The temperature of the samples was recorded during the manufacturing process. The maximum temperature values of the top and middle sections of each sample are shown in Fig. 2. The maximum temperature value increases from the middle to the top section for every investigated sample, indicating that the material experiences a heat accumulation during the manufacturing process. The accumulation phenomenon was particularly evident on sample 3, probably due to its proximity to other samples and limited heat transfer condition (Fig. 1). Moreover, heat transfer mechanism, geometry and positioning of samples have a strong influence on the temperature gradients experienced by the material. [24, 25]. Temperature values obtained by the thermal camera were consistent with the literature .
The sections of samples parallel to the building platform were investigated by optical microscope in order to perform relative density analysis. The porosity values of samples are reported in Fig. 3 and range between 0.02% and 0.05%. Mean Standard deviation of the results was found as 0.005%.The middle section of the samples shows lower porosity values than those of the top section, probably due to the different thermal experience during the process. Indeed, higher maximum temperature values can promote the formation of a larger quantity of sputters, exhaust gasses and residues of burnt materials during the laser scan [27–30]. These phenomena can also cause increased porosity.
The section of as-built samples parallel to the building platform was etched by Kroll’s reagent and investigated by optical and SEM microscopes. Surfaces of the samples were observed with 50x and 200x OM magnification, as reported in Fig. 4. The martensitic α' phase is noticeable within the microstructure of all investigated sections, in good agreement with literature results [31, 32].
SEM analyses were carried out on samples to observe the microstructure of the as-built material. Results are reported in Fig. 5. Martensitic laths with different thicknesses are visible within the microstructure of the as-built material. In particular, thin martensitic laths were recognized on the top surface of the samples (Fig. 5A-B), while thick martensitic laths were observed in the middle section (Fig. 5C-D). It was thought that the higher maximum temperature values achieved in the top section as seen (Fig. 2). This could lead to lower cooling rates due to lower temperature gradient, which was related to cooling through the melting temperature to mean layer temperature. Mean layer temperature increased during manufacturing from 35oC to 75oC. Although this phenomenon was assumed to promote the formation of thicker martensitic needles due to the temperature gradient of the layer, microstructure was observed opposite. It was occurred owing to more effectual repetitive laser heating and cooling cycle in the middle section than the top section. Similar to this study, it was found that primary and secondary α′ martensites were a little coarsening, while the finer ternary and quartic martensite laths were forming . The High magnification SEM images are shown in Fig. 6. Similar in the literature, twins were seen on the primary martensitic laths. Besides, primary, secondary and ternary α' martensitic laths were ascertained in the as-built microstructure. Thermal history induced severe internal stresses caused dislocations such as twins.
Figure 5. SEM images of Back sample, top surface (A), Center sample, top surface (B), Back sample, middle surface (C), Center sample, middle surface (D)
SEM images reported in Fig. 7 shows the evidence of white spots within the microstructure of the as-built material. These spots can be ascribed to the β phase, probably related to agglomerations of vanadium atoms. Indeed, the β phase can appear in form of white spots due to the higher vanadium content that results in a brighter contrast [33, 34].
Finally, the widths of primary martensitic laths were measured in order to understand the influence of the building position and maximum temperature of samples on the final microstructure. Results were reported in Fig. 8. The widths of martensitic lines were measured on 7 different laths with SEM images. Mean standard deviation of the measurements was calculated as ± 0.135 µm. Thinner martensitic laths are visible on top sections of both center and back samples. Besides, each sample had a different mean width of martensitic laths depending on the temperature distribution during the manufacturing process. The samples which are placed nearby the argon gas inlet were cooled faster according to temperature values of samples (Fig. 2). Thus the maximum temperature results observed higher far samples from the gas inlet. This phenomenon led to emerging thinner martensitic lines on neighboring fields to argon gas flow due to lower temperature gradient. Moreover, front areas of each sample had thicker martensitic lines than the back sides of the samples as seen in the Fig. 5. While coarser primary martensite laths were formed, finer grains were existed due to forming secondary, tertiary and quartic martensite laths. Besides, in nearby areas such as edges and corners of two adjacent samples, secondary heat effects were noticed by analyzing the width of martensitic lines according to the observations in Fig. 5. In general, it is believed that the width of martensitic laths on sample 3 is larger than that of sample 5 due to heat concentration on the center of the building platform.
Micro hardness tests were performed both on top and middle sections of samples on center point, side edges and corners. Micro hardness values of the center point of samples are shown in Fig. 9, Fig. 10 and Fig. 11, respectively. Top section of the samples shows lower micro-hardness values than those of middle sections, probably due to the complexity (further α phase transformations) of the microstructures. Although the primary martensitic lath width of the top section was finer, additional martensitic lines (finer than the primary martensitic lath) on the middle section lead microstructure to more complex than the top section. Though the primary martensitic laths of the middle areas were larger than the top areas, middle areas were observed more complex than the top areas in the light of Figs. 5 and 6. Significantly, lower hardness values were noticed on sample 3 than sample 5 due to coarser laths. Since the position of the back sample is closer to the gas inlet, higher cooling rate can be experienced by the material, leading to higher micro-hardness values. The hardness of the edges and the corners were similar. Even though the hardness values at the center of the samples were distinctly constant, however, the edges and the corners of the samples ought some scattered values of micro hardness, since the cooling rate of the side surfaces were higher than the center areas owing to heat transfers. Accordingly, the hardness on edges and corners were ranged between 365–400 HV and micro hardness values at the center points varied between 390–400 HV. The results were similar with previous studies wherein the hardness of the Ti6Al4V material in PBF additive manufacturing was measured between 330–400 HV .
Mean standard deviations of micro hardness values were found as ± 9.75HV. According to previous results, the micro-hardness of samples fluctuates depending on the section and position of the samples on the building platform. Side surfaces, edges and corners were adequate owing to the resistance to friction, internal/residual stresses and deformations. Nevertheless, in case of the reaching excessive hardness, outer surfaces would be brittle. Therefore, unexpected failure mechanisms could be occurred.