Water jet machining of NiTi shape memory alloy(SMA)

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Water jet machining of NiTi shape memory
alloy(SMA)
Presented by Ch. Kumara swamy Roll.No143503
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A shape-memory alloy (SMA, smart metal, memory
metal, memory alloy, smart alloy) is an alloy
that "remembers" its original shape and that when
deformed returns to its pre-deformed shape when
heated.
What is a Shape Memory Alloy
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What are the difficulties

• Conventional machining of SMAs leads to sever
  tool wear
• Undesirable strain hardening effect
• High cutting forces
• Water jet machining used for milling surface.
• This is expected to generate
• Low cutting normal forces(10-40N)
• No thermal distortion on the work piece
  superficial layer.
• Still there are some problems associated with is
  technique.

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Phase transformation plays a key role in the
unique behavior of SMAs. temperature measured
during water jetting (W t )
Zone 1 at W t M f , the machining process (e.g.
PWJ) will overall perform at 100 martensite
phase. Zone 2 at M s ltW t ltA f , the machining
process (e.g. AWJ) will overall perform at
martensiteaustenite phase (i.e. mix of
martensite and austenite). Zone 3 at W t A f ,
the machining process (e.g. conventional
machining) will overall perform at 100 austenite
phase.
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Experimental procedure
Ni49.8 Ti50.2 shape memory alloys were used for
experiments the following values Ms 74 C Mf50
C As86 C  Af110 C

• Experiments were divided into two parts
• Plain Water jet(PWJ)milling and
• Abrasive water jet(AWJ)milling.
• Filtered(10microns abrasive)and fresh water was
  employed as
• a machining tool for PWJ milling.
• For AWJ milling, an addition of garnet was
  employed

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Results of PWJ milling of NiTi (Zone 1)

• Since the PWJ milling process carried out at
  low(21.5 to 23.5?C) temperature.
• PWJ operates upon NiTi alloy is within the
  martensitic phase only (i.e. at TltA s 86?C).
• Surface generated by PWJ, it was found to be
  relatively smooth
• except some locally formed craters.
• Why craters are forme??
• This not because of low yield strength and high
  ductility of NiTi.

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When investigating the martensitic NiTi milled
surface generated by PWJ
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• Water-hammer pressure Pm in PWJ milling process
• Since the martensite has low yield strength
  (70140MPa)
• while the theoretical Pm in PWJ can be
  730MPa.
• Permanent plastic deformation the twinned
  martensite
• undergoes de-twinning process easily.
• Water jet impinge onto the work piece surface at
  different
• angles (jet defocusing effect)
• With different water hammer pressures.
• The kinetic energy generated by water droplets
  varies locally
• within the milling area

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Results of AWJ milling of NiTi (Zone 2/3

• Abrasive waterjet (AWJ) process will lead to a
  comparatively greater thermal effect than PWJ
  milling because of the high velocity of the
  abrasives impacting the target surface.
• Crystal structure may in Austenite or mixture of
  Martensitic.
• Localized areas impacted by the high-velocity
  abrasive particles can reach a temperature
  greater than that of Af, the melting of materials
  was observed.

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Sparks were observed during AWJ milling of this
SMA thus temperature locally reaches up to 1300?C.
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• Pseudoelasticity of NiTi alloy can decrease with
  cyclic deformation the amount of irreversible
  marten site still increases after a period of
  time due to the accumulation of permanent defects
  such as dislocations.
• Martensite twin boundaries can be considered with
  lower wear resistance characteristics and are
  more prone to surface crack initiation.

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References

• M.C. Kong, D. Axinte, W. Voice (2011).
  Challenges in using water jet
• machining of NiTi shape memory alloys An
  analysis of
• controlled-depth milling.
• Journal of Materials Processing Technology
  211 (2011) 959971
• Axinte,D.A.,Kong,M.C.,2009.An integrated
  monitoring method
• to supervise water-jet machining CIRP.
• Annals Manufacturing Technology 58 (1),
  303306.

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