Primary Processing Parameter Effects on Defects and Fatigue in Alloy 718

Abstract

To guarantee the quality of additively manufactured parts, the processing-structure-property-performance (PSPP) relationship must be understood. Despite the current applications of additive manufacturing (AM) in various industries, a suitable understanding of material performance under fatigue loading conditions is lacking. Fatigue performance is largely driven by defects in the material, such as porosity and surface roughness. AM process optimization often is completed through trial-and-error iterations misguided by average values to characterize defects that do not capture the fatigue-limiting behavior. This work investigated the PSPP relationship for a range of AM processing parameters, resulting in variations in porosity and surface roughness for alloy 718 fabricated in the laser powder bed fusion AM process. Nondestructive and destructive characterization methods are used to determine the relationships between varied bulk and contour parameters to porosity and surface roughness characteristics. Porosity coupons with varied bulk parameters were built in the vertical orientation and the internal porosity size and shape metrics were measured using computed tomography. Fatigue testing on machined bars at the nominal parameter set was performed to determine the baseline performance of the AM material. Fatigue bars with as-built surfaces and varied contour parameters were built in the vertical orientation to determine the relationship between surface roughness and fatigue life. The surfaces were characterized before testing using areal surface metrology to determine the relationship of processing parameters to the standard height-based measurement methods. By using these methods, the determined PSPP relationships for porosity and surface roughness can be used to guide process optimization and to inform the qualification strategy for AM components.