Abstract: The solidification process of a conventional superalloy, IN718, was investigated by confocal scanning laser microscope(CSLM). The liquid fraction during solidification was obtained as a function of real time and temperature in reference with the in-situ observation. The characteristics of L→γ transformation were analyzed and the γ growing rate of each stage was also calculated.Scheil equation was employed to predict the segregation behavior, and the predict results are in consistence with the experimental results. As a result, the confocal scanning laser microscope shows a great potential for solidification process research.
1 Introduction
IN718 alloy is a kind of precipitation-strengthened wrought superalloy widely used in the gas turbine industry [1−2], Because of its excellent balance of properties and reasonable cost, IN718 is accounting for more than 50% of commercial superalloy productions in the world[3]. With the development of land-based power generation and aircraft propulsion, scaling-up of components has become the necessity. The size of IN718 ingot produced by VIM-ESR-VAR triple melting has increased dramatically over the past 10 years in response to market demands[4−6]. However, the solute segregation problem, mainly niobium segregation, is a big obstacle for producing large size IN718 ingots.Particularly, some macro-segregations defects such as freckles and white spots formed during the solidification process will lead to entire failure for the whole ingot. So,the driving force for studying the solidification behavior of IN718 superalloy still exists.
In the previous studies, a lot of methods have been employed to investigate the solidification behavior of IN718 superalloy, including DTA, high temperature freezing and computational modeling. For example ,KNOROVSKY et al[7] obtained transformation temperatures by DTA and presented a solidification diagram for an idealized IN718. WANG et al[8] used high temperature freezing method to keep the initial liquid state of alloy, expecting to analyze original solidification process. Thermo-calc and JMatPro software were also employed to calculate the liquid composition and equilibrium diagrams of IN718. These methods provide a variety of valuable solidification information and can be used to predict segregation during melting or casting[8−9].
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