SPECIAL PARAMETERS FOR NiTi SHAPE MEMORY ALLOYS WIRE
DRAWING PROCESS
Leonardo K. Kabayama, William M. M. Menezes, Odair D. Rigo, Jorge Otubo
[email protected] – Praça Marechal Eduardo Gomes, 50 – Vila das Acácias –
São José dos Campos – São Paulo – CEP 12228-900 – Tel: 12 3947-590
Instituto Tecnológico de Aeronáutica – ITA
ABSTRACT
The wire drawing process of NiTi shape memory requires a combination of
special parameters to obtain a production of wires with good surface finishing. These
kind of wires are largely used in aerospace, robotics and biomedical applications.
This work presents the development of the special parameters, which includes
reduction schedule, intermediary annealing temperature and surface treatment, in the
wire drawing process of NiTi shape memory alloys.
1 – INTRODUCTION
NiTi shape memory alloy (SMA) have been used in several applications in
aerospace, automobilist and medical areas, because its functional and mechanical
properties are superior that the properties shown by other shape memory alloys(1).
Among these various applications, NiTi SMA has been used in rod or wire form for
thin tubes, springs and non regular forms(2).
In wire form, NiTi alloys with shape memory effect or superelasticity are used in
thermostatic valves actuator spring, endodontic instruments, orthodontics arc wires,
cellular phone antenna and eyeglass frames, as shown in figure 1.
18º CBECiMat - Congresso Brasileiro de Engenharia e Ciência dos Materiais, 24 a 28 de Novembro de 2008, Porto de Galinhas, PE, Brasil.
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Figure 1 – NiTi superelastic wire applications(3).
Wire drawing is the cold plastic forming process that produces rod, wires or
tubes with several geometries, but the application of this metal forming process in
NiTi SMAs requires special processing parameters(4) that involves heat and surface
treatments, and the basic wire drawing parameters like area reduction, process
speed and die geometry.
NiTi alloys can achieve 30 to 50% area reduction(5), but in cold metal forming
this material presents severe work hardening, reducing its formability, principally in
wiredrawing; therefore, heat treatment of annealing is required to improve the
formability of NiTi alloys(2). These inter-annealing presents a lost in area reduction of
drawn wires after the heat treatments due to its shape memory effect.
The mean problem in NiTi SMA wire drawing is the non lubricity of the lubricant
inner wire drawing die. This lubrication failure causes wire galling. Galling is
concerned with removal of material from wire and this removed material welds itself
to the surface of the die(6). This material welded in the die surface may cause
scratches or grooves in wire surface, variations in wire diameter or wire rupture inner
the die. To improve the lubricity or lubricant adhesion on wire surface, a suitable
surface treatment is required with a heat treatment in non controlled atmosphere that
produce a thin oxide layer in wire surface. This oxide layer promote lubricant
adhesion in wire surface improving lubricant carrying to inner of the wire drawing die
and reducing the friction between wire and die surfaces(7).
This works presents the special parameters required in wire drawing process of
NiTi shape memory alloys. This special parameter is necessary to produce thin wires
with better surface quality.
2 – MATERIALS AND METHODS
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A vacuum induction melting (VIM) process is used to produce the NiTi alloy with
the chemical composition showed in table 1, which was used in this work.
Table 1 – Chemical composition of the analyzed NiTi alloy.
Ni (%wt)
Ni (%at)
Ti(%at)
C (%wt)
O (%wt)
55,21
49,94
50,06
0,110
0,0390
These VIM NiTi alloy ingot was primary hot rolled and hot and cold swaged to
adequate its dimension to a proper dimension used in wire drawing. The ingot
dimension was reduced to a rod with 2,22mm in diameter and from this diameter a
wire drawing process was realized in a single pass wire drawing machine shown in
figure 2.
Figure 2 – Single pass wire drawing machine.
In the wire drawing process a set (figure 3) of tungsten carbide wire drawing
dies was used. The die geometries referred to 15% area reduction in each pass are
shown in table 3. As lubricant to prevent contact of inner die surface and wire surface
a MoS2 grease was used, in a system that simulate a pressure die.
18º CBECiMat - Congresso Brasileiro de Engenharia e Ciência dos Materiais, 24 a 28 de Novembro de 2008, Porto de Galinhas, PE, Brasil.
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Figure 3 – Tungsten carbide wire drawing die set.
Table 3 – Wire drawing die geometries.
Diameter
Reduction
(D)
Angle (2α)
[mm]
[°]
Bearing Length (Lp)
Acumulated Area
Reductions [%]
Lp/D [%]
Lp [mm]
01
2,1742
0,6523
2,33
02
2,0045
0,6014
16,98
03
1,8480
0,5544
29,44
04
1,7038
0,5111
40,02
05
1,5708
0,4712
49,02
06
1,4482
0,4345
56,66
07
1,3352
0,4006
63,16
08
1,2310
0,3693
68,69
09
1,1349
0,3405
73,38
10
1,0463
0,3139
77,38
11
0,9647
0,2894
80,77
15
30
A successful NiTi SMA wire drawing requires special process parameters that
will be discussed in this work.
First is necessary to ensure that the NiTi rod which will be processed has a
surface free of defects like surface cracks or kneaded chippings caused by prior
rolling and swaging processes as shown in figure 4.
18º CBECiMat - Congresso Brasileiro de Engenharia e Ciência dos Materiais, 24 a 28 de Novembro de 2008, Porto de Galinhas, PE, Brasil.
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Figure 4 – Surface defects.
A penetrating liquid analysis (figure 5) was done to identify these surface
defects and to eliminate defects a hard pickling process (figure 6) with an acid
solution with 50% HF and 50% HNO3 was used to remove defects from rod surface.
Figure 5 – Penetrating liquid analysis.
Figure 6 – Acid pickling process.
After eliminating surface defects a annealing is recommended to improve the
material formability and to adequate the rod surface preventing material galling
(figure 7) in die surface and wire scratching. These conditions are obtained by heat
18º CBECiMat - Congresso Brasileiro de Engenharia e Ciência dos Materiais, 24 a 28 de Novembro de 2008, Porto de Galinhas, PE, Brasil.
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treatment on a tubular furnace at 600~650°C during 5 minutes depending on rod
diameter.
Figure 7 – Material galling and wire scratching problems.
Pointing the wire is required to insert the wire into wire drawing die, and a
pickling process is user for it. This pickling process is done with the same acid
solution used for remove surface defects (50% HF and 50% HNO3) and dipping the
wire extremity into the solution until its diameter is reduced to die diameter.
Heat treatments for annealing and for wire straightening process were realized
in a tubular furnace, shown in figure 8.
Figure 8 – Tubular furnace
The final wire must to be straightened to be used in some applications, and the
straightening process is done in a tubular furnace at a temperature range of 450 to
500°C under a tension stress of 35 to 100MPa(5) applied on the wire as shown in
figure 9.
18º CBECiMat - Congresso Brasileiro de Engenharia e Ciência dos Materiais, 24 a 28 de Novembro de 2008, Porto de Galinhas, PE, Brasil.
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Figure 9 – Straightening process.
3 – RESULTS AND DISCUSSIONS
The special parameters proposed in this work in the method section produced
good results in production of wires with surface finishing better than a prior process
developed to NiTi SMA wire drawing. In the first process the obtained wire was
shown in figure 4 which presents surface defects, and in the actual process the final
drawn wire is shown in figure 10.
Figure 10 – Final wire obtained in this work with magnification of 50x.
With surface oxidation proposed, the galling problem shown in figure 7 was
eliminated. This surface treatment was done with inter-annealing in tubular furnace at
600~650°C during 5 minutes, contradicting the literature that presents a heat
treatment at 550°C during 10 minutes(2), because a long process duration produce
excessive surface oxidation.
18º CBECiMat - Congresso Brasileiro de Engenharia e Ciência dos Materiais, 24 a 28 de Novembro de 2008, Porto de Galinhas, PE, Brasil.
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Wire drawing speed must be controlled to prevent wire vibrations that may
cause lubrication failures and surface damages. Wire vibrations may be reduced
using a mechanical system to tension the wire in die entrance.
Finally, in literature the straightening process was made in furnace ata
temperature range of 450 to 500°C with 30~100MPa tension stress(5), but in this work
the better result is obtained with temperature of 450°C during 10 minutes with the
same tension. The obtained straightened wire is shown in figure 11.
Figure 11 – Final drawn and straightened wire.
4 – CONCLUSIONS
The wire drawing process applied to NiTi shape memory alloys is possible using
the following parameters:
- Surface defect removing and pointing with acid pickling with 50% HF and 50%
HNO3;
- Annealing with surface oxidation treatment on non controlled atmosphere in a
tubular furnace at 600~650°C during 5 minutes, depending on wire
diameter;
- Lubrication with MoS2 grease in a system that simulate a pressure die;
- Straightening process in tubular furnace at 450°C during 10 minutes with
125MPa tension.
Those special parameters presented above were effective in obtaining NiTi
SMA wires with better surface finish.
18º CBECiMat - Congresso Brasileiro de Engenharia e Ciência dos Materiais, 24 a 28 de Novembro de 2008, Porto de Galinhas, PE, Brasil.
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5 – ACKNOWLEDGEMENTS
We sincerely thank to FAPESP (grant #06/60849-6) for financial support. We
also thank many researchers at ITA for providing valuable comments.
6 – REFERENCES
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