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Oil-soluble ionic liquids (ILs) have recently been demonstrated as effective lubricant additives of friction reduction and wear protection for sliding contacts. However, their functionality in mitigating rolling contact fatigue (RCF) is little known. Because of the distinct surface damage modes, different types of surface protective additives often are used in lubricants for sliding and rolling contacts. Therefore, the lubricating characteristics and mechanisms of ILs learned in sliding contacts from the earlier work may not be translatable to rolling contacts. This study explores the feasibility of using phosphonium-phosphate, ammonium-phosphate, and phosphonium-carboxylate ILs as candidate additives in rolling–sliding boundary lubrication, and results suggested that an IL could be either beneficial or detrimental on RCF depending on its chemistry. Particularly, the best-performing phosphonium-phosphate IL at 2% addition made a low-viscosity base oil significantly outperform a more viscous commercial gear oil in reducing the RCF surface damage and associated vibration noise. This IL generated a thicker, smoother, and more homogeneous tribofilm compared with commercial additives, which is likely responsible for the superior RCF protection. Results here suggest good potential for using appropriate IL additives to allow the use of low-viscosity gear and axle fluids for improved efficiency and durability.

To understand the process-microstructure relationships in additive manufacturing (AM), it is necessary to predict the solidification characteristics in the melt pool. The present study investigates the influence of Marangoni driven fluid flow on the predicted melt pool geometry and solidification dynamics using a continuum finite volume model (FVM). A calibrated laser absorptivity was determined by comparing the model predictions (neglecting fluid flow) against melt pool dimensions obtained from single laser melt experiments on a nickel super alloy 625 (IN625) plate. Using this calibrated efficiency, the predicted melt pool geometries agree well with experiments across a range of process conditions. Using a surface tension gradient recommended for IN625 tends to overpredict the influence of convective heat transfer, but the use of an intermediate value reported from experimental measurements of a similar nickel super alloy produces excellent results. Despite its significant effect on the melt pool geometry predictions, fluid flow was found to have a small effect on the predicted solidification conditions compared to processing conditions. This result suggests that under certain circumstances, a model only considering conductive heat transfer is sufficient for approximating process-microstructure relationships in laser additive manufacturing. Extending the model to multiple laser passes then showed that fluid flow also has a small effect on the solidification conditions compared to the transient variation within the process. Limitations of the current model and areas of improvement, including uncertainties associated with the phenomenological inputs to the model, are discussed.