Acta Polytechnica Vol. 43 No. 3/2003 Die Design for Near-Net Shape Forging J. iermdk, G. Grff fly lqfer d,iscnlws the use of conuentiorm,lforging equrpmentfor the manufacture ofpreciseforging uithoutfl^ash. Ananalys,is,is presented of the foaors influencing the forging process and the conditions for estabtuhing thu iin"lq"i 1"7 Srgt baseil on practical ixperikce at the SKO.D.A Auto forge. The main problemis usually the excess aolumz of billet mnterinl. Thrie basic forrns of die designfor ouerioad. protection are illustrated. and, discussed^ Keyaords: llashless forging, dic design, accornrnod,ation space, spring-operated dni,ce. I INTRODUCTION The production contract between a customer and a forg- ing company starts with the initial cusromer inquiry and is concluded by the customer's final approval. The customer's technical specification contains requirements with which both the forger and final forging must conform. The specification usually covers: o Forging material - chemical composition, heat treatment. o Drawings and dimensions - forging drawing and final ma- chine drawing. o Macroscopic grain flow and grain size requirement. o Post production tests (destructive and/or non-destructive) and quality management. o Number of items to be produced, rerms of supply. As the contract will be agreed upon partly on the basis of the forging drawing, this drawing is of great importance. The forging drawing must state the tolerance range within which the forging should be produced. A forging design is based on part shape complexiry material composition, required preci- sion, production run, and type of forging machine used. The drawing of a forged part rhat has been accepted by the customer is the only valid document for inspection of the foryed part. The forging design procedure is therefore one of the most important stages in forging production. The designer starts fiom the final machine drawing including details and dimensions of the machining locations, relevanr information on machining operations and the function of the component. After the type of production unit has been selected, forging allowances and machining allowances are added to the component profile. Iinally, the forging toler- ances are added versus the material and forging complexity factor. An important result is that the forging design may sub- stantially influence the production run and the auxiliary steps, tool design and its lifetime complexity, as well as mate- rial costs due to allowances. 2 Near-Net shape forging The term near-net shape forging indicates that some but not all of the forging surfaces require only minimal machin- ing or f,rnishing. This term is used in connecrion with the term precision forging. Precision folging is also described as close-tolerance forging in onder to emphasise the goal of 28 achieving, solely through the forging operation, the dimen- sional and surface finish tolerances required in the finished part. The precision of a forging is defined in terms of its conformity to the finished-part requirements as regards over- all geometry dimensional tolerance, and surface finish. The motivation for precision forgrng is as follows: o Reduction of material costs, which are abour the half of the total cost of forging. A significant amount of input material (5 up to 30 Vo) is associated with the forging flash. The design of precision forging usually minimizes and some- times completely eliminates flash. r Reduction of costs associated with the machining allow- ance. These costs include not only the labour and indirect costs of the machining and finishing operations but also the cost. of the excess raw marerial that is Iost during machining. o The mechanical properties of precision forging are often superior to those offorging that has undergone extensive machining. This occurs because the forged microstructure is preserved intact in precision forging. As already mentioned, the main reason for installing ofthe methods ofnear-net shape fiorging is to reduce produc- tion costs, since the total forging costs usually consists mainly of material costs. This requirement can be achieved by chang- ing the forging shape ind/or by reducing the machinirig allowances [l].An example of these shape changes is given in Fig.l. Fig. l: Possible changes of forging shape. Adapred from "Schmiedeteile" [l]. The forging shape is related to the die design. A more complicated shape usually means a more complicated tool, and therefore a much higher tooling cosr. This approach is justified with increasing size of the production run, if we con- Acta Polytechnica Vol. 43 No. 3/2OO3 Table l: Composition of finished Part costs Pmductionrun Total cost Machining cost Foreinc production cost Material cost Toolinc cost Low (Fig. la) 100 Vo 52Vo 12Vo 22Vo 14Vo Medium (fig. lb) 86Vo 38Vo 20Vo 20Vo 2270 High (Fig. tc) 72 Vo 19 Vo 30 Vo 19 Vo 32Vo sider the total costs for {inished part production' The total costs are the sum of the forging costs and the machining cost. The total production costs decrease even if the costs of the forged part increase, as the structure of the costs changes. Roughly estimated, the percentage share for a given part is given in Thble l. 3 Basic factors influencing near'net shape forging 3.1 Forged material The behaviour of the fbrged material is characterised by its chemical composition, mechanical, chemical and process- ing properties. The forging temperature and the appropriate range influence the formability, the surface quality, the di- mension tolerances, and the wear of the dies. Decreased formability me?rns that the metal flow is restricted, and higher pressure is necessary. The wear of the die increases and the die life decreases. 3.2 Forging shdpe dfficulty The shape ofa forging has a strong influence on the final cost of tooling. Increasing forging shape difficulty means that the flow of the material is more complicated. It is therefore is necessary to use more preform operations, so the die life of the tool decreases, the tooling design becomes more com- plicated, and supplementary operations, such as punching, sizing etc., are often needed. 3.3 Billet aolurne aaridtion If the input material is in the form of a ba4 the accuracy to which billets may be prepared depends on the variation of the cross-section and the variation in cut-off length. Variation in cross-section depends on the tyPe of bar and its quality (black bar, bright bar, peel bar). Two methods are available for sepa- rating: sawing and shearing. Shearing is preferred' as no material is wasted. Volume variations in billets cut frorn bars decrease with increasing height-to-diameter ratio. The great- est accuracy of volume in the case of black bars is obtained when the ratio of the height to the diameter of the billet is equal to the ratio of the variation in height to the variation indiameter [2]. In the case of bright bars, volume accuracy is dependent on bar diameter. 3.4 Flash Flash consists of flash land and gutter. It s€rves to ac- commodate the excess material and to create the necessary pressure in the die cavity. Excess material is usually needed to iompensate for variations in billet volume and die cavity growth due to wear. On the other hand, flash land involves increasing the forging load. The load necessary to forge a shape can be reduced dramatically by eliminating this flash land. At the same time the material flow should be carefully checked, because this change can cause the formation offlow defects and/or under-filling of the die cavity' To eliminate normal flash it is necessary to change the die design. The reason is that excess billet volume may cause overloading of machines, over-stressing of dies, and the forged height will depend on the billet volume. 3.5 Machining allowances Machining allowances refer to the amount of material that is to be machined from the forging to obtain the finished part' Allowances always mean extra material, and they increase the costs. Machining allowances are dependent on the material forged (degree of formability), on the type of production unit being used, on the maximum dimensions of the part' and on customer requirements. Therefore an agreement must be made benveen the forger and the customer. Machining allow- ance values are not given in the current EU standards. The recommended European values are between minimum lmm and maximum 5 mm, per surface [3]. In SKODA Auto forge shop practice, the amount of machining allowances is chosen according to the following considerations: e Type of input bar (peeled bar - rolled bar). r Tool design (accommodation of excess billet material, po- sition of parting plane). o Technical state of the forging machine (tolerances, mis- match, temperature stability, ,m, tp ). o Billet shape, volume and precision of dimensions (method ofbar separation). i.6 Draft angle The forging must be able to be removed from the tooling after the forging process is completed. A slight draft should be added to parallel surfaces to facilitate ejection. The value of the draft angle depends on the die design and the method of removal from the die cavity (ejection by pins or manual removal). Elimination of draft is limited by the capacity of the ejection mechanism of the forging equipment' by the strength of the workpiece material at the ejection tempera- ture, and by the wear of the tooling and/or damage to the surface of the workpiece. A specific role is also played by the version of the given forging machine and by the actual state of ejecting mechanism. The basic rule is : o A greater value of the draft angle means more excess mate- rial. This material should either be removed (more ma- chining) or, if it is left, there will be heavier forging. o A lower value means that the number of tool refurbish- ments is smaller. The share of the tooling cost in the forging price will be higher' In forge shop practice it is necessary to look for a compromise solution. 29 Acta Polytechnica Vol. 43 No. 312003 3.7 Fillets and. conrers Corners and fillets are curved connecting surfaces that smoothly unite the interseCting sides of forged elements, such as ribs and webs [4]. They enhance the ability of these ele- ments to withstand applied mechanical loads. Their design must satisfy both the requirements of metal flow in forging and the cost considerations arising from metal use and re- moval of metal by machining. The SKODA Auto forge shop practice is as follows: Largerforgingfllef"s (larger corner radius of the die) mean: o The wear resistance of the die is greater, and the lifetime and forging precision increase. r The relative share of the tooling cosr in the total forging cost decreases. o The metal flow in the die cavity is betrer. r The machining of the forging deteriorares. Lower diz fitlztu (lower corner radius of forging) mean: o The metal flow in the die cavity deteriorates. . A corner of a forging may be unfilled, otherwise a higher load and/or a higher forging temperature is necessary. T'he thermal load of the die increases and the die life decreases. o The maximum stress load in a given part of the die during forging increases. o Changes in die design may be necessary, the use of die in- serts increases the tooling cost. 3.8 Parting line position Theparting line is the projected line around the periphery of a forging that is defined by the adjacent and mating faces of the forging dies when the dies are closed. If the parting line remains straight around the periphery of the forging, it will lie in the forging plane. Theprging plane corresponds to rhe plane of the mating die surfaces [4]. The forying plane is normal to the direction of closure of the dies. The shape and position of the parting line controls the metal flow, and influ- ences the general type of process and equipment used. Once the parting line is located, the depth and position of the impressions in the upper and lower dies are fixed. The place- ment of the parting line ensures that the principal grain flow direction within the forgingwill be parallel to the direction of the principal loading. Thus the choice of proper placement should take into account: o First - the manufacturing technique, i.e., forging methods, number of forging steps, etc, e Second - metal flow optimisation according to the actual type of forging and forging sequence (preform stage or final stage, etc.). 4 Customer's order for supply of precise forgings Let us suppose that a forge has received an order to pro- duce near-net shape forgings. The first thing to be done is to evaluate the forge's own possibilities, which are substantially influenced by the forging design. This evaluation should be done within nvo basic domains: 30 L Produ.ction mnch,ines and, equipment o Capacity of the forging machines in the forge shop. The nominal load is a function of the forging size. o Ways of preforming, taking inro account the shape diffi- culty factor of the forging. o Number of preform steps in a particular forging machine that will be used for production. o Production run (batch size), with reference ro auromarion (need for robots, manipulators, ransfer feeder, etc.). o Method of die holder fastening. o Presence of a stripping and ejection mechanism in a partic- ular forging machine. o Manner and method of billet preparation (quality of sepa- rating, obtainable weight tolerances, need for cross wedge rolling machine, roll forging machine, etc.). o Ways of heating the billers (chamber furnaces, type of atmosphere used, induction heater, etc.). o Ways of heat treatment (type, size and capacity of heating and annealing furnaces, etc.). 2. Know-how r Possible use of numerical simulation (sofnvare and hard- ware used - 2D or 3D, CAD/CAM methods, etc.). o Knowledge of basic and special forging merhods (techno- logical causalities, special preforming methods, die design, etc.). o Customer's requirements regarding certification (VDA, ISO, etc.). The decision whether to accept or turn down the contract is based on an evaluation of the above-mentioned items. This decision should take into account any further technical speci- fications and delivery conditions from the customer. After taking an affirmative decision, the economic aspects of the contract are elaborated. The share of individual costs in the forging price at the SKODAAuto forge (roughly esrimared) is as follows: Material costs 20-40 Vo Personnel costs 20Vo Tooling costs 30-40 Vo Overhead costs l0 Vo Obviously, the materials and tools form a substantial part of the price. Hence it is necessary to consider very carefully if high forging precision, e.g., together with a low mn size, is acceptable. The question is whether the cost saving due to limited machining is worth with increased tooling cost and the higher requirement for manufacturing production. The same consideration should be applied to different qualities of input material (type of bars). A low machining allowances value requires higher surface quality, and the price of the input material increases. 5 Die design for near-net shape forging The basic problem in near-net shape forging is how to eliminate the influence of excess material. This excess mate- rial results from the permissible tolerances of billet volume Acta Polytechnica Vol. 43 No. 3/2003 variation, initial forging temperature variation, and the fact that we should include abrasive wear in the calculation (the internal die cavity volume gradually increases). This excess material in standard closed die forging process is accumu- lated in an external flash, which is rimmed. In flashless forging we need to look for some accommodating space in the die cavity. The region in which this space is situated will mostly depend on the shape of the component. We have three basic solutions: l. Part of the forging remains underfilled within the ap- proved dimensional tolerances. 2. The use of so called internal flash. If there is no central hole, some other part of the internal web, that will be punched can be used for this purpose. 3. Part of the forging can increase its dimensions. This may be the internal part in the case ofan extruded hub, or the wall height of an extmded cup. It can be also the height of the gear rim or the flange height, etc. 5.1 Forging into closed dies with accommodation in the corners of the Iorglng This method of near-net shape forging seems to be very easily feasible. Cavity dies are completely closed and billet of constant volume is put inside. The shape of forging should have some region (usually peripheral edge radii), which may remain underfilled. This principle is used by HATEBUR forging machines. If we submit this process to analysis we recognise that the approach is rather ambitious. The cavity volume in relation to the admissible variation of the edge radii is very small. The dimensional accuracy of the billet should be very high (weight tolerance 0.5-l Vo) and a narrow temperature range (:-r30'C) is necessary. The foregoing conditions are rather difficult to execute if using a standard forging press. To ensure the stability of the process it is to a very limited extent possible exploit the elastic deformatiqn (stiffness) of both the machine and the tool. When the die cavity is completely filled, some excess billet volume can be accommodated in machine distortion. The ex- ample of such a forging with unfilled corners is in the Fig. 2' Fig. 2: Accommodation space in unfilled corners This method has advantages in the simplicity of the die design and the low number of forging operations,,but on the other hand precise billet volume is necessary and there is a risk of press overloading. To avoid this danger we must keeP a narrow range of technological process conditions, e.9., tem- perature, press adjustment, lubrication, etc. 5.2 Forging into closed. dies with accotnmodation into internal flash This method is also called flashless forging. It is usually used when forging axisymmetrical parts with a central hole. The excess material is accommodated in the central part of the forging in the form of an "internal flash", and is removed by punching. This is done when producting rings, wheels and gears. The die design for individual preform steps should be done in such a manner that the metal flow in the finished die cavity is in the central web only. Other die cavity parts have to be properly filled without forming a fin of material bem'een the punch and the container in the die closure position. Die duign rules for prefonn operations. l) The goal of preforming operations is to distribute the material in such a manner that the material flow in the finished forging operation will be optimal. This means ensuring a minimal path of deformation and a minimal degree of deformation. Short flow distances and low val- ues of contact pressure diminish abrasive wear of the die. 2) The basic goal ofpreform operation is to create a "space" in the forging axis region that will be used for accommo- dating of excess material in the next forging step, see Fig. 3. The dimensions and shape of this cavity are Pro- jected according to type offorging and the shape ofthe die, with goal of making the flow of material as simple as possible. 3) In the case of preforming there is no accommodation space in the die cavity. Thus the volume of the cavity in a preform die should be a little larger than the maximum possible billet volume. The designer Proposes the shape of the cavity and its dimensions in such a way that some space remains unfilled even if the billet volume and forg- ing temperature are at their upper limits. This space should be positioned at the place that is the last to be filled. Generally, this is the region of die closure. This is a safety precaution against press overloading and Prevents creating a fin. Die design ruLes for Jinkh operation. The shape and dimensions of this final cavity correspond to the forging shape, with the exception of creating an accom- modation space in the central region of the forging axis. This space must be designed in such a manner as o to be able to accommodate the maximum possible billet excess volume, and . to ensure perfect filling of the die cavity for the whole range .of billet volume and technological conditions. This method has the advantages that more complicated shapes tan be forged and the volume accuracy of a billet may be lower because the accommodation space is greater. On the other hand, this means higher material consumPtion. The excess material in an internal flash has to create sufftcient pressure for filling. The forging Process itself requires more forging operations. The outcome is that it is necessary to en- sure precise positioning of the preform, we need a larger JI Acta Polytechnica Vol. 43 No. 3/2003 Fig. 3: Accommodation space in the central internal web working (clamping) area in the forging machine, and the shift output is lower. 5.3 Forging into closed dies with accornmo dation into for gtn g height The principle of this rechnique is given by the fact that the excess material is "transferred" into the forging height - ei- ther the total forging height or the height of a part of the forging - according to the current preform volume. -I'his is achieved by special die design when the upper die or a part of die is sprung. Special multi-part dies with springing should be used [2]. In the SKODA Auro forge, a special die holder has been developed, which can change the dies and spring- -operated elements quickly t5]. An important condition when using this technique is that these spring elements should have a given pressure value preset, so that the relative movement can occur only under given conditions. The metal flow in a spmng die can be divided inro rwo steps: o First step: The die cavity gradually fills. Towards the end, when the metal reaches the peripheral corners, the load starts increasing. o Second step: When the die cavity is completely full the significant increase in forging load produces a reiative movement of the main surface'of the die and the excess material can be accommodated in the space that arises. The lirniations of using this techni4ue as follows: l) Maximum billet excess volume: The height of forging has a certain tolerance. The maximum volume to be ac- commodated is the product of this value and the billet cross-section. This volume is compared with the billet volume tolerance given by the type of bar, its dimensions CFlralion 3 ,'.it]!r {$l l$mp.}rautre {(iJ and the method of separation. According ro practical ex- perience in the SKODA Auto forge, the weight tolerance of billets used for close die forging of gears is -r l0 g. This is the case for forging with outer diamerer 70 mm and height tolerance r-0.4 mm. This value corresponds with the range of l-2 Vo according to rhe forging weight. This precision ofseparation can be achieved by using standard shears. 2) Forging dimensions: A rough esrimarion of the forging load is based on the forging cross-secrion and the forging pressure. In this case the forging load should be increased for about a value of load generared by the load-limiting device. At this force, the device should act against the movenlent of the die, even if there is no material. The design of such a device fiig) for eliminating this load of about 0.8 to 1.8 MN ( e.g., for case of rransmission wheels) is rather spatially demanding. To decrease this load, a multi-part device can be used and the pressure acting on the "floating" part of die only can be eliminated. 3) General rule for die design: In this case it is not necessary to use a preform forging stage, because the excess mate- rial is accommodared in the finished operation. f'he first forming step is upsetting between the flat dies in rhe same manner as fbr other methods. 'Ihe shape of the final die cavity corresponds to the forging shape. Ifit is necessary to eliminate just a part of the load, split dies should be used. This forging method is generally comparable with forging where the accommodation space is in the corners of the forg- ing. The principle diflerence is that the accuracy of billet dimensions can be lower because the accommodation space may be greater. Adjusting the machine stroke is also easier here. The forging height is a function of rhe billet volume, 3 5 ti I -l.lto -6.595 Fig. 4: Accommodation space in different forging height 32 Acta Polytechnica Vol. 43 No. 312003 Table 2 Method of accommodation Advantages Disadvantages Space in edge radii Simple die design. Low number of forging operations' Precisb billet volume. Narrow range of process conditions. Risk of press overloading. Space in internal flash Lower billet volume accuracy. Complicated shapes can be forged. Low shift ofitput, more operations. Precise positioning of preform. Higher material consumption. Space in forging height Lower billet volume accuracy. High shift output. Easy machine adjustment. More sophisticated and more exPen- sive tool design. Special spring-operated device. but without the influence of press stiffness. The main dis- advantage is that more sophisticated tool design is necessary. A special spring-operated device is needed. 6 Conclusion The production ofnear-net shape forgings' using conven- tional folging equipment, is practically possible. Elimination of the conventional flash involves solving of a basic problem, which is how to accommodate the excess billet material. This study has concentrated on forgings ofrotational shape. Three different methods of die design were developed. A r6sumd of advantages and disadvantages is given in Thble 2. The deci- sion on which method use, depends on the forying dimen- sions, forging shape complexity, run size, and the stability of the process conditions. Future research will focus on forgings of oblong shapes. The fundamental condition for successful solution is the use of advanced numerical simulation software' 7 Acknowledgments This research has been conducted in the framervork of CTU in Prague research project No. J04/98:212200008 and in collaboration with the SKODA Auto forge shop. 8 References tll Schmiedeteile: Gestaltung, Anwendur-rg, Beispiele' lVeb site: http://www. ids.wsm-net. de l2l Dean, T. A.: Die Tech'nologl' The Feasibilit2 of Flnshless Forgzng.In: "Metallurgia and Metal Forming"' Redhill t3l t4l t5l (England): Fuel & MetallurgicalJournals Ltd., Novem- ber 1977, p.488-a98. iermiik, J., Vrabec, M., Hor6k, J. : ConQuter Aifu d Optirni- sation of Die Duign and Production. In: International Conference on Advanced Engineering Design. Glasgow, University of Glasgow, 2001, p. 300-304. Forgtng Design HandbooA. American Society for Metals' Metals Park, Ohio. 1972. Gr 6f , G., eermiik, J. : Close - Die Fktsltle ss F orging in Automo' tiue Industry. In: Journal for Technology of Plasticity. Novi Sad (Yugoslavia): Fac. of Technical Sciences. Inst. for Prod. Eng., Vol. 26, No. 2, 2001, p. 1-10. Dr. Jan iermdk e-mail: jan.cermak@fs.cvut.cz Department of Manufacturing Engineering Czech Technical University in Prague Faculty of Ir{ec}ranical Engineering Technickd 4 166 07 Prague 6, Czech RePublic Ing. Gustav Grdf e-mail : gustav. graf@skoda-auto.cz Department of Forge TechnologY SKODA Auto, a.s. V. Klimenta 869 293 60 Mladii Boleslav, Czech Republic 33 Scan28 Scan29 Scan30 Scan31 Scan32 Scan33