Contrasting the Role of Pores on the Stress State Dependent Fracture Behavior of Additively Manufactured Low and High Ductility Metals

Abstract

This study investigates the disparate impact of internal pores on the fracture behavior of two metal alloys fabricated via laser powder bed fusion (L-PBF) additive manufacturing (AM)—316L stainless steel and Ti-6Al-4V. Data from mechanical tests over a range of stress states for dense samples and those with intentionally introduced penny-shaped pores of various diameters were used to contrast the combined impact of pore size and stress state on the fracture behavior of these two materials. The fracture data were used to calibrate and compare multiple fracture models (Mohr-Coulomb, Hosford-Coulomb, and maximum stress criteria), with results compared in equivalent stress (versus stress triaxiality and Lode angle) space, as well as in their conversions to equivalent strain space. For L-PBF 316L, the strain-based fracture models captured the stress state dependent failure behavior up to the largest pore size studied (2400 µm diameter, 16% cross-sectional area of gauge region), while for L-PBF Ti-6Al-4V, the stress-based fracture models better captured the change in failure behavior with pore size up to the largest pore size studied. This difference can be attributed to the relatively high ductility of 316L stainless steel, for which all samples underwent significant plastic deformation prior to failure, contrasted with the relatively low ductility of Ti-6Al-4V, for which, with increasing pore size, the displacement to failure was dominated by elastic deformation.

Special Issue: The Science and Technology of 3D Printing

“…This Special Issue contains eleven articles, including three reviews [ 3 , 4 , 5 ], one perspective article [ 6 ], and seven research articles [ 7 , 8 , 9 , 10 , 11 , 12 , 13 ] from leading institutes in the United States, China, Australia, Germany, Sweden, the Netherlands, Slovakia, the Czech Republic, Egypt, and United Arab Emirates. These articles cover the 3D printing of diverse materials such as metallic materials [ 5 , 7 , 8 , 9 , 10 , 11 , 12 , 13 ], composites [ 4 , 6 ], and soft materials [ 3 ].…”

“…The articles in this Special Issue cover a wide variety of experimental [ 8 , 9 , 10 , 11 , 13 ], theoretical [ 12 ], and data-science [ 7 ]-based research on the science and technology of 3D printing. For example, experimental investigations were performed to identify the most important factors that affect the microstructure and properties of Ti-6Al-4V parts printed using powder bed fusion [ 9 , 11 ].…”

“…In particular, the effects of powder morphology, preheating temperature, and post-process heat treatment on the size of α-grains, the hardness, and the tensile properties were studied. In addition, formation mechanisms of defects such as pores and cracks, and their harmful effects on the mechanical properties of printed parts were investigated [ 8 , 10 , 12 , 13 ]. A high-speed synchrotron X-ray imaging technique [ 10 ] was used to reveal the mechanisms of the evolution of pores during powder bed fusion.…”

“…A new computer-aided quality (CAQ) technology [ 13 ] was proposed and used to determine the amount of porosities in the parts printed using powder bed fusion. The mechanical properties of stainless steel and titanium alloy parts were shown to be affected by the presence of pores [ 8 ]. A theoretical model was proposed and used to identify the conditions of crack growth and to discuss the detrimental effects of cracking on the mechanical properties of printed parts [ 12 ].…”

Investigation on Strain Hardening and Failure in Notched Tension Specimens of Cold Rolled Ti6Al4V Titanium Alloy

“…The damage and fracture behaviors of ductile material are strongly influenced by the stress state [ 7 , 8 ]. For example, during tension loading condition the ductile damage is mainly caused by the formation and growth of voids, whereas under compression and shear loadings, the dominant damage mechanism is the nucleation and growth of shear cracks [ 9 ].…”