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Research Papers

Predicting Microstructure From Thermal History During Additive Manufacturing for Ti-6Al-4V

[+] Author and Article Information
Jeff Irwin

Department of Mechanical and
Nuclear Engineering,
The Pennsylvania State University,
17 Reber Building,
University Park, PA 16801
e-mail: jei5028@psu.edu

Edward W. Reutzel, Jay Keist, Abdalla R. Nassar

Applied Research Laboratory,
The Pennsylvania State University,
University Park, PA 16801

Pan Michaleris

Associate Professor
Department of Mechanical and
Nuclear Engineering,
The Pennsylvania State University,
University Park, PA 16801;
Pan Computing LLC,
State College, PA 16803

Manuscript received September 8, 2015; final manuscript received April 25, 2016; published online June 23, 2016. Assoc. Editor: Donggang Yao.

J. Manuf. Sci. Eng 138(11), 111007 (Jun 23, 2016) (11 pages) Paper No: MANU-15-1466; doi: 10.1115/1.4033525 History: Received September 08, 2015; Revised April 25, 2016

Due to the repeated thermal cycling that occurs with the processing of each subsequent layer, the microstructure of additively manufactured parts undergoes complex changes throughout the deposition process. Understanding and modeling this evolution poses a greater challenge than for single-cycle heat treatments. Following the work of Kelly and Charles, a Ti-6Al-4V microstructural model has been developed which calculates the phase fractions, morphology, and alpha lath width given a measured or modeled thermal history. Dissolution of the alpha phase is modeled as 1D plate growth of the beta phase, while alpha growth is modeled by the technique of Johnson–Mehl–Avrami (JMA). The alpha phase is divided into colony and basketweave morphologies based on an intragranular nucleation temperature. Evolution of alpha lath width is calculated using an Arrhenius equation. Key parameters of the combined Kelly–Charles model developed here are optimized using the Nelder–Mead simplex algorithm. For the deposition of two L-shaped geometries with different processing parameters, the optimized model gives a mean error over 24 different locations of 37% relative to experimentally measured lath widths, compared to 106% for the original Kelly–Charles model.

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Figures

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Fig. 1

Equilibrium α and β phase fractions from Ref. [21]

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Fig. 2

An illustration of the concept of equivalent time

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Fig. 3

Kinetic parameter for α growth from Ref. [21]

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Fig. 4

A flowchart for the thermomicrostructural model

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Fig. 5

The order of the eight deposition hatches for odd layers (left) and even layers (right)

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Fig. 6

Locations of the cross sections (dashed lines) for α lath width measurement, superimposed on the 4 s dwell build

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Fig. 7

Two representative micrographs used for measuring lath width. The top two are original grayscale images, while the lower two are black and white binary images after the thresholding process (a) 0 s dwell, z = 5 mm, w¯  = 0.62 μm, grayscale, (b) 4 s dwell, z = 30 mm, w¯  = 1.58 μm, grayscale, (c) 0 s dwell, z = 5 mm, w¯  = 0.62 μm, binary, and (d) 4 s dwell, z = 30 mm, w¯  = 1.58 μm, binary.

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Fig. 8

Temperature profile ( °C) at the end of the final laser pass for 0 s dwell

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Fig. 9

α lath width (μm) after the part has cooled (a) 0 s dwell and (b) 4 s dwell

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Fig. 10

Experimental lath width measurements compared to the results of two different microstructure models. In Fig. 10(b), the results of the original model are off the scale, with lath widths as large as 3.2 μm (a) validation, 0 s dwell, one-bead leg, (b) calibration, 0 s dwell, three-bead leg, (c) validation, 4 s dwell, one-bead leg, and (d) calibration, 4 s dwell, three-bead leg.

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Fig. 11

Phase fractions, lath width, and temperature versus time for both models at a single point in the middle of the three-bead leg (a) Kelly–Charles model and (b) optimized model

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Fig. 12

Colony-α phase fraction after the part has cooled (a) 0 s dwell and (b) 4 s dwell

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Fig. 13

The reheating bands, as observed in a cross-section macrograph (left) and in the simulation results (right), for the one-bead leg of the 0 s dwell part

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Fig. 14

Cross sections of both legs of both parts. From top to bottom: one-bead leg 0 s dwell, three-bead leg 0 s dwell, one-bead leg 4 s dwell, and three-bead leg 4 s dwell.

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