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Computational Evaluation of Temperature-Dependent Microstructural Transformations of Ti6Al4V for Laser Powder Bed Fusion Process

Abstract

In-situ decomposition in the laser powder bed fusion process enables the α’ martensite to transform into lamellar (α + β) microstructures to achieve superior mechanical properties, yet special conditions are required for the formation of decomposition, and these conditions are difficult to predict. In this context, an efficient model has been developed to evaluate the ever-changing thermal behavior of the multi-tracks and multi-layer laser scanning process. The model includes empirical approaches to determine conductivity enhancement factor in the z-direction (λz) and absorptivity for Ti6Al4V. Furthermore, the temperature-dependent microstructural transformations in relation to process parameters and the required stages of martensite decomposition are explained. The model produced consistent results for parameters selected from the literature that allow martensite decomposition. In addition, parameters were estimated for a powder layer thickness of 30 μm and a laser with a beam diameter of 85 μm, where martensite decomposition would be difficult. A cuboid sample was designed to be manufactured on a commercial machine. Despite the limitations in the machine, the martensite decomposition was able to be initiated in the center of the sample by enlarging its dimensions. This shows that lamellar structures with a layer thickness of 30 micrometers can be produced under favorable conditions.

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Fig. 1

Source melts the powders laid in the bed

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Abbreviations

cp :

Specific heat capacity, J/kgK

CR:

Cooling rate, K/s or °C/s

D:

Beam diameter, µm

E:

Energy density, J/mm3

FOD:

Focal offset distance, mm

h:

Hatch distance, µm

hc :

Convection heat transfer coefficient, W/m2K

k:

Thermal conductivity, W/mK

L:

Latent heat of fusion, kJ/kg

m:

Number of tracks

n:

Number of layers

P:

Laser power, W

PD:

Power density, MW/cm2

Q:

Heat, W/m3

q:

Heat flux, W/m2

r:

Beam radius, µm

Tsub :

Substrate temperature, K

Tc :

Certain temperature, K

Ts :

Solidus temperature, K

Tl :

Liquidus temperature, K

Tβ :

Beta transus temperature, K

Tms :

Martensite start temperature, K

Tmd :

Martensite decomposition temperature, K or °C

Tp :

Peak temperature, K

Tvs :

Temperature value stored, K

t:

Time, s or ms

ti :

Inter-layer delay time, s

V:

Scan speed, m/s or mm/s

X:

Spatial ordinate, m

Y:

Spatial ordinate, m

Z:

Spatial ordinate, m

ϕ:

Powder bed porosity

T:

Absorption coefficient

ρ:

Density, kg/m3

ε:

Emissivity

λ:

Thermal conductivity factor

ψ:

Layer thickness, µm

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Acknowledgment

Experimental works were funded by Turkish Aerospace Inc. Rotating Wing Technology Center Project under the Grant #DKTM2018/05.

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Correspondence to Ayse Kubra Yildiz.

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Yildiz, A.K., Mollamahmutoglu, M. & Yilmaz, O. Computational Evaluation of Temperature-Dependent Microstructural Transformations of Ti6Al4V for Laser Powder Bed Fusion Process. J. of Materi Eng and Perform (2022). https://doi.org/10.1007/s11665-022-06767-8

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  • DOI: https://doi.org/10.1007/s11665-022-06767-8

Keywords

  • additive manufacturing
  • laser powder bed fusion
  • martensite decomposition
  • microstructure
  • multilayer modeling