In-situ tailored grain refinement of 316L stainless steel via site-specific titanium inoculation using laser directed energy deposition

Controlling grain structure in metal additive manufacturing is critical to mitigating anisotropic mechanical properties caused by long columnar grains forming along the build direction, reducing hot cracking susceptibility, and enhancing strength via the Hall-Petch effect. This study investigates in situ grain refinement of 316L stainless steel fabricated by Directed Energy Deposition (DED) through direct injection of titanium (Ti) particles into the 316L melt pool. Unlike premixed powders—which can lead to non-uniform transport due to differences in density, morphology, and carrier gas interaction, and limit composition flexibility—direct Ti injection enables real-time control over alloying. A comprehensive parametric study was conducted by varying nozzle velocities up to 6000 mm/min, Ti mass flow rates, and the number of remelting passes to assess their effects on the transition from coarse columnar to fine equiaxed grains. Experimental results showed that Ti addition promotes equiaxed grain formation primarily via heterogeneous nucleation, attributed to the formation of nano-precipitates such as titanium oxides within the melt pool. Complementary Multiphysics simulations provided insights into melt pool dynamics, convection- and diffusion-driven Ti dissolution, and informed Thermo-Calc predictions to assess phase stability and avoid brittle Fe-Ti intermetallics. Low Ti mass flow rates refined grains effectively without inducing phase transitions, whereas higher Ti contents led to brittle intermetallic phases, cracking, and partial transformation from austenite to ferrite. This integrated experimental-modeling approach demonstrates a viable strategy for achieving tailored microstructural control in DED through Ti inoculation and optimized processing parameters.