Microstructure and room-temperature tensile property of Ti-5.7Al-4.0Sn-3.5Zr-0.4Mo-0.4Si-0.4Nb-1.0Ta-0.05C with near equiaxed β grain fabricated by laser directed energy deposition technique

doi:10.1016/j.jmst.2021.03.012

Abstract

Abstract:

Near-equiaxed β grain was achieved in the near-α Ti60 (Ti-5.7Al-4.0Sn-3.5Zr-0.4Mo-0.4Si-0.4Nb-1.0Ta-0.05C) titanium alloy via laser directed energy deposition (LDED). The microstructural evolution along the building direction and the room-temperature tensile properties along the horizontal and vertical directions (building direction) were systematically studied through SEM and OM. EBSD and XRD were utilized to accurately demonstrate the texture of the α and β phases. The results showed that the α phase presented a low texture intensity, which was ascribed to the weak textured β grain with near-equiaxed morphology, since there are Burgers orientation relationships during the β→α transition. In addition, numerical simulation, combined with the CET curve of Ti60 alloy considering the effect of multi-composition, was utilized to elucidate the formation mechanism of the near-equiaxed β grains. Furthermore, according to the solidification theory, we proposed that the solidification temperature range ΔT_f was more accurate than the growth restriction factor Q in predicting the formation tendency of equiaxed β grain in different titanium alloys. Tensile results showed that the horizontal and vertical samples had similar strength, while the former exhibited larger elongation than the latter. The effect of the near-equiaxed β grain and the internal α phase on mechanical properties were revealed at last.

Key words: Laser directed energy deposition, Near-α titanium alloy, Equiaxed β grain, Room-temperature tensile property

MengCheng Deng, Shang Sui, Bo Yao, Liang Ma, Xin Lin, Jing Chen. Microstructure and room-temperature tensile property of Ti-5.7Al-4.0Sn-3.5Zr-0.4Mo-0.4Si-0.4Nb-1.0Ta-0.05C with near equiaxed β grain fabricated by laser directed energy deposition technique[J]. J. Mater. Sci. Technol., 2022, 101: 308-320.

Figures/Tables 20

Table 1 Chemical composition of Ti60 powders used in the experiments.

Al	Sn	Zr	Mo	Si	Nb	Ta	Fe	C	O	N	H
6.0	3.6	3.6	0.32	0.36	0.42	1.0	0.05	0.047	0.07	0.009	0.0018

Fig. 1. (a) Morphology and (b) particle size distribution of Ti60 powder.

Table 2 LDED process parameters.

Laser power (W)	Scanning velocity (mm/s)	Powder feeding rate (g/min)	Spot diameter (mm)	Overlap rate (%)	Vertical increment (mm)
1000	8	4-5	1.2	35	0.6

Fig. 2. LDED-built Ti60 alloy. (a) Scanning strategy. (b) Position of tensile coupons in the as-built block.

Table 3 Physical properties of Ti60 alloy used in the FE modeling [36].

Temperature, T (°C)	100	200	300	400	500	600	700	800	900
Density, ρ (kg/m³)	4530	4530	4530	4530	4530	4530	4530	4530	4530
Specific heat,Cρ (J/(kg°C))	496	520	544	566	585	615	663	707	729
Thermal conductivity, λ (W/(m°C))	6.21	7.38	8.55	9.64	10.7	12.1	13.8	15.4	16.4

Fig. 3. (a) Schematic diagram of finite element mesh of the LDED-built Ti60 alloy. (b) The locations of the nine points selected for thermal analysis during the solidification process.

Fig. 4. Microstructure of the LDED-built Ti60 sample. (a) Prior-β grain characteristics. (b) α phases in the layer band structure. (c) Intergranular α phases. (d) Intragranular α phases. (e) High magnification image of layer band structure. (f) High magnification image of α-colony.

Fig. 5. EBSD results of the LDED-built Ti60 alloy. (a) Inverse pole figure of the α phases. (b) Reconstruction of the β grains. (c) Pole figure of the α phases. (d) Pole figure of the reconstructed β grains.

Fig. 6. Pole figures of the α phases by XRD.

Fig. 7. Room-temperature tensile properties of the LDED-built Ti60 alloys.

Fig. 8. Fractography of the LDED-built Ti60 alloys. (a) Overview of the fracture of the horizontal sample. (b) High-magnification SEM figures of the fiber zone of the horizontal sample. (c) Overview of the fracture of the vertical sample. (d) High-magnification SEM figures of the fiber zone of the vertical sample.

Fig. 9. Formation mechanism of near-equiaxed β grains: (a) Simulation of temperature field of melt pool during solidification process. (b) Evolution of the solidification conditions (G and V) as the depth z (z?=?0 corresponding to the surface) of the local the melt pool, (c) Schematic diagram of the formation of the near-equiaxed grain structure.

Fig. 10. (a) CET curves of the Ti60 alloy and the solidification conditions during the LDED process. (b) Direction of temperature gradient in the melt pool

Fig. 11. Schematic diagrams of (a) constitutional undercooling zone and (b) Ti-X phase diagram.

Fig. 12. Summary of the solidification range ΔTf and growth restriction factor Q in different titanium alloys

Fig. 13. Longitudinal sections of the fractures. (a) and (b) Horizontal sample. (c) and (d) Vertical sample.

Fig. 14. Morphologies of the α laths at different positions of the as-built sample. (a) Bottom. (b) Middle. (c) Top. (d) Relationship between the average α laths width and micro-hardness.

Fig. A1. Fitting curve of the relationship between lnΔT and lnV

Fig. A2. CET curve of Ti60 alloy

Table A1. Physical property parameters used in Ti60 alloy calculation.

Symbol	Parameters	Value	Symbol	Parameters	Value
$T_{0}^{L}$	Liquidus temperature	1952.06 K	m_Zr	Slope of the liquidus surface of Zr	-2.24 K/wt.%
C_Al	Concentration of Al	5.70%	m_Mo	Slope of the liquidus surface of Mo	2.69 K/wt.%
C_Sn	Concentration of Sn	4.00%	m_Si	Slope of the liquidus surface of Si	-19.1 K/wt.%
C_Zr	Concentration of Zr	3.50%	m_Nb	Slope of the liquidus surface of Nb	1.67 K/wt.%
C_Mo	Concentration of Mo	0.40%	m_Ta	Slope of the liquidus surface of Ta	2.04 K/wt.%
C_Si	Concentration of Si	0.40%	Γ	Gibbs-Thomson coefficient	2.89 × 10^-7 K m
C_Nb	Concentration of Nb	0.40%	D_Al	Liquidus diffusion coefficient of Al	5.05 × 10^-9 J/mol
C_Ta	Concentration of Ta	1.00%	D_Sn	Liquidus diffusion coefficient of Sn	4.19 × 10^-9 J/mol
k_Al	Solute distribution coefficient of Al	1.15	D_Zr	Liquidus diffusion coefficient of Zr	2.57 × 10^-9 J/mol
k_Sn	Solute distribution coefficient of Sn	0.60	D_Mo	Liquidus diffusion coefficient of Mo	2.56 × 10^-9 J/mol
k_Zr	Solute distribution coefficient of Zr	0.82	D_Si	Liquidus diffusion coefficient of Si	2.59 × 10^-9 J/mol
k_Mo	Solute distribution coefficient of Mo	1.35	D_Nb	Liquidus diffusion coefficient of Nb	2.56 × 10^-9 J/mol
k_Si	Solute distribution coefficient of Si	0.50	D_Ta	Liquidus diffusion coefficient of Ta	2.55 × 10^-9 J/mol
k_Nb	Solute distribution coefficient of Nb	1.24	a₀	Characteristic length scale for solute trapping	3.00 × 10^-10 m
k_Ta	Solute distribution coefficient of Ta	1.47	N₀	Nucleation density	2.00 × 10¹⁵ m^-3
m_Al	Slope of the liquidus surface of Al	2.41 K/wt.%	μ_k	Linear kinetic coefficient	2.91 K/(s m)
m_Sn	Slope of the liquidus surface of Sn	-3.78 K/wt.%	ΔT_n	Nucleation undercooling	2.5 K

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