Technical field

The invention relates to a method for the preparation of metal foam, in which a metal melt containing up to 25 % by volume of at least one stabilizer is stirred, wherein gas is blown into it, which bubbles through the melt and foams it, thereby forming foamed metal melt which, when solidified, forms metal foam.

In addition, the invention relates to a device for performing this method. Background art

Metal foam, or also a metal cellular system, is a specific material which is based on metal (most often aluminium or its alloy) and whose structure is lightened by air bubbles. The usual porosity is between 75 and 90 % by volume. Despite this relatively high porosity, however, the material retains the mechanical properties of the parent metal and achieves a high strength-to- weight ratio; moreover, it is characterized by good thermal insulation properties, its ability to dampen sound and shock and to shield certain types of ionizing radiation (X-rays, gamma rays and neutron radiation). The use of this material makes it possible to reduce the weight of various components and entire structures without reducing strength and durability. Thanks to this, metal foam is usable in many different fields - in building industry (e.g., as an insulating building material), transport (weight reduction of vehicles, especially vessels), military industry (production of lightweight yet effective armour), medical industry (production of lightweight prostheses), aerospace/space industry, etc. Two fundamentally different methods are currently used to prepare metal foam.

The first method consists in forcing a suitable additive into the structure of molten metal, this additive being removed at some stage in the preparation of the foam. As a suitable additive, metal hydrides (most often titanium hydrides) are used, which decompose at temperatures above the melting point of aluminium (660 °C) to produce hydrogen and CO ₂ , which increase the volume of the parent metal and create a great number of cavities within its structure - see, e.g., US 2983597, US 3005700 or US 4973358. In addition, the use of salt (NaCI, melting point 801 °C), which remains embedded in the structure of the metal in a solid state and is washed out when the metal cools, is also known. The second method consists in foaming a metal melt by blowing a suitable gas into it (e.g., nitrogen, air, argon); in addition, in some cases, a suitable stabilizer, e.g., particles of silicon carbide or aluminium oxide, etc., is present in the metal melt, which by its presence stabilizes the gas bubbles formed. The disadvantage of existing methods for the preparation of metal foams is that the structure of the metal foam and the arrangement and shape of the cavities within the structure is not, and in principle cannot be controlled. The bubbles of the gas bubbling through the melt tend to agglomerate into larger units, which are pushed by the buoyancy force to the melt surface. The resulting structure of the metal foam is thus purely random and is not uniform in the entire volume of the metal foam (due to which the mechanical properties of the metal foam may not be uniform either). The individual pores in the foam structure have an uneven size - a diameter of approx. 3 to 10 mm, or even up to 25 mm, while their walls are only approx. 50 pm thick, due to which the foam does not achieve the required parameters and collapses under load. The density of the foam thus prepared is determined by the size and density of the pores, the specific technological parameters of its preparation, the chemical composition of the base metal, type and amount of the stabilizer/stabilizers, type and amount of the material for pore formation, and usually ranges from 100 kg-m ^{- 3} to 540 kg-m ^{- 3} .

The objective of the invention is therefore to provide a method for the preparation of metal foam which would eliminate the disadvantages of the background art and allow to control the shape and distribution of cavities in the structure of the metal foam. The objective of the invention is also to provide a device for performing this method. Principle of the invention

The objective of the invention is achieved by a method for preparing metal foam from a metal melt, in which the metal melt, containing up to 25 % by volume of at least one stabilizer, is stirred, and gas is injected into it, which bubbles through the melt and foams it, thus forming foamed metal melt which, when solidified, forms metal foam, wherein the principle of the method according to the invention consists in that the metal melt is during its foaming exposed to a translational electromagnetic field with a magnetic induction amplitude with a size of 2 to 6 mT, preferably 2.5 to 4.5 mT. As a result, Lorentz forces f _L are generated in the melt, which act on the gas bubbles in the melt and enhance their stabilization by the stabilizer/stabilizers and accelerate the movement of the stabilized bubbles towards the surface of the melt, which prevents them from agglomerating into larger units.

The translational electromagnetic field is created, e.g., by an electromagnetic coil which is powered by at least three-phase electric current and which is divided into at least six electrically separated segments arranged into groups, within which the individual segments are connected in an antiparallel manner, wherein each group comprises the same number of segments and all the segments of one group are supplied by the same phase of electric current, different from the other groups, one segment of each further group being physically arranged between two segments of one group.

In a preferred variant of embodiment, the translational electromagnetic field is generated by an electromagnetic coil powered by a three-phase electric current, the electromagnetic coil being divided into six segments, wherein the first segment is connected to the fourth segment in an antiparallel manner, the second segment is connected to the fifth segment in an antiparallel manner and the third segment is connected to the sixth segment in an antiparallel manner.

The first segment and the sixth segment of the electromagnetic coil have preferably 1100 to 1600 turns, the second segment and the fifth segment of the electromagnetic coil have preferably 1050 to 1550 turns and the third segment and the fourth segment of the electromagnetic coil have preferably 600 to 1050 turns. The objective of the invention is also achieved by a device for the preparation of metal foam, which comprises a foaming vessel with at least one passage for a shaft of a stirrer and a supply pipe of pressure gas, wherein at least one electromagnetic coil is arranged outside the foaming vessel along at least part of its clear height H, the electromagnetic coil being divided into at least six electrically separated segments grouped together, within which the segments are connected in an antiparallel manner, wherein each group comprises the same number of segments and one segment of each further group being physically arranged between two segments of one group, wherein each group of segments is connected to a different phase output of the power source, or is provided with means for connection thereto.

In a preferred variant of embodiment, the electromagnetic coil is divided into six segments, wherein the first segment is connected in an anti-parallel manner to the fourth segment, the second segment is connected to the fifth segment and the third segment is connected to the sixth segment. Preferably, the first and the sixth segment have 1100 to 1600 turns, the second and the fifth segment have 1050 to 1550 turns and the third and the fourth segment have 600 to 1050 turns.

The foaming vessel preferably has the shape of an inverted truncated cone, which prevents central vortex formation when stirring the melt.

The upper inner diameter D ₂ of the foaming vessel is preferably equal to half the clear height H of this vessel and the ratio D ₂ /D ₁ of the upper inner diameter D ₂ of the foaming vessel and the lower inner diameter D ₁ of the foaming vessel is preferably equal to 1.1 to 1.2.

Description of the drawings

In the enclosed drawings, Fig. 1 schematically shows a cross-section of one variant of a device for the preparation of metal foam according to the invention, Fig. 2 is a cross-section of a second variant of this device with a different arrangement of gas supply to the metal melt, and Fig. 3 is a cross- section of a third variant of this device with a preferred construction variant of an electromagnetic coil. Fig. 4 shows schematically a suitable interconnection of the segments of an electromagnetic coil for the case where this coil contains 6 segments. Fig. 5 schematically shows the connection of a three-phase electric current source formed by a control autotransformer with compensation capacitors and resistors. Fig. 6a schematically shows the force acting on a gas bubble in the molten metal during the preparation of metal foam according to the invention near the centre of a foaming vessel, and Fig. 6b shows the force acting on the gas bubble in the metal melt during the preparation of metal foam by the method according to the invention near the outer wall of the foaming vessel.

Examples of embodiment

The method for the preparation of metal foam according to the invention and the principle of the device for performing the method will be explained below with reference to Figs. 1 to 6b, which show schematically the cross- sections of three exemplary embodiments of this device, the advantageous interconnection of the segments of the electromagnetic coil, the advantageous connection of the three-phase electric current source and the force acting on the gas bubble in the molten metal near the centre and the outer wall of the foaming vessel. The device for the preparation of metal foam according to the invention comprises a foaming vessel 1, which can be closed with a lid 2. The foaming vessel 1 and all its components are made of a suitable refractory material, such as steel 1.4841 (AISI314, CSN 17252), steel 1.4845 (AISI 310, CSN 17252), silica, etc. In the lid 2 of the foaming vessel 1 is formed a passage 23 for a supply pipe 3 of pressure gas which is provided outside the foaming vessel 1 with a pressure gas inlet 31 for connecting an unillustrated source of pressure gas, and a passage 24 for a shaft 4 of a stirrer 41 , the shaft 4 being connected to a drive 42 arranged outside the foaming vessel T In the embodiment shown in Figure 1 , the shaft 4 of the stirrer 41 is accommodated for part of its length in the pressure gas supply pipe 3, so that only one passage 23=24 is formed in the lid 2 of the foaming vessel 1; in the variants of embodiment shown in Figs. 2 and 3, the shaft 4 of the stirrer 44 and the pressure gas supply pipe 3 pass through the lid 2 separately. The advantages of this variant include the possibility of using a pressure gas supply pipe 3 of a smaller inner diameter and the associated formation of gas bubbles of smaller dimensions. The conduction of the supply gas pipe 3 and the shaft 4 of the stirrer 44 through the lid 2 of the foaming vessel 1 is the most advantageous variant; in general, however, the supply gas pipe 3 and/or the shaft 4 of the stirrer 44 can be introduced into the interior of the foaming vessel 1 differently, e.g., through a passage/passages in its shell and/or bottom, etc.

In a preferred variant of embodiment shown in Figs. 1 to 3, the foaming vessel 1 is connected in its upper part by a conduit 5 made of a refractory material to the inner space of a foundry mould 6. The inner space of the foundry mould 6 is further connected, if necessary, to a vacuum pump 7, preferably on the opposite side to which the conduit 5 opens.

Around the entire circumference of the foaming vessel 1 is arranged at least one layer of thermal insulation 8 formed by a known heat-resistant insulating materials, e.g., based on aluminium oxide fibers, silica fibers, etc. The bottom and the lid 2 of the foaming vessel 1 are preferably provided with the same thermal insulation; optionally, the bottom and/or the lid 1 of the foaming vessel 2 are made directly (at least partially) of such a material.

The foaming vessel 1 preferably has the shape of an inverted truncated cone. This shape allows optimal movement of the gas bubbles blown by the supply pipe 3 into the metal melt 9 towards the conduit 5 and at the same time prevents central vortex formation when the metal melt 9 is stirred. In the most preferred variant of embodiment, the upper inner diameter D ₂ of the foaming vessel 1, i.e., the inner diameter of the foaming vessel 1 at its upper end, is equal to half the clear height H of this vessel 1 and the ratio D ₂ /D ₁ of the upper inner diameter D ₂ of the foaming vessel 1 and the lower inner diameter Di of the foaming vessel 1, i.e., the inner diameter of the foaming vessel 1_at its lower end, is equal to 1.1 to 1.2. During the experiments, it was found that in this construction of the foaming vessel 1, each bubble of the melt 9 during its movement captures the optimal amount of particles of the stabilizer/stabilizers (see below) to achieve high stability of the prepared metal foam. In other variants of embodiment, however, it is possible to use a foaming vessel 1 of essentially any other shape. In a preferred variant of embodiment, the stirrer 41 is provided with two superimposed impellers 43, each of which comprises four evenly spaced blades 44 made of refractory steel. In the most preferred variant of embodiment, these blades 44 have an inclination of 30 to 45° relative to the longitudinal axis of the shaft of the stirrer 44, and the stirrer 41 is arranged in the foaming vessel 1 such that the height h of the lower edge of the blades 44 of its lower impeller 43 above the bottom of the foaming vessel 1 is equal to 1/3 to 3/4 of the diameter d of these blades 44. In other embodiments, however, it is possible to use a stirrer 41 of any other construction or type - the stirrer 41 may, for example, comprise a different arrangement and/or a different number of impellers 43 (including one) and/or blades 44, or it may comprise the so-called turbine blades - i.e. blades whose surface is arranged parallel or nearly parallel to the longitudinal axis of the shaft of the stirrer 44, etc.

Around the circumference of the foaming vessel 1, preferably on the outer side of the thermal insulation 8, is arranged an electromagnetic coil 10 provided with unillustrated means for connection to an unillustrated electric current source. This electromagnetic coil 10 is preferably arranged along the entire height of the foaming vessel 1, as shown in the embodiment in Fig. 1 , Fig. 2 and Fig. 3; however, in unillustrated variants of embodiment, it may be arranged only along its clear height or only along part of its (clear) height. The electromagnetic coil 10 is designed to generate a translational electromagnetic field and is preferably formed by electrically separated segments 101, 102. 103. 104, 105, 106. The magnetic induction B of the electromagnetic coil 10 is characterized by the Biot-Savart law. To generate a suitable translational electromagnetic field, an electromagnetic coil 10 with segments 101. 102, 103, 104, 105, 106 is used, the number of which is given by an integer multiple of the number of phases of the supply electric current, whereby the minimum multiple is double. The minimum number of phases of the supply current is three phases - with a lower number of phases, there would be an undesired oscillation of the generated electromagnetic field; the minimum number of segments of the electromagnetic coil 10 is therefore 6. With a sinusoidal supply, each segment 101. 102, 103, 104, 105, 106 of the electromagnetic coil 10 creates a partial stationary sinusoidal field at each point of the melt 9. The sum of all these partial stationary sinusoidal fields powered by an electric current with a phase shift then creates a uniform translational electromagnetic field acting over the entire height of the coil 10.

In other variants, an electric current with a time waveform other than sinusoidal may be used to supply the individual segments 101. 102, 103. 104, 105, 106 of the electromagnetic coil 10, but it must be a defined, preferably symmetrical waveform.

Fig. 3 shows schematically an embodiment of the device for preparing metal foam according to the invention with an electromagnetic coil 10 composed of six electrically separated segments 101. 102, 103. 104, 105, 106, which is symmetrical with the centre of symmetry between the segments 103 and 104. The first segment 101 and the fourth segment 104 (counted from the bottom), the second segment 102 and the fifth segment 105 and the third segment 103 and the sixth segment 106 are interconnected in an antiparallel manner and are connected to an unillustrated source of three-phase current - each pair of segments 101. 104 and 102, 105 and 103, 106 is connected to one of its phase outputs - see Fig. 4, which shows schematically a preferred interconnection of the segments 101. 104 and 102, 105 and 103, 106 of the electromagnetic coil 10, when the start of the winding of the first segment 101 is connected to the start of the winding of the third segment 103, the start of the winding of the second segment 102 is connected to the start of the winding of the fifth segment 105 and the start of the winding of the third segment 103 is connected to the start of the winding of the sixth segment 106, etc.

If the number of phases of the supply current is f, the individual segments 101. 102, 103. 104, 105, 106 of the electromagnetic coil 10 are powered by a current with a phase shift of 0°, 3607f, 2x3607f to (f-1)x3607f. If the power supply is carried out, e.g., from a network where the phase angle between two voltage waves is equal to 3607f, and due to the inductances of the segments 101. 102, 103, 104, 105, 106 of the electromagnetic coil 10, the current passing through is shifted relative to the voltage, and this shift may be different for each segment 101. 102, 103, 104, 105, 106 of the electromagnetic coil 10, it is necessary to compensate for this shift, preferably, e.g., by capacity connected in series. The capacitor size is calculated applying Kirchhoff's second law. The suitable source for powering the electromagnetic coil 10 is, for example, a three-phase current source consisting of a control autotransformer with compensation capacitors and resistors, which eliminate the phase shift of current and voltage and thus ensure the same current values in all phases - see, e.g., Fig. 5. In such a case, both of each pair of segments 101. 104 and 102, 105 and 103, 106 of the electromagnetic coil 10 are always powered in the same way, without a current-to-voltage shift. As a result, both of each pair of segments 101. 104 and 102, 105 and 103, 106 of the electromagnetic coil 10 generate the same partial stationary electromagnetic field with the same phase and the same time waveform.

Between each of the two segments 101. 102, 103, 104, 105, 106, which are interconnected in an antiparallel manner, one segment 101. 102, 103, 104, 105, 106 of each further group of antiparallel manner interconnected segments 101. 102, 103, 104, 105, 106 are arranged. In a preferred variant of embodiment of the electromagnetic coil 10 with six segments 101. 102, 103, 104, 105, 106, the first and the sixth segment 101 and 106 have 1100 to 1600 turns, the second and the fifth segment 102 and 105 have 1050 to 1550 turns (i.e. approx. 65 to 140 % of the number of the turns of the first and sixth segment 101. 106), the third and the fourth segment 103 and 104 have 600 to 1050 turns (i.e. approx. 37 to 95 % of the number of the turns of the first and sixth and sixth segment 101. 106).

The intensity of the translational electromagnetic field generated by the electromagnetic coil 10 depends on many factors, such as the radius R (D/2) of the foaming vessel 1; the design dimension D/H of the foaming vessel 1; the value of the criterion number F of the translational electromagnetic field; the specific electrical conductivity of the melt 9 s; the wave number of the translational electromagnetic field a _m ; and the density p and kinetic viscosity v of the foamed melt 9. The density and kinetic viscosity of the melt 9 are influenced by the amount of the stabilizer added (e.g., ceramic particles - see below). To optimally influence the foaming process, the criterion number F of the translational electromagnetic field is F = 1 -10 ² to 1 -10 ³ , preferably F = 1 · 10 ² to 5-10 ² . Meeting this condition contributes to the creation of a very homogeneous translational electromagnetic field, which produces an axisymmetric flow in the melt 9 without oscillations in the velocity field (see below).

The foaming vessel 1 is mounted in the interior of the electromagnetic coil/coils 10 preferably detachably, or the electromagnetic coil/coils 10 is/are mounted detachably in the structure of the device for the preparation of metal foam according to the invention.

For the preparation of the metal foam is used melt 9 of the respective metal , e.g., aluminium, tin, zinc, copper, alloys of any of these metals, etc., heated above the liquidus temperature (i.e., the temperature at which the metal is liquid in its entire volume), in which at least one suitable refractory stabilizer is dispersed. The refractory stabilizer increases the viscosity of the metal melt 9, wherein its particles adhere due to their high affinity to the metal melt 9 at the melt 9 - gas interface and thus reinforce the walls of the bubbles formed, thereby contributing to the stabilization of the foam formed. Such stabilisers include, for example, various types of ceramic particles (0.1 to 100 pm in size) which are added to the melt in amounts of up to 25 % by volume, preferably 10 to 20 % by volume. A suitable stabilizer is, for example, silicon carbide with a particle size of 5 to 20 pm, in an amount of 10 to 22 % by volume, aluminium oxide with a particle size of 5 to 20 pm in an amount of 10 to 17 % by volume, titanium diboride, zirconium dioxide, silicon nitride, etc. with a particle size of 5 to 20 pm in an amount of 20 to 25 % by volume, dried water glass with a particle size of 0.5 to 25 pm in an amount of 15 to 22 % by volume, etc. From the ecological point of view, magnesium oxide in the form of particles with a size of 5 to 30 pm in an amount corresponding to a maximum of 20 % by volume, is also a suitable stabilizer. In addition to this stabilizer/these stabilizers, the stabilization of the formed foam is to some extent also aided by alloying elements contained in the alloy, such as silicon (up to 7 % by weight in the case of aluminium alloys) or magnesium (1 to 5 % by weight in the case of aluminium alloys), etc. to a certain extent. Moreover, these alloying elements improve the foundry properties of the respective metal (fluidity and run-in) and thus contribute to a good filling of the cavity of the used foundry mould 6 with the foamed melt 9. The stabilizer particles have preferably a regular shape - i.e., the ratio of their greatest length to largest diameter is at most 2:1.

A suitable gas is then blown into the metal melt 9 via the supply pipe 3 from an unillustrated external source. The gas bubbles through the metal melt 9 and foams it - forming bubbles which rise upwards to its surface, where is formed a layer of foamed melt 9, which leaves spontaneously via the conduit 5 into the inner space of the foundry mould 6, in which it subsequently cools and solidifies. The pressure (in the order of 10 ² Pa), which is preferably created in the inner space of the mould 6, represents a suitable chemically inert environment, which prevents oxidation of the hot components and contributes to the creation of a better surface of the resulting metal foam (without the formation of oxides), and at the same time simplifies the entry of the foamed metal melt 9 into the foundry mould 6. However, in an unillustrated variant, the foamed melt 9 can solidify directly in the foaming vessel 1. A suitable gas used for foaming the metal melt 9 is especially air, preferably preheated to 50 °C and with an oxygen content increased by 10 % (relative to the atmospheric content, i.e., approx. 31 % by volume), but it is also possible to use argon of 99.95 purity or nitrogen, or another inert gas, the use of which eliminates completely the risk of reaction (oxidation) with the melt 9. When using stabilizing particles based on silicon carbide, it is advantageous to blow nitrogen into the melt 9. The gas is blown into the metal melt 9 near the bottom of the foaming vessel 1, which makes it possible to saturate the entire column of the metal melt 9 with it. The total amount of gas injected is 20 to 25 % of the melt volume. The minimum pressure of this gas is 0.2 MPa, the maximum is 0.3 MPa (at a flow rate during laboratory experiments of 1 to 3 l-min ^-1 , or 2 to 4 l-min ^-1 ).

Under normal circumstances, the formed gas bubbles tend to rise rapidly upwards to the surface of the melt 9 due to the large buoyancy forces which overcome the relatively low density of the metal melt 9, expanding and spreading due to the decrease of metallostatic pressure. This is in the method according to the invention eliminated by the action of the electromagnetic field (see below) in combination with the stabilizer/stabilizers.

The metal melt 9 is stirred with the stirrer 41 before being foamed in the foaming vessel T Momentum transfer, in the case of using a rotary stirrer 41 with blades 44, takes place by pressing the rotary blades 44 onto the melt 9, so that part of the metal melt 9 in front of the blade 44 penetrates the surrounding melt 9 and part of the metal melt 9 is set in motion in the direction of rotation of the stirrer 41_. In this case, a so-called primary melt 9_stream is formed, which emerges downwards from the rotor region of the stirrer 41 towards the bottom of the foaming vessel 1 and then rises upwards along the walls of the foaming vessel 1 and carries the gas bubbles up to the melt surface 9. This primary stream simultaneously transfers momentum to the surrounding melt 9 by turbulent and viscous friction, wherein a secondary melt 9 stream is formed oriented in the opposite direction to the melt 9 primary stream. Immediately behind the rotary blade 44, a vacuum is created, which causes the melt 9 to be sucked in from the surroundings of the stirrer 41. By extruding and sucking in the melt 9, a turbulent vortex is created around the rotary blades 44 of the stirrer 41 , which helps to homogenize the melt 9 and homogeneously disperse the stabilizer particles therein.

After homogenization, the melt 9 is exposed to the translational electromagnetic field generated by the electromagnetic coil 10 with a magnetic induction magnitude of 2 to 6 mT, preferably 2.5 to 4.5 mT. At the same time, an electric current is induced in the melt 9, resulting in Lorentz forces f _L acting on the melt 9. These forces induce an imbalance in the melt 9, which causes movement of the melt, which in turn affects the behaviour of the bubbles of the gas blown into the melt 9. In addition to the Lorentz forces f _L , the gas bubble is subject to the buoyant force Fvz and the drag force FOD (see Figs. 6a and 6b), whereby it applies: where: s is the specific electrical conductivity coefficient of the melt [m-Ω ^-1 ], [S-n ^-1 ]; w is the angular frequency [s ^-1 ], w= 2·p·ί, f = 50 Hz;

B ₀ is the induction of the magnetic field (without the effect of fluctuation) [T], a _m is the wave number of the translational magnetic field [m ^-1 ], a _m ~ 1 ;

R is the radius of the vessel where the translational electromagnetic field acts [m]; e _z is the unit vector of cylindrical coordinates; and where:

R is the radius of the gas bubble [m]; h is the dynamic viscosity of the melt [Pa s; kg s ^-1 m ^-1 ]; v is the velocity of the flow of the gas bubble [m s ^-1 ]; VB is the bubble volume [m ³ ], assuming the air bubble is spherical, then VB = 4/3 p-R ³ ;

PK is the density of the melt [kg m ³ ]; g is the gravitational acceleration [m s ^'2 ]; v is the kinetic viscosity of the melt [m ² s ^-1 ]. Under the action of the translational electromagnetic field, it is possible to derive the action of Lorentz forces f _L on the gas bubbles either in the downward or upward direction, as required, by the orientation of the translational electromagnetic field. These forces act normally downwards in the middle of the foaming vessel 1, slowing down the movement of the gas bubbles (arrow u in Fig. 6a) towards the melt 9 surface and contributing to the gas bubbles being surrounded by as many particles of the stabilizer/stabilizers as possible, which increases their stability and facilitates the formation of their walls, and near the walls of the foaming vessel 1 upwards, when they contribute to the rapid rise of the gas bubble to the surface of the melt 9 (arrow u in Fig. 6a). When providing the criterion number of the translational electromagnetic field F = 1 -10 ² to 1 *10 ³ , preferably F = 1 -10 ² to 5-10 ² , the main intensity of the downward movement of the melt 9 is in the middle part of the foaming vessel; the upward movement near the walls of the foaming vessel 1 is less intense (according to simulation calculations it is only about 1/3 of the intensity in the middle part of the foaming vessel 1). With the opposite orientation of the translational electromagnetic field, the sense of action of the Lorentz forces f _L is opposite. In any case, the Lorentz forces f _L accelerate at some stage the movement of the gas bubbles sufficiently stabilized by the stabilizer/stabilizers towards the surface of the melt 9, which prevents their agglomeration into larger units.

The flow rate of the melt 9 depends on the intensity of the magnetic induction of the translational electromagnetic field, or on the criterion number F of this field, wherein where: s is the specific electrical conductivity coefficient of the melt [m-W ^-1 ], [S m ^-1 ]; w is the angular frequency [s ^-1 ], ω= 2·π·f, f = 50 Hz;

B ₀ is the induction of the magnetic field (without the effect of fluctuation) [T], [kg-A ^-1 -s ^-2 ]; a _m is the wave number of the translational magnetic field [m ^-1 ], a _m ~ 1 ;

R is the radius of the vessel where the translational electromagnetic field [m] acts; p is the density of the melt, which is acted upon by the translational electromagnetic field [kg m ^-3 ]; v is the kinetic viscosity of the melt [m ² s ^-1 ], v = h/r.

The action of the Lorentz forces f _L disturbs the balance of the melt 9 and creates an additional flow in it, which causes formation of finer gas bubbles, and at the same time the Lorentz forces also act on large gas bubbles in the melt, causing them to divide into several smaller gas bubbles and prevent their further agglomeration, and also support the stability of the individual bubbles and their further existence. As a result (in combination with the stabilizer/stabilizers used), the individual bubbles of the melt 9 do not tend to agglomerate or collapse. The foamed metal melt 9 is then not only stable, but also has a uniform distribution and pore size throughout the volume. This foamed metal melt 9 flows spontaneously through the conduit 5 into the inner space of the foundry mould 6 (preferably preheated), in which it solidifies; optionally, it solidifies directly in the foaming container 1. The vacuum (in the order of 10 ² Pa), which is preferably created in the inner space of the mould 6, represents a suitable chemically inert environment, which prevents oxidation of the hot components and contributes to creating a better surface of the resulting metal foam (without formation of oxides), and simplifies the entry of the foamed metal melt 9 into the foundry mould 6.

In general, the conditions for the creation of the translational electromagnetic field can be defined as follows:

- electric current for the power supply of the electromagnetic coil 10 - minimum number of phases f = 3 (with phase shift 0°, 3607f, 2x3607f to (f-1)x3607f)

- number N of the segments 101, 102, 103, 104, 105, 106 of the electromagnetic coil 10 N = n f, wherein n is an integer > 2

- the segments 101. 102, 103, 104, 105, 106 of the electromagnetic coil 10 are arranged in groups within which all the segments 101. 102, 103, 104, 105, 106 are interconnected in an antiparallel manner, wherein the number of these groups is equal to the number of phases f of the electric current

- magnetic induction amplitude B = 2 to 6 mT, criterion number F = 1.10 ² to 1.10 ³ , preferably F = 1.10 ² to 5- 10 ²

Four examples are given below to illustrate the preparation of metal foam by the method according to the invention. A melt 9 of aluminium alloy EN AC 44300 was prepared in a graphite crucible to which 12 % by volume of silicon carbide particles with a size of 20 pm was added during melting and then 2 % by weight of pure magnesium was added. The temperature of the melt was 760 °C and its density was 2615 kg/m ³ . Subsequently, 10 % by volume of particles of magnesium oxide (MgO) with a size of 20 pm was added to the melt 9. This dispersion system was then stirred with a stirrer at 1000 rpm for 5 minutes. After stirring, it was poured into a foaming vessel 1 preheated to a temperature of 450 ° C, whereby its surface reached to the lower edge of a conduit 5 connecting the foaming vessel 1 to a foundry mould 6. Then the foaming vessel 1 was inserted into the interior of an electromagnetic coil 10 composed of six segments 101. 102, 103, 104, 105, 106, wherein the first segment 101 and the fourth segment 104 (counted from the bottom), the second segment 102 and the fifth segment 105 and the third segment 103 and the sixth segment 106 were interconnected in an anti-parallel manner and each pair was connected to one phase output of a three-phase current source. The first segment 101 and the sixth segment 106 of the coil 10 had each 1200 turns (0.5 mm CuSm), the second segment 102 and the fifth segment 105 of the coil 10 had each 1050 turns (0.5 mm CuSm) and the third segment 103 and the fourth segment 104 of the coil 10 had each 775 turns (0.5 mm CuSm). An alternating voltage of 40 V was applied to each pair of segments 101 and 104, 102 and 105, 103 and 106 of the electromagnetic coil 10 and a current of 0.5 A, with a frequency of 50 Hz, flowed through them, generating a translational electromagnetic field with a magnetic induction amplitude of 6 mT, which acted on the melt 9 in the foaming vessel 1. The criterion number F of the translational magnetic field thus generated was F = 1 -10 ² . The melt 9 was stirred with a stirrer 4, which rotated at a speed of 1500 rpm, and argon at a pressure of 0.2 MPa was blown into it. The foamed melt 9 flowed spontaneously through the conduit 5 into the inner space of the foundry mould 6 preheated to a temperature of 200 °C, in which a vacuum of the order of 10 ² Pa was created and in which the foamed melt 9 solidified. The foaming process took place for 8 minutes. The result was solid aluminium foam with a porosity of 68 % by volume containing regularly spaced pores with a diameter of 3 to 6 mm, with a wall thickness of 60 to 70 pm in its structure covered by a continuous envelope. The density of this foam was 520 kg/m ³ .

Example 2 A melt 9 of aluminium alloy EN AC 42 100 was prepared in a graphite crucible to which 10 % by volume of silicon carbide particles with a size of 20 pm was added during melting. The temperature of the melt was 750 °C and its density was 2675 kg/m ³ . Subsequently, 10 % by volume of particles of aluminium oxide (AI2O3) with a size of 15 pm was added to the melt 9. This dispersion system was then stirred with a stirrer at 1000 rpm for 5 minutes. After stirring, it was poured into a foaming vessel 1 heated to a temperature of 450 °C, whereby its surface reached the lower edge of a conduit 5 connecting the foaming vessel 1 to a foundry mould 6. Then the foaming vessel 1 was inserted into the interior of an electromagnetic coil 10 composed of six segments 101. 102, 103, 104, 105, 106, wherein the first segment 101 and the fourth segment

104 (counted from the bottom), the second segment 102 and the fifth segment

105 and the third segment 103 and the sixth segment 106 were interconnected in an anti-parallel manner and each pair was connected to one phase output of a three-phase current source. The first segment 101 and the sixth segment 106 of the coil 10 had each 1350 turns (0.5 mm CuSm), the second segment 102 and the fifth segment 105 of the coil 10 had each 1215 turns (0.5 mm CuSm) and the third segment 103 and the fourth segment 104 of the coil 10 had each 810 turns (0.5 mm CuSm). An alternating voltage of 40 V was applied to each pair of segments 101 and 104. 102 and 105, 103 and 106 of the electromagnetic coil 10 and a current of 0.25 A, with a frequency of 50 Hz, flowed through them, generating a translational electromagnetic field with a magnetic induction amplitude of 2 mT, which acted on the melt 9 in the foaming vessel T The criterion number F of the translational magnetic field thus generated was F = 2.5- 10 ² . The melt 9 was stirred with a stirrer 4, which rotated at a speed of 1500 rpm, and air at a pressure of 0.3 MPa was blown into it. The foamed melt 9 flowed continuously spontaneously through the conduit 5 into the inner space of the foundry mould 6 preheated to a temperature of 200 °C, in which a vacuum of the order of 10 ² Pa was created and in which the foamed melt 9 solidified. The foaming process took place for 10 minutes. The result was solid aluminium foam with a porosity of 65 % by volume, containing regularly spaced pores with a diameter of 3 to 5 mm, with a wall thickness of 65 to 70 pm in its structure covered by a continuous envelope. The density of this foam was 550 kg/m ³ .

Example 3

A melt 9 of aluminium alloy EN AC 42200 was prepared in a graphite crucible to which 10 % by volume of silicon carbide particles with a size of 20 pm was added during melting. The temperature of the melt was 735 °C and its density was 2660 kg/m ³ . An additional 12 % by volume of particles of silicon carbide (SiC) with a size of 20 pm was then added to the melt 9. This dispersion system was then stirred with a stirrer at 1000 rpm for 5 minutes. After stirring, it was poured into a foaming vessel 1 heated to a temperature of 450 °C, whereby it reached with its surface the lower edge of a conduit 5 connecting the foaming vessel 1 to a foundry mould 6. Subsequently, the foaming vessel 1 was inserted into the interior of an electromagnetic coil 10 composed of six segments 101. 102, 103, 104, 105, 106, wherein the first segment 101 and the fourth segment 104 (counted from the bottom), the second segment 102 and the fifth segment 105, as well as the third segment 103 and the sixth segment 106 were interconnected in an anti-parallel manner and each pair was connected to one phase output of a three-phase current source. The first segment 101 and the sixth segment 106 of the coil 10 had each 1450 turns (0.5 mm CuSm), the second segment 102 and the fifth segment 105 of the coil 10 had each 1390 turns (0.5 mm CuSm) and the third segment 103 and the fourth segment 104 of the coil 10 had each 750 turns (0.5 mm CuSm). An alternating voltage of 40 V was applied to each pair of segments 101 and 104. 102 and 105, 103 and 106 of the electromagnetic coil 10 and a current of 0.3 A, with a frequency of 50 Hz, flowed through them, generating a translational electromagnetic field with a magnetic induction amplitude of 2.5 mT, which acted on the melt 9 in the foaming vessel T The criterion number F of the translational magnetic field thus generated was F = 1 · 10 ³ . The melt 9 was stirred with a stirrer 4, which rotated at a speed of 500 rpm, and argon at a pressure of 0.2 MPa was blown into it. The foamed melt 9 flowed continuously spontaneously through the conduit 5 into the inner space of the foundry mould 6 preheated to a temperature of 200 °C, in which a vacuum of the order of 10 ² Pa was created and in which the foamed melt 9 solidified. The foaming process took place for 7 minutes. The result was solid aluminium foam with a porosity of 63 % by volume containing regularly spaced pores with a diameter of 4 to 6 mm, with a wall thickness of 50 to 70 pm in its structure covered by a continuous envelope. The density of this foam was 560 kg/m ³ . Example 4

A melt 9 of aluminium alloy EN AW6063 was prepared in a graphite crucible to which 8 % by volume of silicon carbide particles with a size of 20 pm was added during melting. The temperature of the melt was 760 °C and its density was 2675 kg/m ³ . Subsequently, 12 % by volume of particles of aluminium oxide (Al ₂ 0 ₃ ) with a size of 10 pm was added to the melt 9. This dispersion system was then stirred with a stirrer at 1000 rpm for 3 minutes. After stirring, it was poured into a foaming vessel 1 heated to a temperature of 450 °C, whereby its surface reached the lower edge of a conduit 5 connecting the foaming vessel 1 to a foundry mould 6. Then the foaming vessel 1 was inserted into the interior of an electromagnetic coil 10 composed of six segments 101. 102, 103, 104, 105, 106, wherein the first segment 101 and the fourth segment

104 (counted from the bottom), the second segment 102 and the fifth segment

105 as well as the third segment 103 and the sixth segment 106 were interconnected in an anti-parallel manner and each pair was connected to one phase output of a three-phase current source. The first segment 101 and the sixth segment 106 of the coil 10 had each 1575 turns (0.5 mm CuSm), the second segment 102 and the fifth segment 105 of the coil 10 had each 1125 turns (0.5 mm CuSm) and the third segment 103 and the fourth segment 104 of the coil 10 had each 725 turns (0.5mm CuSm). An alternating voltage of 40 V was applied to each pair of segments 101 and 104, 102 and 105, 103 and 106 of the electromagnetic coil 10 and a current of 0.4 A, with a frequency of 50 Hz, flowed through them, generating a translational electromagnetic field with a magnetic induction amplitude of 4.5 mT, which acted on the melt 9 in the foaming vessel 1. The criterion number F of the translational magnetic field thus generated was F = 5- 10 ² . The melt 9 was stirred with a stirrer 4, which rotated at a speed of 1500 rpm and air at a pressure of 0.2 MPa was blown into it. The foamed melt 9 flowed continuously spontaneously through the conduit 5 into the inner space of the foundry mould 6 preheated to a temperature of 200 °C, in which a vacuum of the order of 10 ² Pa was created and in which the foamed melt 9 solidified. The foaming process took place for 7 minutes. The result was solid aluminium foam with a porosity of 65 % by volume containing in its structure regularly spaced pores with a diameter of 3 to 4 mm, with a wall thickness of 65 to 70 pm in its structure covered by a continuous envelope. The density of this foam was 540 kg/m ³ .