EP0259232B2 - Easily workable and weldable aluminium alloy, and process for its manufacture - Google Patents

Description

The invention relates to a weldable boilermaking aluminum alloy containing essentially Si, Mg and Cu and its manufacturing process.

Alloys of the 6000 series according to the nomenclature of the Aluminum Association have been developed mainly in the form of profiles, although some of these allallies such as 6061 or 6082, are commonly found in the form of sheets or strips, intended for stamping.

Alloys belonging to this family have been described in French patents FR-A-2,375,332 and FR-A 2,360,684.
These alloys, less loaded with magnesium than the conventional 6000 alloys, which are not far from the stoichiometry Mg ₂ Si, are on the other hand much richer in silicon.

Patent application FR 2 375 332 describes a process in which an alloy rich in Si is treated with so as to obtain a fine sub-micron precipitation (0.1 to 0.5 μm) of Si in supersaturation; this size is intermediate between the eutectic phases present in the alloy and that of the hardening phases usually observed in Al-Si-Mg-Cu alloys.

This precipitation of Si, if it presents according to the authors a certain number of advantages, has also some disadvantages.

In fact, excessively large silicon precipitates reduce the deformation cappacities of the material and more the corrosion resistance of the alloy in its conditions of use is weakened by their presence.

French patent application FR 2 360 684 describes an Al-Si-Mg-Cu alloy containing at least one of the elements which inhibit recrystallization from the Mn, Cr, Zr group.

• 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 le produits épais;
• il confère à l'alliage un comportement à la corrosion assez médiocre;
• il n'est pas adapté aux homogénéisations de courte durée, telle que celles généralement obtenues dans des fours à passages.

However, the presence of these latter elements is not favorable. Mn in particular has several drawbacks:

• 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.

The problem posed to those skilled in the art is therefore obtaining a stampable Al-Si-Mg-Cu alloy. and weldable, free from the drawbacks mentioned above and which has mechanical properties satisfactory in the hardened state, good capacity for cold deformation in the quenched state, good resistance to corrosion following a simple heat treatment, which excludes the presence of any submicron phase precipitation essentially consisting of Si.

According to the invention, the alloy comprises (in% by weight) contents of Si and Mg defined by the trapezoid of coordinates: Yes Mg AT) 0.5 0.1 B) 0.5 0.2 VS) 1.3 0.5 D) 1.3 0.1 Cu 0.1 - 1.5 Mn 0 - 0.15 Fe 0 - 0.35 others each ≤ 0.05 total other ≤ 0.15

remains Al and is free of submicron precipitates essentially consisting of Si.

Below the minimum values of the main elements (Si, Mg, Cu) the mechanical characteristics desired in the treated state are not reached.

For Si ≧ 1.3% the heat treatment for complete solution is difficult to apply industrially, as will be explained below.

For Mg ≧ 0.5%, difficulties during hot transformation appear (embrittlement) and the drawing ability is reduced.

It can also be observed that the maximum Si / Mg ratio (BC side of the trapezoid) remains equal to or greater than 2.6 approximately so as to limit the precipitation of Mg ₂ Si during solidification as much as possible. Thus, the fine precipitation Mg ₂ Si present in the alloy results only from the heat treatments undergone.

For Cu ≧ 0.5%, the corrosion resistance as well as the drawing ability are reduced.

The secondary elements are limited for the following reasons: As explained above, the presence of Mn is not desirable; however it was admitted up to 0.15% maximum due to possible contamination in this element, due to recycling of waste. It should be noted that the alloy does not contains no intentional additions of Cr and / or Zr.

The Ti associated with B controls, as is known, the fineness of the primary crystallization of the products rough casting (plates, strips, billets, etc ...) and allows homogenizations and solutions more short, in particular with regard to the processing of flat products (sheets, strips). The contents effective are Ti <0.1% and B <0.05%. The Fe content is limited to 0.35% to avoid the formation of coarse primary compounds containing Fe (AlMnFeSi type).

A preferred composition of the alloy according to the invention (% by weight) is as follows: content of Si and Mg included in the trapezium having for apex: Yes Mg AT" 0.65 0.18 B " 0.65 0.2 VS" 0.95 0.28 D " 0.95 0.2 Cu = 0.10-0.25 Mn ≤ 0.15 the limits of the other elements being the same.

Another preferred composition of the alloy according to the invention corresponds to the contents of Si and Mg, delimited by the trapezoid A'B'C'D 'of Fig. 1 with Cu: 0.1-0.25 and Mn ≦ 0.15, the limits of the others elements being the same.

The production range of the alloys according to the invention is defined in claim 6.

However, to obtain good properties of the alloy, in particular a lower grain fineness at 80 µm on average, these operations must be carried out under fairly narrow conditions.

Thus, to limit the time for subsequent dissolution, it is preferable to homogenize well the alloy avoiding burning it by fusion of the eutectic phases. High homogenization temperature between 550 ° C and 570 ° C with a holding time of 6 to 24 hours is desirable. Homogenization is preferably preceded by a slow rise in temperature.

The hot transformation is carried out by any known means (rolling, spinning, forging, etc.) However, this must then be carried out so as to avoid coarse recrystallizations in progress of operation.

In the case of sheets and strips, these coarse hot recrystallizations are the source of lines of Macroscopic deformations, visible after stamping, therefore unacceptable for this application.

Therefore, the temperature at the end of hot transformation, to avoid these recrystallizations, must be imperatively between 270 ° and 340 ° C.

After possible cold transformation, the alloy is put into a complete solution. This takes place in the temperature range between 540 and 580 ° C, preferably between 550 and 570 ° C, targeting the temperature of 560 ° C.

Given the voluntary absence of recrystallization inhibiting elements (Mn, Cr, Zr), the rise in temperature before dissolving must be rapid (V ≧ 10 ° C / sec) and dissolving preferably performed either in a through furnace 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 a hour. The sheets and strips thus obtained have good isotropy and grain size average not exceeding 60 µm.

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

After cold forming or assembly operations such as welding, the parts undergo hardening, under the usual conditions; the hardening is due to the precipitation of the Mg ₂ Si phase and of the complex AlCuMg, AlCuMg Si phases. Tempering is typically carried out between 8 to 12 h at 165 ° C.

It should be noted that in certain cases, the baking of surface coatings such as varnishes, although shorter, ipso facto performs this treatment.

The invention will be better understood with the aid of the following examples illustrated by FIG. 1 which represents the domain of composition of the elements Si and Mg of the alloy, and FIG. 2 which represents the domain of dissolving or homogenizing an alloy according to the invention, on a vertical section of the state diagram Al, Mg, Si at 0.2% Mg.

In FIG. 2, we find in (1) the solvus curve, in (2) the solidus curve and in (3) the eutectic plateau, which gather at point E.

Dissolution (or homogenization) must be carried out in the single-phase field and in particular under the temperature conditions represented by the rectangle FGHI for the general range and F'G'H'I 'for the preferred range.
It is obvious from these curves that for high Si contents, the treatment is delicate, since a small variation with respect to the set temperature leads either to a precipitation of Si if the temperature drops, or to a "burning" of the metal if the temperature rises.

This heat treatment therefore requires a precise industrial tool.

Example 1

A plate (1500x400 mm ² ) of the following composition (% by weight): Si 0.90; Mg 0.30; Cu 0.20; Fe 0.25; Ti 0.03, was cast by the conventional semi-continuous process. This plate was homogenized 10 h at 555 ° C (scalped to 1500 x 420 mm ² ) then hot rolled up to 4 mm thick with finishing between 320 and 300 ° C. The coils thus obtained were cold rolled up to 1.25 mm thick.

The dissolution of these was carried out in a passage oven at a speed of 20 m / min, the temperature holding time of 560 ° C. being of the order of 1 minute and the rate of rise in temperature of the order of 25 ° C / sec.

The mechanical characteristics measured in the rolling direction, in the transverse direction and in the 45 ° direction of the rolling direction are collated in the following table:

These measurements show that the product obtained is relatively homogeneous and isotropic.

The anisotropy was estimated by making buckets and measuring the rate of the horns according to the standard AFNOR NF-A-50-301. This value is equal to 7%. The grain size measured by metallography is 40 µm.

Sheets cut from the dissolved metal were finished by shaping parts automotive body, in this case a front cover.

After stamping, it was coated with a protective coating (paint) before undergoing a cooking for 1.5 h at 180 ° C.

The mechanical characteristics obtained as a function of the local hardening rate are as follows: Work hardening rate (%) Rp0.2 (MPa) Rm (MPa) AT % 0 225 285 15 5 250 290 10 10 265 295 8

Example 2

Electrode en "Mallory 328" de forme tronconique avec angle au sommet de 60° et diamètre de pastille Ø 5,5 mm.

Force d'appui : 400 kg

Intensité : 27 000 A

Fréquence : 2 Hz.

A sheet of the same composition as that of Example 1 was welded to another sheet of the same composition by spot welding, under the following conditions:

"Mallory 328" tapered electrode with 60 ° apex angle and Ø 5.5 mm pellet diameter.

Support force: 400 kg

Intensity: 27,000 A

Frequency: 2 Hz.

The assembly was then brought, in an oven, to 165 ° C for 10 h.
The shear strength of the welded joints thus obtained is of the order of 280 MPa.
We can see the good properties obtained after welding and tempering.

The alloy according to the invention has the following advantages:
This alloy is delivered in T4 state to transformers.
In this state, the alloy is ductile and lends itself well to deformation, its maturation at ambient temperature being very low.
The cold-deformed part acquires better resistance characteristics by work hardening, at least locally in the most deformed zones; the softening due to annealing during the welding operation is partially offset by the structural hardening during the final tempering (T6).

To obtain the most ductile state, the metal only undergoes after quenching the finishing operations (such as as dressing, leveling, etc.) strictly necessary.

The alloys according to the invention are mainly used in the fields of automobile bodywork and box.

Claims (9)

1. A weldable aluminium alloy which can be worked in sheet form characterised in that it contains (in percent weight) proportion of Si and Mg which are delimited by the trapezium ABCD whose co-ordinates are as follow: Si Mg A 0.5 0.1 B 0.5 0.2 C 1.3 0.5 D 1.3 0.1 Cu 0.10 - 0.5 Mn 0 - 0.15 Fe 0 - 0.35 others each ≤ 0.05 total others ≤ 0.15 balance Al. and in that it is free from submicron precipitates consisting mainly of silicon.
2. An alloy according to claim 1 characterised in that it contains (in percent by weight) proportons of Si and Mg which are delimited by the trapezium A" B" C" D" whose co-ordinates are as follows: Si Mg A" 0.65 0.18 B" 0.65 0.2 C" 0.95 0.28 D" 0.95 0.2 Cu 0.1 - 0.25 Mn ≤ 0.15.
3. An alloy according to claim 1 characterised in that it contains (in percent by weight) proportions of Si and Mg which are delimited by the trapezium A'B'C'D' represented in Figure 1, with Cu 0.1 - 0.25.
4. An alloy according to one of claims 1 to 3 characterised in that the mean grain size is smaller than 80 µm.
5. An alloy according to one of claims 1 to 4 characterised in that the mean grain size is smaller than 60 µm.
6. A process for producing the products according to one of claims 1 to 5 comprising continuous or semi-continuous casting of blanks, an optional homogenisation operation, a hot transformation operation, an optional cold transformation operation, a solution treatment carried out in the single-phase range, a quenching operation, a cold shaping operation and finally an artificial ageing operation characterised in that the final hot transformation operation takes place at between 270 and 340°C.
7. A process according to claim 6 characterised in that homogenisation or complete solution treatment are carried out at between 540 and 580°C.
8. A process according to claim 7 characterised in that homogenisation or solution treatment are carried out at between 550 and 570°C.
9. A process according to one of claim 7 and 8 characterised in that the solution treatment is preceded by a rise in temperature at a rate of higher than 10°C second.

Images