Composition-Dependent Energy Density Control for Defect Mitigation in Laser Directed Energy Deposition of SS316L–IN718 Functionally Graded Materials
International Journal of Precision Engineering and Manufacturing, 2026 (SCI-Expanded, Scopus)
- Yayın Türü: Makale / Tam Makale
- Basım Tarihi: 2026
- Doi Numarası: 10.1007/s12541-026-01597-3
- Dergi Adı: International Journal of Precision Engineering and Manufacturing
- Derginin Tarandığı İndeksler: Science Citation Index Expanded (SCI-EXPANDED), Scopus, Aerospace Database, Compendex, INSPEC, Materials Science & Engineering Collection (ProQuest), Technology Collection (ProQuest)
- Anahtar Kelimeler: Directed energy deposition, Energy density, Functionally graded materials, IN718, Microstructure, SS316L
- Gazi Üniversitesi Adresli: Evet
Özet
Functionally graded materials (FGMs) fabricated by laser-based directed energy deposition (LDED) offer a promising route for integrating dissimilar alloys; however, achieving low-defect multilayer structures remains challenging due to composition dependent thermal behavior, which directly affects defect formation and microstructural stability. This study systematically investigates the role of composition-dependent energy density (ED) control in governing defect formation, microstructural evolution and mechanical response in multilayer SS316L-IN718 FGMs. Three ED strategies were systematically compared: a composition-mismatched under-optimized strategy (FGM 1), a high-input (delayed-reduction) strategy (FGM 2) and a composition-adaptive gradient strategy (FGM 3). Optical and electron microscopy revealed that insufficient energy density resulted in incomplete melting, lack of fusion defects and the highest porosity (2.47% in FGM 1), whereas optimized ED conditions significantly improved metallurgical bonding and interfacial continuity. While the FGM 2 minimized bulk porosity to 0.87% through severe layer remelting, it triggered intense thermal confinement and macrostructural residual strain volatility. Conversely, the composition-adaptive gradient strategy (FGM 3) successfully mitigated this thermal bottleneck by systematically scaling down the heat input, achieving chemically uniform transitions and suppressing excessive molten pool stirring despite a negligible fraction of isolated micro-pores (1.11% porosity). EBSD results demonstrated that while FGM 2 produced continuous columnar grains within localized analysis zones, the resulting matrix remained highly strained. Microhardness mapping perfectly reflected these global thermal evolution trends, confirming that the composition-adaptive gradient strategy (FGM 3) yielded an exceptionally uniform mechanical response with exceptionally narrow local hardness dispersion (minimal error bars), whereas FGM 2 suffered from massive local hardness fluctuations (large error bars) induced by thermal accumulation strains. A critical transition zone centered around 25 wt% IN718 was identified, where precise thermal input control is required to suppress defect formation. The findings establish process-structure relationships that provide practical guidance for defect mitigation and process window design in multi-material LDED fabrication by proving that trading a minor fraction of local density can successfully pre-empt catastrophic internal stress concentrations in graded structures.