PSI - Issue 46

Raviraj Verma et al. / Procedia Structural Integrity 46 (2023) 175–181 Raviraj Verma/ Structural Integrity Procedia 00 (2021) 000–000

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The LPBFed Ti-6Al-4V alloy is subjected to cyclic loading with different combinations of stress ratios and maximum stresses, which have enabled understanding the cyclic load response of materials. The results are separated for stress ratios, R, -1 as completely reversed loading (Fig. 3(a)), 0 as repeated loading (Fig. 3(b)), and 0.1 as fluctuating loading (Fig. 3(c)) applied at a frequency of 10 Hz. The completely reversed loading scenario has shown the most depreciating fatigue response of the material in which endurance at a maximum stress value of 340 MPa is not attained. The repeated and fluctuating loading could quickly enter the HCF regime and show an endurance limit above 550 MPa with its mathematical asymptotic behaviour as observed from Fig. 3(b) and (c), respectively. This signifies those microstructural characteristics of LPBFed Ti alloys might not be favorable for compressive cyclic loading conditions. This conclusion is also realized while looking 0 R and 0.1 R on S-N curve in Fig. 3(d). It is observed that tension-tension loading is more suitable with LPBFed material due to the significant amount of � martensitic phase and suitable residual stresses present in the alloy. The martensitic phase can only be evolved at rapid solidification rate below β-transus limit in Ti-6Al-4V alloy. However, in case of repeated and fluctuating loading, the material withstands either at zero or tensile regime and exhibit improved fatigue strength. The fatigue strengths are approx. 560 MPa and 600 MPa corresponding to 0 R and 0.1 R stress ratios, respectively. This maximum stress value is well below the yielding limit of 912 MPa and hence, it is justifiable to model by Basquin’s Law stress life criteria. This shows a huge difference of at least 220 MPa between completely reversed and repeated loading. The simulation results assume material as pure homogeneous, but the approximation is through material properties influenced by its microstructure. Also, the experimentally examined microstructure has recorded acicular martensitic phases of hcp and metastable � (Borrego et al. 2018).

Fig. 3. The fatigue behaviour of LPBeded Ti-6Al-4V alloy at varying stress ratios, (a) completely reversed cycle at R = -1, (b) repeated cycle at R = 0, and (c) fluctuating loading at R = 0.1. All the stress ratios compared for high cycle fatigue at log scale to conclude fatigue strength with respect to 1 million cycles.