Process design in flashless forging of rib web-shaped plane-strain components by the finite element method

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ELSEVIER Journal of Materials Processing Technology 47 (1995) 291 309 Journal of Materials Processing Technology Process design in flashless forging of rib/web-shaped plane-strain components by the finite element method B.S K a n g * , J.H Lee, B.M K i m , J.C C h o i "Pusan National University, Research Institute ~2/ Mechanical Technology, Pusan 609-735, South Korea (Received July 5, 1993) Industrial Summary In this work preforming operations in the forging of rib/web-shaped plane-strain components are designed by the rigid-plastic finite element method to obtain flashless products, the height-to-width ratios of rib the geometry used for the analysis and design being 1.0, 2.0 and 3.0 The two design criteria of flashless geometrical filling and an even distribution of effective strain in the final products are investigated in controlling the preform configuration and establishing a systematic procedure of process design One preforming operation is designed for flashless forging with a sound distribution of effective strains at ratios of 1.0 and 2.0 The case of a ratio of 3.0 also needs one preform to satisfy flashless geometrical filling, but has high effective strain, a further preforming operation needing to be added to avoid the high value of effective strain: thus the case of a ratio of 3.0 is designed as two preforming operations The resulting preform configurations in plane-strain forging are compared with those of axisymmetric forging of rib/web components It is noted that the flashless forging of plane-strain rib/web components requires a unique preforming operation to make the metal flow slower at the central part Introduction Closed-die forging is one of the most important metal forming processes in industry, a billet of simple cross-section, round or square, being deformed plastically in closed-die forging by applying compressive force through two or more dies to obtain a more complex desired shape In the conventional closed-die forging process, the formation of flash restricts the lateral flow of material and thus facilitates the filling of the die cavity, the excess material of the flash being trimmed upon the completion of the forging *Corresponding author 0924-0136/95/$09.50 ~C~ 1995 Elsevier Science S.A All rights reserved SSDI - ( ) 3 - 292 B.S Kang et al / Journal ~[" Materials Processing Technology 47 (1995) 291-309 process There is normally about 50% of the material lost as scrap in the form of flash and scale losses, and the trimming process requires additional machining cost [1-3] Process design in die forging involves many areas, such as the determination of required processes, die design, preform design and the selection of the process conditions Recent development in the field of forgings has had the objectives of reducing costs by minimizing scrap losses and expensive secondary operations such as machining, reducing preforming steps and improving forging tolerances Forging difficulties in the closed-die situation may not be related to the occurrence of fracture but rather to a poor grain-flow pattern and lack of die filling Thus, one of the most important aspects of the closed-die forging process is the design of preforms to achieve adequate metal distribution without any defects [4,5] Computer-aided approaches have been proposed for preform and die design, research on this subject having been carried out in several countries [6-15] CAD systems have been developed for preform and die designs of rib/web type forgings using empirically established design rules [6 9] Recently, some useful results have been published in which the finite-element method was applied to preform design in metal forming processes Rebelo and co-workers [10] have proposed a new approach, called the backward tracing scheme, using the capabilities of the finite element method, and tested it for preform design in shell nosing The concept of the approach is to trace the loading path of a forming process backwards from a given final configuration It has been applied to preform design in various metal forming processes, such as plane-strain rolling [11], ring rolling [12], the forging of a disk [13], the forging of an airfoil section blade [14] and axisymmetric H-shaped cross-sections [ 15] In this study, the closed-die forging of H-shaped plane-strain components with different rib height-to-width ratios is investigated and designed, the main objective of the design being to obtain preforms that satisfy the design criterion of flashless geometrical filling with sound distribution of effective strain in the final products This study also attempts to establish systematic procedures for accomplishing the objectives of the preform design Problem description and design procedure The problem explored in the present work is the design of preforms for H-shaped plane-strain closed-die forging without flash using the finite-element method An example of preform design for steel finish forgings of various H-shapes is shown in Fig [1] Conventional closed-die forging of this type is designed to produce proper filling of the die with formation of flash The main goal of the present study, however, is to find preforms which not form any flash when H-shaped finisher dies are used Flashless forging is a type of impression-die forging in which the dies are designed not to include a flash area, the pattern of metal flow being very complex, and depending on the design of the preforms Thus, the most important aspect of the flashless closed-die forging process is the design of preforms to achieve B.S Kang et al./Journal of Materials Processing Technology 47 (1995) 291 309 F//~//"/~ " V//,~///3 I 293 U pse t s tock I Pre form None Finish h=b h= 2b h=5b Fig Preformssuggested by Lange et al [1] for the H-shaped closed-die forging adequate metal distribution without any defects The preform design in this study has wide applicability, since H-shapes are one of the most common cross-sectional shapes of structural components The dies shown in Fig 2(a)~c) are called finisher dies I, I! and III, respectively, according to their rib height-to-width ratios (H/B) A finisher die is used for the forging of the final H-shaped forging products The choice of parting line is based on grain-flow and die manufacturing problems [2] The computational results depend on forging parameters such as interface friction, forging speed and material properties The following computational conditions are Used: friction factor at the die-workpiece interface, m = 0.1; frictional stress = mk, where k is the shear strength; forging speed, 0.2; work-hardening material, 6-/Yo = (1 + g/0.319)°34, where 6" is the effective stress, Yo is the initial yield strength and ~, is the effective strain As mentioned earlier, many design procedures for the preform design in axisymmettic closed-die forging have been used in practical cases for forged rib/web-type components free from forging defects such as cracks, laps, shuts or folds Systematic procedures are also needed in a numerical approach for the preform design under the plane-strain condition In this study, the design procedure to find the preforms to achieve flashless closed-die forging is developed for H-shaped plane-strain components, the procedure consisting of the following steps (see Fig 3) Step Obtain information on material flow from the loading simulation of the initial stock of rectangular cross-section with a finisher die Step Design a test preform in the light of the information obtained in Step 1, and carry out loading simulation Step Design a modified preform using the information obtained in Step Step Check the preform by loading simulation to see whether or not it satisfies the final design conditions such as die filling and effective strain distribution If the 294 B.S Kang et al./Journal of Materials Processing Technology 47 (1995) 291 309 PARTING LINE , 5.0 ~ t!5 , LOWER DIE (a) B=I,0 ~ ~ 3,5 "I 5.o + f , \ (b) B~0.75 (c) Fig Dimensions and configurations of finisher dies: (a) finisher die IIH/B = 1.0); (b) finisher die II (H/B = 2.0); and (c) finisher die Ill (H/B = 3.0) preform satisfies the final design conditions, the process stops here: otherwise a possible preform shape is o b t a i n e d , then going back to Step The r i g i d - p l a s t i c finite-element m e t h o d is used for the analysis and design in the study F o r s y m m e t r y reasons, only one q u a r t e r of the deformed region is considered in the numerical simulation B.S Kang et al./ Journal of Materials Processing Technology 47 (1995) 291 309 I 295 FINAL PRODUCT I SIMULATION OF INITIAL STOCK AND DESIGN TEST PREFORM l SIMULATION OF TEST PREFORM I I-¢~-PREFORM DESIGN 1LOADING SIMULATION [ 1MPROVE I PREFORM uo l Fig Schematic flow chart of the design procedure Rigid-plastic finite element formulation The theory and the procedure of the rigid-plastic finite element method, which latter has proven to be one of the most effective method in simulation of metal forming process available at the present time [10, 16, 17], can be found in the literature [18, 19] The first-order variation of the functional for rigid plastic material model, based on the extremum principles and the incompressibility penalty function, can be written as V V S~ where = ~ / ( a l j a ' i y / , ~ = ~ / ~ (~.ij~ij)l 2, ~ = ~ii, and a~j, iij, ~, v and K are the deviatoric stress tensor, the strain rate tensor, the surface traction tensor, the velocity tensor and the penalty constant with a large positive value, respectively Eq (1) is discretized according to the standard procedure of the finite-element method and becomes a set of non-linear equations with the nodal velocities as the unknowns, which can be solved iteratively using the Newton Raphson method The stiffness equations obtained finally are KAu,+I = F, (2) where K = K(u,) is the stiffness matrix, related to the nodal velocities, u,, obtained from the nth iteration and F is the equivalent nodal force vector After solving Eq (2) with respect to Au, + 1, the assumed velocity field is updated by u, + :¢Au, + 1, where the deceleration coefficient, ~, is taken as 0.0 -G
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