Towards in-situ grain boundary engineering in additively manufactured stainless steel 316 L via reused powder

Grain boundary engineering (GBE) is an established microstructural design strategy to improve mechanical properties and minimize corrosion susceptibility in polycrystalline materials by promoting a high fraction of low-energy grain boundaries (GBs) such as Σ3 boundaries. Traditional GBE utilizes complex cycles of mechanical deformation and annealing to engineer the microstructure of metals and alloys to unlock such superior properties. However, thermomechanical processing is unsuitable for near-net-shape parts produced by additive manufacturing (AM), as it would irreversibly alter their precision-engineered geometries. An innovative solution involves adapting GBE by modulating the strain energy during AM, to generate sufficient driving force for recrystallization. Nonetheless, achieving complete recrystallization in AM microstructures generally requires additional post-AM annealing, which remains time- and energy-consuming. Here, we highlight the formation of a GBE-like microstructure characterized by fine grains and a high density of Σ3 boundaries directly in the as-built LPBF 316L stainless steel, achieved through powder recycling. We argue that this is based on the nucleation of twin-related austenitic grains directly from the solidifying liquid via icosahedral short-range ordering and a modified solidification pathway, where ferrite initially forms and subsequently undergoes a massive transformation into austenite. We also show how these as-built microstructures can be further improved via ex-situ GBE through post-AM annealing, enabling additional property optimization. Our new in-situ AM GBE route paves the way for the design of high-performance metal AM parts with superior properties, using recycled powder.