Microstructure–Property Correlation in a Laser Powder Bed Fusion Processed High‐Strength AF‐9628 Steel

Laser powder bed fusion additive manufacturing (LPBF‐AM) of a low‐alloy, high‐performance AF‐9628 steel results in exceptionally high strength and good ductility. The reasons for such mechanical properties are investigated through detailed microscopy performed at several length scales. Thus, the characterization of melt pool, porosity, grain morphology, phases, and dislocations is performed in the as‐printed material. The as‐printed material consists of only 0.004 vol% of uniformly distributed porosity, single‐phase martensitic laths with an average lath size of ≈2.5 μm, the absence of carbides indicating interstitial trapping of C atom, and high dislocation density in the martensitic laths. Experimental data through microscopy are then fed in analytical models for calculating strengthening contributions from various strengthening mechanisms. Calculated yield strength agrees well with experimentally determined value, and therefore, activation of various strengthening mechanisms is established in as‐printed AF‐9628. Laser powder bed fusion additive manufacturing of a low‐alloy AF‐9628 steel results in an ultra‐high yield strength of 1.5 GPa and a ductility of 10%. Detailed microscopic investigation at several length scales shows martensitic laths with high dislocation density. Solid solution strengthening due to interstitial C atoms and high dislocation density contribute to the strengthening of the material.