Development of a novel differential velocity sideways extrusion process for forming curved profiles with fine grains and high strength

The aim of this study is to develop a novel process, differential velocity sideways extrusion (DVSE), for forming curved profiles with fine grains and high strength. In this new forming-bending-refining process, billets are used as the work-piece material to directly form curved profiles with certain cross-sections in order to increase the manufacturing efficiency and decrease the bending defects in conventional bending process. The DVSE process has been studied in this thesis by using forming experiments, microstructure characterisation experiments, finite element (FE) modelling and theoretical modelling. A tool set enabling sideways extrusion to be performed using opposing punches moving with different velocities was designed and manufactured. Plasticine was used as a model work-piece material and a series of compression tests were undertaken, to determine its constitutive properties and gain an estimate of work-piece die friction for use in process simulation. Feasibility studies for the DVSE process were carried out through a series of designed experimental programmes on plasticine, in which punch/extrusion velocity ratio, extrusion ratio and die land length were process parameters. Ultimately, trial tests using AA1050 at room temperature and AZ31 at elevated temperatures were conducted. Effects of extrusion velocity ratio, extrusion ratio, die land length, forming temperature and strain rate on profile curvature were studied. The microstructure evolution of the formed curved AA1050 bar by DVSE at room temperature was studied through EBSD. The evolution of grain structure and texture of formed curved AZ31 bars at different DVSE process conditions (temperature and strain rate) was investigated through optical microscopy and EBSD, and the optimum temperature and strain rate condition for obtaining fine equiaxed and homogeneous microstructure was identified. The different grain refinement mechanisms of AA1050 and AZ31 during the DVSE process were revealed. Micro-hardness of formed curved AA1050 and AZ31 bars was examined. Process mechanics of DVSE were modelled using FE modelling and upper bound theorem. The extent of work-piece flow velocity gradient across the die exit orifice, which causes curvature, was identified. A dead zone of roughly triangular shape, which exists on the chamber wall opposite the die exit orifice, was determined. The effective strain of the formed curved profiles was studied to confirm the rise of severe plastic deformation (SPD). The effective strain rate in the intersection regions of the channels was investigated to identify the source of severe plastic deformation. An analytical upper-bound-based model has been developed with the consideration of the determined dead zone. The extrusion force and curvature predicted by the analytical method agreed reasonably well with results from experiments and FE modelling. Discussions were made about the correlations between experimental and modelling approaches and results. The relationships between mechanical properties (yield strength, ultimate tensile strength, and elongation to failure) and microstructures (grain size, micro-texture) of formed curved profiles were correlated. From the experimental and modelling work, it has been demonstrated that the DVSE process proposed in this thesis is an effective way to efficiently form curved aluminium and magnesium profiles with controlled curvature and improved properties.Open Acces

One fluid might not rule them all

In this proceeding, we present our recent investigations on hydrodynamic collectivity in high-multiplicity proton--proton collisions at $\sqrt{s}=$ 13 TeV using the VISHNU hybrid model with different initial condition models, called HIJING, super-MC and TRENTo. We find that with carefully tuned parameters, hydrodynamic simulations can give reasonable descriptions of the measured two-particle correlations. However, multi-particle single and mixed harmonics cumulants can not be described by hydrodynamics with these three initial conditions, even for the signs in a few cases. Further studies show that the non-linear response plays an important role in the hydrodynamic expansion of the p--p systems. Such an effect can change $c_2\{4\}$ from a negative value in the initial state to a positive value in the final state. The failure of the hydrodynamic description of multi-particle cumulant triggers the questions on whether the hydrodynamics can rule all collision systems, including p--p collisions at the LHC.Comment: 4 pages, 4 figures, Quark Matter 2020 conference proceedings (accepted

Bending Behaviour Analysis of Aluminium Profiles in Differential Velocity Sideways Extrusion Using a General Flow Field Model

The work in this paper concerns an analytical model for quantitatively describing the bending behaviour of aluminium profiles produced in a novel extrusion process: the differential velocity sideways extrusion (DVSE), in which two opposing rams with a velocity of v1 and v2 were employed, respectively. The analytical model was built on the basis of the upper bound theorem utilising a general streamline equation controlled by a shape factor n, and the curvature was calculated using the material flow velocity gradient across the die exit orifice. The predicted material flow velocity across the die exit orifice, and extrudate curvature agreed well with the finite element (FE) modelling results, which were found to be irrespective of the shape factor n of the streamline equation. For a given extrusion ratio, the minimum value of n = 2 leads to the minimum and closest theoretical extrusion pressure, the n value for obtaining the best approximated mean effective strain of the extruded profile increases with the increase of the velocity ratio v2/v1 , and the value of n = 3.5 gives the closest mean effective strain as a whole

Evolution and control of the phase competition morphology in a manganite film

The competition among different phases in perovskite manganites is pronounced since their energies are very close under the interplay of charge, spin, orbital and lattice degrees of freedom. To reveal the roles of underlying interactions, many efforts have been devoted towards directly imaging phase transitions at microscopic scales. Here we show images of the charge-ordered insulator (COI) phase transition from a pure ferromagnetic metal with reducing field or increasing temperature in a strained phase-separated manganite film, using a home-built magnetic force microscope. Compared with the COI melting transition, this reverse transition is sharp, cooperative and martensitic-like with astonishingly unique yet diverse morphologies. The COI domains show variable-dimensional growth at different temperatures and their distribution can illustrate the delicate balance of the underlying interactions in manganites. Our findings also display how phase domain engineering is possible and how the phase competition can be tuned in a controllable manner.Comment: Published versio

OPML: A One-Pass Closed-Form Solution for Online Metric Learning

To achieve a low computational cost when performing online metric learning for large-scale data, we present a one-pass closed-form solution namely OPML in this paper. Typically, the proposed OPML first adopts a one-pass triplet construction strategy, which aims to use only a very small number of triplets to approximate the representation ability of whole original triplets obtained by batch-manner methods. Then, OPML employs a closed-form solution to update the metric for new coming samples, which leads to a low space (i.e., $O(d)$) and time (i.e., $O(d^2)$) complexity, where $d$ is the feature dimensionality. In addition, an extension of OPML (namely COPML) is further proposed to enhance the robustness when in real case the first several samples come from the same class (i.e., cold start problem). In the experiments, we have systematically evaluated our methods (OPML and COPML) on three typical tasks, including UCI data classification, face verification, and abnormal event detection in videos, which aims to fully evaluate the proposed methods on different sample number, different feature dimensionalities and different feature extraction ways (i.e., hand-crafted and deeply-learned). The results show that OPML and COPML can obtain the promising performance with a very low computational cost. Also, the effectiveness of COPML under the cold start setting is experimentally verified.Comment: 12 page

Forming-based geometric correction methods for thin-walled metallic components:a selective review

Geometric correction processes contribute to zero-defect manufacturing for improved product quality. Thin-walled metallic components are widely used in numerous applications such as electric vehicles and aircraft due to the lightweight feature, facilitating to achieve zero-emission goals. However, many components suffer geometric imperfections and inaccuracies such as undesired curvatures and twists, seriously affecting subsequent manufacturing operations, for example, automatic welding and assembly. Geometric correction techniques have been established to address these issues, but they have drawn little attention in the scientific community despite their wide applications and urgent demands in the industry. Due to the strict geometric tolerances demanded in high-volume automated production, it is urgent to increase the knowledge needed to develop new techniques to address future industrial challenges. This review paper presents an overview of typical geometric defects in thin-walled components and clarifies the associated underlying generation mechanisms. Attempts have also been made to discuss and categorize geometric correction techniques based on different forming mechanisms. The challenges in correcting complex thin-walled products are discussed. This review paper also provides researchers and engineers with directions to find and select appropriate geometric correction methods to achieve high geometric accuracy for thin-walled metallic components.</p