Numerical Simulation of the Formation Conditions of Castings with Exothermic Sleeves

INTRODUCTION

A basic condition for ensuring the soundness of a casting is to create the required solidification sequence of its different parts, ensuring reliable feeding and directing shrinkage defects to removable parts. The allocation of sufficiently sized risers with rational design is a crucial part of the technological solution. The task of economizing metal in the casting, while maintaining or increasing feeding efficiency, can be achieved by using materials that insulate the riser location area or transfer heat to it.

The use of casting process simulation systems is a standard modern practice for the virtual diagnosis of the technology used, allowing verification and correction of riser dimensions and design. One popular software in this field, distinguished by the high accuracy of its shrinkage model, is "PoligonSoft."

When modeling the manufacturing conditions of a casting in a mold containing special thermal insulation parts, it is sufficient to specify the necessary thermophysical properties in the corresponding volume of the 3D model to account for their effect. However, in simulations involving exothermic materials (thermite, which releases heat), a special model is required that includes a description of the reaction initiation conditions during the thermal interaction of the material with the liquid metal and the dynamics of its progress with active heat emission [1−4].

Practice of Using Exothermic Materials

Currently, it is common for exothermic inserts in molds (exothermic sleeves, riser caps, blankets, etc.) not to be manufactured in-house but to be purchased in finished form from specialized manufacturers and suppliers. To meet consumer needs, a wide range of these inserts is offered with various sizes and shapes [5−7]. Modern foundries in Russia and abroad actively use exothermic materials, primarily for the production of steel and iron castings.

The use of exothermic materials may be driven not only by economic reasons and maximizing the riser's operational duration. In some cases, installing exothermic sleeves may be the only option to ensure the integrity of the fed area of the casting due to the limited space available for riser placement, as determined by the design.

Exothermic sleeves (see Fig. 1) can be equipped with insulating sand inserts connected at the bottom, which are used to form a narrowed metal section, the riser neck. This reduces the complexity of separating the riser from the casting and decreases the amount of machining required in the corresponding area. If the neck's width, formed by the central channel of the insulating insert, is chosen correctly, the supply of liquid metal continues throughout the riser's operational time. Premature solidification of the riser neck does not occur due to its intense heating and low heat dissipation from the insulating insert.

In most cases, exothermic inserts are made from a mixture of aluminum powder or shavings and iron oxide, bonded with a binder. Upon contact with molten metal, this material ignites at relatively low temperatures, in the range of several hundred degrees [8, 9]. The oxidation reaction is accompanied by intense heat release (Q): Fe2O3 + 2Al → Al2O3 + 2Fe + Q. During the relatively short duration of the reaction within the insert's volume, the released heat heats the riser, creating a heated zone around it that includes the area filled with reaction products and the adjacent layers of the mold. In most cases, the insert maintains its integrity after the reaction is completed, allowing it to function as a porous insulating material and facilitating measures to prevent contamination of the return mix.

Fig. 1. Production of identical castings using risers with comparable feeding efficiency:
‍1 — Heated, reduced-size riser; 2 — Traditional riser.

Mathematical Model of Exothermic Mixture Functionality

The model for the behavior of exothermic inserts during the solidification of castings, implemented in the latest version of "PoligonSoft" 2024.0, replaced the previously used simplified model, which was based on the assumption that the temperature of the relevant section of the mold remained constant during the thermite reaction.

The new functionality is based on an adapted model in which the insert material is endowed with a set of effective thermophysical properties, including the enthalpy of transformation (amount of heat released). This approach allows for a sufficiently accurate simulation of the thermal behavior of the material without needing to calculate the interaction reaction development of the exothermic mixture components. The effective thermal properties of the material include a "correction" to account for phenomena beyond the normal conditions for solving Fourier's equation and are also tied to the specific model and size of the insert or its group.

The Fourier equation to be solved within the volume of the heated exothermic insert has the following general form:

where t is the temperature; τ is time; C(t), ρ(t), and λ(t) are the temperature-dependent values of heat capacity, density, and thermal conductivity of the exothermic mixture, respectively; I is the heat source function, which is expressed as:

where m is the fraction of transformed material, varying from 0 to 1 over the total reaction time τ_ign and L is the specific thermal effect of the transformation.

Initially, I=0. The activation moment of the heat source in a specific section of the insert is determined when the ignition temperature τ_ign​ is reached, which starts the reaction time counter τ_r (from 0 to τ_ign ​) and the fraction of transformed material. Heat release stops when m=1, and the mixture in that section is considered "consumed."

The initial data for solving equations (1) and (2) are the preset properties of the insert: C(t), ρ(t), λ(t), L, and m=m(τ_r) ; m is defined in the range [0, 1] as a time-dependent function of the reaction τ_r within the interval [0, τ_ign​].

At the contact points of the insert with the metal, mold, and surrounding environment, third-type boundary conditions are applied. The heat transfer coefficients at the boundaries with the metal and mold can also be individually defined special data, due to the specific surface characteristics of the insert.

The traditional approach in competing programs does not allow for separate input of the exothermic mixture's thermophysical properties after the oxidation reaction's completion; it commonly uses averaged constant property values. This assumption is questionable due to the significant transformation of the mixture's structure and its changing thermophysical properties, potentially reducing the accuracy of solidification calculations for large castings with massive risers, whose operation continues long after the sleeve's "combustion." The model implemented in the PoligonSoft software allows for the flexible accounting of the mixture's state during and after the thermal reaction.

The task of obtaining the properties of exothermic inserts for solidification calculations has been resolved by including a corresponding database in PoligonSoft that contains over 30 designations. Selecting appropriate sleeve properties is facilitated by the presence of analogs among products made by different manufacturers. The database can be expanded with user data extracted from bibliographic sources or from the results of experimental property determination [10-12].

Examples of Calculations with the New Model

The functionality and adequacy of the implemented model can be demonstrated by conducting a series of computational experiments with identical small steel ingots, equipped with risers of different designs (see Figure 2). The mass of the ingot after separating the riser is approximately 6 kg, the average diameter is 93 mm, and the height is 120 mm.

Fig. 2. Finite element models of ingots with risers of different designs:
‍a — Traditional; b — With exothermic sleeve; c — Same as b, without heating.

The traditional method for calculating the size of the riser is related to the preliminary determination of the thermal modulus of the casting M_c​, which is the ratio of its volume V_c to the cooled surface area F_c​. The thermal modulus of the riser M_r​ (volume V_r​ in relation to the area F_r) should be 15 to 25% greater than the modulus of the fed part. This method is necessary for selecting the exothermic sleeve from manufacturers' catalogs. These catalogs, along with the product size characteristics, indicate the effective thermal modulus that can be achieved by installing the sleeve of a specific model. Assigning a riser with an exothermic sleeve (see Fig. 2b) is equivalent to installing a traditional riser with the same thermal modulus (see Fig. 2a). The series of calculations also includes a variant with a riser design identical to the one formed with an exothermic sleeve but without the sleeve itself (see Fig. 2c). The data on the materials used, the geometric characteristics of the ingot, and the risers are presented in Table 1.

Table 1. Characteristics of the conditions for producing the samples

The thermophysical characteristics of the mold materials and the properties of the exothermic sleeve (see Fig. 3), corresponding to the manufacturer's data, have been imported from the PoligonSoft database.

Fig. 3. Thermal and exothermic properties of the insert presented in the material properties editor of PoligonSoft.

The comparison of the results of the calculation of the thermo-phase fields during solidification and the distribution of shrinkage in the castings is shown in Fig. 4. To clearly illustrate the solidification conditions in the casting, a visualization mode has been activated for areas where the volume of the liquid phase exceeds the isosurface level of 50%. This visualization method is useful for identifying hot spots, determining the thermal causes of shrinkage formation, etc.

In variants "a" and "b," the liquid phase concentration zone has a pronounced conical shape with an upward widening, indicating a sufficient metal reserve in the risers to feed the lower sections of the ingot. In variant "c," the riser does not contain a sufficient reserve of liquid metal and solidifies before the ingot, leading to the formation of a distinct hot spot. The shrinkage forecast is consistent: with a traditional or heated riser, the shrinkage is fully contained within them, while with a reduced unheated riser, the ingot exhibits numerous defects. Additionally, it is important to note the efficiency of using the riser's volume to compensate for shrinkage. A large part of the heated riser contains shrinkage defects, whereas the traditional riser, nearly twice the volume, is occupied by these defects only by one-third.

Fig. 4. Forecast of the resulting shrinkage (top) and characteristic distribution of temperatures and liquid phase fraction 8 minutes after pouring (bottom) for ingots with risers according to variants "a," "b," and "c".

The calculation results showed that the sleeve complies with the equivalent thermal modulus M_r declared by the manufacturer and provides good reproducibility of the expected result in practice with the new exothermic material behavior model implemented in PoligonSoft.

Fig. 5 shows some details of a project analyzing the casting technology for the "Pump Casing" made of 10Х18Н10Т steel in a sand-resin mixture mold. The liquid metal mass for the part is 350 kg, with an average wall thickness of 30-40 mm and a maximum dimension of 730 mm. To effectively feed the hot spots, risers with exothermic sleeves were installed. The installation of ring-shaped cast iron chills is intended to prevent the formation of hot spots in the lower areas of the casting with difficult feeding conditions. Calculations in PoligonSoft indicated that using the selected exothermic sleeves provides sufficient liquid metal retention time in the risers (see Fig. 5b), contributing to a reliable compensation of hot spots located beneath them, although the cooling scheme in one area required adjustments.

Fig. 5. Numerical analysis of the solidification conditions for the "Pump Casing" casting:
‍Top — Finite element model of the casting in a sand mold (hidden) with chills and riser sleeves (general view and section);
Bottom — Distribution of the liquid phase and temperatures in the casting, 10 minutes after pouring.

Conclusion

The exothermic material behavior model implemented in "PoligonSoft" allows for calculations with satisfactory accuracy for casting parts or ingots with heated areas and predicting defect distribution. Simulations are recommended for the development and optimization of such technologies, enabling the selection of the most suitable insert models in terms of size, design, and exothermic properties. The ability to adjust the properties of the exothermic insert, considering the transformation of its structure during and after the thermite reaction, allows for precise simulation configuration.

REFERENCES

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Translated by  A.J. Camejo Severinov
Original text in Russian‍