PSI - Issue 37

G. Macoretta et al. / Procedia Structural Integrity 37 (2022) 632–643 G. Macoretta, B. D. Monelli / Structural Integrity Procedia 00 (2019) 000 – 000

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sporadic pore, Table 3. The adoption of the A1 and A2 sets produced an increase in the dimension and frequency of the pores, as shown in the sections reported in Fig. 4. The pores present in the baseline condition are small (less than 50 μ m) and round, suggesting that they arise from entrapped gas bubbles. On the other side, the pores present in the A1 and A2 conditions are larger, more elongated, and piled in regions where a meltpool with irregular geometry is present, hinting that they are produced by local instabilities of the meltpool, which create spots of lack of fusion or cavities. As confirmed by the numerical results, reported in Table 3, the sets A1 and A2 produced a not significant reduction of the material density but introduced larger pores having an aspect ratio (the ratio of the major and minor axes of the particle’s fitted ellipse) greater than 2, pointing out the elongated shape of the defects. The reported value of the pore aspect ratio is the average value calculated for the first 5 largest pores of each set.

Table 3. Porosity analysis

Parameters set

Baseline

A1

A2

Long.

Section plane

Transv.

Long.

Transv.

Long.

Transv.

99.97

99.95

99.84

99.79

99.71

99.88

Density

Max pore area, μm 2

817

1454

4818

2146

3411

1804

Pore aspect ratio

1.33

1.20

1.81

2.15

2.29

2.51

The pores are significantly more frequent in proximity to the specimen surface, suggesting that an important role was played by the process parameters adopted for the specimen contours, whose quality can be thus improved by the adoption of specific parameters with lower values of energy density. The microstructure produced by the three sets of parameters in the inner region of the specimen, namely the region where the investigated hatching parameters are intended to be used, are quite similar in terms of meltpool geometries and precipitates segregation on the meltpool boundaries, Fig. 5.

Fig. 4. Metallographic examinations in longitudinal direction: (a) baseline , (b) A1 , (c) A2 process parameters set, respectively.