Bulk Metal-forming Processes Research Articles

Fundamental investigations on the material flow at combined sheet and bulk metal forming processes

The class of combined sheet and bulk metal forming (SBMF) processes denotes the forming of sheet metals with typical bulk forming operations. As a consequence, simultaneous 2D and 3D stress and strain states are present. Furthermore the local shape elements formed out of the sheet plane are in the magnitude of the sheet thickness. This paper deals with process combinations upsetting/lateral extrusion and deep drawing/upsetting. It has been shown that an insufficient mould filling is nearly inevitable using conventional tooling systems. Hence, the prospect of newly developed forming approaches will be discussed.

Modelling and optimisation of incremental bulk metal-forming processes for near net shape components

In this paper, we present a method to accelerate incremental bulk metal-forming simulations by using different mesh discretisations in two depending meshes. This multimesh method is preventing a loss of local resolution by using a special data transfer. Two example processes show the working of this method and the archieved acceleration.

Validation and application of a screw method for strain measurement in bulk metal forming

A new method for measurement of plastic strain inside a deforming body is advanced and validated through a simple cylinder upsetting experiment. It is also applied to a ring for demonstrating this method in a ring compression test. The experimental results and numerical simulation show good agreement. In contrast with other physical modelling methods, this method utilizes the real metal for a sensor and workpiece, rather than using substitute materials, and can observe and measure material flow and plastic strain inside the specimen without splitting it before deformation. It can be used to study material flow and to predict strain distribution in general bulk metal-forming processes such as upsetting, extrusion, and die forging. It is also useful for verification of the numerical simulation methods when the material model and the processing parameters were uncertain and/or not easily verified.

Study of cold tube drawing by finite-element modelling

Abstract The potential for optimising commercial cold drawing of steel tubes by reducing the number of drawing passes whilst minimising the induced tensile residual stresses has been investigated. Modelling techniques have been used based on a commercial implicit finite-element (FE) code. The FE model takes into account the elastic–plastic response of the tube, the elastic response of the tools and the adiabatic and frictional heating effects during drawing. The model has been validated against the measured surface temperature, drawing load and mandrel reaction force. Key model outputs such as residual stresses, drawing force and temperature distribution have been predicted. In addition, an optimisation procedure has been developed based on the Cockcroft–Latham workability failure criterion to determine whether it is possible to reduce the number of drawing passes for a given size reduction. The optimisation procedure can be used to design optimum dies and mandrels for tube drawing and minimise the time and cost associated with lengthy works trials in bulk metal-forming processes.

FINITE ELEMENT REMESHING: A METAL FORMING APPROACH FOR QUADRILATERAL MESH GENERATION AND REFINEMENT

The paper presents an automatic finite element remeshing system for quadrilateral elements consisting of modules for mesh generation, densification, smoothing and interpolation of field variables. The mesh generator takes into account the contour of the old mesh, eventual interference with dies and the plastic deformation of the material. An initial coarse mesh is created by utilizing a grid-based approach for creating well-shaped internal elements, in conjunction with a nodal connection approach based on constrained Delaunay triangulation, for linking with the boundary. Subsequent local mesh refinement is performed according to parameters depending on past, present and predicted future deformation related field variables; being, respectively, the strain gradient and strain rate distribution in relation with the velocity field, element size and quality. Smoothing is accomplished using an iterative Laplacian repositioning method. As illustrated in the presented examples this overall strategy ensures a robust and efficient remeshing scheme for finite element simulation of bulk metal-forming processes. © 1997 by John Wiley & Sons, Ltd.

Friction in bulk metal forming: a general friction model vs. the law of constant friction

Commercially available finite-element programs for the simulation of bulk metal-forming processes usually model the frictional restraint acting at the interface between the workpiece and the tools according to the law of constant friction. Such description is often inadequate and does not represent the state-of-the-art in tribology. In the present paper it is shown that the application of the general friction model, developed by Wanheim and Bay, involves a major improvement in the ability to simulate processes where low tool-workpiece interface stresses may prevail. This is confirmed by experimental and numerical investigations into the upsetting of a semi-tapered specimen between parallel dies. Additionally, it has led to the proposal of a new ring-compression test geometry intended to complement the conventional ring test for the calibration of friction models under conditions where the normal stresses over considerable parts of the tool-workpiece interface may be lower than the yield stress of the material.

Measurement of friction and material flow-stress by a plane-strain compression tribometer

With the development of computer technology, many metal-forming processes have been investigated by numerical analysis [1]. A knowledge of friction and flow-stress material is extremely important for performing this analysis. The interfacial friction condition and the material flow stress are amongst the main concerns in metal-forming process design and analysis. Until now, there has not been a universal test method for friction and material flow-stress in bulk metal-forming processes [2]. The present paper introduces a recently developed, easy-to-handle, low cost, accurate tribometer which can measure the friction and material flow-stress in a simple plane-strain compression test. This tribometer can reproduce many characteristics of bulk-forming processes. The data acquisition is realized with a PCL 718 card. Menu controlled data processing software has been developed which can obtain the friction coefficients μ, m and q and the material flow curve σ f = f( ε i) by processing the same experimental data. Compared with the ring-compression test, this new method can obtain friction coefficients μ, m, q continuously without interrupting the experiment, which would change the interfacial conditions uncontrollably. Compared with the Rastegaev test, the material flow-stress can be obtained up to a fairly large strain with good accuracy. The newly developed tribometer can be used to evaluate the tribological properties of lubricants, tool materials, tool surface finishes and tool coatings. It can also be developed for use in elevated temperature measurements. Experimental results have been analyzed which show that the friction coefficients in the models commonly used (Coulomb, Von Mises, Ramaekers-Kals [3]) are not constant during the forming process, the R-K model forecasting the deformation load best amongst the three models.

A study of the heat-transfer coefficient as a function of temperature and pressure

The accuracy of thermal-mechanical models of the hot/warm forging process depends on the proper description of the boundary conditions. At the hot workpiece-cold die interface this requires knowledge of the heat-transfer coefficient. A method for the determination of the heat-transfer coefficients in bulk metalforming processes is presented. The technique consists of two steps. The first involves measuring the temperature distributions within two dies, one of which simulates the cold forming tool and the other the hot workpiece. The dies are brought into contact under closel controlled conditions. The second step makes use of the finite-element simulation of the resulting heat-transfer problem. The interfacial heat-transfer coefficient is treated as an unknown variable and is determined using a non-linear optimization technique, forcing the measured and the computed temperature distributions to be as close as possible. Examples of the interfacial heat-transfer coefficients as functions of the time of contact are given for different surface temperatures and interfacial pressures for 303 stainless steel. An empirical relationship is then developed, giving the coefficient of heat transfer as a function of time, temperature and interfacial pressure. The predictive capability of the relationship is substantiated by comparing its output to data obtaibed with the original values of the coefficient. The empirical relationships may be of use in the planning of hot-forming processes when the contact times are in excess of half of a second.

A practical stress-strain-strain rate-temperature constitutive relation of the power form

Many technical processes have been analysed using a double-power constitutive relation of the form: σ = B σ ϵ n ϵ ̇ m Such an equation can be used, for example, in the simulation of many bulk metal-forming processes, in forming-limit diagrams, and in many other situations. The problem, however, arises when this constitutive relation has to be used at different temperatures. Earlier analysis has indicated that in such relations, material constants such as the plasticity modulus B(T), the strain-hardening exponent n(T) and the logarithmic rate-sensitivity m(T) are temperature-dependent. A new approximation for these parameters as a function of temperature is proposed. Within the framework of temperature generalization all material constants are identified for three materials: commercially pure polycrystalline aluminium, OFHC copper and steel. Special attention is paid to 1018 cold-rolled steel (1018 CRS) where the response of the material to plastic deformation is more complicated. These three materials are analysed over a wide range of temperatures: θ < T < T m/2 and a wide range of shear strain rates: 5×10 −4 s −1 < Γ ̇ < ∼5×10 2 s −1 , where T m is the melting temperature. In addition, the generalized form of the constitutive relation is discussed for the whole temperature range, i.e. θ < T < T m . For the purpose of demonstration, several characteristic material responses during plastic deformation at different temperatures and different strain rates have been simulated using a computer program and computer graphics: the program is available from the author. Results are shown in the form of plots, along with experimental points. The predictions of the calculations are confirmed by experimental data.

Friction stress and normal stress in bulk metal-forming processes

When analysing pressure distributions in bulk metal-forming processes, it is conventional to adopt either Amonton's friction law, τ = μ p , or the law of constant friction stress. Neither of these two laws however, is applicable universally. In this paper a general friction model including both laws is presented. Using a slip-line analysis of the plastic deformation of surface asperities, the influence on friction stress of normal pressure, friction factor and surface roughness (defined by asperity slope) is expressed analytically. Examples of applications of the friction model when analysing bulk metal-forming processes are given: the tool/workpiece interface pressure and friction stress distributions are estimated for simple upsetting, forward bar extrusion and plate rolling. The calculated pressure distibutions show good accordance with those measured experimentally.

Testing the plastic behaviour of metals by torsion of solid and tubular specimens

In torsion tests the flow curves of metals can be determined at high strains and high strain-rates, thus simulating many bulk metal-forming processes. Experimental details concerning specimen geometry and the evaluation of flow curves from the test results are described. For obtaining high strain-rates to simulate industrial hot-forming, it is recommended to use extremely short specimens of high diameter/length ratio. For such specimens the “effective length” participating in deformation depends not only on the specimen geometry but also on the material: methods of determining “effective lengths” are described. For evaluating flow curves from measured torque—twisting angle curves, it is recommended to calculate stress and strain for a “critical radius” at 3 4 of the specimen radius where stress and strain are almost independent of the strain-hardening exponent of the material and of the notch effect. Uncertainty as to the yield criterion is discussed. Information regarding other sources of uncertainty - such as anisotropy and adiabatic heating at high strain-rates - can be obtained by comparing the test results for solid and tubular specimens. For this purpose the determination of “effective lengths” and “critical radii” is also described for tubular specimens. The predictions of the calculations are confirmed by experiments.

Friction in finite-element analyses of metalforming processes

In recent years the application of finite-element methods in studies of bulk metalforming processes has received considerable attention. However, the inclusion of accurate surface conditions has not been sufficiently examined. Much of the theoretical work has been performed with unrealistic conditions, frequently the two limits of zero and sticking friction. This paper discribes the development of a method for introducing friction into the finite-element analysis. This is achieved by the inclusion of a layer of elements whose stiffness is modified by a function of the interfacial shear factor. To investigate the proposed approach it is applied to the friction sensitive process of ring compression. A critical experiment is performed where the properties of an aluminum ring and of an applied surface layer of lead are known prior to the test. Excellent agreement is found both in the diametral changes and in the profile of the inner surface.

Friction and lubrication in bulk metal-forming processes

The type of lubrication regime which occurs in a metal forming operation has a strong influence on the frictional conditions as well as on other important factors such as product surf ace finish and tooling wear rate. The characteristics of the different types of regimes which can occur are reviewed, together with what is known about their incidence in drawing, extrusion, rolling, and upsetting operations. In the light of this information it is evident that the commonly used methods of characterizing friction can often lead to erroneous results. Suggestions for improved methods of characterizing friction are made.