Search

Ultra-high rotational speed micro friction stir welding is the combination of micro friction stir welding (μFSW) and ultra-high rotational speed FSW to overcome the practical difficulties such as increased heat loss from the workpiece, forging force requirement and fixturing issues associated with μFSW by using ultra-high rotational speeds. This work deals with the CFD modelling of ultra-high rotational speed micro friction stir welding of AA1100 to investigate the heat generation, temperature distribution and material flow in the weld zone at ultra-high tool rotational speeds. The temperature-dependent material properties and coefficient of friction are used in this study. A partial sliding-sticking contact condition is assumed between the tool and workpiece, and the possibility of partial melting occurring at high rotational speed is incorporated using boundary conditions. The predicted temperature field agrees well with the experimentally measured temperature results. The thermo-mechanically affected zone (TMAZ) predicted in the numerical simulation is also comparable with the micrographic studies. It is observed that the contribution of plastic heat generation is more than that of frictional heat generation at high rotational speeds, and partial melting does not occur. Welding speed does not have a significant influence on the peak temperature at high rotational speeds. Micro friction stir welding can be successfully performed at ultra-high rotational speeds to overcome the disadvantages and practical difficulties associated with low rotational speeds.

Liquid layering is considered to be one of the factors contributing to the often anomalous enhancement in thermal conductivity of nanoparticle suspensions. The extent of this layering was found to be significant at lower particle sizes, as reported in an earlier work by the authors. In continuation to that work, an investigation was conducted to better understand the fundamental parameters impacting the reported anomalous enhancement in thermal conductivity of nanoparticle suspensions (nanofluids), utilizing equilibrium molecular dynamics simulations in a copper-argon system. Nanofluids containing nanoparticles of size less than 6 nm were investigated and studied analytically. The heat current auto-correlation function in the Green-Kubo formulation for thermal conductivity was decomposed into self-correlations and cross-correlations of different species and the kinetic, potential, collision and enthalpy terms of the dominant portion of the heat current vector. The presence of liquid layering around the nanoparticle was firmly established through simulations that show the dominant contribution of Ar-Ar self-correlation and the trend displayed by the kinetic-potential cross-correlation within the argon species.

The impact of elastic spheres in presence of adhesion has been extensively studied due to its wide-ranging significance. A major emphasis of such studies is the energy loss occurring during impacts. Energy loss due to elastic stress waves is of special significance, as it could be the only active energy loss mechanism in many cases. In this work, we use finite element method to study the impact of elastic spheres on a rigid half-space in presence of adhesion. Adhesion, caused by van der Waals force, is modeled as a body force. This formulation helps to avoid many of the restrictions imposed by the commonly-used surface-force model of adhesion. The energy loss in adhesive impact of elastic spheres is predominantly due to the stress waves caused by adhesion-induced instability, which is absent below a critical sphere radius. Therefore, the coefficient of restitution increases with increase in sphere radius below this critical value, while it shows a reducing trend above it, within the range of sphere radii considered in this study. Therefore, our study indicates that the variation of critical velocity for capture with sphere radius will not be monotonic, unlike presented in many of the previous studies. (C) 2017 The Authors. Published by Elsevier Ltd.