PSI - Issue 13

Yanzeng Wu et al. / Procedia Structural Integrity 13 (2018) 890–895

891

Yanzeng Wu et al. / Structural Integrity Procedia 00 (2018) 000 – 000

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Studies show that the static performance of many LMD titanium alloys is found to be reasonably comparable in terms of the static properties to traditional wrought materials. By reason of the advantages of the LMD technology there is a strong tendency to apply it even where not only the static but also fatigue properties are decisive for reliable and long term operation. Due to the thermal effect on the previous deposited layers executed by the melting pool during the LMD process, the heat affected bands (HAB) exists in the deposited layers, which lead to a periodic fluctuation in the fatigue crack growth rate (Lu et al. (2017)). The crack tip opening displacement (CTOD), as a classical parameter in elastic-plastic fracture mechanics, has an importance for fatigue crack growth analysis. It was demonstrated that there is a relationship between CTOD and the crack growth rate (FCGR). Shahani et al. (2009) proposed a relation between FCGR and CTOD similar to Paris law. Antunes et al. (2017) found a relation between FCGR and the plastic CTOD range independent of stress ratio. Thus, in this work an in-situ optical microscopy fatigue testing combined with the digital image correlation (DIC) technique is conducted to investigate the CTOD variation within one cyclical loading with and without the influence of HAB in laser melting deposited Ti-6.5Al-3.5Mo-1.5Zr-0.3 titanium alloy (named TC11 in China). 2. Experimental methods In-situ fatigue tests were performed to achieve the high resolution investigation for the CTOD variation , some details of the experimental set-up for the in-situ optical microscopy experiment are shown in Fig. 1.

Fig. 1. In-situ experimental setup

2.1. Specimen and microstructure The LMD system was used to fabricate plated-like TC11 samples in fine powder form with globular particles 100 - 300 μm diameter range. Two clamped single edge notched tensile (SENT c ) specimens (Fig. 2a) with width W =8 mm, length L =56 mm and thickness T = 1 mm fabricated from the LMD TC11 sample were tested at constant amplitude (CA) fatigue loading. Before the fatigue crack growth tests, the specimens were polished and then etched with Kroll's reagent, which is easy to observe the macrostructure during the in-situ fatigue testing. The edge notch of length 1 mm was processed on specimen by wire electrical discharge machining (WEDM), and then the specimens were pre-cracked under a servo-hydraulic fatigue test system until the initial crack length reaches about 2 mm. Following this, the specimen is loaded in a 5 kN in-situ fatigue stage under cyclic loading and observed under the optical microscope (OM). All fatigue tests were conducted at the stress ratio of 0.1 applying 150 MPa as maximum stress level. To ensure the crack growth is stable, the measurements were carried out after about 2000 cycles loading in the in-situ fatigue stage until the crack length is equal to 2.40 mm. To study the effect of HAB on CTOD, the crack tips with the same crack lengths in the two specimens are located within different zones of macrostructure: case 1 for crack tip in HAB zone (Fig. 2c) and case 2 for crack tip in non-HAB zone (Fig. 2d). The metallographic structures for HAB and non-HAB zone are shown in Fig. 3a and 3b respectively. As for the HAB, ultrafine basket-weave microstructure forms with superfine lamellar α and β , which is related with the rapid cooling rate during the LMD process. However, a special bimodal microstructure is absent in the non-HAB zone, which is clearly distinguishable from that of the HAB. In comparison, the acicular α of the large α colonies within HAB is much finer and the volume fraction of α phase within HAB is also higher than that within non-HAB zone.