DE69212533T2 - Process for improving the transverse isotropy of a thin product made of an AA 7000 aluminum alloy - Google Patents

Description

The invention relates to a method for improving the isotropy of the mechanical tensile properties in each plane containing the transverse short direction of thick products made of Al alloy with microstructure hardening belonging to the 7000 series according to the designation of the Aluminum Association.

It is known that the thick aluminum alloy wrought products of this type have a zone of weak ductility associated with weak mechanical tensile strengths in each plane containing the transverse short (TC) direction in a zone located at about 40-60º from this transverse short direction.

This generally leads to a lack of durability under mechanical stresses that occur in this zone either during the manufacture or during the use of such materials.

Thermomechanical treatments known as ITMT, which involve a sequence of deformations and heat treatments, are also known from the paper by JC WILLIAMS and EA STARKE entitled "The role of Thermomechanical Processing in Tailoring the Properties of Aluminum and Titanum Alloys", published in the proceedings of the ASM Materials Science Seminar "Deformation Processing and Structure" in Saint-Louis, Missouri, USA, in October 1982. These ITMT treatments lead to a fine equiaxial structure with complete recrystallization, which results in a reduction in the anisotropy of the treated product. These complex treatments are not used because of their high costs, with the exception of the production of superplastic sheets. not used industrially.

The problem of anisotropy in the transverse short extensibility is solved by applying the manufacturing process according to the invention as defined in claim 1; claims 2-5 relate to preferred embodiments of this process.

According to the invention

(a) such a product is cast in the form of blocks (plates or ingots) of an alloy of the 7000 series as designated by the Aluminium Association, containing not more than 90 % of the maximum contents of Fe and/or Si and preferably not more than 75 % of these.

At least one of the following elements is tolerated up to the indicated contents: Zr ≤ 0.15, Mn ≤ 0.45, Cr ≤ 0.23, Hf ≤ 0.15 Ti ≤ 0.10, whereby the total content is less than 0.90%.

Zr is preferred for thick products, such as medium and thick sheets and forged or stamped products with a thickness of ≥ 20 mm.

Cr + Mn is preferred for thin products (< 15 mm).

b) The product is homogenized in such a way as to dissolve the cast eutectics and to obtain sufficiently fine intermetallic compounds; preferably Zr phases (Al3Zr in spherical form) with a diameter of ≤ 30 nm and Al18Cr2Mg3 and/or Al20Cu2Mn phases in the form of platelets or rods with a maximum dimension of less than 500 nm.

c) This homogenised product is shaped into a wrought product, such as sheets, profiles, etc., under such conditions that the final product, after solution annealing, has a degree of recrystallisation of more than 15% and preferably more than 25% and less than 35%, measured by scanning a microsection in a zone located in the middle third of the product.

d) The hot-formed product is then heat-treated by solution annealing, quenching, possibly controlled plastic deformation and artificial ageing, preferably over-tempering, to obtain the final product

It is known that in order to obtain the indicated degrees of recrystallization, the end of the hot deformation must be carried out at a sufficiently low temperature (T ºC), typically below 390 ºC and preferably below 370 ºC, these temperatures depending in particular on the nature and composition of the alloy in question, the thickness of the product, and a sufficiently high degree of deformation is generally > 100% and preferably > 150%. The degree of deformation is defined by S - s/s x 100, where

S is the cross-sectional area in the deformation process when it reaches the temperature T ºC, and

s is the cross-sectional area after deformation.

The invention, which also relates to the products obtained by the process according to the invention, will be better understood with the help of the following examples illustrated by the following figures:

Fig. 1 shows the relationship between ΔA/A (hereinafter defined) in the longitudinal/transverse short (L-TC) plane and the degree of recrystallization in half-thickness for thick sheets of 7010.

Figure 2 shows a comparison of the isotropy (the strain %) in the transverse long/transverse short (TL/TC) plane of thick sheets of 7010 in the T 7651 and T 7451 tempers.

In all the following examples, the anisotropy in the plane under consideration is characterized by the parameter ΔA/A, where ΔA is the tensile strain difference between the TC direction and the direction corresponding to the strain minimum with respect to the transverse short strain A.

EXAMPLE 1 Influence of the degree of recrystallization (see also Fig. 1)

Using 90 mm thick sheets of 7075 with degrees of recrystallization of 5 and 25%, respectively, measured by half-thickness image scanning, the following results were obtained in the L-TC plane. Degree of recrystallization direction of strain

EXAMPLE 2 Influence of purity

Sheets of 100 mm thickness made of alloys 7075 and 7475 with the following Fe and Si contents:

were obtained according to a same manufacturing and heat treatment range. The results obtained in the L-TC plane are the following: Alloy Direction

EXAMPLE 3 Influence of the starting state (Fig. 2)

Statistical comparison of the arithmetic mean of the deviation ΔA between the states T 74 and T 76 for laboratory treated sheets with a thickness of 80, 105 and 150 mm.

It is found that the T 74 state is more favorable than the T 76 state.

Claims (8)

1. Process for improving the isotropy in any plane containing the transverse short direction of thick products made of Al alloys with hardening of the 7000 series, according to which: a) a composition containing not more than 90% of the maximum contents of Fe and Si according to the Aluminum Association is cast, b) this composition is homogenized, (c) the homogenised product is hot-worked under conditions such that the degree of recrystallisation of the final product is greater than or equal to 15% and less than 35%, and d) the product is subsequently heat treated by solution annealing, quenching, possibly controlled deformation and artificial ageing to obtain the final product. 2. Process according to claim 1, characterized in that the composition contains Fe and Si contents which reach at most 75% of the maximum contents of the Aluminum Association. 3. Process according to one of claims 1 to 2, characterized in that the hot deformation leads to a degree of recrystallization greater than or equal to 25%. 4. Method according to one of claims 1 to 3, characterized in that the alloy is an alloy containing at most 0.15% Zr, 0.45% Mn, 0.23% Cr, 0.15% Hf, 0.10% Ti, where Zr + Mn + Cr + Hf + Ti ≤ 0.90%. 5. Method according to one of claims 1 to 4, characterized in that the artificial aging to a T74 state according to the Aluminum Association. 6. Thick product obtained according to one of claims 1 to 5, characterized in that the relative tensile elongation reduction ΔA/A between the transverse short (TC) direction and the direction corresponding to the minimum elongation in each plane containing a transverse short direction is less than 35%. 7. Thick product according to claim 8, characterized in that in each plane containing a short transverse direction A % > 3. 8. Product according to one of claims 6 or 7, characterized in that the Al₃Zr phases have a diameter of less than or equal to 30 nm and in that the Al₁₈Cr₂Mg₃ and/or Al₂₀Cu₂Mn phases are in the form of platelets or rods whose maximum dimension is less than 500 nm.