Process modelling of linear friction welding (LFW) between AA2124/SICp composite and unreinforced alloy

In the present study, the Linear Friction Welding (LFW) process between a bar of Metal Matrix Composite (MMC) AMC225xe (AA2124 with 25% SiC particulate reinforcement) and a bar of unreinforced base alloy was simulated using the commercial finite element package ABAQUSTM. Fully coupled implicit thermo-mechanical analysis procedure was employed, with semi-automatic re-meshing using Python scripting and output database scripting methods for extracting deformed configurations. Due to the large deformation near the weld region, multiple analyses were carried out between each re-meshing stage in order to limit the element distortion. Comparison of the simulation results with the experimental data collected during welding, and with post-weld optical section micrograph has shown satisfactory agreement

Residual stress and distortion in samples of complex geometry subjected to spray quenching

Paper presented at the 9th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Malta, 16-18 July, 2012.This paper discusses the residual deformation modelling of a complex shaped stainless steel AISI316L component heated to 900C and quenched with a water spray subcooled to 80K. The paper shows that accurate deformation modelling of the problem necessitates not only an adequate mathematical formulation of the physical system but also the application of precise thermal and mechanical boundary conditions. Correlations of experimentally measured spatio-temporal heat transfer coefficients found valid for spray Reynolds numbers, Res Gdd f , of 6×10-2 < Re < 3 and spray Weber numbers, 2 s d f We G d , of 4.6×10-7 < We < 4.1×10-2 used in the calculation are presented. These boundary conditions are used in a finite element model (FEM) to enable predictions of the deformation history of a complex geometry subjected to spray quenching. The results of the modelling are compared to the deformed shape of the processed component. The excellent agreement between the modelling and experiment suggest that the method used to analyse this problem is consistent and may be applied to determine the residual stress and strain formation of complex shaped components subjected to a spray quench with great accuracy. The results of this study provide the opportunity to do life assessments with reliable residual stress and strain predictions which may be applied to components such as electronic chips, aluminium extrusions or metal sheet rollers.dc201

Investigation of changes in crystalline and amorphous structure during deformation of nano-reinforced semi-crystalline polymers by space-resolved synchrotron saxs and waxs

Complex structural changes occur in semi-crystalline polymers during deformation. In (nano-)filled systems the situation becomes even more complicated, since not only phase changes may take place, but also local (interfacial) failure between phases may occur. To help identify specific processes taking place within these systems, simultaneous small- and wide-angle X-ray scattering (SAXS/WAXS) measurements were performed using synchrotron radiation during in situ deformation. Using a highly focused beam, spatially resolved local information can be extracted by scanning the beam across the deformed/damaged region within the sample. The characteristic changes in the different phases are presented and discussed. While the study of WAXS patterns gives insight into the orientation and dimensions of the crystallites, SAXS provides information about the mutual arrangement of phases and the interfacial failure phenomena. Based on the analysis of the results obtained in our experiments it will be shown that the first changes in the crystalline phase appear long before macroscopic yielding of the sample is reached, i.e. the onset of irreversible deformation takes place. In the post-yield regime radical changes are observed in both the long- and short-range structures. It is concluded that the presence of nano-fillers exerts a strong influence on the establishment of microcrystalline structure, and hence also on the deformation behaviour at the microscopic scale

Temporal build-up of electromagnetically induced transparency and absorption resonances in degenerate two-level transitions

The temporal evolution of electromagnetically induced transparency (EIT) and absorption (EIA) coherence resonances in pump-probe spectroscopy of degenerate two-level atomic transition is studied for light intensities below saturation. Analytical expression for the transient absorption spectra are given for simple model systems and a model for the calculation of the time dependent response of realistic atomic transitions, where the Zeeman degeneracy is fully accounted for, is presented. EIT and EIA resonances have a similar (opposite sign) time dependent lineshape, however, the EIA evolution is slower and thus narrower lines are observed for long interaction time. Qualitative agreement with the theoretical predictions is obtained for the transient probe absorption on the $^{85}Rb$ $D_{2}$ line in an atomic beam experiment.Comment: 10 pages, 9 figures. Submitted to Phys. Rev.

Strengthening mechanisms in an Al-Fe-Cr-Ti nano-quasicrystalline alloy and composites

We report a study of the structure-processing-property relationships in a high strength AlFeCrTi nano-quasicrystalline alloy and composites containing 10 and 20 vol% ductilising pure Al fibres. The superimposed contributions of several different strengthening mechanisms have been modelled analytically using data obtained from systematic characterisation of the monolithic alloy bar. An observed yield strength of 544 MPa has been substantiated from a combination of solid solution strengthening, work hardening, precipitation hardening and Hall-Petch grain size dependent effects. These materials have been shown by other authors in previous published work to be highly sensitive to the size distribution of particles in the powder from which they are made, and the subsequent thermomechanical processing conditions. The processing condition employed in this study provided micron-sized grains with a strong [111] preferential orientation along the extrusion direction and a bimodal size distribution of the icosahedral nano-quasicrystalline precipitates. Both were deemed to be a significant contributor to the high yield strength observed. The addition of pure Al fibres was found to decrease the yield strength linearly with increasing Al content, and to augment the ductility of the composites.Industrial collaborator ALPOCO Ltd. (and more specifically Steve McArthur) provided the powders. Dr. Karen Kruska and Dr. Alan Xu assisted with sample preparation of FIB lift-out specimens of the atomised powder for TEM analysis. EPSRC Project EP/E040608/1 provided financial support. M. Galano thanks the RAEng for their support by means of a Research Fellowship. F. Audebert and M. Galano thank PICT-Oxford2010/2831. F. Audebert also thanks UBACyT20020130100663 and FONARSEC FS Nano 2010/11 for financial support.Peer Reviewe

Atom-optics hologram in the time domain

The temporal evolution of an atomic wave packet interacting with object and reference electromagnetic waves is investigated beyond the weak perturbation of the initial state. It is shown that the diffraction of an ultracold atomic beam by the inhomogeneous laser field can be interpreted as if the beam passes through a three-dimensional hologram, whose thickness is proportional to the interaction time. It is found that the diffraction efficiency of such a hologram may reach 100% and is determined by the duration of laser pulses. On this basis a method for reconstruction of the object image with matter waves is offered.Comment: RevTeX, 13 pages, 8 figures; minor grammatical change

Analysis of increasing torque with recurrent slip in interference-fits

Previous research associated with interference-fitted assemblies has shown that as recurring slip occurs (i.e. load to total slip, unloading and reload to total slip) there is an observed increase in the holding torque after each loading cycle. The aim of this work was to identify the reasons for this ‘torque strengthening’ phenomenon. The work also has industrial relevance in the optimum design of interference-fitted rolls used for the hot rolling of steel sections. Previous work has shown that the major contributors to the overall holding torque were the interface pressure, material properties and the coefficient of friction between component materials. In this work, neutron diffraction tests and crack compliance tests showed no correlation between the interface pressure and increased holding torque. Meanwhile, experimental holding torque tests on sample interference-fits showed that for each recurring holding torque failure (slip) in a test cycle, the holding torque increased. Subsequent wear investigations showed that the wear of the surfaces increased throughout the testing and once a specific type of wear had occurred through a ‘ploughing’ mechanism, significant damage could be done to the more expensive shaft component. These observations suggest that an effective increase in the coefficient of friction between shaft and hub is responsible for the increase in holding torque, while the same level of interface pressure is maintained throughout slipping. The research provides a basis for the optimisation of interference-fit design in order that the working lives of expensive shafts, which are prone to damage through ploughing, and brittle hubs, which are prone to sudden fracture, are maximised when experiencing recurrent slipping

The solution of axisymmetric crack problems in inhomogenous media

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