RU2339485C2 - Method for manufacturing foundry from metallic melt - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: at crystallisation of metallic melt it is implemented uniform volumetric cooling of melt with velocity (2-10)°S/s in created by means of centrifuge gravitational field. Gravitation coefficient is used from the range from 10 till 1000 depending on specified size of foundry grain. At uniform and slow enough cooling of melt grain size of crystallize phase according to growth of gravitation coefficient from 10 till 1000 changes by anomalously. At first grain size is reduces and then at appointed gravitation coefficient for each type of melt it is decreased till receiving of mono-structures.

EFFECT: receiving of foundry without porosity and non-junctions; increasing of foundries density and decreasing of gas content.

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Description

The invention relates to foundry.

The invention considers a method for controlling grain size to provide the possibility of obtaining castings of any configuration and from any metal melts having a single predetermined structure over any sections, which is adequate to producing castings without anisotropy of service properties.

Currently, in the manufacture of castings from homogeneous melts to control their structure, methods are used (in full) either by grinding, for example using ultrasonic fields, or known methods of growing monostructures.

If it is necessary to obtain castings from heterogeneous melts, various methods of modifying and processing with ultrasonic electromagnetic fields are used to grind the structure. These funds are the only ones to ensure the formation of castings with a given structure and, therefore, with specified service properties, ceteris paribus.

A known method of producing monostructures, which is based on the creation in the melt of supercooling, corresponding (approximately) to the maximum linear crystal growth rate (Csochralski JZ, Physik. Chem. 1917, Bd 92, S.219 .; Chalmers B. Principles of Solification, 1968 , p.280).

The effectiveness of the application of the above methods to a large extent depends on the type of melt, the volume of the casting, heat removal conditions (rate of temperature decrease, direction of operation of refrigerators). Moreover, due to the practical impossibility of identifying the conditions of heat removal in the periphery of the melt and from its central zones, the casting is added, of course, to the anisotropy of the grain size score.

Closest to the proposed one is a method of manufacturing casting from a metal melt by directional crystallization in a gravitational field created by centrifugation with volumetric cooling of the melt at a rate of (0.1 ÷ 0.5) ° K / s with a gravity coefficient equal to or more than 10 (International publication WO 94/02270, publ. 02/03/1994).

The known method does not provide a casting with a given structure, because it does not take into account the dependence of grain size on the coefficient of gravity.

The present invention is directed to solving the technical problem of producing castings of any configuration and from any metal melts having a single predetermined structure over any sections, which is adequate to producing castings without anisotropy of service properties.

The specified technical result is achieved by a method of manufacturing a casting from a metal melt by directional crystallization in a gravitational field created by centrifugation with uniform volumetric cooling of the melt at a rate of (2 ÷ 10) ° C / s with a gravity coefficient selected from a range from 10 to 1000 depending on a given grain size castings.

The present invention is illustrated by the following drawings:

figure 1 schematically shows the normalized dependencies of Tamman;

figure 2 is a graph of the magnitude of the grain of the casting from the created subcooling;

figure 3 - diagram of the initial potential relief;

4 is a diagram of a distorted potential terrain;

figure 5 shows a graph of the magnitude of the grain of the casting on the coefficient of gravity;

6 shows a graph of grain size changes in samples from A99;

Fig.7 shows a graph of the change in grain size in samples from AL4.

The possibility of obtaining castings with structures from monometallic to ultrafine from any metal melts with a change in the intensity of the gravitational field of the centrifuge is theoretically predicted and experimentally confirmed.

The scientific significance of the phenomenon is to obtain fundamentally new information about the behavior of metal melts in artificial gravitational fields during crystallization. Of greatest interest is the process of nucleation and growth of the crystalline phase under uniform cooling. In contrast to the available information on a monotonic decrease in grain size with increasing gravity coefficient, information on the existence of a clearly nonlinear given dependence is obtained and substantiated.

The practical value of the invention lies in obtaining a universal method for controlling the structure of the casting in the range from single-crystal to ultrafine without the use of modification and vibration means.

The theoretical proof is as follows.

, однозначно приведет к получению отливки с максимально измельченной структурой за счет форсированного роста количества зародышей (даже в гомогенной системе) при ограниченном объеме расплава - без введения дополнительных модификаторов.Analyzing the normalized dependences of Tamman (Fig. 1), one can imagine that the formation in the melt of supercooling equal to

, will unequivocally lead to the production of castings with a finely divided structure due to a forced increase in the number of nuclei (even in a homogeneous system) with a limited melt volume - without the introduction of additional modifiers.

, приведет к форсированному росту линейных размеров кристаллов, что имеет место для систем гомогенного и гетерогенного типа.Obtaining hypothermia corresponding

, will lead to a forced increase in the linear sizes of crystals, which is the case for systems of a homogeneous and heterogeneous type.

Summarizing the last conclusions, it is possible to synthesize an algorithm for controlling the grain size of a casting when supercooling is a controlling parameter. In this case, the dependence of the grain size of the casting (produced by any method) on the value of the magnitude of supercooling has the form shown in figure 2.

The required subcooling in the melt (except for conventional cooling paths) can be created by forming a pressure field in the melt that is distributed in any way. Given that the dependence of the crystallization temperature of the melt has the form:

Where

,

- температуры кристаллизации при давлении Рх и нормальном Ро;

,

- crystallization temperature at a pressure of Px and normal Po;

α is the coefficient of the adopted model,

:we can obtain the desired dependence of the amount of supercooling ΔТ on Px under the assumption that the melt is thermally stabilized at the level of crystallization temperature

:

Values of α for a number of metals:

Aluminum - 6.4 · 10 ^-2 ° C m ² / Mn,

Iron - 3.0 · 10 ^-2 ° C m ² / Mn,

Copper - 4.2 · 10 ^-2 ° C m ² / Mn,

Nickel - 3.7 · 10 ^-2 ° C m ² / Mn.

It follows that to create a noticeable hypothermia (adequate metastability intervals), it is necessary to form a pressure of several tens of Nm / m ² in the melt, which is simply unacceptable in the production of, for example, shaped castings in ceramic forms.

, и затем, сформировав в расплаве давлениеOne can imagine a slightly different way of using pressure for forced growth of the solid phase either by increasing the nucleation rate n ^* (ΔT) or the linear growth rate V ^* (ΔT) of the crystals. Indeed, using the dependence of the degree of melt overcooling on the cooling rate, it is possible to initially subcool the entire melt to a level, for example, equal to

, and then, forming a melt pressure

get the desired result on the structure formation of the casting. A similar result can be obtained in relation to the problem of structure enlargement:

,

, так как:In this case, much lower pressures are required for stable control of the casting structure.

,

, as:

÷

превышает 10¹⁰, нетрудно рассчитать требуемое давление, необходимое для увеличения скорости зародышеобразования в заданное количество pas Z:Given the fact that the value, for example, of the derivative d _n ^* / d (ΔT) in the interval

÷

exceeds 10 ¹⁰ , it is easy to calculate the required pressure necessary to increase the nucleation rate in a given amount pas Z:

which, ultimately, will determine the grain size of the castings.

In directional solidification of melts and their uniform volume cooling centrifuge nonlinear gravitational fields to a reduced effect on the mechanism and n ^* V ^* obtained by the pressure due to the presence of the centrifugal force F _c:

where γ is the specific gravity of the melt,

X - removal of the melt from the axis of rotation;

Kg - gravity coefficient,

distortion is added due to F _{c of the} initial potential relief and, as a result, the emergence of significant prerequisites for forced growth of the solid phase.

Consider the initial potential relief (figure 3).

An atom, located in a potential well (Xi) due to the symmetry of the potential relief, can equally likely move in the + X or -X direction. Tamman dependency analysis

where U ⁰ is the height of the potential relief;

R is the gas constant;

B is the constant of the substance;

σ is the surface tension;

That is the melting point

leads to the conclusion that an identical increase in n (ΔT) can be obtained by increasing ΔT or decreasing U ⁰ .

The presence of the centrifugal force F _c equal to:

where m ¹ is the mass of the atom;

ω is the angular velocity of rotation of the melt;

with potential energy

leads to a distortion of the initial potential relief (figure 4). In fact, to move an atom from the X _i hole to X _{i + 1} , the activation energy is needed

For an alternative movement in the -X direction, the similar energy is:

The total change in activation energy is equal to:

In the absence of F _n (3) melt flow atoms to the solid phase can be determined as follows:

Where

n _k is the number of atoms in contact with the nucleus;

ν is the atomic vibration frequency;

ρ is the probability of the atom moving in the direction of attachment;

a similar stream of “holes” moves, naturally, in the -X direction (hereinafter, it is not mass transfer that is meant, but a monatomic displacement to the boundary of the phase distribution).

The flow of atoms in the + X direction (without output) is equal to:

where S is the surface of the attachment of atoms;

.

.

A joint analysis of expressions (14) and (15) shows that their ratio is equal to:

In real conditions of foundry at Kg >> 1, the value Z >> 1 even without taking into account the corresponding supercoolings ΔT. This circumstance explains the possibility of the formation of a solid phase in nonlinear gravitational force fields.

In principle, it is possible to evaluate the effect of Kg adequate in intensity, reduced (or recalculated) to ΔT

where Tc is the value of subcooling obtained in a natural way.

Thus, some adequacy is proved by the effect on the formation of a solid phase from the melt in the presence of an external non-uniform gravitational field, for example, created by a centrifuge.

In this regard, the dependence (similar to FIG. 2) of the size of the casting grain D on the coefficient of gravity K, starting from 10 (the minimum threshold, below which the nucleation pattern repeats the traditional crystallization processes existing today), is obvious (FIG. 5). The grain size D (obtained in statics) decreases slightly with increasing K, which is explained by a fracture of the dendrides and modification of the melt by fragments. Further, the growth of K leads to crystallization in the maximum zone of the dependence n ^* (ΔТ) or V ^* (k _g ), that is, in the zone of a sharp increase in grain size, up to the monostructure. With a further increase in the gravitational coefficient, crystallization is carried out in the zone of maximum n ^* (ΔT) or n ^* (k _g ), which leads to a sharp increase in the number of crystallization centers, i.e., to a decrease in grain size at Kg = Kц,

After reaching the minimum grain size Du it starts to increase slightly, which can obviously be explained by the concomitant increase in pressure in the melt, a decrease in diffusion, and an increase in viscosity.

The considered plot (Fig. 5) can take place, naturally, only in the case when the induction to the beginning of crystallization is the presence of a gravitational force field of the required intensity and quasithermostation of the melt.

During the research, work was carried out on the following metals and alloys: A99, VAL5, VAL8, AL4, R9, ZhS6K, PC-A10Mg.

In the course of the experiments, the range of gravity coefficients from 10 to 1000 was studied in detail at the rates of concomitant cooling of the working melt (2-10) ° C / s.

In this case, ultrafine monostructures were also obtained, moreover, the monostructures were obtained without seeds, and the single crystal at a growth rate of (0.5-1) mm / C had a dislocation density of no higher than 5 · 10 ⁶ cm ^-2 .

For experiments, we used a crystallizer for industrial use (KP-1000) - Popov's mold - the maximum ingot diameter was 1000 mm. An ingot weighing 100 kg and a diameter of 1000 mm was obtained in the KP-1000 crystallizer from material A99. 6 and 7 show technological graphs of grain size for A99 and AL4.

A method of manufacturing a casting was carried out as follows.

The melt, heated to a temperature of 800 ° C, was poured into a centrifugal crystallizer pre-spun to a predetermined speed, the mold of which was lined with graphite to reduce the rate of heat loss by the melt, which, together with overheating of the melt being poured, ensured the maximum permissible rate of volumetric cooling of the melt equal to 10 ° C / s . The centrifuge turns (gravity coefficient) were set corresponding to the required size of the castings in accordance with the previously obtained dependences of D on K (Fig.6, 7).

Example 1

To obtain castings from A99 aluminum with a grain size of 1-2 mm, a centrifuge was turned on at a speed of 1200 rpm, which corresponds to K = 200 (Fig. 7). Volumetric cooling was carried out at a rate of 8 ° C / s. Measurement of the grain size of the obtained casting showed a value of 1.5 mm.

Example 2

To obtain a casting from A99 aluminum with a single-crystal structure (D = ∞), a centrifuge was turned on at a speed of 350 rpm, which corresponds to K = 50 (Fig. 7). Volumetric cooling was carried out at a rate of 2 ° C / s. Measurement of the grain size of the obtained casting showed a single crystal structure.

Example 3

To obtain an AL4 aluminum casting with a grain size of 4 mm, a centrifuge was turned on at a speed of 150 rpm, which corresponds to K = 10 (Fig. 7). Volumetric cooling was carried out at a rate of 7 ° C / s. Measurement of the grain size of the obtained casting showed a value of 4.2 mm

Example 4

To obtain an AL4 aluminum casting with a grain size of 2 mm, a centrifuge was turned on at a speed of 600 rpm, which corresponds to K = 20 (Fig. 7). Volumetric cooling was carried out at a rate of 10 ° C / s. Measurement of the grain size of the obtained casting showed a value of 2.05 mm

Thus, practical confirmation of the theoretical premises has been obtained.

Based on the present invention, the completely scientific problem of dynamically stimulating nucleation and crystalline phase growth in nonlinear stationary force fields can be solved. In addition, it becomes possible to build analytical deterministic algorithms for confident control of the structure formation of castings.

The practical role of the invention lies in the possibility of producing castings of any configuration and from any metal melts having a single predetermined structure over any sections, which is adequate to producing castings without anisotropy of service properties.

In addition to the noted, you should pay attention to some side effects:

- lack of pores;

- lack of nonsense;

- a significant reduction in gas content (more than 10 times);

- increase the density of castings.

Claims (1)

A method of manufacturing a casting from a metal melt by directional crystallization in a gravitational field created by centrifugation with uniform volumetric cooling of the melt at a rate of 2 ÷ 10 ° C / s with a gravity coefficient selected from a range from 10 to 1000 depending on the given grain size of the casting.

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