H. L. Gegel

Modeling of dynamic material behavior in hot deformation: Forging of Ti-6242

A new method of modeling material behavior which accounts for the dynamic metallurgical processes occurring during hot deformation is presented. The approach in this method is to consider the workpiece as a dissipator of power in the total processing system and to evaluate the dissipated power co-contentJ = ∫oσ ε ⋅dσ from the constitutive equation relating the strain rate (ε) to the flow stress (σ). The optimum processing conditions of temperature and strain rate are those corresponding to the maximum or peak inJ. It is shown thatJ is related to the strain-rate sensitivity (m) of the material and reaches a maximum value(Jmax) whenm = 1. The efficiency of the power dissipation(J/Jmax) through metallurgical processes is shown to be an index of the dynamic behavior of the material and is useful in obtaining a unique combination of temperature and strain rate for processing and also in delineating the regions of internal fracture. In this method of modeling, noa priori knowledge or evaluation of the atomistic mechanisms is required, and the method is effective even when more than one dissipation process occurs, which is particularly advantageous in the hot processing of commercial alloys having complex microstructures. This method has been applied to modeling of the behavior of Ti-6242 during hot forging. The behavior of α+ β andβ preform microstructures has been exam-ined, and the results show that the optimum condition for hot forging of these preforms is obtained at 927 °C (1200 K) and a strain rate of 1CT•3 s•1. Variations in the efficiency of dissipation with temperature and strain rate are correlated with the dynamic microstructural changes occurring in the material.

A new yield function for compressible PM materials

Abstract A new yield function for compressible P M materials has been derived based upon a yield criterion postulated by the authors. This function was experimentally verified for the uniaxial state of compressive stress using the P M aluminum alloy X7091 as a model material, and excellent agreement was found between theoretical and experimental results for the density dependence of the yield and geometrical hardening. Yield surfaces for various density levels have been generated in a three-dimensional principal-stress space using computer graphics.

Extrusion through controlled strain rate dies

The workability of a material during deformation processing is determined by (a) the die geometry which, in turn, determines the flow field during deformation, and, (b) the inherent workability of the material under the imposed processing conditions of strain rate and temperature. Most common alloys have good inherent workability and can be successfully formed over wide ranges of temperature and strain rate. Products can be successfully formed from these alloys even with dies which impose large variations in strain rate during deformation. However, many of the new alloys and composites can be deformed only in very narrow processing regimes, and control of the strain rate during deformation of such materials becomes important. For example, extrusion of a whisker-reinforced aluminum alloy composite is possible only when the strain rate is controlled to within one order of magnitude. This paper describes the development of a method for obtaining preliminary shapes of controlled strain rate extrusion dies, a special case being the constant strain rate die. The theoretical basis for such die design processes is presented, followed by some examples of die geometries. Since this design procedure ignores the material flow properties, the designed die shapes must be verified using the finite element method or physical modeling. Results of simulations with the program ALPID are also presented.

Physical modeling of metalworking processes—I: Determination of large plastic strains

This article discusses the application of the visioplasticity method to the evaluation of large plastic strains such as those occurring in metalforming. Although this method can be used for any mode of deformation, its application to plane-strain deformation is treated here. The distortion of a quadrilateral element of a grid is tracked to compute strains during deformation. In each case any two lines of the quadrilateral can be used (length before and after deformation and direction cosines before deformation) to determine strains in the element. The method has been verified by application to basic cases of deformation such as uniaxial compression, tension, pure shear, and rotation of elements. The effect of choice of lines upon the results of strain calculation is also discussed.

Computer-Aided Process Modelling of Hot Forging and Extrusion of Aluminum Alloys

Summary Research on process modeling of hot forging and extrusion of aircraft structural parts has been conducted and powerful and efficient software developed for the analysis of metal flow during forging of complex shapes using the rigid-plastic and rigid-viscoplastic finite-element method. New constitutive equations and flow rules have been developed for fully dense as well as partially dense P/M material and this information integrated into the analysis software through a material data base. A new concept for the design of streamlined dies of arbitrary shape has been developed and used in the manufacture of dies for the extrusion of a complex structure from a newly developed SiC composite aluminum alloy. Visualization and understanding of the complex three-dimensional geometry are enhanced by the use of shaded color graphics.

Process Modeling of P/M Extrusion

A powerful and efficient thermoviscoplastic finite-element program called ALPIP has been combined with advanced CAD/CAM methods for die design to permit mathematical and physical modeling of the extrusion of P/M materials. New constitutive equations and flow rules have been developed for porous materials as Well as for the case of fully dense sintered and hot-pressed billets. These new equations were integrated into the analysis software through a material data base. The modeling results clearly show that the streamlined die geometry strongly influences the state of stress and uniformity of metal flow, which subsequently controls the quality of the product in terms of, density, microstructure, and mechanical properties. Special attention is being given to the extrusion of RSR aluminum alloy powders as well as aluminum alloy powders containing SiC whiskers.

CAD/CAM of streamlined extrusion dies

A new concept has been developed for the design of streamlined dies for the extrusion of “difficult-to-extrude” metal-matrix composite materials and P/M alloys. Based on this concept, CAD/CAM packages have been developed to facilitate the design and manufacture of these complex dies. The packages are interactive and user friendly and can help the user in arriving at an optimum design with relative ease within a short time. Use of 3- D graphics, hidden line removal, and shaded color aid the user in visualizing and understanding the complex die geometries. Two methods are presented for the manufacture of electrodes used for electro-discharge machining of dies. The first is by the use of a turnkey CAD/CAM system and the second is by the use of a special software known as CUTTER. Several electrodes (round to rounds, as well as round to shapes) have been machined using a three-axis vertical milling machine—the dies have been electrodischarge machined—and extrusion trials have been performed on several materials. The results have been very promising. It is concluded that streamlined dies have definite advantages over other dies when extruding difficult-to-extrude materials such as composites and P/M alloys.

Physical modeling of metalworking processes—II: Comparison of visioplastic modeling and computer simulation

The analytical modeling of physical processes is an integral part of scientific and technical research. Physical build-and-test procedures used for designing forging dies are prohibitively expensive and result in long lead times in obtaining satisfactory designs. In the present study the wedge test was advanced to the level of a standard laboratory test in order to verify the analytical results of a viscoplastic finite-element program, ALPID (Analysis of Large Plastic Incremental Deformation), which was developed to simulate the metal flow in deformation processes such as forging and extrusion. Wedge-shaped specimens were machined from plates of 1100-F and 6061-T6 aluminum alloy and the grids engraved on the meridian plane by means of a CNC engraver. The specimens were compressed in segmented dies at room temperature. The undeformed and deformed grids were digitized, and the true effective strains were calculated using a computer program developed for that purpose. The effective strains were then displayed as contour plots for comparison with the ALPID-generated strain values. Comparison of the experimental and ALPID results indicates that the values predicted by the ALPID code are very near the experimental values. The minor differences in the results are attributed to unavoidable experimental errors.