K.J. Weinmann

Evaluation of lubrication and friction in cold forging using a double backward-extrusion process

Abstract In cold-forging operations, the material flow and the quality of forged parts are functions mainly of the tool geometry, the amount of deformation or strain, and the conditions at the workpiece/tool interface (e.g., friction, relative surface velocity, surface finish and heat transfer). While many of these parameters are well known and controllable, the influence of friction is often difficult to predict and depends on a variety of factors. To reduce the amount of friction in cold forging, billets are generally coated (e.g., phosphate coating) and lubricated. In this paper a friction test, based on a double backward-extrusion process, is proposed and examined in order to obtain information on lubrication quality. In this test, the upper punch moves downwards, while the lower punch and the die are stationary. An FEM analysis, using the program deform , has been conducted for different area reduction ratios and billet heights. The simulation results show reasonable agreement with the results of experimental studies performed in Germany some years ago. The reduction ratio that gives the greatest differences in extruded cup heights was selected for the test design and the influence of friction shear factors between m = 0.08 and m = 0.20 was investigated. Based on the FEM simulations, calibration curves were established. Using these calibration curves and measuring only the heights of the extruded cups and the punch stroke in experiments enables the quantification of the friction factor and the evaluation of the lubrication conditions under production conditions.

Prediction of forming limit curves of sheet metals using Hill's 1993 user-friendly yield criterion of anisotropic materials

Abstract A user-friendly yield criterion was proposed by Hill in 1993, which utilizes five independent material parameters in representing the yield locus. In the present investigation, an attempt is made to analyze forming limits in sheet metals based on this yield criterion and the M–K approach. Comparison of the predicted results with experimental data indicates that Hill’s 1993 yield criterion is able to characterize the localized necking of both aluminum and AK steel. A parametric study is carried out to investigate the influence of material parameters (r 0 , r 90 , σ 0 , σ 90 and σ b ) on forming limits, which shows that the shape of the yield locus has a significant influence on limit strains.

The Bauschinger Effect of Sheet Metal Under Cyclic Reverse Pure Bending

Abstract In some sheet metal forming operations, metal flow involves a series of bending and unbending steps. In any such bending sequence the occurrence of the Bauschinger effect is likely. Correct process modelling of such forming operations therefore requires inclusion of the Bauschinger effect. This paper presents results of an experimental procedure developed for the determination of the Bauschinger effect. Sheet strips were subjected to a series of bending and reverse bending steps by means of a pure moment bending device. Results show the Bauschinger effect in steel and aluminum sheet as a function of strain amplitude, prestrain, and sheet thickness.

Effect of deformation-dependent material parameters on forming limits of thin sheets

Material properties are deformation history dependent. To take this fact into consideration in forming limit analysis, the material parameters are defined as functions of strain using the Voce equation. These history-dependent material parameters are incorporated in the M–K analysis based on Hills 1993 yield criterion in which all material parameters are independent, so that the effect of each of these history-dependent parameters on forming limits can be investigated individually. The analysis shows that history-dependent material properties have a significant influence on forming limits. An increasing r-value will increase the limit strain under plane strain (FLD0), which is different from the traditional M–K analysis. Comparison of predicted results with experimental data illustrates that the consideration of history-dependent material properties can improve forming limit predictions considerably.

A strip-drawing simulator with computer-controlled drawbead penetration and blankholder pressure

Abstract Failure in the stretch-drawing of sheet metal panels is caused predominantly by either wrinkling or splitting. Wrinkling, which can occur in the flange inside the blankholder or in the unsupported side wall of the stamping part, is due to compressive circumferential stresses that result in buckling of the sheet. Splitting, on the other hand, is caused by excessive tensile stresses. Both modes of failure are influenced by blankholder force (BHF). Material flow is often modified locally by the insertion of drawbeads into the tooling. In industrial practice, drawbeads are a fixed component of the tooling and thus exercise their controlling function with a set penetration. A multiple-action hydraulic sheet-metal strip-drawing test apparatus has been constructed for the purpose of studying the effectiveness of feedback controls in sheet-metal forming. This device simulates the flow of sheet-metal in the blankholder and over the die shoulder in the stamping of sheet-metal panels. Its special features consist of the ability to adjust both the BHF and the drawbead penetration while the strip is being drawn and to measure forces in the sheet on either side of the die shoulder. Feedback control of the strip-drawing process using a proportional plus integral controller is demonstrated and the results are presented in the paper. The controller adjusts the drawbead position to eliminate the deviation of the punch force from the desired force. The results indicate that the addition of feedback control to the process reduces the effects of undesirable input variations as well as of disturbances on the output.

A Model for Improving Economic Performance of a Demanufacturing System for Reduced Product End-of-Life Environmental Impact

Abstract A demanufacturing facility benefits the environment by removing end-of-life products from the disposal waste stream. The facility dismantles end-of-life products and then places salvaged components into inventory for subsequent sale to remanufacturers and other firms. Lack of profitability will cause a demanufacturing facility to abandon its mission, with the environment suffering as a result. A model is presented for a demanufacturing facility that describes the disassembly process activities, the accumulation of component inventories, component sale based on market price behavior, and inventory management costs. The effect of three selling policies on financial performance is investigated. Selling policy is seen to be the dominant factor in determining profitability, with transaction and holding costs also influencing the performance.

Development of a Computer-Controlled Drawbead Simulator for Sheet Metal Forming

Summary A multiple-action hydraulic sheet metal strip drawing test device has been constructed for the purpose of studying the effectiveness of feedback controls in sheet metal forming. The apparatus simulates the flow of sheet metal in the blankholder and over the die shoulder during the stamping of sheet metal panels. Its special features consist of the ability to adjust both blankholder force and drawbead penetration while the strip is being drawn. A force transducer was developed to measure the forces in the sheet on either side of the die shoulder. Measurements from this unique transducer determine both the drawbead restraining force in the simulator and friction between the sheet and the die shoulder. Data demonstrating the capabilities of the equipment are presented.

On the Determination of the Coefficient of Friction and the Friction Factor by the Strip-Tension Friction Test

Abstract The strip tension friction test was used to investigate friction conditions in the forming of thin aluminum sheet without lubrication. In this test. which simulates metal flow around the leading face of a punch. the level of friction is judged based on tool-workpiece interaction when a metal strip, bent in the shape of a U. is stretched plastically around two fixed cylindrical friction pins. Friction is described by means of both friction coefficients and friction factors. Variables were pin diameter. surface condition of aluminum, and test orientation with respect to rolling direction. The effects of elastic and plastic deformation states in the straight portions of the strip upon friction conditions are also addressed.

Friction studies in sheet metal forming based on a unique die shoulder force transducer

Abstract A transducer developed for measuring tension in metal strip as it moves over the die shoulder was used to conduct friction studies. The portion of the transducer simulating the die shoulder is cylindric and mounts in the body on a pair of bearings. Incorporated into the design is a means to selectively lock the die shoulder. Strip tension measurements from tests with the die shoulder locked and unlocked are necessary before friction information is extracted. The transducer is used in a forming die simulator with a controlled drawbead to evaluate how drawbead penetration speed influences die shoulder friction.

On Predicting the Forming Limit Diagram for Automotive Aluminium Sheet

Abstract Aluminum has considerable potential as a material for automotive panels. The ability to predict forming limit diagrams accurately for aluminum is important for preventing failure in the forming process. A method is proposed to develop forming limits using Hills 1993 yield criterion. Predicted limit strains based on both Hills 1948 and 1993 yield criterion are compared with experimental data for Aluminum 6111-T4. It is found that Hills 1993 yield criterion can characterize localized necking in aluminum sheets well, while Hills 1948 criterion cannot. The critical thickness rule is found useful in predicting the left hand side of FLDs.