EP0020282B1 - Process for the manufacture of hollow bodies of an aluminium alloy, and products thus obtained - Google Patents

Description

This invention relates to a method for manufacturing hollow bodies of aluminum alloy and to the products thus obtained, which have high ductility (in the long direction) and high toughness (in the cross direction) when they are treated with resistance levels higher than 660 MPa.

It is known that the alloys A-ZBGU (or 7049A) according to the AFNOR 50-411 standard, the analysis of which is given in Table 1, are particularly used in the manufacture of hollow bodies under pressure, because of the high mechanical characteristics which they acquire in the quenched-tempered state (state T6).

We know in particular the alloy described in Russian patent SU-A-155 001, which is an alloy type A-Z8GU for obtaining spun tubes which have a high mechanical resistance (Rm in length ≥ 680 MPa) but by against for which the long and transverse direction ductility is very limited (A% ≥ 2%). It should be noted that the Cr content in this alloy is limited to 0.1% by weight.

However, such alloys are not always reliable in the sense that premature ruptures or bursts are sometimes observed during hydraulic tests to control such hollow bodies subjected to internal pressure.

The aim of this invention is therefore to solve this problem by a suitable choice partially covering the field of alloy 7049A, which makes it possible to obtain, according to the process, products having high ductility and toughness characteristics, and, by therefore, great job security.

• 1 ) Essentiellement en diminuant les teneurs des éléments Cr, Mn et Zr, qui sont connus pour être des inhibiteurs de recristallisation dans les alliages d'AI (voir ALTENPOHL, »Un regard à l'intérieur de l'aluminium«, éd. française, 1976, p 148).
  Or, en vue d'obtenir les très hautes caractéristiques mécaniques recherchées, l'alliage est, en effet, utilisé à l'état non recristallisé avec effet de presse, même après les traitements de mise en solution, trempe et revenu.
• 2) En augmentant au-delà des limites habituelles les teneurs en éléments principaux tels que Zn, Cu, Mg.
• 3) En limitant à des niveaux bas ou très bas les teneurs des éléments mineurs (Fe, Si, Ti) ou même des impuretés telles que le V.

This goal is surprisingly achieved:

• 1) Essentially by decreasing the contents of the elements Cr, Mn and Zr, which are known to be inhibitors of recrystallization in AI alloys (see ALTENPOHL, "A look inside aluminum", French ed. , 1976, p 148).
  However, in order to obtain the very high mechanical properties sought, the alloy is, in fact, used in the non-recrystallized state with press effect, even after the dissolution, quenching and tempering treatments.
• 2) By increasing beyond the usual limits the contents of main elements such as Zn, Cu, Mg.
• 3) By limiting the contents of minor elements (Fe, Si, Ti) or even impurities such as V. to low or very low levels.

The general composition of the alloys according to the invention is as follows (by weight):

In a preferred composition, the V content is limited to a content of less than 0.01%.

In the process, the products are transformed in the following way: - homogenization between 460 and 490 ° C of the cast billets, hot deformation between 320 and 420 ° C, including possibly the shrinking of one (or both) end ( s) in the case of the production of hollow bodies, dissolving between 460 and 480 ° C. and tempering suitable for obtaining a breaking load (long direction) and a limit burst stress (transverse direction) greater than or equal to 660 MPa .

Under these conditions and, for a breaking load equal to 660 MPa, the elongation in the long direction is greater than 9%; this elongation is measured over a base length 10 = 5.65√S, S being the section of the test piece. The hot deformation is preferably carried out by reverse spinning; homogenization, dissolution and income conditions may be different from those indicated above without departing from the scope of the invention.

dans laquelle

• e: est l'épaisseur minimum du tube ou du corps creux (supposé sensiblement cylindrique circulaire),
• D: est le diamètre moyen du cylindre, soit
  
• p: est la pression d'éclatement.

The bursting stress (RE) during the hydraulic test is given by the classic formula:

in which

• e: is the minimum thickness of the tube or hollow body (assumed to be substantially circular cylindrical),
• D: is the mean diameter of the cylinder,
  
• p: is the burst pressure.

It has been found that the elements Cr, Mn, Zr have an unpredictable synergistic effect, that is to say that their overall action on the mechanical characteristics is very much greater than the sum of the individual actions of each of them. This effect is clearly demonstrated in the examples given below. It was therefore not at all obvious to choose this particular combination of contents of these elements to obtain the desired properties.

• - 200 g d'alliage environ sont refondus à 735°C±5°°C dans un creuset de graphite poteyé d'alumine.
• - on soumet ensuite l'ensemble à un refroidissement lent en four, à raison de 0,5 à 1°C/min. suivi d'un palier de 2 h à une température supérieure à 2°C à 4°C à celle du début de solidification de l'alliage (liquidus), puis on sort le creuset à l'air pour assurer une solidification rapide.
• - l'examen en micrographie optique au grossissement x 100 à 500 d'un échantillon poli prélevé dans la moitié inférieure du lingotin ainsi obtenu, ne révèle que des amas de constituants intermétalliques primaires ou de particules intermétalliques individuelles massives, non dendritiques, de forme générale polygonale, de longueur inférieure à 35 µm dans leur plus grande dimension.

The alloys according to the invention meet the following control test:

• - About 200 g of alloy are melted at 735 ° C ± 5 °° C in a crucible of alumina-coated graphite.
• - The assembly is then subjected to slow cooling in the oven, at a rate of 0.5 to 1 ° C / min. followed by a 2 h stage at a temperature above 2 ° C to 4 ° C at that of the beginning of solidification of the alloy (liquidus), then the crucible is removed in the air to ensure rapid solidification.
• - the examination in optical micrography at 100 × 500 magnification of a polished sample taken from the lower half of the ingot thus obtained, reveals only clusters of primary intermetallic constituents or of individual massive non-dendritic intermetallic particles, of general shape polygonal, of length less than 35 µm in their largest dimension.

The particles are considered to be part of a cluster when the interparticle distance is less than or equal to the largest dimension of the particle considered. In this case, the length taken into account is the cumulative length of the maximum dimensions of each particle in the cluster. The invention will be better understood and illustrated by the following examples:

Example 1

The alloys marked 1 to 12, the compositions of which (% by weight) are given in Table II, were vertically semi-continuously cast in 0 185 mm billets, which were homogenized 24 h at 450 ° C. These billets were machined at 0 170 mm and drilled with a central hole 0 70 mm for reverse spinning of tubes 0 82 x 67.5 mm at a temperature of 365 ° C.

• - mise en solution à 460° C pendant 45 minutes
• - trempe à l'eau froide (10-15° C)
• - revenu à 125° C pendant 20 h

The tubes were then treated as follows:

• - dissolved at 460 ° C for 45 minutes
• - cold water quenching (10-15 ° C)
• - returned to 125 ° C for 20 h

The tubes thus obtained were subjected to tensile tests in a direction parallel to the generatrices of the tube and to bursting tests under hydraulic pressure (longitudinal tear). The breaking load (Rm), the elastic limit (R 0.2), the elongation (A%) and the bursting stress (RE) were noted.

The results obtained are reported in Table III.

The individual effects of the additions of 0.07% Cr (rep. A), 0.08% Zr (rep. B) and 0.15% Mn (rep. C) are shown in Table IV. It can be seen that the sum of the individual effects (lines A + B + C) is much lower than that of the joint additions (line D according to the invention) of all of these elements on the traction characteristics and particularly spectacularly on the elastic limit and the elongations. However, it has no significant effect on the burst strength.

Thus, the synergistic effect of these elements, unpredictable a priori, is well demonstrated.

Furthermore, it can be seen that the characteristics targeted are not achieved for alloys 1 to 8, the composition of which is outside the scope of the invention, while alloys 9 to 12, according to the invention, achieve them.

Example 2

Three semi-continuous castings of alloy 7049A, outside the composition limits of the present invention, were carried out. The analyzes obtained are given in Table V.

These were transformed into tubes under the conditions of Example 1 by reverse spinning, and these were shrunk and treated in the T6 state by dissolving at 465 ° ± 5 ° C, 45 min, quenching water and tempering 125 ° C, 20 h. The breaking stresses under hydraulic test, calculated as indicated above, are also given in Table V. It can be seen that they are much lower than the target limit value (660 MPa).

• - prélèvement de 200 g de métal dans les billettes coulées en série continue,
• - fusion du prélèvement à 735° C±5° C,
• - refroidissement à 632°C à raison de 0,5 à 1°C/minute,
• - maintien 2 h à 632° C (début de solidification de l'alliage à 628° C),
• - sortie du four et refroidissement rapide.

On metal in accordance with the invention (Rep. 12, table II) and not in accordance with the invention (Rep. E, table V), we carried out the following solidification test:

• - removal of 200 g of metal from the billet cast in continuous series,
• - fusion of the sample at 735 ° C ± 5 ° C,
• - cooling to 632 ° C at a rate of 0.5 to 1 ° C / minute,
• - 2 h hold at 632 ° C (start of solidification of the alloy at 628 ° C),
• - exit from the oven and rapid cooling.

On the alloy in accordance with the invention, the micrographic structure of the ingot in its lower 1/3 is represented by FIG. 1 at 200 magnification. No compound larger than 35 microns in its largest dimension is observed. In addition, all the compounds out of solution are observed in the interdendritic spaces. A good part of them is moreover resolved by subsequent heat treatments.

On the contrary, in the case of the alloy leaving the field of the invention; (Cr 0.22%, Mn 0.27%, Zr 0.13%), it is possible to observe primary intermetallic compounds of polyhedral form, of dimension greater than 100 microns and grouped in colonies (Figure 2). These crystals cannot be confused with those of FIG. 1, neither by their size, nor by their situation, nor finally by their evolution during transformation. They undergo, in fact, no modification by the effect of heat treatments. They fragment and align, remaining contiguous, in the main direction of the deformation, with all the consequences that this configuration implies on the fragility of the product.

Claims (3)

1. A process for producing a hollow body which is capable of resisting an internal pressure, having a tensile stress and a bursting stress which are higher than or equal to 660 MPa and an elongation at rupture in the lengthwise direction of higher than or equal to 9% when Rm =660 MPa, characterised by the following steps:

- an alloy is produced, containing (% by weight):

- the homogenised alloy is cast and shaped in a hot condition in the form of a tube, preferably by indirect extrusion and one end or both ends is or are constricted in the hot condition, and

- it is subjected to heat treatment by being put in solution, hardening and tempering (state T6).

2. A process according to claim 1 characterised in that the alloy contains less than 0.01% V (by weight).

3. A product produced in accordance with one of claims 1 and 2 characterised in that the cast structure of an ingot resulting from the product and having been subjected to the following solidification test:

- 200 g of alloy approximately is re-melted at 735°C±5°C in a graphite crucible with a coating of alumina therein,

- the assembly is then subjected to slow cooling in a furnace, at a rate of from 0.5 to 1°C/minute followed by a plateau of 2 hours at a temperature which is 2 to 4°C higher than that at the commencement of solidification of the alloy (liquidus), then the crucible is brought out into the air to provide for rapid solidification,

has on micrographic section only accumulations of primary intermetallic compounds, the cumulated length of which is less than 35 ₁1m and/or isolated particles of primary intermetallic compounds the largest dimension of which is less than 35 µm.

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