Issue 50

L. Romanin et al., Frattura ed Integrità Strutturale, 50 (2019) 251-263; DOI: 10.3221/IGF-ESIS.50.21 254 welding line. Because starting and ending lack of full penetration could be neglected, the current has been therefore slightly lowered to 14.5 mA in the last welding test (Test 4). Optical and Scanning Electron Microscopes (with microprobe EDS) were used for the metallurgical investigation. For optical microscope (OM) analysis, the specimens have been etched with a solution made of nitric acid (HNO 3 ) and chloridic acid (HCl) (1/3 parts in volume). Precipitates were analyzed in terms of shape, dimension and chemical composition. Detected X-ray spectra have been compared with previous ones published in literature with the aim to find matching patterns and recognize phases with a reasonable degree of accuracy. Micro-segregation patterns across the dendritic substructure of the FZ were finally checked. Figure 1: 3D representation (a) and top view (b) of the specimen showing the holes position for the thermocouples. Figure 2: Clamping configuration and EBW in progress. Accelerating voltage (kV) Beam current (mA) Focus current (mA) Spot diam. (mm) Freq. (Hz) Speed (mm/s) Notes Test 1 150 15 347 1.5 300 12 - Test 2 150 15 347 1.5 300 12 - Test 3 150 15 347 1.5 300 12 - Test 4 150 14,5 347 1.5 300 12 Thermocouples Table 2: EBW tests parameters E XPERIMENTAL RESULTS Metallurgical results The FZ shape is shown in Fig. 3a. The grains are dendritic and oriented along the heat flux direction (Fig. 3b). Fig. 4 reveals both the characteristic epitaxial grains grow at the FZ/HAZ interface and a narrow HAZ of about 0.2 mm. Grains, in the parent material, maintains an equiassic morphology. No grain growth was detected in the HAZ, as it could be observed in Fig. 4. This is because EBW is characterized by a rapid heating and cooling, greatly reducing the material