Spatially engineered strength: Thermomechanical notch strengthening in additively manufactured 316L-IN718

Abstract

• In the presence of weak material interfaces, notch strengthening is viable for many notch geometries. • Notch strength increased by 40% and 108% with 316L-IN718 at room and elevated temperature respectively. • Thermal expansion mismatch induces compressive residual stress which increases the strength of the interface with temperature. This study demonstrates notch strengthening using multi-material additive manufacturing (MMAM) where multiple materials are deposited to create a functionally graded component. It is hypothesized that MMAM can improve the overall part performance. To that end, rectangular bars of stainless steel 316L and 316L-Inconel 718 (316L-IN718) were printed by directed energy deposition. Tensile tests determined that 316L exhibited better tensile properties than 316L-IN718 interface. Digital image correlation shows brittle-fracture at the 316L-IN718 interface. A stress concentration factor design tool was developed, to design a double-edge notch geometry where failure occurs in the notch rather than in the 316L base or 316L-IN718 interface. Many notch dimensions are viable. A single set was selected for evaluation. Notch strengthening was achieved with a 40% and 20% increase in strength and toughness respectively. Tests conducted at 650°C showed a remarkable 108% and 51% further enhancement in the properties. At elevated temperature, thermal expansion mismatch coupled with interface topology induce beneficial compressive residual stress. Optical microscopy reveals residual meltpool beads at the interface. Laser ablation laser ionization, time of flight, mass spectrometry (LALI-TOF-MS) shows a steep compositional gradient and segregation within the interface. Scanning electron microscopy shows elongated grains with a strong segregation of Mo and Cr-rich laves. Bands of chevron shaped laves are interrupted by lave-free bands with high Fe; indicative of Marangoni mixing of the two alloys. Finite element analysis shows that compressive stress and plastic strain at the interface. This thermally activated strengthening mechanism increases with temperature. Overall, MMAM notch strengthening is viable.