PSI - Issue 24

Jacopo De Nisi et al. / Procedia Structural Integrity 24 (2019) 541–558

548

Paolo Folgarait et al./ Structural Integrity Procedia 00 (2019) 000 – 000

8

For groups A2, B2, and C (with solution annealing before ageing) (see Table 4 ) a fully equiaxial microstructure has been found at the same magnitude, with homogeneously dispersed precipitates. For all groups the apparent precipitates’ density increases with incremen tal ageing temperature till a critical value (laying in the range 480÷550°C), then decreases (see Table 5 ). This phenomenon is clear in case group C (H1025), where the coarsening mechanism of dispersed precipitates becomes increasingly effective. The effect of the incremental holding time (from 1 to 4 hours) on the apparent precipitates’ density seems to be slighter or negligible. More investigations are needed to better understand the effect of solution annealing on precipitates’ density after ageing. HRC hardness HRC hardness indentation tests have been performed on transverse sections of 3D printed (heat treated and as built) according to ASTM E18-19 Standard. Each hardness specimen preventively submitted to finishing to obtain perfectly parallel surfaces and smoothing till the aimed surface to be tested. Indentation tests performed on a calibrated universal hardness equipment AFFRI model LD250/D with load cells. Each test point has been resulted in the average among 6 (six) independent measurements, assessed on samples taken from different tensile specimens (equally selected among horizontally and vertically fabricated). Experimental data are summarized in Table 6 . 3.2. Hardness survey

Table 6 . HRC hardness indentation tests after heat treatment conditions performed on L-PBF 15-5 specimen.

Position

Group 0

Group A1

Group A2

Group B1

Group B2

Group C

A

39,4

44,7

44,1

46,0

43,7

37,5

B

38,9

45,0

44,2

45,8

43,4

37,4

C

39,4

44,4

44,2

45,8

43,4

37,5

D

39,5

44,8

44,2

45,9

43,5

37,8

E

39,4

44,4

44,4

45,9

43,2

37,1

F

40,0

44,9

44,4

46,1

43,3

37,6

G

39,1

44,7

44,3

46,6

43,3

37,4

H

39,2

44,8

44,3

46,5

43,4

37,8

I

39,5

44,8

44,1

46,1

43,5

37,9

average HRC

39,4

44,7

44,3

46,1

43,4

37,5

st. dev.

0,60

0,44

0,23

0,43

0,36

0,47

st. dev./average [%] Requirements ASTMA564/A564M

1,53

0,99

0,51

0,93

0,83

1,26

38,0 * 35,0 * The reference value assumed for the 0 group ( as-built condition, no heat treatment applied) is that for solution treated specimen. Results showed ( Fig. 3 ) that for each group the (measured) hardness is independent on the specific location on the building platform. In fact, all trends show to be linear and flat (= very low slope = nearly constant behavior within the building positions, within the experimental uncertainty), in particular for the 0 group ( as-built condition). For those cases for which the linear correlation relationship shows a slight (positive or negative) slope (for example B1 and C group), the maximum hardness value excursion is included within the error bar. For all groups the full error bar is equivalent to 2σ ( - σ; + σ; symmetric), σ being the standard deviation, calculated for each group as per Table 2 . For 40,0 40,0 40,0 40,0