Solidification behavior of Inconel 713LC gas turbine blades

Inconel is a superalloy of nickel that is comprised of chromium and iron, and portrays excellent resistance to corrosion at high temperatures. Inconel-713LC is a low carbon nickel-based superalloy, generally produced by investment casting, which is widely used in hot section components of gas turbines engines. The efficiency and reliability of a gas turbine heavily depends on the material from which it is crafted from. To this end, Inconel-713 low carbon alloy is the most suitable material for the fabrication of turbine blades that are subject to tensile loading at high temperature. Research has shown that the excellent strength of this alloy is mainly provided by the γ’ coherent precipitates (an intermetallic Ni₃(Al, Ti) compound) within the γ matrix. Unfortunately, despite much having been done on this, there is no single report on a comprehensive analysis of the solidification behavior of Inconel-713LC gas turbine blades under a variety of cooling conditions during electron beam welding.

Dr. Mohsen K. Keshavarz in collaboration with Professor Sylvain Turenne at École Polytechnique Montréal and Dr. Ali Bonakdar at Siemens Canada developed a physical model that could be used to predict the evolution of the microstructure of the Inconel-713 low carbon alloy in the fusion zone. They also aimed at establishing the empirical correlation between the secondary dendrite arm spacing in the fusion zone and cooling rate. Their work is currently published in Journal of Manufacturing Processes.

The outcome of this research can be employed to determine the hot crack index which assists in evaluating the risk of crack formation during cooling and also in locating the spots where the phenomenon is likely to occur. The team utilized the results of a numerical model for the electron beam welding process to compute the cooling rates during solidification of the weld. Eventually, the solidification behavior of the Inconel-713LC under a variety of cooling conditions during electron beam welding was investigated.

The authors observed that the rapid solidification of fusion zone led to very fine dendritic microstructure of the weld. Additionally, by incorporating the calculated cooling rates with the microstructural observations, they introduced an empirical relation between cooling rate and secondary dendrite arm spacing. To be more precise, the calculated cooling rates during solidification of electron beam welding of Inconel-713LC showed some high magnitudes ranging from 700 K/s to more than 5600 K/s at different points in the fusion zone.

The study has successfully presented a model that accurately predicts the evolution of the microstructure of the Inconel-713 low carbon alloy in the fusion zone. From the analysis undertaken, it has been revealed that the segregation of elements related to the formation of NbC occurs in the interdendritic regions in the fusion zones and at the grain boundaries in heat affected zone and parent material. To this end, the results and observations of this work can be used to understand hot cracking causes in the electron beam welding of Inconel-713LC and reduce its occurrence.