# Mechanical and Metallurgical Properties of CO\({}_{2}\) Laser Beam INCONEL 625 Welded Joints
## 3 Results and Discussion  
### Defects and Metallurgy  
The weldments were found by visual inspection free from surface defects. NDT X-ray radiography tests detected defects such as macroporosity in the fusion zone (FZ) or incomplete penetration (Figure 5). ASME SEC IX-2017 standard was used for the quality testing of the weldments. It was observed that all weldments had straight white lines indicating acceptable weldments. Underfill and some porosities were identified in trial 1, while incomplete penetration was observed in trial 2 and trial 4; moreover, to better verify the weld's internal quality, macrographs of the weld bead cross-section were obtained from all trials and deeply examinated. Welding of high-strength alloys is always challenging in welding research aimed at achieving good metallurgical and mechanical properties by reducing the heat input. The observed macrostructure as a function of process parameters is shown in Figure 6. Weld pool dimensions were carefully measured. Full penetration at the highest heat inputs values of 180 and 198 J/mm was observed. In trial 1, in-line gas porosity was found (Figure 7), whereas trial 3 reached full penetration without detectable porosity. The weld pool width at the cap is wider and decreases at the root. Table 3 summarizes the main geometrical parameters of the FZ. It is observed that the weld pool's Y-shape is due to the laser beam diffraction promoted by the plasma formed at the top of the plate, called 'plume'. It is worth noting that the depth to width ratio is less than 2 (trials 1 and 3), which is much lower than that reached by arc welding processes (often greater than 2). A very narrow heat-affected zone characterizes all joints (Figures 7c and 8c), while the FZ microstructure shows the typical dendritic microstructure grown in the heat dissipation direction (Figures 7b and 8b). Moreover, the typical epitaxial grain growth is observed at the FZ/HAZ interface, as illustrated in Figures 7c and 8c. Figure 5: Weldments radiographic images (**A**–**D**). Figure 6: Macrographs of Inconel 625 laser beam welded joints as a function of process parameters (laser power and welding speed). \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Trial** & **Heat Input** & **Weld Pool** & **Weld Pool** & **Weld Pool** & \\
**No.** & **[J/mm]** & **Width (Cap)** & **Height** & **Width (Root)** & **Penetration** \\  & & **[mm]** & **[mm]** & **[mm]** & **[mm]** \\ \hline
1 & 180 & 3.3 & 5.5 & 1.1 & Full \\
2 & 120 & 2.6 & 4.3 & 1.2 & Partial \\
3 & 198 & 3.5 & 5.4 & 1.4 & Full \\
4 & 132 & 2.3 & 3.9 & 0.9 & Partial \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Weld pool geometry as a function of heat input. Figure 7: **(a)** Macrograph of Inconel 625 laser beam welded joint (weld 1) showing gas porosity on FZ and optical microscope images of the (**b**) weld cap and (**c**) HAZ. No solidification or liquidation cracking at the FZ/HAZ interface occurred in the present Inconel 625 butt-welded joints. This is a consequence of the lower heat input of the CO\({}_{2}\) laser beam process compared to arc welding processes where, in contrast, such defects were detected, as reported by [PERSON] [3], [PERSON] [26; 30], [PERSON] [31] and [PERSON] [32]. The microstructure is refined at the FZ/HAZ interface and became coarse, moving toward the weld centerline as a consequence of the competitive grain growth phenomenon, so that favorably oriented (FO) grains, whose preferred growth angle relative to the temperature gradient direction is small, grow preferentially relative to unfavorably oriented (UO) grains, which are characterized by a larger preferred growth angle. The XRD analysis as shown in Figures 9 and 10 revealed the precipitate of secondary phases in the fusion zone. The precipitates observed are Cr\({}_{2}\)Ni\({}_{3}\), Mn\({}_{3}\)Si, Al\({}_{2}\)FeSi, NbNi\({}_{3}\) and Cr\({}_{2}\)Ni\({}_{3}\), Fe\({}_{2}\)N\({}_{3}\), Al\({}_{2}\)FeSi, NbNi\({}_{3}\) in weld 1 and weld 3, respectively.