USINAGE D'UN METAL A MEMOIRE

MACHINING OF A MEMORY METAL

Abrégé français

On fabrique par extrusion un tube en métal à mémoire à base de NiTi, ce qui consiste à: a) commencer à partir d'une ébauche de métal à mémoire; b) pratiquer un perçage central dans ladite ébauche; c) introduire un noyau dans le perçage central, l'ébauche étant encastrée dans une enveloppe et entourée également, de préférence, par deux pignons, ce qui l'isole de l'atmosphère ambiante; d) chauffer et extruder l'ébauche encastrée; e) supprimer les extrémités de l'ébauche extrudée; f) supprimer également le noyau et l'enveloppe. Ceci permet de fabriquer des tubes en métal à mémoire à base de NiTi d'une longueur inégalée jusqu'à présent, telle que >= 1/2 m.

Abrégé anglais

A NiTi-based tube of memory metal is produced by extrusion, whereby: a) one
starts off from a blank of a memory metal; b) a central hole is drilled in
said blank; c) a core is introduced into the central hole, whereafter the
blank is encased by a casing, and preferably also by two gables, whereby the
blank is excluded from the surrounding atmosphere; d) the encased blank is
heated and extruded; e) the ends of the extruded blank are removed; and f)
also the core and the surrounding casing is removed. In this way, it has been
accomplished to produce NiTi-based tubes of memory metal of hitherto
unattained length, such as >= 1/2 m.

Revendications

Claims
1. Process for the production of tubes of memory metal, characterized in
that
a) one starts off from a blank of a memory metal,
b) a central hole is drilled in said blank,
c) a core is introduced into the central hole, whereafter the blank is encased
by a casing, and preferably also by two gables, whereby the blank is excluded from the
surrounding atmosphere,
d) the encased blank is heated and extruded,
e) the ends of the extruded blank are removed, and
f) also the core and the surrounding casing is removed.
2. Process according to claim 1, characterized in that the starting blank is
cylindrical.
3. Process according to claim 1 or 2, characterized in that the memory
metal is NiTi-based.
4. Process according to claim 3, characterized in that the memory metal
comprises 35 to 60 % by weight of nickel, 35 to 60 % by weight of titanium and 1 to 30
% by weight of niobium.
5. Tube-shaped product consisting mainly of NiTi-shaped memory metal,
characterized in that it is produced by extrusion.
6. Tube-shaped product according to claim 5, characterized in that it
contains 35 to 60 % by weight of nickel, 35 to 60 % by weight of titanium and 1 to 30 %
by weight of niobium.

7. Tube-shaped product according to claim 6, characterized in that the
average length of the niobium particles is ~ 15 µm.
8. Tube-shaped product according to claim 6 or 7, characterized in that its
length exceeds 1/2 m.

Description

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WO 96/17698 = PCT/SE95/01463
' Machinin~ of a Memor~T Metal
t
The present invention relates to a process for the extrusion of tubes of a
5 memory metal, in particular a memory metal of the type NiTi and a tube produced
according to this process.
So called memory metals relate to a group of materials which are
characterized by their deviating thermo-mechanical properties. To these belong inter alia
NiTi-based alloys and copper alloys of so called ~-brass. The compositions~ properties
10 and applications of the memory metals are known by a number of publications in this
area (see, e.g.~ Walter S. Owen: Shape memory effects and applications. an overviev~.
Shape memory effects in alloys, edited by Jeff Perkins, 1975, Plenum Press New ~ork;
Process of Int. Symposium on shape memory effects and applications, Toronto~ Canada,
19. - 22.5.1975). The memory metals are characterized in that their phase transformation
15 (transition from a martensitic to an austenitic state and vice versa) takes place within a
very limited temperature range in the order of magnitude 30~C.
The reason for extrusion of memory metal being desirable is that these
materials are difficult to machine by cutting tools. because of rapid wear of the tool and
slow production. Thus~ it would be very advantageous to be able to produce these tubes
20 by e~trusion instead of cutting m~rhinin~, e.g. when producing couplings and bit rings.
However. a heat treatment~ such as an extrusion~ of a memory metal of NiTi-type is very
difficult. since this material has a high affinity for oxygen at hi~h temperatures. which
leads to a strong oxidation and wasted material.
Thus, a primary object of the present invention is to provide a process that
25 makes possible an extrusion of memory metal in general and of NiTi-based memorv
metal in particular.
A second object ofthe present invention is to provide an extruded tube of
memorv metal in ~eneral and of NiTi-based memory metals in particular.
For illustrative but non-limiting purposes~ the invention will now be
30 described in more detail with reference to the appended figure which schematically
shows the production of a tube accordin~ to the present invention.

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The used memory metal according to the exemplified embodiment consists
of a NiTi-based alloy with an addition of niobium. The amounts can suitably vary from
35 to 60 % by weight Ni, 35 to 60 % b.w. Ti and 1 to 30 % b.w. Nb, preferably from 40
to 60 % b.w. of Ni and Ti, respectively and 1 to 20 % b.w. of Nb. In the concrete
example, the concentrations of both Ni and Ti were 45 ~- 2 % b.w. and Nb was 10 + 4 %
b.w., plus naturally occurring hll~uliLies.
In accordance with the invented process, one starts off from a solid bar 1 of
memory metal, see step a). Through and concentrically with this bar. a hole 2 is drilled in
a conventional manner, which hole thus should be centered around the central axis of the
bar, in accordance with step b). If the bar 1 is not perfectly round or is uneven and/or
rough on the outer surface, then a turning operation should be effected in order to attain
sufficiently good surface smoothness and cylindricity. This is suitable in order to later
attain a good fit in the cap S; cf. lln~lerne~th In this case, the turning should be perforrned
after the central drilling.
After the central drilling, possible turning and degreasing, e.g. by a suitable
alcaline solution~ the pre-prepared blank 3 is capped in, primarily for the exclusion of the
oxygen of the air; see step c). However~ before this encasing takes place~ a core 4 of
suitable fit and of the same length as the blank 3 is suitably introduced into the hole 2 .
Preferably~ there is a gap between the blank and the core for the introduction of a suitable
release agent~ such as talcum or a talcum-cont~ining mixture. Such a core 4 is used
instead of an internal mandrel at the extrusion~ since the encasing prevents the use of
such a mandrel. Further~ a sleeve or cap S of equally good fit is threaded over the
envelope surface of the blank 3~ which cap has the same length as the blank 3. After this.
two gable pieces 6 and 7 are welded to and around the end surfaces of the casing 5. The
front gable 7~ as seen in the direction of extrusion~ is suitably rounded off around the
edge. which is beneficial for the extrusion.
The materials used for the the core 4. the casing 5 and the gables 6, 7~
respectively. can of course vary within broad limits~ but at the tests the following alloys
were used. % bv weight being meant:

CA 02206310 1997-0~-28
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Table 1
Steel C Si Mn Cr Ni
Casing+gables SS21720,20 0,30 1,5 <0,3
Core 15M13~2* 0.75 - 13 -
* De.~ign~tion of Sandvik; also called Hadfield steel
In order to become extrudable, the ready blank or block 8 consisting of the
blank 3, the core 4, the casing 5 and the gables 6, 7, were heated to about 1 040~C and
lubricated with glass on the outside. However, the extrusion temperature may vary. to a
large extent depending upon the memory metal alloy, and may suitably lie between 900
and 1150~C, preferably between 1000 and 1050~C.
After this, the thus pre-prepared extrusion block was extruded according to
step d). The dimension of the ready tube may vary within wide limits~ depending upon
the dimensions of the blank, the casing, the core and to which dimension one extrudes.
For instance. if a thick-walled casing is used, then a ready-extruded tube of a smaller
diameter is obtained. Further. a thicker core gives a more thin-walled tube. If one
extrudes to a small diameter, of course a lon~er tube is obtained than if one extrudes to a
large diameter.
After extrusion, the ends are cut according to step e). Generally it mav be
said that a typical extension factor before and after extrusion (i.ealso after cutting) is
from 7 to 18 times, preferably from 10 to l S times.
After cutting. cooling and straightening (e.g.! by press or roller
straightening). the casing 5 is removed. which can be effected in different ways. Usually.
the surface of the tube 3' has to be turned clean to become sufficiently smooth. which
involves that the casing 5 can be turned away in the sarne working operation. The casing
can also be picL;led awav in a suitable pickling bath.

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The core 4' may also be removed in different ways. When producing long
tubes, for in~t~nce up to 10 m, the core is removed by extending it by between 10 and 30
%, preferably by about 20 %. Then the diameter of the core llimini.~hes uniformly along
its whole length and may then easily be removed, whereby a ready tube 3' remains ? see
5 step f). In order to extend the core, a piece of the tube has to be turned away at each end,
in order to grip the part of the core that protrudes by said tl-ming, in a traction m~chine
At shorter tube pieces the core may be drilled out instead of being extended.
Example
A memory metal accoding to the above composition is prepared to a read~
extrusion block according to steps a-b-c. the block having the following dimensions:
Blank diameter: 71 rnm
Blank diameter including casing: 77 mm
Core (in "Hadfield steel"): 31 mm
Length of blank and casing: 350 mm
The block was extruded at 1040~C at a pressing force of 868 Mpa (8850
kp/cm~) and with a pressing speed of 135 mm/s. Thereby. an extruded block with casing
with a diameter of 19.1 mm was obtained, the ready tube having a diameter of 172 mm.
The wall thickness of the ready tube was about 4,6 mm and its length was about 4500
20 mm.
The dimension ranges that are possible depend of course on the magnitude
of the extrusion press that is used. In the example above, a press with a maximal pressing
force of about 1300 Mpa (13 300 kp/cm2) is used. and a maximal starting diameter of 77
mm. Thus. this means that the blank including the casing shall have a diameter that does
25 not exceed 77 mm. On these conditions. the dimension range of the ready tube is from l ()
to 47 mm. Said minimum diameter implies a tube length of about 20~6 m. while said
maximum diameter results in a tube length of about 0.94 m, all under the above
mentioned test parameters. Shorter and longer blanks than the above mentioned ones are
of course also feasible. for instance down to 200 mm and up to 400 mm. The wall
30 thickness can be ~ aried by optional dimensioning of the core and may suitablv lie
between 2 and 15 rnm.
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If a larger press is used, then the ~limen~ion range can be increased upwards
to larger diameters of the finished tube. Thus, with a suitable press. tube diameters of up
to about 100 mm may be reached.
The extrusion of the thus ~nc~ce~l NiTi-based memory metal showed
S surprisingly good results. Thus, it turned out to be fully feasible to produce tubes from
bar material~ which tubes then may be m~(~.hinP~l further to the desired product. This
involves inter alia that the necessity of the hitherto required cutting m~hiningdimini.~hes dramatically, or even is entirely elimin~te-l, which brings considerable
savings in time and costs. Memory metal of NiTi-type is difficult to machine by cutting?
since it has a large wear resistance. Further. it was observed that the memory metal with
gables, casing and core was verv easy to extrude.
The fact that one needs to drill a hole into the bar I is not a major
disadvantage, having regard to the fact that, according to the working example, the bar
extends only about 30 cm, in comparison with a number of meters after extrusion. In this
context it should be underlined that such long tubes of memory metal (e.g., 2 1/2 m) are
very difficult to produce by cutting machining due to insufficient centering accuracy
when drilling.
Some studies in light microscopy were made of the microstructure of the
memory metal before and after extrusion. From these it is inter alia clear that a more
homogenous structure was obtained in the extruded tube, with more evenly distributed
niobium particles. This evenerdistribution is surprising per se. The function of the
niobium particles is to increase the temperature at which the memory metal automaticall~
reestablishes its original shape~ since said particles counteract the reestablishment of the
original shape of the metal.
A measurement in a bar before extrusion and in a tube after finished
extrusion gave the following results with reference to niobium particles:
Bar: Average length: 79 ~lm
Average width: 1,5 llm
Tube: Average length: 6~9 ~m
Average width: 1,4 ~m

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These measurements show that the average length of the niobium particles
is stronly :reduced at extrusion. No extruded tube had an average length for niobium
particles of more than 15 ~lm.
A comparative test was also made with an identical blank, which however
had not been encased but had only been provided with a core 4. An entirely inferior
product was obtained, with a number of cracks. This is supposed to depend on the strong
oxidation that is caused by the free access of air.
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