3.2.1. Microstructure Evolution

In the process of WAAM, the heat cycle repeats over and over again. Some typical phenomena are caused by complex thermal cycles such as rapid heating, fast solidification in the fusion zone, various temperature gradients, a large amount of heat accumulated, reheated, and then recooled. The non-equilibrium thermodynamics governs the physical characteristics of microstructure formation in the WAAM process. Thus, analyzing microstructural characteristics and evolution is helpful for describing the manufacturing process [21]. Based on the thermal history illustrated above, the microstructure can be divided into three main morphologies: (1) columnar, (2) mixture of columnar and equiaxed, and (3) equiaxed. In other words, during the WAAM process, the microstructure can be divided into three different regions: (1) columnar grain zone, (2) boundary zone, and (3) equiaxed grain zone, as shown in Figure 6. The evolution of the microstructure is influenced by two crucial control parameters including the temperature gradient (G) and solidification velocity (SV) [22]. Based on solidification theory, the ratio of the G to SV determines the forms of the microstructure. When the G/SV is extremely large, the solidification microstructure contains planar grains, as shown in Figure 6. In this condition, the melt metal acts upon the former solidified layer and solidifies rapidly, G tends to be infinite, and VS tends to be zero. When the G/SV is a relatively high value, the major solidification microstructure is columnar grain, as shown in Figure 6; at that moment, the heat dissipation that comes from the substrate results in a rapid cooling speed and high temperature gradient. The melt metal solidifies quickly and then the columnar grain is obtained. In a general way, the core of the molten pool is finally solidified. The temperature gradient and solidification velocity tend to be low, resulting in smaller G/SV values, giving rise to the equiaxed grains as shown in Figure 6. In a word, heat is mostly transmitted through the substrate or formerly deposited layers, and different regions of the molten pool appear to have various temperature gradients and solidification velocities, which eventually influences the evolution of the microstructure.

**Figure 6.** Three typical regions of microstructure in the process of WAAM.

During the deposition process, the temperature gradient dominates the directional growth of the microstructure and affects its morphology. The growth direction is usually perpendicular to the boundary of the molten pool [23]. The shape and growth of dendrites in different deposited layers vary as a function of heat dissipation, as shown in Figure 7. In this work, the component consists of deposited layers and its macrostructure at cross section is shown in Figure 7a. Region b is close to the substrate, representing a rapid cooling rate and large heat dissipation, which indicates a high temperature gradient. In this region, the heat dissipation of metal liquid is perpendicular to the substrate during the cooling and solidification processes. The direction of heat flow is therefore perpendicular to the interface, leading to the fact that solidification is directional. Thus, the structure of columnar grain formed is vertical to the fused interface. The typical and directional columnar microstructure in region b is shown in Figure 7b. Along with the deposition progress, the effect of heat dissipation from the substrate decreases by increasing the distance to the substrate, resulting in the decrease in the temperature gradient. The columnar microstructure formed appeares less typical than that in region b, and the details of the microstructure in region c are shown in Figure 7c. As deposition continued, region d is hardly influenced by the substrate, but is only affected by the thermal cycles between layers. At that time, the temperature gradient is relatively low, resulting in the inapparent directional growth of the microstructure. Simultaneously, region d undergoes a long period of heating, leading to a coarse microstructure. The conditions of region d are shown in Figure 7d. At the end of the deposition, the microstructure of regions e and f on the top of component are presented in Figure 7e,f, separately. In both regions, the columnar grain is rarely observed, while a uniform and fine microstructure appeare. To some extent, the characteristics of the microstructure are disclosed due to rapid cooling resulting from contact with the environment and no subsequent heating.

**Figure 7.** Microstructure evolution for different parts of the as-deposited component: (**a**) Macrostructure of the cross section; (**b**) Region b; (**c**) Region c; (**d**) Region d; (**e**) Region e; (**f**) Region f.
