EP0356356A1 - Method for reducing damage during a superplastic deformation - Google Patents

Abstract

The invention relates to a method making it possible to reduce the damage during a superplastic deformation. This method consists, at a given temperature and in microstructural and rate of deformation conditions resulting in the superplastic behaviour of the alloy in question, in applying successive partial deformations ( epsilon > 0) during a time (t) which are separated by periods of rest ( epsilon = 0) during a time (t'). The value of t and t' are between 0.5 and 10 min and preferably between 1 and 3 min. This method makes it possible to obtain good tensile or fatigue mechanical characteristics without resorting to the use of a superimposed isostatic pressure. <IMAGE>

Description

The invention relates to a method for reducing damage during superplastic deformation of metals or metal alloys.

It is known that superplastic deformation manifests itself for certain metals or alloys by elongations at break in continuous traction, greater than 100% under particular conditions of temperature, microstructure and rate of deformation.

formule dans laquelle
σ est la contrainte appliquée et
ε̇ est la vitesse de déformation rationnelle
soit $\frac{\text{∂ε}}{\text{∂t}}$

, ε étant la déformation rationnelle
on admet que le matériau est superplastique lorsque m ≳ 0,3Superplasticity is generally characterized by the sensitivity parameter to the strain rate

formula in which
σ is the applied stress and
ε̇ is the rational strain rate
is $\frac{\text{∂ε}}{\text{∂t}}$

, ε being the rational deformation
we admit that the material is superplastic when m ≳ 0.3

However, the maximum elongation capacity is limited by the appearance and coalescence of intergranular decohesions, which lead to premature rupture during this deformation; this cavitation is also detrimental to the traction characteristics and especially to the fatigue resistance as this is reported for example in "M.W. MAHONEY and C.W. HAMILTON Superplatic Aluminum evaluation, Final Report AFWAL-TR 81-3051. January 1981".

The known means for avoiding or at least delaying the appearance of this cavitation consists in superimposing on the forming efforts an isostatic pressure (see for example C.C. BAMPTON and R. RAJ, Acta Metallurgica, Vol. 30, 1982, p. 2043). This isostatic pressure must be equivalent to half or a third of the material flow stress and is generally in the vicinity of 3 MPa.

As a result, this method then leads, for its implementation, to robust and complex and therefore costly tools.

Recall finally that the damage of the material is generally evaluated either by variation of density of the material or by micrographic way.

The method according to the invention makes it possible to minimize the damage, without using isostatic pressure.

This method therefore consists, at a given temperature and under the microstructural conditions and of deformation speed causing the superplastic behavior of the alloy considered, in applying successive partial deformations (ε> 0) during a time (t) separated by periods of rest (ε = 0) for a time (t ′). The values of t and t ′ are a function of the nature, the microstructure of the alloy considered, the total deformation undergone, the temperature and the speed of deformation.

These values must therefore be determined for each particular case, but in general the values of t and t ′ are typically between 0.5 and 10 min and preferably between 1 and 3 m in.

• La figure 1 représente des résultats d'essais en sollicitation uniaxiale comparés entre la méthode continue (1) et la méthode avec repos (2).
• La figure 2 représente une coupe axiale d'un embouti circulaire en alliage 7475, selon la nomenclature de l'Aluminium Association.
• La figure 3 représente la variation de la densité (d) en fonction de la déformation rationnelle (ε) pour la méthode classique (continue) avec ε = 2x10⁻⁴s⁻¹ - courbe 1 - et la méthode selon l'invention - courbe 2.
• La figure 4 donne la comparaison des propriétés de traction (charge de rupture Rm, limite élastique Rp0,2 et allongement A% dans le sens long (L) et travers long (TL)) de la tôle initiale déterminées en A (fig. 1) correspondant aux deux méthodes 1 et 2 ci-dessus et à l'alliage non déformé (0).
• Les figures 5 et 6 représente une coupe micrographique dans l'épaisseur du produit selon les méthodes (1) et (2) pour ε = 1,4.

The invention will be better understood with the aid of the following examples illustrated by FIGS. 1 to 6

• FIG. 1 represents the results of tests in uniaxial stress compared between the continuous method (1) and the method with rest (2).
• Figure 2 shows an axial section of a circular stamped alloy 7475, according to the nomenclature of the Aluminum Association.
• FIG. 3 represents the variation of the density (d) as a function of the rational deformation (ε) for the conventional method (continuous) with ε = 2x10⁻⁴s⁻¹ - curve 1 - and the method according to the invention - curve 2 .
• Figure 4 gives the comparison of the tensile properties (breaking load Rm, elastic limit Rp0,2 and elongation A% in the long direction (L) and long traverse (TL)) of the initial sheet determined in A (fig. 1 ) corresponding to the two methods 1 and 2 above and to the non-deformed alloy (0).
• Figures 5 and 6 represents a micrographic section in the thickness of the product according to methods (1) and (2) for ε = 1.4.

The following tests were carried out on a 2 mm thick sheet in alloy 7475 in the superplastic state, the chemical composition of which is as follows (% by weight): Zn Cu mg Mn Cr IF Fe Ti 5.8 1.60 2.14 <0.02 0.22 0.05 0.06 0.05 remains A1, and whose grain size is 13 µm;
This was formed at 516 ° C at the average speed of ε̇ = 3.10⁻⁴ sec⁻¹ with (or without) rest in uniaxial loading and of ε̇ = 2x10⁻⁴s⁻¹ with (or without) rest in biaxial loading ..

EXAMPLE 1

The uniaxial stress tests were carried out continuously - curve 1, fig. 1 or according to the method claimed with the times (t, t ′) indicated in minutes; the damage was measured as a function of the deformation by the relative variation in density (Δd / d) determined by a picnometric method.

We can see that for all the tests (1,1) - curve 2 fig. 1 - and compared to the continuous method, the damage is reduced by a factor of 10 approx. for extensions of the order of 140%; this factor still remains around 3.5 for elongations close to 220%.

EXAMPLE 2

A blank ̸ 330x320 mm taken from the above sheet was formed by stamping in the form of a circular stamping (biaxial stress) whose axial section is given in fig. 2 on the one hand in continuous deformation (1) and on the other hand with deformation cycles (1 min) followed by rest (1 min), etc.

The evolution of the damage according to the local strain: ε = Ln (E / e), with E initial thickness and e final thickness, is given in figure 3.

The mechanical tensile characteristics, averaged over 4 tests, were determined in the bottom of the stamping, in the long and long transverse direction of the initial sheet 100 mm from the center in the T76 state, according to the designation of the Aluminum Association.

The results are reported in table I attached and graphically represented in FIG. 4.

The compared micrographs are given in FIGS. 5 and 6 for ε = 1.4 (A% 300).

The fatigue tests carried out under repeated stresses on long-drawn samples taken from the bottom of the stamps in the states defined above are as follows: State 0̸ R σ max (MPa) Lifespan * (Kilocycles) 1 0.1 200 128.8 2 0.1 200 177.8 * Average of 12 tests. TABLE I MECHANICAL CHARACTERISTICS Rep. Deformation rate Direction of withdrawal RO, 2 (MPa) Rm (MPa) AT (%) max mini difference avg. max mini difference avg. max mini difference avg. 0 Witness L 471 457 14 463 527 512 15 520 16.6 8.1 8.5 12.3 ε = 0 TL 454 445 9 449 503 501 2 502 9.6 7.2 2.4 8.6 1 2 10⁻⁴s⁻¹ L 415 392 23 404 479 439 40 453 2.7 1.4 1.3 1.9 ε = 1.4 TL 401 355 46 384 468 436 32 447 7.4 1.2 6.2 3.2 2 with relaxation ε = 1.4 L 407 335 72 373 four hundred ninety seven 459 38 480 8.9 1.5 7.4 5.6 2.10⁻⁴s⁻¹ TL 451 332 119 400 498 473 25 485 5.9 4.4 1.5 5.1

Claims (4)

1. Method of deformation of a metal or alloy under superplastic conditions with a view to reducing damage, characterized in that at a given temperature, the material undergoes a succession of partial deformations (ε> 0) over time ( t) separated by rest intervals (ε = 0) during time (t ′). 2. Method according to claim 1 characterized in that t and t ′ are between 0.5 and 10 min. 3. Method according to one of claims 1 or 2 characterized in that for the alloy 7475 deformed around 516 ° C with a speed close to ε = 2.10⁴ sec⁻¹, the periods of partial deformation (t) last 1 to 3 min and rest periods (t ′) from 1 to 3 min. 4. Method according to claim 3 characterized in that the periods of deformation and rest are close to 1 min.

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