The Importance of Subsurface Residual Stress in Laser Powder Bed Fusion IN718

The residual stress (RS) in laser powder bed fusion (LPBF) IN718 alloy samples produced using a 67°‐rotation scan strategy is investigated via laboratory X‐ray diffraction (XRD) and neutron diffraction (ND). The location dependence of the strain‐free (d0) lattice spacing in ND is evaluated using a grid array of coupons extracted from the far‐edge of the investigated specimen. No compositional spatial variation is observed in the grid array. The calculated RS fields show considerable non‐uniformity, significant stress gradients in the region from 0.6 to 2 mm below the surface, as well as subsurface maxima that cannot be accounted for via XRD. It is concluded that failure to determine such maxima would hamper a quantitative determination of RS fields by means of the stress balance method. Failure to consider the subsurface residual stress (RS) values when using the stress balance (SB) method for RS analysis will lead to a spurious computation (left bottom). Hence, a reliable quantitative evaluation of the RS state in a laser powder bed fusion IN718 allow requires a proper determination of the subsurface RS state (right bottom).