PSI - Issue 34

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Harry O. Psihoyos et al. / Procedia Structural Integrity 34 (2021) 253–258 Harry O.Psihoyos et al. / Structural Integrity Procedia 00 (2021) 000 – 000

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This criterion accounts for the melt pool mode of keyhole transition as described by Cunningham et a l. (2019) and may be non-conservative for keyhole defect prediction but it can serve as initia l indicator of keyhole defects. These criteria were used for the characterization for melt pool characteristics of the SLM Ti-6Al-4V fatigue specimenpredictedwith thermal history in ANSYSAdditive Science. 3.2. Fatigue crack growthproperties of SLMTi-6Al-4V Fatigue crack growth properties of SLM Ti-6Al-4V used for fatigue life estimation can significantly affect the results. Due to the size defects which act as initia l cracks, long fatigue crack growth data may lead to overestimation of fatigue life of AM materia ls. In long cracks the effect of the plastic zone on the crack tip is more considerable compared to the small cracks. Therefore, the crack closure effect in small crack growth is negligible. As a result, the effective stress ranges for sma ll cracks are greater than those of long cracks for the same applied stress ranges and the sma ll crack growth rates can be significantly faster than the corresponding ra tes of long cracks (Zha i et a l., 2016). For this reason, the ava ilable small fatigue crack growth data from the work of Zha i et a l. (2016) were considered in the present ana lysis. Zha i et a l. (2016) compared the long and sma ll fatigue crack growth properties of laser engineered net shaping (LENS) Ti-6Al-4V with conventiona l mill-annea led one for R=0.1. It should be noted that despite that LENS AM process present some differences with SLM process the resulting microstructure of the processed materia ls is similar, thus, LENS fa tigue crack growth data can used in the present ana lysis. Table 2 shows the fatigue crack growth threshold, fracture toughness and the need tensile mechanica l properties for fatigue life simula tion. Table 2: Selected material properties of AM Ti-6Al-4V used in fatigue life simulation Mechanical properties Value Fatigue crack growth threshold, ΔΚ τη 2.5 MPa√m Fracture toughness , Κ Ic 53 MPa√m Ultimate strength, S u 1198 MPa Yield strength, S y 1135 MPa Young Modulus, E 110 GPa 3.3. Fatigue life prediction model – Linear elastic fracturemechanics simulation Fatigue life estimation of SLM Ti-6Al-4V specimen was performed in AFGROW software. AFGROW is a fracture mechanics and FCG ana lysis software tool that enables the prediction of fatigue lives of components. AFGROW provides a series of stress intensity factors (SIF) solutions or K-solutions for the fatigue life estimation. Based on the location of critica l defect on cross-section of the specimen, the appropriate K-solution will be chosen. In this work, a linear elastic fracture mechanics (LEFM) approach was used for fatigue life estimation , thus, no crack closureeffect was considered. 4. Results 4.1. Critical areas for defect formation The examination of melt pool was performed ma inly in the gage length region of the fatigue specimen. The genera l trending was that the melt pool located at the surface of specimen did not fulfill the LOF criterion. Therefore, the melt pool locations at surface of specimen were deemed as susceptible areas for the formation of lack-of-fusion defects. This trending agrees with experimenta l observation in the work of Du et a l. (2021) where LOF defects proved more prone to be produced in subsurface area due to imperfect connection between contour and inner scanning. The predicted melt pool characteristics in the gage length region did not present any keyhole mode according to keyhole defect criterion. From a ll the layers whose externa l surface was susceptible for LOF defect, the layer of the cross-section with the sma llest diameter was the most critica l, as the developed stresses are the highest.