PSI - Issue 19

S. Periane et al. / Procedia Structural Integrity 19 (2019) 415–422 S.Periane et al./ Structural Integrity Procedia 00 (2019) 000 – 000

417

3

4. Results and discussion

4.1. Microstructural observation The samples were cut to dimension of 10x10x10 mm 3 and mechanically polished and then etched by Kalling’s reagent. The below figure 2 shows the microstructure of the sample in the as-built and after heat treated such as: 1. HIP (1160°C and 102 MPa pressure for 3 hours) as per ASTM F3055 to reduce porosity. 2.AHT (720°C for 8 hrs, furnace cooled for 1.5 hrs again heated at 620°C for 8 hrs and then air cooled to room temperature) to enhance precipitate formation to get better mechanical properties.

Fig. 2. (a) asbuilt; (b) after heat treatment (HIP+AHT)

a) b) Fig. 2. (a) Microstructure of asbuilt sample; (b) microstructure of HIPed + AHT sample

The microstructure is then observed using optical microscope Keyence VHX 2000 with 500X magnification. In figure 2 (a) the as-built microstructure shows the melt tracks due to laser intensity, the maximum depth of a melt track overlapping into subsequent layer is about 120 µm. Presence of many pores marked in red circle are present at middle and at the periphery of the melt tracks. In order to increase the density HIP heat treatment was performed followed by AHT heat treatment shown in figure 2 (b). The microstructure in figure 2 (b) is completed changed compared to the as-built in figure 2 (a) due to the formation of the precipitates and total dissolution of the melt tracks.

4.2. Mechanical properties of the as-built and heat-treated sample

The Vickers hardness test was conducted on the as-built and heat-treated samples by applying a load of 20kg for 15s as per ASTM E384 standard. About 25 indents were performed in 5 rows for each sample in both as-built and in heat treated conditions. Figure 3 shows the hardness value for as-built it is around 297HV whereas for heat treated sample the hardness value has increased to about 433HV. i.e. around 31% increase in the hardness value after heat treatment mainly due to the formation of the precipitates (strengthening phase) and reduction of porosity.

Fig. 3. Vickers hardness value of as-built and heat-treated samples