Water jet machining of NiTi shape memory alloy(SMA)

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Presented by Ch. Kumara swamyRoll.No:143503

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.

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What is a Shape Memory Alloy

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What are the difficulties

Conventional machining of SMAs leads to sever tool wearUndesirable strain hardening effectHigh 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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Alloys

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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 overallperform at 100% martensite phase.Zone 2: at M s <W t <A f , the machining process (e.g. AWJ) will overall perform at martensite–austenite phase (i.e. mix of martensiteand 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 experimentsthe following values: Ms =74 C; Mf=50 C; As=86 C;  Af=110 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 T<A 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 (70–140MPa) 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) 959–971

Axinte,D.A.,Kong,M.C.,2009.An integrated monitoring method to supervise water-jet machining CIRP. Annals Manufacturing Technology 58 (1), 303–306.

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