Laser powder bed fusion in situ alloying of AISI 316L-2.5%Cu alloy: microstructure and mechanical properties evolution

This work investigates the effects of copper addition on the microstructure and mechanical properties of AISI 316L austenitic stainless steel fabricated by the laser powder bed fusion (L-PBF) method. The outcomes reveal that the copper atom dissolves into iron and forms a complete austenitic structure under the condition of the L-PBF process. Microstructural observations demonstrate that the microstructure of the new alloy is characterised by columnar grains consisting of finer cellular structures, as compared to the as-built AISI 316L. The appearance of such a finer sub-structure could be originated from the effect of copper on the cooling rate during the L-PBF process. The energy-dispersive X-ray spectroscopy maps indicate that the distribution of copper in the AISI 316L matrix is homogeneous, and no significant segregation of elements in the matrix is revealed. The results of the tensile tests show that the ultimate tensile strength of AISI 316L-Cu alloy is 558 MPa, whereas the yield strength value and the tensile elongation are 510 MPa and 30.4%, respectively. Two mechanisms of solid solution strengthening, and refinement of cell sizes improve the mechanical properties of AISI316L-Cu alloy compared with AISI 316L one. The microscale fractography of the fracture surface shows ductile fracture with massive dimple networks and brittle fracture with a quasi-cleavage plane, which may indicate the melt pool boundary. All these results confirm that the development of new alloys following the in situ alloying approach is economical and reliable.