Manufacturing of thin-walled Ni-based superalloy castings using alternative thermal insulating module to control solidification

Journal Pre-proof Manufacturing of thin-walled Ni-based superalloy castings using alternative thermal insulating module to control solidification D. Szeliga

PII:

S0924-0136(19)30476-5

DOI:

https://doi.org/10.1016/j.jmatprotec.2019.116503

Reference:

PROTEC 116503

To appear in:

Journal of Materials Processing Tech.

Received Date:

7 July 2018

Revised Date:

4 November 2019

Accepted Date:

11 November 2019

Please cite this article as: Szeliga D, Manufacturing of thin-walled Ni-based superalloy castings using alternative thermal insulating module to control solidification, Journal of Materials Processing Tech. (2019), doi: https://doi.org/10.1016/j.jmatprotec.2019.116503

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Manufacturing of thin-walled Ni-based superalloy castings using alternative thermal insulating module to control solidification D. Szeliga a,b a

Department of Materials Science, Faculty of Mechanical Engineering and Aeronautics,

Rzeszow University of Technology, 12, Powstancow Warszawy Avenue, 35-959 Rzeszow, Poland b

Research and Development Laboratory for Aerospace Materials, 4, Żwirki i Wigury Str., 35-

*

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036 Rzeszow, Poland

Corresponding author: e-mail: [email protected]

Abstract

Novel cylinder-shaped thermal insulation with variable thickness of wall along its height,

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called insulating module, which can be used several times, was successfully developed and then tested during manufacturing of equiaxed grain casting plates and airfoils made of IN

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713C Ni-based superalloy. In the casting plate produced without insulation cooling rate reached approx. 0.45 K/s and centerline shrinkage porosity was formed below threshold

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Niyama (Ny) value of 18 (Ks)0.5/cm. Application of insulating module or standard insulation blankets resulted in decrease of cooling rate to approx. 0.15 K/s and increase of Ny value to 30 (Ks)0.5/cm at half plate height compared with casting without insulation. For those

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solidification conditions, the maximum average area fraction of shrinkage porosity reached 0.41 and 0.14 % for the plate manufactured without insulation and with insulating module, respectively. It was found that secondary dendrite arm spacing (SDAS) depended on average

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cooling rate v and changed in casting according to determined relation

SDAS

 42.6v

 0.371

.

Increase of thermal conductivity from 0.23 to 2.5 W/mK resulted in unfavorable rise of

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cooling rate by approx. 60 % at casting height 88 mm. The gap between mold and inner surface of module could achieve up to 7 mm and did not cause increase of cooling rate. Comparison of results for different types of thermal insulations showed that the insulating module could successfully replace standard insulation blankets.

Keywords: Thermal insulating module, shell mold, Ni-based superalloy, solidification, shrinkage porosity. 1. Introduction 1

The investment casting method is used to produce ceramic shell molds needed for obtaining Ni-based superalloy castings. In this way, the complex thin-walled equiaxed grain castings such as turbine blades and vanes are mainly produced by pouring the molten metal into a preheated ceramic mold as presented by Pattnaik et al. (2012). During cooling of the mold and the solidification of liquid alloy, the equiaxed grains and undesirable defects form in the volume of casting. Torroba et al. (2014) found that shrinkage porosity in castings was a major problem during the manufacture of vanes. Roskosz et al. (2006) also studied the size and morphology of shrinkage porosity in the turbine blades. The pores mainly form inside the casting and therefore the X-ray method is required to detect and evaluate their fraction.

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Carlson and Beckermann (2009) reported that shrinkage porosity appeared in castings mainly as a result of poor feeding of interdendritic channels with liquid metal during the last stage of solidification. Niyama et al. (1982) established that an appropriate control of the heat flow and local solidification parameters in the casting significantly reduced the tendency to the

formation of shrinkage pores. Sigworth and Wang (1993) showed that continuous liquid metal

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feeding into the next volumes of interdendritic channels took place through tapered liquid

feeding channel due to axial temperature gradient and directional heat flow along the casting.

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Therefore the cooling rate and temperature gradient are the most often controlled with different techniques in selected volumes of equiaxed grain casting..

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The control of turbine blade solidification is very often carried out using a thermal insulation blanket. It is placed on the surface of the mold in its different areas in order to reduce the cooling rate of selected parts of casting. Torroba et al. (2014) and Pieczaba et al.

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(2010) showed how to apply the insulation blankets on the mold during the production of vanes and blades. Szeliga et al (2014) used a silicon carbide chill in the area of hot spot formation in order to eliminate shrinkage porosity by increasing the cooling rate in the

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selected areas of casting. Other methods of shrinkage porosity control were developed based on the directional solidification technique in which the G/R parameter (temperature

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gradient/solidification rate) was controlled along the casting height by withdrawing the filled mold at specific rate, from the heating to cooling zone of the furnace. Ferro and Shendye (1996) presented a thermally-controlled solidification (TCS) method for the production of large and complex geometries of Ni-based superalloy castings with equiaxed grain (disks and rings). Naik (2012) additionally increased the cooling rate of casting by directing the gas stream as a cooling medium on the surface of the mold. Zheng et al. (2014) and Jie et al. (2016) modified the TCS method into the interdendritic-melt solidification control (IMSC) technique and the modified thermally controlled solidification (MTCS). Song (2003) or 2

Czekaj and Karwinski (2014) conducted the rapid solidification of Al by immersing the filled mold into an oil bath or liquid polymer, respectively. In the water cooling-controlled solidification method, presented by Zhang et al. (2017), the filled mold was placed inside a tank, which was then gradually filled with water. Manufacturing techniques of Ni-based thin-walled equiaxed grain castings are being still developed. It should be emphasized that only some of the presented methods can be introduced in the production of nickel superalloy castings. The liquid cooling media have too low boiling point, so that they cannot be effectively used for cooling the mold filled with nickel superalloy. The TCS method and its further modifications (IMSC and MTCS) enable

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the production of large and complex castings. However, the time of preheating and keeping the poured mold in the furnace is longer compared with the process in which the thermal

insulation blankets are used. In turn, the application of insulation blankets to the mold surface is not only difficult to carry out but also time-consuming. In this technique, the mold is often wrapped with blankets, which are connected together using a high-temperature adhesive.

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Therefore, providing the same solidification conditions in following processes of casting is difficult, despite the use of similar parameters of manufacturing like pouring and mold

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temperature. This is often caused by uneven densification of insulation blanket during its installation on the mold surface. The shell molds with attached blankets are less recyclable

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and also cause environmental pollution.

The standard process of manufacturing of thin-walled Ni-based superalloy investment casting was improved in the present work. A novel insulating module for controlling the

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solidification of blade, instead of the insulation blankets, has been successfully developed in the paper. The control of solidification and thermal parameters in the casting was carried out by an appropriate selection of the shape and thickness of the module wall, relative to the

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shape of mold and the gating system. The insulating module can be applied many times depending on its shape and used material. It is mounted and dismounted on the mold very

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easily and quickly and can be used repeatedly. Based on the results of investigation, it was found that insulating module allows the effective control of solidification and can replace the standard insulation blankets as well as can shorten the manufacturing process of Ni based superalloy of the complex thin-walled equiaxed grain castings.

2. Methodology 2.1. Experimental procedures

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Casting of plates and blade airfoils made of nickel superalloy, were prepared using the investment casting method (Fig. 1). The temperature distribution, microstructure and shrinkage porosity in the castings manufactured with the use of different thermal insulation were determined. Geometric patterns of casting plates with dimensions 170x45x5 mm were designed. The airfoils of the blades of the same height as plates were also adopted in the investigations. The wax patterns of the pouring cup and five plates or airfoils were evenly distributed on the gating system and then were attached together in order to manufacture the wax model assembly (Fig. 2a and b). The ceramic tubes with outer diameter of 1.5 mm, were then introduced to the wax model to provide the protection of thermocouples against the

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direct influence of the liquid alloy. The wax assemblies were the basis for manufacturing 3 molds with plates (Fig. 2c) and 2 molds with airfoils (Fig. 2d). Thermal insulations in the

form of blankets or module were used to control the solidification of castings. Three layers of blanket with the same thickness of approx. 13 mm and various heights were applied on the

ceramic mold with the plates (Fig. 2e). The first and next layers of blankets (marked as 1, 2

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and 3) were placed at the distance of 27, 72 and 112 mm from the casting base, respectively. In turn, the same insulating module was mounted on the mold with casting plates and then it

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was used to manufacture the airfoils (Fig. 2 g and h). A cylinder-shaped insulating module with the inner diameter of 170 mm and height of 215 mm (Fig. 2f) had the shape similar to

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the applied insulation blankets. The wall thickness of the module was gradually reduced from 30 to 10 mm along the height. The module was made in the vacuum forming process from a mixture of fibers, organic and inorganic binders of chemical composition Al2O3 34 %, SiO2

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49 %, ZrO2 17 %. One mold with plates and one with airfoils were left without the thermal insulation (Fig. 2c and d). The protective tubes with thermocouples were connected to the mold, before applying the thermal insulation (Fig. 2c and d). Type S thermocouples with

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diameter 0.2 mm were used to temperature measurements at test points located at the symmetry axis of plate casting at the height of T4-14 mm, T3-49 mm, T2-88 mm and T1-132

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mm. The temperature in the airfoil was measured only in one point at distance 88 mm from the casting base. The obtained cooling curves were used for the verification of numerical simulation and the calculation of axial temperature gradient and cooling rate. The details of the calculation method are given in Appendix A.

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Fig.1. Scheme of furnace for production of Ni-based equiaxed castings using ceramic shell mold and insulating module

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The mold was introduced into the cooling chamber of the Bridgman vacuum furnace and was placed on the thermal insulating layer to prevent thermal influence of chill plate on

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the solidification of casting (Fig. 1). The chill plate with the mold was then moved to the heating area of the furnace and vacuum was pumped. The molds with the insulation blankets and the module were preheated to 1250 oC, while the mold without insulation was preheated

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to 1215 oC. Different temperatures of molds were assumed, due to the effect of thermal insulation, which caused the decrease of the mold temperature. In that way, a similar value of the mold temperature was obtained in all experiments performed. The preheated mold was

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filled with the liquid IN 713C nickel superalloy at 1480 oC. The amounts of 4.2 and 3.8 kg of the melt were poured into the molds with the plates and airfoils, respectively. The mold

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withdrawal from the heating to cooling area of the furnace started 10 s after filling was accomplished. Melting and solidification processes of the superalloy were carried out in

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vacuum.

Shrinkage porosity and dendritic microstructure were observed on 4 cross-sections

located at the distance of 14, 49, 88 and 132 mm from the plate or airfoil base. The dendritic microstructure was revealed by etching using the reagent with following chemical composition CH3COOH–33cm3 + HNO3–33cm3 + H2O–33cm3 + HF–1cm3. Secondary dendrite arm spacing (SDAS) was determined on each cross-section of casting, based on 15 measurements in its middle area, using the following relation:

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SDAS



L

n

(1)

 1

where: L is length and n is number of secondary dendrite branches. The shrinkage porosity in the castings was detected using the X-ray method and the microscopic examination. Radiographs were taken of the casting plates, which were cast into the molds without thermal insulation and equipped with the insulating module. The light optical microscopy and digital image analysis software were used to measure the area fraction of shrinkage porosity on 4 cross-sections of the plate. 30 individual images with shrinkage porosity were taken on each of them. The number and surface area of pores in the images were determined using Leica software. The total area fraction of shrinkage porosity was then calculated on each cross-

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section.

Fig. 2. Wax assembly with plates (a), blades (b) and ceramic molds with mounted thermocouples (c and d). Thermal insulation in the form of three blankets (e) or module (f-i)

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placed on ceramic mold with plate (g) and airfoils (h). Dimensions and shape of standard blankets (j) and module (k). T1-T4 are points of temperature measurement. 1-3 show layers of blankets 2.2. Numerical simulation The numerical simulation of solidification was carried out using ProCAST software in order to determine the solid fraction, average cooling rate, secondary dendrite arm spacing and values of Niyama in the casting plates manufactured without and with the use of insulating module. The three-dimensional geometric model of assembly with the casting plates and insulating module were designed (Fig. 3). The model of the inner surface of the

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furnace and thermal insulation (located on the chill plate), was additionally taken into account in the numerical simulations. The finite element mesh was generated on geometric models

and the 8 mm layer of ceramic shell mold was then created. The mesh size was 3, 5, 15, 10 and 50 mm for the casting plate, gating system, insulating plate, insulating module and

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enclosure, respectively. Thermophysical parameters of the casting, mold, insulating module and boundary conditions were assumed for the geometric models. Their values are given in

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Appendix B.

The temperature of mold and liquid alloy was assumed based on experimental temperature measurements. The molds were preheated to 1215 and 1250 oC for the process

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without and with the insulating module, respectively. In the conducted numerical simulations, the mold withdrawal from the heating chamber of the furnace to cooling one was neglected.

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Due to very short time of withdrawal of the filled mold and its high rate (approx. 2200 mm/min) the change of enclosure temperature of the mold was assumed in simulations. Therefore, after 10 s from filling the mold, the enclosure temperature changed from 1215 °C

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(mold without insulation) and 1250 °C (mold with insulating module) to 20 °C. The simulations of solidification were carried out to determine the effect of material and

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dimensions of insulating module on cooling rate of the plate (Table 1). Hence, the thermal conductivity, emissivity and internal diameter of the module were checked. The simulations of solidification of airfoils (experiment 2 and 5) and plates (experiment 3) were not carried out.

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Rys. 3. Finite element mesh: a) model assembly, b) mold, insulating plate and chill plate, c) insulating module and d) enclosure

insulation Shape of casting

Thermal conductivity of mold and module, W/mK

Emissivity of insulation

1 (E, S) 2 (E) 3 (E) 4 (E, S) 5 (E) 6 (S) 7 (S) 8 (S) 9 (S) 10 (S)

Plate Airfoil Plate Plate Airfoil Plate Plate Plate Plate Plate

Table B2 Table B2 Table B2 Table B2 and B3 Table B2 and B3 Table B2 and 0.8 Table B2 and 2.5 Table B2, Table B3 Table B2, Table B3 Table B2, Table B3

0.6 0.6 0.6 0.6 0.6 0.9 0.6 0.6

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Type of insulation Without Without Blanket Module Module Module Module Module Module Module

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Inner dimension of blankets or module, mm 156 170 170 170 170 170 160 210

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Designation of experiment (E) and numerical simulation (S)

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Table 1. Parameters of numerical simulations (S) and experiments (E) for various types of

3. Results and Discussion

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3.1. Thermal analysis and validation of numerical simulation

The experimental and numerical thermal analyses were carried out to determine the

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solidification process of plates and airfoils, made without and with the use of various types of thermal insulation, applied on the mold. The established temperature distribution was the basis for the verification of boundary conditions and thermophysical parameters of materials. It was also applied to calculate the cooling rate and the axial temperature gradient in the castings (see appendix A). The predicted cooling curves were determined in the measurement points, based on the results of temperature field in the solidifying casting (Fig. 4). The simulated cooling curves, values of cooling rate and axial temperature gradient were then compared with their measured values, confirming good agreement (Figs. 5 and 6). 8

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Fig. 4. Temperature distribution on the surface, longitudinal section of castings and molds with (a, b) and without (c, d) insulating module in dependence on time

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The cooling curve analysis allowed better understanding of the process of pouring the metal into the mold and solidification of castings (Fig. 5). Filling the mold cavity with molten

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metal at 1480 oC resulted in an intensive increase of temperature measured by thermocouples. It was found that the melt had various temperatures at point 1 in all the experiments carried out, despite the same pouring temperature. The highest temperatures of melt was approx.

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1440, 1380 and 1410 oC after 3s for the mold with the insulating module, insulation blanket and without insulation, respectively (Fig. 5 a-c). The melt temperature was approx. 1335 °C in the cavity of airfoil area at point 2, and was significantly lower compared with its values

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inside the plate area of the mold (Fig. 5 d). Such a large difference of the melt temperatures could be caused by the duration of filling process, the deflection rate of crucible, the method

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of directing the melt stream into the mold and casting thickness. 10 s after the cavity was filled with the melt, the mold was withdrawn from the heating to cooling area of the furnace.

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Then, the temperature of the melt significantly decreased until the liquidus temperature was reached and the solidification started. A very similar shape of cooling curves was found in the castings manufactured with the use of insulating module and blankets mounted on the ceramic mold (Fig. 5 b and c).

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Fig. 5. Temperature distribution in plate (a-c) and airfoil (d). Castings were manufactured

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without (a, d) and with insulating module (b, d) and blankets (c). Comparison of experimental and simulated cooling curves at points located at distance 1-14 mm, 2-49 mm, 3-88 mm and

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4-132 mm from the casting base (a, b)

Fig. 6. Experimental (M), simulated (S) cooling rate (a) and axial temperature gradient (b) along plate height. Cooling rate in the airfoil was determined at height 88 mm (a)

The predicted and measured temperature distributions were used for establishing the average cooling rate of casting during its solidification (Fig. 6 a). For the mold without

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thermal insulation, the cooling rate reached a similar value along the whole height in the area located close to the plate symmetry axis (Fig. 6 a). However, it significantly increased in the external edge areas, compared with the symmetry axis area (Fig. 7 a). The use of thermal insulation of various thicknesses along the mold height caused a gradual change of cooling rate (Fig. 6 a). It was found that it attained the largest value in the bottom part of the plate and then decreased along the casting height towards the gating system. Near the edge areas, the cooling rate reached a similar value to that in the symmetry axis area (Fig. 7 b) in contrast to the casting without insulation. Such a cooling rate distribution in the casting promoted the formation of favorable directional solidification in the plate. The measured cooling rate was

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also determined in the airfoil at the distance of 88 mm from its base (Fig. 6 a). It was found that it increased almost twice in the airfoil compared with the plate, both for the mold without and with the insulating module.

The temperature distribution and cooling curves allowed establishing the axial

temperature gradient at temperature Tc=1248.5 oC (see appendix A). The actual temperature

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gradient was determined in test points 2 and 3, while its predicted values were calculated along the axis of plate casting (Fig. 6 b).

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It was found that the axial temperature gradient gained the highest values along the height of the casting manufactured with the use of insulating module, compared with the

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process without thermal insulation (Figs. 6 b, 7 c and d). It reached the largest value in the bottom part of the plate and then gradually decreased along its height. It increased in the upper part of casting, in the contact area of plate and the gating system. For the mold without

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the insulation, the predicted axial temperature gradient changed to a greater extent than for the

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mold with insulating module.

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Fig. 7. Cooling rate (a, b) and axial temperature gradient (c, d) distribution on surface of plate

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manufactured without (a, c) and with (b, d) insulating module

The predicted solidification process of the plate was determined, based on solid

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fraction distribution (Fig. 8). It was found that the castings manufactured without thermal insulation solidified in a different way and in a shorter time, than the ones with the insulating

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module. In the standard case, the edges of the plate solidified first (Fig. 8 a-e). Then, the solidification proceeded from the area of edges to the direction of symmetry axis. However, in the casting manufactured with the use of insulating module, the solidification was better

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controlled and continued in the favorable, directional way (Fig. 8 f-j). It was found, that the bottom part of the casting solidified at the beginning, and then the next volumes followed towards the gating system. The insulating module also provided a smaller curvature of the

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solid fraction field in both, the edge and the axial areas of casting (Fig. 8 f-j), compared with

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the process without thermal insulation.

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insulating module (f-j) in dependence on time

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Fig. 8. Solid fraction field at casting surface produced without (a-e) and with the use of

3.2 . Microstructure

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The dendritic microstructure was observed along the plate and airfoil height, manufactured without thermal insulation and with the insulating module (Fig. 9). The microstructure characterization was carried out by the measurement of secondary dendrite

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arm spacing (SDAS) (Fig. 10 a). It was observed that SDAS changed in a small range along the plate height, and its average value was approx. 58 m, for the casting manufactured without insulation. The application of insulating blankets or the module with varying

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thickness for the control of solidification also resulted in the change of SDAS along the plate height. It was found that the SDAS had a similar value at the bottom part of the casting,

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independently of the production method of the casting. The SDAS gradually increased with increasing of both, the distance from the casting base and the thickness of thermal insulation blanket and the module wall. It was observed that the SDAS in the casting manufactured with insulating module had

similar values to the results obtained with the blankets. The insulating module was also applied to manufacture the airfoil castings (Fig. 10). A similar change of SDAS was observed along the height in both airfoil and plates. However, in the airfoil, those values were smaller (approx. 20 m and 10 m), compared with the plate manufactured with the use of insulating 13

module and without insulation, respectively (Fig. 10). The relationship between dendritic microstructure, insulation thickness and cooling rate was found. The increase of insulation thickness led to the rise of thermal resistance and decrease of heat extraction from the surface of mold to its ambience. Therefore, the cooling rate of the casting was also lower when the insulation thickness increased. Secondary dendrite arm spacing is often defined as the dependence on average cooling rate v according to equation: SDAS

 av

n

(2)

where: a and n are material constants determined experimentally. In order to describe the

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dendritic microstructure more precisely, SDAS in dependence on average cooling rate v of airfoil and blade was assessed (Fig. 11). The experimental results were fitted in equation 2 and the material constants a = 42.6 and n = -0.371 were then calculated for the IN713C

superalloy. According to the obtained results, SDAS decreased with increasing cooling rate of

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the casting (Fig. 11).

Fig. 9. Dendritic microstructure on the cross-section of plate (a-c) and airfoil (d, e) at distance

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of 88 mm from its base, which were manufactured without thermal insulation (a, d) and using insulating blankets (b) and modules (c, e)

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Fig.10. Secondary dendrite arm spacing (SDAS) along the height of casting plate and airfoil

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manufactured with and without blankets as well as insulating module

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Fig. 11. Secondary dendrite arm spacing in dependence on cooling rate

The experimentally obtained material constants of a and n were applied to establish the predicted distribution of SDAS, along the plate height, using the mapping factor procedure

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of ProCAST software: M  aR G v b

c

n

(3)

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where: a, b, c, n are constants and R, G, v are solidification rate, temperature gradient and cooling rate, respectively. The constant values of a = 42.6, b = 0, c = 0 and n = -0.371 were used to obtain the relationship described with equation 2. Good agreement was found between the predicted and measured value of SDAS (Fig. 12 a). Its distribution in near the edge area achieved a similar value as in the middle part of the plate manufactured with the insulating module (Fig. 12 c). However, the application of the module brought about the inhomogeneous SDAS distribution along the plate height caused by a change of cooling rate (Fig. 12 a, c). Moreover, such a dendritic microstructure was the result of directional solidification required 15

to feed various volumes of the casting and it prevented the formation of disadvantageous shrinkage defects.

using mold with (c) and without (b) insulating module

3.3. Shrinkage porosity formation

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Fig. 12. Predicted (S) and measured (M) value (a) of PDAS along height of plate produced

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The aim of this study was the development of the method to control both, the

solidification and shrinkage porosity formation, along the casting height, using a novel thermal insulating module that could be applied again in the next process. The analysis of

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shrinkage porosity was carried out in order to confirm the effective reduction of pore size in the castings by applying the insulating module (Figs. 13 and 14). The total area fraction of

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shrinkage porosity was established on the surface of cross-sections 1-4 of the plate. Fig. 13 shows a typical arrangement of pores on the cross-section at the distance of 88 mm from the base of plate which was manufactured without (Fig. 13 a, d) and with the use of blankets (Fig.

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13 b, e) and the insulating module (Fig. 13 c, f). It was found that the largest pores were mainly located in the middle area of the cross-section of the plate manufactured without

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thermal insulation (Fig. 13 a, d). For the casting manufactured using the blankets and insulating module, the size of pores was the smallest and they were uniformly distributed at

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the whole cross-section (Fig. 13 b, c, e, f).

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Fig.13. Example of shrinkage porosity on the cross-section located at distance 88 mm from

the base of plate manufactured: a, d) without thermal insulation; b, e) with blankets; c, f) with

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insulating module

Fig. 14. Example of shrinkage porosity on the cross-section placed at distance 88 mm from the base of airfoil produced without (a, d, e) and with (b, c) insulating module

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The analysis of pores was also carried out on the cross-sections of airfoils manufactured without and with the insulating module (Fig. 14). It was observed that the highest shrinkage porosity occurred for the process without insulating module, mainly in the area of cross-section, where the airfoil thickness was the largest (Fig. 14 a, d). The application of the same insulating module as in the previous experiment with the plate resulted in a change of solidification conditions of the airfoil. The pores were favorably and uniformly distributed on the cross-section area of the airfoil, similarly as it happened in the plate (Fig. 14

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b, c).

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Fig. 15. Average area fraction of shrinkage porosity at cross-section located at various distances from base of plate manufactured without thermal insulation and using blankets or

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insulating module

The average area fraction of shrinkage porosity was determined on 1-4 cross-sections

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of plates to analyze the shrinkage porosity formation along the casting height (Fig. 15). It was found to be the smallest in the bottom part of plate (cross-section 4) manufactured without and with the thermal insulation (the blankets or the module). Then, it increased intensively

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with increasing distance from the base of casting manufactured without the insulation. On the remaining cross-sections of casting it changed a little for the process with the use of module

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or blankets. In addition, the castings manufactured with the insulating module revealed smaller shrinkage porosity, compared with the castings produced using the standard blankets. It was established that the area fraction of pores increased, especially in the upper part of the casting. The reason for this phenomenon may be a slight change in the local solidification parameters along the casting height. According to Ny criterion (equation 4), the shrinkage porosity increases when temperature gradient and the cooling rate grow. Figure 11 shows that SDAS decreases with increasing cooling rate. It was found that SDAS reaches slightly smaller values at plate height 132 mm than at 88 mm for the mold with the insulation blankets. Hence, 18

the cooling rate also arrives at a similar value at the analyzed height. However, SDAS and cooling rate change to a larger extent in the casting with insulating module. Also, the calculated axial temperature gradient is greater (G = 21.1 cm) than in the casting with insulation blankets (G =17.5 K/cm) at the casting height of 88 mm (Fig. 6b). Hence, for the process with the insulating module, the Ny value increase results in the reduction of shrinkage porosity at casting height 88 and 132 mm, compared with the casting manufactured using insulation blankets. The predicted area of shrinkage porosity in the plate (Fig. 16) was calculated using the well-known Niyama criterion: (4)

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Ny  G /

where G is temperature gradient and v is cooling rate determined at Tc temperature near the end of solidification. Niyama et al. (1982) used mainly that criterion in order to assess

shrinkage porosity in cast-steel cylinders. Carlson et al. (2005) also successfully predicted

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shrinkage porosity formation in Ni-based superalloy castings according to that criterion. Guo et al. (2015) reported that the shrinkage porosity was formed below some critical threshold of Ny value, which was determined by comparing the simulated distribution of those results with

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the actual location of shrinkage porosity in the casting. In this study the plates were radiographed in order to detect the location of shrinkage porosity. Hence, the correlation

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between the distribution of Ny and the location of shrinkage porosity was carried out for the casting plate manufactured with and without the insulation module (Fig. 16). The sample radiograph and the Ny distribution showed that the centerline shrinkage porosity occurred

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mainly along the band located at the symmetry axis of plate for the process without the module (Fig. 16 a-c). Such a similar distribution of shrinkage porosity was also observed in

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the microscopic examinations (Fig. 13 a). The largest pores were mainly located in the middle area of the cross-section of the plate. The Ny value increased intensively, while the shrinkage porosity decreased with the rise of distance from the symmetry axis towards the edge area

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(Figs. 13 a and 16 a-c). The approx. threshold value of Ny was determined to be18 (Ks)0.5/cm, below which the centerline shrinkage porosity could occur. Unfortunately, the criterion did not provide information about the volume fraction of shrinkage porosity and amount of pores. However, if the Ny value decreased, the shrinkage porosity became larger. In the casting manufactured with the use of insulating module, the average Ny was significantly higher than in the plate without thermal insulation (Fig. 16 e). The smallest Ny value reached approx. Ny=25 (Ks)0.5/cm at the distance of 88 mm from the casting base and was higher than the

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critical threshold value. The analysis of X-ray images (Fig. 16 d) and the optical micrograph of the cross-section (Fig. 13 c) revealed that large pores and centerline shrinkage porosity did

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not occur at that distance.

Fig. 16. Predicted distribution of Ny value (c, e) and X-ray images (a, b, d) for castings

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manufactured with (d, e) and without insulating module (a-c)

The distribution of Ny value (Fig. 16) and solid fraction (Fig. 8) as well as cooling rate

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(Fig. 7 a, b) were used for establishing the way of plate solidification and the shrinkage porosity formation. The lower part of the casting located close to its base, solidified first. In

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that area, the cooling rate was the highest and similar for all the castings. In the casting without the module, the cooling rate reached the similar value along the height (Fig. 6 a). Based on the numerical simulation results, it was found that the areas near

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the plate edges, at half its plate height, solidified first and then the mushy zone moved towards the casting axis. It induced radial dendritic growth and produced an equiaxed grain structure. For that stage of solidification, sufficient liquid metal feeding into interdendritic

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channels from the area near the symmetry axis, was maintained. It was established that the smallest pores were mainly located in the nearly external edge areas of casting where the

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value of Ny was approx. 40 (Ks)0.5/cm (Fig. 13a, 16c). According to Plancher et al. (2019) such small pores could be formed between the secondary dendrite arms as a result of arms growth and coalescence during the early stages of solidification. During the following solidification process, the mushy zone continuously moved from the left and right side to the middle area of the plate. Finally, the residual liquid metal solidified in the symmetry axis area of the plate, where Ny reached the smallest value of approx. 4 (Ks)0.5/cm. When the temperature of liquid metal became lower than the liquidus along that area, the primary

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dendrites nucleated and then their arms grew and the interdendritic channels appeared. At the last stage of solidification, liquid metal feeding through them down into the isolated liquid region stopped. If this region becomes totally surrounded by the solid, shrinkage porosity will form in the interdendritic space during cooling of the liquid metal, as presented Zheng et al. (2014). In the present study the uniform cooling rate occurred along the symmetry axis area (Fig. 6a) of casting without insulation. Hence, the presented manner of cooling and solidification without feeding, promoted the centerline shrinkage porosity formation along the symmetry axis of the plate. The application of insulating module with variable thickness resulted in a significant

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change of solidification conditions in the plate. Based on the performed analysis of cooling rate, SDAS and the shape of solid fraction contour it was concluded that the directional

solidification occurred along the plate height in the direction of gating system. The high axial temperature gradient and continuous movement of the mushy zone along the casting height

provided successive feeding the next volumes of interdendritic channels by the liquid metal.

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Those solidification conditions resulted in a significant decrease of shrinkage porosity and caused the removal of large pores in the middle part of casting.

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During solidification of a long casting, the solid fraction contours create a tapered liquid feeding channel characterized by the angle, as reported by Sigworth and Wang (1993).

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When this angle is smaller than a certain critical one, the isolated liquid region and then shrinkage porosity are formed in the interdendritic regions during the last stages of solidification. Whitesell and Overfelt (2001) claimed that centerline shrinkage porosity

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decreased when the angle and axial temperature gradient increased. In this study, the amount of shrinkage porosity was different along the casting height, because the changing

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solidification conditions resulted in uneven solidification.

3.4. Effect of thermophysical parameters and geometry of module on solidification of

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casting

The insulating module must meet appropriate both, the construction and material

requirements, in order to provide the proper solidification control of Ni-based superalloy castings. The microstructure and shrinkage porosity formation depend on cooling rate of casting as described earlier. Therefore, the influence of selected thermophysical parameters like thermal conductivity and emissivity and the inner module diameter on average cooling rate along the axis symmetry of plate was examined (Fig. 17).

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Fig. 17. Influence of thermal conductivity and emissivity as well as inner diameter of

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insulating module on average cooling rate along plate height

The cooling rate was determined assuming the thermal conductivity of the module equal to 0.8 and 2.5 W/mK. It was found that thermal conductivity resulted in the highest

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change of cooling rate. Its value increased when the thermal conductivity grew, especially for the distance range of approx. 50 to 170 mm from the plate base, relative to the results obtained for the experiment with the insulating module (Fig. 17). However, the increase of

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emissivity of module surface from  = 0.6 to  = 0.9 did not cause noticeable rise of cooling rate. Szeliga et al (2017) stated that the heat flow from the casting to its surrounding depended

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mainly on the layer characterized by the lowest heat transfer coefficient. For the mold with the module, the summary heat transfer coefficient hs can be calculated according to equation: hs=hc+hc-m+hm+hsm+hsmd1+hmd+hsmd2

(5)

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where, heat transfer coefficient consists of the coefficients of casting hc, casting-mold hc-m, mold hm, surface of mold hsm, inner surface of module hsmd1, wall of module hmd and external

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surface of module hsmd2.

For the module of thickness l=30 mm and average value of thermal conductivity =0.23 W/mK (Table B3) the heat transfer coefficient equaled to hmd=l =7.7 W/m2K, while

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its value for the mold was hm=225 W/m2K assuming =1.8 W/mK and l =8 mm. Therefore, the heat transfer coefficient was significantly smaller for the module in comparison with the mold, and its value would mainly affect the cooling rate of the casting in the process with the use of insulating module (Fig. 17). The emissivity of module and casting-mold heat transfer coefficient changed the cooling rate to a lesser extent, which was also confirmed by Szeliga et al. (2017) for a standard manufacturing process of casting.

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It was found that the gap between the inner surface of module and ceramic mold also had a strong impact on cooling rate and control of solidification. The increase of internal diameter of insulating module from d = 170 to 210 mm led to the increase of cooling rate to a similar value as for  = 0.8 W/mK. In the next simulation, the inner diameter was reduced to the smallest value equal to d =160 mm, which was induced by the outer shape of the mold. The cooling rate for the smallest gap thickness obtained in that manner slightly increased. It was found that the gap between the mold and the inner surface of module could be even up to 7 mm thickness and it would not affect unfavorably the directional solidification of the casting. The possibility of maintaining such large and various gap thicknesses has practical

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significance. Thus, the manufacture of insulating module and its adjustment to the shape of mold with changing wall thickness will be simplified, during the preparation process of manufacturing complex castings of nickel superalloys.

4. Conclusions

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Based on the analysis of research results, following conclusions were drawn:

1. The solidification parameters and shrinkage porosity along the height of casting with

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insulating module, reached very similar values to those with standard insulating blankets. Therefore, insulating modules can be successfully used repeatedly to control both,

attached to the mold surface.

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solidification and shrinkage porosity formation, instead of standard insulating blankets

2. The SDAS in dependence on average cooling rate of airfoil and plate was determined SDAS

 42.6v

 0.371

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according to equation

for the IN713C Ni-based superalloy. SDAS values

were smaller (approx. 20 m and 10 m) in the airfoil, compared with the plates

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manufactured with and without the use of insulating module, respectively. 3. The largest average area fraction of shrinkage porosity was 0.41 %, 0.21 % and 0.14 % at the cross-section of the plate manufactured without insulation, with blankets and insulating

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module, respectively. The approx. threshold value of Ny was 18 (Ks)0.5/cm, below which the centerline shrinkage porosity could occur in the casting without insulation. 4. The thermal conductivity of module resulted in the highest change of cooling rate of plate, while the module surface emissivity did not cause noticeable effect on cooling rate. Increase of thermal conductivity from 0.23 W/mK (temperature 1200 oC) to 2.5 W/mK caused rise of cooling rate approximately by 60% at casting height 85 mm. The gap between mold and

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inner surface of module could achieve up to 7 mm thickness and it did not unfavorably affect solidification of casting.

Appendix A. Calculation of average cooling rate and axial temperature gradient The obtained cooling curves were the basis for the determination of the average cooling rate between the temperature of liquidus 1325 oC and that of the solidus 1240 oC (Fig. A1a) using equation: v 

TL  TS tL  tS

(A1)

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where: TL and TS are temperatures of liquidus and solidus, respectively; tL and tS are times required to reach the liquidus and solidus temperature at test point, respectively. Axial

temperature gradient Gz at point zi, of the casting and for critical temperature TC=1248.5 oC (Fig. A1b) was calculated from equation:

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 dT  Gz     dz  z  z i

(A2)

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The temperature values between points zi+1, zi, zi-1 (Fig. A1) determined by fitting second-order

T(z)  a 2 z  a 1 z  a 0 2

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polynomial for time ti, when the point zi reaches critical temperature TC=1248.5 oC: (A3)

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where: a2, a1, a0 are polynomial coefficients, z is distance between points zi+1, zi, zi-1. TC temperature was calculated based on dependence: (A4)

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T C  T S  (T L  T S )  0 . 1

The distribution of predicted temperature gradient at the critical temperature and average

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cooling rate, were also calculated with the use of the ProCAST software.

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Fig. A1. Scheme of determination of: a) average cooling rate, b) axial temperature gradient based on cooling curves, where: Gz is axial temperature gradient at the point zi, TL and TS are liquidus 1325oC and solidus temperature 1240oC, tL and tS are times required to reach the liquidus and solidus temperature at test point, TC is critical temperature at height zi, Ti+1 and Ti1 are

temperature at height zi+1 and zi-1, a2, a1, a0 are polynomial coefficients, z is distance

between points zi+1, zi, zi-1,

Appendix B. Thermophysical parameters of Ni-based superalloy, ceramic mold, insulating module and boundary conditions

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Thermophysical parameters were assumed for geometric models. Density and specific heat were assumed for the castings made of IN 713C Ni-based superalloy, according to Wang and Overfelt (2001) works (Table B1). The temperature of both, liquidus 1325 °C and solidus 1240 °C as well as the solid fraction for the alloy was determined experimentally. The values of specific heat, thermal conductivity and density for the ceramic mold were adopted from the

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data specified by Xu et al. (2016) (Table B2). The thermophysical parameters of insulating module and insulating plate placed on the chill plate are shown in Table B3. Suitable

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boundary conditions were then assumed for the geometric models. Szeliga et al. (2014) widely described the boundary conditions used in the numerical simulation of solidification of

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Ni-based superalloy castings. Casting-mold interface heat transfer coefficient (IHTC), was assumed in dependence on temperature on the basis of performed calculations conducted by Szeliga et al. (2017). The chill plate-insulation IHTC equaled to 20 W/m2K. The emissivity of

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mold surface, depending on temperature, was assumed after Szeliga et al. (2014), while it was 0.6 for the insulating module.

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Table B1. Thermophysical parameters of IN713C Ni-based superalloy Thermal conductivity W/mK 9.37 13.4 18.4 18.6 24.3 26.4 30.5 29 31 32

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Temperature, o C 24 200 500 649 982 1093 1290 1320 1400 1450

Temperature, o C 200 600 1000 1070 1100 1115 1130 1150 1215 1235 1245 1250

Specific heat, kJ/kg K 0.47 0.58 0.90 1.03 1.19 1.43 1.00 0.78 0.86 1.07 1.38 1.75

Temperature, o C

Solid fraction

Temperature, o C

Density, kg/m3

1240 1247 1253 1260 1266 1272 1276 1281 1285 1289 1294 1298

1 0.999 0.996 0.990 0.981 0.968 0.946 0.913 0.861 0.814 0.762 0.714

20 100 200 300 400 500 600 700 800 900 1000 1100

7905.2 7883.6 7853.3 7822.4 7790.6 7757.7 7722.6 7684.4 7641.9 7593.6 7537.1 7468.0

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1260 1265 1270 1275 1285 1290 1295 1300 1305 1310 1320 1330 1420

3.17 4.05 4.61 4.67 4.86 5.21 5.38 5.38 5.17 3.78 0.84 0.83 0.85

1302 1306 1311 1315 1317 1319 1321 1324 1325

0.658 0.585 0.491 0.372 0.302 0.226 0.139 0.048 0

1200 1245 1325 1500

7390.2 7324.8 7023.5 6972.8

Table B2. Thermophysical parameters of ceramic shell mold

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Specific Temperature, heat, °C kJ/kgK 200 0.75 400 0.9 600 1.1 800 1.4 1000 1.6 1200 1.9

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Density, kg/m3

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20 299 600 800 1200

Thermal conductivity W/mK 0.5 0.65 0.85 1.1 1.8

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Temperature, °C

Table B3. Thermophysical parameters of insulating module Specific heat, kJ/kgK

330

1.13

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Density, kg/m3

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200 400 600 800 1000 1200

Thermal conductivity W/mK 0.08 0.09 0.11 0.16 0.19 0.23

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Temperature, °C

Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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