WO1994023076A1

WO1994023076A1 - Metallic composite and process for producing the same

Info

Publication number: WO1994023076A1
Application number: PCT/EP1994/000916
Authority: WO
Inventors: Jean-Michel Lauradoux; Jean Eugène Joseph WEGRIA
Original assignee: Union Miniere France S.A.
Priority date: 1993-04-05
Filing date: 1994-03-22
Publication date: 1994-10-13
Also published as: FR2703694A1; AU6504294A; FR2703694B1

Abstract

A metallic composite consists of a matrix made of a zinc-based alloy and of a reinforcing phase made of a skeleton of mild steel fibres. This composite is produced by securing a felt or knitted fabric made of mild steel fibres in the molding cavity of a pressure molding machine and by then molding the alloy in said machine.

Description

METAL COMPOSITE AND PROCESS FOR ITS MANUFACTURE

The present invention relates to a metallic composite consisting of a matrix of a zinc-based alloy and a reinforcing phase formed by a skeleton of steel fibers.

Such composites are described in the articles "Zn-AI matrix composites: investigation of the thermal expansion, creep resistance and fracture toughness" by MA Dellis et al., Materials Science and Engineering, A135 (1991) 253-257, and "Mechanical properties of zinc-aluminum matrix composites reinforced by unoriented fibers "by MA Dellis et al, Proceedings of the 12th Risa International Symposium on Materials Science: Métal Matrix composites - Processing, microstructure and properties. Editors: N. Hansen et al., Ris0 National Laboratory, Roskilde, Denmark, 1991, p. 299-304.

These known composites consist of a ZA8 matrix (Zn - 8 Al -1 Cu - 0.02 Mg) which is reinforced by a felt of stainless steel fibers: "Fecralloy" (Fe - 20 Cr - 5 Al) or "316 L" (Fe- 18 Cr - 8 Ni). This reinforcement by stainless steel fibers significantly reduces the creep rate at 150 ° C of the ZA8.

These composites have the drawback of requiring an expensive reinforcing material for their manufacture, namely stainless steel fibers.

The object of the present invention is to provide a composite as defined above, which avoids the drawback of known composites and which nevertheless has a creep resistance comparable to that of these known composites.

For this purpose, according to the invention, the steel fibers are mild steel fibers. In fact, the mild steel fibers are much cheaper than the stainless steel fibers and the Applicant has found that they produce a reinforcing effect quite comparable to that due to stainless steel fibers.

The choice of mild steel fibers to reinforce zinc alloys was not at all obvious, since on the one hand we were aware of the great reactivity of mild steel towards zinc, reactivity to which uses for example the process of galvanizing by immersion, and that on the other hand one was convinced until now that it was necessary to use a material with low reactivity vis-à-vis zinc to reinforce the zinc alloys, like that spring from the article "Processing and properties of Zn-AI alloy matrix composites reinforced by bidirectional carbon tissues" by F. Vescera et al, Proceedings of the 12th Ris0 International Symposium on Materials Science, p. 719-724.

It should be noted here that the document EP-A-0304167, which essentially relates to a process and apparatus for continuously casting a metal profile reinforced with fibers, gives the following information concerning the fibers and metals which can be used in this method and apparatus "Examples of suitable fibers include steel wire and ceramic fibers (eg alumina fibers, carbon fibers and silicon carbide fibers) Each of the types of fibers, which are suitable for use in the production of fiber-reinforced metal composites, and the preforms made therefrom may similarly be used, the metal may be any metal which is capable of forming the metal matrix of a fiber-reinforced metal composite Typical metals include aluminum, copper, magnesium, zinc and lead, as well as the alloys of each of these metals "The method and the apparatus are illustrated by the description of an embodiment, in which d '' on the one hand aluminum or an aluminum alloy and on the other hand fibers whose composition is not disclosed

In this context it should be noted that the term "steel" covers an iron-based alloy, which is malleable in the cast state in a certain temperature range and which contains manganese, usually carbon and often other alloying elements, while mild steel is carbon steel with a maximum of about 0.25% C, carbon steel being a steel containing up to about 2% carbon and only residual amounts of other elements with the exception of those a / stops for deoxidation, with silicon usually limited to 0.60% and manganese to about 1.65% (Metals Handbook, 8th Edition Vol 1 Properties and Selection of Metals, American Society for Metals, 1961, p 7.25 and 36)

The fiber skeleton advantageously has a density of 0.5 to 3.0 g / cm ³ or, in other words, a density of 6.5 to 38% of that of mild steel, which corresponds to a vacuum of 62 to 93.5% When the density is less than 0.5 g / cm ³ , the reinforcing effect becomes very low When the density is greater than 3.0 g / cm ³ , the composite is difficult to manufacture

The zinc-based alloy is preferably an alloy containing aluminum, in particular an alloy containing as alloying elements, in% by weight, ie 3.9-4.3 Al, 0.5-3 , 5 Cu and 0.02-0.06 Mg or 3.9-4.3 Al and 0.03-0.06 Mg or 6.0-8.0 Al and 3.2-4.3 Cu or 3.6 -3.9 Al, 3.3-3.7 Cu, 0.04-0 06 Mg, 0.02-0.03 Be and 0.08-0 11 Ti i.e. 8.2-8.8 Al, 0 , 9-1, 3 Cu and 0.02-0.03 Mg or 10.5-11.5 Al, 0.5-1, 25 Cu and 0.015-0.030 Mg or 25.0-28.0 Al, 2 , 0-2.5 Cu and 0.010-0.020 Mg or 0.01-0.03 03 Al, 1.0-1.5 Cu and 0 12-0 16 Ti or 5 6-6.0 Al and 1.0- 1.5 Cu, the remainder being zinc and unavoidable impurities Such an alloy is very fluid and can easily be molded under pressure, which particularly facilitates the manufacture of the composite, as will be seen below.

The present invention also relates to a process for manufacturing the composite according to the invention, this process being characterized in that a felt or a knitted fabric of mild steel fibers, hereinafter called "preform", is fixed in the molding cavity of a pressure molding machine and the alloy is then molded in this machine.

It is known that pressure molding is the art of producing parts of precise dimensions, by forcing, under high pressure, the molten metal into the mold cavity or cavity of a permanent mold. This mold is constituted by a set of metal elements making, in the "closed" position, the envelope of the molding cavity and of the various channels of the supply system through which the liquid alloy is injected. The mold is constructed in two main parts: a fixed part, called "fixed block" and a mobile part called "mobile block". The contact surface of the movable block and the fixed block is called "joint plane". The ejection of the molded parts is carried out by the ejectors: rods, generally cylindrical, which pass through the movable block, are flush with the impression and which, by their movement, drive the parts out of the mold (see in particular "Technology of the foundry in metal molds. Pressure foundry, 2nd revised edition 1977 "Technical Editions of the Foundry Industries, 12 avenue Raphaël - 75016 Paris).

In a particular embodiment of the process of the invention, the preform is made to occupy only a predetermined part of the molding cavity, thereby producing a cast part of which a predetermined part is formed by the composite and the rest by the alloy. . It is obvious that the preform will be placed where the intensity of the stresses in the part to be produced will be the highest.

The preform can be fixed in the mold cavity in different ways.

Thus it can be fixed by magnetization, mild steel being magnetic opposite to stainless steel. The movable block or fixed block of the molding machine must then be made of a non-magnetic material, for example beryllium copper, and equipped with a suitable magnet or electromagnet near the place where the preform must be fixed .

It is also possible to fix the preform in the mold cavity by wedging it between a wall of the cavity and a false ejector, that is to say a rod which, like the ejectors, passes through the movable block and which is actuated so to exert a strong pressure on the preform in the mold cavity. Yet another method of fixing consists in providing the fixed block, opposite a false ejector with a protuberance and in wedging the preform between this protuberance and the false ejector thereby maintaining the preform at a predetermined distance from the walls of the cavity One can also use a preform whose dimensions are such that it spans the mold cavity in at least one direction and then fix this preform by crushing its ends, which exceed the cavity, in the joint plane of the mold, which allows a particularly effective maintenance of the preform within the molding cavity in the extension of the joint plane With this method of fixing it is necessary to cut out of the molded composite the crushed parts of the preform

As for the injection of the alloy, it is obvious that this must be done so that the alloy does not solidify before having impregnated the preform The parameters to be checked for this purpose include the temperature of the alloy, the temperature of the mold, the temperature of the preform and the duration and speed of injection. Furthermore, it is particularly advantageous to ensure that the time elapsing between the impregnation of the preform and the solidification of the alloy be as short as possible and preferably less than 50 milliseconds Since the injection conditions required to achieve this result depend on a large number of factors such as the composition of the alloy and the volume and the density of the preform, these conditions must be determined experimentally for each particular case

The present invention further relates to a process for manufacturing a preform for use in the process just described. This process for manufacturing a preform consists in compressing a loose mass of mild steel fibers so as to produce a body with a density of 0.5 to 3.0 g / cm ³ and to subject this body to a heat treatment between 600 and 1000 ° C., under a controlled atmosphere or under vacuum and for a time long enough for the fibers to be able to freeze in the shapes and positions imposed by compression

As a loose mass of mild steel fibers, commercially available iron wool can be used. This wool consists of long, mildly intertwined mild steel fibers, which generally have a triangular cross section, the dimensions of which vary. from 5 to 200 μm Given the intermingling of fibers in iron wool, the properties of a composite incorporating a preform made of an iron blade will be isotropic

If we want to produce a composite with anisotropic characteristics, we will use a preform made from a mass of fibers aligned in the same direction It may be advantageous to mix during the production of preforms fibers of large and small sections, for example of fibers with a "diameter" of 100 to 200 μm and fibers with a "diameter" of 5 to 35 μm Commercially available mild steel fiber knits can be used directly as a preform as long as they have the required density. If this is not the case, it is possible to superimpose several layers of knitting and to compress this assembly in the mold cavity. Preferably, knitwear is used, the fibers of which have a diameter greater than 100 μm.

Other details and particularities of the invention will emerge from the description of an embodiment of the method for manufacturing the composite according to the invention, given below by way of non-limiting example and with reference to the drawings below. joints. In these drawings: FIG. 1 schematically represents an enlarged vertical section through the center of the cavity and perpendicular to the joint plane of a conventional mold of a hot chamber pressure molding machine, this mold being intended to mold the test tube shown in bold lines in Figure 4; - Figure 2 shows the same section of Figure 1, but this time of a mold adapted to mold the test piece of Figure 4 with a composite core;

- Figure 3 shows the mold of Figure 2 with a preform fixed in the cavity;

- Figure 4 schematically shows a plan view of the left half of the mold cavity of Figure 2; and - Figure 5 shows a creep curve of a test piece with composite core molded in the mold of Figure 2. In the drawings the same reference numerals designate identical elements. The conventional mold 1 consists of a fixed block 2 and a movable block 3, which touch in the joint plane 4 and form the envelope of a molding cavity 5 and an injection channel 6. The cavity molding 5 has the shape of the test piece 7 whose total length is 110 mm, the useful length of 50 mm and the useful section of 20 x 3 mm.

The mold 8, which is suitable for carrying out the method of the invention, consists of a fixed block 9 and a movable block 10. This mold 8 has, just like the conventional mold 1, a molding cavity 5 and a channel d injection 6 on the side of the movable block 10, but it also has an injection channel 11 on the side of the fixed block 9 as well as an upper housing 12 and a lower housing 13 for the crushed ends 14 and 15 of a preform 16. The upper housing 12 is equipped with two studs 17 to which the preform 16 is hung before the mold is closed. The alloy Zn - 7 Al - 3.2 Cu is molded in the conventional mold 1: the temperature of the liquid alloy is 420 ° C., the temperature of the walls of the mold of approximately 180 ° C., the duration of injection of approximately 15 milliseconds and the time interval between the injection of the liquid alloy and the ejection of the molded part of approximately 4 seconds. The test piece thus obtained is subjected to a creep test at 100 ° C under a stress of 40 MPa there is a deformation of 5% after 10 hours

The same alloy is molded in mold 8, in which a preform made from iron wool has been fixed, by crushing in the joint plane. The preform therefore consists of a felt of mild steel fibers. a density of 1, 2 gr / cm ³ and a thickness of 1, 3 mm The fibers have a triangular section with a size of 20 to 50 μm The molding conditions are identical to those used with the conventional mold. part removed from the mold the crushed parts of the preform and a test piece having a composite core according to the invention is thus obtained. This test piece is subjected to the creep test described above and the result of this test is shown in the diagram in the figure. 5 with the duration in hours on the abscissa and the elongation in% on the ordinate This result is entirely comparable to what the Applicant has found by using preforms made of stainless steel fibers A second test tube with composite core, made of e the same way as the first, was subjected to an examination by scanning electron microscopy contrary to what one could have feared, no reaction between fibers and matrix was observed, moreover, it was noted that the matrix completely permeated the preform

Claims

1. Metallic composite consisting of a matrix of a zinc-based alloy and a reinforcing phase formed by a skeleton of steel fibers, characterized in that the steel fibers are mild steel fibers.

2. Composite according to claim 1, characterized in that the skeleton of steel fibers has a density of 0.5 to 3.0 g / cm ³ .

3. Composite according to claim 1 or 2 characterized in that the alloy is a zinc-based alloy containing aluminum.

4. Composite according to claim 3, characterized in that the zinc-based alloy contains as alloying elements, in% by weight, ie 3.9-4.3 Al, 0.5-3.5 Cu and 0.02-0.06 Mg or 3.9-4.3 Al and 0.03-0.06 Mg or 6.0-8.0 Al and 3.2-4.3 Cu or 3.6 -3.9 Al, 3.3-3.7 Cu, 0.04-0.06 Mg, 0.02-0.03 Be and 0.08-0.11 Ti or 8.2-8.8 Al , 0.9-1, 3 Cu and 0.02-0.03 Mg or 10.5-11.5 Al, 0.5-1, 25 Cu and 0.015-0.030 Mg or 25.0-28.0 Al , 2.0-2.5 Cu and 0.010-0.020 Mg i.e. 0.01-0.03 Al, 1.0-0-1,

5 Cu and 0.12-0.16 Ti or 5,

6-6.0 Al and 1.0-1.5 Cu, the remainder being zinc and unavoidable impurities.

Method of manufacturing the composite according to one of claims 1-4, characterized in that a felt or a knitted fabric of mild steel fibers, hereinafter called "preform", is fixed in the molding cavity of a pressure molding machine and then the alloy is molded in this machine.

Method according to claim 5, characterized in that the knitted fabric is compressed between two walls of the molding cavity.

7. Method according to claim 5 or 6, characterized in that the preform is used to occupy only a predetermined part of the mold cavity, thereby producing a cast part of which a predetermined part consists of the composite and the rest by the alloy.

8 Method according to claim 7, characterized in that the preform is fixed in the cavity of molding by magnetization.

9. Method according to claim 7, characterized in that the preform is fixed in the mold cavity using a false ejector.

10. Method according to claim 7, characterized in that a preform having such dimensions is used that it spans the mold cavity in at least one direction and the preform is fixed by crushing it in the joint plane of the mold .

11. Method according to one of claims 5-10, characterized in that a hot chamber pressure molding machine is used.

12. A method of manufacturing a preform for use in the method according to one of claims 5-11, characterized in that a loose mass of mild steel fibers is compressed so as to produce a body of a density from 0.5 to 3.0 g / cm ³ and this body is subjected to a heat treatment between 600 and 1000 ° C, under controlled atmosphere or under vacuum and for a time long enough for the fibers to freeze in the forms and positions imposed by compression.