WO2002060618A1 - Method for processing and casting oxidisable alloys - Google Patents

Abstract

The invention relates to a method for processing an oxidisable molten alloy, such as an aluminium casting, and for casting said processed molten alloy in order to produce parts in the cavities of a mould (1) which are filled with molten alloy via a downward channel (4). The inventive method consists in depositing a dose (23) comprising at least the oxidisable alloy elements in a chamber (6) provided in the mould (1) above the downward channel (4) and isolated from said channel by means of a meltable pellet (9) having a calibrated thickness. A volume of unprocessed molten alloy corresponding to the volume of the mould (1) cavities is poured into said chamber (6) and subsequently said volume of alloy is processed in the chamber (6) by the melting of the oxidisable elements in the dose (23) and said processed volume is then transferred automatically towards the mould (1) cavities once the pellet (9) has melted. A funnel (14) is mounted on top of said chamber, the lower end of which is provided with an axial-centrifugal diffuser which communicates a swirling movement (18) to the alloy bath during the casting thereof in the chamber.

Description

 Process for the treatment and casting of oxidizable alloys.

The subject of the invention is a method of treating an oxidizable molten alloy, such as aluminum casting, and of casting said treated molten alloy in order to produce parts in the cavities of a mold supplied with alloy in fusion by a downspout.

The development of certain techniques requires the availability of materials, and in particular of cast irons, capable of retaining their mechanical qualities and their qualities of resistance to oxidation for increasingly high temperatures.

This is particularly the case in the automotive industry where progress in engine performance with multi-valve cylinder heads, turbochargers, heat shields, and the need for pollution control with catalytic converters, is driving to a continuous increase in the temperature of the exhaust gases. Consequently, the materials used to make certain parts, such as the exhaust manifolds and the casings of the turbochargers, must be capable of undergoing increasingly severe thermodynamic stresses, be they maximum temperatures, gradients thermal shock, thermal shock, mechanical stress, hot fatigue or thermal fatigue. In this context, investigations have been undertaken for several years by the founders to develop alloys to produce molded parts, in particular exhaust manifolds, which meet the criteria of the specifications of engine manufacturers and which are more economical than collectors produced by mechanically welded steel tubes, or half-shells made of stamped and welded stainless steel sheet. Among the alloys likely to meet the needs, cast iron weakly alloyed with aluminum seems the most appropriate to satisfy, at low cost, the severe conditions of use of the collectors. EP 0 534 850 has already proposed a family of spheroidal or vermicular graphite cast irons containing an aluminum content of between 0.5% and 2.5%. However, the development of these castings which are weakly alloyed with aluminum by a conventional process, which consists in adding aluminum in the oven or in the bag is made very difficult. Indeed, the presence of aluminum generates the formation of alumina skins in the bath, because the cast iron is continuously oxidizable due to the oxidability of aluminum. These alumina skins will, on the one hand, foul the refractory of the oven and of the ladle, and will, on the other hand, disturb the good flow of the liquid metal and form inclusions in the castings. In addition, due to the low density of aluminum, the homogenization of molten iron is made difficult.

EP 0 534 850 plans to implement current techniques for developing these castings which are weakly alloyed with aluminum and he mentions that it is only necessary to introduce aluminum as late as possible.

US 4,424,853 represents the state of the art closest to the invention. This document describes a process for treating and casting an oxidizable molten alloy, under inert gas, in which a dose consisting of at least the oxidizable elements of the alloy is deposited in a chamber provided in the mold above. of the pouring channel and isolated from the latter by a pellet of fusible material having a calibrated thickness, a volume of untreated molten alloy is poured into said chamber corresponding to the volume of the mold cavities, which results in said chamber the treatment of said volume of alloy by fusion of the oxidizable elements of the dose, and the automatic casting of said volume treated towards the cavities of the mold after the fusion of the pellet.

Thus, the addition of oxidizable elements is carried out in an upper chamber of the mold when the volume of untreated molten alloy is poured. Thus, the walls of the furnace and of the ladle are never in contact with the oxidizable elements, which eliminates their fouling. In addition, the content of oxidizable elements is easily adjustable, since it suffices to produce homogeneous and precise doses per type of mold. Since the mold is for single use, any clogging of the chamber wall will have no effect on subsequent pouring. Finally, the pellet separating the chamber from the pouring channel having a calibrated thickness, its destruction by fusion will occur at a precise moment after the start of the casting operation, for the same type of molds given and a temperature of untreated alloy given. In this document, the chamber has a cover provided with an orifice through which the molten metal is poured.

The object of the invention is to ensure better mixing of the alloy before it is poured into the mold.

The invention achieves its object by the fact that the chamber is surmounted by a funnel forming a ceiling, and the volume of untreated alloy is poured into said funnel, this funnel comprising at its lower end an axial-centrifugal diffuser which imparts a vortex movement to the alloy bath residing in said chamber. This chamber is preferably cylindrical.

This arrangement ensures good homogenization of the alloy.

Very advantageously, the chamber and the mold cavities are inerted before casting the volume of untreated alloy molten in the chamber.

The invention also relates to a device for implementing the method comprising a foundry mold having internal cavities capable of being supplied with molten alloy by a pouring down channel formed in said mold, characterized in that the foundry mold comprises above the pouring channel a chamber isolated from said channel by a pellet of fusible material, and by the fact that said device also comprises a funnel surmounting said chamber and forming its ceiling, said funnel having at its lower end an axial-centrifugal diffuser capable of imparting a vortex movement to the alloy bath when it is poured into said chamber.

Advantageously, the funnel is separable from the foundry mold. Other advantages and characteristics of the invention will emerge on reading the following description given by way of example and with reference to the accompanying drawings, in which: FIG. 1 is a sectional view of a foundry mold according to the invention. invention fitted with a funnel for carrying out the process according to the invention; Figure 2 is a top view of the funnel surmounting the upper chamber of the mold of Figure 1; Figure 3 is a side view of the funnel tube showing the orifices of the diffuser; and Figure 4 is a bottom view of the funnel tube.

Reference 1 designates a disposable foundry mold intended for molding metal parts. This mold 1 is produced in the traditional way in two parts, namely a lower part 1a and an upper part 1b joined together according to a joint plane 2. The two parts 1a and 1b have, on either side of the plane of joint 2, indentations, not shown in the drawings, which, when the two parts 1 a and 1 b are joined along the joint plane 2, form cavities intended to receive a molten alloy by means of channels 3 connecting the cavities to a pouring channel 4 formed in the upper part 1b. After the alloy contained in the cavities and channels has solidified, the mold 1 is destroyed and the metal from the channels 3 is removed. This casting molding technique has been known for a very long time and does not require further explanation.

As can be seen in FIG. 1, the upper part of the mold 1b has in its upper face 5 a preferably cylindrical chamber 6 in the axis X from which the pouring channel 4 opens. This chamber 6 has a volume at least equal to the volume of the mold cavities 1 and the feed channels 3.

The bottom 7 of the chamber 6 has a bowl 8 into which the pouring channel 4 opens. The orifice of the pouring channel 4 is closed by a disc of fusible material 9 placed in the bottom of the bowl 8. The role of the pad 9 will be explained later in this memo.

The chamber 6 is closed by a cover 10 which has a peripheral wall 11 supported by its base 12 in a groove 13 formed in the upper face 5 of the mold 1 around the orifice of the chamber 6, and a middle part 14 in funnel shape connected to the upper edge 15 of the peripheral wall 11. The middle part 14 comprises a tube 16 of axis X which extends in the chamber 6. The lower end 17 of the tube 16 is disposed above the bowl 8 and above the bottom plane 7 of the chamber 6. A diffuser 18 axial-centrifugal is disposed in the lower end 17 of the tube 16. This diffuser 18 comprises a body 19 of axis X closing the lower end 17 of the tube 16 and a plurality of orifices 20 formed in the wall of the tube 16 and putting the interior of the tube 16 into communication with the chamber 6. The geometry of the orifices 20, with respect to the axis X, is calculated in such a way that the ejection of a molten alloy through the orifices 20 causes a movement swirling of the molten bath poured into the chamber 6. The body 19 has a point 21 directed upwards. The reference 22 designates a conduit connecting the upper face 5 of the mold 1 to the joint plane 2 intended to be connected to a source of inert gas under pressure, nitrogen for example, before a casting operation in order to introduce this gas inert in the cavities of the mold 1, the channels 3, the downspout 4 and the chamber 6. The cover 10 is made from an aggregate of grains of sand and resin.

The fusible material constituting the pellet 9 can be of different types, for example steel, cork, cardboard and sodium silicate. This pellet 9 has a determined thickness, so that it melts at the desired time after the introduction of an untreated molten alloy into the chamber 6, which makes it possible to automatically start feeding the mold 1.

The mold 1 described above with the chamber 6, surmounted by a cover 10 in the form of a funnel, is particularly suitable for the treatment and the casting of oxidizable alloys, such as a cast iron weakly alloyed with aluminum.

Before proceeding to the casting of this alloy, there is available in chamber 6 a dose 23 of treatment products in the form of granules or solid blocks. This dose contains in particular the oxidizable elements of the alloy after casting. The quantity of each of these elements depends on the volume of cast alloy and the desired content of each of the elements. It is also possible to have a dose 24 containing the inoculants necessary for the treatment in chamber 6. When it is for example an alloy of cast iron with aluminum, a dose 24 containing an inoculant and a dose 23 is used. of an alloy aluminum / magnesium containing the aluminum and magnesium contents necessary to properly spheroidize the cast iron and to achieve the desired percentage of aluminum in the castings.

According to the method of the invention, a volume of untreated molten alloy is poured into the funnel 14 of the cover 10 after the inerting has been carried out. The volume of alloy poured is equal to the volume of metal required to produce parts using the mold 1.

This volume of untreated alloy poured into the funnel 14 flows through the tube 16 and is ejected into the chamber 6 through the orifices 20 of the diffuser 18. The jets of untreated molten metal form an identical angle with respect to radial planes passing through the X axis and through the ends of the orifices 20. This arrangement causes a swirling movement of the bath of molten metal poured into the chamber 6. The molten bath causes the elements of the dose 23 to melt, and also the melting of the material constituting the pellet 9.

The oxidizable elements of dose 23, in particular aluminum and magnesium, have a melting temperature significantly lower than the melting temperature of the material constituting the pellet 9. The elements of dose 23 are quickly melted and the vortex generated by the diffuser 18 results in good homogenization of the molten alloy in the chamber 6.

The treatment of the alloy is thus carried out in chamber 6. The addition of the oxidizable elements, in particular aluminum, is thus carried out just before casting into the mold 1. Thanks to the prior inerting, the aluminum introduced into the alloy is not in contact with oxygen, and the treated alloy is normally free of alumina skins detrimental to good mechanical strength of the castings.

As soon as the pellet 9 is melted, the volume of treated alloy contained in the chamber 6 flows through the casting down channel 4 towards the cavities of the mold 1.

The drop in temperature of the alloy between the start of its introduction into the funnel 14 and its pouring into the mold 1 will be compensated for by overheating of the untreated cast iron. By way of example, four exhaust manifolds have been produced by developing an aluminum cast iron by the method described above.

The metallurgical study carried out on these castings highlighted the effectiveness of the process.

In fact, according to spectroscopic studies carried out on a few samples taken from the collectors, at the level of the pipes, the collar and the flanges, the aluminum and magnesium contents are close to the targeted contents and are homogeneous, as shown. in Table 1. The samples observed present the expected structure, namely a ferritic matrix, with less than 10% of perlite, and a high level of spheroidal graphite GS (A + B), as shown in Table 2 .

In addition, radiographs and micrographic observations of the castings highlighted the absence of inclusions of the alumina skin type. This confirms the effectiveness of the inerting and the settling of the liquid metal in the chamber 6.

The introduction of an inert gas, such as nitrogen, into the chamber 6 and into the cavity of the mold 1, before the pouring of the liquid metal into the chamber 6, in fact ensures the maintenance of satisfactory inclusive cleanliness in the castings.

To improve, if necessary, the inclusiveness of the metal, it will be possible to filter the alloy treated in the mold 1, knowing that the useful filtration surfaces must be sufficient to avoid premature clogging of the filters.

TABLE 1

TABLEAU 2

TABLE 2

Claims

1. Process for treating an oxidizable molten alloy, such as aluminum casting, and for casting said treated molten alloy in order to produce parts in cavities of a mold (1) supplied with alloy melting by a downspout (4), process in which a dose (23) consisting of at least the oxidizable elements of the alloy is deposited in a chamber (6) formed in the mold (1) at the above the downspout (4) and isolated from the latter by a pellet of fusible material (9) having a calibrated thickness, a volume of untreated molten alloy corresponding to the volume is poured into said chamber (6) cavities of the mold (1), which causes in said chamber (6) the treatment of said volume of alloy by fusion of the oxidizable elements of the dose (23), and the automatic pouring of said treated volume towards the cavities of mold (1 ) after the pellet (9) has melted, characterized in that the chamber (6) is surmounted by a funnel (14) forming a ceiling, and by the fact that said volume of untreated alloy is poured into said funnel (14), said funnel (14) comprising at its lower end a diffuser ( 18) axial-centrifugal which imparts a vortex movement to the alloy bath when it is poured into said chamber (1). 2. Method according to claim 1, characterized in that the chamber (6) is cylindrical. 3. Method according to one of claims 1 or 2, characterized in that one proceeds to the inerting of the chamber (6) and the mold cavities (1) before the casting of the volume of molten alloy in the bedroom (6). 4. Method according to any one of claims 1 to 3, characterized in that the dose (23) comprises aluminum and magnesium. 5. Device for implementing the method according to any one of claims 1 to 4, comprising a foundry mold (1) having internal cavities capable of being supplied with molten alloy by a downspout ( 4) formed in said mold (1) - characterized by the fact that the foundry mold (1) comprises, above the pouring channel (4), a chamber (6) isolated from said channel (4) by a pellet of fusible material (9), and by the fact that said device further comprises a funnel (14) surmounting said chamber (6) and forming its ceiling, said funnel (14) having at its lower end an axial-centrifugal diffuser (18) capable of imparting a vortex movement to the bath alloy during its casting in said chamber (6). 6. Device according to claim 5, characterized in that the funnel (14) is separable from the mold (1).