Microstructural evolution and stress state related to mechanical properties of electron beam melted Ti-6Al-4V alloy modified by laser shock peening

doi:10.1016/j.jmst.2019.11.039

Abstract

Abstract:

This work characterizes microstructural evolutions of electron beam melted (EBM) Ti-6Al-4V alloy modified via laser shock peening (LSP). The depth stress distribution and tensile properties of EBM Ti-6Al-4V alloy were measured before and after LSP. The results indicate that microstructure consists of β phase with 7.2% ± 0.4% vol.% and balance α lamellar in EBM sample, and the α lamella was refined into nano-equiaxed grains and submicro-equiaxed grains after LSP. The dominant refinement mechanism is revealed during LSP. Stacking faults were found in the LSP-treated sample, and their corresponding planes were determined as (0001) basal plane, (10$\bar{1}$0) prismatic plane, and (10$\bar{1}$$\bar{1}$) pyramidal plane obtained by high resolution transmission electron microscopy. The subgrains and high-angle grains formed during dynamic recrystallization were identified by selected area electron diffraction pattern. The LSP treatment produces a significantly residual compressive stress approximately -380 MPa with the depth of compressive stress layer reaching 450 μm. Strength and elongation of the EBM sample were significantly increased after LSP. The strength and ductility enhancements are attributed to compressive stress, grain refinement and grain gradient distribution of α phase.

Key words: Electron beam melting, Ti-6Al-4V alloy, Laser shock peening, Microstructural evolution, Stress state, Mechanical properties

Liang Lan, Xinyuan Jin, Shuang Gao, Bo He, Yonghua Rong. Microstructural evolution and stress state related to mechanical properties of electron beam melted Ti-6Al-4V alloy modified by laser shock peening[J]. J. Mater. Sci. Technol., 2020, 50: 153-161.

Figures/Tables 17

Table 1 Compositions of received Ti-6Al-4V powder.

Al	V	C	Fe	O	N	H	Ti
6.0	4.0	0.03	0.1	0.1	0.01	<0.003	Bal.

Fig. 1. Ti-6Al-4V powder used in the EBM process at low (a) and high (b) magnification.

Fig. 2. Schematic of LSP principle for EBM sample.

Fig. 3. OM images of (a) top, (b) side view, and (c) corresponding SEM image of Ti-6Al-4V titanium alloy samples prepared by EBM.

Fig. 4. XRD patterns obtained from EBM Ti-6Al-4V sample before (a) and after (b) LSP.

Table 2 EDS analysis of the β-Ti and α-Ti phases (wt%).

Phase	Al	V	Ti
α-Ti	6.84	2.37	90.79
β-Ti	4.34	12.85	82.81

Fig. 5. TEM images of microstructure and corresponding SAED patterns in the EBM Ti-6Al-4V specimen before (a-c) and after (d-f) LSP.

Fig. 6. In-depth residual stress distribution of EBM Ti-6Al-4V sample before and after LSP.

Fig. 7. Engineering stress-strain curves before and after LSP.

Table 3 Mechanical properties of EBM Ti-6Al-4V samples before and after LSP.

Property	Before LSP	After LSP
UTS (MPa)	916.00 ± 12.73	961.00 ± 6.16
YS (MPa)	817.00 ± 5.66	830.30 ± 5.79
El (%)	9.30 ± 1.56	10.47 ± 0.99

Fig. 8. SEM images of fracture morphologies of EBM Ti-6Al-4V samples before (a, b) and after (c, d) LSP.

Fig. 9. Perfect dislocations in EBM sample before LSP: (a) individual dislocations; (b) GBDs and IDs.

Fig. 10. TEM images of stacking faults and perfect dislocations in nanograin region (a, c) and submicro-grain region (b, d) in EBM sample after LSP.

Fig. 11. HRTEM image and corresponding FFT of (0001) basal plane stacking faults (a), (100) prismatic plane stacking faults (b) and (10) pyramidal plane stacking faults (c).

Fig. 12. TEM identification of subgrains in submicro-scale grains region: (a) bright field image; (b) SAED pattern; (c-e) dark field images by operation of different diffraction spots.

Fig. 13. Formation of nanograins in the surface layer: (a) bright field image; (b) diffraction rings of polycrystal; (c, d) dark field images of nanograins from the recrystallization of different deformation twins; (e) dark field image of nanograins from recrystallization of α matrix.

Fig. 14. Schematic of the microstructural evolution of the EBM sample before (a) and after (b) LSP.

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