IL47001A - Process for heat treating metal alloys particularly aluminium-based alloys - Google Patents

Abstract

1499934 Alloy treatment in pressure die casting SOC DE VENTE DE L'ALUMINIUM PECHINEY 3 April 1975 [4 April 1974 13650/75 Heading B3F In a process for treating a metal alloy in the form of a mixture of liquid and solid phases, the normally cast alloy is heated to a temperature between the liquidus and solidus points to liquefy a well defined proportion, greater than 40% by weight, and is maintained for a period of a few minutes to a few hours, preferably 5 minutes to 1 hour, so that the original dendritic phase at least starts to globularize. The solid/liquid material thus formed may be pressure cast into a mould at pressures similar to those used in casting liquid metal, with the advantage of reducing die wear, better quality and greater throughput, since the heat needed to be removed by the die is reduced. The treated material may be cooled and reheated without affecting the thixotropic characteristics. Examples of aluminium alloys of the A-S9U3 [Al-9%, Si-3% Cu) type and A-4USG [0À2% Fe, 4À3% Cu, 0À75% Si, 0À5% Mg and 0À6% Mn) type continuously cast in the normal manner and then reheated to 580‹ and 630‹ C. respectively show that the time held at temperature significantly affected the thixotropic casting properties of the materials. [GB1499934A]

Description

nvxona miioao:. oin «wy ia*» ^"« Process for heat treating metal alloys particularly aluminium based "alloys SOCIETE DE VENTE DE L' ALUMINIUM PECHINEY C. 44705 This invention, which is the outcome of work by Mr. Serge BERCOVICI, relates to a process which, applied to metal alloys and, in particular, to lightweight aluminium- based alloys, provides them with a range of particular properties enabling them to be shaped in a form in which they consist of a mixture of solid phases and liquid phase sufficiently similar to that of so-called thixotropic substances for it to be utilised in the majority of processes used for shaping in the liquid state.

A substance is said to be thixotropic when its viscosity is not constant, but is governed by movements imparted to it. This is the case, for example, with clays of the bentonite type or of the so-called "moving sand" type.

Attempts have already been made to exploit the thixotropic behaviour of certain metal alloys. French Patent No. 2,141,979 of 15th June, 1972 in the name of the Massachusetts Institute of Technology claims "a process for the preparation of a solid/liquid mixture for use in processes for casting a metallic composition, characterised in that it comprises increasing the temperature of the metallic composition until it is in the liquid state, followed* by cooling to produce a certain degree of solidification of the liquid and vigorously stirring the liquid/solid mixture until approximately 65% b weight of the mixture thus formed is in liquid form and comprises individual degenerated dendrites or nodules." As stated in the above-mentioned patent, this liquid/solid mixture has thixotropic properties. If stirring is stopped while keeping the temperature constant, the metal bath has a consistency similar to that of a solid but, under the. effect of sufficiently intense stirring, it returns to a viscosity closer to that of a liauid.

When the liquid/solid mixture is cooled from a temperature T whilst its solidification -is completed by casting by any process, the product, by virtue of the ·)■ particular structure of the primary phase solidified in the form of globules, becomes thixotropic again when it is reheated to this temperature T. The process described in the above-mentioned Patent is not easy to work on an industrial scale, because the crucible in which the product is prepared has to rotate about its axis and the metal bath has to be stirred by means of two paddles rotating in opposite directions. It is obvious that the use of this apparatus in practice involves. significant problems in regard to the treatment of large quantities of alloys which have high melting points and are aggressive with respect to conventional mechanical construction materials.

Attempts have been made by the authors of the above-mentioned Patent Specification to obtain this thixotropic state, in particular in billets of A 380 alloy, which is an aluminium alloy containing 8.5% of and 3.5%of copper, by more simple means, such as reheating to a temperature which had to correspond to 40% of solid phase (approximately 1030°F, i.e. approximately 555°C). Unfortunately, these attempts were unsuccessful (Die Casting Engineer, vol 17, No.4, 1973, page 51).

Accordingly, the present invention which obviates the disadvantages of the prior art and considerably widens its range of application,1 relates to a process for obtaining metal alloy in the form of a mixture of solid phases and liquid phase in such a proportion that the alloy is able temporarily to change into the liquid state when an external stress is applied to it during its shading in a mould, returning instantaneously to the solid state when the stress is removed. The process according to the invention comprises heating the alloy to a temperature between the solidus point and liquidos point selected in such a way that the proportion by weight of liquid phase is equal to at least^ 40% and preferably equal to at least 60%, and keeping it at this temperature for a period ranging from a few minutes to a few hours, preferably from 5 minutes to 60 minutes, so that the primary dendritic structure has at least begun to develop into a globular form.

The dimensions of these globules are governed by the fineness. of the initial dendritic structure, but are generall in the range from about 100 to 400 micrometres.

The limits to the liquid phase/solid phase ratio corresponding to the application of the invention vary somewhat according to the type of alloy in question. They are also governed by the handling requirements of the treated products. An aluminium-based alloy treated in accordance with the invention and containing from 40 to 50% of liquid phase has the external appearance of a solid. It may be handled with minimum precautions. If it is subjected to any kind of violent shock or dropped, it may collapse. Towards 80% of liquid phase, the product is more like a paste and less easy to handle, although there is nothing to prevent it from being introduced in this paste-like form into the container of a moulding machine.

If, as a result of an error in temperature, it is found that the liquid phase/solid phase ratio is not the one required there is nothing to prevent this ratio from being modified by reheating or cooling the product and keeping it at the new temperature for a sufficiently long period in accordance with the invention.

Similarly, it has been found that, when an alloy treated in accordance with the invention is cooled to any temperature below the solidus point, and subsequently reheated to a temperature between the solidus point and the liquidus point, , corresponding to a proportion by weight of liquid phase of greater than 35%, the alloy immediately recovers its thixotropic properties, which shows that it has undergone a permanent modification in structure and taken on the appearance of a new structure.

Products of metal alloys, especially lightweight aluminium-based alloys, prepared in accordance with the invention from cylindrical billets cast in sand moulds, chill moulds, continuously or semi continuously, may be used for feeding a pressure casting machine.

When introduced into the container of the machine,: they behave in the same way as a liquid when the piston begins to apply its pressure to them. The solid-liquid mixture may thus fill all the "details" of a mould without the force applied by the piston being appreciably greater than that applied for injecting the same alloy heated to a temperature above the liquidus point, i.e. completely in liquid form.

These products may also be used in any casting processes in which the low-viscosity state, similar to the liquid state, is made to appear temporarily, for example by means of mechanical vibrations.

This new method has a certain number of advantages over conventional pressure moulding: - less wear of the moulds cast under pressure by virtue of the fact that, since the temperature of the injected metal is lower, the tendency towards blocking decreases as does the extent of heat exchanges, thereby reducin the thermal shocks in the mould'; - an increase in output due to the reduction in the amount of heat to be dissipated, some of the metal already being solidified when it is injected into the mould; - better stability of the mouldings due to their compactness and to the absence of pores.

The process according to the invention is illustrated in the accompanying drawings and following Examples.

Figure 1 is a micrograph, with a magnification of 50 times, of an AS9U3 alloy (aluminium-base, silicon 9%, copper 3%) conventionally cast in an ingot mould. The dendritic structure is clearly visible.

Figure 2 shows the same alloy kept for one hour at 580°C in accordance with the invention, which corresponds to a concentration by weight of liquid phase of approximately 70%. It can be seen that the network of aluminium dendrites has almost disappeared and has been replaced by globules regular both in shape and distribution.

The structure shown in Figure 2 may be compared with that of a wet shaped sand which, although containing a large percentage of liquid phase, nevertheless retains sufficient consistency both to maintain the shape given to it and to be handled with caution.

The following Examples are given purely by way of illustration and are not in any way intended to limit the scope of the invention.

EXAMPLE 1 AS9U3 alloy (aluminium-base, silicon 9.2%, copper 3.1%, iron 0.8%) was continuously cast in the usual way in the form of a round billet 63 mm in diameter. This billet was cut into sections 100 mm long which were then machined to a diameter of 55 mm.

A mould intended for the production of small mouldings in the form of caps 80 mm in diameter and 2 mm thick was installed in a horizontal cold-chamber pressure casting machine with a closing force of 400 tonnes.

A first section of A-S9U3 was preheated to a temperature of 580°C. At this temperature, approximately 70% of the alloy is in liquid form. This section is immediately introduced into the container of the pressure casting machine, followed by injection. Under these conditions, a partly "fed" moulding with a very poor surface is obtained. In addition, the pressure applied by the piston is greater than that observed with liquid metal. Accordingly, the metal does not have any thixotropic properties. By contrast, another section of A-S9U3, kept at the same temperature of 580°C for two hours, introduced into the container of the machine and injected into the mould by means of the piston, behaves in exactly the same way as a liquid. Throughout injection, the pressure is entirely comparable with that observed with liquid metal.

The moulding obtained by this process is thus perfectly finished and has an equally perfect surface.

EXAMPLE 2 A round 63 mm diameter billet of A-U4SG (iron 0.2%, copper 4.3%, silicon 0.75%, magnesium 0.50%, manganese 0.60%) was continuously cast. This billet was cut into sections 100 mm long which were again machined to a diameter of 55 mm.

The same mould as described in the preceding Example was installed in a horizontal cold-chamber pressure casting machine with a closing force of 400 tonnes.

One section cut from the round 63 mm diameter billet was preheated to a temperature of 630°C and kept at that temperature for one hour, corresponding to a liquid fraction of approximately 45%.

This section was then immediately introduced into the container of the pressure casting machine and injected by means of the piston. The metal behaves in exactly the same way as a liquid during injection. The pressure of the piston, is comparable with that observed during the injection of the same liquid alloy. The moulding obtained is perfect and has an 'excellent surface.

EXAMPLE 3 The same section as in Example 2 was preheated for one hour to 630°C so as to produce approximately 45% of liquid phase, subsequently cooled to ambient temperature, reheated to 630°C and immediately introduced into the container of the same pressure casting machine and injected into the mould.

The pressure required was the same as for the injection of the same alloy in liquid form, and the moulding obtained was perfect and had an excellent surface.

Claims (6)

47001/2 CLAIMS:

1. A process for treating metal alloys or shaping them in the form of a mixture 6 liquid phase and solid phases temporarily behaving in the same way as a low-viscosity liquid, wherein the alloy is heated in solid form to an intermediate temperature between the solidus point and liquidus point so as to liquefy a well defined proportion by weight of the alloy of greater than 35 % and preferably greater than 40 and wherein the above-mentioned temperature is maintained for a period ranging from a few minutes to a few hours, preferably from 5 to 60 minutes, so that the solid dendritic phase has at least begun to develop into the globular form.

2. A process for treating metal alloys as claimed in Claim 1 as applied to lightweight aluminium-based alloys.

3. · A process for treating metal alloys as claimed in Claim 1 , wherein the alloy thus treated is cooled to any temperature below the solidus point and then reheated to an intermediate temperature between the solidus and liquidus points corresponding to a proportion by weight of liquid phase of greater than 35 $ and preferably greater than 40

4. Products of aluminium alloys obtained by the process claimed in Claim 2 or 3*

5. » A process for the production of pressure castings wherein the product according to Claim 4 is introduced into the container of an extrusion press.

6. A process for casting of the product of Claim 4, wherein the low-viscosity state similar to the liquid state i3 made to appear temporarily. HE:gd