RU2762692C1 - Method for casting ring blanks from aluminum or magnesium alloys - Google Patents

Abstract

FIELD: foundry production.

SUBSTANCE: invention relates to the field of foundry production of aluminum and magnesium alloys, in particular to the production of castings by centrifugal casting, and can be used for the manufacture of large-sized ring castings, mainly thick-walled, in a mold with a vertical axis of rotation. A method for producing annular blanks from aluminum or magnesium alloys involves feeding the melt into a metal mold with a vertical axis of rotation through a distributor. The melt is fed through a distributor equipped with four nozzles symmetrically arranged in one horizontal plane, of which two nozzles have channels with a diameter of 10-25 mm and a length equal to 0.7-0.9 of the inner radius of the cast workpiece, and two nozzles have channels with a diameter of 1-3 mm and a length equal to 0.1-0.2 of the inner radius of the cast workpiece, while part of the melt in an amount of 5-10% is fed into nozzles with a length equal to 0.1-0.2 of the inner radius of the cast workpiece, for its dispersion into drops with a diameter of 1-3 mm and the formation of supercooled partially crystallized droplets when they move to the inner surface of the mold, followed by their mixing with the bulk of the melt supplied through nozzles with a length equal to 0.7-0.9 of the inner radius of the cast workpiece, and the initiation of crystallization centers in the specified bulk of the melt. The production of annular castings with an adjustable structure and a homogeneous, fine grain in cross-section is ensured.

EFFECT: low anisotropy of the mechanical properties of the semi-finished product after rolling is ensured, as well as an increase in the metal utilization factor by reducing the tolerances for machining.

4 cl, 2 dwg, 1 tbl, 1 ex

Description

The invention relates to the field of foundry production of aluminum and magnesium alloys, in particular to the production of castings by the method of centrifugal casting, and can be used in mechanical engineering, rocketry and aircraft construction for the manufacture of large-sized annular castings, mainly thick-walled, into a mold with a vertical axis of rotation.

In the scientific and technical literature (Evstigneev AI, Shchekin AV and others. Calculation of technological parameters of chill and centrifugal casting. Komsomolsk-on-Amur: KNAGTU, 1996, - 73 p.) Set forth the general principles of centrifugal alloy casting.

There is a known method of casting a steel alloy onto a rotating element, which sprays the metal, and the latter is cooled and solidified on the inner surface of the rotating mold. The rotation axes of the element and the shape are not parallel. A decrease in the average cooling rate is provided, which contributes to the formation of a dendritic phase (US Patent No. 6797081, V22D, 13/10, 2004).

A device for producing ring castings from magnesium alloys by the method of centrifugal casting in an inert gas medium is known (RF Patent No. 2520249, V22D, 13/04, 2006).

A known method of producing a structural material from aluminum-based alloys with a magnesium content of 10-15%, which consists in carrying out crystallization in a rotating crystallizer with a gravity coefficient of 180-250 units, with a melt life time of 12-15 s / kg at a melt cooling rate of ≤5 ° C / s; then the resulting ingot is rolled (RF Patent No. 2380453, V22D, 2010).

The disadvantage of this method is that at a low cooling rate of the alloy (≤5 ° C / s), the casting has an extremely heterogeneous and rough structure, as well as strong liquation. In addition, the established concept of "melt lifetime" does not currently exist in the scientific and technical literature.

There is a known method of producing metal powder by extracting from the melt: solid phase particles 0.5-100 microns in size are introduced into the melt from the outside in an amount of 0.1-5.0% by weight, and the phase is chosen so that its properties differ sharply from the rest of the material (AS USSR No. 1135555, B22F 9/10, 1985).

Close to the claimed proposal is a method for producing glass-type castings: refractory solid particles of a different chemical composition with a density of more than 8000 kg / m ³ are fed into the melt stream. After pouring 50% of the melt, the supply of particles is stopped, the mold is brought into rotation after the bottom part has solidified and until complete solidification (RF Patent No. 2557854, V22D 13/04).

However, this method is applicable only to obtain a powder from high-temperature steel alloys: the hardened alloy with brittle particles of the introduced phase is easily milled. Common to the claimed invention is only the principle of introducing solid particles into the melt.

As a prototype, a method for producing large-sized annular semifinished products from deformable aluminum alloys by the method of centrifugal casting in an inert gas environment was chosen, characterized in that the melt is fed into the mold with the maximum possible second flow rate equal to 1-4% per second at the beginning of casting and 0.02- 0.08% per second at the end of casting of the total mass of the casting with a gradual decrease in consumption inversely proportional to the casting time, and the rotational speed of the mold is smoothly increased by 0.4-6.0% per minute of the initial speed, followed by deformation processing of the casting by rolling (RF patent No. 2487776, V22d 13/02, 2013).

The implementation of the prototype casting method was carried out in a pilot version at one of the enterprises under the following conditions:

- Amg6 alloy, ring casting size: 1200 × 700 mm, height 250 mm;

- casting weight 400 kg;

- steel mold without external cooling;

- casting temperature 760 ° С;

- the jet was directed to the bowl in the center of the rotating mold;

- casting speed (metal consumption per second): at the beginning of casting 8 kg / s and at the end of solidification of the casting 3.5-4.0 kg / s;

- casting solidification time: 10-15 min;

- initial speed of rotation of the mold: 300 rpm with a gradual increase to ~ 450 rpm for 8 minutes.

The casting speed was controlled using a dispenser with a stopper.

The disadvantages of this method are the extremely heterogeneous resulting structure of the casting and large allowances for machining (turning) the workpiece. In addition, when the melt is fed into the mold by a jet, it breaks up on a rotating substrate into chaotic bunches and drops.

The technical objective of the present invention is to develop a method for producing annular castings, including large-sized ones, with an adjustable structure and a sufficiently uniform and fine grain in cross-section, as a result of which a low anisotropy of the mechanical properties of a semi-finished product after rolling is achieved, as well as an increase in the metal utilization rate by reducing tolerances for mechanical processing.

The distinctive features of the proposed method are as follows:

- a part of the melt supplied to the casting mold is formed in the form of small supercooled drops to initiate crystallization centers in the casting due to a special distributor and branch pipes;

- at the beginning of casting, the mold is rotated at a reduced speed equal to 40-60% of the calculated one, and the latter is limited by the permissible ultimate strength of the mold material;

- at the end of casting, defined as the end of solidification of the casting, the casting speed is sharply increased to replenish the shrinkage of the last casting layer;

- the design of the distributor makes it possible to form melt jets, bring them as close as possible to the inner surface of the future casting and control the alloy crystallization process.

The time of casting into a mold of a certain mass of the melt should be less than the solidification time of the casting, which is determined experimentally.

The criterion for limiting the speed of rotation of the mold is the maximum allowable speed (n), which is calculated from the allowable ultimate strength of the mold material by the formula:

где

where

k = 0.2-0.8 - safety factor;

– допустимое напряжение материала формы;

- allowable stress of the mold material;

D is the inner diameter of the mold.

If at the initial moment of casting the mold rotation speed is high, then the first portion of the melt is "washed out" on the vertical surface of the mold with a thin layer, which solidifies so quickly that subsequent portions of the melt do not weld with it and an annular discontinuity is formed, which must then be mechanically removed.

It is proposed to start casting at a low (40-60% of the design) rotation speed, when a paraboloid of rotation begins to form in the mold, rising to a small height. Subsequently, with a smooth increase in the rotation speed, the melt rises along the wall of the mold to the height of the mold, eliminating the possibility of smearing with a thin layer.

At the end of the casting, the centrifugal force is practically close to zero and does not compensate for the volumetric shrinkage of the casting; moreover, there is not enough liquid melt. As a result, it is necessary to increase the machining allowance for the inner part of the casting in order to remove the loose layer.

To reduce the above negative phenomena, it is proposed to increase the metal consumption at the end of casting for about 20-50 seconds.

An inherent feature of the centrifugal casting method is the inhomogeneity of the structure of the casting: coarsening of the grain from the outer surface to the inner, because the heat sink goes through the wall of the mold. To reduce the inhomogeneity, it is necessary to create in the solidifying melt many centers of nucleation of crystals of the α-phase and, as a consequence, to increase the cooling rate of the casting. For this, part of the mass of the supplied metal (from 5 to 15%) is dispersed into drops with a diameter of 1-3 mm.

In analogs and in the prototype, liquid metal is fed onto a substrate, from which it is thrown onto the inner surface of the mold in ragged, shapeless jets.

In the proposed version, the free flight path of the metal melt is quite large: for example, for an annular casting with dimensions: 1500 × 900 mm, it will range from 750 mm (start of casting) to 450 mm (end of casting). FIG. 1 shows an installation for implementing a method for producing ring castings from light alloys. The stream of metal 1 comes from the nozzle 2 through the metal receiver into the distributor 3 and further into the nozzles 4. In Fig. 2 shows the distributor 1 with nozzles 2. The distributor, made in the form of a cup, serves to eliminate excessive fragmentation of the melt stream. Replaceable nozzles 2 are attached to the distributor in one horizontal plane. The nozzles are pipes lined from the inside with tubes with calibrated holes (channels). The maximum length of the nozzles (from the axis of rotation) is close to the inner radius (surface) of the finished casting. The branch pipes are attached to the distributor by means of union nuts 3. If necessary, additionally, very short nozzles 4 are used, in the lining of which there are holes (channels) with a diameter of 1-5 mm in the end part for dispersing the melt into drops. Flying the above distances, the droplets cool, partially crystallize at an increased cooling rate. Getting on the inner surface of the mold, they mix with the main mass of the melt, contributing to the production of fine grains and reducing the irregularity of the casting along the radial section. At the same time, it is not recommended to have more than 10% of the mass of such particles, since the fluidity of the alloy will decrease, which will lead to scattered porosity in the casting.

Depending on the height of the casting, the branch pipes can be located in 2 or more rows in height. In addition, the entire valve assembly can be moved vertically. This allows you to maintain the fluidity of the melt and cast a billet of the required height without increasing the casting temperature, because the latter reduces heat removal from the mold and coarsens the structure of the casting. The branch pipes are attached to the glass using union nuts.

The best casting mode is when the casting speed forms a constant thickness of the melt layer over the constantly thickening layer of the solidifying alloy. The solidification rate should be slightly less than the alloy build-up rate. Thus, the liquid layer is maintained during casting with a decrease in the current value of the inner radius of the casting. The casting speed must be continuously reduced, thereby maintaining the desired ratio of two important parameters - the speed of solidification and the casting speed.

Example 1 of casting according to the proposed modes

Maximum permissible design speed of rotation of the mold:

где

where

k - safety factor (taken equal to = 0.8);

– допустимое напряжение материала формы (для машиностроительной углеродистой стали 40 кг/мм²);

- permissible stress of the material of the form (for machine-building carbon steel 40 kg / mm ² );

D is the inner diameter of the mold (1.2 m).

Initial speed: 236 rpm (50% of rated 472 rpm). In 5 s after the start of casting, the rotation speed was gradually increased to 472 rpm.

In the first 3-4 s, the melt was fed into the mold with a flow rate of 12 kg / s (~ 3% of the casting mass of 400 kg), after which the casting speed decreased to values of 3-5 kg / s. Approximately 40-45 seconds before the end of the solidification of the casting, the metal consumption sharply increased to the initial one, i.e. 12-13 kg / s.

A distributor was used in the form of a glass with a diameter of 120 mm with 4 branch pipes: two with a length of 0.9 r = 0.9 - 350 = 315 mm (from the axis of rotation of the glass) and two with a length of 0.2 r = 70 mm. The nozzles were lined from the inside, the diameters of their channels were 15 and 2 mm, respectively.

The total casting time was 9-9.5 minutes.

The total cross-sectional area of holes with a diameter of 2 mm was equal to 4% of the same area of channels with a diameter of 15 mm.

Both castings (according to the prototype and according to the claimed method) were turned on the end surfaces and then their hardness HB was measured in thickness. All areas with HB <80 kg / mm ² were considered necessary for machining.

To assess the microstructure, samples were cut at half the thickness of the workpiece, closer to the inner surface. On the same samples, the density of the metal was determined by the hydrostatic method.

The results obtained are presented in Table 1, where R is the outer radius of the casting; r is the inner radius of the casting.

Claims (4)

1. A method of producing annular billets from aluminum or magnesium alloys, including feeding a melt into a metal mold with a vertical axis of rotation through a distributor, characterized in that the melt is fed through a distributor equipped with four branch pipes symmetrically located in one horizontal plane, of which two branch pipes have channels with a diameter of 10-25 mm and a length equal to 0.7-0.9 of the inner radius of the cast billet, and two branch pipes have channels with a diameter of 1-3 mm and a length equal to 0.1-0.2 of the inner radius of the cast billet, while a part of the melt in an amount of 5-10% is fed into nozzles with a length equal to 0.1-0.2 of the inner radius of the cast workpiece, for dispersing it into drops with a diameter of 1-3 mm and the formation of supercooled partially crystallized drops when they move to the inner surface of the mold with their subsequent mixing with the bulk of the melt supplied through the nozzles with a length equal to 0.7-0.9 of the inner radius of the cast billet ki, and the initiation of crystallization centers in the specified bulk of the melt. 2. The method according to claim 1, characterized in that at the beginning of casting, the melt is fed for 2-4 seconds at a flow rate equal to 3-10% per second of the mass of the cast billet, at a mold rotation speed equal to 40-60% of the maximum permissible speed, and then increase the speed of rotation of the mold to the maximum permissible for 3-10 seconds and reduce the casting speed in the range of 3-5% per second of the mass of the cast workpiece, and 30-45 seconds before the end of solidification of the cast workpiece, the casting speed is increased to the original value. 3. The casting method according to claim 1, characterized in that the melt is fed through a distributor made in the form of a glass with a height of 0.6-0.9 from the height of the cast billet, located coaxially with the mold and made with the possibility of its vertical movement, and the total area the cross-section of the channels of the branch pipes with a diameter of 1-3 mm is equal to 1-5% of the cross-sectional area of the channels of the branch pipes with a diameter of 10-25 mm. 4. The method according to claim 1, characterized in that after the start of casting, after 2-4 seconds, the outer surface of the mold is cooled with a water-air mixture at a rate of 2-10 liters per second during the entire time of solidification of the cast billet.

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