Cracking behavior and control of β-solidifying Ti-40Al-9V-0.5Y alloy produced by selective laser melting

doi:10.1016/j.jmst.2019.08.026

Abstract

Abstract:

A β-solidifying Ti-40Al-9V-0.5Y (at.%) alloy with a high cracking sensitivity has been successfully fabricated by selective laser melting (SLM) in this study. The influence factors for cracking sensitivity, cracking behavior and crack inhibition mechanism were investigated. The results show that the effects of process parameters on cracking sensitivity strongly depend on the cooling rate in molten pool with different heat transfer modes. The conduction mode with higher cooling rates exhibits a higher cracking sensitivity in comparison to the keyhole mode. Microstructure characteristics and phase transformations controlled by cooling rate determine the inherent ductility of β-solidifying γ-TiAl alloys during SLM. On this basis, the formation and inhibition mechanism of solidification and cold cracking are proposed. Finally, the crack-free Ti-40Al-9V-0.5Y sample with fine equiaxed microstructures and favorable mechanical properties (microhardness of 542 ± 19 HV, yield strength of 1871 ± 12 MPa, ultimate strength of 2106 ± 13 MPa and ultimate compressive strain of 10.89 ± 0.57%) can be produced by SLM. The strengthening mechanism can be attributed to grain refinement and precipitation strengthening.

Key words: Selective laser melting, β-solidifying γ-TiAl alloy;, Cracking behavior, Cracking control, Microstructure, Phase transformation

Piao Gao, Wenpu Huang, Huihui Yang, Guanyi Jing, Qi Liu, Guoqing Wang, Zemin Wang, Xiaoyan Zeng. Cracking behavior and control of β-solidifying Ti-40Al-9V-0.5Y alloy produced by selective laser melting[J]. J. Mater. Sci. Technol., 2020, 39: 144-154.

Figures/Tables 17

Fig. 1. Ti-40Al-9V-0.5Y powders: (a) particle morphology, (b) particle size distribution.

Table 1 Processing parameters applied in experiments.

Parameters	Value
Laser power P (W)	200
Scanning velocity V (mm/s)	100-1000, with a step of 100
Hatch spacing S (mm)	0.08
Layer thickness δ (μm)	50
Phase angle (degree)	90

Fig. 2. Measure methods to determine the solidification time of liquid-state molten pools. (a) The laser melts powders at point A (t0); (b) point A is still in liquid state as the laser moves to point B at t0+14.71 ms; (c) point A just solidifies at t0+22.79 ms; (d) the solidified state of point A remains unchanged at t0+26.94 ms.

Table 2 The average ts under different scanning velocities.

V (mm/s)	100	200	300	400	500	600	700	800	900	1000
t_s (ms)	39.75± 1.29	22.79 ± 5.35	3.53 ± 0.25	2.92± 0.13	2.49 ± 0.09	2.14 ± 0.07	1.26 ± 0.04	1.03 ± 0.06	0.70 ± 0.02	0.57 ± 0.05

Fig. 3. Surface morphologies, metallographic photographs and molten pool characteristics of the cylinder samples deposited at different scanning velocities. (a, d, g) 100 mm/s≤V≤200 mm/s; (b, e, h) 200 mm/s<V≤500 mm/s; (c, f, i) 500 mm/s<V≤1000 mm/s. (Note that red dotted line represents the boundary of molten pool.).

Fig. 4. Crack density and porosity of SLM-processed samples deposited at different scanning velocities.

Fig. 5. Aspect ratios of molten pools deposited at different scanning velocities (the insert displays the corresponding depth and width of molten pools).

Table 3 Thermal-physical properties of γ-TiAl alloy.

Thermo-physical parameters	Value	Ref.
Density ρ (kg/m³)	3800	[24]
Thermal conductivity K (W·m^-1·K^-1)	14.9	[24]
Specific heat capacity C (J·kg^-1·K^-1)	650	[24]
Boiling point T_b (K)	3315	[6]
Laser absorption coefficient A	0.25	[25]

Fig. 6. Cooling rates in molten pools under different scanning velocities. ($\dot{T}_{max}$: the cooling rate in the center of molten pool.).

Fig. 7. Microstructures and phase characteristics of molten pools for cracking and crack-free samples. (a, d) SEM images (red dotted line represents the boundary of molten pool); (b, e) Inverse pole figures (IPFs) for zones marked by the blue and yellow rectangles, respectively; (c, f) phase maps corresponding to b and e, respectively. (Note that red, blue, and yellow represents the B2, α2 and γ phases, respectively.).

Fig. 8. (a) Section of Ti-Al binary phase diagram (the red dotted line shows the solidification path of Ti-40Al); (b) XRD spectra of the crack-free and cracking cylinder samples.

Fig. 9. EPMA maps of element distribution for the crack-free sample.

Fig. 10. Pole figures (PFs) corresponding to the zones marked by the yellow rectangle in Fig.7(d) for the crack-free sample. (a) β0-TiAl (B2) phase; (b) α2-Ti3Al phase; (c) γ-TiAl phase.

Fig. 11. Schematic illustration of cracking behavior and crack inhibition mechanism for SLM-processed Ti-40Al-9V-0.5Y alloy.

Fig. 12. Microhardness distributions in the directions of X, Y and 45° for the crack-free sample deposited at 200 mm/s.

Fig. 13. (a) Compression stress-strain curves of the three crack-free Ti-40Al-9V-0.5Y samples at 200 mm/s. (b) Compression properties of the crack-free SLM-processed Ti-40Al-9V-0.5Y alloy, the as-cast and SLM-processed TNM alloys.

Fig. 14. SEM images of compression fracture surface for the crack-free sample at (a) a low magnification, and (b) a high magnification.

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