Development of High Density Parts in the Low-Alloy, High-Performance Steel AF9628 Using Laser Powder Bed Fusion

A process parameter development study was performed in order to determine the printability of a low-alloy, high-performance steel, AF9628, using laser powder bed fusion. A weld track study was performed using 40 distinct laser power and speed combinations in order to determine which combinations produced acceptable conduction welds that would yield high-quality parts. Ten combinations with three distinct hatch spacing values were selected to create cylindrical specimens for porosity studies. Eight out of the ten combinations resulted in parts that were 99.5% dense. Two combinations that produced 99.9% dense parts were used to create tensile specimens. Tensile testing revealed that the ultimate tensile strength (UTS) for as-printed specimens manufactured using both of the processing conditions was significantly higher (24%) than the literature values for wrought AF9628. Heat-treating the specimens reduced their UTS values, but they still exceeded the literature value by 8%. Hardness measurements indicate that the Vickers hardness is approximately 10% lower for the as-printed specimens when compared to the literature value for wrought AF9628, while it is at least 6% greater for the heat-treated specimens than the wrought AF9628. Electron backscatter diffraction results showed that the as-printed microstructure exhibited features typical of the martensitic transformation in quench-and-temper steels

Relationship Between Molecular Contact Thermodynamics and Surface Contact Mechanics

Measurements have been made of the adhesion and friction forces between organic monolayers in heptane/ acetone mixtures using an atomic force microscope (AFM). It has been found that the contact mechanics are best modeled by treating the friction force as the sum of a load-dependent term (attributed to “molecular plowing”) and an areadependent term attributed to shearing (adhesion). The relative contributions of plowing and shearing are determined by the coefficient of friction, μ, and the surface shear strength τ. The transition from adhesion- to load-determined friction is controlled by the solvation state of the surface: solvated surfaces represent a limiting case in which the shear term approaches zero, and the friction-load relationship is linear, while in other circumstances, the friction-load relationship is nonlinear and consistent with Derjaguin−Muller−Toporov mechanics. A striking correlation has been observed between the concentration-dependence of the association constant (Ka) for the formation of 1:1 hydrogen-bonded complexes and the pull-off force Fa and surface shear strength τ for the same molecules when one partner is immobilized by attachment to an AFM probe and the other is adsorbed to a surface. Analysis of the concentration-dependence of Fa and τ enables the prediction of KS with remarkably high precision, indicating that for these hydrogen bonding systems, the tip−sample adhesion is dominated by the H-bond thermodynamics. For mixed monolayers, H-bond thermodynamics dominate the interaction even at very low concentrations of the H-bond acceptor. Even for weakly adhering systems, a nonlinear frictionload relationship results. The variation in τ with the film composition is correlated very closely with the variation in Fa. However, the coefficient of friction varies little with the film composition and is invariant with the strength of tip−sample adhesion, being dominated by molecular plowing and, for sufficiently large concentrations of hydroxyl terminated adsorbates, the disruption of intramonolayer hydrogen bonding interactions