Raj Balendra

Measurements of thermal contact conductance

Abstract During forging, the transfer of heat between the component, the tools and the environment has an impact on tool-life and the accuracy of the formed component. Consequently, the measurement of thermal contact conductance is of increasing interest to researchers and industrial engineers participating in the manufacture of high-precision components by plastic deformation. It is recognised that thermal contact conductance is a function of several parameters, the dominant ones being the type of contacting materials, the macro- and micro-geometry of the contacting surfaces, temperature, the interfacial pressure, the type of lubricant or contaminant and its thickness. A new steady-state method and measurement equipment are proposed in which the measurements are conducted on thin cylindrical specimens, which are retained under pressure between two tools. A clear advantage of this method is the ability to measure the thermal contact conductance under precisely controlled conditions. Due to the small aspect ratio of the specimen, the applied pressure may be of the same magnitude as that prevailing in industrial bulk-metal forming processes. In the present paper some experimental results on the dependence of h on the pressure and the specimen texture are presented.

A new method of measuring thermal contact conductance

Abstract The measurement of thermal contact conductance between forging work-material and tools is of interest to the assessment of temperature-change induced component-form errors derived during metal-forming operations and also to tool-life. Methods of measurement of the thermal contact conductance are reviewed and evaluated and a new steady-state methodology and equipment for the measurement of conductance are proposed. Experiments were conducted on thin cylindrical specimens retained under axial pressure between two tool surfaces. A clear advantage of this method is the ability to measure the conductance under continuously sustained thermal conditions and under pressures in excess of the yield strength of the work-material. Experimental results are provided and experimental errors are defined.

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.

FE simulation of the influence of thermal and elastic effects on the accuracy of cold-extruded components

The thermal and elastic behaviour of both the component and the forming tool have a significant effect on accuracy of high-precision forming operations. FE simulation of material flow, temperature distribution, die deflection and dimensional variation of the component during extrusion, punch retraction, component ejection and cooling was conducted using a coupled thermo-mechanical approach for the forward extrusion of pure aluminium and low-carbon steel in tools of steel. Heat generated by the deformation of the work material and that generated by friction at the component–die interface, heat transfer from the component to the die, cooling of the component, die deflection due to elasticity and temperature changes and the dimensional variation of the component resulting from thermal and elasticity factors with different process conditions were investigated. The results show that the thermal and elastic behaviour of the process have a significant influence on the accuracy of formed components not only during tool loading and unloading, but also during component ejection and cooling. The thermal behaviour of the component has a greater impact on component form errors than has the elastic behaviour of the tool and the component for aluminium components. For low-carbon steel, the elastic behaviour of the tool and the component has a more dominant influence on the accuracy of the component. Changes in the process conditions including the punch velocity, the interfacial friction, the die transition radii and the die land also affect the accuracy of the component. Temperature changes in the die during a single forming cycle do not cause significant changes in its dimension; consequently, die deflection is dependent mainly on the elastic deformation.

Effect of secondary yielding on nett-shape forming

Abstract Nett-shape forming requires better understanding of the phenomena affecting component accuracy. Some of these are subtle or not fully recognised and are, therefore, being disregarded. This paper reveals secondary yielding in some cold bulk metal forming operations. Secondary yielding of a component results from unloading by punch removal and die contraction. The process of closed-die upsetting of a cylindrical workpiece is examined. The theoretical analysis of this process explains the notion of secondary yielding. The main factors affecting secondary yielding are the flow stress of the workpiece material and the elastic properties of both the workpiece and the die. Secondary yielding refers to the plastic flow of material during the unloading cycle and, therefore, affects the final dimensions of the workpiece. The present analysis, which has been carried out in terms of the elastic strains of both the workpiece and the die, enables formulation of the die compensation procedure for this case. It appears that the die compensation procedure used for processes which involve secondary yielding is different from that for processes characterised by purely elastic unloading.

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.

Evaluation of elasticity and temperature effects on the dimensional accuracy of back-extruded components using finite element simulation

The elastic characteristics and the thermal changes of both the forming tools and the work material influence the dimensional accuracy of formed engineering components. Using finite element (FE) simulation, the influence of each was evaluated for the cold back-extrusion of low-carbon steel and pure aluminium. The results indicate that the elastic behaviour of the tools and the work material has a greater influence on the dimensional accuracy than the temperature changes for the forming of low-carbon steels. However, temperature has a greater influence than does the elasticity of the tools and the work material when pure aluminium was extruded.

Die-elasticity for precision forging of aerofoil sections using finite element simulation

Abstract Precision forging of material into turbine blades needs a clear understanding of the prevailing parameters in forging, appropriateness of preform design, process simulation and techniques for compensating component form errors due to die-elasticity. With this in mind, simulations are conducted to analyses the material flow during forging of aerofoil sections, forging force history, contact pressure distribution between die and component, and the elastic deflections of the forging dies are investigated using finite element simulation. Further, the compensation of die-elasticity is proposed by modifying die profiles in response to die deflections based on the nominal dimensions of forging dies. The minimisation of form errors of aerofoil sections due to die-elasticity is derived by iterations using FE. The results obtained enable the quantitative estimation of die-elasticity in precision forging of aerofoil sections, and the technique for compensating component-form errors to achieve net-shape forming production.

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.