When pulsed laser beams deposit spatially and temporally localized power on the metallic surfaces, microstructure and mechanical properties could change significantly. During and after each pulse, the exposed surface experiences a cycle of thermal processes involving extremely high heating and cooling rates, which consequently cause significant microstructure evolution. Changes in mechanical strength follow such microstructural changes. In this work, the influence of pulsed laser processing (PLP) on microstructural evolution, resulting hardness and wear resistance of Ti6Al4V surfaces is investigated. Average pulse power is varied from surface heating to melting regimes in processing. A 2D axisymmetric finite element model of a single pulse is utilized to obtain the heating and cooling histories, and a phase transformation mapping procedure is used to obtain the microstructural evolution. Melting (MZ) and heat affected zones (HAZ) observed at the cross sections of the processed surfaces are found to correlate with the predicted temperature field. The resulting phases in those zones are predominantly martensitic α' and α phase; higher laser power produces thicker martensitic α' surface layers several microns into the processed surface. Nanoindentation, nano/microscale and mesoscale wear tests are then applied to the processed surfaces to identify the effects of microstructure on hardness and wear resistance. The surface processed by higher laser power shows higher hardness and wear resistance compared to the surface processed by low power laser. The hardness and wear resistance of the surface processed by low power laser shows no obvious change from substrate material. Mixture of hard martensitic surface layer and underlying ductile substrate resulting from PLP facilitate superior resistance to abrasive and partly low cycle fatigue wear.

Interfacial damping in assembled structures is difficult to predict and control since it depends on numerous system parameters such as elastic mismatch, roughness, contact geometry, and loading profiles. Most recently, phase difference between normal and tangential force oscillations has been shown to have a significant effect on interfacial damping. In this study, we conduct microscale (asperity-scale) experiments to investigate the influence of magnitude and phase difference of normal and tangential force oscillations on the energy dissipation in presliding spherical contacts. Our results show that energy dissipation increases with increasing normal preload fluctuations and phase difference. This increase is more prominent for higher tangential force fluctuations, thanks to larger frictional slip along the contact interface. We also show that the energy dissipation and tangential fluctuations are related through a power law. The power exponents we identify from the experiments reveal that contacts deliver a nonlinear damping for all normal preload fluctuation amplitudes and phase differences investigated. This is in line with the damping uncertainties and nonlinearities observed in structural dynamics community.

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Seemingly-stationary (pre-sliding) interfaces between different materials, parts and components are major sources of compliance and damping in structures. Classical pre-sliding contact models assume smooth elastic contact and predict that frictional slip leads to a well-defined set of stiffness and damping nonlinearities. However, reported data deviate from those predictions, and literature lacks a conclusive evidence leading to those deviations. In this work, the authors measure tangential stiffness and damping capacities inside a scanning electron microscope (SEM) while monitoring contacts between a rigid spherical probe and two materials (high density polyethylene-HDPE and polyurethane elastomer). Measured force, displacement, contact area, stiffness and damping are then compared to predictions of classical models. In-situ SEM images synchronized to the tangential force-displacement responses are utilized to relate the degree of plasticity and geometric alterations to stiffness and damping nonlinearities. In agreement with the classical models, increasing tangential loads cause softening in contacts under light normal preloads. In contrast, stiffness for HDPE increases with increasing tangential loads at heavy normal preloads due to plasticity and pile-ups over the contact. Material damping is prevalent for all loading cases in polyurethane samples thanks to nearly fully adhered contact, whereas for only light tangential loads in HDPE. With increasing tangential loading, specific damping capacity of HDPE contacts increases 10-fold. This nonlinear increase is due to plastic shearing and frictional losses induced by tangential loading. Those findings suggest that predictive interface models should include geometric alterations of contact, plasticity and material damping.