3.2.2. Phase Transformation

It is well know the phase transformation and microstructure development of the metal component fabricated by WAAM are completely different compared to that of conventional manufacturing. The WAAM fabricated components undergo a slower cooling rate and gradual/equilibrium thermodynamics processes, thereby obtaining more homogeneous properties. In this study, the phase transformation at different positions of (from bottom to top) the component is investigated and reported in Figure 8, in which Figure 8a shows the details of the typical phases of the bottom part. Here, the phase transformation analysis is based on phase morphology and the continuous cooling transformation diagram of a high strength steel with a similar chemical composition [24]. At the beginning of deposition, since the bottom layer is affected by the substrate and is then cooled rapidly, thus martensite transformation occures. It plays a tempering role under the influence of a long-term thermal cycle in the subsequent deposition layer, with the occurrence of tempered sorbite transformation. Along with the tempered sorbite transformation, the major phase transformation is bainite due to the intermediate cooling rate. Under the action of tempering, tempered bainite is achieved. Thus, the phase composition at the bottom is tempered bainite and tempered sorbite. As deposition continues, the part close to the bottom moves away from the substrate gradually, lowering the cooling rate and heat dissipation, thereby reducing the content of tempered sorbite. At this time, the phase composition at the bottom is tempered bainite and less tempered sorbite, as shown in Figure 8b. As deposition height increases heat accumulation approaches saturation state, while the heat input and output are in balance, which presents a lower temperature gradient and cooling rate, resulting in the fact that it could not achieve the transformation requirement of tempered sorbite. As a consequence, the phase composition at the middle is almost made up of tempered bainite, as shown in Figure 8c. When deposition arrives at the end, the melt pool emerges at a rapid cooling rate due to mass heat dissipation with the environment, which allows the tempered sorbite transformation to occur again [25]. At the same time, some ferrite transformation takes place due to the lack of subsequent heating. Under this circumstance, the phases on the top are composed of tempered bainite, tempered sorbite, and ferrite, as reported in Figure 8d. In general, the complex phase transformation is caused by distinct thermal cycles and various cooling rates. This phenomenon eventually results in different phase transformation in diverse regions.

**Figure 8.** Phase transformation of different parts of the component: (**a**) bottom part; (**b**) part close to the bottom part; (**c**) middle part; (**d**) top part.
