IE47282B1 - A method of extruding high-strength aluminium alloys and products obtained by such a method - Google Patents

Abstract

1. A method of extrusion, followed by quenching, of high-strength based aluminium alloys of series 2 000 and 7 000, wherein the extrusion ratio is from 20 to 200, characterised in that : a) extrusion is effected with a lubricant, at a speed of over 15 m/minute ; b) the extrusion temperature is between Ts -50 degrees C and Tm -180 degrees C, Ts being the equilibrium solidus temperature of the alloy, and is adjusted within that range so that the temperature of the metal leaving the die is between Tm and Ts , Tm being the minimum temperature at which the said alloy is solution heat treated ; c) the metal is quenched from extrusion heat at the usual cooling speeds for the alloy in question.

Description

This invention concerns a method of extruding highstrength aluminium alloys for the manufacture of tars, tubes or sections.

High-strength aluminium alloys are understood as heing 2 alloys that have a breaking strength of over 35 kg/mm after welding and heat treatment. Those referred to particularly are Al-Cn-Mg alloys (Cu >y}>, Mg > 0.5$), the best known heing 2014, 2017 A, 2050 and 2024, and Al-Zn-Mg-Cu alloys, the best known heing 7075, as defined in Drench Standard AFNOR NF A-02-104.

Direct extrusion of these alloys in a jacket generally takes place at fairly low speeds, say 2 to 8 metres per minute, with extrusion ratios that do not generally exceed 40, When the alloys are draw in a jacket at high speeds and/or with, high extrusion ratios, the temperature of the product leaving the die is such that surface cracks appear or the metal overheats at least at the surface.

French Patent No. 2,273,077 describes the extrusion of soft alloys of the 6000 series, at speeds of about 20m/mn, and extrusion ratios of over 100. It happens occasionally that some of these alloys, such as 2030, have to be quenched on the press, that is to say, the extruded product has to be quenched as soon as it leaves the die. The extrusion speed used in this case is generally lower than that used liken there is no quenching on the press. If the metal is to form a solid solution satisfactorily, it is necessary for the temperature at which it emerges to be accurately contolled.

It should he between the temperature at which the alloy forms a I solid solution and that at which cracks or overheating appear; nevertheless, the formation of a solid solution must he satisfactory. For these reasons, the temperature at which extrusion begins is generally very close to that at which the solid solution is formed, so that the hillet does not show intense precipitation, which would make it difficult for a solid solution to form. On the other hand, the extrusion speed is reduced either to avoid overheating during drawing, with the danger of cracks or incipient melting appearing (partial fusion of metastable or non-metastahle eutectics), or specifically because a high extrusion temperature, close to the solution-heat-trcatment temperature, is less favourable at high speed than a lower one, because of the dangers of cracks or incipient melting appearing.

Finally, a satisfactory compromise, which it is already difficult to obtain with the usual extrusion ratios of 10 to 25, becomes 3till more difficult at higher extrusion ratios because of the greater heating which they entail.

In any case, it is difficult to control the discharge temperature owing to the development of frictional forces and thus heating throughout the extrusion operation.

There are some methods of increasing the extrusion speed.

For example, attempts have been made to increase it by reducing friction within the cylinder containing the metal and against the die, particularly by using a lubricant.

This can be done with a conventional lubricant, e.g. a grease with molybdenum bisulphide and/or graphite added, ii certain precautions are taken to keep the lubricant permanently in contact with the metal.

It is equally possible to use a vitreous lubricant of a 3 4 viscosity between 10 and 10 poises between 400 and 650°C for example. Under these conditions a vitreous lubricant,containing elements such as P^O^ - B^O^ - Κ^,Ο - Νβ,,Ο, enables speeds of over 100 m/minute to be obtained. Such a method is claimed in Irish Patent No. 44,975 to which attention is directed.

The present invention aims at improving these quick extrusion methods. It makes it possible to obtain products with excellent surface condition and good mechanical and metallurgical properties under very favourable economic circumstances.

The present invention is based on the discovery that, under certain conditions, quenching on the press can he combined with high extrusion ratios and a high extrusion speed for these alloys. The high extrusion speed - provided that it is sufficiently high - is even favourable to satisfactory control of the formation of a solid solution before quenching.

The method comprises extruding higli-strengoh aluminium alloys with a lubricant, with an extrusion ratio between 20 and 200 and at an extrusion speed of over 15 m/minute (and preferably over 50 m/minute) under the following conditions: (a) the extrusion temperature, i.e. the temperature of the billet at the time of compression, must he between Tg -180°C and Tg -50°C, Tg being the equilibrium solidus temperature of the alloyj (b) for every extrusion ratio the extrusion temperature must be adjusted, so that the discharge temperature is between Tffl and T and preferably between T and T - 3θ°C, T being the s mam minimum temperature at which the alloy is solution-heat-treated in cases where quenching takes place in a furnace in the conventional way; (c) the metal must be quenched from the extrusion temperature on leaving the press (i.e. instead of undergoing cooling followed by re-heating prior to quenching) at the normal cooling speeds for that alloy and possibly after* a time lag such that tbe temperature does not drop below the critical temperature, i,e. the temperature at which precipitation appears and destroys the dissolving action (about 450° for 2017 Λ and 20'jd), before the metal is placed in the quenching medium.

As an illustration average values of T and T for the 5 JQ main high-strength alloys are given in the table below: alloy Tg (»c) T (°C) m v ' 2014 535 490 2017 A 535 490 2024 515 480 2050 530 490 7075 535 450 These values obviously depend on the exact composition of the alloy. In the case of alloys in series 2,000 of the following composition: Cu 3.5 to 5% Mg 0.4 to 1$ Si < 1.2 % possibly Pb 0.3 to 1.4#, the remainder being aluminium with impurities or secondary added elements such as Fe, Cr, Ti, or Mn in the usual concentrations, the extrusion temperature is from 350°C to 480°C and the tenperature at which the alloy leaves the die is from 480°C to 53C°C and preferably from 480 to 500°C.

It will be found that with this method the temperature of discharge after extrusion can be satisfactorily controlled, because the high extrusion speeds and/or the reduced friction give quasi-adiabatic conditions during extrusion. Thus, exception a small portion at the end of the extruded product, which can be removed, the temperature is uniform with a variation not exceeding 10 C degrees from one end of the extruded metal to the other. It is also an advantage that extrusion takes a veiy short time, less than 3θ seconds and even 20 seconds, however long the billet (this is selected from industrial lengths).

The very short extrusion times allow for more uniform lubrication from one end of the extruded metal to the other and improve its appearance. It may be helpful not to place the metal in the cooling fluid immediately it leaves the die. A time lag before quenching - when this is carried out - enables the solution-heat-treatment to be completed. Tha time lag is from 15 to 9θ seconds, its length being determined by two conflicting requirements: firstly, the need to increase the solution-heat-treatment time in order to improve the metallurgical quality of the extruded product, and secondly, the need to avoid the critical precipitation range, which would impair mechanical properties and make the alloy sensitive to intercrystalline corrosion.

Preliminary homogenisation of the billet is generally favourable from the point of view both of good appearance and ease in forming a solution; such homogenisation takes place between 480 and 520°C for 1 to 24 hours.

It may also he advantageous to heat the billet, before extrusion, to a temperature close to the normal temperature at which the alloy forms a solid solution; this is often over 20 C degrees ahove the extrusion temperature. In this case the billet is cooled rapidly, in less than 3 minutes and preferably less than 1 minute, e.g. hy a spraying arrangement that avoids shrinkage cracks down to the extrusion temperature. This arrangement ensures that the alloy dissolves more completely and avoids over-large precipitates during extrusion. Xn this way the heat given out hy the extrusion process will enable the metal to dissolve within a very short time after leaving the die.

Any lubricant and any lubricating method that will allow for the above-mentioned extrusion ratios and speeds may he used for extrusion. Hydrostatic extrusion may he put in the same category as lubricated extrusion.

It is particularly advantageous to use a lubricant that is soluble in the quenching medium. This makes it possible to combine quenching and lubricant removal in one operation on the press.

For example, the above-mentioned, vitreous product (PgOg + BgOj + KgO + NagO) which is soluble in water or in a liquid containing over 80½ oi water may- he used as the lubricant, the remainder being quenching additives. The lubricant can then be removed instantly from the extruded product.

Quenching may take place in known manner either hy spraying or hy immersion.

The invention will he illustrated in the following Examples.

Example 1 Billets 100 mm in diameter made of 2017A and 2050 are homogenised for 6 hours at 500°C in a static air furnace, then cooled in ambient air without any special precautions.

The billets, cut into blooms, are reheated to 400°C in 5 minutes, then extruded in a press with a lubricant in the form of bars 22 mm in diameter (drawing ratio 22) at a speed oi 70 m/minute. Under these conditions extrusion takes 15 seconds.

The temperature at which the bars emerge is from 490 to 500°C for both alloys (this applies from one end of the bars to the other).

Quenching with water is then carried out as follows; (a) immediately after extrusion (b) 45 seconds after extrusion (c) 90 seconds after extrusion After being quenched the bars are stretched by 3½ so as to give a state T3 with a diameter of 21.7 mm.

Various characterising tests are carried out in com47282 parison with alloys treated conventionally, viz. hy homogenisation at 500°C and extruding in a jacket at 35O°C and 4-5 lg/mn:(i) stretching approximately 50$,* (ii) separate solution-heat-treatment, taking minutes at 490°C, and quenching with water; (iii) straightening.

For bars extruded with a lubricant (whether it is vitreous or a graphite-like substance containing molybdenum sulphide), the tolerances in all cases are such that, even after a simple 3$ gauging, the bare comply with characteristics of stretched bars.

The characteristics obtained are set out in the table below, which also gives the conversion conditions: Alloy Extrusion Time lag between extrusion and quenching Yield strength IiO.2-MPa Mechanical characteristics Tensile strength Itm-Ml’a Elongation Λ5 - 0 286 393 12 2017 A lubricated 45 s 307 416 16 90 s 287 392 13.2 non-lubricated 569 459 15.8 0 348 445 12 2030 lubricated 45 s 338 440 14.5 90 s 341 443 13.6 non-lubricated 374 448 16.7 Intercrystalline corrosion tests are carried out in all eases. No susceptibility is detected apart from cases where the time lag between quenching and extrusion is 90 seconds. This is explained hy the fact that the metal has come within the precipitation range of Al-Cu-Mg and precipitation is intense at the temperatures envisaged (=400°C).

Tests of fatigue in rotary tending do not reveal any significant differences between the products of extrusion with a lubricant and those of extrusion without lubricant.

The mechanical characteristics are in all cases found to conform with the standards enforced. Nevertheless, in the case of alloy 2017 A the mechanical characteristics are slightly inferior to those, obtained after solution-heat-treatment in a furnace, particularly at the level of elongations at tensile strength. The highest elongations are obtained when there is a time lag of 45 seconds between extrusion and quenching. Now for this period the temperature of the bar is approximately 450“C, that is to say, at the limit of the precipitation threshold. It is thus an optimal time for the extrusion operation envisaged, and enables the metal to dissolve more completely.

Example 2.

Billets 800 mm in diameter, made of 2017 A and 2030, are homogenised for 6 hours at 500°C then cooled in ambient air without ai?y special precautions.

The billets are cut into blooms and reheated to 490°C for § hour, cooled rapidly to 400°C (l minute) and finally extruded at that temperature at a speed of 70 n/minute under conditions similar to those in Example 1, that is to say, extruded into bars 22 mm in diameter with a lubricant. The temperature at which the bars emerge is from 490 to 500°C. Quenching with water is then carried out immediately after extrusion. The bars are finally stretched by 3$. They have dimensional characteristics appropriate to stretched bars. The mechanical characteristics obtained are as follows: Alloys β 0.2 - MPa Rm - MPa A. - 2017 407 536 15.5 2030 396 500 16 It· will be noted that there is an important gain in the mechanical properties obtained as compared with those in Example 1, due to the preliminary heat treatment prior to extrusion.

Example 3 Billets of 2030 alloy 145 mm in diameter are extruded in the form of square bars 20 x 20 mm (extrusion ratio 44) under the following conditions: extrusion with a vitreous lubricant re-heating 410°C speed 50 m/minute, i.e. about 20 seconds of extrusion quenching with water after holding for 30 seconds after extrusion Taking into account the dimensional tolerances and the surface appearance obtained, a single 3f“ gauging is sufficient to give square bars complying with the standards for stretched bars. The mechanical characteristics obtained after 3/ gauging are: BO 2 b 404 MPa; Em = 491 MPa; A? = 14.8% This example is a good illustration of the fact that shapes other than cylinders can be obtained under favourable conditions.

Example 4 Billets of 2030 alloy 145 mm in diameter are homoI genised and then cooled in ambient air without any special precautions. The billets are cut into blooms, reheated to 37θ°0 and extruded at that temperature with a graphite grease at a speed of 100 m/minute, in the form of bars 12 mn in diameter. The tenperature at which the resultant wire emerges is approximately 510°C and remains constant throughout the extrusion process, which takes ahout 30 seconds. Quenching with water is carried out seconds after extrusion. Finally, the bars are stretched 3½ to give them a diameter of 11.8 mm. The dimensional tolerances are still compatible with those of stretched products.

The mechanical properties obtained are as follows: B 0.2 = 339 MPa; Em = 444 MPa; A5 = 14½ These properties are quite comparable with those obtained in the other examples.

Claims (10)

1. A method of extrusion, followed by quenching, of alloys based on high-strength aluminium of series of 2,000 and 7,000 at an extrusion ratio in the range 20 to 200, in which 5 (a) extrusion is effected with a lubricant, at a speed of over 15 m/minute; (h) the extrusion temperature is between - 50°C and - 180°C, T g being the equilibrium solidus temperature of the alloy, and is adjusted within that range so that the temperature 10 of the metal leaving the die is between T and Τ , T being the —in “TQ minimum temperature at which the said alloy is solution-heattreated; (c) the metal is quenched from the extrusion temperature at the usual cooling speeds for the alloy in question.

2. A method as claimed in Claim 1 in which tiie total extrusion time is less than 30 seconds.

3. , A method as claimed in Claim 1 or 2 in which quenching begins after a delay of from 15 to 90 seconds after the metal leaves the die and at a temperature above the critical precipitation temperature.

4. 5 4. A method as claimed in any of Claims 1 to 3 in which the billet is reheated to a temperature at least 20°C above the extrusion temperature and is then cooled to the extrusion temperature in such a way that the total time between its emergence from the reheating furnace and the beginning of extrusion 10 does not exceed 3 minutes. 5. A method as the lubricant is soluble claimed in any of claims 1 to 4 in which in the quenching medium.

5. 6. A method as claimed in Claim 5 in which the lubricant is a water-soluble vitreous product based on PgQj’ ®2θ3’ ^2θ an ^ 15 NagO.

6. 7. A method as claimed in any preceding claim applied of series 2000 to alloys y containing the following quantities of the chief elements: (in weight %): Cu 3.5 Mg 0.4 Si <. possibly with the addition of 0.5 to 1.4/ of lead, the remainder being aluminium with contents of impurities or secondary elements such as Fe, Mn : Ti or Cr yat the usual level for that type of alloy, in which the temperature at which extrusion commences is between 350 and 480°C, and the 5 temperature at which the billet leaves the die is from 480 to 53O e C.

7. 8. A method as claimed in Claim 7 in which the billet is homogenised for 1 to 24 hours between 480 and 520°C.

8. 9. A method as claimed in Claim 1 substantially as

9. 10 hereinbefore described in any one of the methods set forth in any one of Examples 1 to 4.

10. A quenched extruded product, which is optionally stretched, straightened or gauged after quenching, when obtained by a method as claimed in any one of the preceding claims.