WO2008086872A1

WO2008086872A1 - Inert atmosphere for light metal alloys melting systems and method and system for melting said alloys with the use of said inert atmosphere

Info

Publication number: WO2008086872A1
Application number: PCT/EP2007/011007
Authority: WO
Inventors: Gianluca Porto; Pierangelo Scoero; Alessandro Borri; Raoul Giudici
Original assignee: Rivoira SRL
Current assignee: Nippon Gases Italia SRL
Priority date: 2007-01-15
Filing date: 2007-12-14
Publication date: 2008-07-24
Anticipated expiration: 2009-07-15
Also published as: ITMI20070046A1

Abstract

An inert atmosphere for protecting, stabilising and making inert melt baths in systems for melting light metal alloys, containing pentafluoroethane C2HF5, alone or in a mixture with other gases is disclosed. Compared with the solutions of the prior art, the inert atmosphere of the invention offers the advantage of drastically reducing the environmental impact of the melting process, though increasing the inertisation of the free surface of the molten metal and reducing the amount of dross produced by the melting.

Description

INERT ATMOSPHERE FOR LIGHT METAL ALLOYS MELTING SYSTEMS AND METHOD AND SYSTEM FOR MELTING SAID ALLOYS WITH THE USE OF SAID INERT ATMOSPHERE.

DESCRIPTION

The present invention pertains to an inert atmosphere for systems for melting light metal alloys. The invention also extends to the method and to the system for melting these alloys with the use of said inert atmosphere.

The field of the invention is that of the gases used to form the inert atmosphere necessary to protect the metal alloy, in the molten state, in the baths of the melting furnaces of the plants for melting light metal alloys, in particular magnesium alloys (destined, for example, for production of components for the automobile industry, the aviation industry, etc.).

In the field of this technology, it is necessary to protect the free surface of the molten metal by stabilising it with a protective atmosphere. In the case of magnesium in particular, which is a highly unstable metal in the molten state, the lack of an adequate protection of the bath can cause fires and explosions.

For this purpose, it is known in the art to make inert the free surface of the molten metal with a protective atmosphere, consisting of a mixture of an inert gas (usually CO₂ or N₂) and of an active gas, sulphur hexafluoride (SF₆) and SO₂ being the most commonly used.

However, these known gases have the drawback of being potentially harmful for the environment, cither because they are a source of the greenhouse effect (as in the case of the SF₆ which has a very high GWP), or because they are toxic/corrosive (as in the case of the SO₂). For this reason, the new regulations on the environmental protection aim to reduce drastically the emissions of SF₆ into the atmosphere, with the resulting limitations on industrial use of this gas.

Object of the invention is to provide an inert atmosphere for melting systems for light alloys - in particular for magnesium alloys - which, unlike those of the prior art, besides improving the performance of the bath has a less impact on the environment. A further object of the invention is to provide a system and a method for melting these alloys in which said inert atmosphere is used.

These and other objects arc achieved with the inert atmosphere, the method and the system of claims 1. 6 and 9, respectively. Preferred embodiments of the invention are apparent from the remaining claims.

With respect to the known solutions, the inert atmosphere of the invention offers the advantage of drastically reducing the environmental impact of the melting process, though increasing the inertisation of the free surface of the molten metal and reducing the amount of drosses produced by the melting.

These and other objects, advantages and characteristics emerge from the description that follows of a preferred embodiment of the method and the system of the invention illustrated, by way of a non limiting example, in the figures of the appended sheets of drawings, wherein:

Figure 1 shows a general diagram of a system for melting light metal alloys with the use of the inert atmosphere of the invention;

Figure 2 shows a schematisation of the reactions that take place, on the free surface of the molten metal bath, through the action of the inert atmosphere of the invention;

Figure 3 shows the mixing and delivery system for a variant of the inert atmosphere of the invention.

In the system shown in Figure 1, the melting of a light metal alloy is carried out, in particular of magnesium charged, in the solid state in the form of bars 1, into a melting crucible 2. In said crucible a mass of molten metal 3 is thus formed, onto the free surface

4 of which a stream 6 of inert atmosphere is blown from a plurality of nozzles 5. The molten metal leaving the crucible 2 is then conveyed, though a trap 7 also subjected to the same stream 6 of inert atmosphere, to the casting system 8 for the production of the metal component (for example an automobile or aircraft component, etc.).

According to the invention, said inert atmosphere 6 contains pentafluorethane C₂HF₅ which, on the free surface 4 of the molten mass 3 of the light alloy bath, for example magnesium at 650-750⁰C, causes a lowering of the surface tension of the bath, with the formation of a thin film 9 of magnesium oxide in an even, unbroken layer on the free surface of the molten metal (Figure 2). It has advantageously been observed that the addition of a limited, measured amount of oxygen to the inert C₂HF₅ atmosphere increases the ability of said atmosphere to make inert the bath 3.

According to a variant embodiment of the inert atmosphere of the invention, illustrated in

Figure 3, this consists of a mixture Of C₂HF₅, of O₂ and of CO₂ or of N₂, preferably with the following volume composition:

C₂HF₅ 0.01 - 10% O₂ 0.01 - 20%

CO₂ or N₂ remainder to 100%

Even more advantageously, said mixture is made up of: C₂HF₅ 0.01 - 1% O₂ 0.01 - 5%

CO₂ or N₂ remainder to 100%

Even more advantageously, said gaseous mixture is made up of 0.3% Of C₂HF₅, 1% of O₂ and 98.7% of CO₂ or of N₂

The above described ternary mixture is obtained in the system of Figure 3, which comprises the containers 10. 1 1 and 12 for the storage of O₂, of CO₂ and of C₂HF₅, respectively, in turn sent to the mixer 14 through the separate supply lines 13, 15 and 16, respectively. The pressure, the metering and the flow rates delivered for each individual gas can be controlled on a panel 17. The gaseous stream 18 leaving the mixer 14 makes up the supply to the nozzles 5 of the melting furnace, from which it exits in the form of the inert atmosphere 6 of Figure 1.

Preliminary evaluation test. The inert atmosphere of the invention was tested, under static conditions, on a magnesium bath, in the conditions and with the results set forth in Table 1 below, where C₂HF₅ is designated with the code Rl 25: TABLE 1

From this table, which represents a summary of the results for one of the various days of test in static conditions, it can be seen that the magnesium bath was evaluated at different times (at 1 1.30 a.m., after about 2 hours of operation, and at 2.00 p.m.), excellent stabilisation levels being seen in both cases, with limited formation of dross (stable bath, no smoke or flames, no burning of molten metal and of the poured off dross, less dross produced, no Hashing or sparking of the molten metal).

Test on a pilot system

An inert atmosphere according to the invention was prepared as described in Table 2 below, where the code Rl 25 stands for C₂HFs:

TABLE 2

This atmosphere was fed onto a production island, made to run in a steady state for one week. The results, partially comparative with respect to the average yearly production with the use of SF₆-bascd mixtures, are set forth in Table 3 below: TABLE 3

A considerable increase in the stability of the bath was noted in the system, with clear advantages for the safety of the system and for the working environment in general. Furthermore, a drastic reduction in the formation of dross was noted. The results of the preliminary evaluation test (Table 1 above) are confirmed.

Claims

1. An inert atmosphere for protecting, stabilising and making inert melting baths in melting systems for light metal alloys, characterised in that it contains pentafluorethane C₂HF₅, alone or in a mixture with other gases.

2. An atmosphere according to claim 1, characterised in that it further contains oxygen and at least one inert gas.

3. An atmosphere according to claim 2, characterised in that it consists of a mixture having, in volume, from 0.01% to 10% of C₂HF₅, from 0.01% to 20% of O₂ and the remainder of an inert gas.

4. An atmosphere according to claim 2. characterised in that it consists of a mixture having, in volume, from 0.01% to 1% of C₂HF₅, from 0.01% to 5% of O₂ and the remainder of an inert gas.

5. An atmosphere according to claim 2, characterised in that it consists of a mixture having, in volume, 0.3% of C₂HFj, 1% of O₂ and the remainder of an inert gas (CO₂ or N₂).

6. A method for melting light metal alloys, of the type comprising the steps of melting the metal in a melting crucible and of casting the molten metal, characterised in that it provides for the protection, the stabilisation and the inertisation of the melt bath with an inert atmosphere according to at least one of claims 1 to 5.

7. A method according to claim 6, characterised in that said inert atmosphere is fluxed in the form of a stream (6) onto the free surface (4) of the molten mass (3) of said light metal alloy.

8. A method according to claims 6 or 7, characterised in that said light metal alloy is a magnesium alloy.

9. A melting system for light metal alloys, of the type comprising a crucible (2) for the molten mass (3) of metal alloy and a trap (7) to transfer said molten mass to the casting system (8). characterised in that means (5) for blowing a stream (6) of the inert atmosphere according to at least one of claims 1 to 5 onto the free surface (4) of said molten mass (3) are provided.

10. A system according to claim 9. characterised in that a system for forming an inert atmosphere made up of a mixture of O₂, of CO₂ (or of N₂) and of C₂HF₅ is provided, said system being provided with means for decompressing and metering said gases to a mixer (14) that supplies said mixture to said blowing-in means (5).

11. A system according to claims 9 or 10, characterised in that it is a system for melting magnesium alloys.

12. Use of an inert atmosphere containing C₂HF₅ for the protection, the stabilisation and the inertisation of melting baths in methods and systems for melting light metal alloys, particularly magnesium alloys.