Effect of Microstructural Characteristics on Fracture Toughness in Direct Energy Deposited Novel Ti-6Al-4V-1Mo Alloy

doi:10.1007/s40195-023-01650-4

Abstract

Abstract:

Meeting the damage tolerance requirements for engineering-grade titanium alloys pose a significant challenge in achieving high fracture toughness in direct energy deposition (DED) titanium alloys. This work primarily investigated the relationship between the microstructure and the fracture toughness of DED new Ti-6Al-4V-1Mo alloy. Two types of microstructures were designed via two process strategies: high-line energy density (HE) and low-line energy density (LE). Relative to LE samples, HE samples possess larger-sized microstructural characteristics (coarser grain boundary α (α_GB), larger α colonies, and coarser α laths). Less α/β phase boundaries were formed by coarser α laths in the HE samples, increasing the movement of dislocations, resulting in tensile strength decreasing from 1007.1 MPa (LE) to 930.8 MPa (HE) and elongation increasing from 10.8% (LE) to 15.7% (HE). Also, HE samples exhibited an excellent fracture toughness of 114.0 MPa m^1/2, significantly higher than that of LE samples (76.8 MPa m^1/2). An analysis of crack propagation paths was conducted to investigate the factors contributing to toughening. The primary factor enhancing toughness is the frequent obstruction of cracks by coarse α_GB and large α colonies in HE samples. Particularly, the pretty large-angle deflections induced by the superposition effect of coarse α_GB and large α colonies play a vital of significant role. These factors induced the long and tortuous high-energy pathways, which resulted in ultimately improved fracture toughness. The discovered microstructural toughening mechanisms can serve as a reference for future studies involving titanium alloys, offering insights on how to enhance fracture toughness by achieving similar characteristics.

Key words: Direct energy deposition, Ti-6Al-4V-1Mo, Tensile properties, Fracture toughness, Crack propagation

Chao Xia, Kexin Zhao, Xin Zhou, Yuqi He, Panpan Gao, Hengxin Zhang, Guangrui Gao, Fengying Zhang, Hua Tan. Effect of Microstructural Characteristics on Fracture Toughness in Direct Energy Deposited Novel Ti-6Al-4V-1Mo Alloy[J]. Acta Metallurgica Sinica (English Letters), 2024, 37(1): 119-131.

Figures/Tables 12

Fig. 1 Powder material, DED process, deposited samples, and the tested samples: a powder morphology and distribution diagram of powder particle size; b schematic diagram of DED process and deposited samples; c schematic diagram of sample position, size and appearance of tensile sample and C(T) sample

Table 1 DED process parameters used for Ti-6Al-4V-1Mo alloy

Process strategies	Laser power (W)	Scanning speed (mm min⁻¹)	Spot diameter (mm)	Powder feeding rate (g min⁻¹)	Carrier gas flow rate (L min⁻¹)	Z axis increment (mm)	Overlap ratio (%)
1	3800	950	7	20	10	0.9	50
2	1800	750	4	10	10	0.5	50

Fig. 2 OM images of Morphologies of prior β grains in Ti-6Al-4V-1Mo alloy under two process parameters of high-line energy density and low-line energy density: a, b HE samples; c, d LE samples

Fig. 3 SEM images of microstructural characteristics in the grain of Ti-6Al-4V-1Mo alloy obtained under high-line energy density and low-line energy density: a-c HE samples; d-f SEM images of LE samples

Fig. 4 The α lath width and α colony width distribution diagram of Ti-6Al-4V-1Mo alloy under two process parameters of high-line energy density and low-line energy density: a distribution of α lath width in HE samples; b distribution of α colony width in HE samples; c distribution of α lath width in LE samples; d distribution of α colony width in LE samples

Fig. 5 Mechanical properties: a tensile properties of Ti-6Al-4V-1Mo alloy with tow processes; b yield strength and fracture toughness of reported additive manufactured Ti-6Al-4V alloy, high-toughness titanium alloys, Ti-6Al-4V-xFe alloys, and the DED Ti-6Al-4V-1Mo alloy in this work; c engineering stress-strain curves of LE and HE samples; d true stress-strain curves of LE and HE samples

Fig. 6 Fracture morphologies of DED Ti-6Al-4V-1Mo alloy for C(T) samples obtained under high-line energy density and low-line energy density: a-d HE samples; e-h LE samples

Fig. 7 Dimple size distribution of C(T) samples obtained under high-line energy density and low-line energy density: a HE samples; b LE samples

Fig. 8 Three-dimensional crack propagation path for C(T) sample fracture of DED Ti-6Al-4V-1Mo alloy obtained under two process parameters, the extracted line contour position and the corresponding main crack path: a three-dimensional crack propagation path of HE sample; b three-dimensional crack propagation path LE sample; c the position of four selected contour lines in HE sample; d the position of four selected contour lines in LE sample; e four main crack paths in HE sample; f four crack paths in LE sample

Fig. 9 Length and λ of the main crack path corresponding to the four line contours of DED Ti-6Al-4V-1Mo alloy obtained under high- and low-line energy density: a the length of the main crack; b the value of λ

Fig. 10 Crack propagation paths and the microstructural characteristics near the main crack in the fracture of DED Ti-6Al-4V-1Mo alloy obtained under high- and low-line energy density: a crack propagation paths of HE samples; b-f effect of microstructural characteristics on the crack in HE samples; g micro-void near the main crack of HE sample; h crack propagation paths of LE samples; i, j effect of microstructural characteristics on the crack in LE samples; k micro-void and microcrack near the main crack of LE sample

Fig. 11 Schematic diagram of the effect of different microstructure characteristics on crack deflection and the crack propagation model based on the correlation between crack propagation path and microstructure: a, b the effect of αGB; c, d the effect of α colonies; e combined effect of αGB + large α colonies; f, g the effect of α laths; h, i crack propagation model based on high and normal fracture toughness in DED Ti-6Al-4V-1Mo alloy

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