*3.5. Bead-on-Plate Weldability Test Results*

Figure 8 shows an image of the bead-on-plate tests performed on casting plates from five heats. Both straight and circular weld paths were applied. Circular welds were 15 mm in diameter and the goal was to modify the self-constraint condition. As described above, bead-on-plate tests were carried out both in 9 mm and less than 3 mm thickness plates, after surface grinding and EDM cutting, respectively, employing similar welding parameters as in LBW Varestraint tests. In the case of thinner samples, laser power was decreased to 2050 W to avoid excessive root overhang.

**Figure 8.** Bead-on-plate trials and X-ray digital images showing cuts for cross-section metallographic examination.

Every examined weld was free of cracks in less than 3 mm thickness samples, showing a "bowl-like" welding shape (Figure 9), that is, a shape without characteristic nail head of keyhole mode LBW. However, several cracks were detected in the HAZ in 9 mm thickness samples (Figure 10) welded with similar process parameters (2300 W), following both straight and circular welding paths. Note that in this case, welding resembled "nail or mushroom shape" usually observed in keyhole mode LBW.

Total number of cracks and TCL determined in each cross section were included in Figure 11. For circular welds, average values determined from cross sections A and B were represented. It is clearly shown that straight welds gave rise to longer and higher number of cracks with minor influence of alloy composition.

**Figure 9.** Cross-section of bead-on-plate tests of moulds P and NP with less than 3 mm thickness. Laser power 2050 W.

**Figure 10.** Cross-section of bead-on-plate tests of moulds O, P, and NP with 9 mm thickness. Laser power 2330 W.

**Figure 11.** Number of cracks (blue bars) and TCL (red line) in bead-on-plate cross-sections of moulds O, P, and NP with 9 mm thickness. Laser power 2330 W.

#### *3.6. Microstructural Characterisation of Bead-on-Plate Welding Samples*

The composition of Laves phases in FZ and HAZ/BM (base metal) was analysed by EDX in both 9 and less than 3 mm thickness plates; corresponding values are included in Table 7. It can be observed that Laves phases in FZ had much lower Si, Nb, and Mo contents with respect to the HAZ and BM, and, on the contrary, they were enriched in Ti, Cr, Fe, and Ni elements. The Si content of Laves phase in FZ increased with the Si weight percentage of the alloy, with 0.22% being the lowest in mould O heat. In FZ the area percentage of Laves phases increased with increasing Si content of the alloy. Surprisingly, the area percentage of Laves phases of 3 mm plates of both moulds (P and NP) were lower than in the 9 mm thickness plates. The 9 mm thickness plates evacuated heat more quickly by thermal conduction, and therefore comparatively higher solidification rates and less Laves phases were expected.


**Table 7.** Area percentage and mean chemical composition of Laves phase (in wt %) and standard deviation in FZ and HAZ/BM of bead-on-plate welding samples.

Laves phase was not detected in HAZ and BM of the low Si O alloy. In the high Si alloy (mould P) and the standard Si alloy with low solidification rate and pre-HIP (NP), small quantities of Laves phases in HAZ/BM were still observed, which were higher in the latter. This indicates that the time and temperature of additional heat treatment before HIP were not enough to completely dissolve these phases.

As described above, cracks in HAZ were only observed in the 9 mm thickness plates along the grain boundaries. Higher magnification SEM images are displayed in Figure 12. This micrograph corresponded to mould P (high Si) and it was representative of the rest of the observed cracks in 9 mm thickness samples. These cracks were partially filled with a continuous Laves phase film, which was observed either at the tip of the crack or at the edges of the open crack. The chemical composition of this Laves film matched exactly with the Laves phase composition in FZ (Table 8).

**Figure 12.** (**a**) Grain boundary cracking in HAZ and (**b**) higher magnification of squared zone. Mould P.

**Table 8.** Composition of Laves phases (in wt %) at different locations in 9 mm thickness sample of Mould P.


Besides cracks, there was also evidence of Laves phase liquation observed in the HAZ of moulds P and NP for both less than 3 mm and 9 mm thickness plates. These phases were located in the interdendritic region, also showing the presence of carbides and δ phase (Figure 13).

(**a**) (**b**)

**Figure 13.** *Cont.*

(**c**)

**Figure 13.** (**a**) Phases in the interdendritic region. (**b**) Magnification of the squared zone showing the presence of δ phase, carbides, and Laves phases in base metal (BM) in the interdendritic segregation area. (**c**) Segregation area with evidence of Laves and δ phase liquation in HAZ (NP- less than 3 mm plate).
