Yi Qin

Electronic property and bonding configuration at the TiN(111)/VN(111) interface

Multilayered TiN(111)/VN(111) coatings find many technological applications where the behaviors of their inside interfaces are known or suspected to influence functionalities in such an engineering surface system. Here, we demonstrate, by first-principles calculations on energetics and electronic structures of a total of 36 candidate interfaces, that the preferred geometries (i.e., that having the largest adhesion energy) are those that retain the interface structures as in either of the nitride bulks both atomically and electronically. Using several analytic methods, we have thoroughly characterized electronic states and determined that the interfacial bondings are mainly ionic, yet maintain a small amount of covalent character. The theoretical calculations presented provide insight into the complex electronic properties of the functional TiN/VN interface that could be difficult to obtain by experiment alone but which are practically important for further understanding and improvement of such a multilayered coating at the atomic scale.

FE simulation of the influence of die-elasticity on component dimensions in forward extrusion

The elastic behaviour of the forming die has a direct influence on the quality of the formed component. Simulation of material flow and of die deflection during forward extrusion was conducted using elastic and plastic considerations to establish the influence of die-elasticity on the form of the component under different processing conditions. The simulations show that the dimensions of the work-material are affected by die-elasticity during both, loading and unloading of the die. The change of the die-geometry does not produce obvious differences in the dimensions of the extrudate; however, it influences the geometric errors of the workpiece which is retained in the die. Larger values of friction in the injection chamber result in the corresponding increase of punch pressure which results in larger radial deflection of the die during extrusion (loading), the consequence being a greater variation in extrudate geometry. High levels of friction reduce the elastic contraction of the die during punch retraction (unloading), and hence, larger form-errors are sustained after unloading. The contraction of the die during unloading phase of the process causes plastic deformation of the work-material remaining in the injection-chamber; this plastic deformation is significant to the overall form-error of the component.

Injection forging: engineering and research

Injection forging is a nett-shape manufacturing process which enables the forming of complex component-forms that are difficult to form by conventional metal-conversion processes. The state-of-the-art of the research in this subject is reviewed with reference to different process configurations, forming limits, energy/pressure requirements, process modelling and product forms.

A study on the forming limits of the hydromechanical deep drawing of components with stepped geometries

Abstract FE simulations using ABAQUS/Explicit were conducted to examine the upper and lower forming limits of the hydromechanical deep drawing (HDD) of stepped components. Comparison of FE simulations and experimental results reported previously suggests that FE analysis may replace experimental trials to derive “master curves” of HDD, and hence renders higher efficiency of the tool design and process control of HDD, particularly for the forming of complex component-forms.

FE analysis of a novel roller form: a deep end-cavity roller for roller-type bearings

A novel roller form, a deep end-cavity roller, is proposed for roller-type bearings, with a view to reducing the weight of the structure and the centrifugal forces acting on the outer race of the bearing, and reducing the sensitivity of the bearing performance to the manufacturing precision. FE simulation results suggest that the new design would enable a straight-profile-roller bearing to have a similar performance to that for a logarithmic-profile roller bearing, as the deep end-cavity roller eliminates the sharp edge-stresses at the two apexes of the roller. Compared with the manufacture of a logarithmic-profile roller, the manufacture of a straight-profile deep end-cavity roller is simpler, and less strict on manufacturing precision. The deep end-cavity roller also enables an “in-process” correction of the contact-stress distribution between the roller and the raceway.

Identification and classification of flow-dependent defects in the injection forging of solid billets

Flow-dependent forming defects were identified for the injection forging of solid billets with reference to several process configurations with a view to defining the process range of injection forging. The mechanism of initiation of the defects was analysed based on which the identified defect-forms were classified.

Computer-aided design of nett-forming by injection forging of engineering components

Methodology and supporting techniques for design are developed with a view to defining a design and analysis system for nett-forming by injection forging. Processes and components were classified with reference to the method of injection, billet geometry, constraint to the billet and auxiliary media. FE simulation approaches were developed to predict flow-dependent flaws; the identified flaws were classified with reference to the mechanism of initiation of the flaws. Using these classifications, forming configurations, parameters and tools are either selected for the specified component by referring to the forming of similar products or designed by using proposed procedures; these procedures enable the design of forming operations to achieve flawless, high-precision products.

Optimisation of the lubrication for the extrusion of solid and tubular components by injection forging

Abstract FE simulations were conducted to assess the scale of friction-dependent pressure losses in the injection chamber during the injection forging of solid and tubular components. Several lubrication procedures were then tried with a view to establishing the “optimal” lubrication which would result in a significant reduction of the friction in the injection chamber. A procedure which combined two lubricants was identified as being “optimal” for use in the extrusion of long, shaft-type components. The procedure enabled the pressure losses in the injection chamber to be reduced from 50 to 60% of the applied pressure to 4–6% of the pressure. Experiments and FE simulations of the forming of the tubular components confirmed the significant reduction of the forming force requirement and the improvement of the material flow by the developed procedure.

FE simulation of the development of flaws during injection forging

Several types of flaws develop during the injection forging of components; among these a prominent form results from the instability of the free length of the billet. The material in the die cavity buckles or slides laterally along the anvil; consequently, die filling is effected by asymmetrical deformation of the billet. This FE simulation considers the influence of several parameters which influence the developments of flaws during injection forging. By considering friction conditions at the anvil, the aspect ratio of the primary deformation zone, the exit geometry and the inhomogeneity of the material, the types of flaws which are initiated and the subsequent die filling are simulated using ABAQUS code. Marginal changes in the friction conditions influence the stability of the billet; billets which were unstable when μ = 0.01 were, generally, sufficiently stable when μ = 0.03 to effect flawless die filling. Simulation confirms the experimentally proven, limiting aspect ratio of the primary deformation zone to be between T = 1.6 and 1.8. The deformation of the billet graduates from non-symmetrical deformation at aspect ratios greater than 1.7 to bending at ratios greater than 2.0. larger exit radii improve the flow characteristics and had been shown to reduce the energy requirements; however, the simulation shows that instability would occur at a lower aspect ratio when a large exit radius was incorporated in the injection chamber. Lack of inhomogeneity in the material will also result in instability and asymmetrical die filling at low aspect ratios.

A new approach for the optimisation of the shrink-fitting of cold-forging dies

Nett-shape manufacturing of engineering components is a major objective of modern material-conversion industries: relevant technologies depend on tools in which error-compensation can be effected. A novel die-design approach, known as the least squares approach, to minimise the component errors is presented. Shrink-fitting compensating die structure is employed. The errors caused by die-elasticity, secondary yielding, springback and temperature are considered in the process of minimisation. The main factors that may influence the accuracy of the optimisation procedure are analysed. The final component errors can be controlled to within a few micrometers. The approach is illustrated using axisymmetric closed-die forging.