Rajendram Balendra

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.

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.

Secondary yielding of forged components due to unloading

Abstract Secondary yielding of the workpiece material is a new phenomenon recently recognised, which can occur in some bulk metal forming operations. It is caused by contracting dies as a result of unloading. Since secondary yielding involves dimensional changes of the workpiece, it becomes one of the factors which should be taken into account when attempting net-shape forming. This paper contains a comprehensive study of secondary yielding. A simplified theoretical model will serve as a means of explaining the notion of secondary yielding. It will also be used to predict dimensional changes of the workpiece. Experimental verification of these results will be presented. Observed discrepancies will be rectified by an improved analysis using the finite element technique and thoroughly discussed.

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.

Pressured-assisted injection forging of thick-walled tubes

Abstract Numerous engineering components may be created in a hollow form without detracting from their performance requirements; the difficulty, however, is in the conversion of tubular materials into such components. Hollow components may be formed from tubular materials if these can be prevented from collapsing during the forming cycle. Further, die-filling during injection forging, is especially difficult at the terminal stages of the process; the use of a pressurising medium within the material could effect die-filling more effectively. The scope for the forming of thick-walled tubes into hollow components was investigated using different pressurising media to support the material during the forming cycle. A tubular hexagonal form was the basis for the evaluation of the forming requirements of the process. Several pressurising-media were tested experimentally to determine suitability and the process was stimulated using FE techniques to establish the “optimal” processing sequence. The pressurising-medium influences the forming sequence for the production of a component of acceptable quality. The forming sequence has three distinct stages. Injection forging of the tube has to be proceeded by an initial internal pressurisation while subsequent injection has to be matched by increases in pressurisation; the final stage is characterised by a rapid increase in pressurisation to complete the filling of the die. Failure to establish the correct sequence will result in different forms of failures; the correct sequence will enable the production of a component of “uniform” wall-thickness.

Material-flow considerations for the design of injection forging

During injection forging, the ratio of axial and radial deformation of each section of billet in the diecavity prescribes the pattern of material-flow, thereby determining the scope of the process and the quality of the product. To date, research on the quantification of the pattern of material-flow has been superficial; the models which were used invariably referred to particular conditions. FE simulation and corresponding experimental analyses were conducted with a view to characterising the flow of material during Injection Forging of solid billets and enabling the definition of the practical process range and a proposal for an approach for the design of preforms to eliminate the development of flaws.

Extrusion die for in-process compensation of component-errors due to die-elasticity

A proposed die configuration is designed to effect the in-process compensation of the errors in the form of the extrudate which arise from the elasticity of the extrusion dies. The die is in the form of a truncated, conical-shell which is either on a plane or a spherical bearing surface, to enable it to deflect elastically, thereby changing the dimension of the die orifice. FE simulation of the work-material flow and the die deflection was conducted to verify the feasibility of the proposed configuration. The deflection of the die is a function of the extrusion force and the resulting reduction of the die orifice is in proportion to this force. The simulation enabled optimisation of the geometric parameters of the die with respect to component-form errors.

FE analysis of springback and secondary yielding effect during forward extrusion

Abstract The response of the work-material during forward extrusion and the subsequent unloading process was analysed with a view to examining differences in prediction of component-form errors, when different constitutive models are used. Two types of constitutive models were adopted for the analysis—classical theory of plasticity (CP) with isotropic hardening and non-classical theory of plasticity (NCP). When compared the results of the CP model with the NCP model, the latter shows a slightly smaller maximum punch-force requirement, smaller diameter of the extrudate and larger contraction of the die during unloading. The significant difference in the predicted final dimensions of the extrudate with different constitutive models suggests that more accurate constitutive descriptions on the work-material have to be used for the analysis of component-form errors in precision forming, if more accurate results are to be achieved.

Injection-chamber to die-cavity interface for injection forming

Abstract For a specific die-configuration and forming condition the interface between the injection-chamber and the die-cavity should have an optimal specification; the aim was to define, experimentally, this specification with reference to injection and die-filling characteristics. For different simple radii at the interface, billets of material were injected into die-cavities of different pdz aspect ratios while the punch pressure and energy consumption was recorded. The radius at the interface influence the magnitude of the maximum punch pressure but has only a marginal effect on die-filling requirements. The specific energy expended to effect injection decreased with increases in interface radius but saturated at R = 0.08 that to effect die-filling changed by 5% over the range of radii R = 0 to 0.126. The incidence of the intermittent θ-shear depends on the severity of the interface specifications and on the constraint to radial flow. The optimal radius, regardless of die-cavity configuration, is R = 0.08.

An approach for the forming of large-thickness-flange components by injection forging

Abstract Injection forging has advantages for the nett-forming of flange-typed components. Research, however, has identified that folds occurred on material free-surfaces during the injection upsetting of flanges when the billet-diameter/flange-thickness ratio was beyond 1.2. These occurred at lower aspect ratios than that defined with reference to the stability of the billets. FE simulation and experiments were conducted to identify approaches by which the initiation of folds could be prevented and, hence, the process range of injection forging could be improved. It was recognised that, since the folds develop for conditions under which the billets are stable (no buckling or lateral sliding), it is possible to design a forming route of more than one stage to prevent the development of the flaws and, hence, to extend formability. Through the research, a preforming procedure was developed by means of which the process range of injection forging of solid components can be extended to flange-thickness/billet-diameter ratios of 1.4–1.5 using machined preforms, and to 1.50–1.64 using preformed billets. The proposed preform enabled the forming of several flange-typed components without folds.