Three-Dimensional Dendrite Growth Within the Shrouds of Single Crystal Blades of a Nickel-Based Superalloy
- PDF / 4,872,724 Bytes
- 16 Pages / 593.972 x 792 pts Page_size
- 16 Downloads / 197 Views
UCTION
OWING to the eliminated grain boundaries (GB), single crystal (SC) turbine blades exhibit exceptional properties at elevated temperatures.[1–8] For this reason, they are widely used in aero-engines and have been increasingly introduced into power generation turbines. To obtain higher turbine entry temperatures and therefore increase the efficiency of the engines and turbines, the design of the blades becomes more complex, sometimes incorporating very complex inner cooling
FU WANG, ZINING WU, DEXIN MA, and BU¨HRIG-POLACZEK are with the Foundry Institute, RWTH Aachen University, Intzestrasse 5, 52072 Aachen, Germany. Contact e-mail: [email protected] CAN HUANG and JU¨RGEN JAKUMEIT are with the ACCESS e.V., Intzestrasse 5, 52072 Aachen, Germany. Manuscript submitted March 6, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
passages. In addition to this, refractory elements are increasingly added to the superalloys in order to obtain an excellent high temperature performance.[9–14] However, these optimizations are accompanied by a more inhomogeneous thermal distribution in the SC blades during the directional solidification (DS),[15] and the formation of casting defects caused by geometrical features during directional solidification process becomes an increasingly serious challenge.[16,17] As a result of the abrupt enlargement of the cross section and the different cooling rates of the local melt, the shrouds of turbine blades are considered to be the most sensitive site prone to casting defects. SC growth in the blade’s body can be considered to be vertical, whereas within the shroud, the SC growth becomes three-dimensional (3D) and is more complex than that in the blade body. Stray grains[18–20] and low-angle boundaries (LABs) are always found within the shroud. These are harmful to the high temperature mechanical properties of the SC blades[21,22] because of the absence of GB strengthening elements.
Over the past few decades, many studies have been conducted to investigate the formation of casting defects in the shrouds. The majority of these studies are related to the forming mechanism of stray grains which, as well as their methods of reduction, is now well understood. One study[23] reported that stray grains are caused by a concave liquidus isotherm passing from the blade body to the shroud. The curved liquidus isotherm can lead to isolated, thermally undercooled regions of the melt at the shroud’s extremities. When this local undercooling exceeds the undercoolability of the alloy, new grains may nucleate which form a high-angle boundary with the original grain to form the macroscopic stray grain defects. This phenomenon will be exacerbated when a mold cluster is used.[24] Some researchers[25] investigated the effect of the shroud’s dimension on stray grain formation. One study[26] showed that the alloying elements of superalloys have a substantial influence on the formation of stray grains within the shrouds because of the difference in the undercoolability between these superalloys. Those superalloy
Data Loading...
