PSI - Issue 14

462 14

Anigani Sudarshan Reddy et al. / Procedia Structural Integrity 14 (2019) 449–466 Author name / Structural Integrity Procedia 00 (2018) 000–000

Table 7: Difference between the small scale and the regular test specimen geometry for the DMLS alloys in the as printed and heat treated conditions

Δ0.2% YS (MPa)

ΔUTS (MPa)

S.No

Material

Condition

Δ %Elongation

As printed

-1.5

-87

2

1

IN718

Heat treated

35

26

2.9

As printed

5

-16

19

2

CoCrMo

Heat treated

-10

30

-1

As printed

15

-31

4

3

MS

Heat treated As printed Heat treated

0

-32 -16

1.4 -10

31

4

SS316L

-

As printed

-95

-110

-1.5

5

Ti6Al4V

Heat treated

-134

15

2

Figure 14(a-j) represents the fracture surfaces of the monolithic DMLS alloys in the as printed and heat treated condition. It can be seen that by enlarge all the DMLS alloys shows dimples on the fracture surface indicating ductile fracture in both the as printed and heat treated condition. The depth of the dimples can be indicated as an approximate measure of the ductility. IN718 shows very fine features in the as printed condition (Figure 14(a)) that nearly match the as printed layers and after heat treatment, distinct feature that correspond to the microstructural features of NbC /  -Ni 3 Nb circled as shown in Figure 14(b) corresponding reduced ductility. The ductility of IN718 after heat treatment is commensurate with that of wrought HSIN718 (Kruegar, D.D (1989)). In the case of CoCrMo, some lamellar tearing can be seen in the as printed fractograph (Figure 14(c)) corroborating with the presence of the hcp-beta phase in the as printed microstructure as shown in (Kaustubh Krishna Bawane et al. (2018)). In some sense it appears like a cleavage fracture in appearance characterized by a well-defined crystallographic appearance in the facets. Upon heat treatment the occurrence of an equiaxed recrystallized microstructure renders the alloy with a ductile matrix as indicative of the fractograph in Figure 14d, corresponding to a much higher ductility of 42%. In the case of maraging steel, the fractographs represents that of flat grooves with very fine dimples and a somewhat structureless region (Figure 14e). The sharp drop in ductility after heat treatment (5%) is akin to cold worked steels and in this case the yielding is likely to the strongly localized, as indicated by the fractograph in (Figure 14f). Coarse slip bands are formed providing the crack path and the brittle fracture propagates either in an intergranular (follows grain boundaries) or transgranular (through the grains). In the austenitic SS316L, the fractograph resembles a ductile failure except that the dimples have a characteristic fine feature commensurate with the weld pool width as shown in Figure 14(g). Ti6Al4V alloy (Figure 14 (h)) reveals an intergranular fracture along the alpha grains in the as printed condition, and does not show much of a variation after heat treatment (Figure 14 (i)), thereby corroborating with the microstructures (Figures 5(e) and 6(e)).