EP3831509B1

EP3831509B1 - Apparatus and metallurgical process for the preparation and feeding of semi-solid magnesium alloys in a quasi-liquid state for casting injection machines

Info

Publication number: EP3831509B1
Application number: EP20211682.8A
Authority: EP
Inventors: Fabrizio D'ERRICO; Marco ROMEO
Original assignee: Innsight Srl
Current assignee: Innsight Srl
Priority date: 2019-12-05
Filing date: 2020-12-03
Publication date: 2022-07-20
Anticipated expiration: 2040-12-03
Also published as: EP3831509A1; IT201900023061A1

Description

TECHNICAL FIELD

The present invention is in the metallurgic field of magnesium alloys, in particular, of apparatuses and processes for the preparation of semi-solid magnesium alloys in a quasi-liquid state for casting injection machines.

BACKGROUND

The development of magnesium alloys (abbreviated Mg), which was driven during the last century by the aerospace industry, historically occurred in order to satisfy the needs of the transportation industry, that was seeking advantages on further strategies related to weight reduction, considering the very low specific weight of such alloys.
The viscosity of magnesium (abbreviated Mg) at the molten state is lower than the viscosity of aluminum alloys, allowing an easier filling of the mold cavities, thus allowing to make castings of particular geometrical complexity, even with thin walls. Compared to aluminum alloys, magnesium in a liquid state also has reduced chemical compatibility with steel used for manufacturing shell casting molds; this property allows to markedly reduce the phenomena of gradual wear of steel molds due to liquid metal attack. Although such promising characteristics, Mg alloys show significant problems due to the high reactivity of magnesium metal with oxygen. The high affinity of magnesium for the atmosphere oxygen causes high flammability of Mg and alloys thereof which, once flame-triggered, proceed with an incessant exothermic process of self-combustion, fed by the presence of oxygen (abbreviated O₂) and supported by the flame temperature: once the flame is triggered, in the presence of oxygen, the combustion reaction proceeds forming the combustion product magnesium oxide (abbreviated MgO) and releasing heat. The combustion flame rapidly reaches temperatures between 1 726,85 °C (2000°K) 3 726,85 °C (4000 °K), therefore capable of perforating any crucible, even if made of refractory material.
Due to this problem, Mg alloys require the use of special melting plants and trained personnel, increasing the production cost of Mg alloy castings despite the Mg raw material cost is actually similar to the cost of pure aluminum.
In order to safely control Mg alloys at the molten state, therefore, it is necessary removing O₂ contacting the metal bath, i.e. the primary trigger source of Mg combustion reaction is to be completely eliminated. That is possible by achieving inert atmospheres with respect to melted Mg. Accordingly, there are several techniques in Mg foundries, which are divided in: a) vacuum melting plants, b) melting plants provided with atmospheres being inert and protective of Mg bath. The inert atmospheres usually used are those mixed SF₆ and CO₂, those based on R-134a freon gas, and those based on SO₂. Both SF₆ R-134a freon gas are classified as very high GWP (global warming potential) greenhouse gases, while SO₂ gas, although being a valid alternative to greenhouse gases SF₆ and R-134a from an environmental point of view, requires stringent application protocols due to its high toxicity to the operators.
All these problems related to the uncommon technical skills necessary for magnesium melting, the high investment costs for magnesium die casting nowadays relegate magnesium melting to a market niche in the aerospace, military air force, and automotive field.
Together with the above-cited magnesium technological barrier limiting the widespread use of magnesium, although it is the lightest metal available on Earth, when conventional melting processes are used, metallurgic drawbacks exist (e.g. gravity die casting, pressure die casting, centrifugal die casting) for producing fused pieces.
Starting from the '90s, it was found that the addition of calcium (abbreviated Ca) to magnesium alloy significantly increased the flammability temperature of Mg, as cited by Sakamoto, Mitsuru et al. in "Suppression of ignition and burning of melted Mg alloys by Ca bearing stable oxide film." Journal of Materials Science Letters, 16, (1997): 1048-1050. According to such article, the addition of only 1% by weight of Ca in Mg alloys promotes the melting step and the subsequent casting of Mg alloy in the air without using blanket gases. Currently is known that other elements of Mg alloy such as beryllium (abbreviated Be) and Yttrium (abbreviated Y), together with calcium, are common inhibitors of Mg alloy ignition. The phenomenon regulating the higher protection provided by Ca, Be and Y elements present in the Mg bath against oxygen present in the atmosphere, although little known, it is however known; the volume ratio between the volume of Mg oxide (abbreviated MgO) formed by spontaneous reaction of Mg of the bath with air O₂ and the volume of underlying melted Mg is lower than the unity. This implies that, for each mass unity of melted Mg reacting with atmospheric oxygen, a magnesium oxide film not capable of covering the entire melted Mg metal surface, and therefore not capable of operating its action of continuous, stable protective film, and then perfectly oxygen impermeable unlike melted aluminum, is formed. The result is the presence on the surface of the melted Mg of a film of porous oxide exposing the Mg bath underlying to the oxide film to the oxygen present in the atmosphere, activating the above-mentioned conditions of flame ignition and resulting reaction of exothermic combustion between liquid Mg and the oxygen in the atmosphere.
In particular, Sakamoto demonstrated that the structure of oxide film extending on the Ca-added Mg alloy added is characterized by a very thin and uniform oxide film, which is resistant although the long air expositions of the melted alloy at 973 °K for 60 minutes, confirming that the oxide film formed as Mg alloys with Ca acts as an effective barrier for preventing both oxygen diffusion from air to the melted metal and magnesium volatilization. In the last years, together, not flammable pure-Ca added Mg alloys (and/or with other above-mentioned flame inhibitor elements such as Be and Y) calcium oxide (CaO)-modified magnesium alloys, commercially known as Eco-magnesium alloys (abbreviated Eco-Mg) were developed to be used at low costs similar to the Mg alloys with Ca as a pure alloying element. The use of high-reactive elements with atmosphere oxygen - such as Ca, Be and Y - allow to considerably reduce (theoretically, eliminating) the use of the above-mentioned gases, highly polluting protective gas even using melting plants closed in protective atmosphere usually used for melting conventional Mg alloys, i.e. without adding the above-mentioned three elements.
Patent documents WO 92/15412 A1 , US 2004/129402 A1 , WO 02/18072 A1 and CN 109 666 818 A disclose an apparatus for the casting of a semi-solid magnesium alloy in a quasi liquid state.

DESCRIPTION OF THE DRAWINGS

Figure 1 shows a scheme of the apparatus (17) of the invention used for the preparation of the semi-solid magnesium alloy in a quasi liquid state, wherein:
- Chamber X is the melting chamber adapted to prepare and/or hold the magnesium alloy to the molten state;
- Chamber Y is the chamber wherein the semi-solid magnesium alloy in a quasi-liquid state is mainly made;
- Chamber Z is the dosage wherein the semi-solid magnesium alloy in a quasi liquid state is dosed, through a measuring tube (9), outside the apparatus (17);
- (1) is the opening of the sluice-gate type, or the like in terms of functionality, of chamber X;
- (6) is a communicating channel between chamber X and chamber Y, or between chamber Y and Z, and is adjacent to the bottom or the proximity of the chamber bottom;
- (5) are heating elements;
- (2) is a porous septum, placed on the lower side of chamber X, and allows inert gas insufflation;
- (4) is one or more inserts of high electric and thermal conductivity material, such as graphite or silicon carbide, inserted on the bottom of the chamber;
- (5) optional, an electromagnetic induction block plate (3) outside the bottom immersed in an insulating material (3bis) in order to isolate the bottom of the apparatus (10) from the magnetic field produced by the high voltage alternating current flow and variable frequency within the conductive windings (3ter), according to what is well known in the field, with particular reference to the electromagnetic field laws determining parasitic current formation (or Foucault's currents) induced into a metal conductive mean, both solid and liquid, when it is submerged in a variable magnetic field caused and connected to the alternating electric current circulation in the aforementioned conductive windings (3ter);
- (12) inlet and outlet of the water cooling circuit for cooling the conductive windings and preservation thereof;
- (7) is a functional element for the closing of channel (6), such as, for example, a piston (7), that is an element operated by an external command capable of opening and closing the communicating channel (6) between chamber Y and chamber Z.
- (8) is a valve, located on the upper side of chamber Z which allows insufflating the inert and/or protective gas for the Mg alloy bath;
- (9) is a dosing tube (9) that allows the leaving of the magnesium alloy from the apparatus (17).
Fig. 2 shows the apparatus of the invention, provided with elements for the magnetic stirring such as rotating permanent magnets (10bis), in the pace of the electromagnetic stirring of Fig.1.
Fig. 2-bis shows the detail of an example of the positioning of the rotating permanent magnets (10bis) and rotating plate (10), for the support thereof. According to an alternative embodiment, the rotating magnets (10bis) can be placed on different and smaller plates that are adjacent to each other, instead of being on a single plate (10) as shown in the scheme of Fig.2.
Fig. 3 shows the apparatus provided with a magnetic stirring with rotating permanent magnets placed in parallel to the vertical walls of the apparatus chambers.
Fig. 4 shows an optical microscope micrography of a metallographic sample obtained by a specimen into a graphite ingot mold with the apparatus of the invention already sectioned by a metallographic blanking machine, smoothed and polished for the optical microscopic observation. The observed structure is equiaxed, globular.
Fig. 5 - comparative - shows the microstructure obtained for the same alloy (of Fig. 4) by a conventional melting process obtained without using the apparatus (17), and casting into the same graphite ingot mold by gravity.

DESCRIPTION OF THE INVENTION

The present invention is directed to an apparatus for the preparation of semi-solid magnesium alloys in a quasi-liquid state, under safety conditions, avoiding the use of highly polluting protective gas.
Furthermore, the coupling of the apparatus according to the invention with the process involving the use thereof allows solving the limits related to magnesium melting and its alloys, thus reducing as much as possible the melting temperature and the temperature at which the product is poured. All of this is achieved through the apparatus of the invention and preparing a semi-solid magnesium alloy near to the alloy liquidus temperature, i.e. in a quasi-liquid state, using an oxygen-depleted atmosphere and a particular stirring system that, together with the above-mentioned conditions, allows to avoid the Mg flame ignition.
Furthermore, the process involves a step wherein, under isothermal conditions, the semi-solid magnesium alloy is subjected to stirring for a certain time, with particular stirring systems, which allow obtaining a highlyhomogeneous alloy, a high-pourability globular equiaxed microstructure (see Fig. 4) and having a reduce volumetric contraction at the terminal stage of complete solidification, allowing to obtain complex-shape castings and in the absence of porosity and/or shrinkage cavities typical of solidified castings from liquid conditions.
The invention is also described according to preferred embodiments and the claims, whose definitions are an integral part of the present description.

DETAILED DESCRIPTION OF THE INVENTION

Object of the present invention is an apparatus and a metallurgical process using the same for the preparation of a semi-solid magnesium alloy comprising calcium and/or calcium oxide (CaO) (hereinafter Mg alloy with Ca and/or CaO) semi-solid near to the liquidus temperature but within the range of solidification of the metal alloy, i.e., under conditions of quasi-liquid semi-solid mass.
An object of the invention is an apparatus (17) and a process for dosing the melted metal belonging to the magnesium alloy family, from a closed furnace achieving, inside, an inert atmosphere for Mg alloy casting.
The above-mentioned apparatus has the characteristic of having three separated chambers for the management of the steps of:

1. preparation of the liquid alloy,
2. refining and optionally degassing,
3. stirring and semisolid phase mixing (for the dendritic breaking and the conversion to a low-viscosity globular semisolid state) and thermostatation thereof,
4. dosing into a casting container, which is physically connected, preferably, to an injector.

Once the semi-solid magnesium alloy in a quasi-liquid state is prepared, it is directly dosed into the injection chamber of the pressure casting machine. The above-mentioned three-chamber concept can be intended as one single chamber having three separated areas, for example, made by suitable separation elements, wherein the operations of described above points 1. to 4. are carried.
The apparatus (17) of the present invention for the preparation of a semisolid magnesium alloy in a quasi-liquid state comprises three communicating chambers, where:

the melting chamber X provided with a sluice-gate opening (1), or another type for similar-function and objective, on the upper side, of a communicating channel (6) between the chamber X and the chamber Y adjacent to the bottom, of heating elements (5), of a porous septum (2) on the lower side for inert gas insufflation, and of one or more inserts of graphite or silicon carbide (4) inserted on the bottom, and an electromagnetic induction block plate (3) and/or one or more rotating plate (10) bearing permanent magnets (10bis) and/or rotating permanent magnets (10ter);
the chamber Y provided with heating elements (5), of a communicating channel (6) between the chamber Y and the chamber Z adjacent to the bottom, of a operated element such as, for example, a piston (7) or other element of similar function and objective capable of opening and closing the channel (6) and of one or more inserts of graphite or silicon carbide (4) inserted within the bottom, and an electromagnetic induction block plate (3) and/or one or more rotating plate (10) bearing permanent magnets (10bis) and/or rotating permanent magnets (10ter);
the dosing chamber Z provided with heating elements (5), a valve (8) on the upper side, a dosing tube (9) which allows the leaving of the magnesium alloy from the apparatus (17), and of one or more inserts of graphite or silicon carbide (4) inserted within the bottom, and an electromagnetic induction block plate (3) and/or one or more rotating plate (10) bearing permanent magnets (10bis) and/or rotating permanent magnets (10ter).

The apparatus (17) can also be named furnace, semisolid furnace, dosing furnace, melting furnace, etc..
The apparatus (10) is composed of three communicating chambers, nominate X, Y, and Z, see scheme of Fig.1.
The chamber X is related to the charge and melting of the material that can be already at liquid phase from a standing furnace with higher capacity, or, as in the example of Fig.1, in solid phase as slabs or ingots.
Therefore, the chamber X bearing a sluice-gate opening (1) or other types with similar function and objective which is, for its opening, protected by the inert gas flow to avoid the direct contact between the melting alloy in the chamber X and the new solid or liquid charge introduced.
The chamber X of charging and pre-heating is also provided, on the bottom, with a porous septum (2) made of graphite or in silicon carbide or other inert material, from which inert gas is insufflating, such as nitrogen or argon, for the optional degassing step.
Along the lower wall of the furnace, outside the construction carpentry of the furnace bottom, there is an inductive coil plate (3) generating, for electromagnetic effect, parasitic currents in the liquid metal mass. When such induced currents cross a conductive body, being solid or liquid, in the presence of an external magnetic field, a force (Lorentz force) acting on the conductor itself is provided. Generally, such force is of the rotational-type and, if the conductor is liquid, it is induced to move (i.e. carried away) in "response" to this force. In simple words, this is the principle of electromagnetic stirring.
The inductive coil plate (3), in a preferred low energy consumption form, of the present invention can be replaced by a rotating plate (10) or, alternatively, by adjacent multiple rotating plates (10), bearing permanent magnets (10bis) such as in the example of Fig.2. The permanent magnet plate (10bis) is also illustrated in the exemplified scheme of Fig.2bis. The use of a rotating plate (10) with permanent magnets (10bis), is an efficient alternative to an electromagnetic induction block plate (3): in fact, the magnetic field change necessary to develop within the liquid metal said Foucault's currents, in the embodiment of the invention bearing the magnetic induction plate (10) with permanent magnets (10bis) is typically caused by rotation ω of Fig.2bis of permanent magnets positioned on the rotating plate with NORTH and SOUTH polarity couples. According to the embodiment of the apparatus (17) illustrated in Fig. 2 where (10) and (10bis) are reported, rotation of (10) and (10bis) is preferably carried out preferably by a high-efficiency electric motor and constant couple, such as a brushless motor.
In another embodiment of the invention, reported in Fig. 3, for making the magnetic stirring, instead of the electromagnetic stirring, the rotating plate (10) and related permanent magnets (10bis) can be replaced by permanent magnets (as a circumferential sector) represented in section from elements (10ter) in coaxial rotation with two main cylindrical section crucibles (13) and (14). (13) is chamber Z, while (14) the two chambers Y and X separated by an inner wall almost diametral made in the same crucible (14). The communicating crucibles (13) and (14) are preferably made of high electrical conductivity material and highly permeable to the variable magnetic flow generated by rotation ω1 and ω2 of the permanent magnets around axes (15) and (16). In the embodiment of the invention exemplified in Fig.3 described herein, the coaxial rotation of permanent magnets is carried out preferably by two independent constant torque electric motors of the brushless type connected preferably by two bevel gear groups (14).
The apparatus (17) can comprise one or more rotating plates (10) bearing permanent magnets (10bis). In the case of more than one rotating plate (10), and therefore in the case of multiple rotating plates 10) bearing permanent magnets (10bis), to put in rotation, they are adjacent to each other.
According to an alternative embodiment, the rotating magnets (10bis) can be placed on different and smaller plates, which are adjacent to each other, instead of on a single plate (10) as shown in the scheme of Fig.2.
The bottom of the three chambers is provided with inserts (4) made of highly conductive material preferably graphite or silicon carbide, i.e. highly conductive materials. When the magnetic field produced by the plate (electromagnetic induction block) permeates these inserts, these also respond by generating induced parasitic currents and, due to the high electrical conductivity and high electrical resistivity of such inserts, the Joule effect is very high and such that further heating the liquid metal through which parasitic currents pass.
As support to such heat source, electrical resistors (5) present in Fig.1 in chamber Y, for example, can be placed separately for each chamber to contribute through the submerging temperature auxiliary probes (5bis) to the thermoregulation of the mass present in chamber by suitably balancing the heat supply in order to obtain metal mass thermoregulation at a controlled temperature close to liquid-to-solid state passage.
The dosing chamber, or chamber Z of Fig. 1, is made such that channel (6), which connects it to the heating chamber Z can be closed through an electrovalve controlling a gate piston (7) in order to avoid the liquid mass back from chamber Z to chamber Y, as schematically shown in Fig.1.The electro-controlled piston(7) closes or opens the communication door between chamber Y and chamber Z.
When the door is closed, a valve (8) located on the top of chamber Z is opened and pressurized inert gas is insufflated. The pressurized gas forced metal returning in the dosing tube, allowing the magnesium alloy leaving from apparatus (17) through the connecting tube. The alloy can optionally feed an injector through the dosing tube (9) connecting the apparatus (17) and the injector.
The apparatus (17) is, at least, completed with the following transducers:

a pressure sensor, for determining the pressure in the chamber;
an exhaust gas outlet valve;
Three primary temperature detection probes, to control metal temperature within the three chambers, which must be precisely adjusted at the quasi-liquid semisolid state.

The injector is, optionally, physically connected to the apparatus (17) and receives the metal, from this latter, in a dosed amount and under semisolid conditions.
The apparatus can be provided with, an immersion thermocouple to measure the magnesium alloy temperature, in each of the three chambers. The heating elements (5) are, preferably, electrical resistors.
All the heating elements (5), preferably, are controlled by a PLC to thermoregulating the bath as a function of the instant temperature reading carried out by the auxiliary probe (5bis).
All the heating elements (5), preferably, the electrical resistors work independently. Therefore, they can provide different temperature values to the apparatus chambers.
According to a preferred embodiment, the three heating elements are three electrical resistors operating independently.
Preferably, in the apparatus, each of the three chambers is provided with a pressure sensor.
The apparatus is made of refractory material.
The block plate with coil and/or turns for the electromagnetic induction (3) allows to carry out the electromagnetic stirring. Said plate being optional, meaning that it can be integrated in the apparatus or, alternatively, the apparatus without such plate can be contacted, preferably above, to an outer electromagnetic induction block plate.
According to a preferred embodiment, the electromagnetic induction block plate (3) is integrated in the apparatus (17).
The inductive coil plate (3), of the present invention, can be replaced by one or more rotating plates (10), adjacent to each other, bearing permanent magnets (10bis) such as in the example of Fig.2. As the inductive coil plate (3), the rotating plate (10) bearing permanent magnets (10bis) is optional, meaning that it can be integrated in the apparatus (17) or it can be independent and then external to the apparatus (17). The apparatus (17), in such case, can be arranged adjacent to the rotating plate (10), or more adjacent rotating plates, bearing permanent magnets (10bis), preferably above and/or on the side.
In another embodiment of the invention, illustrated in Fig. 3, for the magnetic stirring, permanent magnets are optionally used (a circumferential sector) (10ter) or rotating permanent magnets or coaxial rotation permanent magnets. Also, in this case, said means generating the stirring are optional, meaning that they can be part of the apparatus (17) or external, and in such case, the apparatus (17) will be arranged adjacent to said means.
According to a preferred embodiment, in the apparatus (17) the electromagnetic induction block plate (3) and/or a rotating plate (10) bearing permanent magnets (10bis) and/or rotating permanent magnets (10ter) are an integral part of the apparatus (17).
In the apparatus of the invention (17) is in fact possible the coexistence of various possible stirring modes, for example, the electromagnetic induction block plate (3) and a rotating plate (10) bearing permanent magnets (10bis) and rotating permanent magnets (10ter); alternatively the electromagnetic induction block plate (3) and one or more from rotating plate (10) bearing permanent magnets (10bis) or rotating permanent magnets (10ter) adjacent to each other; alternatively a rotating plate (10) bearing permanent magnets (10bis) and rotating permanent magnets (10ter); etcetera,.
Said stirring means can be the same or different for each chamber, therefore a stirring mean can be present in a chamber, for example, the stirring in the electromagnetic induction block plate (3) and a different magnetic stirring mean can be present in another chamber, for example, a rotating plate (10) bearing permanent magnets (10bis) or rotating permanent magnets (10ter).
According to a preferred embodiment, the electromagnetic induction block plate (3) and/or a rotating plate (10) bearing permanent magnets (10bis) is located on the bottom of the apparatus and/or outside the bottom. According to a preferred embodiment, the rotating permanent magnets (10ter) are positioned abreast and adjacent to the vertical walls of one or more chambers.
The apparatus optionally comprises a grounding for the electrostatic discharge.
Another object is the metallurgical process using the apparatus of the invention for the preparation of a semi-solid magnesium alloy in a quasi-liquid state comprising the following steps:

a') loading la chamber X of the apparatus (17), through the opening (1), with magnesium alloy comprising calcium and/or calcium oxide and bringing or maintaining it at the molten state;
b') stirring the magnesium alloy at the molten state present in chamber X by electromagnetic stirring and/or magnetic stirring with permanent magnets and/or by bubbling nitrogen or argon gas;
c') allowing the magnesium alloy at the molten state to pass from chamber X to chamber Y;
d') thermoregulating the melted magnesium alloy of step c') until a semisolid magnesium alloy in a quasi-liquid state is formed;
e') lifting piston (7) and allowing the semi-solid magnesium alloy in a quasi-liquid state to pass from chamber Y to chamber Z;
f') lowering the piston closing the communicating channel (6) between chamber Y and chamber Z;
h') insufflating inert gas to exert pressure above the free surface of the semisolid magnesium alloy in a quasi-liquid state and pushing it into the dosing tube (9) and making it coming out the apparatus (10).

The magnesium alloy in step a') can be at the solid, semisolid, or molten state. Therefore, when solid or semisolid, it is brought to molten, or liquid state, in case it is already at the molten state, it is maintained at the molten state.
According to a preferred embodiment, in step a') one or more magnesium alloy ingots from the sluice-gate (1).
According to a preferred embodiment, step a') is performed under an inert gas flow protective atmosphere;
According to a preferred embodiment in step a') the ingot melting is carried out within chamber X through the presence of an already liquid mass.
In step b') the stirring is carried out by electromagnetic stirring and/or magnetic stirring with permanent magnets and/or by bubbling nitrogen or argon gas.
According to a preferred embodiment, in step b') the stirring is carried out by electromagnetic and/or magnetic stirring.
According to a more preferred embodiment, in step b') the melted material is constantly stirred by the block plate of the electromagnetic inductor (3) and/or a rotating plate (or plates) (10) bearing permanent magnets (10bis) and/or rotating permanent magnets (10ter), making the electromagnetic and/or magnetic stirring, respectively. The induced currents allow to maintain the bath of magnesium alloy at the molten state under stirring, and, simultaneously, heat the inserts (4) due to the Joule effect. The heat, produced in the insert by conduction, is transferred to chamber X. According to the same principle, heat is transferred in all three chambers.
In step d'), thermoregulating the melted magnesium alloy of step c'), i.e. brought to temperature, until semi-solid magnesium alloy in a quasi-liquid state is formed; typically, but not necessarily, step d') is conducted by cooling. The thermoregulation usually allows to suitably reduce the magnesium alloy temperature to bring and maintain it at the semisolid state. The heating elements, preferably, the electrical resistors (5) allows to adjust the local temperature in chambers X, Y and Z.
According to a preferred embodiment, the heating in the chambers is carried out by electromagnetic or magnetic stirring, depending on the variable source of the magnetic field, which causes the simultaneous heating of the container of one, two or three cambers, corroborated by the heating provided by the heating elements, preferably electrical resistors, thus allowing the fine and independent regulation of the temperatures in one, two or three chambers.
In step e') the piston (7) is lifted controlled by an electrovalve to feed chamber Z with the metallostatic swing at the level of the chamber X (according to the communicating vessel principle);
In step f') the piston (7) is lowered by closing chamber Z, hermetically.
In step h') inert gas (for example, nitrogen or argon) is insufflated to exert pressure above the free surface of the quasi-liquid semisolid metal and pushing it within the dosing tube (9), from which the alloy comes out the apparatus (9).
The dosing tube (9) can optionally feed an injection group, or a container of the injection group, which then injects the alloy within the press.
Another object is a further metallurgical process using the apparatus of the invention for the preparation of a semi-solid magnesium alloy in a quasi-liquid state comprising the following steps:

a) loading the above-described apparatus with magnesium alloy comprising calcium and/or calcium oxide and bringing or maintaining it at the molten state,
b) insufflating nitrogen-type inert gas through (8), achieving an atmosphere with an oxygen content not higher than 10% mol. inside said apparatus,
c) cooling the melted magnesium alloy of step b) until a semi-solid magnesium alloy in a quasi-liquid state is formed,
d) maintaining the semi-solid magnesium alloy in a quasi-liquid state of step c) under isothermal conditions at the temperature obtained by step c), stirring for at least 20 seconds by electromagnetic stirring and/or magnetic stirring with permanent magnets,
e) discharging the semi-solid magnesium alloy in a quasi-liquid state from the apparatus.

The magnesium alloy in step a) can be at the solid, semisolid or molten state. Therefore, if solid or semisolid it is brought to the molten state, or liquid state, if already at the molten state, is maintained at the molten state.
According to a preferred embodiment, the metallurgical process comprises the following steps:

a) loading an apparatus (17) above-described with magnesium alloy comprising calcium and/or calcium oxide under conditions of complete melting, i.e. at a temperature higher than the alloy liquidus temperature;
b) inside said apparatus, which is top closed with a sealed lid, preferably of the gate-type, achieving an atmosphere with oxygen content in dry atmosphere not higher than 10% mol. by insufflation of inert gas of nitrogen or argon;
c) measuring the bath temperature by means of a submerged thermal probe, thermoregulating the apparatus in order to thermostat the above-mentioned magnesium alloy to a process temperature in a thermal range between 5°C and 15°C below the temperature bath complete melting, this latter referred as liquidus temperature;
d) maintaining the magnesium alloy comprising Ca and/or CaO under isothermal conditions within the above-mentioned range of near-toliquidus semi-solid process described in previous point c) and stirring the mass for a time of at least 20 seconds by electric coil electromagnetic stirring (3) or magnetic stirring with rotating permanent magnets;
e) discharging the semi-solid magnesium alloy in a quasi-liquid state from the apparatus of preparation.

Steps a) and b) are carried out in the chamber X of apparatus (17), steps c) and d) in the chamber Y, and step e) in the chamber Z).
According to an alternative embodiment, the process can involve a variant wherein steps a) and b) are reversed.
The process of the present invention, which is carried out in the apparatus (17), allows the preparation of magnesium alloys comprising Ca and/or CaO under semi-solid conditions avoiding the adoption of conventional highly polluting protective gases (such as SF₆, SO₂, R134a) which, in the conventional processes, on a precautionary basis, however, are necessary although the alloys are Ca- or CaO-added since, during the stirring step, the oxide film stabilized in a static melting stage would continuously removed thus causing the trigger of incipient flame also on the Mg alloys containing Ca/CaO, thus making the preparation of the semi-solid mass unfeasible even for these alloys in a free atmosphere.
The fact of operating the specific stirring magnetically and/or electromagnetically of the semi-solid magnesium alloy comprising Ca and/or CaO in combination with an atmosphere with an oxygen content not higher than 10% mol. within the apparatus (17) allows to completely avoid the trigger and the subsequent combustion of said magnesium alloy.
Furthermore, the fact of operating such specific stirring of the above-cited alloy in a semi-solid state under isothermal conditions, together with the fact that said stirring is carried out by electromagnetic stirring and/or permanent magnets, i.e. it is a stirring wherein no mechanical shearing is applied, instead, under isothermal conditions, the end of solid nucleus enucleation with globular-like equiaxed growth is promoted, allowing to prepare an alloy characterized at the same time by:

a) globular equiaxed microstructure, i.e. not dendritic (see Fig. 4),
b) a low volumetric concentration of the liquid residual mass during the terminal stage of complete solidification of the casting inside the casting mold, where this latter property, i.e. the limited volume reduction of an already semisolid mass, which promotes the production of melted casting in the absence of hot tearings and/or shrinkage cavities (micro/macro shrinkage cavities).

Such benefits affect the manufacturing of objects (in jargon castings) made of magnesium alloy having complex shapes with, if required, thin walls being free of defects.
In other terms, the above-described specific process using stirring modes performed under isothermal conditions and in O₂-depleted atmosphere and in the absence of mechanical stirring (or carried out by electromagnets and/or by rotating permanent magnets) - spontaneously resulting in the homogeneous formation of solidification nuclei with their subsequent equiaxed-type growth, such that to eliminate the above-mentioned lacks typical of conventional solidification (i.e. from 100% liquid conditions).
Therefore, the metal objects or pieces made of magnesium alloy manufactured according to the process of the invention have higher shapedefinition and profile-precision with respect to both those obtained by the current "full-liquid" processes and those obtainable in the case a mechanical stirring is performed in the process of the present invention, in particular minor micro-defects.
The semi-solid magnesium alloy in a quasi-liquid state can have preferably a solid fraction content between about 0% and about 20%.
The quasi-liquid semi-solid magnesium alloy is a magnesium alloy non-flammable under certain conditions, i.e. a magnesium alloy known to be non-flammable, such as Mg-alloys comprising Ca and/or CaO (and, in substitution or in addition to Ca as anti-flammability, also Be and/or Y elements) and it can further comprise other elements usually used in foundry Mg-alloys.
The quasi-liquid semi-solid magnesium alloy must comprise, as well as the typical alloy elements present in the magnesium alloys, Ca and/or calcium oxide such as elements increasing the flammability temperature of conventional magnesium alloys; it can optionally comprise other elements, such as Be, Y together with or alternatively to Ca and/or CaO and/or Aluminum.
The semi-solid magnesium alloy in a quasi-liquid state comprises calcium and/or calcium oxide, and according to a preferred embodiment, the semisolid magnesium alloy in a quasi-liquid state can further comprise aluminum.
The aluminum improves the mechanical properties in the aluminum alloys, but reducing the time of alloy self-combustion trigger in air.
According to a more preferred embodiment, the semi-solid magnesium alloy in a quasi-liquid state comprises calcium and/or calcium oxide in an amount ranging between 1% and 2 % by weight, i.e. weight-on-weight, and aluminum ranging between 2% and 9% by weight.
In step b), an atmosphere with an oxygen content lower than 10% mol. is achieved inside the apparatus. For this purpose, the apparatus is hermetically sealed and provided with a closed heated thermoregulation chamber, closed provided with the porous septum inlet channel (2) to insufflate inert gas such as, for example, nitrogen or argon, and with an outlet channel to remove the air of the atmosphere containing oxygen to reduce O₂ concentration in the apparatus atmosphere below 10% mol. before the beginning of the semi-solid mass stirring.
The term 10% mol. means the oxygen content in dry atmosphere as oxygen moles with respect to the total moles. The oxygen content in dry atmosphere, means a water-free atmosphere, steam or moisture. According to a preferred embodiment, the atmosphere has an oxygen content ranging between 0.1 and 10% mol., more preferably between 0.1% and 5% mol..
Optionally, the outlet channel can be connected with a vacuum pump to accelerate oxygen evacuation from the apparatus.
The quantification of the maximum oxygen content, into the atmosphere of the closed apparatus must be lower than 10% mol. can be carried out on the leaving gas effluent by the conventional oxygen-content detectors currently available on the market.
Step a) of loading the apparatus with melted magnesium alloy can be carried out by simple pouring of the melted Ca and/or CaO-added magnesium alloy prepared in another furnace through an opening, closeable, of the apparatus. Preferably, but not necessarily, the loading is carried out by a closable inlet mouth, maintaining open the inert gas inlet inside the apparatus chamber, such that the loading occurs under a protective atmosphere, and in countercurrent.
In step d), the apparatus chamber is thermoregulated to get the temperature of the melted Ca and/or CaO-added magnesium alloy to a value such that a semi-solid magnesium alloy in a quasi-liquid state is made. The temperature at which the apparatus is set is a temperature below the one at which the alloy passes into a liquid state, but above the minimum temperature wherein the chemical composition of the alloy can locally vary due to the solidification, even partial, of one or more of its constituents; said minimum temperature depends on the composition of the magnesium alloy considered and it must be set such not more than 20% of solid fraction is present in the semi-solid magnesium alloy in a quasi-liquid state.
The temperature of the magnesium alloy can be controlled by a thermocouple submersed in the alloy itself.
According to a preferred embodiment, in step c) the melted magnesium alloy is cooled until a temperature within a ranged between 5°C and 15°C, preferably between 5°C and 10°C, below the liquidus temperature (i.e. specific temperature of solidification beginning during the cooling of the overheated liquid) so as to make a semi-solid Ca and/or CaO-added magnesium alloy in a quasi-liquid state.
According to a more preferred embodiment in step c) the melted magnesium alloy is cooled until a temperature ranging between 570°C and 610°C, even more preferably 580-595°C.
In step c), the apparatus chamber is thermoregulated to maintain the magnesium alloy in such an isothermal temperature condition, below the temperature wherein the alloy passes into a liquid state, but above the minimum temperature for which the subsequent and essential step of isothermal stirring described at point d) makes the nucleation conditions of solid germs and the equiaxed growth thereof without the need of mechanically breaking of dendrites, which are conventional solidification structures.
The temperature of step d) is the same with respect to the temperature obtained at the end of step c).
According to a preferred embodiment, in step d) the isothermal condition is obtained by a constant temperature which is comprised in a temperature range set between 5° and 15°C below the temperature of liquidus of the specific alloy treated.
According to a preferred embodiment, in step d) the isothermal condition is carried out by a constant temperature ranging between 570°C and 610°C, more preferably between 580°C and 600°C, even more preferably between 580-595 °C.
In step d), the stirring under isothermal conditions is carried out for at least 20 seconds, preferably for a time ranging between 20 seconds and 30 minutes, more preferably between 60 seconds and 20 minutes, even more preferably between 60 seconds and 500 seconds.
In step d) the stirring can be carried out by electromagnetic stirring and/or by magnetic stirring with permanent magnets.
According to a preferred embodiment, in step d) the stirring can be carried out by magnetic stirring with rotating permanent magnets.
The magnetic stirring with rotating permanent magnets can be carried out by a rotating plate (10) bearing permanent magnets (10bis) and/or by rotating permanent magnets (10ter) (see Fig. 2 + Fig. 2bis and/or Fig. 3, respectively).
In step d) the stirring can be carried out by magnetic stirring with rotating permanent magnets such as in the example of Fig.2 and Fig.3. when operating under coaxial stirring such as in the example of Fig.3, pushing effects of electromotive forces developed in the liquid or semisolid mass are developed, which are capable of giving an effect of "lifting" in the liquid mass which tends to separate the metal mass from crucible walls, thus reducing also the physiological phenomena of wear if the construction material of crucible is refractory.
The electromagnetic stirring and, more efficiently, the magnetic stirring allow to carry out the stirring action under isothermal conditions thus avoiding the use of mechanical stirrers whose integration inside the apparatus would cause several mechanical problems on one side, and subjecting the semisolid magnesium alloy to shearing, which provides, as already said, worse results in terms of magnesium alloy homogeneity, on the other side. In fact, the stirring system through which carrying out step d) allows to avoid the use of complex mechanical architecture of the apparatus for the preparation of semi-solid magnesium alloys and provide a product with the best homogeneity, or minor defects.
The step e) of discharging the semi-solid alloy in a quasi-liquid state from the apparatus can be carried out by pouring the content of the apparatus into another apparatus, for example in an injector or in a press.
Another object is a process for the preparation of objects or articles consisting of magnesium alloy comprising the following steps:

1) preparing the magnesium alloy comprising calcium and/or calcium oxide in a molten state;
2) preparing the semi-solid magnesium alloy comprising calcium and/or calcium oxide in a quasi-liquid state according to the processes described above;
3) filling an injector with the semi-solid magnesium alloy in a quasi-liquid state of step 2) and injecting it into the mold of a press;
4) molding the object by the press.

The object or article is then discharged from the press.
Considering the characteristics given to the semi-solid magnesium alloy in a quasi-liquid state according to the process described above, the present process provides objects with high shapes definition and, then consequently, of the shapes details.
According to a preferred embodiment, in step 3) the filling of the injector with the semi-solid magnesium alloy in a quasi-liquid state is performed under an atmosphere with an oxygen content not higher than 10% mol..
The apparatus (17) and the metallurgical processes of the present invention was developed for treating magnesium alloys comprising calcium and/or calcium oxide (CaO) affected by the problem of easy inflammability, in the absence of polluting protective gases.

EXPERIMENTAL SECTION

An apparatus (17) as reported in Fig.1 is filled with 2000 g of alloy of the AZ91D series added with 1.5% of CaO. The cited alloy is a commercially available alloy.
The apparatus was fed through a feeding control unit (not shown in the scheme). The feeding control unit provided the conductive coil (3ter) with alternating current between 1000 and 3000 A with a variable frequency con in the range 300-1000Hz.
In order to protect the melting step, the apparatus (17) was closed through the door (1) and then the valve for nitrogen-type inert gas insufflation were opened, and simultaneously the seal valve is opened (not shown in the scheme) to promote atmosphere turnover within the furnace through the progressive evacuation of an oxygen-rich atmosphere.
Controlling through the oxygen-detection sensor (not shown in the scheme of Fig.1) the content of O₂ in the atmosphere present in the melting chamber X, the maximum allowable O₂ amount, or < 10% mol. was provided.
A temperature of chamber of 680°-700°C was then reached within the apparatus (17), in order to promote the complete melting of the mass. The complete melting of the mass occurred in about 40 minutes. After having reached the complete melting, the furnace temperature was brought to the temperature of 580°C and thermoregulated through auxiliary submerging probes (7) and auxiliary resistors of nominal power equal to 3kW, at the target temperature of 590 °C.
The sluice-gate (7) is closed through the control of the operating electrovalve. Therefore, the set-point temperature stabilization equal to 595°C was reached.
After 595°C temperature was reached, the valve was opened by nitrogen inert gas flushing (a pressure of 1,5-3 bar was reached in the chamber Z) in order to promote metal outflow from the nozzle (9).
The liquid mass was therefore cast into a pre-heated graphite ingot mold at 150°C for the examination of the final structure obtained.
For this aim, the casting specimens are sectioned by a metallographic blanking machine, smoothed, and polished for the optical microscope examination. A globular equiaxed structure was observed, as shown in Fig.4. For comparative purposes, in Fig.5 the microstructure obtained for the same alloy by conventional melting, without using the device (17), and casting in the same graphite ingot mold by gravity, is reported.

Claims

Apparatus (17) for the preparation of a semi-solid magnesium alloy in a quasi-liquid state comprising three communicating chambers, where:
- the melting chamber X is provided with a sluice-gate opening (1) on the upper side, a communicating channel (6) between the chamber X and the chamber Y adjacent to the bottom, heating elements (5), a porous septum (2) on the lower side for inert gas insufflation, and one or more inserts of graphite or silicon carbide (4) inserted on the bottom, and an electromagnetic induction block plate (3) and/or one or more rotating plate (10) bearing permanent magnets (10bis) and/or rotating permanent magnets (10ter);

- the chamber Y provided with heating elements (5), a communicating channel (6) between the chamber Y and the chamber Z adjacent to the bottom, a piston (7) capable of opening and closing the channel (6), and one or more inserts of graphite or silicon carbide (4) inserted within the bottom, and an electromagnetic induction block plate (3) and/or one or more rotating plate (10) bearing permanent magnets (10bis) and/or rotating permanent magnets (10ter);

- the dosing chamber Z is provided with heating elements (5), a valve (8) on the upper side, a dosing tube (9) which allows the magnesium alloy coming out from the apparatus (10), and one or more inserts of graphite or silicon carbide (4) inserted within the bottom, and an electromagnetic induction block plate (3) and/or one or more rotating plate (10) bearing permanent magnets (10bis) and/or rotating permanent magnets (10ter).
Apparatus according to claim 1, wherein each of the three chambers is provided with an immersion thermocouple.
Apparatus according to any one of claims from 1 to 2, wherein the electromagnetic induction block plate (3) and/or the rotating plate (10) bearing permanent magnets (10bis) and/or the rotating permanent magnets (10ter) are an integral part of the apparatus (17).
Metallurgical process for the preparation of a semi-solid magnesium alloy in a quasi-liquid state comprising the following steps:
a') loading the chamber X of the apparatus of any one of the claims from 1 to 3, through the opening (1), with magnesium alloy comprising calcium and/or calcium oxide and bringing or maintaining it at the molten state;

b') stirring the magnesium alloy at the molten state present in the chamber X by electromagnetic stirring and/or magnetic stirring with permanent magnets and/or by bubbling nitrogen or argon gas;

c') allowing the magnesium alloy at the molten state to pass from chamber X to chamber Y;

d') thermoregulating the melted magnesium alloy of step c') until a semisolid magnesium alloy in a quasi-liquid state is formed;

e') lifting piston (7) and allowing the semi-solid magnesium alloy in a quasi-liquid state to pass from chamber Y to chamber Z;

f') lowering the piston closing the communicating channel (6) between chamber Y and chamber Z;

h') insufflating inert gas to exert pressure above the free surface of the semi-solid magnesium alloy in a quasi-liquid state and pushing it into the dosing tube (9) it to come out from the apparatus (10).
Metallurgical process for the preparation of a semi-solid magnesium alloy in a quasi-liquid state comprising the following steps:
a) loading the apparatus of any one of the claims from 1 to 3, with magnesium alloy comprising calcium and/or calcium oxide and bringing or maintaining it at the molten state,

b) achieving an atmosphere with an oxygen content not higher than 10% mol. inside said apparatus,

c) cooling the melted magnesium alloy of step b) until a semi-solid magnesium alloy in a quasi-liquid state is formed,

d) maintaining the semi-solid magnesium alloy in a quasi-liquid state of step c) under isothermal conditions at the temperature obtained by step c), stirring for at least 20 seconds by electromagnetic stirring and/or magnetic stirring with permanent magnets,

e) discharging the semi-solid magnesium alloy in a quasi-liquid state from the apparatus.
Process according to any one of claims from 4 to 5, wherein the semisolid magnesium alloy in a quasi-liquid state comprises a content of solid fraction between 0% and 20%.
Process according to any one of claims from 4 to 6, wherein the semisolid magnesium alloy in a quasi-liquid state comprises aluminum.
Process according to claim 7, wherein the semi-solid magnesium alloy in a quasi-liquid state comprises calcium and/or calcium oxide in an amount between 1% and 5% by weight and aluminum in an amount between 2% and 9% by weight.
Process according to any one of claims from 5 to 8, wherein steps a) and b) are reversed.
Process according to any one of claims from 5 to 9, wherein in step d) the isothermal condition is achieved by a constant temperature which is comprised in a temperature range between 5°C and 15°C below the temperature of liquidus of the specific alloy treated.
Process according to any one of claims from 5 to 10, wherein in step d) the time is between 60 and 500 seconds.
Process for the preparation of objects or articles consisting of magnesium alloy comprising the following steps:
1) preparation of the magnesium alloy comprising calcium and/or calcium oxide at the molten state;

2) preparation of the semi-solid magnesium alloy in a quasi-liquid state comprising calcium and/or calcium oxide according to the process of any one of the claims from 4 to 11;

3) filing an injector with the semi-solid magnesium alloy in a quasi-liquid state of step 2) and injecting it into the mold of a press;

4) molding the object by the press;
Process according to claim 12, wherein the step 3) of filling the injector with the semi-solid magnesium alloy in a quasi-liquid state is performed under an atmosphere with a comprised oxygen content not higher than 10% mol..