CA2973937A1 - Process for obtaining a low silicon aluminium alloy part - Google Patents

Abstract

The low silicon aluminium alloy part comprises silicon, magnesium, copper, manganese, titanium and strontium. Said part is obtained by a process according to which: - said alloy is cast in a mould in order to obtain the part, - after casting, the part constituting a still hot preform is removed from the mould, - said preform is cooled and is then subjected to an operation capable of reheating it to a temperature between 470°C and 550°C, - said part is positioned between two shells of a die that complete a cavity having dimensions that are substantially equal, but smaller than that of the mould, - the two shells are pressed strongly against one another in order to exert on the part positioned between said shells a combined pressing and surface kneading effect.

Description

Process for obtaining a low silicon aluminum alloy part The invention relates to the technical sector of the foundry, for the manufacture of aluminum parts, particularly in the field of automotive, aeronautics and more generally, all types industries.
There are many alloys called low silicon. These alloys exhibit high mechanical characteristics after heat treatment T6 (Rp0.2 300 MPa, A% 8%). They are gathered in the 6000 series (Al-Mg-Si) of the classification of aluminum alloys. The most famous are the 6082, 6061, 6151. Many compositions also exist with levels similar to standard alloys, among which may be mentioned for example the document EP 0 987 344.
The alloys mentioned have been developed to obtain products semi-finished products (billets or ingots for forging or rolling) intended to be transformed during hot or cold operations with high rates of deformation (> 50%). In addition, the geometries of these semi-finished products are simple (bar, bar or ingot) which solidifies these alloys with a minimum of defects using processes with high solidification rates. These geometries and these processes lead according to currently mastered techniques, to semi-finished products free from defects among which we can mention: sinkholes, creeks, macro-segregation, macro-precipitation (prevents the formation of precipitates too much coarse,> 100 tm).
From this state of the art, the problem posed to solve the invention is to be able to produce parts responding to high standards of quality and safety, and likely to have complex.
To solve this problem, the subject of the invention relates to a process for manufacturing a low silicon aluminum alloy part, type 6000.
More particularly, the invention relates to a process for obtaining of a low silicon aluminum alloy part, comprising silicon to a level between 0.5 and 3%, magnesium at a rate between 0.65 and 1%, copper at a rate between 0.20 and 0.40%, manganese at a rate of between 0.15 and 0.25%, titanium at a rate between 0.10 and 0.20%, and strontium at a rate between 0 and 120 ppm, according to which:
said alloy is cast in a mold to obtain the part, after casting, the part constituting a preform is demolded hot, said preform is cooled and then subjected to an operation able to heat it to a temperature between 470 C and 550 C.
positioning said part between two shells of a matrix defining a footprint of substantially equal dimensions, but less than that of the mold, - the two shells are strongly pressed against each other to exercise on the part arranged between said shells a combined effect of pressing and surface treatment.
The present invention also objects:
the implementation of the above method in the field automotive or in the aeronautical field;

2 - the use of a part obtained by the process mentioned above 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 process, after cooling the preform, the latter is heated by being placed in an oven tunnel.
It follows from these characteristics that the foundry operation followed by the forge in one stage of the preform do not exhibit the same temperature parameters, solidification rate, strain rate, forging temperature than the prior art methods.
The claimed alloy satisfies these constraints and makes it possible to obtain parts with satisfactory quality, especially if these come under a safety obligation (ground connection piece = pieces of security). =
Among these constraints, we note, as examples:
the geometry of the preform, unlike bars or ingots, includes from its conception the sketches of the functional zones of the piece and can therefore have a complex geometry including ribs or sectional variations leading to isolated masses of liquid metal. These isolated masses can be tolerated by increasing the silicon level (AS7G03 type, standard foundry alloy). A
lowering this rate makes the alloy more sensitive during solidification and leads to more extensive sink defects (porosities) and most important.

3 the solidification interval, 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 in the order of 90 C (655 C ¨ 562 C) by retaining the precipitation of Macroscopic Mg2Si (or silicon) as eutectic pseudo-level.
A large solidification interval leads to a more pasty area extended across the room, so it becomes more difficult to direct the solidification front to reduce defects as it is done traditionally and almost naturally with an AS7G03 alloy.
the AS7G03 has almost no sensitivity to the creek due to the large amount of eutectic that will be able to fill the creeks appear during solidification shrinkage. This is not the case for a alloy low silicon, which has very little eutectic which causes a strong sensitivity to the crack and demand to adapt the composition and master the thermal gradients of solidification.
It is also necessary to adjust the chemical composition to to obtain the best compromise between the parameters of foundry, forge, heat treatment and the desired mechanical characteristics on parts finished. For this purpose we detail below each of the elements of the alloy, their content and the effects that led to the retention of these values:
The silicon content is between 0.5 and 3%. Silicon content less than 1%, leads to the highest elastic limits and elongations high. However, this is the rate for which the alloy is the most sensitive at the creek and the lowest flowability. It is therefore necessary to be able to

4 adapt the silicon content according to the geometry of the part. of the complex geometries will require a higher rate to reduce this crack sensitivity. The maximum rate of 3% corresponding to a rate beyond which the elongation and the elastic limit become too weak for that it is always interesting to produce with an alloy of this type.
The magnesium level is between 0, 65 and 1%. This rate allows to optimize the density of precipitates Mg2Si in the matrix aluminum. It compensates for the decrease in silicon content while having a minimum of macroscopic Mg2Si precipitates that are damaging and must be dissolved or transformed during heat treatment. If the precipitates are too numerous, or too big, heat treatment will not that a weak effect for their dissolution, the critical size of dissolution having been outdated.
The copper content is between 0.20 and 0.40%. This rate allows the Al2Cu precipitates in the matrix and the total absence of precipitated Al2Cu macroscopic. The absence of these precipitates macroscopic techniques makes it possible to maintain high forging temperatures and thus minimizing forging efforts (which is performed in one step).
Indeed, the main precipitates formed in the presence of copper are Al2Cu and AlMgSiCu melting at 490 C and 525 C respectively, their presence prevent forging at higher temperatures without risk of burning of the alloy which would render the parts unusable. This degradation is akin to a destruction of the alloy. Higher copper content also increases the sensitivity to the crack of the alloy, because it remains a eutectic to be solidified at low temperatures (490 C or 525 C) for which mechanical stresses (related to the removal of solidification) exercised on the piece are important.

5 The manganese content is between 0.15 and 0.25%. This rate avoids the formation of AlFeSi precipitates in f3 form (very damaging) and allows to form AlFeMnSi precipitates under form a (Chinese writing less damaging). This allows to maximize 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 strong hardening alloy but also to larger precipitates during the solidification. These large precipitates are penalizing for a good elongation.
However; the alloy according to the invention is intended, as indicated, for Cobapress process, in which one forges in one step, which does not not present the large deformations encountered in forging, rolling or extrusion. These large deformations allow to fragment these fat precipitated and make them a lot less damaging while retaining their hardening effect. In the case of the alloy, according to the invention, the impact of iron precipitates should be minimized as soon as they are poured on the mechanical characteristics. Indeed, their morphology will not be modified, the forge in one step does not distort the room sufficiently to change their morphology. Finally, this level of manganese is adapted to cooling rates obtained during permanent mold casting, compared to these speeds, it promotes the formation of AlFeMnSi precipitates subform a.
The titanium content is between 0.10 and 0.20%. This rate is necessary for efficient grain germination and grain size which has a significant effect on the mechanical characteristics of these alloys.

6 The strontium level is between 0 and 120 ppm. This rate is necessary to have fibrous solidification of the small quantities of eutectic that are formed. This occurs mainly for rates silicon greater than 1.5%.
We have seen that the composition of this alloy is adapted to lead to solidification that will maximize the characteristics in spite of the low levels of deformation encountered during Cobapress process.
However, solidification defects persist, defects in intergranular solidification of local gill-localized shrinkage with a branched and diffuse morphology that weakens the casting.
The Cobapress forging operation allows you to close and refold these defects with a control in design of the rate of deformation. The torque temperature / deformation allows a recovery of defects. The table, below, presents the mechanical characteristics on foundry and on parts, according to the Cobapress process, after treatment Thermal T6 of the low silicon alloy. We can note the improvement of rupture limit Rm and elongation at break:
im ... AIMIEMPUMMUMSURSINeeiempeigggfettreeAleeeet - :: AlMeiC.0 Foundry. 300,315 1.3 Cobapresst "AlrvIgSiCtu + T6 ... e 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 alloys. The rate of

7 silicon, solidification rates and grain refinement lead to of the macroscopic precipitates Mg2Si whose size and morphology facilitates the dissolution during heat treatment.
Referring to the figures of the accompanying drawings showing the micrograph of a room, to show the importance of the rate of manganese and copper. Figure 1 shows a foundry microstructure, without manganese, precipitated in needles, type 13, while Figure 2 shows the monostructure with manganese, precipitated in writing Chinese type a.
Figures 3, 4 and 5 show the elimination of copper precipitates Al2Cu.
In FIGS. 3 and 4, the copper content is greater than 0.40%, which leads to the presence of precipitates Al2Cu., Figure 4 shows an example where we can observe the AlFeMnSi and Mg2Si precipitations surrounded by precipitated Al2Cu.
Figure 5 shows a copper content of between 0.20% and 0.40%, according to the invention, showing a lack of precipitates Al2Cu.

8

Claims

-1- Process for obtaining a low silicon aluminum alloy part, comprising:
silicon at a level of between 0.5 and 3%, magnesium at a level of between 0.65 and 1%, copper at a rate of between 0.20 and 0.40%, - manganese at a rate between 0.15 and 0.25%, titanium at a level of between 0.10 and 0.20%, and strontium at a level of between 0 and 120 ppm, according to which:
said alloy is cast in a mold to obtain the part, after casting, the part constituting a preform is demolded hot, said preform is cooled and then subjected to an operation able to heat it to a temperature between 470 ° C and 550 ° C, positioning said part between two shells of a matrix finishing a footprint of substantially equal dimensions, but less than that of the mold, - the two shells are strongly pressed against each other for exert on the piece arranged between said shells an effect combined pressing and surface treatment.
Use of a part obtained by the process according to claim 1, in the automotive field.
Use of the alloy in the process according to claim 1, in which field of aeronautics.