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Optimization
of NiTinol Shape Setting
through Post Laser Cutting Processing
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©2006
ASM International. All rights reserved.
Presented at ASM International M&PMD Conference, Boston MA,
November 14, 2005 |
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By
Jonathan MacWilliams,
Engineering Manager/Metallurgist,
Norman
Noble Inc.
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| Nitinol
is often used in medical devices for its unique mechanical
properties. It is also used for the ability to produce
complex geometry via shape setting, eliminating costly
and difficult machining operations. The process of shape
setting requires deformation of a finished or partially
finished device. If this deformation is too severe, cracking
can result leaving the device unusable. While multiple
shape setting operations may be used to minimize the possibility
of cracking, limiting the number of these operations is
also important. Each shape setting cycle will affect the
Austenite to Martensite transformation temperature and
mechanical properties, potentially increasing variation
in the end product performance or moving the properties
outside the allowable limits. |
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This paper will study whether the level of achievable
shape setting deformation can be affected by modifying
the post laser cut surface of the pre-shape set component.
The study will evaluate the effect of the laser cutting
processes, and post laser cutting processes, and their
effect on the level of deformation achievable before cracking
occurs. Increasing the deformation achievable with a single
shape setting cycle may minimize the number of cycles required,
decreasing variation of end product performance as well
as decreasing manufacturing cost and complexity. This paper
will present an analysis of the effects of pre-shape setting
surface preparation on the ability to maximize the deformation
achievable during shape setting heat treatment.
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Nitinol
is an important part of the medical device industry. Understanding
the fundamental aspects of processing this alloy is important
for future innovations. One of the most important properties
of Nitinol is the ability of shape setting through heat
treating. The process of shape setting permits geometry
to be produced that may not be cost effective to achieve
with a machining process.
The process of Nitinol shape setting includes controlled deformation
of a part during heat treatment. If the level of deformation required |
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results
in cracking, then multiple shape setting cycles may be
required to achieve the final geometry. Each shape setting
cycle alters the mechanical and transformation properties
of the component, making the prediction of final performance
much more difficult. Also, each shape setting cycle will
require complex shape setting fixturing that will increase
costs. In order to minimize the number of shape setting
cycles, it is important to understand the effects of post
laser processing on the ability to shape set a Nitinol
component. This testing will focus on these effects. |
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Test
samples were produced from 0.35 mm (0.014”) thick
Superelastic Nitinol Sheet. The Active Af of the sheet
was 7.2 °C.
The samples were 12.7 mm (0.500”) in overall length. The cross
section of the test section was 0.35 mm (0.014”) square. Samples
were cut in both the longitudinal and transverse to the rolling direction.
The sample geometry is depicted in Figure 1 below:
Figure
1: Sample geometry indicating orientation to the rolling
direction. Samples are 12.7 mm (0.500”) long.
Each test group
was cut using an Nd:Yag laser, and separated into three
sub-groups. One of the sub-groups was left as-cut, while
the other two were processed with typical post laser
cleaning techniques prior to testing. The post laser
cutting processes were Post Laser Cleaning (Pickling
/ Descaling), and Electropolishing. Each group had a
minimum of 15 test samples. |
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The
sample matrix is presented in Table 1 below:
Table
1: Sample Preparation Matrix.
| Direction |
Group
1 |
Group
2 |
Group
3 |
| With |
As
Cut |
Post
Laser Cleaned |
Electropolished |
| Against |
As
Cut |
Post
Laser Cleaned |
Electropolished |
Testing included deforming the test specimens until failure occurred
(see Figure 2). The amount of deformation achievable was measured
for each sample. This data was then used to characterize the amount
of deformation achievable with each group.
Figure
2: Test Fixture for deforming test samples.
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Each
finishing step changed the surface finish of the test pieces.
The post processing operations also changed the amount
of deformation that could be achieved prior to failure.
The test results are presented in Figure 3:
Figure 3: Scatter plot of
data produced during deformation testing. The
as-cut parts are on the left, the post laser
cleaned are in the middle, and electropolished
parts on the right.
The test results clearly show a change in deformation and surface
finish with each processing step. With each post processing step,
the amount of deformation improved. The most dramatic change can
be seen between the raw laser cut parts and the post laser cut cleaned.
The maximum deformation was achieved after the parts were electropolished.
It can also be seen that the orientation of the samples with respect
to the rolling direction also played a role. This effect was not
as noticeable on the raw laser cut parts. However, as the parts were
post laser processed, the effects of orientation became more evident,
with the electropolished parts showing the most dramatic effects.
This result, though not unexpected, is very easy to overlook during
process design.
To confirm these relationships, an ANOVA (Analysis of Variance) was
performed. The ANOVA indicated that both orientation and post processing
had a statistically significant impact on the amount of deformation
achievable. The results of the ANOVA are represented graphically
as an interaction plot (Figure 4):
Figure
4: Interaction plot of the test data.
Metallographic samples were prepared to examine the effects of laser
cutting on the microstructure of the samples. The metallographic
examination confirms that a thin layer of resolidified material exists
on the laser cut surfaces (see Figure 5). This layer has different
properties than the base material due to its rapidly solidified material.
This change in properties will almost certainly aid in crack formation
during tensile loading, and ultimately result in premature failure.
Figure 5: Optical Photomicrograph
of recast layer present after laser cutting. This example
is approximately 0.0028 mm (0.00011”) thick. 1000X
magnification, Krohl’s Reagent.
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Subsequent examination of the products after post laser
cutting cleaning and electropolishing revealed that this
recast layer had been removed.
SEM fractography was conducted on the fracture faces of each group.
The examination revealed that each processing step increased the
amount of plastic deformation achievable prior to failure (see
Figures 6 and 7). This indicates that the recast layer observed
in the metallographic examination
of the unprocessed laser cut samples likely contributed to a stress
concentrator that allowed a premature failure to occur. The post
processed samples did not show evidence of this recast layer and
subsequently showed fracture faces typical of classical tensile
overload.
Figure 6: SEM Photomicrograph
of fracture face of the laser cut test specimen.
The arrows indicate the fracture initiation site
in the recast layer still present on the surface.
There is microvoid coalescence – indicative
of tensile overload; but note the lack of necking – pointing
to a stress concentrator. 236X magnification.
Figure 7: SEM Photomicrograph
of fracture face of the post laser
cut cleaned test specimen. The arrows
mark the evidence necking, coupled
with microvoid coalescence indicating
a classical tensile overload. 236X
magnification.
Based upon these observations, several relationships
can be drawn:
Laser cutting leaves a layer of resolidified material
on the laser cut edge. This material has different physical properties from the
base material, and also likely contains regions of high residual stresses due
to its rapid solidification. These two properties act as stress concentrators
that promote premature failure during shape setting deformation.
The post laser cleaning processes contribute to the removal
of this layer. The removal of this layer resulted in a drastic improvement in
the amount of deformation achievable during shape setting.
Electropolishing further improves the surface finish,
as can be seen in Figure 3. This improvement in surface finish, however, has
a negligible impact on the amount of deformation achievable prior to shape setting.
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Understanding
the fundamental properties of Nitinol is important for
optimizing process design. The impact of post laser processing,
as well as the orientation of the raw material, should
be considered during process design.
In the case of laser cutting, removal of the resolidified material
is critical. This can be achieved through post laser cutting cleaning,
and subsequent electropolishing. By removing this layer, and considering
texture, the amount of deformation achievable during shape setting
can be maximized. This, in turn, will minimize the number of shape
setting cycles required.
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By
minimizing the number of shape setting cycles, the final
product consistency can be improved. The ability of achieving
final dimensions is maximized, and more importantly, the
final properties are more likely to be within specification.
It is important to note that in this study the amount of material
removed during these processes was not measured or controlled. It
is recommended that any post laser cleaning process be designed to
ensure full recast layer removal to repeat the relationships detected
in this study. |
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Micromachining
Facility & Headquarters - Medtech I
5507 Avion Park Drive, Highland Heights, Ohio 44143
Tel: 800.474.4322 Tel: 216.761.5387 Fax: 216.761.0455
email: sales@normannoble.net |
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Micromachining
Facility - Medtech II
Highland Heights, Ohio 44143 |
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Micromachining
Facility - Collamer
Cleveland, Ohio 44110
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Copyright © 2009
Norman Noble, Inc.
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