*3.3. Microstructure*

Based on the substantial investigation on the macro morphology and phase transformation, the e ffect of specific energy on the crystal growth and microstructure is figured out in this research. Figure 6 presents the SEM morphology of the coatings obtained under the condition of di fferent specific energy. Metallurgical bonding has been obtained between the base metal and the coatings of which the bonding region is mainly composed of columnar crystal and shrinkage cavities, as shown in Figure 6. There is a slight increase in the width of the bonding region from 25.5 μm to 60 μm at the function of specific energy varied from 40.9 to 89.3 J/mm2. With regard to the thermal cycling process, the increase of specific energy leads to the decrease of actual cooling rate and thus provides an opportunity for the mixing of HEA alloy and TC4 substrate. As implied in the detailed image, the transformation from columnar crystal to the cavities is an important feature in this region. It also can be concluded from Figure 6g–i that there are obvious di fferences in the size of columnar crystals under di fferent specific energy. There is a significant increase in the width of the columnar crystal zone from 3.2 μm to 12.1 μm at the function of specific energy varied from 40.9 to 89.3 J/mm2. It is attributed that the higher heat provides more energy for the growth or coarsening of sub-grain. Thus, the columnar crystals grow larger slightly with the increase of specific energy.

The microstructure at the top of the HEA laser cladding layer under di fferent specific energies is presented in Figure 7. Meanwhile, the chemical composition of di fferent regions, which are shown in Figure 7, is exhibited in Table 4. It can be seen from Figure 7 and Table 4 that there is a slight di fference between Case 1 and Case 2, while Case 3 shows an obvious di fference. The microstructure of Case 1 is composed of the Fe-Cr phase (point A) and matrix. Compared to Case 1, the rich-Ti phase is formed with the higher specific energy which provides a promotion to the melting of the TC4 substrate. However, the extremely high specific energy results in su fficient reaction between powders and substrate. Thus, the (Ni, Co, Ti)-rich phase is formed because of the more negative Δ Hmix.

EDS analysis is then carried out along the path from the upper layer to the substrate as marked in Figure 8. Little fluctuation in the relative concentration curves of Cr, Co, Fe, Ti, and Ni elements is found in the upper and middle layers in the coatings. However, the content of all the elements increased and decreased sharply, respectively, in the bottom layers of the coatings, especially in the bonding region. As the content of Ti elements in the substrate is much higher than that of the coatings, the obvious dilution e ffect by the substrate upon the cladded coatings led to the increase of Ti elements and the formation of the bonding region. Besides, the element change of the sample with lower specific energy is strongly sharper than that with a higher specific energy. Among the experiments, increasing specific energy leads to an increase in the dilution ratio. More melted substrate metal is melted to give birth to a higher dilution rate and thus less obvious di fference in the chemical composition between the HEA coating and substrate.

**Figure 6.**(**a**) SEM image of the transverse section of Case 1, (**b**) SEM image of the transverse section of Case 2, (**c**) SEM image of the transverse section of Case 3, (**d**) high-magnification SEM image of bonding region in Case 1, (**e**) high-magnification SEM image of bonding region in Case 2, (**f**) high-magnification SEM image of bonding region in Case 3, (**g**) high-magnification SEM image of the localized region marked in (**d**), (**h**) high-magnification SEM image of the localized region marked in (**e**), (**i**) high-magnification SEM image of the localized region marked in (**f**).

**Figure 7.** SEM images of laser cladded coatings in different region. (**a**) Case 1, (**b**) Case 2, (**c**) Case 3.


**Table 4.** The chemical composition of different points in Figure 7.

**Figure 8.** (**a**) EDS line scanning of Case 1, (**b**) EDS line scanning of Case 2, (**c**) EDS line scanning of Case 3.
