Evolution of Macrosegregation During Three-Stage Vacuum Arc Remelting of Titanium Alloys

doi:10.11900/0412.1961.2022.00544

Acta Metall Sin

2024, Vol. 60

Issue (11): 1531-1544 DOI: 10.11900/0412.1961.2022.00544

Research paper

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Evolution of Macrosegregation During Three-Stage Vacuum Arc Remelting of Titanium Alloys

GUO Jie¹, HUANG Liqing¹^,², WU Jingyang¹, LI Junjie¹, WANG Jincheng¹, FAN Kai²(

)

1 State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi'an 710072, China
2 Hunan Xiangtou Goldsky Titanium Industry Technology Co. Ltd., Changde 415001, China

Cite this article:

GUO Jie, HUANG Liqing, WU Jingyang, LI Junjie, WANG Jincheng, FAN Kai. Evolution of Macrosegregation During Three-Stage Vacuum Arc Remelting of Titanium Alloys. Acta Metall Sin, 2024, 60(11): 1531-1544.

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Abstract

Macrosegregation is a typical solidification defect formed during vacuum arc remelting (VAR) process. This defect adversely affects the property of ingots as the defect sustains even in the subsequent heat treatment process. In the industrial production of titanium alloys, VAR is repeated thrice to eliminate inclusions and improve the homogenization of composition. However, the evolution of macrosegregation during the different stages of the triple VAR process remains unclear. In this study, the melt flow behavior and macrosegregation of titanium ingots in the multistage VAR process are examined via solidification simulations, considering both buoyancy and electromagnetic force. The results show that the strong fluid flow in the upper part of melting pool eliminates nonuniform concentration along the radial direction of the electrode. In contrast, the nonuniform concentration along the axial direction can be inherited in the sequential ingot. However, with the increase in the depth of melt pool, the sustained melt flow from the bottom to upside can reduce the axial macrosegregation delivery. In addition, the use of the previous ingot directly as the electrode for the subsequent remelting process results in severe macrosegregation. However, turning the previous ingot upside-down at least once during the three-stage VAR process can substantially reduce the macrosegregation. Overall, the simulated macrosegregation of Al and V elements in TC4 ingot agree well with that observed in experiment.

Key words: macrosegregation solidification vacuum arc remelting titanium alloy numerical simulation

Received: 25 October 2022

ZTFLH:

TG244

Fund: Research Fund of State Key Laboratory of Solidification Processing in NWPU(2020-TS-06)

Corresponding Authors: FAN Kai, senior engineer, Tel: (0736)7326915, E-mail: fk@xtjtty.com

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	GUO Jie
	HUANG Liqing
	WU Jingyang
	LI Junjie
	WANG Jincheng
	FAN Kai

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https://www.ams.org.cn/EN/10.11900/0412.1961.2022.00544 OR https://www.ams.org.cn/EN/Y2024/V60/I11/1531

Parameter description	Symbol	Value	Unit
Density	ρ	4170	kg·m^-3
Diffusion coefficient for V in liquid	$D l V$	4.0 × 10^-9	m²·s^-1
Diffusion coefficient for Al in liquid	$D l A l$	4.0 × 10^-9	m²·s^-1
Latent heat of fusion	L	3.77 × 10⁵	J·kg^-1
Partition coefficient for V	$k p V$	0.95	-
Partition coefficient for Al	$k p A l$	1.08	-
Liquidus slope for V	m^V	-2.0	K·%^-1 (mass fraction)
Liquidus slope for Al	m^Al	-4.44	K·%^-1 (mass fraction)
Solutal expansion coefficient for V	$β S V$	-0.35	%^-1 (mass fraction)
Solutal expansion coefficient for Al	$β S A l$	0.4	%^-1 (mass fraction)
Heat capacity	c_p	975	J·kg^-1·K^-1
Thermal conductivity	k_T	32.7	W·m^-1·K^-1
Thermal expansion coefficient	β_T	6.5 × 10^-5	K^-1
Viscosity of liquid	μ_l	3.1 × 10^-3	kg·m^-1·s^-1
Electric conductivity	σ	1.0 × 10⁶	S·m^-1
Magnetic permeability	μ₀	1.26 × 10^-6	H·m^-1

Table 1 Physical parameters of the computational model

Fig.1 Schematics of electrode placement in the triple vacuum arc remelting (VAR) process (The first smelting uses a homogeneous electrode, denoted by A. When the previous ingot is placed upright as an electrode, it is represented by D; when the previous ingot is placed upside down as an electrode, it is represented by R)
(a) VAR1-A (b) VAR2-D (c) VAR2-R
(d) VAR3-D-D (e) VAR3-D-R (f) VAR3-R-D (g) VAR3-R-R

Fig.2 Solute distributions and liquid phase flow vectors in the ingot at different time during the first smelting process (Two solid lines represent the weld pool outlines, the same below. C_V—mass fraction of vanadium element)
(a) 1000 s (b) 4300 s (c) 6700 s (d) 8800 s (e) 9000 s (f) 10800 s (g) 12500 s
(a1, b1) enlarged views of liquid phase flow vectors in Fig.2a (a1) and Fig.2b (b1), respectively

Fig.3 Solute distributions and liquid phase flow vectors in the ingot at 1000 s (a1-c1), 2200 s (a2-c2), 3800 s (a3-c3), 5080 s (a4-c4), 6520 s (a5-c5), 8520 s (a6-c6), and 9440 s (a7-c7) during the second smelting process (a1-a7) VAR2-A (b1-b7) VAR2-D (c1-c7) VAR2-R

Fig.4 Radial concentration distribution curves at different time during the second remelting process (The extracted concentration positions are shown by horizontal lines in Fig.3)
(a) comparisons of the three electrode placement results at 2200 s
(b) comparisons of the three electrode placement results at 5080 s

Fig.5 Radial average concentrations in the second remelting electrode (a) and ingot (b) varied with height

Fig.6 Solute distributions and liquid phase flow vectors in the ingot at 1300 s (a1-c1), 2300 s (a2-c2), 3800 s (a3-c3), 8350 s (a4-c4), and 9400 s (a5-c5) during the third smelting process (a1-a5) VAR3-D-D (b1-b5) VAR3-D-R (c1-c5) VAR3-R-R

Fig.7 Radial concentration distribution curves at different time during the third remelting process (The extracted concentration positions are shown by the horizontal lines in Fig.6)
(a) comparison of the three electrode placement results at 1300 s
(b) comparison of the three electrode placement results at 3800 s

Fig.8 Radial average concentrations in the third remelting electrode (a) and ingot (b) varied with height

Fig.9 Comparisons of final concentration distributions of all triple remelting ingots
(a) VAR1-A (b) VAR2-A (c) VAR2-D (d) VAR2-R
(e) VAR3-A (f) VAR3-D-D (g) VAR3-D-R (h) VAR3-R-D (i) VAR3-R-R

Fig.10 Comparisons of the global macrosegregation index (GMI) of each remelting ingot

Fig.11 Simulation results of V element (a1-a7) and Al element (b1-b7) concentration distributions in triple remelting ingot of TC4 alloy (C_Al—mass fraction of aluminum element) (a1, b1) VAR1-A (a2, b2) VAR2-D (a3, b3) VAR2-R (a4, b4) VAR3-D-D (a5, b5) VAR3-D-R (a6, b6) VAR3-R-D (a7, b7) VAR3-R-R

Fig.12 GMI of V element (a) and Al element (b) in TC4 alloy ingot under different melting schemes

Fig.13 Ingot sampling procedure and positions 1-9

Fig.14 Comparisons between the simulation and the experimental results of radial V element (a1-a3) and Al element (b1-b3) concentration distributions in the top (a1, b1), middle (a2, b2), and bottom (a3, b3) parts of the final ingot of TC4 alloy after triple remelting

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