PSI - Issue 14

Manojakumar Chimmat et al. / Procedia Structural Integrity 14 (2019) 746–757 Author name / Structural Integrity Procedia 00 (2018) 000 – 000

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Figures 7a and 8a show representative optical micrographs taken from the CoCrMo Cube and Coupon and SS316L Cube, in the as printed condition. There was no variation in the microstructure between the CoCrMo Cube and Coupon, after etching. A layer-by-layer melt pool structure can be observed having a width of 115-130  m and a depth of 70-80  m for CoCrMo (Fig. 7a). A similar microstructure was observed in the SS316L having a melt pool width of 80  m and a depth of 82  m as shown in Fig. 8a. The melt pool depth and width are representative of the respective process parameters used to build these parts, including layer thickness and laser power. Optical micrograph of CoCrMo Cube after being exposed to 950°C indicates the onset of the recrystallization, as shown in the Fig. 7c. At 1050°C, elongated grains can be observed, as shown in the Fig. 7c, and at 1150°C equiaxed grains with an average grain size of 35-40 µm can be seen (Fig. 7d) indicative of a completely recrystallized microstructure. The optical micrograph of the SS316L, after heat treatment at 650°C shows columnar elongated grains, as shown in the Fig. 8b. The hardness was found to be uniform across the CoCrMo Coupon span, measuring between 420-460 HV. The hardness was found to decrease to 340-380 HV after heat treatment. The hardness in SS316L Cube after HT at 650°C decreases from 210 HV to 185 HV. The decrease in the hardness after heat treatment was because of the stress relieving and recrystallization. The variation for both alloys in the as printed and heat treated condition is shown in the Fig. 9.

b)

Average size 20  m

a)

50 µm

Fig. 2. a) SE image showing size and morphology of as received CoCrMo powder b) powder size distribution.

a)

Average size 55  m

50 µm

Fig. 3. a) SE image showing size and morphology of as received SS316L powder b) powder size distribution