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Prediction of white layer formation in μ-WEDM process of NiTi shape memory superalloy: FEM with experimental verification

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Abstract

Microscopic changes in the surface of nickel-titanium (nitinol) shape memory alloys (SMAs) in micro-wire electro-discharge machining (μ-WEDM) due to the formation of a resolidified layer on the machined surface, called white layer, are one of the main drawbacks in the processing of such alloys. Since these changes significantly affect the shape memory and elastic recovery characteristics of these alloys, reduction of the white layer thickness (WLT) based on the selection of optimum process parameters is essential to raise the quality of the machined parts. In this regard, a finite element model (FEM) has been developed to simulate the effects of μ-WEDM process parameters, including discharge current, pulse on-time, pulse off-time, and servo voltage, on the heat distributing in Ni55.8Ti SMA to predict the WLT. The flushing efficiency of electric discharges and the effect of flow regime of the dielectric fluid on the heat distribution in the workpiece and the formation of the WLT are analyzed. Experimental data are used to verify the accuracy of the FEM. The results show that the developed model can predict the WLT in μ-WEDM process of Ni55.8Ti SMA with an average error of 14%. The effects of discharge parameters on the formation of the WLT are discussed in details based on the results of the FEM.

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Data availability

Data are available within the article and supplementary materials will be available on request from the corresponding author.

Abbreviations

A p :

Cross section of gap entrance (mμ)

C P,f :

Specific heat of fluid in constant pressure (J/kg K)

D :

Diameter of wire electrode (m)

D c :

Depth of molten crater

D h :

Hydraulic diameter (m)

f w :

Energy fraction of workpiece

f tur :

Friction factor of turbulent flow

h :

Gap between cutting wire and kerf wall (μm)

h C :

Convection heat transfer coefficient (W/mμ K)

I d :

Discharge current (A)

k :

Thermal conductivity of workpiece (W/m K)

k f :

Thermal conductivity of fluid (W/m K)

KW :

Kerf width (μm)

Nu :

Nusselt number

P :

Wetted perimeter of micro-channel (m)

PFE% :

Plasma flushing efficiency (%)

Pr :

Prandtl number

q :

Heat flux (W/m2)

R a :

Arithmetic average surface roughness (μm)

Re :

Reynolds number

R i :

Radius of plasma channel

r,z :

Polar coordinate axes

T :

Temperature (°C)

t :

Time (s)

T o :

Initial temperature of workpiece (°C)

T on :

Pulse on-time (μs)

T s :

Surface temperature of workpiece (°C)

U :

Discharge voltage (V)

V :

Average velocity (m/s)

V c(FEM):

Volume of molten crater obtained by FEM (μm3)

WLT(EXP):

White layer thickness obtained by experiments (μm)

WLT(FEM):

White layer thickness obtained by FEM (μm)

x W :

width of workpiece (mm)

α :

Thermal diffusivity (m2/s)

μ f :

Dynamic viscosity of fluid (N s/m2)

ρ f :

Density of fluid (kg/m3)

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Acknowledgments

The authors would like to thank the Micro System Design and Manufacturing Research Center and National Nanotechnology Research Center (UNAM) of Bilkent University for the WEDM and SEM measurements. Especially, we would like to thank the help of Professor Yigit Karpat and Dr. Şakir Baytaroğlu.

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Authors and Affiliations

  1. Faculty of Mechanical Engineering, University of Tabriz, Tabriz, Iran

    Reza Najati Ilkhchi

  2. Department of Mechanical Engineering, Çankaya University, Ankara, Turkey

    Samet Akar

  3. Faculty of Engineering, Atilim University, Ankara, Turkey

    Hassan Ali M. Meshri

  4. Faculty of Engineering, Adana Alparslan Turkeș University of Science and Technology, Adana, Turkey

    Mirsadegh Seyedzavvar

Authors
  1. Reza Najati Ilkhchi
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  2. Samet Akar
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  4. Mirsadegh Seyedzavvar
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Contributions

Samet Akar and Mirsadegh Seyedzavvar have conceived the idea, designed the experiments, and analyzed the results and data. Reza Najati Ilkhchi conducted the finite element simulations. Samet Akara and Hassan Ali M. Meshri conducted the experiments and collected the data. Mirsadegh Seyedzavvar wrote the manuscript getting contribution from all authors.

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Correspondence to Mirsadegh Seyedzavvar.

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Ilkhchi, R.N., Akar, S., Meshri, H.A.M. et al. Prediction of white layer formation in μ-WEDM process of NiTi shape memory superalloy: FEM with experimental verification. Int J Adv Manuf Technol 113, 2805–2817 (2021). https://doi.org/10.1007/s00170-021-06796-y

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