Abdalla S. Wifi

Computer-aided evaluation of workability in bulk forming processes

Abstract A computer-aided workability evaluation system, (CAWES), has been developed and coupled to an elasto-plastic large-strain finite-element package to check for ductile fracture at all locations in bulk formed workpieces, using different workability criteria. To compare the results of these criteria, a relative fracture index, γ , is introduced, which is a dimensionless parameter measuring the proximity of a material to fracture. The new procedure is used to determine the workability limits in upsetting and bolt heading, and it renders itself useful as a design tool. This is demonstrated by an example where a selection is made from two alternative preform designs for the backward extrusion of a cup-shaped product.

3-D Finite Element Modeling of the Welding Process Using Element Birth and Element Movement Techniques

The modeling and simulation of the welding process has been of main concern for different fields of applications. Most of the modeling of such a problem has been mainly in 2-D forms that may also include many sorts of approximation and assumptions. This is due to limitations in the computational facilities as the analysis of 3-D problems consumes a lot of time. With the evolution of new finite element tools and fast computer systems, the analysis of such problems is becoming in hand. In this research, a simulation of the welding process with and without metal deposition is developed. A new technique for metal deposition using element movement is introduced. It helps in performing full 3-D analysis in a shorter time than other previously developed techniques such as the element birth.

A study of workability criteria in bulk forming processes

Abstract This study presents a critical review, and comparison of workability criteria in bulk forming processes. Various well established criteria, are classified into appropriate groups, and a simple comparison between them is performed based on theoretical basis. An elastic-plastic large strain finite element technique is also used to predict workability limits using some of these criteria, with special emphasis on upsetting operations. The workability criteria are investigated based on their ability to predict fracture initiation sites and critical level of deformation up to fracture.

Finite element correction matrices in metal forming analysis (with application to hydrostatic bulging of a circular sheet)

Abstract This paper presents a condensed review of finite element applications in metal forming processes. Difficulties in large strain-elastoplastic analysis of these processes are identified. Correction techniques for potential sources of errors, such as lack of nodal equilibrium, violation of yield conditions, overstiff performance of finite elements, and improper formulation of configuration-dependent problems, are discussed. A main feature of the paper is the account of mixed boundary data in the updated Lagrangian formulation variational procedure. The code developed is applied to the process of the hydrostatic bulging of a circular sheet clamped at its periphery. This is based on a convenient explicit form of the hydrostatic-pressure load correction stiffness matrix for isoparametric and allied types of elements.

An optimum-curved die profile for the hot forward rod extrusion process

Abstract An incremental slab method is implemented to obtain the extrusion pressure of the hot forward rod extrusion process for arbitrarily-curved dies. Two different algorithms utilizing that incremental slab method are developed to obtain the optimum curved die profile that minimizes the extrusion load and stresses at the tool–workpiece interface for the hot forward rod extrusion process. Both techniques are based on the optimization of the effects of friction at the tool–workpiece interface, strain rate, and redundant deformation. The optimum curved die profiles generated by the two different algorithms are found to be very similar. For the material used in the present study which is carbon steel, the shape of the optimum curved die profile is found to depend on the extrusion ratio and Coulomb friction coefficient and not to be affected by the extrusion velocity. The finite element method revealed that the optimum curved die profile developed in the present work produces lower stress levels than those produced using the optimum conical die profile found in the literature.

Study of the Effect of Boundary Conditions on Residual Stresses in Welding Using Element Birth and Element Movement Techniques

The structure in which the welding process is performed highly affects the residual stresses generated in the welding. This effect is simulated by choosing the appropriate boundary conditions in modeling the welding process. The major parameters of the boundary conditions are the method by which the base metal is being fixed and the amount of heat being applied through the torch. Other parameters may include the coefficients of thermal heat loss from the plate which may simulate the media in which the welding is taking place. In modeling the welding process, two-dimensional forms of approximation were developed in analyzing most of the models of such problem. Three-dimensional models analyzing the welding process were developed in limited applications due to its high computation time and cost. With the development of new finite element tools, namely the element movement technique developed by the authors, full three-dimensional analysis of the welding process is becoming in hand. In the present work, three different boundary conditions shall be modeled comparing their effect on the welding. These boundary conditions shall be applied to two models of the welding process: one using the element birth technique and the other using the element movement technique showing the similarity in their responses verifying the effectiveness of the latter being accomplished in a shorter time.

Finite element determination of workability limits for disks and rings under different upsetting conditions

Abstract In the present work, an updated Lagrangian, elasto-plastic large strain finite element code is used to simulate the process of upsetting of disks and rings with full account of the frictional contact boundary conditions. Workability limits are predicted using the strain- based fracture criterion of Kuhn and Lee as well as a stress- based one of Shabaik and Vujovic. It is found that the workability limits predicted by Shabaik fracture criterion fit a Kuhn type fracture line with a slope of minus one half but with ordinate intercepts for the upsetting of disks different than that for rings. The predicted workability limits are compared with those published using rigid plastic finite element solution; some minor differences are reported. Furthermore, a comparative discussion of a number of fracture criteria is given and disagreement among them is highlighted.

A dynamic programming approach for minimizing the number of drawing stages and heat treatments in cylindrical shell multistage deep drawing

Deep drawing is an important sheet metal forming process that appears in many industrial fields. It involves pressing a blank sheet against a hollow cavity that takes the form of the desired product. Due to limitations related to the properties of the blank sheet material, several drawing stages may be needed before the required shape and dimensions of the final product can be obtained. Heat treatment may also be needed during the process in order to restore the formability of the material so that failure is avoided. In this paper, the problem of minimizing the number of drawing stages and heat treatments needed for the multistage deep drawing of cylindrical shells is addressed. This problem is directly related to minimizing manufacturing costs and lead time. It is required to determine the post-drawing shell diameters along with whether heat treatment is to be conducted after each drawing stage such that the aforementioned objectives are achieved and failure is avoided. Conventional computer-aided process planning (CAPP) rules are used to define the search space for a dynamic programming (DP) approach in which both the post-drawing shell diameter and material condition are used to define the states in the problem. By discretizing the range of feasible shell diameters starting from the initial blank diameter down to the final shell diameter, the feasible transitions from state to another is represented by a directed graph, based upon which the DP functional equation is easily defined. The DP generates a set of feasible optimized process plans that are then verified by carrying out finite element analysis in which the deformation severity and the resulting strains and thickness variations are investigated. Two case studies are presented to demonstrate the effectiveness of the developed approach. The results suggest that the proposed approach is a valuable, reliable and quick computer aided process planning approach to this complicated problem.

A Rule-Based Process Design and Finite Element Analysis of Multistage Deep Drawing of Box Shapes

In this paper a computer-aided rule-based process design of multi-staged deep drawing of box shaped shells is developed. A decomposition method is adopted in the algorithm for geometry description of the part under consideration. The shell geometry, tooling dimensions and load required are determined for each stage. A finite element analysis is carried out to verify and adjust the output of this process design algorithm. The deformation severity and the resulting strains and thickness variations are investigated. The forming limit diagram (FLD) is adopted as a basic reference to monitor possible part failure in the process.Copyright

Study of the Effect of Anode/Cathode Geometry on the Yield Rate and Quality of the MWCNTs Synthesized by Submerged Arc Discharging

The main objective of the present paper is to clarify the effect of anode/cathode geometry combinations on the yield rate and quality of the Multiwalled Carbon Nanotubes (MWCNTs) produced by submerged arc discharge technique. The effects of current intensity and the discharging medium (solvent) are also investigated. The morphology and crystalline perfection of the produced MWCNTs are confirmed by transmission electron microscopy (TEM) and Electron diffraction. Thermogravimetric analysis (TGA) is conducted to check the quality of the MWCNTs in a quantitative manner. The flat ended anode/cathode combination of diameters 4 and 12 mm respectively exhibited the highest yield at 70 A using deionized water as solvent. Through careful selection of the process parameters, the yield rate of MWCNTs obtained is found to be higher than most of the reported values in literature. However, the best quality of MWCNTs with purity as high as 95%, average thermal stability of 745°C as well as good batch homogeneity, is obtained with KCl solution and tapered male/female anode combination. The best quality MWCNTs is used successfully as reinforcement for A356 aluminum silicon composite.Copyright