Effect of Multiple Thermal Cycles on Microstructure and Mechanical Properties of Cu Modified Ti64 Thin Wall Fabricated by Wire-Arc Directed Energy Deposition

doi:10.1007/s40195-024-01756-3

Abstract

Abstract:

This study investigated the effect of thermal cycles on Cu-modified Ti64 thin-walled components deposited using the wire-arc directed energy deposition (wire-arc DED) process. For the samples before and after experiencing thermal cycles, it was found that both microstructures consisted of prior β, grain boundary α (GB α), and basketweave structures containing α+β lamellae. Thermal cycles realized the refinement of α laths, the coarsening of prior β grains and β laths, while the size and morphology of continuously distributed GB α remained unchanged. The residual β content was increased after thermal cycles. Compared with the heat-treated sample with nanoscale Ti₂Cu formed, short residence time in high temperature caused by the rapid cooling rate of thermal cycles restricted Ti₂Cu formation. No formation of brittle Ti₂Cu means that only grain refinement strengthening and solid-solution strengthening matter. The yield strength increased from 809.9 to 910.85 MPa (12.46% increase). Among them, the main contribution from solid solution strengthening (~ 51 MPa) was due to the elemental redistribution effect between α and β phases caused by thermal cycles through quantitative analysis. The ultimate tensile strength increased from 918.5 to 974.22 MPa (6.1% increase), while fracture elongation increased from 6.78 to 10.66% (57.23% increase). Grain refinement of α laths, the promoted α′ martensite decomposition, decreased aspect ratio, decreased Schmid factor, and local misorientation change of α laths are the main factors in improved ductility. Additionally, although the fracture modes of the samples in the top and middle regions are both brittle-ductile mixed fracture mode, the thermal cycles still contributed to an improvement in tensile ductility.

Key words: Wire-arc directed energy deposition (wire-arc DED), Ti64-1.2Cu thin wall, Thermal cycles, Microstructure variation, Mechanical properties

Zidong Lin, Xuefeng Zhao, Wei Ya, Yan Li, Zhen Sun, Shiwei Han, Xiaoyang Peng, Xinghua Yu. Effect of Multiple Thermal Cycles on Microstructure and Mechanical Properties of Cu Modified Ti64 Thin Wall Fabricated by Wire-Arc Directed Energy Deposition[J]. Acta Metallurgica Sinica (English Letters), 2024, 37(11): 1875-1890.

Figures/Tables 18

Table 1 Chemical composition of the used materials (wt%)

Materials	Al	V	Cu	Fe	C	Ti
Ti64-1.2Cu wire	4.72	4.08	1.20	< 0.25	< 0.08	Bal.
Wrought Ti64 base	6.28	4.15	-	0.15	0.03	Bal.

Table 2 Deposition and post-heat treatment parameters

Variables	Parameters
Current	200 A
Voltage	16.2 V
Deposition speed	420 mm/min
Step up distance	4.5 mm
Shielding gas flow rate (99.999% Ar)—Torchnozzle	22 L/min
Protective gas flow rate (99.999% Ar)—Tracing shielding	150 L/min
Contact tip-to-work distance (CTWD)—Tracing shielding	1 mm
Wire stick-out distance	15 mm
Interlayer temperature	Room temperature
Polarity	Direct current, reversed polarity (wire: positive, base: negative)
Molten droplet transfer mode	Super active wire process (modification of short-circuit)
Post-heat treatment	830 °C−1 h + Air cooling

Fig. 1 Deposition arrangement of the Ti64-1.2Cu single-bead wall: a the setup of the deposition system, b the physical diagram of the deposition process

Fig. 2 Samples adopted for characterizations: a sample position in the schematic diagram, b sample position in the physical diagram

Fig. 3 XRD test results of the deposited Ti64-1.2Cu wall (Top and Mid in Fig. 2)

Fig. 4 EDS analysis results of Ti64-1.2Cu wall: a top region (Top in Fig. 2), b middle region (Mid in Fig. 2)

Fig. 5 SEM microstructural images of the deposited Ti64-1.2Cu single-bead wall: a cross section (CS in Fig. 2), b-e microstructure of the top sample (Top in Fig. 2), f-i microstructure of the middle sample (Mid in Fig. 2)

Table 3 Size variation of prior β grains and α phases before and after experiencing thermal cycles

Position	Prior β	The aspect ratio of α laths	GB α	Basketweave
Top	592 ± 8 μm	16.24 ± 1.8	Continuous 0.45 ± 0.04 μm	Coarse
Mid	761 ± 10 μm	10.59 ± 1.6	Continuous 0.45 ± 0.04 μm	Fine

Fig. 6 EBSD results of the top and middle samples (Top and Mid in Fig. 2) in the deposited Ti64-1.2Cu wall: a, e, i, m inverse pole figure (IPF), b, f, j, n Kernel average misorientation (KAM), c, g, k, o Schmid factor (SF), and d, h, l, p pole figure (PF)

Fig. 7 TEM graphs of the top and middle samples (TE-Top and TE-Mid in Fig. 2) in the deposited Ti64-1.2Cu wall: a, e TEM bright field image showing the α and β laths, b, f TEM bright field image showing the targeted fast Fourier transformation (FFT) regions, c, g the corresponding SAED patterns from the targeted FFT regions, d, h HRTEM images of α and β interface

Table 4 Elemental composition in the phase of the top and middle regions in the Ti64-1.2Cu wall (at.%)

Top					Middle
Element	Ti	Al	V	Cu	Element	Ti	Al	V	Cu
α phase	93.69 ± 3.68	2.26 ± 0.23	3.26 ± 0.54	0.79 ± 0.23	α phase	94.56 ± 3.13	1.62 ± 0.13	3.45 ± 0.29	0.37 ± 0.13
β phase	94.06 ± 3.65	1.60 ± 0.16	3.97 ± 0.45	0.37 ± 0.23	β phase	84.44 ± 2.86	1.07 ± 0.14	10.64 ± 0.58	3.85 ± 0.38

Fig. 8 Schematic of microstructural variation: a thermal cycles, b top region (Top in Fig. 2), c middle region (Mid in Fig. 2)

Fig. 9 Results of tensile tests

Fig. 10 Fracture morphologies of Ti64-1.2Cu tensile samples: a top region (Top in Fig. 2), b middle region (Mid in Fig. 2)

Fig. 11 Thermo-Calc calculation results of Ti64 alloy with Cu addition: a solidification range, b phase composition at the 4.4 wt% Cu addition, c phase composition at the 4.5 wt% Cu addition

Fig. 12 Maximum solubility of Cu in α-Ti in the adopted Ti64-1.2Cu system

Fig. 13 TEM graphs of heat-treated Ti64-1.2Cu samples: a-c TEM bright field image showing the α laths, β laths, secondary α and dislocation, d-f TEM bright field image showing the Ti2Cu precipitation, g SAED patterns of the region I in (c), h HRTEM images of the region II in (d), i the corresponding FFT and SAED patterns of (h)

Table 5 Parameters for calculating the yield strength increment contributed by the SSS effect

Parameter	Value	Parameter	Value	Parameter	Value
f_β_{, Mid}	18.67%	B_α_{, Cu} (MPa/at.%)	27	B_β_{, Cu} (MPa/at^2/3)	1650
f_α_{, Mid}	81.33%	B_α_{, V} (MPa/at.%)	27	B_β_{, V} (MPa/at^2/3)	879
f_β_{, Top}	5.45%	B_α_{, Al} (MPa/at.%)	40	B_β_{, Al} (MPa/at^2/3)	285
f_α_{, Top}	94.55%	n_α	1	n_β	3/2
X_β_{, Mid, Cu}	3.85 at.%	X_β_{, Mid, V}	10.64 at.%	X_β_{, Mid, Al}	1.07 at.%
X_α_{, Mid, Cu}	0.37 at.%	X_α_{, Mid, V}	3.45 at.%	X_α_{, Mid, Al}	1.62 at.%
X_β_{, Top, Cu}	0.37 at.%	X_β_{, Top, V}	3.97 at.%	X_β_{, Top, Al}	1.60 at.%
X_α_{, Top, Cu}	0.79 at.%	X_α_{, Top, V}	3.26 at.%	X_α_{, Top, Al}	2.26 at.%

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