EP3250722B1 - Process for obtaining a low silicon aluminium alloy part - Google Patents

Description

The invention relates to the technical sector of the foundry, for the manufacture of aluminum parts, particularly in the field of automotive, aerospace and more generally, all types of industries.

There are many alloys called "low silicon". These alloys have high mechanical properties after T6 heat treatment (Rp _0.2 300 MPa, A% 8%). They are collected in the 6000 series (Al-Mg-Si) of the classification of aluminum alloys. The most known are the 6082, 6061, 6151. Many compositions also exist with contents similar to the standardized alloys, among which may be mentioned for example the documents EP 0 987 344 and US 2010/0288401 A1 .

The alloys mentioned have been developed for obtaining semi-finished products (billets or ingots for forging or rolling) intended to be transformed during hot or cold operations with high deformation rates (> 50%). In addition, the geometries of these semi-finished products are simple (bar, bar or ingot) which makes it possible to solidify these alloys with a minimum of defects by using processes with high solidification rates. These geometries and these processes lead, according to currently controlled techniques, to semi-finished products free from defects among which we may mention: shrinkage, cracks, macro-segregations, macro-precipitation (prevents the formation of too coarse precipitates,> 100 μm).

From this state of the art, the problem posed that the invention proposes to solve is to be able to produce parts responding to standards of quality and safety, and likely to have complex shapes.

To solve this problem, the object of the invention relates to a method of manufacturing a piece of low silicon aluminum alloy, type 6000.

• on coule dans un moule ledit alliage pour obtenir la pièce,
• après la coulée, on démoule la pièce constituant une préforme encore chaude,
• on refroidit ladite préforme que l'on soumet ensuite à une opération apte à la réchauffer à une température comprise entre 470 °C et 550 °C.
• on positionne ladite pièce entre deux coquilles d'une matrice définissant une empreinte de dimensions sensiblement égales, mais inférieures à celle du moule,
• on presse fortement les deux coquilles l'une contre l'autre pour exercer sur la pièce disposée entre lesdites coquilles un effet combiné de pressage et de corroyage superficiel.

More particularly, the invention relates to a process for obtaining a low silicon aluminum alloy part, comprising silicon at a level of between 0.5 and 3%, magnesium at a rate of between 0.65 and 1%, copper at a level between 0.20 and 0.40%, manganese at a rate of between 0.15 and 0.25%, titanium at a level between 0.10 and 0.20% , and strontium at a level between 0 and 120 ppm, according to which:

• casting said alloy to obtain the part,
• after casting, the part constituting a still hot preform is removed from the mold,
• said preform is cooled and subjected to an operation capable of heating it to a temperature of between 470 ° C. and 550 ° C.
• said piece is positioned between two shells of a matrix defining a footprint of substantially equal dimensions, but smaller than that of the mold,
• the two shells are strongly urged against each other to exert on the piece disposed between said shells a combined effect of pressing and surface treatment.

• la mise en oeuvre du procédé ci-dessus dans le domaine automobile ou dans le domaine aéronautique ;
• l'utilisation d'une pièce obtenue par le procédé mentionné ci-dessus, dans le domaine automobile ; et
• l'utilisation de l'alliage dans le procédé mentionné ci-dessus, dans le domaine aéronautique.

The present invention also objects:

• the implementation of the above method in the automotive field or in the aeronautical field;
• the use of a part obtained by the method mentioned above, in the automotive field; and
• the use of the alloy in the process mentioned above, in the aeronautical field.

In one embodiment of the method, after cooling the preform, the latter is heated by being placed in a tunnel oven.

It follows from these characteristics that the foundry operation followed by forging in one step of the preform do not have the same temperature parameters, solidification rate, deformation rate, forging temperature as the processes of the prior state of the preform. the technique.

The claimed alloy responds to these constraints and makes it possible to obtain parts with a satisfactory quality, especially if they come under a safety obligation (ground connection piece = safety parts).

• la géométrie de la préforme, contrairement à des barreaux ou des lingots, comprend dès sa conception les ébauches des zones fonctionnelles de la pièce et peut donc avoir une géométrie complexe comprenant des nervures ou des variations de section conduisant à des masses isolées de métal liquide. Ces masses isolées peuvent être « tolérées » en augmentant le taux de silicium (type AS7G03, alliage standard de fonderie). Une diminution de ce taux rend l'alliage plus sensible lors de la solidification et conduit à des défauts de retassure (porosités) plus nombreux et d'un volume plus important.
• l'intervalle de solidification, qui est défini par la différence entre la température de liquidus et la température eutectique de l'alliage considéré. Pour un alliage, type AS7G03 modifié au strontium, cet intervalle est de 50 °C env. (611 °C - 562 °C). Pour un alliage bas silicium type 6000, il est de l'ordre de 90 °C (655 °C - 562 °C) en retenant la précipitation des Mg₂Si macroscopiques (ou du silicium) comme pseudo palier eutectique. Un intervalle de solidification grand conduit à une zone pâteuse plus étendue à travers la pièce, de sorte qu'il devient plus difficile de diriger le front de solidification pour réduire les défauts comme cela se fait traditionnellement et presque naturellement avec un alliage AS7G03.
• l'AS7G03 a une sensibilité presque nulle à la crique du fait de la grande quantité d'eutectique qui va pouvoir combler les criques qui apparaissent lors du retrait en solidification. Ce n'est pas le cas d'un alliage bas silicium, qui comporte très peu d'eutectique ce qui entraine une forte sensibilité à la crique et demande d'adapter la composition et de maîtriser les gradients thermiques de solidification.

Among these constraints, we note, as examples:

• the geometry of the preform, unlike bars or ingots, comprises from its design the blanks of the functional areas of the part and can therefore have a complex geometry comprising ribs or sectional variations leading to isolated masses of liquid metal. These insulated masses can be "tolerated" by increasing the silicon content (AS7G03 type, standard foundry alloy). A decrease in this rate makes the alloy more sensitive during solidification and leads to more numerous and larger volume shrinkage defects (porosities).
• the solidification range, which is defined by the difference between the liquidus temperature and the eutectic temperature of the alloy considered. For an alloy, type AS7G03 modified with strontium, this interval is 50 ° C approx. (611 ° C - 562 ° C). For a low silicon type 6000 alloy, it is of the order of 90 ° C (655 ° C - 562 ° C) by retaining the precipitation of macroscopic Mg ₂ Si (or silicon) as a eutectic pseudo-plateau. A large solidification range leads to a larger pasty area across the workpiece, so that it becomes more difficult to direct the solidification front to reduce defects as is traditionally and almost naturally with an AS7G03 alloy.
• the AS7G03 has an almost zero sensitivity to the crack because of the large amount of eutectic that will be able to fill the cracks that appear during the solidification shrinkage. This is not the case of a low silicon alloy, which has very little eutectic which causes a high sensitivity to crack and requires to adapt the composition and control the thermal gradients of solidification.

It is also necessary to adjust the chemical composition to obtain the best compromise between the foundry, forging, heat treatment parameters and the desired mechanical characteristics on finished parts. For this purpose we detail below each of the elements of the alloy, their content and the effects that led to retain these values:
The silicon content is between 0.5 and 3%. A silicon content of less than 1% leads to the highest elastic limits and elongations. However, this is the rate at which the alloy is most sensitive to crack and has the lowest flowability. It is therefore necessary to be able to adapt the silicon content according to the geometry of the part. Complex geometries will require a higher rate to reduce this crack sensitivity. The maximum rate of 3% corresponds to a rate beyond which the elongation and the yield point become too low so that it is always interesting to produce with an alloy of this type.

The magnesium level is between 0, 65 and 1%. This rate makes it possible to optimize the density of Mg ₂ Si precipitates in the aluminum matrix. It compensates for the decrease in silicon content while having a minimum of macroscopic Mg ₂ Si precipitates that are damaging and must be dissolved or transformed during heat treatment. If the precipitates are too numerous, or too big, the heat treatment will have a weak effect for their dissolution, the critical size of dissolution having been exceeded.

The copper content is between 0.20 and 0.40%. This rate allows the formation of Al ₂ Cu precipitates in the matrix and the total absence of macroscopic Al ₂ Cu precipitates. The absence of these macroscopic precipitates makes it possible to maintain high forging temperatures and thus to minimize forging efforts (which is carried out in a single step). Indeed, the main precipitates formed in the presence of copper are Al ₂ Cu and AlMgSiCu respectively melting at 490 ° C and 525 ° C, their presence would prevent forging at higher temperatures without risk of burning of the alloy that would make the parts unusable. This degradation is similar to a destruction of the alloy. A higher copper content also increases the crack sensitivity of the alloy, because there remains a eutectic to be solidified at low temperatures (490 ° C or 525 ° C) for which the mechanical stresses (related to the removal of solidification) exercised on the piece are important.

The manganese content is between 0.15 and 0.25%. This rate avoids the formation of AlFeSi precipitates in β-form (very damaging plate) and makes it possible to form AlFeMnSi precipitates in α-form (Chinese writing less damaging). This maximizes the finished part elongation resulting from the Cobapress process. This effect is most often used with larger amounts of manganese and iron, these two elements leading to a hardening of the alloy but also to larger precipitates during solidification. These large precipitates are penalizing for a good elongation. However, the alloy according to the invention is intended, as indicated, the Cobapress process, which is forged in a single step, which does not have the large deformations encountered in forging, rolling or extrusion. These large deformations can break these large precipitates and make them much less damaging while maintaining their hardening effect. In the case of the alloy, according to the invention, the impact of the iron-based precipitates on the mechanical characteristics should be minimized as soon as they are poured. Indeed, their morphology will not be changed, the forge in one step does not deform the room enough to change their morphology. Finally, this manganese content is adapted to the cooling rates obtained during casting in permanent mold, with respect to these speeds, it promotes the formation of AlFeMnSi precipitates α-form.

The titanium content is between 0.10 and 0.20%. This rate is necessary for efficient seed germination and fine grain size which has a significant effect on the mechanical characteristics of these alloys.

The strontium level is between 0 and 120 ppm. This rate is necessary to have a fibrous solidification of the small amounts of eutectic that are formed. This occurs mainly for silicon levels higher than 1.5%.

It has been seen that the composition of this alloy is adapted to lead to a solidification which will maximize the mechanical characteristics despite the low levels of deformation encountered during the Cobapress process.

However, defects of solidification persist, defects of intergranular solidification of localized shrinkage at the grain boundaries with a branched and diffuse morphology which weakens the casting.

The forging operation Cobapress allows to close and rewrite these defects with a control in design of the rate of deformation. The temperature / deformation couple allows a rectification of the defects. The table below shows the mechanical properties on casting and parts, according to the Cobapress process, after T6 heat treatment of the low silicon alloy. We can note the improvement of rupture limit Rm and elongation at break: STATES Rp0.2 [Mpa] Rm [Mpa] AT% [%] AlMgSiCu + T6 foundry 300 315 1.3 cobapress ™ AlMgSiCu + T6 300 340 8 Rp = Elastic limit
Rm = Mechanical resistance
A% = Lengthening

Finally, this composition makes it possible to reduce the complexity of the usual heat treatment for Al-Mg-Si-Cu type alloys. The rate of silicon, solidification rates and grain refinement lead to macroscopic Mg ₂ Si precipitates whose size and morphology facilitate dissolution during heat treatment.

Referring to the figures of the accompanying drawings showing the micrograph of a room, to show the importance of the manganese and copper. The figure 1 shows a foundry microstructure, without manganese, precipitated "in needles", type β, while the figure 2 shows the monostructure with manganese, precipitated "in Chinese writing", type α.

The Figures 3, 4 and 5 show the elimination of Al ₂ Cu copper precipitates.

To the Figures 3 and 4 , the copper content is greater than 0.40%, which leads to the presence of Al ₂ Cu precipitates. figure 4 shows an example where one can observe the AlFeMnSi and Mg ₂ Si precipitation surrounded by precipitates Al ₂ Cu.

The figure 5 shows a copper content of between 0.20% and 0.40%, according to the invention, showing an absence of Al ₂ Cu precipitates,

Claims (3)

1. A method of obtaining a part made of low-silicon aluminum alloy, containing:

   - silicon at a content lying in the range of 0.5% to 3%;

   - magnesium at a content lying in the range of 0.65% to 1%;

   - copper at a content lying in the range of 0.20% to 0.40%;

   - manganese at a content lying in the range of 0.15% to 0.25%;

   - titanium at a content lying in the range of 0.10% to 0.20%; and

   - strontium at a content lying in the range of 0 ppm to 120 ppm;

   the remainder of the alloy consisting of aluminium and possible impurities,
   wherein the method comprises the steps of :

   - casting said alloy in a mold so as to obtain the part;

   - after the casting, demolding the part constituting a preform that is still hot;

   - cooling said preform and then subjecting said preform to an operation adapted for reheating sais preform to a temperature lying in the range of 470°C to 550°C;

   - positioning said part between two shells of a die that defines a cavity of dimensions substantially equal to but less than those of the mold; and

   - strongly pressing the two shells together to exert on the part disposed between said shells a combined effect of pressing and surface kneading.
2. Use of a part obtained by the method according to claim 1, in the automobile sector.
3. Use of the alloy in the method according to claim 1, in the aviation sector.

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