Process qualification of laser powder bed fusion based on processing-defect structure-fatigue properties in Ti-6Al-4V

Abstract

The aim of this manuscript is to give a compact overview of the results that illustrate the applicability of processing-structure-property relationships in the increasingly important context of 3D printing of metals. A process qualification approach based on the physics-based understanding of defect formation in laser powder bed fusion (L-PBF) additive manufacturing (AM) is investigated for an aerospace-grade titanium alloy (Ti-6Al-4V). A physically interpretable qualification approach is critical for enabling L-PBF part certification for structure-critical applications. This approach relies on systematic experimentation, characterization, testing, and data analysis tasks including design of experiments varying power and velocity to generate varying defect populations, process window development based on defect structure, high throughput fatigue testing, and fractography, 2D porosity characterization, and use of extreme value statistics to develop a porosity metric that, in turn, could have predictive power for the variation in fatigue performance. Results from four-point bend fatigue tests demonstrate that a process window can be defined based on this key mechanical property. This relatively high throughput approach can, in turn, support a reduced set of round bar fatigue tests typically used for qualification. Overall, the proposed ecosystem for process qualification of L-PBF AM shows promise and is expected to apply to other materials and powder bed fusion AM technologies. • A delineated process window for L-PBF printing with Ti-6Al-4V was identified for both porosity and fatigue in power and velocity space with all other variables fixed, which is in good agreement with previous work. • There is a distinct increase in porosity as one moves into the keyhole range (low speed, high power) or towards lack-of-fusion (high speed, low power). • Extreme value analysis (EVA) is found to be beneficial in quantifying pore sizes and frequencies. • The fatigue life measured in 4-point bend fatigue mirrors the variations in porosity, i.e., the fatigue life decreases as porosity increases and is maximal within the process window. • 4-point bend fatigue is found to be practical for screening printed material quality.