Issue 50

L. Romanin et al., Frattura ed Integrità Strutturale, 50 (2019) 251-263; DOI: 10.3221/IGF-ESIS.50.21 253 helpful in correlating microfissures occurring at the grain boundaries, under the nail head of the bead, with process parameters. Laki et al. [20] studied the EBW of 30HGSA steel using a conical heat source. They also found good consistency with empirical data. In a more recent investigation, Laki et al. [21], from a series of 49 weld experiments, created a Partial Least Square model of the FZ and subsequently defined the heat source utilizing mesh segments and constant power density on each segment. The phenomenological approach has been used by Palmer et al. [22] when they simulated the EBW of 304L stainless steel. They included thermal conductivity and viscosity to account for enhanced heat and mass transfer due to turbulence in the weld pool and found that convective heat transfer was very significant in determining the weld geometry. However, this type of analysis is not convenient for a subsequent mechanical analysis because it involves results mapping from a Finite Volume Method to a Finite Element Method code. It is pointed out that only few researchers have investigated in the past the effects of high-power density welding techniques on Ni-base wrought superalloys. Some works can be found about laser welds in wrought Waspaloy [2] but specific information on the high-energy beam welding of other Ni-based superalloy have not yet fully established. In the recent literature some works focused their attention to the precipitation of Laves phase particles [23] or equilibrium and metastable phase/grain morphology [24] during laser additive manufacturing or rapid solidification of Ni-based superalloys. Such work uses quite complex models solved by the phase-field method or multi-scale model combining the finite element method and stochastic analysis. The JMAK-type model was instead applied by different researcher [25–27] with the aim to predict the microstructure evolution in nickel-based superalloys. In particular the Avrami exponent for  ’ precipitation was determined to be in the 1.5-2.3 range, suggesting spherical or irregular growth. A methodology was also developed to take into account the temperature dependence of the rate coefficient k(T) in the non-isothermal JMA equation. Finally, it is interesting to mention a recent review paper about welding characteristics of aerospace materials that highlight the potentiality of high-power density welding processes (LW, EBW) compare to the traditional arc welding technologies [28]. In this work, a numerical model of EBW of Inconel 625 has been developed. The phenomenological approach proposed is based on the calibration of the source parameters on the basis of macrographs and temperature measurements. Experimental and numerical results were found in good agreement and suitable for a future mechanical analysis. The obtained results will be used also for a further development of the thermal-metallurgical model that includes phase precipitation and dissolution [29]. Moreover, the utilized welding process did not produce any grain growth or detrimental phases such as σ, μ and Laves Phases in more than trace amounts as it will be described afterwards. Hardness measurement has been presented, as well. M ATERIAL AND E XPERIMENTAL P ROCEDURE he chemical composition of the specimens has been measured by Energy Dispersive X-ray spectroscopy (EDS) and is summarized in Tab. 1. Ni Cr Fe Mo Nb+Ta C Mn Si Al Ti Co 58.30 20.39 3.94 8.91 3.76 - - 0.32 - 0.22 - Table 1 . Chemical composition calculated by EDS on specimen Test 1 (wt%) Four butt-welded joints were produced, each one obtained by electron beam welding two plates which dimensions are 16 mm x 72 mm x 2.5 mm (Fig. 1). Since no filler material has been used, the joining edges have been prepared to obtain the best contact. In order to validate thermal analysis results, two K-type thermocouples have been inserted at 2.5 mm and 6 mm far from the weld seam, as shown in Fig. 1. The holes, with a diameter of 1 mm, have been realized with Electro Discharge Machining. Picotech TC-08 Termocouple Data Logger has been used for data recording. The plates to be welded were first clamped as shown in Fig. 2 and then tack welded in six points, one every 10 mm, in order to avoid plates detachment due to thermal expansion. Weld tacking increased also the overall specimen temperature because of the small mass of the specimens. Welding tests have been carried out according to the parameters defined in Tab. 2 and using a vacuum chamber with a pressure of 4 10-4 mbar. Full penetration was not reached in the first 10 mm of the welding line. However, starting and ending points are not taken into account in this work. In Tests 2 and 3, the current proved to be too high since a crater formed at about 2/3 of the T