RU2106226C1 - Molten metal stamping method - Google Patents

Abstract

FIELD: foundry, in particular, casting of metals, including high-melting temperature and chemically active metals. SUBSTANCE: method involves producing melt; feeding it into die and subjecting to the action of increased pressure. During feeding of molten metal into die, metal is cooled to temperature, at which phase transformation time is reducing to zero value. Increased pressure is provided by collision between melt and die. Upon collision, melt is exposed to additional pressure pulses, when required. EFFECT: increased efficiency, simplified method and improved quality of stamped products. 2 cl, 3 dwg, 1 tbl

Description

 The invention relates to the field of foundry and can be used for casting any metals, including refractory and chemically active. The closest technical solution is the method of casting with crystallization under pressure, in which pressure is used as a factor of effective influence on the solidification and the processes occurring during it - shrinkage, gas evolution, cracking, segregation, including the preparation of the melt in a separate melting chamber, moving the melt from the melting chamber with using a filling device into a stamp, followed by pressing it using a piston or punch.

 Injection crystallization casting methods provide a high density of castings close to the density of deformable workpieces and a fairly uniform structure.

 This method has found application for the manufacture of flanges, gears, pistons, cylinder blocks, die inserts and molds.

 The aim of the invention is to increase the efficiency of use and expand technical capabilities by obtaining products of a particularly complex shape from any metals, including refractory and chemically active, as well as composite metal-non-metal products.

 This goal is achieved by the fact that in the known method of stamping liquid metal, which includes the transfer of a deformable metal billet into the liquid phase, moving it into a stamp, followed by exposure to increased pressure, characterized in that it is cooled to a temperature when the melt moves into the stamp at where the phase transition time tends to zero, and the pressure is obtained due to the collision of the melt with the stamp. And in addition, after the collision of the melt with the stamp on the melt is affected by additional pressure pulses.

 The proposed method implements the installation shown in FIG. one; in FIG. Figure 2 shows the dependence of the change in the volume V of a substance under the action of pressure P with phase transformations in time t; in FIG. Figure 3 shows the diagrams of cooling curves of pure metal depending on the cooling rate, showing the dependence of the time t of the transition of the metal from one phase to another on the supercooling temperature T.

 The installation includes a melting chamber 1, in which a remelted metal billet 2 is placed, upon heating of which a molten bath 3 is formed, upon reaching the melt 3 of the lower part of the billet 2, pressure is applied to it (gas pressure, magnetic field, explosion, mechanical shock, etc. ) to move it through the pipeline 4, where it is subcooled to the required temperature and gets into the stamp 5, where it is processed by high pressure.

A schematic representation of a liquid stamping method is shown in FIG. 2 Hugoniot curve and discharge unloading, which shows the dependence of the change in the volume V of the substance under the action of pressure P with phase transformations. The process is carried out according to the curve 0-1-2-3-4: supercooled at a pressure of P ₀ to a volume of V ₁ (V _P → V ₁ ) in the area 0 - 1, then with a slight increase in pressure from P ₁ to P ₂ is the formation crystal with a significant decrease in volume (V ₂ << V ₁ ) in the interval 1 - 2 due to the transition of the liquid phase into the solid phase 2 - 3 corresponds to a further compression of the already formed crystalline metal structure, and with a significant increase in pressure from P ₂ to P ₃ It is a slight decrease in metal volume (V ₂ → V ₃₎ have been carried out here about ess OMD stage 3 - 4 shows that the reverse phase transition occurs and therefore the final amount of the metal V _k in this case is less than the initial melt volume V _p, as The thermal regime of the process was selected so that the thermal energy Q _kp , which promotes the transfer of the solid phase of the metal into the melt, was scattered into space before pressure treatment.

If the metal is not supercooled, then the process will go according to the section of the curve 3 - 5, while the newly molten metal receives additional energy due to pressure, which increases its temperature and thereby increases the volume (V ₄ > V _p ). From the graph of the timeline of FIG. 2 it can be seen that the formation u of crystal growth in section 1 - 2 occurs during the crystallization time t _cr = t ₁ - t ₂ several times less than t _cr = t _about - t _{to the} action of the shock wave, therefore, the formed crystal has a very fine structure .

In order for the metal to enter the solid phase from the pressure exerted on it and not transfer to the melt after removing this pressure, it is required that the amount of heat Q _P lost by the metal during its transportation to the stamp and being in the stamp during compression and final filling of the cavities stamp, exceeded the amount of heat Q _cr allocated by the metal during its crystallization.

It is known that the crystallization process develops if the metal temperature T _{k is} lower than the melting temperature T _S by δT, called the degree of supercooling of the system, which is influenced by the cooling rate ν ^ox (Fig. 3).

With increasing speed (ν $\genfrac{}{}{0ex}{}{\text{ox}}{}$$\genfrac{}{}{0ex}{}{}{\text{3}}$ > ν $\genfrac{}{}{0ex}{}{\text{ox}}{}$$\genfrac{}{}{0ex}{}{}{\text{2}}$ > ν $\genfrac{}{}{0ex}{}{\text{ox}}{\text{1}}$ ) cooling, the degree of supercooling of the system (δT ₃ > δT ₂ > δT ₁ ) also increases, but the phase transition time from liquid metal to solid decreases (δt ₃ <δt ₂ <δt ₁ ). Therefore, it is possible to choose such a degree of subcooling δT of the metal when the phase transition time δt tends to zero. The portions δt of the curve in FIG. Figure 2 shows the time of compensation for the latent heat of crystallization Q _{cr by the} heat Q _p dissipated into space, but if before the start of crystallization the heat of scattering into space was equal in magnitude to the heat of crystallization Q _p = Q _cr , then δt → 0. From this condition, it is possible to calculate the degree of melt supercooling δT, after which the crystallization time tends to zero
Q _p = C • m • δT,
Q _cr = m • q _k ,
δT = q _c / s,
where c is the specific heat of metal;
m is the mass of crystallized metal;
q _to - specific heat of crystallization of the metal.

For example, for Ti, the supercooling temperature δT, at which the crystallization time δT → 0, is
δT = q _c / s = (392 kJ / kg) / (0.53 kJ / (kg • ^o C)) = 739 ^o C.

Moreover, according to the theory of amorphization of pure metals, pure titanium is able to supercool to a temperature T _K equal to 0.25 T _S at a sufficiently high (10 ^{8 o} C / s) cooling rate, i.e. δT can reach 1252 ^o C.

The amount of heat of dissipation Q _p lost by the liquid phase of the metal is calculated by the formula
Q _p = A • q _T • t,
where A is the surface area of the cooled substrate in contact with the molten metal, m ² ;
q _T - specific heat flux, Wt / m;
δt is the contact time of the liquid phase of the metal and the substrate, s.

δt = α • T _ω
where α is the contact heat transfer coefficient;
T _ω is the temperature difference between the melt and the substrate.

 Another important parameter determining the feasibility of the implementation of the liquid stamping process, affecting the phase transformations in metals, is pressure.

It is known that high (over 1000 MPa) pressures significantly (up to 25%) reduce the melt volume, and at the same time, the melting temperature increases, i.e. harden. An increase in the solidification temperature with increasing pressure is observed in those metals that occupy a smaller volume in the solid state than in the liquid state. To transfer a liquid metal into a solid state, it is necessary to apply a pressure P to the melt, which compresses the melt to a density ξ corresponding to the solid-state state near the melting temperature T _S.

 For example, to transfer molten Ti into the solid phase near its melting temperature, compression by ξ = 1% is necessary.

The required melt compression can be determined according to the theory of compression by shock waves
ξ = (ν _in / C _o ) • 100%,
where ν _in - the compression rate of the substance;
C ₀ is the propagation velocity of a longitudinal wave in a substance.

From here, the required collision rate of the melt with the walls of the die cavities can be calculated.
ν _in = ξ • C _o / 100%.
For example, to transfer a Ti melt at the melting temperature T _S to a solid state, it is necessary to compress the metal due to the impact of the melt and the stamp with a velocity ν _of = 27 m / s, which was confirmed in our experiments on a pilot plant. At the pilot plant, cladding with a thin (1 mm thick) Ti layer of a nozzle of a rocket engine made of carbon (carbon-carbon composite material) was carried out, which confirmed the possibility of using the liquid stamping process to obtain products from alloys and mixtures of metals and nonmetals.

The compression time of the body by shock pressure
δt _c = 2l / C _o ,
where l is the length of the body.

For l = 0.5 m for Ti we have δt _c = 7.4 • 10 ^-4 s.
From the above example, it follows that the time of applying pressure P to the melt is so short that very small crystals have time to form, there is no time for growth and therefore no dendritic structure is formed. Moreover, the intercrystalline space formed by reducing the volume of the metal when it enters the crystal is continuously filled by forcing the newly formed crystals to be compressed relative to each other under the same high pressure, thereby reducing intercrystalline voidness, since the formation of dislocations is complicated in comparison with the usual conditions of solidification of the melt.

 The paper presents an enlarged approximate method for calculating modes for liquid stamping. The basic parameters calculated for it for carrying out the liquid stamping process for some metals are given in the table.

 The installation allows the implementation of a method of stamping liquid metal with the following advantages: in contrast to the prototype, the finished part is obtained with an internal crystalline structure of the same parameters over the entire cross section of the product.

 From the above diagram of this process it follows that the time of applying pressure to the melt in the die is so short that very small crystals have time to form, there is no time for their growth and therefore a coarse-grained structure does not form. Moreover, the intercrystalline space formed by reducing the volume of the metal when it enters the crystal is continuously filled by forcing the newly formed crystals to be compressed relative to each other under the same high pressure, and the preliminary supercooling of the melt allows preserving the obtained intercrystalline structure throughout the entire volume of the product, what is impossible to achieve according to the prototype scheme.

Claims (2)

 1. A method of stamping a liquid metal, including obtaining a melt, moving it into a stamp, subsequent exposure to it with increased pressure, characterized in that while moving the melt into the stamp it is cooled to a temperature at which the phase transition time tends to zero and the pressure is created due to the collision of the melt with the stamp.  2. The method according to p. 1, characterized in that after the collision of the melt with the stamp on the melt is additionally affected by pressure pulses.

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