EP0259232A1 - Easily workable and weldable aluminium alloy, and process for its manufacture - Google Patents

Abstract

The invention relates to a weldable boilermaking aluminum alloy containing essentially Si, Mg and Cu and its manufacturing process. This alloy includes (by weight%) Si and Mg contents defined by the trapezoid of coordinates: <IMAGE> Cu: 0.1 - 0.5 Mn: 0 - 0.2 Ti: 0 - 0.1 Fe: 0 - 0.35 Others each: <= 0.05 Total <= 0.15 Rest Al. The manufacturing range includes semi-continuous or continuous casting of blanks, possible homogenization, hot transformation completed in the field 270 ° C-340 ° C, a possible cold transformation, a complete solution, a shaping by stamping, bending, bending, etc ... and an income.

Description

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

The alloys of the 6000 series according to the nomenclature of the Aluminum Association have been developed essentially in the form of profiles, although some of these alloys such as 6061 or 6082, are commonly found in the form of sheets or strips, intended for the 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, not far from the Mg₂Si stoichiometry, 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 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, also has some disadvantages.

In fact, excessively large silicon precipitates reduce the deformation capacities of the material and, moreover, the corrosion resistance of the alloy under 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 recrystallization inhibiting elements from the group Mn, Cr, Zr.

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 effects similar to those of Mn.

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

Cu 0,1 - 0,5
Mn 0 - 0,2
Ti 0 - 0,1
Fe 0 - 0,35
autres chacun ≦ 0,05
total ≦ 0,15
reste Al.According to the invention, the alloy comprises (in% by weight) contents of Si and Mg defined by the trapezoid of coordinates:

Cu 0.1 - 0.5
Mn 0 - 0.2
Ti 0 - 0.1
Fe 0 - 0.35
others each ≦ 0.05
total ≦ 0.15
remains Al.

Below the minimum values of the main elements (Si, Mg, Cu) the desired mechanical characteristics 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 processing appear (embrittlement) and the ability to stamp 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 Mg₂Si during solidification as much as possible. Thus, the fine precipitation Mg₂Si 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 has been admitted up to 0.2% maximum due to possible contamination in this element, due to the recycling of waste. It should be noted that the alloy does not contain intentional additions of Cr and / or Zr.

The Ti associated with B controls, as is known, the fineness of the primary crystallization of the raw casting products (plates, strips, billets, etc.) and allows shorter homogenizations and solutions, in particular in which concerns the processing of flat products (sheets, strips). The effective contents 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).

Cu = 0,10-0,25
Mn = 0-0,15
Ti = 0 - 0,1
Fe 0 - 0,3
autres chacun ≦ 0,05
total ≦ 0,15
rest Al.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:

Cu = 0.10-0.25
Mn = 0-0.15
Ti = 0 - 0.1
Fe 0 - 0.3
others each ≦ 0.05
total ≦ 0.15
rest Al.

The range of manufacturing of the alloys according to the invention generally comprises the 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, in particular a fineness of grain of less than 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 the alloy well, avoiding burning it by fusion of eutectic phases. Homogenization at high 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 during operation. In the case of sheets and strips, these coarse hot recrystallizations 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 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.

Taking into account the voluntary absence of recrystallization inhibiting elements (Mn, Cr, Zr), the temperature rise before dissolving must be rapid (V ≧ 10 ° C / sec) and dissolving preferably carried out either in a 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 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 welding, the parts undergo hardening income, under the usual conditions; the hardening is due to the precipitation of the Mg₂Si 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 field of composition of the elements Si and Mg of the alloy, and FIG. 2 which represents the field of dissolution or homogenization of an alloy according to the invention, on a vertical section of the state diagram Al, Mg, Si at 0.2% Mg.

In Figure 2, we find in (1) the solvus curve, in (2) the solidus curve and in (3) the eutectic plateau, which are grouped 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 slight 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 mm²) 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 for 10 h at 555 ° C (scalped to 1500 x 420 mm²) then hot rolled to 4 mm thick with finishing between 320 and 300 ° C. The coils thus obtained were cold rolled to 1.25 mm thick.

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

Mechanical characteristics measured in the rolling direction, in the cross direction and in the 45 ° direction of the direction of the rolling are gathered 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 AFNOR NF-A-50-301 standard. 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 of the automobile body, in this case a front cover.

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

The mechanical characteristics obtained as a function of the local hardening rate are as follows:

Example 2

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 apex angle 60 ° and pellet diameter ⌀ 5.5 mm.
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 undergoes after quenching only the finishing operations (such as dressing, leveling, etc.) which are strictly necessary.

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

Claims (9)

1. A cauldronable and weldable aluminum alloy characterized in that it contains (by weight%) Si and Mg contents delimited by the trapezoid ABCD whose coordinates are:

Cu 0.1 -0.5
Mn 0-0.2
Fe 0-0.35
others each ≦ 0.05
Total ≦ 0.15
remains Al. 2. Alloy according to claim 1, characterized in that it contains (in% by weight) contents of Si and Mg delimited by the trapezoid A'B'C'D 'whose coordinates are:

Cu 0.1-0.25
Mn ≦ 0.15. 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. Method for obtaining the products according to one of claims 1 to 3, comprising continuous or semi-continuous casting of blanks, possible homogenization, hot transformation, transforma tion when cold, solution treatment, quenching, shaping by stamping, bending, bending, etc ... and finally an income characterized in that the final hot transformation takes place between 270 and 340 ° C. 5. Method according to claim 4, characterized in that the homogenization or the complete solution are carried out between 540 and 580 ° C. 6. Method according to claim 5 characterized in that the homogenization or the dissolution takes place between 550 and 570 ° C. 7. Method according to one of claims 5 or 6 characterized in that the dissolution is preceded by a rise in temperature at a speed greater than 10 ° C / sec. 8. Use of an alloy according to one of claims 1 to 3 in the field of automobile bodywork. 9. Use of an alloy according to one of claims 1 to 3 in the field of housing.

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