RS57888B1 - Process for obtaining a low silicon aluminium alloy part - Google Patents

Description

Description of the invention

[0001] The present invention relates to the technical field of casting for the production of aluminum parts, especially in the automotive industry, aeronautics and generally speaking, in all types of industry.

[0002] There are many alloys known as "low silicon" alloys. These alloys have high mechanical properties, after heat treatment T6 (Rp0.2 300 Mpa ; A% 8%). They are grouped into 6000 series (Al-Mg-Si) classification of aluminum alloys. The most famous are 6082, 6061, 6151. There are also many compositions with similar content for standard alloys, among which we can mention, for example, documents EP 0987344 and US 2010/0288401 A1.

[0003] The alloys mentioned above were developed for the production of semi-finished products (ingots or bars for forging or rolling) in order to process them in hot or cold processes with a high degree of deformation (> 50%). In addition, the geometries of these semi-finished products are simple (strips, tapes or rods), which allows these alloys to be solidified with a minimum of irregularities using high-hardening processes. These geometries and procedures lead, according to the techniques they have mastered, to semi-finished products, free from defects such as: contraction, crack, macro-segregation (separation), macro-precipitation (preventing the occurrence of too coarse deposition, > 100 mm).

[0004] Based on the technique of the present invention, the problem presented by the present invention is the possibility of producing parts that meet a high quality and safety standard and the probability of complex shapes.

[0005] To solve this problem, the subject of the present invention relates to a process for producing a part of a low-silicon aluminum alloy, type 6000.

[0006] More precisely, the present invention relates to a process for obtaining a part made of an aluminum alloy with a low degree of silicon, which contains silicon in a ratio between 0.5 and 3%, magnesium in a ratio between 0.65 and 1%, copper in a ratio between 0.20 and 0.40%, manganese in a ratio between 0.15 and 0.25%, titanium in a ratio between 0.10 and 0.20% and strontium at a rate of 0 to 120 ppm, according to which:

- said alloy is placed in a mold to obtain a part,

- after casting, the formed part, which is still a hot preform-preform, is separated,

- said preform is cooled and subjected to a heating operation capable of heating the preform to a temperature between 470 ̊C and 550 ̊C.

- said part is positioned between two die shells to define an impression of the same size as one die shell, equal to but smaller than the mold,

- the two shells are pressed strongly towards each other in order to exert pressure on the part located between the mentioned shells, to achieve the combined pressure of the forged surface.

[0007] Also, the purpose of the present invention is also for

- implementation of the above-mentioned procedures in the automotive industry or the field of aeronautics;

- the use of the part obtained by the above-mentioned procedures in the field of the automotive industry i

- the use of alloys from the above procedures in the field of aeronautics.

[0008] In the implementation procedures, after cooling the preform, the form is heated by placing it in tunnel furnaces.

[0009] From these characteristics it follows that the casting operation is followed by the forging of the preform in one step and will not have the same parameters of temperature, deposition rate, deformation rate, forge temperature as previous processes recorded in the profession.

[0010] The alloy defined here fulfills these requirements and allows obtaining parts of satisfactory quality, especially if they are part of the safety obligation (earthing part = safety part).

[0011] Among these limitations are, for example, the following:

- preform geometry, unlike rods and bars, includes preforms from the functional phase of part design and can therefore have complex geometries including ribbed or transverse variations, leading to isolation of the mass of liquid metal. These isolated masses can be "tolerated" by increasing the silicon content (type AS7G03, standard casting alloy). Decreasing this rate allows for the sensitivity of the alloy during solidification and leads to multiple defect contractions (porosity) and a larger volume.

- The solidification interval, which is defined by the difference between the liquid (liquid) temperature and the eutectic temperature (freezing or melting temperature of the eutectic) for the alloys under consideration. For an alloy, type AS7G03 modified with strontium, this interval is approximately 50 ̊C (611 ̊C - 562 ̊C). For the low-silicon type 6000 alloy, this is about 90 ̊C (655 ̊C - 562 ̊C) which results from the retention of macroscopic Mg2Si (or silicon) deposits as a pseudo eutectic deposit. A large curing interval leads to an increase in the area of the passage space, which makes it difficult to direct the face of the piece. Defect reduction hardening is both traditional and almost natural done using AS7G03 alloy.

-AS7G03 has almost insensitivity to cracking due to the large eutectic volume that will be able to fill cracks that appear during contraction (shrinkage) in the hardening process. This is not the case with alloys with a low percentage of silicon, which have very small eutectic volumes that lead to high susceptibility to cracks and require adjustment of the composition and control of the thermal gradient (slope) during solidification.

[0012] It is also necessary to adjust the chemical composition to obtain the best compromise between casting, forging and heat treatment and the desired mechanical characteristics of the finished parts. To this end, each of the alloying elements, their content and the effects that lead to their retention are detailed below, values:

The silicone content is between 0.5 and 3%. Silicone content of less than 1% leads to yield strength (tensile strength) and maximum elongation. However, this is the percentage at which the alloy is most susceptible to cracking and has the lowest flowability. Therefore, it is necessary to modify the silicone contents according to the geometry of the part. Complex geometries will require a higher percentage to reduce cracking sensitivity. The highest percentage of 3% corresponds to the rate above which the elongation and yield strength (tensile strength) become too low to be always interesting for the production of an alloy of this type.

[0013] The magnesium content is between 0.65 and 1%. This rate optimizes the Mg2Si deposit density in the aluminum matrix. This compensates for the reduction in silicon content while having a minimum of macroscopic Mg2Si deposits, which are harmful and must be dissolved or transformed during heat treatment. If the deposits are numerous or too large, the heat treatment will have only a small effect on their dissolution (decomposition), as the size of the critical decomposition is exceeded.

[0014] The copper content is between 0.20 and 0.40%. This rate allows the formation of Al2Cu precipitates in the matrix and the complete absence of macroscopic Al2Cu precipitates. The absence of these macroscopic deposits enables the maintenance of a high forging temperature and thus reduces the forging strength (which takes place in a one-step operation). Indeed, the main precipitates formed in the presence of copper are Al2Cu and AlMgSiCu, with melting temperatures of 490 ̊C and 525 ̊C respectively, their presence would prevent forging at higher temperatures without the risk of burning the alloy rendering it unusable. This degradation (destruction) is similar to the destruction of the alloy. A higher copper rate also increases the risk of the alloy being susceptible to cracking, as it still remains eutectic to solidify at high temperatures. At low temperatures (490 ̊C or 525 ̊C) the mechanical efforts exerted on the part (related to the collection of deposits) are important.

[0015] Manganese content is between 0.15 and 0.25%. This rate prevents the formation of AlFeSi precipitates under the β pattern (highly damaging plaque) and allows the formation of AlFeMnSi precipitates in the α pattern (Chinese writing of minor damage). This maximizes the elongation of the finished part resulting from the Cobapress process. This effect is much more common in use with higher amounts of manganese or iron, both of which lead to a strong strengthening of the alloy but also to greater precipitation during hardening. These large deposits are detrimental to good elongation. However, the alloy according to the present invention is intended, as indicated, for Cobapress processes, according to which the forging is in one step, which does not lead to large deformations in the encounter with forging, rolling or pushing. These large deformations allow large deposits in the piece and make it less harmful while simultaneously preserving the hardening effect. In the case of alloys, depending on the subject invention, the influence of iron-based deposits on the mechanical properties should be minimized during casting itself. Their morphology will not be further modified, since forging in one step does not deform the part sufficiently to change its morphology (shape). Finally, this manganese content is adjusted for the cooling rates obtained during permanent mold casting, where relative to the rates it leads to the formation of AlFeMnSi also in the α pattern.

[0016] The titanium content is between 0.10 and 0.20%. This rate is necessary for effective grain growth and fineness of granulation, which has a significant effect on the mechanical properties of these alloys.

[0017] The content of strontium is in the range from 0 to 120 ppm. This rate is necessary for fibrous solidification from the small amount of eutectic that is formed. This occurs mainly in the case of a silicon rate higher than 1.5%.

[0018] We have seen that the composition of these alloys is adapted to lead to solidification that allows the increase of their mechanical characteristics despite the low level of deformation encountered during Cobapress processes.

[0019] However, there are solidification defects, intergranular solidification defects, localized shrinkages, at the granule joints, with branched and diffuse morphology that weaken the cast part.

[0020] The operation of "Cobapress" forging allows to close and weld these defects (defects do not have to be flaws, often a defect in the material is very useful) with a superior design for the degree of deformation. The temperature/strain combination allows the defects to be welded together. The table below has the mechanical characteristics of cast parts and parts, according to the Kobapress process, after heat treatment of T6 low-silicon alloy. Observed improvements in fracture strength Rm and fracture elongation:

Condition Rp0.2 Rm [Mpa] A% [%]

[Mpa]

Foundry 300 315 1.3

Cobapress tm 300 340 8

Rp= Elastic limits

Rm= Mechanical resistance

A%= Elongation

[0021] Finally, this composition reduces the complexity of the usual heat treatment for the Al-Mg-Si-Cu type alloy. Silicon content, solidification rates, and refinement glanuralization fineness lead to the deposition of macroscopic Mg2Si whose size and morphology (shape) facilitate dissolution during heat treatment.

[0022] The picture references in the attached drawings are micrographs of the part, and to show the importance of copper and manganese levels. Figure 1 shows the microstructure of the casting free of manganese, deposition "in needles", type β, while Figure 2 shows the monostructure with manganese, deposition "in porcelain writing", type α.

[0023] Figures 3, 4 and 5 show the elimination of Al2Cu copper deposits.

[0024] In Figures 3 and 4, the copper content is greater than 0.40%, which leads to the presence of Al2Cu deposits. Figure 4 shows an example where AlFeMnSi and Mg2Si precipitates can be seen surrounded by an Al2Cu precipitate.

[0025] Figure 5 shows a copper content between 0.20% and 0.40%, depending on the subject invention, showing the absence of Al 2 Cu deposits.

Claims (3)

Patent claims 1. A method of obtaining a part made of low-silicon aluminum alloy, comprising: - silicon content ranging from 0.5% to 3%; -magnesium content ranging from 0.65% to 1%; -copper content ranging from 0.20% to 0.40%; -manganese content ranging from 0.15% to 0.25%; -titanium content ranging from 0.10% to 0.20%; and -strontium content ranging from 0 ppm to 120 ppm; The rest of the alloy containing aluminum and possible impurities, wherein the method comprises the steps: - casting the mentioned alloy in a mold to obtain a part; - after casting, decapping the part that constitutes the preform while it is still hot; - cooling the said preform and then subjecting the said preform to an operation adjusted for reheating at a temperature ranging from 470 ̊C to 550 ̊C; -positioning of the said part between two shells of the casting that defines a cavity with dimensions essentially the same but smaller than that of the mold; and -strong pressing of the two shells to act on the part that is placed between the mentioned shells, with a combined effect of pressing and crushing the surfaces. 2. Use of the part obtained by the method according to patent claim 1, in the automotive industry sector. 3. Use of the alloy from the method according to patent claim 1, in the aviation sector.