Laser powder bed fusion (LPBF) is a bottom-up manufacturing technique using a high-energy laser to selectively melt metallic alloys, enabling the creation of complex microstructures. While rapid cooling rates and process stochasticity can lead to unpredictable properties, LPBF's layer-wise powder deposition method unlocks unprecedented opportunities for in-situ alloying and microstructure engineering. This work introduces a novel blend of 316 L stainless steel with less than two percent of aluminium, demonstrating the potential for readily tunable microstructures leading to versatile mechanical properties – either as-built or following a post-process heat treatment. Compared to conventional fully austenitic LPBF 316 L, this new alloy solidifies into a combination of large BCC delta-ferrite grains and fine FCC gamma-austenite grains. Medium temperature furnace heat treatments significantly reinforce the BCC phase through the formation of NiAl B2 precipitates, leading to hardness values up to 708 HV. Furthermore, in situ selective laser heat treatments allow the transformation from columnar to equiaxed microstructure in a very short time. This unique alloy therefore appears well suited for tailoring local mechanical properties of phases through appropriate ex situ or in situ heat treatments, paving the way for designing composite-like materials within a single build.