PSI - Issue 39

Gonzalo M. Domínguez Almaraz et al. / Procedia Structural Integrity 39 (2022) 281–289 Author name / Structural Integrity Procedia 00 (2019) 000–000

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applications are: submarines structures, where corrosion is of main concern, downhole shafts and wellhead parts for oil and gas industries, and cryogenic storage tanks [7-10]. In aeronautical industry, blades and discs are subjected to severe operating conditions during flying, such as: 1) aggressive environment: high temperatures, pollution of air or fuel, solid particles, etc., 2) high stresses, associated with: vibrations, centrifugal and aerodynamic forces, and 3) thermal stresses related to thermal gradients [6, 10]. Previous investigations [11-14], have revealed two relevant factors to improve the performance of superalloys: 1) modification of the elements and its quantities present in the alloy; for example, addition of Al, Ti, Nb or Ta, which are used to improve the volume fraction of the phase γ’’ (Ni3Al or Ni3Ti with FCC structure), destined to phase hardening in Inconel 718; 2) use the appropriate thermal treatments such as: solution annealing and aging. The main purpose and contribution of this study have been: investigate the effect of consecutive heating treatments performed on the Inconel-718 alloy, with aim to improve its ultrasonic fatigue endurance. The first heat treatment applied to the as-received material is solution annealing, destined to reduce hardness in order to machine the ultrasonic fatigue specimens. The second heat treatment was double aging, with aim to recuperate the mechanical properties of the as received material. The first heat treatment, also known as solubilization, was carried out at temperature and time stay proposed in this paper; whereas the second one (double aging) was performed according bibliographic references. At the best of authors' knowledge, no information is available in literature concerning the ultrasonic fatigue endurance of Inconel 718, at room temperature and after two consecutive heat treatments: solution annealing and double aging. In a recent paper [15], the effect of pre-corrosion on the ultrasonic fatigue endurance of Inconel 718 was studied; the present paper concerns the effect of two in row heats treatments on the ultrasonic fatigue endurance of this alloy. 2. Materials and Methods. I-718 round bars were received from the Mega-Mex company (Texas, USA), with dimensions: 0.5 inch in diameter and 12 inches in length. The chemical composition (in weight %), was: 0.62Al, 0.94Ti, 2.89Mo, 5.13Nb, 18.46Cr, 18.7Fe and balance Ni. Other elements present in I-718 alloy were: Bi, Pb, Se, S, Ca, B, P, Ta, Mg, C, Cu, Si, Mn and Co; nevertheless, these elements were in chemical composition lower than 1% in weight. In Table 1 are shown the main monotonic mechanical properties of Inconel 718. Ultrasonic fatigue tests require to evaluate the natural frequency of oscillation of testing specimens in longitudinal direction; this task was performed by numerical modal analysis using the finite element method. The mechanical properties of: density, Young’s Modulus and Poisson’s ratio were introduced in the numerical model in order to obtain its natural frequency of vibration in longitudinal direction. The obtained natural frequency of vibration and dimensions of the testing hourglass shape specimen under resonance are shown in Figure 1(a). The numerical modal analysis was carried out taking the material as: homogeneous, isotropic and linear. Numerical value for the natural frequency of oscillation in longitudinal direction was 20,101.7 Hz; this frequency is close to the ultrasonic fatigue machine oscillation frequency of 20 KHz; the ultrasonic fatigue tests are available with 20 KHz  300 Hz. All ultrasonic fatigue tests were carried out at zero mean stress (R=-1), and at room temperature. The ultrasonic fatigue machine and specimen are illustrated in Figure 1(b), whereas Figure 1(c) presents thermographic images during ultrasonic fatigue testing. Thermographic images show higher temperatures at zones corresponding to higher stresses, located at the neck section of testing specimen: the neck section undergoes the higher values of stresses and the lower or null displacements. Resonance condition implies a stationary elastic wave along the testing specimen, inducing high stress at its neck section and higher displacements at its ends. The ultrasonic fatigue tests were obtained using a self-designed and self-constructed ultrasonic fatigue testing machine, presenting the following main features: high frequency generator (20 KHz), with 1100 watts of power, connected to piezoelectric transducer to convert ultrasonic electronic signal to ultrasonic mechanical vibration, coupled to an amplifier horn. The modality of tests was displacement controlled; calibration was carried out with an Table 1. Main monotonic mechanical properties of Inconel 718. Density (Kg/m 3 ) E (GPa) Poisson rate (-)  y (MPa 20  C)   (MPa 20  C) Elong. (%) break 8220 210 0.294 1100 1375 25