IL47002A - Process for heat treating of 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

m*nano maioaoa oma Vi3»e †-ion p mooiaa maioaoa tm'oa Process for heat treating of metal alloys particularly aluminium based alloys SOCIETE VENTE DE L* ALUMINIUM ECHINEY . 44706 This inven on, 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 state where they consist; of a mixture of solid phases and liquid phase maintaining the solid form. The invention also relates to the products obtained by this process and to their use in processes for. shaping or forming in the solid state.

It is known that, if a metal alloy in the solid state is progressively heated, the first traces of liquid phase appear from the moment when the solidus point is reached, and the solid phase/liquid phase proportion increases progressively until total liquefaction is obtained at the moment when the liquidus point is reached.

It has surprisingly been found that, when a solid metal alloy is heated to a temperature between the solidus point and the liquidus point, the branched network of primary phase dendrites, although remaining largely solid during this treatment, undergoes structural degradation and progressively changes into a globular form in which the dimensions of the globules are governed by the fineness of the initial dendritic structure, but are generally in the range from about 100 to 400 micrometres.

It has also been found that an alloy, kqpt between the solidus and liquidus points, retains the external, appearance of a solid body as long as the proportion of liquid phase does not exceed approximately 60 %. However, a product of this kind has to be handled with precautions and any excessive shock or force frequently results in its complete or partial collapse. When the proportion of liquid phase falls to around 50 Yo and, preferably, to below 35 %, handling may be carried out with minimum precautions. Below approximately 20 % of liquid phase, the product may be handled in substantially the same way as an ordinary solid.

It has been found that an alloy kept between the solidus and liquidus points and at such a temperature that the proportion of liquid phase is less. than 40 % is particularly suitable for shaping by any of the processes normally applied to solid alloys, such as drawing, extrusion, rolling, die stamping, stamping, forging, etc.

The transition of the solid phase of the dendritic structure into a globular structure provides the alloy treated in accordance with the invention with considerably improved plastic flow properties in the solid state which may be utilised both for accelerating the flow rates and for reducing the effort required from machines. These new properties, which are also part of the invention, correspond to a particular state of the alloy which may be described as "rheotropy".

It should be emphasised that "rheotropy" is not simply a result of the increase in temperature because, in view of the difficulties involved in processes for transforming certain solid alloys, it could be assumed that it is sufficient to increase the drawing temperature for example in order to reduce the effort required of the press.

However, experience has shown that the temperature of 450°C, which is normally used for drawing alloys such as AU4SG, is a maximum temperature which is never exceeded in industrial practice. Beyond this temperature, the plasticity of the metal, which is exceeded by a maximum between 420 and 450°C, tends to decrease, whilst on the other hand the dynamic effort generated at the level of the die brings about such an increase in temperature that the profile issues in partly liquid form, so that it cannot be used.

By contrast, drawing in the rheotropic state generates relatively few dynamic efforts at the level of the die and a very gentle cooling flux is sufficient for solidifying, in the extruded profile, the small quantities of liquid phase which are' uniformly distributed therein and which do not affect its cohesion. In addition, the wear of the tooling used is greatly reduced.

The effect of the process according to the invention may be illustrated by comparing the micrographic structures of an aluminium alloy AU4 GS containing approximately 47o of copper, 1 % of silicon, 0.8 % of manganese and 0.5 % of magnesium.

Figure 1 , which is a micrograph with a magnification of 50 times, shows the structure of an AU4 GS alloy conventionally obtained by the semi-continuous casting of a billet. The dendritic structure is clearly visible.

Figure 2 is a micrograph with an enlargement of 50 times of the same alloy treated in accordance with the invention: it has been kept at 620°C for 1 hour, which corresponds to a proportion by weight of liquid phase of approximately 25 %.

The micrographs were taken after "SEG0L" attack, comprising an anodic attack in a mixture containing lOOOcc of orthophosphoric acid and 30 g of chromic anhydride for a period of 1 to 2 minutes at a temperature of 90°C under such a d.c. voltage that the current density in the test 2 specimen is approximately 2mA/cm . The black areas in Figure 2 are the globules of primary phase emanating from degeneration of the dendrites. Spherical "rosettes" with white outlines are often found inside the globules, emanating from droplets of liquid trapped inside during the heat treatment.

The period for which the heat treatment has to be applied to bring about the appearance of the globular structure and to impart the plastic flow properties in the solid state according to the invention varies according to the type of alloy and, for a given alloy, according to the heat-treatment temperature which also determines the liquid phase/solid phase ratio. For example, it ranges from a few minutes to 4 hours, and preferably from about 5 to 60 minutes, for aluminium-zinc-magnesium alloys or aluminium-silicon-copper magnesium alloys.

Cylindrical billets obtainedby chill casting, continuous casting or semi-continuous casting treated in accordance with the invention may be used for feeding a drawing press for example. After introduction into the container of the machine, they may be extruded in the form of profiles provided that suitable cooling means are available both for the die and for the profile issuing therefrom, for example cooling with water or air for example, etc. These cooling means should be sufficiently effective to solidify substantially all the liquid fraction contained in the drawn billet.

The advantages of this process are as follows: a much higher drawing ratio and/or drawing speed; a considerably reduced drawing pressure and, hence, reduced wear of tooling.

It has also been found that » when a billet treated in accordance with the invention is cooled either to ambient temperature or to any temperature below the solidus point, and subsequently re-heated to a temperanre between the solidus .and liquidus points corresponding to less than 40 % of liquid phase, the billet recovers its rheotropic properties, which shows that it has undergone a permanent . modification in its structure and taken on a new structure. This reheating may be carried out at a temperature above, equal to or below the temperature of the initial treatment, according to the required proportion of liquid phase.

The process according to the invention is illustrated by, but by no means limited to, the following Examples. All the drawing tests of Examples 1 to 7 were carried out in an 800 tonne Loewy press.

EXAMPLE 1 .

Three freshly cast cylindrical billets 100 mm in diameter and 300 mm long of AU4SG alloy (chemical composition: aluminium base, iron 0.42 %, silicon 0.91 %, copper 4.24 %, manganese 0.82 %, magnesium 0.51 %) were kept for 15 minutes at temperatures of, respectively, 585°C (billet A), 595°C (billet B) and 605°C (billet C), corresponding to respective proportions of liquid phase of approximately 6 % (billet A), approximately 8 % (billet B) and approximately 13 % (billet C).

They were then introduced during three successive tests into the container, preheated to 420 - 450°C, of the 800 tonne drawing press and immediately drawn at a rate of 8 m/minute in the form of a profile with a rectangular cross-section measuring 40 χ 3 mm. The water-cooling of the die was adjusted in such a way that the temperature of the profile on leaving the die is approximately 450°C. The billets were handled under the usual conditions and drawing carried out satisfactorily. Approximately 15 metres of profile with a perfect surface were obtained for each billet. The mechanical properties of the profiles were measured on test specimens: immediately after drawing, ' - after ageing for 8 hours at 175°C, - following quenching in water after 2 hours at 505°C, followed by ageing for 8 hours at 175°C (state T6).

The following results were obtained: Billet A/585°C Billet B/595°C Billet C/605°C ' STATE YS UTS R YS UTS : E YS UTS E hb hb % hb hb % hb hb % Immediately •after drawing .22.5 35.4 22.2 .24.1 38.1 24.2. 24.8 34.9 23.5 'After 8 h at •175°C .31.1 37.1 12.7 .35.4 40.2 12.7 33.3 37.7 12.9 'After 2 h at .505°C, 'Quenching in iwater, 8 h at •175°C (state .41.4 46.9 14.8 .42.4 47.1 13.3. 43.0 47.6 A . i T6> .

These properties, which are substantially independent the treatment temperature, are entirely satisfactory and comparable with those obtained vuth conventionally drawn products.

EXAMPLE 2 A 100 mm diameter billet of AU4SG with the same composition as in Example 1 was treated in accordance with the invention for 15 minutes at 572°C to produce approximately 4 % of liquid phase. It was then drawn into a 20 mm diameter round bar (corresponding to a drawing ratio of 25) a a rate of 3 metres per minute. The average pressure level on the body of the press was 150 bars.

The same test carried out by the conventional hot-drawing process on a billet which had riot been treated in accordance with the invention required a pressure of 220 bars, i.e. a pressure higher by approximately 50 %.

The drawn bars were found to have the following properties: It can be seen that, in tine case of the bar drawn in acco dance with the invention, it is possible simply by ageing for 8 hours at 175°C to obtain mechanical properties better than those of the equivalent state in conventional drawing. EXAMPLE 3 Two 100 mm diameter billets of AZ5G (aluminium base, zinc 4.40-%, magnesium 1.18 %) were drawn, one by the conventional process and the other after treatment in accordance with the invention (30 minutes at 620°C to produce 4 % of liquid phase).

A 20 mm diameter round bar was drawn at a rate of 13.2 metres per minute.

The pressure required amounted to 155 bars for the untreated billet and to 118 bars for the billet treated in accordance with the invention, corresponding to a reduction of 24 %.

EXAMPLE 4 Two billets of AZ5G (same composition as in Example 3) were drawn using a bridge die to obtain a tubular profile with a square cross-section measuring 25 x 25 mm and a thickness of 2 mm, one untreated and the other treated in accordance with the invention for 20 minutes at 625°C to produce 7 % of liquid phase.

The pressure required amounted to 280 bars for the untreated billet and to 238 bars for the billet treated in accordance withi the invention, corresponding to a reduction of 17 %.

EXAMPLE 5 Two billets of AU4SG with the same composition and dimensions as in Example 1 were drawn using a bridge die, one untreated and the other treated in accordance with the invention for 30 minutes at 585°C to produce 6 % of liquid phase, to form a tubular profile with a cross-section measuring 25 x 25 mm and a thickness of 2 mrn.

Drawing of the AU4SG using a bridge die was not carried out in the usual way. In this particular case, it produced mediocre results and a poor surface appearance, and required a pressure of 290 bars which is close to the permitted limit for an 800 tonne press.

By contrast, drawing of the billet treated in accordance with the invention produced excellent results and only required a pressure, of 210 bars.

EXAMPLE 6 A billet of AU4SG with the same composition and dimensions as in Examp-le 1 was treated in accordance with the invention for 15 minutes at 620°C so as to produce 257o of liquid phase. It was then introduced into the container of the drawing press. In order to avoid any risk of deformation, the billet was treated and transported from the reheating furnace to the press in a horizontal semicircular cradle.

A profile measuring 40 x 3 mm was extruded without difficulty under a pressure which did not exceed 220 bars. The 800 tonne press used for this test does not enable AU4SG to be drawn into a profile measuring 40 x 3 mm under the usual conditions (100 mm billet preheated to 420 - 450°C) Accordingly, the process according to the invention affords a significant advantage in this particular case.

EXAMPLE 7 A cylindrical billet, of AU4SG alloy 100 mm in diameter a 300 mm long was kept for 15 minutes at 585°C to produce approximately 6 % of liquid phase, subsequently cooled to ambient temperature, reheated to 595°C and immediately introduced into the container, preheated to 420 - 450°C, of the 800 tonne drawing press. It was immediately drawn at a rate of 8 metres per minute into a profile of rectangular cross-section measuring 40 x 3 mm. The water cooling of the die was adjusted in such a way that the temperature of the profile issuing from the die is approximately 450°C.

The results of this test were the same as in Example 1B, which shows that simple reheating to the selected temperature immediately restores a billet previously treated in accordance with the invention and then cooled to its rheotropic properties. · t EXAMPLE 8 .. ■ \. ; A compressor rod of AU4SG was produced by die stamping (distance between the axes of the piston and jack: 100 mm). Normally, the production of a compressor rod involves rough shaping and finishing.

By re-heating the billet for 15 minutes to 595°C so as to produce 8 % of liquid phase, it was possible to form the rod in a single die-stamping operation with a pressure of 40 bars in the hydraulic circuit of the press as against 100 bars under normal conditions.

Claims (6)

47002/3 CLAIMS t r:

1. A process for treating metal alloys for shaping them in the form of a mixture of solid phases and liquid phase maintaining the solid form, wherein the alloy is heated to an intermediate temperature between the solidus point and the liquidus point so as to liquefy a well de ined proportion by weight of the alloy of less than 40% and preferably less than 35%, and wherein the temperature is maintained for a period ranging from a few minutes to a few hours, preferably from 5 minutes to 60 minutes, so that the solid dendritic phase has at least begun to change into a 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 teating 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 less than 40% and preferabl less than 35%.

4. A produc of aluminium alloys obtained by the process claimed in Claim 2 or 3·

5. A process for extrusion of the products according to Claim 4 in the form of a mixture of solid phases and liquid phase, to form profiles of any section such as bars, wires and tubes comprising carrying out the extrusion in a drawing press. 47002/2

6. A process according to claim 5, wherein the drawing press is equipped with mesns for cooling the die and the drawn product. 7· A process for working up of a product according to claim 4» comprising hot plastic deformation, such as rolling, forging, stamping, die stamping or swagging the product, preceidod and/or followed "by any conventional heat treatment. HE.gd