B.M. Kim

Application of neural network and FEM for metal forming processes

This paper proposes a new technique to apply the artificial neural network in metal forming processes. A three-layer neural network is used and a back propagation algorithm is employed to train the network. It is determined by applying the ability of function approximation of the neural network to the initial billets which satisfy the minimum of incomplete filling in the die cavity. The die geometry for cylindrical pulley is designed to satisfy the design conditions of the final product. The proposed schemes have been successfully adapted to find the initial billet size for axisymmetric rib-web product and to design the die geometry for cylindrical pulley. The neural network may reduce the number of finite element simulation for designing the die of forging products and further it is usefully applied to multi-stage process planning.

Die design optimization for axisymmetric hot extrusion of metal matrix composites

Abstract Strain rate sensitive materials such as Ti alloys, superplastic materials and metal matrix composites (MMCs) can be deformed only in very narrow range of strain rate. In this work, a new process design method for controlling strain rate in workpiece during hot extrusion process is proposed. In this approach, a coupled numerical approach of finite element analysis and optimization technique to optimal profiled die which yields more uniform strain rate distribution in the deforming region is applied to the hot extrusion process of MMCs. Extrusion die profiles are defined by Bezier curves, and FPS (flexible polyhedron search) method is used as optimization technique. The change of relative deviation of strain rate, the progressive development of die profiles with increase of iteration for optimization and the corresponding strain rate distributions are investigated. In addition, the die profiles by optimization scheme for different extrusion ratios are compared with those by analytical solution.

Effects of surface treatments and lubricants for warm forging die life

Abstract In most metal forming applications, the surface treatments and lubricants are the main factors influencing die life, since they directly determine the interface friction and thermal insulation. In general, in hot and warm forging processes, the surface hardness of the die is decreased while the temperature of the die increased during repeated operations, which induce thermal softening. The thermal softening causes the plastic deformation of die despite the forming load not being changed. In this study, experiments and numerical analyses were performed under various conditions: two kinds of surface treatment, two lubricants, different initial billet temperatures and different loads; to investigate the effects on thermal softening and the amount of heat-transfer. Carbon nitride (CN x ), ion-nitride and no surface treatment for the dies were used and oil-based and water-based graphite were used as lubricants. Experiments were also performed to take the heat-transfer coefficient into account with the combination of surface treatments and lubricants. The coefficients determined were then used in a finite element model for the analysis of the backward extrusion process, the results produced were compared being experiment.

Determination of welding pressure in the non-steady-state porthole die extrusion of improved Al7003 hollow section tubes

Abstract Porthole die extrusion has a great advantage in the forming of hollow section tubes that are difficult to produce by conventional extrusion with a mandrel on the stem. Because of the complicated structure of the die assembly, the extrusion of hollow section tubes has been investigated experimentally. Analytic approaches that are useful in profitable die design and in the improvement of productivity are inevitably demanded. In porthole die extrusion the welding strength promises the possibility of producing sound hollow section tubes. Welding strength is affected by many parameters, such as extrusion ratio, extrusion speed, die shape, porthole number, bearing length, billet temperature and mandrel shape. In this paper, parameters such as billet temperature, bearing length and product thickness, are examined. The welding pressures are examined through 3D simulation of the non-steady state and compared with experimental results. The objective of this study is to analyze the metal flow and to predict the welding pressure of product by FE simulations and experiments. These results will be applied to porthole die design in the future.

The effect of die shape on the hot extrudability and mechanical properties of 6061 Al/Al2O3 composites

Abstract Metal matrix composites (MMCs) fabricated by the compocasting process show a homogeneous distribution of the reinforcing fiber in the matrix. Microstructural observation of hot extruded MMCs reveals that as the extrusion ratio increases, the fiber alignment becomes improved, but fiber fracture occurs more severely. The mechanical properties of hot extruded MMCs are better than those of the matrix metal, with the exception of the elongation at failure, and are not influenced significantly by the extrusion temperature. The tensile strength and hardness of MMCs are improved to a greater degree by hot extrusion using a constant-strain-rate die. Also, there exists a critical extrusion ratio that gives maximum strength, which is 5.44 in this study. Additionally, fractograph of the tensile specimen of extruded MMC indicates the ductile fracture behavior of the matrix.

Prediction of die wear in the wire-drawing process

Abstract In cold-forming processes, failure must be considered before die design, because high working pressure acts on the die surface. One of the main reasons for die failure in industrial application of metal-forming technologies is wear. Die wear affects the tolerances of formed parts, metal flow and costs of processes, etc. The only way to control these failures is to develop methods which allow the prediction of the die wear and which are suited to being used in the design stage in order to optimize the process. In this paper, the wire-drawing process is simulated by the rigid-plastic finite-element method and the results used for predicting the die wear by abrasive-wear theory. The effects of the temperature rise on the wear profiles of the dies are investigated. The simulation results are compared with measured die profiles.

Prediction of welding pressure in the non-steady state porthole die extrusion of Al7003 tubes

In this study, a visual sensor system for weld seam tracking the I-butt weld joints in GMA welding was constructed. The sensor system consists of a CCD camera, a diode laser with a cylindrical lens and a band-pass-filter to overcome the degrading of image due to spatters and arc light. In order to obtain the enhanced image, quantitative relationship between laser intensity and iris number was investigated. Throughout the repeated experiments, the shutter speed was set at 1-milisecond for minimizing the effect of spatters on the image, and therefore most of the spatter trace in the image have been found to be reduced. Region of interest was defined from the entire image and gray level of searched laser line was compared to that of weld line. The differences between these gray levels lead to spot the position of weld joint using central difference method. The results showed that, as long as weld line was within 15from the longitudinal straight fine, the system constructed in this study could track the weld line successful1y. Since the processing time reduced to 0.05 sec, it is expected that the developed method could be adopted to high speed welding such as laser welding.

A study for the constitutive equation of carbon steel subjected to large strains, high temperatures and high strain rates

Abstract In this paper, presented is a study for predicting the flow stress during hot deformation of carbon steel where the strain rate reaches as high as 3000xa0s −1 at high temperature (700–1100xa0°C). We have analyzed thoroughly Shida’s constitutive equation generally being used within the strain rate limit of 100xa0s −1 at elevated temperature. It was then assumed possible to obtain the constitutive equation which can depict the flow stress behavior of material beyond strain rate of 100xa0s −1 by modifying the Shida’s constitutive equation. The validity of the modified constitutive equation has been examined using split Hopkinson pressure bar (SHPB) experiment at high temperature. The high temperatures of SHPB test were obtained by enclosing the specimen in a clam-shell radiant-furnace. It has then been applied to hot rod rolling mill for predicting the surface temperature of workpiece where material throughout rolling experiences high strain rate (3000xa0s −1 ) and subsequently significant heat generation due to high strain rate plastic deformation. Results showed the modified constitutive equation could predict the flow stress value with a reasonable extent of error, in comparison with the one obtained from the SHPB experiment. The predicted surface temperatures of the workpiece during hot rod rolling were also in agreement with measured ones.

METHODOLOGY OF PREFORM DESIGN CONSIDERING WORKABILITY IN METAL FORMING BY THE ARTIFICIAL NEURAL NETWORK AND TAGUCHI METHOD

Abstract This study describes a new method of preform design in multi-stage metal forming processes considering workability limited by ductile fracture. The finite element simulation combined with ductile fracture criterion has been carried out in order to predict ductile fracture. The artificial neural network (ANN) using Taguchi method has been implemented for minimizing objective functions relevant to the forming process. The combinations of design parameters used in finite element simulation are selected by an orthogonal array in a statistical design of experiments. The orthogonal array and the results of simulation are used as training data of ANN. A cold heading process is taken as an example of designing a preform which does not form any fracture in the finished product. The results of analysis to validate the proposed design method are presented.

Optimal die profile design for uniform microstructure in hot extruded product

Abstract The objective of this study is to design the optimal die profile that can yield more uniform microstructure of a hot extruded product. The microstructure evolution, such as dynamic and static recrystallization, and grain growth, is investigated by using the program combined with Yada and Senumas empirical equations and rigid-thermoviscoplastic FEM. The die profile of hot extrusion is represented by a Bezier–curve to generate all the possible die profiles for a given extrusion ratio and initial radius of the billet. In order to obtain the optimal die profile which contributes to uniform microstructure of the product, the FPS (Flexible Polyhedron Search) method is applied to this study. To show the validity and effectiveness of the result of this study, an experiment of hot extrusion is performed, and the result of simulation is in good agreement with that of the experiment.