PSI - Issue 14

Author name / Structural Integrity Procedia 00 (2018) 000 – 000

8

Manojakumar Chimmat et al. / Procedia Structural Integrity 14 (2019) 746–757

753

As printed Heat treated

0 100 200 300 400 500

1150 C

650 C

Hardness (HV)

CoCrMo

SS316L

Fig. 9. Hardness of the as printed and the heat treated CoCrMo Coupon and SS316L Cube

3.2 Residual stress on CoCrMo Cube and Coupon Figure 10a shows a representative plot of 2θ v s sin 2 ψ from CoCrMo Cube sample taken from the side XZ surface, for calculating the residual stress. It can be seen that 10 ψ angles were used to fit the line. The reliability is an indicator of the goodness of fit and values between ±20-30 MPa was taken as an acceptable value including instrumental and measurement and surface deviations in measurement. In Fig. 10a, a reliability of 10 MPa was obtained. A representative plot of 2θ vs sin 2 ψ taken from the XZ surface after grit blasting is shown in Fig. 10b. It can be seen that the positive slope of the curve is indicative of compressive residual stresses. Figure 11a shows the variation of residual stress in the as printed and grit blasted CoCrMo Cube. All the surface stresses are tensile in the as printed condition, with the top most surface showing a maximum tensile stress of 265 MPa. All the sides of the CoCrMo Cube measured nearly half that of the top surface, ranging between 130-200 MPa. The residual stress measured in the current study is much lower as compared with the residual stress reported by Bawane et al. 2018 where a stress as high as 900 MPa has been reported in the as printed condition for CoCrMo Coupon. However, the high residual stresses resulted in a part distortion in the as printed condition as shown in Fig. 13. Subsequent process optimization has led to lower residual stresses as measured in the Cube and Coupon studied in this paper. Upon grit blasting, it can be seen that all the surfaces are compressive in nature and after heat treatment, the residual stresses are seen to undergo an interesting evolution, being tensile at a lower temperature of 950°C, followed by a slight compressive stress at 1050°C and then finally at 1150°C, is nearly stress relieved and shows a near zero residual stress. A similar trend is seen for the CoCrMo Coupon, as shown in the comparative plot in Fig.12, with a maximum tensile stress of 246 MPa in the as printed condition, a maximum or average compressive stress of 300 MPa after grit blasting and finally a completely stress relieved state after 1150°C. The findings of the current study reveals that there is no significant difference between Cube and Coupon.