Microstructure Evolution and Residual Stress Redistribution in Selective Laser Melted TA15 Titanium Alloy Under Severe Shot Peening Treatment

doi:10.1007/s40195-025-01909-y

Abstract

Abstract:

A gradient nanostructured layer was fabricated on the surface of TA15 (Ti-6Al-2Zr-1Mo-1V) alloy (produced by selective laser melting) using severe shot peening (SSP). This study focuses on the evolution of the microstructure and the mechanism of grain refinement in TA15 titanium alloy during SSP treatment. Transmission electron microscopyand Rietveld refinement methods were employed. The residual stress and microhardness variations with depth were also characterized. The results show: (1) At the initial stage of deformation, plastic deformation is primarily accommodated through twinning and dislocation slip. (2) As the strain increases, twinning disappears, and dislocations interact to form tangles. Some dislocations annihilate and rearrange into subgrain boundaries, subdividing the original grains into subgrains. (3) With continued dislocation activity, the subgrain size decreases until nanocrystals are formed through the dynamic rotational recrystallization. SSP introduced compressive residual stress (CRS) in the near-surface layer of the material, with the maximum CRS of approximately −1141 MPa observed in the subsurface layer. It also induced work hardening, increasing the surface hardness to approximately 479 HV. However, the surface roughness increases, leading to a slight deterioration in surface quality.

Key words: Severe shot peening, Selective laser melting, TA15, Residual stress, Gradient nanostructured layer, Microstructure

Ang Yin, Wenbo Li, Chengxi Wang, Vincent Ji, Chuanhai Jiang. Microstructure Evolution and Residual Stress Redistribution in Selective Laser Melted TA15 Titanium Alloy Under Severe Shot Peening Treatment[J]. Acta Metallurgica Sinica (English Letters), 2025, 38(11): 1953-1964.

Figures/Tables 14

Fig. 1 Morphology and particle size distribution of TA15 powder

Table 1 Chemical composition of TA15 powder (wt%)

	Ti	Al	V	Zr	Mo	Si	Fe
Min	Bal	5.50	0.80	1.50	0.50	—	—
Act	Bal	6.55	1.92	1.92	1.46	0.02	0.027
Max	Bal	7.10	2.50	2.50	2.00	0.15	0.25

Fig. 2 Schematic diagrams of a SSP treatment and b scanning strategy

Table 2 Parameters for residual stress and XRD measurement

	S₁ (MPa⁻¹)	S₂/2 (MPa⁻¹)	Voltage (kV)	Current (mA)	Step (°)	Scan speed (°/s)
XRD	-	-	40	30	0.02	2
Residual stress	2.83 × 10^-6	11.88 × 10^-6	30	25	-	-

Fig. 3 Surface morphology of the sample: a 3D morphology, b SEM images

Fig. 4 XRD patterns of the sample under different depths

Fig. 5 Fitting results by Rietveld refinement (50 μm)

Fig. 6 a Average domain size and microstrain of the sample at different depths, derived from the fitting results of the Ti (101) planes, b lognormal distribution of domain size at various depths, with the Y-axis representing the number fraction

Fig. 7 TEM images at about 250 µm below the treated surface: a bright field, b dark-field TEM image of deformation twinning corresponding to a, c corresponding SAED patterns of deformation twinning in b, d β-Ti, e EDS mapping of d

Fig. 8 TEM images at different depths below the treated surface: a, b 180 μm; c, d 150 μm; e SAED pattern obtained from the region enclosed by the white circle in d

Fig. 9 TEM images at different depths below the treated surface: a 110 μm; b 80 μm; c 50 μm; d 30 μm; e 15 μm; f dark field of e. The SAED patterns in c-e are obtained from the region enclosed by the white circles

Fig. 10 HRTEM images at about 15 µm from the treated surface: a, b bright field and corresponding fast fourier transform images; c schematic diagram of diffraction spots in b; d inverse fast fourier transform of (020) crystal plane of FCC-Ti; e diagram of interplanar spacing measurement

Fig. 11 Grain refinement mechanism of SLM-TA15 alloy during SSP

Fig. 12 Depth distributions of residual stress and microhardness after SSP

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