EP0375572B1 - Aluminium alloy for cupping, containing silicon, magnesium and copper - Google Patents

Abstract

The invention relates to an Al alloy for stamping, essentially containing Si, Mg and Cu, and to its process of manufacture. This alloy comprises (in % by weight) contents of Mg and Si which are within a polygon ABCDE having the following coordinates: <IMAGE> the copper content being between 0.5 and 0.8%, the contents of the elements Cr and Zr being less than 0.2% each, the Mn content being </= 0.3%, the Fe content being </= 0.35% and the content of the other elements being individually </= 0.05%, and in total </= 0.15%, the remainder being Al. The range of fabrication methods comprises semi-continuous or continuous ingot casting, optional homogenisation, hot transformation ending in the range 270-320 DEG C, optional cold transformation, complete dissolution, shaping by stamping, folding, roll bending etc., and annealing. This alloy is principally employed in panels for automobile bodywork. <IMAGE>

Description

The invention relates to an Al alloy essentially containing Si, Mg and Cu intended for the manufacture of stampable sheets or strips, in particular sheets used in automobile bodywork.

100 MPa, Rm

200 MPa, R 0,2 étant la limite élastique à 0,2% de déformation résiduelle et Rm la charge de rupture).In French patent FR-2 601 040, a composition of boilable and weldable Al alloys is disclosed, usable in the treated state (quenched / tempered) after shaping between the quenched state and the tempered state. However, for certain applications such as that indicated above, the strength characteristics of this alloy in the quenched-cured state remain insufficient (R 0.2

100 MPa, Rm

200 MPa, R 0.2 being the elastic limit at 0.2% of residual deformation and Rm the breaking load).

It has been found that the moderate increase in the Cu content makes it possible, surprisingly, to improve the strength characteristics, without loss of the ductility characteristics.

US-A-4,614,552 describes an Al alloy for stamping containing essentially from 0.65 to 0.79% (by weight) Cu; from 0.62 to 0.82% Mg; from 0.6 to 1.0% Si; 0.1 to 0.5% Mn; up to 0.4% Fe; up to 0.10% Ti; remains Al.

• . les teneurs en Si et Mg sont comprises dans un rectangle dont les sommets sont :
  
• . la teneur en Cu étant comprise entre 0,5 et 0,8
• . la teneur en Mn inférieure ou égale à 0,3%
• . la teneur en Fe inférieure ou égale à 0,35%
• . les teneurs en Cr et Zr inférieures à 0,2% chacune
• . autres éléments
     chacun ≦ 0,05%
     total ≦ 0,15%
• . reste Al

According to the invention, the alloy responding to the problem posed above has the following weight composition:

• . the Si and Mg contents are included in a rectangle whose vertices are:
  
• . the Cu content being between 0.5 and 0.8
• . the Mn content less than or equal to 0.3%
• . Fe content less than or equal to 0.35%
• . Cr and Zr contents of less than 0.2% each
• . other elements
  each ≦ 0.05%
  total ≦ 0.15%
• . stay Al

For the minimum values in main elements the minimum mechanical characteristics indicated above are not reached.

For Si greater than the DE limit, the risks of precipitation of submicroscopic elementary Si exist if the dissolution is carried out at too low a temperature.

For Mg greater than the line BC, (ie Si / Mg <2 approximately) coarse precipitations of Mg sontSi are possible during solidification and therefore harm the ductility and drawing properties.

For Cu ≧ 0.8%, the gain in strength in mechanical characteristics becomes negligible; moreover, the corrosion resistance and the drawability decrease.

It is possible to add anti-recrystallizing elements to the composition such as Mn, Cr and / or Zr, however in limited quantities.

• . il donne naissance à la solidification des composés intermétalliques à base de Fe, Mn, Si qui réduisent la capacité de déformation de l'alliage et peuvent initier des décohésions et ruptures, lors des opérations de mise en forme;
• . il augmente la vitesse critique de trempe et limite donc les possibilités de traitements thermiques pour les produits épais;
• . il confère à l'alliage un comportement à la corrosion assez médiocre;
• . il n'est pas adapté aux homogénéisations de courte durée, telles que celles généralement obtenues dans des fours à passage.

The Mn content is further limited since this element has the following disadvantages in higher quantities:

• . it gives rise to the solidification of intermetallic compounds based on Fe, Mn, Si which reduce the deformation capacity of the alloy and can initiate decohesions and ruptures, during shaping operations;
• . it increases the critical quenching speed and therefore limits the possibilities of heat treatments for thick products;
• . it gives the alloy a fairly poor corrosion behavior;
• . it is not suitable for short-term homogenizations, such as those generally obtained in passage ovens.

Cr and Zr have similar effects to those of Mn, and are therefore also superiorly limited.

A preferred composition of the alloy according to the invention therefore comprises an Mn content of less than or equal to 0.2% and contents of Cr and Zr ≦ 0.05% each.

The manufacturing range used generally includes the following operations:
. continuous or semi-continuous casting of blanks, possible homogenization, hot transformation, possible cold transformation, dissolution and tempering.

However, to obtain good properties of the alloy, these operations must be carried out under fairly narrow conditions.

Thus, to limit the time for subsequent dissolution, it is preferable to homogenize the alloy well, avoiding burning it by fusion of the eutectic phases. Homogenization at high temperature between 520 ° C and 560 ° C with a holding time of 6 to 24 hours is desirable. Homogenization is preferably preceded by a slow rise in temperature.

In the case of sheets and strips, coarse hot recrystallizations (grain size greater than 80 μm) are the source of macroscopic deformation lines, visible after stamping, therefore unacceptable for this application.

Therefore, the temperature at the end of hot transformation, to avoid these recrystallizations, must imperatively be between 270 ° and 320 ° C.

After possible cold transformation, the alloy is put into a complete solution. This takes place in the temperature range between 520 and 560 ° C, and preferably between 530 ° and 550 ° C by targeting the temperature of 550 ° C.

In the voluntary absence of recrystallization inhibiting elements (Mn, Cr, Zr ≦ 0.05% each), the rise in temperature before dissolving must be rapid (V ≧ 10 ° C / sec) and dissolving preferably performed either in a pass-through oven or in a sheet-to-sheet processing oven.

The processing time varies from a few seconds to a few minutes, without being able to exceed one hour. The sheets and strips thus obtained have good isotropy and an average grain size generally not exceeding 60 μm.

The quenching must be rapid and depends on the thickness of the product. For sheets and strips, it is generally carried out in calm or pulsed air.

After the cold forming operations such as stamping, bending, bending, etc. and / or assembly such as spot welding, the parts undergo hardening income, under the usual conditions; the hardening is due to the precipitation of the Mg₂Si phase and of AlCuMgSi complex phases. Tempering is typically carried out between 8 and 12 hrs around 165 ° C.

It should be noted that in certain cases, the baking of surface coatings such as varnishes or paints, although shorter, and generally at higher temperature, ipso facto performs this treatment.

The invention will be better understood with the aid of FIG. 1 giving the field of composition of alloys in the Mg-Si plane and is illustrated by the following example:
- A 1100x300x2850 mm3 plate of the following weight composition was poured semi-continuously:

This plate was homogenized at 530 ° C-8h and immediately hot rolled; up to 4 mm thick, the final rolling temperature was 300 ° C; the strip thus obtained has been cold rolled until 1.2 mm thick, then dissolved in a passing oven at 550 ° C (about 1 min at temperature) at a speed of 20 m / min and quenched in a mist (air + water).

The results obtained on sheet 1.2 mm thick in states T4 and T6, in the long transverse direction, are as follows:

By way of comparison, an alloy with the following weight composition:
If: 1%; Mg: 0.4%; Cu: 0.15%; Mn: 0.1%; rest Al and usual impurities having followed the same range of transformation and heat treatment as above, has the following characteristics in state T4:

Claims (7)

1. An aluminium alloy for the production of drawable sheets characterized in that it has the following composition (% by weight):

   . the Mg and Si contents are comprised within a rectangle A'CDE' with the following co-ordinates:

   

   . the Cu is comprised between 0.5 and 0.8%

   . the elements Cr and Zr less than 0.2% each

   . Mn ≦ 0.3%

   . Fe ≦ 0.35%

   . other elements: each ≦ 0.05%, total ≦ 0.15%

   . the balance being Al
2. Alloy according to claim 1, characterized in that the Mn content is less than or equal to 0.2% and Cr & Zr < 0.05% each.
3. Alloy according to one of claims 1 or 2, characterized in that the average grain size is less than 80 µm and preferably less than 60 µm.
4. Process for obtaining products according to one of claims 1 to 3, comprising the continuous or semi-continuous casting of ingots, optional homogenization, hot working, optional cold working, solution treatment, quenching operation forming operation, for example by drawing, folding, bending, and finally artificial aging characterized in that the final hot working takes place between 270 and 320°C.
5. Process according to claim 4, characterized in that the homogneization or complete solution treatment takes place between 520 and 560°C.
6. Process according to claim 5, characterized in that the homogenization or solution treatment takes place between 530 and 550°C.
7. Process according to one of claims 5 or 6, characterized in that the solution treatment is preceded by a rise in temperature at a speed which is greater than 10°C/sec.

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