OBRABOTKAMETALLOV technology Vol. 27 No. 1 2025 The microstructure of the samples from Inconel 625 obtained by wire arc additive manufacturing (WAAM) technology is shown in Fig. 2. The micrograph obtained with an optical microscope clearly shows an elongated, cellular structure in the centre of the sample. A characteristic feature is the presence of bright inclusions (probably secondary phases or oxide particles) in the intergranular regions (between dendrites). This indicates the complexity of phase transformations and possible inhomogeneities occurring during the wire depositing process. The dendritic structure is also clearly visible in the image. Dendrites formed during solidification of molten Inconel have prominent first-order axes extending in one direction. The second-order axes are usually much less pronounced or absent, indicating the specificity of crystallisation caused by rapid wire depositing processes. This microstructure feature is related to the high cooling rates characteristic of WAAM. In addition, the microstructure analysis indicates a noticeable texture in the grains. The presence of texture may indicate the preferred directions of crystallite growth due to the directional heat flow and stress state of the material during wire depositing. These texture and dendritic structure features significantly affect the mechanical properties of the finished WAAM-derived Inconel 625 samples. The different cooling rates characteristic of the wire arc additive manufacturing (WAAM) process result in the formation of different grain sizes in the Inconel 625 samples. The microstructure is characterised by a dendritic structure where the crystals are elongated along the direction of heat dissipation. This direction coincides with the direction of layer build-up during WAAM. A pattern of grain size increase with distance from the substrate is observed. In the studied samples, the length of dendrites reached 0.3–0.5 mm. This is consistent with the data of other researchers studying the microstructure of Inconel 625 obtained by additive methods. For example, in the study [6] a grain size of approximately 1 mm was observed in Inconel 625 samples produced using selective laser melting (SLM). The difference in grain size can be explained by the different cooling rates characteristic of WAAM and SLM. The results also confirm the data of [11, 16], where it is shown that in the lower part of the Inconel 625 sample (closer to the substrate), equiaxed grains predominate. As the distance from the substrate increases and new layers are deposited, the grains elongate along the direction of heat extraction, texture develops, and their length significantly increases. The coincidence of the observed patterns with the results of [11, 16] confirms the results and allows us to conclude about the influence of heat flow and cooling rate on the formation of microstructure in Inconel 625 samples obtained by WAAM method. Microhardness The microhardness of the samples was determined using the Vickers method at a 1 kgf load with a dwell time of 10 s. Values were calculated as the average of twenty indentations at different locations. Microhardness measurements (Fig. 3) indicate that the hardness at the center of the samples is lower compared to the edges. The central areas of the sample cool more slowly than the peripheral ones. The outer areas, dissipating heat more rapidly to the surrounding environment, solidify faster, resulting in a finer-grained microstructure and consequently higher hardness. Conversely, the central areas cool more slowly, promoting the growth of larger grains and leading to a reduction in hardness. This aligns favorably with the results of our metallographic analysis. This structural variation will influence the processing parameters of the workpieces and the cutting forces generated during machining. Fig. 2. Microstructure of the samples obtained using WAAM technology Fig. 3. Microhardness of the samples obtained using WAAM technology
RkJQdWJsaXNoZXIy MTk0ODM1