and then preferably grew along [100] direction. The crystallization of the eutectic along [110] always lagged behind that in [100] direction. The eutectic growth in [100] direction terminated on impinging the edge of the dendrites or another eutectic island. The end of the eutectic island’s solidification terminates due to the encroachment of the eutectic liquid/solid interface at the dendrites or another eutectic island in [110] direction. The distribution of the alloying elements depended on the crystalline axis. The degree of the alloying elements’ segregation was lower along [100] than [110] direction with increasing distance from the eutectic island’s center.

Microstructural evolution of aluminium-copper alloys during the downward directional solidification process

Abstract The microstructural evolution of Al–Cu alloys during the downward directional solidification process was investigated. At the planar-to-cellular transformation point, the planar liquid/solid (L/S) interface broke down at the centre. This was contrasted with the behaviour in the liquid metal cooling process, where the interface broke down at the periphery. The critical withdrawal rate at this point was higher than the theoretical value. In addition to this, the variation in the primary dendrite arm spacing (λ1) as a function of the withdrawal rate (V) at constant GL for the Al-2.0 wt.% Cu alloy agreed with the conventional processes. The comparison of λ1 in our experiment to the calculated value λ1 using the Kurz–Fisher, Ma and Trivedi models showed that λ1, calculated by these models, overvalued our experimental results. However, the λ1 calculated from the Hunt model agreed well with the experimental values of λ1. When we reduced the diameter of the sample from 13 mm to 9 mm and maintained the other parameters constant, the L/S interface retained a planar shape. This indicated that the L/S interface was more stable in the smaller sample than that in the larger. This result contrasted with the result in the liquid metal cooling process.

Freckle Defect Formation near the Casting Interfaces of Directionally Solidified Superalloys

Freckle defects usually appear on the surface of castings and industrial ingots during the directional solidification process and most of them are located near the interface between the shell mold and superalloys. Ceramic cores create more interfaces in the directionally solidified (DS) and single crystal (SX) hollow turbine blades. In order to investigate the location of freckle occurrence in superalloys, superalloy CM247 LC was directionally solidified in an industrial-sized Bridgman furnace. Instead of ceramic cores, Alumina tubes were used inside of the casting specimens. It was found that freckles occur not only on the casting external surfaces, but also appear near the internal interfaces between the ceramic core and superalloys. Meanwhile, the size, initial position, and area of freckle were investigated in various diameters of the specimens. The initial position of the freckle chain reduces when the diameter of the rods increase. Freckle area follows a linear relationship in various diameters and the average freckle fraction is 1.1% of cross sectional area of casting specimens. The flow of liquid metal near the interfaces was stronger than that in the interdendritic region in the mushy zone, and explained why freckle tends to occur on the outer or inner surfaces of castings. This new phenomenon suggests that freckles are more likely to occur on the outer or inner surfaces of the hollow turbine blades.