Enhancing the Mechanical Properties Induced by Ta Microalloying in TIG-Welded Ti2AlNb-Based Intermetallic Alloy

doi:10.1007/s40195-024-01784-z

Abstract

Abstract:

During the tungsten inert gas (TIG) welding process of Ti₂AlNb alloy, high heat input leads to the formation of coarse grains, which are detrimental to the mechanical properties of welded joints. To address this problem, Ta microalloyed welding wires were developed to enhance the strength of the welded joints. The Ta-modified fusion zone (FZ) exhibited a well-defined structure with a smooth, defect-free surface. Systematic analysis of the microstructure evolution and mechanical properties of the welded joints revealed that the Ta element completely dissolves into the FZ. During solidification, a significant constitutional undercooling effect occurs, promoting the columnar-to-equiaxed transition (CET) and reducing grain size from 187.42 to 133.49 μm. Mechanical properties tests indicated that with increased Ta content, the strength of the welded joints initially increased and then decreased. When the Ta content in the welding wire was 1 wt%, the joints showed the best performance, with a tensile strength of 909.36 MPa and an elongation of 1.21%. Compared to the welded samples without Ta, the tensile strength and elongation increased by 153.01 MPa and 0.53%, respectively. Grain refinement and increased dislocation density were the main reasons for the improved mechanical properties. However, excessive Ta content led to significant the intragrain misorientation, increasing the joint’s anisotropy and causing uneven deformation during tensile testing. Therefore, further addition of Ta did not substantially enhance the tensile properties of the joint. Additionally, the paper provides a detailed analysis of the low elongation observed in the joint. After welding, dislocations were neatly arranged in the FZ, forming numerous parallel dislocation walls, leading to local stress concentration and accelerating crack initiation and propagation. Consequently, the elongation at the weld was lower than that of the base metal (BM). This research offers a new approach to improve the mechanical properties of Ti₂AlNb alloy during welding.

Key words: Ti₂AlNb base alloy, Tungsten inert gas (TIG) welding, Grain refinement, Ta microalloying, Constitutional undercooling

Hao Zhang, Le Zai, Xiaohuai Xue. Enhancing the Mechanical Properties Induced by Ta Microalloying in TIG-Welded Ti₂AlNb-Based Intermetallic Alloy[J]. Acta Metallurgica Sinica (English Letters), 2025, 38(3): 419-434.

Figures/Tables 19

Fig. 1 a XRD patterns, b microstructure of Ti-22Al-25Nb-0.5Mo Alloy

Table 1 Chemical composition (wt%) of Ti-22Al-25Nb-0.5Mo (BM) and four kinds of alloy welding wires

Materials	Element (wt%)
Materials	Ti	Al	Nb	Mo	Ta
Ti-22Al-25Nb-0.5Mo (BM)	Balance	10.80	42.21	0.89	-
Ta-0#	Balance	10.35	41.63	-	-
Ta-0.5#	Balance	10.63	42.10	-	0.56
Ta-1#	Balance	10.16	42.36	-	0.98
Ta-1.5#	Balance	10.24	41.68	-	1.59

Table 2 Mechanical properties of Ti-22Al-25Nb-0.5Mo (at room temperature)

BM	Yield strength (MPa)	Tensile strength (MPa)	Elongation (%)	Hardness (HV)
Ti-22Al-25Nb-0.5Mo	961.36	1079.36	10.96	322.67

Fig. 2 a TIG welding process, b the surface appearance of the joints fabricated by TIG, c dimensions of the welding assembly, d geometry of the tensile sample

Table 3 Parameters for TIG process

Base current (A)	Peak current (A)	Frequency of the pulse current (Hz)	Frontal protection of argon gas (L/min)	Back protection of argon gas (L/min)	Welding speed (mm/min)	Voltage (V)	Purity of Argon (%)
70	130	15,000	15	5	200	12	99.999

Fig. 3 XRD analysis of FZ with Ta-0, Ta-0.5, Ta-1, and Ta-1.5

Table 4 Lattice parameter (a, Å) and lattice strain (∆a/a, %) of FZ of the four kinds of joints

Samples	a (Å)	∆a/a (%)
Ta-0	3.2394	-
Ta-0.5	3.2355	0.120
Ta-1	3.2329	0.201
Ta-1.5	3.2306	0.272

Fig. 4 Room temperature stress-strain diagram: a engineering tensile stress-strain curves for Ta-0, Ta-0.5, Ta-1, and Ta-1.5, b tensile properties

Fig. 5 SEM micrographs of Ta-0 a and Ta-1 b, including FZ (A), PMZ (B), HAZ (C), and BM (D). The histogram at the lower right is the phase composition and proportion of each region

Fig. 6 EDS analysis: elemental distribution of a selected region in the FZ of Ta-0 a and Ta-1 b, EDS mapping results of Ta-0 c and Ta-1 d

Fig. 7 Comparison of microstructural evolution between FZs of Ta-0, Ta-0.5, Ta-1, and Ta-1.5: a-d IPF, Ta-0, Ta-0.5, Ta-1, and Ta-1.5, respectively, e-h grain boundaries maps, Ta-0, Ta-0.5, Ta-1, and Ta-1.5, respectively, i-l KAM, Ta-0, Ta-0.5, Ta-1, and Ta-1.5, respectively, and corresponding legends are on the right

Fig. 8 Graphical presentation of EBSD analysis for FZ of Ta-0, Ta-0.5, Ta-1, and Ta-1.5: a, b B2 grain size and its distribution, c, d KAM for B2 grains

Fig. 9 TEM images from the FZ of Ta-1: a low multiple and b high multiple of BF image, respectively, c SAED patterns of the red circle marked regions in a

Fig. 10 SEM fracture surfaces of Ta-0 a, b, Ta-0.5 c, d, Ta-1 e, f, and Ta-1.5 g, h. The right column images show the corresponding higher magnification of the selected rectangular regions in the left column images

Fig. 11 a-d Optical microstructure of the joints fabricated of Ta-0, Ta-0.5, Ta-1, and Ta-1.5, respectively; e-h hardness profiles for Ta-0, Ta-0.5, Ta-1, and Ta-1.5, respectively. Including FZ (A), PMZ (B), HAZ (C), and BM (D)

Fig. 12 Comparison of hardness statistics of each part of Ta-0, Ta-0.5, Ta-1, and Ta-1.5 welded joints from Fig. 11e and f. Including FZ (A), PMZ (B), HAZ (C), and BM (D)

Fig. 13 Mechanism schematics of the microstructure evolution of the studied samples: a Ta-0 and b Ta-1 are the microstructure solidification process of liquid-solid; c influence of temperature gradient on growth mode and nucleation; d the effect of Ta element on the constitutional undercooling

Table 5 Physical parameters and mechanical features of the Ti alloys involved in calculations

Parameter		Physical description	Value	Refs
B_i	Ta	Strengthening coefficient of solute Ta	164 MPa·at.^−2/3	[43]
	Al	Strengthening coefficient of solute Al	285 MPa·at.^−2/3	[43]
	Nb	Strengthening coefficient of solute Nb	71 MPa·at.^−2/3	[43]
X_i	Ta-0.5	The relative atomic mass of the Ta element in the Ta-0.5 joint	0.1 at.%	Experimental value
	Ta-1	The relative atomic mass of the Ta element in the Ta-1 joint	0.2 at.%	Experimental value
	Ta-1.5	The relative atomic mass of the Ta element in the Ta-1.5 joint	0.3 at.%	Experimental value
k_y		Hall-Petch constant	0.75 MPa·m^1/2	[44]
α		Dislocation interaction constant	0.3	[45]
M_T		Taylor factor	2.8	[43]
G		Shear modulus	39 GPa	[43]
b		Magnitude of Burgers vector	0.28 nm	[43]
μ		Step size used in EBSD testing	5 μm	Experimental value
k_HV		Hall-Petch slope in Vickers hardness	80 HV·mm^1/2	[42]

Fig. 14 Strengthening contributions of the studied samples

References 45

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Enhancing the Mechanical Properties Induced by Ta Microalloying in TIG-Welded Ti₂AlNb-Based Intermetallic Alloy

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