*3.2. Surface Topography*

Next to the build-up volumes and diameters of the bars, the topographical properties of the WAAM bars were specified. It can be seen from the results in Figure 7 that the conventional GMAW led to a non-uniform surface with a minimum to maximum range of 1.4 mm, determined along a measuring section of 30 mm. The surface topography of the CMT standard specimen was smoother and the layers were built more regularly. The waviness was in the order of 0.89 mm. In comparison, the CMT cycle step process led to the most uniform and even surface. The range between the minimum and maximum was 0.35 mm.

**Figure 7.** Surface laser scans along the build-up direction.

#### *3.3. Microstructure and Hardness*

Subsequently, deposited layers reheat weld metal during the manufacturing process. The reheating causes a change of the microstructure. Especially, the refined geometry of the bars manufactured here lead to heat accumulation and comparably long t8/5-times in combination with high peak temperatures. Micrographs showing the weld layer geometry and the resulting hardness are given in Figure 8.

**Figure 8.** Hardness distribution in the build-up direction.

The averaged hardness increased with decreasing energy input. In the conventional GMAW process (9 kJ per layer), the hardness ranged from 128 to 163 HV1; in the standard CMT process (4.2 kJ per layer), the hardness varied between 146 and 176 HV1; and in the process with the lowest energy input per weld point (3.35 kJ), the hardness increased from 150 HV1 in the lower layers to 223 HV1 in the higher layers. In the pictures on the bottom of Figure 8, one can see that due to the high heat input in the conventional GMAW weld, the layers melted together and an even homogenous secondary microstructure developed. Hence, the hardness did not vary as much as in the CMT cycle step specimen. Looking at the two CMT processes, one characteristic is the hardness increase at the transition zone between two layers. Right when the next layer started, the hardness rose significantly and decreased subsequently.

Figure 9a shows the microstructure of the conventional GMAW specimen consisting of mostly ferrite and small fractions of bainite and perlite. Here, the grains were comparatively large. The grain growth is most likely a result of the peak temperatures well above the austenitization temperature, A3, and the comparably long holding times.

In general, the microstructure of the CMT processes in the middle and on the top of each layer is a fine-grained bainite-ferrite structure. In the micrographs in Figure 9b,c, the grains appear smaller in the area of high hardness and a higher amount of bainite can be detected. Figure 9c shows at the right side the top layer of the bar, which was welded last. The resulting primary microstructure not affected by subsequent weld layers is composed of ferrite and bainite fractions in an acicular structure. However, the secondary microstructure resulting from both CMT processes did not vary significantly while the high energy input of conventional GMA welding led to a significantly lower, but also more homogeneous, hardness. This is expected to result in a variation of the tensile properties compared with Section 3.6.

**Figure 9.** Micrographs of the specimen manufactured with (**a**) conventional GMAW, (**b**) CMT standard, and (**c**) CMT cycle step.

General effects of cooling rates on microstructure and hardness are given in welding time-temperature-transformation (TTT) diagrams. Figure 10 shows the transformation behavior of a G4Si1 at rapid cooling. However, the chemical composition of this welding wire varies from the one used here, but general effects can be explained. Fast cooling rates lead to more bainite and less ferrite, resulting in higher hardness. Low cooling rates result in low hardness and mainly a ferrite/pearlite microstructure. Accordingly, the hardness of deposited G4Si1 can vary depending largely on the welding parameters.

**Figure 10.** Welding time-temperature-transformation (TTT) diagram for G4Si1 and peak temperatures of 1350 ◦C, according to [41].

The fine-grained microstructure is an indicator for low heat input while welding subsequent layers and moderate peak temperatures above A3. The slightly harder regions at the layer interface were reheated less during subsequent thermal cycles. Thus, annealing effects are less prominent. The result is a heterogeneous microstructure in the build-up direction with varying mechanical properties.
