*2.3. Metallographic Characterisation*

As-cast, heat-treated, and bead-on-plate welding samples were characterised by optical microscopy (OM) in a LEICA MEF4 microscope (LEICA MICROSYSTEMS GmbH, Wetzlar, Germany) and field emission scanning electron microscopy (FESEM) with a ZEISS Ultra Plus microscope (CARL ZEISS AG, Oberkochen, Germany). Energy-dispersive X-ray (EDX) spectroscopy analysis was conducted in the FESEM microscope to determine local chemical composition of precipitates and phases. Average chemical composition of the Laves phases was determined through EDX analysis of 5 phases for each casting heat. For the metallographic analysis, cross sections were prepared by grinding and polishing using standard procedures. The area percentage of carbides was determined by image analysis of 5 images obtained by OM at 100× (HAZ and BM) and 500x (FZ) using Leica application suite V4.2. The area percentage of Laves phases + carbides was determined through SEM at 500× in the heat-affected zone (HAZ) and in the base metal (BM), and at 2500× in the fusion zone (FZ). Finally, the Laves phase area percentage was obtained by subtracting the carbide area percentage obtained by OM from the area percentage of Laves phases + carbides obtained by SEM.

The microsegregation degree of alloying elements was evaluated by the segregation ratio (SR), which can be calculated using the following formula [27]:

$$\text{SR} = \text{C}\_{\text{i:IR}} / \text{C}\_{\text{i:DC}} \tag{2}$$

where Ci,IR is the maximum concentration of element i in interdendritic region, and Ci,DC is the minimum concentration of element i in dendrite core. For SR < 1, solute elements tend to segregate to dendrite core during solidification; when SR > 1, the alloying elements partition towards the interdendritic region. If SR values are close to 1, the corresponding elements do not favourably segregate to any region. The concentration of each element was determined by EDX analysing 8 points in 3 different regions; an example is shown in Figure 3.

**Figure 3.** Location of EDX analysis points for determining the segregation ratio.

Grain size was revealed by etching with Kalling 2 reagent. Dimensions of at least 10 grains were analysed for each casting heat in cross-sections. The grain size was determined from the horizontal and vertical mean intersection lengths between 2 grain boundaries of grains.

The solidification path and evolution of Laves phases were studied by Thermo-Calc software (using TCNI10 and MOBNI4 databases). Scheil simulation, considering back diffusion of alloying elements in the primary phase, was performed at a high cooling rate of 100 ◦C/s, similar to the expected welding cooling rates, for alloys with different Si contents (moulds O, E, and P).
