FORM 2
THE PATENTS ACT, 1970
(39 of 1970)
& The Patent Rules, 2003
COMPLETE SPECIFICATION
1. TITLE OF THE INVENTION:
NICKEL ALLOY
2. APPLICANT:
Name: SNECMA
Nationality: France
Address: 2, boulevard du Général Martial Valin, 75015 Paris, France.
3. PREAMBLE TO THE DESCRIPTION:
The following specification particularly describes the invention and the manner in which
it is to be performed:
2
The subject of the invention is a nickel alloy.
It has been conceived to improve the foundry manufacture of certain
parts of complex shape such as distributor blades used in engine stators in aeronautics.
These parts are constituted of blades fitted into platforms, the assembly of which on an
engine constitutes a ring. Certain of these parts are traditionally constructed from a nickel
alloy known as René 125, but which is particularly liable to produce cracks on moulding,
during the solidification of the molten metal, which can constitute an important cause of
rejects.
An alloy of composition analogous to that of René 125 was thus
searched for in order to conserve for the main part the favourable properties of this alloy,
but which would be much less prone to the formation of cracks.
According to the invention, the alloy proposed here comprises, in a
nickel base, from 9.5% to 9.90 % by weight of cobalt, 8.70% to 9.00% by weight of
chromium, 6.65% to 7.05% by weight of tungsten, 3.67% to 3.87% by weight of tantalum,
1.77 % to 1.97% by weight of molybdenum, 0.10% to 0.12% by weight of carbon; and it is
differs from the conventional composition of René 125 by levels of 4.80 % to 5.00% by
weight of aluminium, 1.48% to 1.52% by weight of hafnium, 2.28% to 2.33% by weight of
titanium, 0.0005% to 0.01% by weight of boron. The alloy does not in principle comprise
other ingredients, except at much lower levels than those mentioned until now, or only
present as impurities. It is thus underlined in particular that zirconium, quite abundant in
René 125, is eliminated entirely or almost entirely from the present alloy (at the most
0.007% by weight), as are phosphorous and sulphur (at the most 0.001% each by weight).
Certain other criteria, corresponding to secondary aspects of the invention, can still
advantageously be respected as will be detailed hereafter.
This alloy resists the hot formation of cracks much better than
René 125; it may thus be employed in processes of foundry manufacturing parts of
complex shapes.
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The table below indicates the respective compositions of the alloy
René 125 (in nominal values) and of the alloy according to the invention, in minimum
values and maximum values. Only significant elements have been reported, others being
in general present as impurities.
TABLE (% by weight)
Element Co Cr W Al Ta Ti
René 125 10.00 9.00 7.00 4.80 3.80 2.50
Invention
(min)
9.50 8.70 6.65 4.80 3.67 2.28
Invention
(max)
9.90 9.00 7.05 5.00 3.87 2.33
Element Mo Hf C Zr B P S
René 125 2.00 1.55 0.09 0.05 0.015 <0.01 <0.075
Invention
(min)
1.77 1.48 0.10 0 0.005 0 0
Invention
(max)
1.97 1.52 0.12 0.007 0.010 0.001 0.001
Several indications on the effects obtained are given below. It appeared
during tests that boron, zirconium, silicon, sulphur, phosphorous, hafnium and titanium
were favourable to the appearance of cracks while hot, and this is why the levels of these
elements are reduced compared to the composition of the starting alloy René 125. From a
quantitative viewpoint, titanium and hafnium saw their levels the most reduced, and
zirconium, phosphorous and sulphur have become elements present only as traces,
whereas zirconium was necessarily present and at a non-negligible level in the starting
alloy. The level of certain other elements, which are not incriminated as favouring the
appearance of cracks, has also been reduced significantly in order to increase the level in
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the nickel base, since it has been noted that cracks were less numerous with a high nickel
level.
The favourable effect of the reduction or the quasi-elimination of these
elements may be explained in this way: the particles that they form accumulate at the
joints of grains of the alloy during solidification. The internal stresses that develop upon
solidification tend to produce fissuring at the grain joints, and all the more readily while
the latter are still molten. The reduction in the level of these elements favours the
solidification of the grain joints at temperatures closer to that of the grains and thus
enhances the cohesion of the alloy.
More detailed indications on the different elements are given hereafter.
Boron and zirconium: it appeared that these elements were those that
most thwarted solidification at the grain joints, and were thus responsible for
solidification at a temperature well below that of the grains themselves in René 125. Their
level is thus considerably reduced or even eliminated in the alloy of the invention.
As regards boron, much lower probabilities of cracks (four times lower
or even less) than in René 125 have nevertheless been observed at the levels proposed in
the invention, so that the total elimination of this element is not recommended.
Phosphorous and sulphur merit the same commentaries, but their
importance is less since their level is already reduced in René 125.
Titanium and hafnium: their effect was the same but less important,
such that their total elimination is not recommended, a slight reduction in the level
sufficing to reduce enormously the probabilities of cracks. In the same way as for boron,
the levels proposed in the invention led to probabilities of cracks at least four times less
than in René 125 (the comparative tests each time related to a single variant element).
Total level of hafnium, titanium and aluminium: the reject rate had a
low levelling off between around 8.73% and 8.77% (total of levels), then increased
sharply, then being multiplied by at least around four. Aluminium does not have in itself
the harmful role of the other elements mentioned above and its level is even increased in
the invention, but this criterion shows that it cannot be in excess. The total level is
advantageously below the latter figure.
5
Nickel: the reject percentage of parts due to cracks dropped from a level
of 59.71% of nickel, and reached a levelling off corresponding to very low reject rates
from around 59.83%. The level is advantageously above the latter figure.
Silicon: it is at the most 0.10% by weight in René 125, and
advantageously present as traces in the invention.
The heat treatment used for the tests as for the manufacture by
moulding of the bladed distributors was a T3R, namely a heating to 1175° Celsius for 30
minutes, then a cooling to 1095° Celsius in 6 to 10 minutes, then a cooling in the oven to
650° Celsius, then a cooling in air, and an annealing at 815° Celsius for 16 hours under
vacuum or protective atmosphere.
6
We claim:
1) Nickel alloy, comprising 9.50% to 9.90% by weight of cobalt, 8.70% to
9.00% by weight of chromium, 6.65% to 7.05% by weight of tungsten, 3.67% to 3.87% by
weight of tantalum, 0.10% to 0.12% by weight of carbon, 1.77% to 1.97% by weight of
molybdenum, characterised in that it comprises 4.80% to 5.00% by weight of aluminium,
1.48% to 1.52% by weight of hafnium, 2.28% to 2.33% by weight of titanium, 0.005% to
0.01% by weight of boron, and less than 0.007% by weight of zirconium, the remainder
being nickel and optionally other elements present as impurities.
2) Nickel alloy according to claim 1, characterised in that it comprises
even less than 0.001% by weight of phosphorous and less than 0.001% by weight of
sulphur.
3) Nickel alloy according to any of claims 1 or 2 characterised in that it
comprises more than 59.83% by weight of nickel.
4) Nickel alloy according to any of claims 1 to 3 characterised in that it
comprises less than 8.77% by total weight of titanium, hafnium and aluminium.
5) Nickel alloy according to any of the preceding claims, characterised in
that it is used to manufacture in a foundry a bladed distributor part of an aeronautics
engine stator.
6) Nickel alloy according to claim 5, characterised in that it is used with a
T3R heat treatment.