Tailoring grain morphology in Ti-6Al-3Mo through heterogeneous nucleation in directed energy deposition

doi:10.1016/j.jmst.2021.01.056

Abstract

Abstract:

A common challenge in direct energy deposition (DED) is eliminating the anisotropy in mechanical performance associated with microstructure and the formation of coarse columnar grains. In this work, a heterogeneous nucleation mechanism was introduced into the melt pool, and, from this mechanism, an almost fully equiaxed grain morphology was obtained in the DED of Ti-6Al-3Mo. Three types of grain morphologies in DED Ti-6Al-3Mo, including full columnar grains, near-equiaxed grains and almost fully equiaxed grains were obtained from premixed and satellite powder blends from Ti, 6 wt.% Al and 3 wt.% Mo, respectively. Combined with the analysis of the interactions between powder particles and the melt pool in DED, the formation mechanism of the equiaxed grains caused by the incomplete melting of high melting point Mo particles was revealed. As the prior-β grains transformed from coarse columnar grains to fine-equiaxed grains, the strong <100> fiber texture along the deposition direction was weakened, while the size of the α-laths in the prior-β grains slightly decreased, and the selection of α-variants was weakened. Due to the transformation of the prior-β grains from coarse columnar grains to fine-equiaxed grains, the tensile strength of the deposited samples increased from 982 MPa to 1082 MPa, while the yield strength increased from 840 MPa to 922 MPa, and the elongation of the as-deposited alloy also increased from 9.0 % to 9.8 %, which confirmed that the presence of fine-equiaxed grains is beneficial to the strength and plasticity of the DED alloy. This work further demonstrates the role that satelliting powders can play in terms of enhancing the columnar to equiaxed transition (CET) behavior associated with DED.

Key words: Direct energy deposition, Titanium alloy, Grain morphology, Microstructure, Mechanical properties

Fengying Zhang, Panpan Gao, Hua Tan, Yao Li, Yongnan Chen, Min Mei, Adam T. Clare, Lai-Chang Zhang. Tailoring grain morphology in Ti-6Al-3Mo through heterogeneous nucleation in directed energy deposition[J]. J. Mater. Sci. Technol., 2021, 88: 132-142.

Figures/Tables 14

Fig. 1. SEM morphologies of different powders used for DED Ti-6Al-3Mo alloy: (a) Ti, (b) Al, (c) Mo, (d) directly mixed Ti +6 wt.% Al +3 wt.% Mo powder blends, (e) and (f) satellite Ti +6 wt.% Al +3 wt.% Mo powder blends.

Fig. 2. Schematic of the DED process and the fabricated samples showing the outline of the associated tensile test specimen.

Table 1 DED process parameters used for Ti-6Al-3Mo alloy.

Experiment	Laser power (W)	Scanning speed (mm s^-1)	Spot diameter (mm)	Powder feeding rate (g min^-1)	Carrier gas flow (L min^-1)	Z axis increment (mm)	Powder used
E1	1800	10	4	6	7	0.3	Directly mixed powder
E2	1500	8	4	20	10	0.8	Directly mixed powder
E3	1500	8	4	20	10	0.8	Satellite powder

Fig. 3. The microscopic solidification microstructure of the DED Ti-6Al-3Mo samples prepared by: (a-d) E1 experiment from directly mixed Ti +6 wt.% Al +3 wt.% Mo, (e-h) E2 experiment from directly mixed Ti +6 wt.% Al +3 wt.% Mo, and (i-l) E3 experiment from satellite Ti +6 wt.% Al +3 wt.% Mo.

Fig. 4. Grain size distribution of DED Ti-6Al-3Mo alloy obtained in E1, E2 and E3, respectively.

Fig. 5. EBSD results for the E3 sample with equiaxed grains: (a) IPF map along the deposition direction for the α phase, (b) the misorientation distributions of α-phase in the scanning region, (c) IPF map along the deposition direction for the reconstructed β grains, and (d) the pole figures of the α-phase and β-phase.

Fig. 6. Schematic diagram showing the penetration of Mo into lower temperature regions within the melt pool and failing to melt completely.

Fig. 7. The influence of the powder particle size on the particle speed after entering the melt pool.

Fig. 8. The calculated results of the position and the corresponding running speeds of the powder particle with the diameter range from 5-100 μm during the process of breaking through the gas/liquid interface of the melt pool: (a) 5 μm, (b) 20 μm, (c) 40 μm, (d) 60 μm, (e) 80 μm, and (f) 100 μm.

Fig. 9. EDS mapping results of unmelted Mo particle in the deposited sample from satellite Ti +6 wt.% Al +3 wt.% Mo powder in E3 experiment: (a) SEM, (b) Ti, (c) Al, and (d) Mo.

Fig. 10. Microstructure in the prior-β grains in different experiments: (a, b) columnar grains obtained from E1 experiment, (c, d) near-equiaxed grains obtained from E2 experiment, and (e, f) equiaxed grains obtained from E3 experiment.

Fig. 11. EBSD analysis of DED Ti-6Al-3Mo samples from E1, E2 and E3 experiments: (a-d) IPF map along the deposition direction, pole figures and the proportion of the α variants of E1 sample, (e-h) IPF map along the deposition direction, pole figures and the proportion of the α variants of E2 sample, and (i-l) IPF map along the deposition direction, pole figures and the proportion of the α variants of E3 sample.

Table 2 Average tensile properties of DED Ti-6Al-3Mo alloy from E1, E2 and E3 experiments.

Sample	Tensile strength (MPa)	Yield strength (MPa)	Elongation (%)
E1	982	840	9.2
E2	1120	1021	8.9
E3	1082	922	9.8

Fig. 12. Fracture morphology of tensile specimens for different experiments: (a, b) E1 sample, (c, d) E2 sample, and (e, f) E3 sample.

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