Issue 35

R. Konečná et alii, Frattura ed Integrità Strutturale, 35 (2016) 31-40; DOI: 10.3221/IGF-ESIS.36.04 34 specimen, which were mechanically polished. The resolution of determination of the crack tip position by means of CCD cameras with suitable optics was 0.01 mm. The measurement fulfilled the requirements of the ASTM E647-08 standard. The fatigue growth tests were started at load amplitudes resulting in initiation of a fatigue crack from the starting chevron notch after some hundreds of thousand cycles. When the crack appeared on both lateral sides of the specimen the load amplitude shedding procedure fulfilling the load ratio condition R = 0.1 was applied to reach the crack growth rate of about 1 x 10 -6 mm/cycle. Then the load amplitude was held constant and the practical determination of the crack growth rate was started after the crack grew through the area influenced by the load shedding procedure. The lowest crack growth rates of the order of 1 x 10 -7 mm/cycle in the threshold region were measured by load shedding method. The load was reduced in steps which were at the most of 10 % of the preceding value. As valid data were considered only those which were determined after the crack had grown through the corresponding cyclic plastic zone and when the average crack increment on both sides of the specimen was larger 0.1 - 0.2 mm. R ESULTS Microstructure s-fabricated CT specimens produced by SLM exhibit strong directionality of the microstructure. A three- dimensional cube (Fig. 5) shows the characteristic structure on section planes whose orientation with respect to the CT specimen is shown in Fig. 3. The z-axis coincides with the build direction; the x-y plane is parallel to the build plane. The scanning laser beam direction lies in x-y plane, which is perpendicular to the build direction. The fatigue crack in CT specimen propagated macroscopically in the x-z plane against the build direction. Figure 5 : Layered structure of SLM Inconel 718, LM. The micrographs taken at lower magnifications by light microscopy reveal the SLM process characteristics: line-by-line and then layer-by-layer building-up of the bulk (Fig. 5). In x-z and y-z planes the cut of melted tracks shows series of arcs produced by the laser energy distribution. The melt pools and layered structure appear as the result of laser beam alternating motion in x-y plane. Each next layer overlaps the previous melt pool tracks and due to heating influences the microstructure of the previous layer. All melted tracks are closely stacked to form a good metallurgical bonding between two neighboring layers. Defects in the form of small microshrinkages between the connected layers were found only rarely in the material. Similarly, small gas pores manifesting themselves as black round spots were observed only locally. The observed layer structure and melt pool tracks are typical features of the SLM production in various materials when the build strategy is of the type used in this study. The melt pool width on metallographic section in x-z plane (yellow arrow in Fig. 6a) varies approximately from 50 to 100 µm. The average layer thickness is of about 40 µm. The white arrows in Fig. 6a, c indicate melt pool boundaries. Big columnar grains oriented in the build direction are present in the structure. They appear as dark and elongated columns (Fig. 6a) and are characterized by two different types of microstructure, which are shown at higher magnification in Fig. 6b, c. The microstructure corresponding to the region 1 (Fig. 6a) is formed by very small cuboidal particles of γ ʹ precipitates in solid solution γ (Ni-Cr) fcc matrix (Fig. 6b). The intermetallic γ ʹ precipitates of Ni 3 (Al, Ti, Nb) composition have also the fcc structure. Fig. 6c shows a detail of the elongated (microdendritic) structure in region 2 (Fig. 6a). This structure is a result A x-y z-y x-z