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An Experimental and Numerical Analysis of the Dieless Drawing of Nickel
Titanium Alloy
E. Twohig 1, 2 , P. Tiernan 1, 2 , S.A.M. Tofail 2 , and J. Butler 2
1 Dept. of Design and Manufacturing Technology, University of Limerick, Limerick, Ireland
2 Materials and Surface Science Institute, University of Limerick, Limerick, Ireland
eoghan.twohig@ul.ie
Abstract
The considerable die wear that occurs at the tool-die
interface during production of wire using a
conventional drawing method of a die to reduce its
diameter, coupled with the additional lubrication and
pre-cleaning costs, add considerably to the overall cost
of the process, [1]. A process known as dieless
drawing has the potential to reduce the costs of
production and improve the properties of the finished
product, [2]. An experimental rig for dieless drawing
was previously designed, built and successfully
operated at the University of Limerick [3]. The next
step in the development of this process is to develop an
accurate, reliable finite element (FE) model capturing
the heating/cooling rate, applied force, and strain rate
characteristics of the process as applied to the shape
memory alloy Nickel Titanium. Good agreement has
been shown between experimental and numerical
results.
1. Introduction
The process of dieless drawing is an innovative
method for the production of wire without the use of
reduction dies, which is best suited to material forms
that currently have high production costs and are
difficult to produce using the conventional method.
The shape memory material, Nitinol presents such
difficulties during production due to its high toughness
and work-hardenability [4] . Nitinol is widely used in
wire and tube form in the biomedical industry as
guidewires for deploying stents, angioplastic balloons
and filters, and as self-expanding stents which are
precision laser-cut from thin walled tubing [5, 6] .
2. Experimental Method
An initial test of ten NiTi rods of 5mm diameter
was carried out on the dieless drawing machine.
Successful rod drawing tests were carried out at
temperatures of 800 o C. An initial drawing velocity of
3mm/min and heater/cooler assembly velocity of
12mm/min was applied to the rod, giving a process
ratio of 0.25. As the process ratio determines the
percentage reduction in cross-sectional area of the rod,
the drawing velocity was incrementally increased
giving process ratios of 0.33, 0.42 and 0.5.
3. Results and Discussion
The percentage reduction per pass was measured
over the steady state length of drawn rod to be 51.8%
179
to a diameter of 3.47mm for a rod drawn with a
drawing velocity of 5mm/min and a heating/cooling
velocity of 12mm/min. The final rod length was
extended by 143mm. A smooth, even, oxidized surface
resulted from the single pass dieless draw.
The finite element model developed has accurately
predicted the process deformation. The heating/cooling
rate and interaction between the heater, the cooler, the
rod surface and the environment were found to be the
main controlling factors on results obtained from the
model.
The experimental results have shown the
possibility to reduce the number of passes and
subsequent heat treatments required in fabricating
Nitinol wire.
4. Conclusion
The cross-section of 5mm diameter NiTi rods were
successfully reduced using the dieless drawing method
at temperatures of 800 o C.
Further work is required to define the material
behavior under load at elevated temperature. A more
accurate definition of the interaction between the rod
and the heating/cooling environment of the machine is
also required.
5. References
1. Naughton, M.D. and P. Tiernan, Requirements of a
dieless wire drawing system. Journal of Materials
Processing Technology, 2007. 191(1-3): p. 310-
313.
2. Tiernan, P. and M.T. Hillery, Experimental and
numerical analysis of the deformation in mild steel
wire during dieless drawing. Proc IME J Mater Des
Appl, 2002. 216: p. 167 - 178.
3. Carolan, R. and P. Tiernan, Computer controlled
system for dieless drawing of tool steel bar. Journal
of Materials Processing Technology, 2009. 209(7):
p. 3335-3342.
4. Russel, S.M. Nitinol Melting and Fabrication. in
Shape Memory and Superelastic Technologies.
2001. California, USA: SMST, California.
5. Barras, C.D.J. and K.A. Myers, Nitinol - Its Use in
Vascular Surgery and Other Applications.
European Journal of Vascular and Endovascular
Surgery, 2000. 19(6): p. 564-569.
6. Duerig, T.W., A.R. Pelton, and D. Stöckel, An
overview of nitinol medical applications. Materials
Science and Engineering A, 1999. 273-275: p. 149-
160.