Influence of molten pool thermal history on weld quality and mechanical properties in laser welding of Inconel 718

Laser welding in keyhole mode typically exhibits keyhole-induced porosity, which adversely affects the mechanical performance of welded components; therefore, the present study aims to develop a real-time monitoring system using an IR pyrometer to capture the molten pool thermal history of Inconel 718 and establish its correlation with keyhole porosity quantified through micro-CT analysis. Welding was performed at welding speeds of 1200–2000 mm/min, corresponding to linear energies of 100–60 J/mm. Micro-CT analysis revealed that lower welding speeds (1200 mm/min) resulted in reduced porosity attributed increased molten-pool lifetime allowing the entrapped gas to escape, whereas higher speeds (2000 mm/min) promoted keyhole instability and gas entrapment due to reduction in molten pool lifetime. The as-welded samples exhibited ultimate tensile strength (UTS) values ranging from ∼765±33 MPa (2000 mm/min) to ∼850±41 MPa (1200 mm/min), compared to ∼920±28 MPa for the base material (BM), with corresponding elongation decreasing from ∼29±2.9 % to ∼19±2.5 % and the BM ∼53±3.8 %. The hardness in the as-welded condition found to be ∼303±27 HV 0.5 . These variations were attributed to the combined effects of porosity and Nb and Mo rich Laves phase formation. Post-weld heat treatment significantly improved mechanical performance. Solution treatment reduced strength (∼685±48–740±45 MPa) and hardness (∼246±53 HV 0.5 ) due to dissolution of strengthening precipitates, but improved ductility (up to ∼65±2.2 %). In contrast, precipitation hardening increased UTS to ∼1220±14–1250±11 MPa and hardness to ∼520±50 HV 0.5 (∼1.8-fold improvement compared to the base material) through the formation of γ′/γ″ precipitates, though ductility decreased (∼5 ± 1.1 % to ∼11±2.3 %) due to coarse porosity level.