Professor Liu Jing's Team At Wuhan University Of Science And Technology: The Effect Of Wire Arc Additive Manufacturing On The Fatigue Behavior Of Ta15 Titanium Alloy

TA15 titanium alloy (Ti-6Al-2Zr-1Mo-1V) is a near-α-type medium-strength titanium alloy with high aluminum content, which has been widely used in aerospace technology and other fields. As the components required for aircraft become more and more complex, traditional forging methods limit the development and use of complex parts. Wire arc additive manufacturing (WAAM) technology can directly manufacture fine and complex structural parts. The aerospace field attaches great importance to it, and researchers have conducted a lot of research on it. In practical applications, the flight state of the aircraft is constantly changing, and the stress on the components is also changing all the time. Under the state of alternating stress load, fatigue failure is one of the important reasons leading to accidents. Therefore, the research on the fatigue properties of TA15 titanium alloy manufactured by wire arc additive manufacturing has important guiding significance and application value in practical applications.

At present, the research on the tensile mechanical properties and fatigue properties of TA15 titanium alloy is relatively extensive. LIU Y et al. studied the laser additive manufacturing of TA15-Ti2AlNb alloy and found that after annealing, a fine basket-like structure was obtained. performance. Ren Yuhang et al. studied the fatigue crack propagation behavior of laser-deposited TA15 titanium alloy, and found that the structure after heat treatment is a basket structure, and the fatigue crack growth path is tortuous, mainly parallel or perpendicular to the α-sheet propagation, and a large number of secondary cracks appear . The raw material of selective laser melting is powder, and the heat source is laser, while the raw material of wire arc additive manufacturing is wire material, and the heat source is electric arc. SZOST BA et al. studied the microstructure of Ti-6Al-4V alloy produced by WAAM process, and found that coarse columnar β grains were found at the bottom of the molten pool, which seriously affected the mechanical properties of the material, while the fine β grains And acicular structure will improve the mechanical properties of the material. In order to control the anisotropy of mechanical properties caused by the coarse original β grains, DONOGHUE J et al. added in-situ rolling to the WAAM process, which can significantly refine only the original β grains and improve the comprehensive mechanical properties of the material. Li S et al. obtained the needle-like basket structure of TA15 titanium alloy at 990°C for 1 h and then air-cooled. It was found that when the stress ratio R=0.1, the lamellar structure has a higher resistance to fatigue crack growth than the two-state structure. , and when the crack propagates in the lamellar structure, the path is more tortuous, and the dual-state structure is composed of two forms of α phase, namely equiaxed structure and lamellar structure, which are evenly distributed.

In summary, different processes and heat treatments will result in different microstructures, which will lead to changes in the mechanical properties and fatigue properties of TA15 titanium alloys. TA15 titanium alloy is an important alloy for aerospace applications, and there are few studies on the fatigue characteristics of its wire-arc additive manufacturing.

The team of Professor Liu Jing from Wuhan University of Science and Technology published the article "The influence of stress level on the fatigue behavior of TA15 titanium alloy manufactured by wire arc additive manufacturing" in the journal "Special Casting and Nonferrous Alloys" in Volume 42, Issue 12, 2022. In this paper, the fatigue performance and fatigue fracture failure behavior of annealed TA15 alloy were studied by in-situ rolling wire arc additive manufacturing of TA15 alloy. The results show that the rolling and longitudinal microstructures of the annealed TA15 alloy are dominated by basket microstructures formed by overlapping α/β sheets, and equiaxed grain distribution exists locally. By fitting the S-N curve, the fatigue limit σf of TA15 titanium alloy is 311 MPa. The fracture of the sample includes the fatigue crack source area, the fatigue crack growth area and the transient fracture zone. The cracks are all initiated from the surface. At the same time, there are a large number of fatigue striations and secondary cracks in the crack propagation zone, and the transient fracture zone is composed of equiaxed dimples. , showing typical quasi-cleavage fracture characteristics. Under low cyclic stress, the crack mainly propagates through or along the α/β lamellar structure, and the crack propagation path is longer; under high cyclic stress, the crack begins to propagate along the original β grain boundary, and the secondary crack propagation path is shorter. Short but high in number. Fatigue cracks tend to propagate along relatively weak positions in the tissue, and there are crack branches around the secondary cracks.

【Test materials and methods】

TA15 titanium alloy wire is selected as the raw material, and the deposition process is carried out on a custom-made flat rolling mill. The whole system is controlled by a computer, and the roll rolls on the surface of the sample along with the arc at the front end according to the trajectory specified by the arcing signal. The chemical properties of the sample The ingredients are shown in Table 1. The size of the prepared plate is 180 mm × 160 mm × 8 mm, as shown in Figure 1a. Heat treatment was carried out in a vacuum furnace with an inert protective gas, the annealing process was 800 °C × 40 min, and air cooling. The phase analysis of the TA15 sample was carried out by ZSXPrimus IV wavelength dispersive X-ray fluorescence spectrometer with a slow sweep of 2°. The yield strength of the sample was measured to be 891 MPa, and the elongation was 12%. Samples were taken along the X direction on the XOY plane, and the fatigue sample size is shown in Figure 1b. The fatigue test was carried out on the INSTRON 8801 fatigue testing machine. Five groups of different stress levels were selected and carried out at dry room temperature. Sine wave loading was adopted, the stress ratio R=0.1, and the loading frequency f=15Hz. Cut the metallographic sample near the fracture surface and observe the microstructure, then inlay, pre-grind and polish the sample, and use Kroll etchant (HF-HNO3-H2O volume ratio 1:6:7) for corrosion; A Zeiss Axioplan 2 imaging optical microscope was used to observe the microstructure and a Nova 400 Nano field emission scanning electron microscope equipped with X-ray energy dispersive analysis (EDS) was used to observe the sample microstructure, fatigue fracture morphology and crack propagation path, and analyze the fatigue fracture mechanism.

Figure 1 WAAM forming TA15 titanium alloy original slab

【Graphic results】

The spectrum of TA15 titanium alloy mainly presents peaks related to the hexagonal close-packed (hcp-Ti) crystal structure, which are all α phases, and there are peaks corresponding to high-temperature β phases of body-centered cubic (bcc), with low peak intensity.

Fig.2 XRD diffraction pattern of TA15 titanium alloy annealed sample formed by WAAM

After annealing, the TA15 titanium alloy formed by WAAM is mainly a basket-like lamellar structure, and at the same time, some equiaxed grains are aggregated and distributed in the same original β grain.

Fig.3 Metallographic microstructure of annealed TA15 titanium alloy formed by WAAM

(a) Longitudinal (b) Rolling to (c) Enlarged at 1 place (d) Enlarged at 2 places

(a) Original β grains (b) Lamellar structure (c) Inhomogeneous structure (d) Equiaxed grains

Fig.4 Rolled SEM microstructure of WAAM formed TA15 titanium alloy in annealed state

                                                   (a) lamellar structure (b) Mo element (c) V element (d) Ti element (e) Al element (f) Zr element

Fig.5 Lamellar structure and EDS analysis of annealed TA15 titanium alloy formed by WAAM

Usually, deformation in the α+β two-phase region can obtain an equiaxed structure, and then recrystallization annealing above the two-phase region can obtain a titanium alloy containing a two-state structure, which has better strength and plasticity. However, compared with the two-state structure, the TA15 alloy with lamellar structure will have a higher fatigue limit, which is related to the higher fatigue crack growth resistance of the lamellar structure. The smaller original β grains and fine lamellar structure in the sample can improve the fatigue strength of titanium alloy, obtain higher yield strength and the ability to resist fatigue crack growth. TA15 titanium alloy formed by WAAM is a typical near-α-type titanium alloy with high thermal stability and significantly improved fatigue strength compared with traditional cast TA15 alloy. Through investigation, it was found that TA15 titanium alloy prepared by laser deposition and traditional forging process The alloy will have a relatively high fatigue limit (200~650MPa), but laser deposition can only prepare small-sized components, while traditional forging cannot prepare complex large-scale components well. When the service temperature reaches 400-500 ℃, the performance advantages of TA15 titanium alloy are obvious, and the strength is improved.

Fig.6 Fatigue S-N curves of TA15 titanium alloy in annealed state formed by WAAM

As the load decreases, a large number of secondary cracks appear in the fracture of TA15 titanium alloy, and the fatigue striations gradually increase, the spacing decreases, and the prolongation of life is mainly reflected in the increase of the fatigue crack source area and crack propagation area.

Fracture is divided into fatigue crack source zone, fatigue crack growth zone and transient fracture zone. Under low cyclic stress, the crack mainly propagates through or along the lamellar structure, and the secondary crack propagation path is long and tortuous; under high cyclic stress, the crack begins to propagate along the original β grain boundary, and the secondary crack propagation path is relatively slow Short but high in number. These secondary cracks can consume the energy of the crack tip in time, and can effectively delay the fatigue crack growth rate of TA15 titanium alloy.

Fig.7 Fatigue fracture morphology of TA15 titanium alloy in annealed state formed by WAAM

(a) 800MPa (b) 668MPa (c) 490MPa

Fig.8 Fracture morphology of the fatigue source zone in the annealed state of TA15 titanium alloy formed by WAAM

(a) 668 MPa (b) Enlarged at A (c) 579 MPa (d) Enlarged at B

Fig.9 Fracture morphology of the fatigue crack propagation zone in the annealed state of TA15 titanium alloy formed by WAAM

(a) 757 MPa, (b) 668 MPa, (c) 579 MPa, (d) 490 MPa (e) long secondary crack (f) short secondary crack

Fig.10 Fracture morphology of fatigue transient fracture zone of TA15 titanium alloy formed by WAAM in annealed state

(a) 757 MPa, (b) 668 MPa, (c) 490 MPa

Fig.11 Morphology of the crack propagation zone on the fatigue fracture side of TA15 titanium alloy formed by WAAM in the annealed state

(a) The main crack passes through the lamellar structure, 490MPa (b) The main crack propagates along the β grain boundary, 668MPa (c) The main crack deflects and crack branches, 490MPa (d) The crack branch, 668MPa (e) The main crack deflects and Secondary crack, 490MPa (f) Main crack propagates along lamellar structure and secondary crack, 490MPa

(a) Low cyclic stress, (b) High cyclic stress

Fig.12 Schematic diagram of fatigue crack growth mechanism of TA15 titanium alloy formed by WAAM in annealed state