Additively manufactured hierarchical stainless steels with high strength and ductility

Abstract

Many traditional approaches for strengthening steels typically come at the expense of useful ductility, a dilemma known as strength-ductility trade-off. New metallurgical processing might offer the possibility of overcoming this. Here we report that austenitic 316L stainless steels additively manufactured via a laser powder-bed-fusion technique exhibit a combination of yield strength and tensile ductility that surpasses that of conventional 316L steels. High strength is attributed to solidification-enabled cellular structures, low-angle grain boundaries, and dislocations formed during manufacturing, while high uniform elongation correlates to a steady and progressive work-hardening mechanism regulated by a hierarchically heterogeneous microstructure, with length scales spanning nearly six orders of magnitude. In addition, solute segregation along cellular walls and low-angle grain boundaries can enhance dislocation pinning and promote twinning. This work demonstrates the potential of additive manufacturing to create alloys with unique microstructures and high performance for structural applications.

Microstructure, Solidification Texture, and Thermal Stability of 316 L Stainless Steel Manufactured by Laser Powder Bed Fusion

“…In metallic materials manufactured by LPBF, microsegregations are not uncommon and they can cause undesirable effects like hot cracking [41,42]. In LPBF 316 L steel, segregation of Cr and Mo on cell boundaries has been reported [22,43,44]. In the present investigation, TEM EDX and APT were carried out to study microsegregation in LPBF 316 L austenitic stainless steel.…”

“…cell boundaries has been reported [22,43,44] Figure 5a was cut out from the cell body, while Specimen 2 was from the region containing the cell boundary, which does not contain a boundary and, therefore, does not contain any segregation of elements at the atomic scale. Figure 5c illustrates the size of the APT specimen, which was obviously much smaller when compared to the cell boundary thickness.…”

“…Quite remarkable variations of yield strength (300-600 MPa), ultimate tensile strength (350-650 MPa), and elongation (10-60%) are usually observed [5,6,[18][19][20][21]29,36,44,46,47]. Higher strength could be associated with fine structure, high dislocation density, cellular structure, and the presence of nanoparticles.…”

“…A lower elongation in LPBF material is associated with high strength; additionally, porosity and internal defects contribute to a large scatter in elongation values. A summary of the mechanical characteristics of LPBF 316 L steel can be found in [5,6,44]. Figure 8 illustrates the dependence of the hardness of AM and conventional 316 L steel on the inversed square root of primary cell spacing.…”

Processing Parameter DOE for 316L Using Directed Energy Deposition

“…MAM components undergo thermal cycles that are unique from other fabrication options. Large thermal gradients can occur, where the temperature can vary between 40 • C and 1000 • C within a short distance (mm's) in the build, and it can also experience rapid solidification (~10 3 -10 8 K/s) [6,7]. To evaluate the effect of these unique thermal cycles seen in MAM, empirical methods are often used to correlate thermal cycles to a set of processing parameters, without grasping the complexity of the essential foundations of these effects (i.e., what role does the heat input have on solidification).…”