PSI - Issue 14

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

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Figure 6 (a-e) represents the optical micrographs of polished and etched surfaces along the build direction for all alloys in heat treated condition. Partial recrystallization with elongated grains can be observed after heat treatment in IN718, as shown in Figure 7(a). After heat treatment at 1150ºC, CoCrMo exhibited an equiaxed microstructure with grain size of about 37-42 µm Figure 7(b)). Figure 7(c) is a representative micrograph from maraging steel. There appears to be no significant change to the microstructure as compared to the as printed microstructure after this heat treatment as seen from the presence of the weld pools (Figure 7c). SS316L shows a columnar grain structure after the heat treatment as represented in Figure 7(d). DMLS Ti6Al4V shows a characteristic biphasic  +  phase for Ti6Al4V after heat treatment as shown in Figure 7(e).

a

b

CoCrMo

Maraging steel

c

IN718

100 µm

100 µm

100 µm

d

SS316L

e

Ti6Al4V

100 µm

100

Fig. 6. Representative optical micrographs taken from heat treated DMLS samples after etching (along the build direction) (a) IN718, (b) CoCrMo, (c) Maraging steel, (d) SS316L, (e) Ti6Al4V

0 200 400 600 800

IN718

CoCrMo Maraging steel

SS316L

Ti6Al4V

Microhardness (HV)

As printed Heat treated

Fig. 7. Microhardness of monolithic DMLS alloys in the as printed and heat treated condition

Figure 7 shows a comparison of the microhardness for all the alloys in the as printed and heat treated condition. For CoCrMo, SS316L and Ti6Al4V there is a drop in hardness after heat treatment by 12%, 12% and 17% respectively. The drop in hardness may be due to recrystallization with equiaxed grains for CoCrMo, stress relieving which makes material little softer for SS316L and conversion of  -phase laths to biphasic  +  phase for Ti6Al4V after respective