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ORIGINAL ARTICLEAn integrated computer-aided decision support systemfor die stresses and dimensional accuracyof precision forging diesNecip Fazil Yilmaz & Omer EyerciogluReceived: 29 January 2007 /Accepted: 14 January 2008 /Published online: 28 February 2008# Springer-Verlag London Limited 2008Abstract Precision forging is a field in which decisionsupport systems can be effectively and widely appliedand depends on knowledge and rules derived from thepast experience of forging die design engineers. Precisecomponents are becoming quite important in attempts toreduce cost and improve reliability. There are thus manyapplication areas in which the rules themselves becomeinherent to the parts or the processes. In forging diedesign, dimensional accuracy is one of the main goals.The load carrying capacity and life of any forged productis greatly affected by its dimensional accuracy. To predictthe precise dimension of the part and determine the diedimension for precision forging, it is necessary to analyzethe factors which affect dimensional accuracy. Dimen-sional evolution of die and product should be analyzed ateach stage of forging. In this study, both radial andtangential stresses are encountered in the determination ofdie stresses since cylindrical workpieces were used. Inorder to sustain dimensional accuracy of the forging die,differences between the forging product and the die insertsuch as elastic die expansion and product contraction arepresented.Keywords Precisionforging.Diestress.Decisionsupportsystem1 IntroductionPrecision forming processes and dimensional accuracy offorged components have a special place in forging. Due toits economical benefits, precision forming is one of themost important goals for metal forming technology toachieve. The higher dimensional accuracy of forged partshas been looked at for precision manufacturing in theforging industry, together with die life. Dimensions of theforged part are likely to be different from that of the diecavity due to the elastic characteristics of the die andworkpiece and thermal influences. Among these features,the elastic behaviour of the tool and work material havegreater influence on the dimensional accuracy 1.Elastic characteristics of the die and workpiece could bevaried according to the shape of the part, even for the samematerials. Therefore, designers should fully recognize theelastic deformation of the die and workpiece for eliminatingtrial-and-error. Many researchers have studied die cavitycompensation experimentally or numerically in search of thebetterdie, dielife, and processdesign 210. As for numericalstudies, Takshashi and Brebbia 11 analyzed the forging diestress with the boundary element method. Sadeghi and Dean12 studied the dimensional accuracy of precision forgedaxisymmetric components. Eyercioglu 13 and Dean alsostudied design and manufacture of precision gear forging dies.Also, several studies referred to the dimensional accuracy andsome numerical studies such as FEM, upper bound elementaltechnique (UBET), and the slab method were proposed forelastic characteristics of the forging tool 1418.Gerhard and Altan 19 stated that the structural analysis ofthe die and the prediction of stresses and elastic deflections areuseful from die life perspective. Especially in hot forging, dieInt J Adv Manuf Technol (2009) 40:875886DOI 10.1007/s00170-008-1402-zN. F. Yilmaz (*):O. EyerciogluMechanical Engineering Department, University of Gaziantep,27310 Gaziantep, Turkeye-mail: .trstresses, consisting of mechanical contact and thermal stresses,govern die fatigue, surface cracking, and crack growth, andconsequently they influence die life and profitability.The deformation patterns experienced in most formingoperations are very complicated, and thus it is not possibleto describe the patterns in quantitative statements. Thedeformation patterns of the workpiece geometry andmaterial in the forming zone are influenced by a numberof important parameters such as friction condition, lubrica-tion, temperature, velocity, boundary conditions, materialproperties, workpiece, and tool geometry. The optimumdesign of the metal forming process requires the knowledgeof the influences of these parameters as well as theinteraction among these parameters on the process mechan-ics in order to understand a certain metal forming process.In order to obtain the desired geometry and mechanicalproperties, the process parameters must be accurate, welldesigned, and properly controlled 20, 21.The design of forging die, prediction of requiredload, and thus dimensional accuracy can be handled byhighly experienced tool designers using a combinationof accumulated knowledge based on industrial experi-ence. Apart from the mathematical calculations on diestress and forging load, the necessity of the processplanner to take into account the empirical rules andgained knowledge derived from industrial experienceprovides an ideal scenario for the implementation offorging die design 22.2 General die design assumptionDie design is influenced by several factors which will beassociated with the type of product and its shape andindividual circumstances, but chiefly by the strengthrequirements. It should be realized that, with the complexstress distribution existing in a forging die, design in termsof support requirements is not particularly precise. Thesituation arises because, for example, the distribution andmagnitude of radial pressure exerted by the work material isnot known with certainty. In addition, the work materialduring forging of products is moving; therefore, steady stateconditions are not achieved due to the continuouslychanging pressure distribution. But in general it is assumedthat steady state stress conditions are present and there is auniform internal pressure along the whole length of the die23, 24. These assumptions permit calculations based on thetheory of thick-walled hollow cylinders to be carried out.The upper bound elemental technique (UBET) incorpo-rates the advantages of both the upper bound theorem andthe finite element method to provide more accuratepredictions of important parameters such as strain rates,die load, and die cavity filling when compared to the othermethods. UBET is perfect for initial stages of theoptimization algorithms, where it is necessary to reachnear-optimum solutions as quickly as possible.The stresses in dies arise mainly from the high level ofinternal pressure during forging. However, the pressure isnot constant over the whole length of the die. Since it isconcentrated in the portion of the die that is in contact withthe deforming workpiece, the pressure will vary duringforging and the length of the pressurised region will alsochange. The dimension of the forging is different from thedie because of several factors: The die insert is shrink fitted into the outer ring causingan extraction of the die cavity (Ue). In hot forging, the die may be heated prior to forgingand further heated by the hot billet during forging. Thiscauses the die insert to expand (Ut). Contraction occurs during cooling from forging tem-perature to room temperature (Uc). In electrodischarge machining of the die components,spark gap occurs between electrode and workpiece.This decreases the die cavity size (G).As seen in Fig. 1, if the radius of the workpiece isassumed to be equal to the original die radius R0; thus, thefinal radius of the die R4will be:R4 R0UeUtC0UcC0G3 Calculation formulae3.1 Calculation of the elastic die expansion (Ue)In order to calculate the changes in workpiece dimensionsdue to elastic deflection of the die, the elasticplasticdeformation of the workpiece has to be considered.Assuming that the workpiece is stressed uniformly by thedie and always remains cylindrical at the maximum forgingR0R1R2R3R4UeUtUcGFig. 1 Half section of a cylindrical forging of die insert 25876 Int J Adv Manuf Technol (2009) 40:875886load, the die deflection is elastic and uniform along its axis.Ignoring the friction on workpiecedie interfaces, work-piece dimensions change when the punch load is appliedand removed. Also, changes in workpiece dimensions occurduring ejection 25.In order to calculate the amount of expansion of the dieunder radial pressure, an initially stress-free duplex cylinderis considered. By applying the punch load on theworkpiece, two modes of deformation will occur. First,the workpiece will deform elastically and when the punchpressure becomes equal to the yield stress of the workpiecematerial, plastic deformation starts and simple compressioncontinues until the workpiece touches the die wall. Forcontinuity across the interface, the hoop (tangential) strainsfor insert and shrink ring must be equal at this point,q1 q2.q1Pi1C0nb2a2C16C17b2a2C011C0ud1Ed1Pi1C0nb2a2C011ud1Ed11q2nPic2b2C011C0ud2Ed2nPic2b2C16C17c2b2C011ud2Ed22The subscripts 1 and 2 refer to die insert and shrink ring,respectively. When the maximum load is exerted on theworkpiece, the radial stress will be greater than its yieldstrength. After reaching such a condition, if the punch loadis removed, the die will compress the workpiece plasticallyuntil the radial stress on the workpiece is reduced to twiceits shear yield stress (Sy). By using Trescas yield criterion,the total amount of radial expansion of the workpiece (U)atthe end of this stage can be calculated by:Uaaa21C0ud1b21ud1C02n2SyC02ab2PpEd1b2C0a23At the end of the forging process, the punch pressure is zeroand the radial stress (2Sy) is still acting on the workpiece.On ejection, its radius will expand elastically and theamount of recovery (s) can be calculated by assuming acylindrical state of stress (sr sq) and by placing z=0,such that:s 1C0uwEw2Sya 4where Ewand ware the Youngs modulus and Poissonsratio of the workpiece material, respectively. The totalbaTiTpFig. 2 Temperature distribution along the die radius in hot forgingDie Ring azbcabcFig. 4 Die insert and shrink ring dimensionsab0 ()0 (+)r ()To - Tp=0Ti - Tp= Fig. 3 Radial and tangential stress distributions due to outwardtemperatureInt J Adv Manuf Technol (2009) 40:875886 877change in the workpiece dimensions due to elastic dieexpansion is given by:Ueaa21C0ud1b21ud1C02n2SyC02ab2PpEd1b2C0a21C0uwEw2Sya53.2 Calculation of the thermal die expansion (Ut)In hot forging, dies are preheated to prevent cracking of thedie components and to reduce the cooling rate of theworkpiece. Some heat is transferred from the workpieceduring forging which further heats the die. The combinationof these two sources of heat causes the die to expand.The temperature distribution along the radius of the diewith a preheat temperature of Tpand bore diameter of Tiisgiven in Fig. 2. The preheat temperature is assumedconstant throughout the die, but the heat transferred fromthe workpiece produces an outward heat flow with radialtemperature gradient. Assuming uniform preheating, the diewall will expand freely. The magnitude of the radialexpansion (Utp) at any radius can be determined as:Utp radTpC0TrC0C16where Tris room temperature, Tpis preheat temperature, anddis the coefficient of thermal expansion of the die material.The temperature increase on the inner surface of the dieand stress distributions are shown in Fig. 3. Thus, the radialdisplacement at any radius r due to thermal stresses can befound with:UtsC0dT3 bC0aC0 1da2b2ab1r 2dC01r21C0db3C0a3b2C0a2C20C217Total die expansion (Ut) due to temperature will then be:Ut UtpUts8Top Frame Die Geometry Forging Load Geometry Die Assembly Material Parent Frame Fig. 5 General frame structureFriction Flow Stress FORGING LOAD Contour Frame Remove Frame Region Frame DATABASELubricationGoodAveragePoorDryINFERENCE ENGINEFig. 6 Framework for forgingload frame878 Int J Adv Manuf Technol (2009) 40:8758863.3 Calculation of the thermal product contraction (Uc)Theamountofshrinkageafterhotformingoperationsdependson the working temperature and coefficient of thermalexpansion of the forged material. Assuming that shrinkagetakes place radially, and the finish forging temperature isuniform, the amount of radial contraction at any radius is:Uc rawTfC0TrC0C19where Tfis the forging temperature, wis the coefficient ofthermal expansion of the workpiece, and r is the radius of theworkpiece before contraction. In order to achieve closedimensional tolerances on forgings, die dimensions shouldbe closely controlled. From the foregoing it is apparent thatknowledge of the magnitude of the above factors should beobtained before appropriate die and electrode dimensions aredetermined.Using the above analysis, the parameters affecting forgingdimensions were calculated and for a given condition theprofileofthe die was determined. A program hasbeenwrittento perform these calculations and to create the correctedforging product dimension for die. Die insert and shrink ringdimensions (Fig. 4) are then given in Eqs. 1017.b aQ110c aQ11z b:SyE1K1C0Q21C18C1912Q Q1:Q213Q11211K1C18C19C0PPs14Q2 Q1:K1p15PP PiSydie16K1SydieSyring17Fig. 9 Friction calibration curve in terms of m 27Table 1 Aluminum ring test dataLubricated Dry (ground) Dry (rough)Do1(mm) 30 30 30Do2(mm) 37.7 38.5 38Di1(mm) 15.2 15.2 15.2Di2(mm) 14.8 13.5 11.2H1 (mm) 10 10 10H2 (mm) 5.65 5.35 5.3% H 43.5 46.5 47% D 2.63 11.18 26.3Load (ton) 25 30 35m 0.25 0.4 0.605101520051015DHP(TON)Fig. 7 Disc forging for aluminiumALUMINIUM0501001502000,00 0,10 0,20 0,30 0,40STRAIN (DH)STRESS(MPa)Fig. 8 Stressstrain curve for aluminiumInt J Adv Manuf Technol (2009) 40:875886 879where a is the die insert inner radius, b is the die insertouter radius, c is the shrink ring outer radius, z is theinterference, and Piis the inner pressure.4 General structure of the systemA general structure for building up an inference and controlengine for the decision-support expert system as well as analgorithm for finding a compromise solution for the diestress and dimensional accuracy of the product is achieved.By using an intelligent, knowledge-based object-orientedsystem, high precision manufacture of product has been putinto perspective. Knowledge representation in this workwas structured in the network representation. Parent frames(geometry, forging load, diegeometry,dieassembly,material) are connected to the top frame. Each parent framealso has child frames. General frame structure is shown inFig. 5.Parent frames are used to describe the general class ofobjects. In a database, the data definition of a recordspecifies how the data is stored so that the database cansearch and sort through the data. To actually enter thevalues into the system, child frames and instances areformed to represent the specific objects. Prediction offorging load has vital importance for the dimensionala b c 4017.53031.1Fig. 10 U-shaped product withdifferent sizes of specimen880 Int J Adv Manuf Technol (2009) 40:875886accuracy and die life. This frame has six child frames and itis defined as one of the main frames of the developedsystem (Fig. 6).Contour frame This is the child frame of forging loadparent frame. This frame takes its knowledge from thegeometry parent frame. In order to determine the forgingload, the contour frame is the first frame that is to be fired.The entities are searched to find the inclined lines and arcs.During this process, related rules are fired so that theentities found are inclined line or arc.Remove frame This is the child frame of forging loadparent frame. In this frame, removed entities are stored inthe database. There are two instances. One of them containsthe knowledge about inclined lines and the other containsarcs.Region frame This frame is the child frame of forging loadparent frame. The geometry decomposition is made by theknowledge taken from this frame. Vertical and horizontallines are drawn from the corners to the corresponding line.In this way, rectangular regions are obtained. The knowl-edge about the regions are stored in the database.Friction frame One side of the region contacts one of thematerial, die, or punch. Therefore, each side must bechecked and friction factor must be determined. This frameis used for the determination of sides, whether it contactsthe material, die, or punch.c 2050ab4012.53022.2Fig. 11 T-shaped product withdifferent sizes of specimenInt J Adv Manuf Technol (2009) 40:875886 881Lubrication frame This frame takes its knowledge fromfriction frame and adds its own knowledge. This frame hasfour slots: good lubrication, average lubrication, poorlubrication, and no lubrication (dry). These slots arerequired from the user. The entered values are used forthe determination of friction factor for each side of theregion and therefore for all forging products.Flow stress frame Deformation characteristics of eachmaterial are different from the other materials. The flowstress value changes for all deformation conditions.Therefore, this property of the material must be in hand.5 ExperimentationIn the experiments a hydraulic press which has a capacityof 600 kN was used. A graphitewater based lubricant wasused as a lubricant. Great care was taken to ensure that allthe working surfaces were completely and evenly lubricat-ed. As a die insert material, AISI A10 air hardeningmedium alloy cold worked tool steel was used. The tool setcomprised essentially a container, punch, ejector, andbolster.U-shaped, T-shaped, and taper shaped aluminium prod-ucts were forged. Experiments were carried out at roomtemperature. Three different sizes of cylindrical aluminiumbillets were used. Products which have a dimension of40 mm in outside diameter and 20 mm in height wereobtained from stock bars and hollow bars.5.1 Disc forgingA disc forging compression test was carried out to determinethe stress-strain curveforaluminium.To thisaim,incrementalcompression was perform