Method of raising the recrystallization temperature of aluminium and of its alloys

The invention relates to a method for the deformational transformation of aluminum and its alloys comprising the steps of deforming the aluminum in the solid state and annealing the deformed aluminum at a temperature at which recrystallization occurs. The recrystallization temperature of the aluminum is raised and the grain size is minimized by adding to the aluminim 5 to 1000 ppm uranium prior to the deformation. The method is particularly applicable to the production of aluminum based sheets intended to be subjected to heating at a relatively high temperature, for example the heating which accompanies enamelling or brazing operations, where the heating operation could change the mechanical properties of the sheets.

The invention relates to a method whereby it is possible to raise the 
recrystallisation temperature of aluminium and its alloys and to minimise 
the grain size. 
It is a known fact that in dimensional transformations of a metal in the 
solid state, such as rolling, for example, a phenomenon occurs which is 
termed hammer-hardening, that is to say the crystalline structure of the 
metal is altered: faults, dislocations and cells of hammer-hardening 
appear. 
If this metal is annealed, it develops towards a more stable condition of 
equilibrium which depends upon the temperature and length of the annealing 
process. 
For example, in a first so-called restoration stage, a restructuring of the 
metal takes place which tends to organise linear defects in a polygonised 
wall. Then, in a stage referred to as primary recrystallisation, almost 
perfect grains appear in certain regions and develop until they come in 
contact with one another. Finally, the number of grains diminishes to 
bring about the most stable recrystallised structure which corresponds to 
a minimal surface area of grain joints. 
It is likewise well-known that the addition of certain elements to alloys 
during their processing or even the presence of certain impurities can 
have an effect of slowing down this evolution, that is to say the 
temperature at which primary recrystallisation starts is then higher and 
that for a given temperature the size of the grains formed is smaller. For 
instance, numerous authors have reported the delaying effect of zirconium 
for concentrations of around 2000 ppm when it is precipitated finely into 
the sub-joints at the moment of annealing. The same goes for iron but at 
lower concentrations of around a few hundred ppm. 
The Applicants have found that this slowing-down effect could also be 
obtained by the addition of uranium but entailing the use of far smaller 
quantities of this element than of zirconium and iron since the effect 
appeared when concentrations were as low as 5 ppm. Hence the method which 
is the object of the invention, which makes it possible to raise the 
recrystallisation temperature of aluminium and its alloys and minimise the 
grain size, being characterised in that between 5 and 1000 ppm of uranium 
are added at the moment of processing. 
The slowing-down effect increases with the uranium concentration but 
reaches a maximum of about 200 ppm. 
The existence of a limitation on the efficacy of the retarding influence 
for strong concentrations of uranium seems due to the fact that only the 
uranium which is in solid solution prior to the annealing has any effect. 
This is confirmed by experiments which have shown that to obtain a similar 
effect it required less uranium when the metal is subjected to an 
homogenisation operation following casting, at an elevated temperature 
instead of a simple reheating at a lower temperature. For practical 
purposes, the optimum concentration is around 50 ppm in the first case and 
150 ppm in the second. 
The Applicants have likewise found that in the case of a simple reheating, 
the more iron contained in the metal, the more it was possible to reduce 
the quantity of uranium and still obtain a similar effect. 
Therefore, there is a combined effect of these two elements which makes it 
possible, according to the greater or lesser purity of iron in the metal 
used, to supplement the effect of this element by a small quantity of 
uranium. 
To this retarding effect of the uranium must likewise be added the other 
effect which, if one nevertheless exceeds the recrystallisation 
temperature, is that of minimising the size of the grains.

As it happens, there are three aluminium alloys of type 1085 complying with 
the standards of the Aluminium Association and having the following 
composition: 
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Content of impurities in ppm 
REF 
Si Fe Cu Mn Mg Cr Ni Zn Ti V B Ga 
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A 200 
630 
&lt;20 
&lt;20 
&lt;10 
&lt;10 
180 
90 300 
50 17 80 
B 200 
630 
&lt;20 
&lt;20 
&lt;10 
&lt;10 
210 
90 280 
40 12 80 
C 260 
700 
&lt;20 
&lt;20 
&lt;10 
&lt;10 
170 
80 260 
50 13 90 
__________________________________________________________________________ 
Starting from each of these, a series of seven ingots were prepared and 
given references 1 to 7 for the alloy A, 8 to 14 for alloy B and 14 to 21 
for alloy C, the alloys being such that in each series the uranium 
contents are respectively 0, 20, 50, 100, 200, 500 and 1000 ppm. The 
ingots are then subjected to the following changes: 
ingots 1 to 7 were homogenised for 60 hours at 620.degree. C., then 
quenched in water, cold rolled to a thickness of 0.45 mm, the resultant 
sheet being annealed for 1 hour at 350.degree. C.; 
ingots 8 to 21 were reheated to 465.degree. C. and maintained at this 
temperature for 5 hours, then naturally cooled, cold rolled down to a 
thickness of 0.45 mm, the resultant sheet being annealed for 30 minutes at 
310.degree. C. 
The granular structures observed on the annealed plates obtained from the 
21 ingots are shown in FIGS. 1 to 21 corresponding to the references of 
the ingots. 
They make it possible to show that the following results of crystallisation 
are obtained: 
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Ref. 
Content in 
A B C 
U (ppm) (homogenised) (reheated) (reheated) 
______________________________________ 
0 E.R. grain size 
fr = 95% fr = 80% 
heterogeneous 
20 E.R. grains finer 
fr = 80% fr = 80% 
and more homogeneous 
grains coarse 
50 fr &lt; 10% fr = 50% fr = 40% 
a few grains near 
N.R. grains coarse 
the edge thoroughly 
100 fr = 15% fr = 50% fr = 40% 
coarse lining N.R. grains coarse 
thoroughly 
200 fr = 15% fr &lt; 30% fr = 40% 
fine lining grains coarse 
500 fr = 20% fr &lt; 30% fr = 40% 
very fine lining grains coarse 
1000 fr = 20% fr &lt; 30% fr = 40% 
very fine lining finer grains 
______________________________________ 
E.R.: entirely recrystallised 
N.R.: not recrystallised 
fr: fraction recrystallised. 
From this Table it can be deduced that: 
the effect of the uranium on the rate of recrystallisation is quite 
substantial as from 50 ppm, 
the effect is quite considerable in the case of homogenisation. When the 
metal is only reheated, it requires more uranium to achieve a similar 
effect, 
in the case of the reheated metal, the higher the iron content of the metal 
the more pronounced is the effect of the uranium (comparison of content 
reference C&lt;content ref. B), 
the effect of the uranium shows no further increase beyond 200 ppm. 
Consequently, the addition of uranium at contents comprised between 50 and 
200 ppm has a retarding effect in an alloy of type 1085 and therefore and 
raises the recrystallisation temperature. The optimum concentration 
depends upon the range of transformation of the metal: 
50 ppm approx. if the metal is homogenised 
150 ppm approx. if it is reheated. 
Furthermore, with effect from 200 ppm, the uranium diminishes considerably 
the enlargement of the grain particularly in the case of homogenised 
alloys at high temperature. 
This invention is applied particularly to the production of aluminium based 
sheets intended to be subjected to heating at relatively high temperature 
such as, for example, that which accompanies enamelling or brazing 
operations, without this treatment possibly changing the mechanical 
properties of the said sheets.