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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Optical microscopy was carried out using Nikon Eclipse MA200 optical microscope with a Clemex image analysis system. The porosity was evaluated along (longitudinal) and transverse to the build direction. Electrolytic etching of CoCrMo and SS316L was carried out using 5% hydrochloric acid at 6V for 10 seconds and 10% oxalic acid at 3V for 30 seconds, respectively. Vickers microhardness measurement was carried out using Shimadzu microhardness tester using a 300 gm load,10 s dwell time. Residual stress measurements were carried out on the as printed, grit blasted and heat-treated samples using a Rigaku Automate-II micro-area X-ray residual stress machine. The X-ray wavelength used for both the alloys was Cr- Kα source (λ =2.29 A°). Residual stress measurement was carried out using (220) diffraction peak of FCC Cobalt and (220) diffraction peak for SS316L corresponding to FCC Fe, using Young’s modulus 235 MPa for CoCrMo and 200 MPa for SS316L . The parameters used for residual stress measurements is listed in Table 3. A graph comprising graph was plotted, considering a minimum of 7 and maximum of 13 “ Ψ” values and via a fitting a least square regression line the slope “M” was measured. As per the convention a positive slope in the graphs indicates compressive and a negative slope indicates tensile stresses. The stress was calculated by multiplying the stress constant (K) with the slope M. The stress constant “K” (-754.39 MPa/deg for CoCrMo and -663.57 MPa/deg for SS316L) was calculated from the elastic constants.

(1)

Where, the constant K (MPa/deg) :

=

0 .

Tilt of diagram M = 0 = Bragg angle without strain, E = Young’s modulus , :

= Poisson’s ratio

Surface residual stresses were measured on Cube faces marked as XZ, YZ, top and bottom (opposite of top) surfaces as shown in Fig. 1a. In some locations where the reliability of measurements was not good, electropolishing was carried out to remove 50-100  m from the surface using (80:20: Methanol: Perchloric acid, 5V at the rate of 40 µm/min for CoCrMo and 95:5: Ethanol: Perchloric acid, 35V at the rate of 45 µm/min for SS316L). For the Cube samples a minimum of three measurements were taken at different points along the longitudinal direction to compute the average residual stress. For the Coupon sample, surface residual stresses were measured at four points along the longitudinal direction on both sides i.e. Side A and Side B (other side of the Coupon), as shown in Fig. 1b. CoCrMo was studied for Cube and Coupon geometries and SS316L was studied for the Cube geometry. 3. Results and Discussion 3.1 Microstructural characterization: SEM of the starting powders reveal a spherical morphology with particle size in the range of 5-45 µm, with an average of 20 µm for CoCrMo and 20-90 µm with an average of 32 µm for SS316L, respectively as shown in representative micrographs in Fig. 2 (a-b) and Fig. 3 (a-b) respectively. Figure 4 shows the surface roughness of CoCrMo Cube and Coupon and SS316L Cube in the as printed and grit blasted condition, along and perpendicular to the build direction. The surface roughness for as printed CoCrMo Cube and Coupon ranges between 1-5 µm and 3-5 µm, along the longitudinal and transverse directions respectively. The surface roughness for grit blasted CoCrMo Cube and Coupon ranges between 4-5 µm and 3-4 µm, along the longitudinal and transverse directions respectively The surface roughness for as printed SS316L Cube ranges between 8-9 µm and 9-11 µm, along the longitudinal and transverse directions respectively. SS316L records a slightly higher roughness as compared to CoCrMo. Figure 5 shows representative optical micrographs of CoCrMo in the unetched condition, taken from the Cube and Coupon. There was no significant difference in the porosity between the CoCrMo Cube and Coupon. The average porosity was around 0.043±0.04%, with little or no variation in the distribution along the build direction. Figure 6 shows the porosity distribution along the longitudinal and transverse section of the as deposited SS316L Cube. The average porosity in the longitudinal direction is around 0.038% and in transverse direction it is 0.128%.